The location of spinal neurons with long descending axons (long descending propriospinal tract neurons) in the cat: a study with the horseradish peroxidase technique

Silencing long-descending inter-enlargement propriospinal neurons improves hindlimb stepping after contusive spinal cord injuries

Spinal locomotor circuitry is comprised of rhythm generating centers, one for each limb, that are interconnected by local and long-distance propriospinal neurons thought to carry temporal information necessary for interlimb coordination and gait control. We showed previously that conditional silencing of the long ascending propriospinal neurons (LAPNs) that project from the lumbar to the cervical rhythmogenic centers (L1/L2 to C6), disrupts right-left alternation of both the forelimbs and hindlimbs without significantly disrupting other fundamental aspects of interlimb and speed-dependent coordination (Pocratsky et al., 2020). Subsequently, we showed that silencing the LAPNs after a moderate thoracic contusive spinal cord injury (SCI) resulted in better recovered locomotor function (Shepard et al., 2021). In this research advance, we focus on the descending equivalent to the LAPNs, the long descending propriospinal neurons (LDPNs) that have cell bodies at C6 and terminals at L2. We found that conditional silencing of the LDPNs in the intact adult rat resulted in a disrupted alternation of each limb pair (forelimbs and hindlimbs) and after a thoracic contusion SCI significantly improved locomotor function. These observations lead us to speculate that the LAPNs and LDPNs have similar roles in the exchange of temporal information between the cervical and lumbar rhythm generating centers, but that the partial disruption of the pathway after SCI limits the independent function of the lumbar circuitry. Silencing the LAPNs or LDPNs effectively permits or frees-up the lumbar circuitry to function independently.

Terminal organization of the corticospinal projection from the arm/hand region of the rostral primary motor cortex (M1r or old M1) to the cervical enlargement (C5-T1) in rhesus monkey

High-resolution anterograde tracers and stereology were used to study the terminal organization of the corticospinal projection (CSP) from the rostral portion of the primary motor cortex (M1r) to spinal levels C5-T1. Most of this projection (90%) terminated contralaterally within laminae V-IX, with the densest distribution in lamina VII. Moderate bouton numbers occurred in laminae VI, VIII, and IX with few in lamina V. Within lamina VII, labeling occurred over the distal-related dorsolateral subsectors and proximal-related ventromedial subsectors. Within motoneuron lamina IX, most terminations occurred in the proximal-related dorsomedial quadrant, followed by the distal-related dorsolateral quadrant. Segmentally, the contralateral lamina VII CSP gradually declined from C5-T1 but was consistently distributed at C5-C7 in lamina IX. The ipsilateral CSP ended in axial-related lamina VIII and adjacent ventromedial region of lamina VII. These findings demonstrate the M1r CSP influences distal and proximal/axial-related spinal targets. Thus, the M1r CSP represents a transitional CSP, positioned between the caudal M1 (M1c) CSP, which is 98% contralateral and optimally organized to mediate distal upper extremity movements (Morecraft et al., 2013), and dorsolateral premotor (LPMCd) CSP being 79% contralateral and optimally organized to mediate proximal/axial movements (Morecraft et al., 2019). This distal to proximal CSP gradient corresponds to the clinical deficits accompanying caudal to rostral motor cortex injury. The lamina IX CSP is considered in the light of anatomical and neurophysiological evidence which suggests M1c gives rise to the major proportion of the cortico-motoneuronal (CM) projection, while there is a limited M1r CM projection.

Anatomical Plasticity of Rostrally Terminating Axons as a Possible Bridging Substrate across a Spinal Injury

Transfer of information across a spinal lesion is required for many aspects of recovery across diverse motor systems. Our understanding of axonal plasticity and which subpopulations of neurons may contribute to bridging substrates following injury, however, remains relatively incomplete. Most recently, attention has been directed to propriospinal neurons (PSNs), with research suggesting that they are capable of bridging a spinal lesion in rodents. In the current study, subpopulations of both long (C5) and short (T6, T8) PSNs-as well as a supraspinal system, the rubrospinal tract (RST)-were assessed following low thoracic (T9) hemisection in the cat using the retrograde tracer Fluoro-Gold. Acutely, within 2 weeks post-hemisection, the numbers of short and long PSNs, as well as contralateral RST neurons, with axons crossing the lesion were significantly decreased relative to uninjured controls. This decrease persisted bilaterally and was permanent in the long PSNs and the contralateral red nucleus (RN). However, by 16 weeks post-hemisection, the numbers of ipsilesional and contralesional short PSNs bridging the lesion were significantly increased. Further, the number of contralesional contributing short PSNs was significantly greater in injured animals than in uninjured animals. A significant increase over uninjured numbers also was seen in the ipsilateral (non-axotomized) RN. These findings suggest that a novel substrate of undamaged axons, which normally terminates rostral to the lesion, grows past a thoracic lesion after injury. This rostral population represents a major component of the bridging substrate seen and may represent an important anatomical target for evolving rehabilitation approaches as a substrate capable of contributing to functional recovery.

Propriospinal Neurons: Essential Elements of Locomotor Control in the Intact and Possibly the Injured Spinal Cord

Propriospinal interneurons (INs) communicate information over short and long distances within the spinal cord. They act to coordinate different parts of the body by linking motor circuits that control muscles across the forelimbs, trunk, and hindlimbs. Their role in coordinating locomotor circuits near and far may be invaluable to the recovery of locomotor function lost due to injury to the spinal cord where the flow of motor commands from the brain and brainstem to spinal motor circuits is disrupted. The formation and activation of circuits established by spared propriospinal INs may promote the re-emergence of locomotion. In light of progress made in animal models of spinal cord injury (SCI) and in human patients, we discuss the role of propriospinal INs in the intact spinal cord and describe recent studies investigating the assembly and/or activation of propriospinal circuits to promote recovery of locomotion following SCI.

Terminal organization of the corticospinal projection from the lateral premotor cortex to the cervical enlargement (C5-T1) in rhesus monkey

High-resolution tract tracing and stereology were used to study the terminal organization of the corticospinal projection (CSP) from the ventral (v) and dorsal (d) regions of the lateral premotor cortex (LPMC) to spinal levels C5-T1. The LPMCv CSP originated from the postarcuate sulcus region, was bilateral, sparse, and primarily targeted the dorsolateral and ventromedial sectors of contralateral lamina VII. The convexity/lateral part of LPMCv did not project below C2. Thus, very little LPMCv corticospinal output reaches the cervical enlargement. In contrast, the LPMCd CSP was 5× more prominent in terminal density. Bilateral terminal labeling occurred in the medial sectors of lamina VII and adjacent lamina VIII, where propriospinal neurons with long-range bilateral axon projections reside. Notably, lamina VIII also harbors axial motoneurons. Contralateral labeling occurred in the lateral sectors of lamina VII and the dorsomedial quadrant of lamina IX, noted for harboring proximal upper limb flexor motoneurons. Segmentally, the CSP to contralateral laminae VII and IX preferentially innervated C5-C7, which supplies shoulder, elbow, and wrist musculature. In contrast, terminations in axial-related lamina VIII were distributed bilaterally throughout all cervical enlargement levels, including C8 and T1. These findings demonstrate the LPMCd CSP is structured to influence axial and proximal upper limb movements, supporting Kuypers conceptual view of the LPMCd CSP being a major component of the medial motor control system. Thus, distal upper extremity control influenced by LPMC, including grasping and manipulation, must occur through indirect neural network connections such as corticocortical, subcortical, or intrinsic spinal circuits.

Computational modeling of spinal circuits controlling limb coordination and gaits in quadrupeds

Interactions between cervical and lumbar spinal circuits are mediated by long propriospinal neurons (LPNs). Ablation of descending LPNs in mice disturbs left-right coordination at high speeds without affecting fore-hind alternation. We developed a computational model of spinal circuits consisting of four rhythm generators coupled by commissural interneurons (CINs), providing left-right interactions, and LPNs, mediating homolateral and diagonal interactions. The proposed CIN and diagonal LPN connections contribute to speed-dependent gait transition from walk, to trot, and then to gallop and bound; the homolateral LPN connections ensure fore-hind alternation in all gaits. The model reproduces speed-dependent gait expression in intact and genetically transformed mice and the disruption of hindlimb coordination following ablation of descending LPNs. Inputs to CINs and LPNs can affect interlimb coordination and change gait independent of speed. We suggest that these interneurons represent the main targets for supraspinal and sensory afferent signals adjusting gait.

An attempt to explain the Vojta therapy mechanism of action using the surface polyelectromyography in healthy subjects: A pilot study

BACKGROUND

Rehabilitation according to Vojta is a neurophysiological method used to obtain reflex responses in muscles following stimulation of particular activation zones.

OBJECTIVES

This study aims to objectively evaluate the muscular responses following stimulation according to Vojta's method. The possible routes of spinal transmission responsible for the phenomenon of muscle activation in upper and lower extremities are considered.

METHODS

Polyelectromyographic (pEMG) recordings in the upper and lower extremities in healthy volunteers (N = 25; aged 24 ± 1 year) were performed to find out the possible routes of spinal transmission, responsible for muscle activation. The left acromion and right femoral epicondyle were stimulated by a Vojta therapist; pEMG recordings were made including the bilateral deltoid and rectus femoris muscles.

RESULTS AND DISCUSSION

Following acromion stimulation, muscle activation was mostly expressed in the contralateral rectus femoris, rather than the contralateral deltoid and the ipsilateral rectus femoris muscles. After stimulation of the lower femoral epicondyle, the following order was observed: contra lateral deltoid, ipsilateral deltoid and the contra lateral rectus femoris muscle. One of the candidates responsible for the main crossed neural transmission involved in the Vojta therapy mechanism would be the long propriospinal tract neurons.

The neural control of interlimb coordination during mammalian locomotion

Neuronal networks within the spinal cord directly control rhythmic movements of the arms/forelimbs and legs/hindlimbs during locomotion in mammals. For an effective locomotion, these networks must be flexibly coordinated to allow for various gait patterns and independent use of the arms/forelimbs. This coordination can be accomplished by mechanisms intrinsic to the spinal cord, somatosensory feedback from the limbs, and various supraspinal pathways. Incomplete spinal cord injury disrupts some of the pathways and structures involved in interlimb coordination, often leading to a disruption in the coordination between the arms/forelimbs and legs/hindlimbs in animal models and in humans. However, experimental spinal lesions in animal models to uncover the mechanisms coordinating the limbs have limitations due to compensatory mechanisms and strategies, redundant systems of control, and plasticity within remaining circuits. The purpose of this review is to provide a general overview and critical discussion of experimental studies that have investigated the neural mechanisms involved in coordinating the arms/forelimbs and legs/hindlimbs during mammalian locomotion.

Long-Distance Descending Spinal Neurons Ensure Quadrupedal Locomotor Stability

Locomotion is an essential animal behavior used for translocation. The spinal cord acts as key executing center, but how it coordinates many body parts located across distance remains poorly understood. Here we employed mouse genetic and viral approaches to reveal organizational principles of long-projecting spinal circuits and their role in quadrupedal locomotion. Using neurotransmitter identity, developmental origin, and projection patterns as criteria, we uncover that spinal segments controlling forelimbs and hindlimbs are bidirectionally connected by symmetrically organized direct synaptic pathways that encompass multiple genetically tractable neuronal subpopulations. We demonstrate that selective ablation of descending spinal neurons linking cervical to lumbar segments impairs coherent locomotion, by reducing postural stability and speed during exploratory locomotion, as well as perturbing interlimb coordination during reinforced high-speed stepping. Together, our results implicate a highly organized long-distance projection system of spinal origin in the control of postural body stabilization and reliability during quadrupedal locomotion.

Interlimb Coordination during Tied-Belt and Transverse Split-Belt Locomotion before and after an Incomplete Spinal Cord Injury

Coordination between the arms/forelimbs and legs/hindlimbs is often impaired in humans and quadrupedal mammals after incomplete spinal cord injury. In quadrupeds, the forelimbs often take more steps than the hindlimbs, producing a two-to-one forelimb-hindlimb (2-1 FL-HL) coordination. In locomotor performance scales, this is generally considered a loss of FL-HL coordination. Here, FL-HL coordination was quantified before and 8 weeks after a lateral spinal hemisection at the sixth thoracic segment in six adult cats. Cats were tested during tied-belt locomotion (equal front and rear speeds) and transverse split-belt locomotion with the forelimbs or hindlimbs stepping faster. The results show that consistent phasing between forelimb and hindlimb movements was maintained after hemisection, even with the appearance of 2-1 FL-HL coordination, indicating that new stable forms of coordination emerge. Moreover, transverse split-belt locomotion potently modulated interlimb coordination and was capable of restoring a one-to-one FL-HL coordination with a faster treadmill speed for the hindlimbs. In conclusion, the results suggest that neural communication persists after an incomplete spinal cord injury, despite an unequal number of steps between the forelimbs and hindlimbs, and that interlimb coordination can be modulated by having the forelimbs or hindlimbs move at a faster frequency. We propose that locomotor recovery scales incorporate more sensitive methods to quantify FL-HL coordination, to better reflect residual functional capacity and possible cervicolumbar neural communication. Lastly, devising training protocols that make use of the bidirectional influences of the cervical and lumbar locomotor pattern generators could strengthen interlimb coordination and promote locomotor recovery.

Frontal and frontoparietal injury differentially affect the ipsilateral corticospinal projection from the nonlesioned hemisphere in monkey (Macaca mulatta)

Upper extremity hemiplegia is a common consequence of unilateral cortical stroke. Understanding the role of the unaffected cerebral hemisphere in the motor recovery process has been encouraged, in part, by the presence of ipsilateral corticospinal projections (iCSP). We examined the neuroplastic response of the iCSP from the contralesional primary motor cortex (cM1) hand/arm area to spinal levels C5-T1 after spontaneous long-term recovery from isolated frontal lobe injury and isolated frontoparietal injury. High-resolution tract tracing, stereological, and behavioral methodologies were applied. Recovery from frontal motor injury resulted in enhanced numbers of terminal labeled boutons in the iCSP from cM1 compared with controls. Increases occurred in lamina VIII and the adjacent ventral sectors of lamina VII, which are involved in axial/proximal limb sensorimotor processing. Larger frontal lobe lesions were associated with greater numbers of terminal boutons than smaller frontal lobe lesions. In contrast, frontoparietal injury blocked this response; total bouton number was similar to controls, demonstrating that disruption of somatosensory input to one hemisphere has a suppressive effect on the iCSP from the nonlesioned hemisphere. However, compared with controls, elevated bouton numbers occurred in lamina VIII, at the expense of lamina VII bouton labeling. Lamina IX boutons were also elevated in two frontoparietal lesion cases with extensive cortical injury. Because laminae VIII and IX collectively harbor axial, proximal, and distal motoneurons, therapeutic intervention targeting the ipsilateral corticospinal linkage from cM1 may promote proximal, and possibly distal, upper-limb motor recovery following frontal and frontoparietal injury.

Dense distributed processing in a hindlimb scratch motor network

In reduced preparations, hindlimb movements can be generated by a minimal network of neurons in the limb innervating spinal segments. The network of neurons that generates real movements is less well delineated. In an ex vivo carapace-spinal cord preparation from adult turtles (Trachemys scripta elegans), we show that ventral horn interneurons in mid-thoracic spinal segments are functionally integrated in the hindlimb scratch network. First, mid-thoracic interneurons receive intense synaptic input during scratching and behave like neurons in the hindlimb enlargement. Second, some mid-thoracic interneurons activated during scratching project descending axons toward the hindlimb enlargement. Third, elimination of mid-thoracic segments leads to a weakening of scratch rhythmicity. We conclude that densely innervated interneurons in mid-thoracic segments contribute to hindlimb scratching and may be part of a distributed motor network that secures motor coherence.

Central neuropathic pain in MS is due to distinct thoracic spinal cord lesions

Objective

To determine a neuro-anatomic cause for central neuropathic pain (CNP) observed in multiple sclerosis (MS) patients.

Methods

Parallel clinical and neuro-anatomical studies were performed. A clinical investigation of consecutively acquired MS patients with and without CNP (i.e. cold allodynia or deep hyperesthesia) within a single MS center was pursued. A multivariate logistic regression model was used to assess the relationship between an upper central thoracic spinal cord focus to central pain complaints. To identify the hypothesized autonomic interneurons with bilateral descending projections to lumbosacral sensory neurons, retrograde single- and double-labeling experiments with CTb and fluorescent tracers were performed in three animal species (i.e. rat, cat, and monkey).

Results

Clinical data were available in MS patients with (n = 32; F:23; median age: 34.6 years (interquartile range [IQR]: 27.4–45.5)) and without (n = 30; F:22; median age: 36.6 years [IQR: 31.6–47.1]) CNP. The value of a central focus between T1–T6 in relation to CNP demonstrated a sensitivity of 96.9% (95% confidence interval [CI]: 83.8–99.9) and specificity of 83.3% (95% CI: 65.3–94.4). A significant relationship between CNP and a centrally located focus within the thoracic spine was also observed (odds ratio [OR]: 155.0 [95% CI lower limit: 16.0]; P < 0.0001, two-tailed Fisher exact test). In all animal models, neurons with bilateral descending projections to the lumbosacral superficial dorsal horn were concentrated in the autonomic intermediomedial nucleus surrounding the mid-thoracic central canal.

Interpretation

Our observations provide the first evidence for the etiology of CNP. These data may assist with the development of refined symptomatic therapies and allow for insights into unique pain syndromes observed in other demyelinating subtypes.

A novel growth-promoting pathway formed by GDNF-overexpressing Schwann cells promotes propriospinal axonal regeneration, synapse formation, and partial recovery of function after spinal cord injury

Descending propriospinal neurons (DPSN) are known to establish functional relays for supraspinal signals, and they display a greater growth response after injury than do the long projecting axons. However, their regenerative response is still deficient due to their failure to depart from growth supportive cellular transplants back into the host spinal cord, which contains numerous impediments to axon growth. Here we report the construction of a continuous growth-promoting pathway in adult rats, formed by grafted Schwann cells overexpressing glial cell line-derived neurotrophic factor (GDNF). We demonstrate that such a growth-promoting pathway, extending from the axonal cut ends to the site of innervation in the distal spinal cord, promoted regeneration of DPSN axons through and beyond the lesion gap of a spinal cord hemisection. Within the distal host spinal cord, regenerated DPSN axons formed synapses with host neurons leading to the restoration of action potentials and partial recovery of function.

A novel population of cholinergic neurons in the macaque spinal dorsal horn of potential clinical relevance for pain therapy

Endogenous acetylcholine (ACh) is a well-known modulator of nociceptive transmission in the spinal cord of rodents. It arises mainly from a sparse population of cholinergic interneurons located in the dorsal horn of the spinal cord. This population was thought to be absent from the spinal cord of monkey, what might suggest that spinal ACh would not be a relevant clinical target for pain therapy. In humans, however, pain responses can be modulated by spinal ACh, as evidenced by the increasingly used analgesic procedure (for postoperative and labor patients) consisting of the epidural injection of the acetylcholinesterase inhibitor neostigmine. The source and target of this ACh remain yet to be elucidated. In this study, we used an immunolabeling for choline acetyltransferase to demonstrate, for the first time, the presence of a plexus of cholinergic fibers in laminae II-III of the dorsal horn of the macaque monkey. Moreover, we show the presence of numerous cholinergic cell bodies within the same laminae and compared their density and morphological properties with those previously described in rodents. An electron microscopy analysis demonstrates that cholinergic boutons are presynaptic to dorsal horn neurons as well as to the terminals of sensory primary afferents, suggesting that they are likely to modulate incoming somatosensory information. Our data suggest that this newly identified dorsal horn cholinergic system in monkeys is the source of the ACh involved in the analgesic effects of epidural neostigmine and could be more specifically targeted for novel therapeutic strategies for pain management in humans.

Ascending and descending propriospinal pathways between lumbar and cervical segments in the rat: evidence for a substantial ascending excitatory pathway

Precise mechanisms are required to coordinate the locomotor activity of fore- and hind-limbs in quadrupeds and similar mechanisms persist to coordinate movement of arms and legs in humans. Propriospinal neurons (PSNs) are major components of the networks that coordinate these mechanisms. The b subunit of cholera toxin (CTb) was injected unilaterally into either L1 or L3 segments in order to label ascending and descending propriospinal pathways. Labelled cells were examined with light or confocal microscopy. Cells projecting to lumbar segments were evenly distributed, bilaterally throughout all cervical segments. However many more cells were labelled from L1 injections than L3 injections. Roughly 15% of cells in both sides of the C2 segment was found to be immunoreactive for calretinin and a small number (4%) was immunoreactive for calbindin. Axons projecting from L1 to cervical segments formed predominant ipsilateral projections to the cervical intermediate grey matter and ventral horn. Very large numbers of terminals were concentrated within the ventrolateral motor (VLM) nuclei of C7-8 segments but there was sparse innervation of the contralateral nucleus. The vast majority (85%) of these axon terminals in the ipsilateral VML was immunoreactive for the vesicular glutamate transporter 2 (VGLUT2) and the remaining 15% was immunoreactive for the vesicular GABA transporter (VGAT); many of these contained GABA and/or glycine. Inhibitory and excitatory terminals were also found in the contralateral VLM. Most of the terminals in the VLM made contacts with motoneurons. The major finding of this study is the existence of a substantial excitatory propriospinal pathway that projects specifically to the VLM. Motoneurons in the VLM supply muscles of the axilla therefore this pathway is likely to have a profound influence on the activity of the shoulder joint. This pathway may synchronise lumbar and cervical pattern generators and hence the coordination of locomotor activity in the fore- and hind limbs.

Coordination between the fore- and hindlimbs is bidirectional, asymmetrically organized, and flexible during quadrupedal locomotion in the intact adult cat

Despite the obvious importance of inter-girdle coordination for quadrupedal locomotion in terrestrial mammals, its organization remains poorly understood. Here, we evaluated cycle and phase durations, as well as footfall patterns of four intact adult cats trained to walk on a transverse split-belt treadmill that could independently control fore- and hindlimb speed. When the hindlimbs walked at faster speeds than the forelimbs, an equal rhythm was always maintained between the fore- and hindlimbs, even at the highest fore-hindlimb speed ratio of 1:3 (0.4:1.2 m/s). The locomotor pattern adjusted through changes in both hindlimb stance and swing phase durations, whereas only the forelimb stance phase was affected. In such conditions, when fore- and hindlimb values were compared to those obtained at matched speeds during tied-belt walking (i.e. predicted values based on treadmill speed), hindlimb cycle, stance and swing durations were consistently longer than predicted. On the other hand, forelimb cycle and stance durations were shorter than predicted but only at the highest split-belt speed ratios. Forelimb swing durations were as predicted based on front-belt speed. The sequence of footfall pattern when hindlimb speed was faster was identical to tied-belt walking. In stark contrast, when the forelimbs walked at slightly faster speeds than the hindlimbs, the rhythm between the fore- and hindlimbs broke down. In such conditions, the locomotor pattern was adjusted through changes in stance and swing phase durations in both the fore- and hindlimbs. When the rhythm between the fore- and hindlimbs broke down, hindlimb cycle and phase durations were similar to predicted values, whereas forelimb values were shorter than predicted. Moreover, several additional sequences of footfall patterns were observed. Therefore, the results clearly demonstrate the existence of a bidirectional, asymmetric, and flexible control of inter-girdle coordination during quadrupedal locomotion in the intact adult cat.

Cervical multisegmental motor responses in healthy subjects

STUDY DESIGN

Experimental design.

OBJECTIVES

This descriptive design study presents multisegmental motor responses in the upper extremities after stimulation of cervical spinal vertebrae.

SETTING

Neuro Lab of Texas Woman's University, School of Physical Therapy, Texas, USA.

METHODS

In trial 1, C7 spinal segment was electrically stimulated in 13 healthy subjects using surface electrodes while recording responses from abductor digiti minimi (ADM), abductor pollicis brevis (APB), flexor carpi radialis (FCR) and biceps brachii (BB) in the right upper extremity. In trial 2, C7 stimulation was carried out while recording responses from the APB and BB bilaterally. In trial 3, C7 stimulation was carried out while recording responses from the flexor hallucis brevis (FHB), soleus, vastus medialis (VMO) and gluteus medius (GM) in the right lower extremity. The amplitude of the signal and the deflection latency were the measured parameters. Descriptive statistics were completed on the data.

RESULTS

Results showed response amplitudes in all muscles of the upper extremities ranging from 328 to 1239 μV, with the largest recorded from the APB and ADM, then the FCR and BB. Muscular responses were recorded simultaneously in bilateral muscles. Response latency was comparable, in bilateral similar muscles, and was varied from 6 to 16.5 msec, being longer in the ADM and APB, shorter in FCR and shortest in the BB. No lower limb muscles responded to C7 spinal stimulation, using current setup/method.

CONCLUSION

These responses appear to be caused by stimulating the dorsal roots or motor nuclei of the cervical region and could be useful in testing patients with cervical spinal disorders.

The role of propriospinal interneurons in recovery from spinal cord injury

Over one hundred years ago, Sir Charles Sherrington described a population of spinal cord interneurons (INs) that connect multiple spinal cord segments and participate in complex or 'long' motor reflexes. These neurons were subsequently termed propriospinal neurons (PNs) and are known to play a crucial role in motor control and sensory processing. Recent work has shown that PNs may also be an important substrate for recovery from spinal cord injury (SCI) as they contribute to plastic reorganisation of spinal circuits. The location, inter-segmental projection pattern and sheer number of PNs mean that after SCI, a significant number of them are capable of 'bridging' an incomplete spinal cord lesion. When these properties are combined with the capacity of PNs to activate and coordinate locomotor central pattern generators (CPGs), it is clear they are ideally placed to assist locomotor recovery. Here we summarise the anatomy, organisation and function of PNs in the uninjured spinal cord, briefly outline the pathophysiology of SCI, describe how PNs contribute to recovery of motor function, and finally, we discuss the mechanisms that underlie PN plasticity. We propose there are two major challenges for PN research. The first is to learn more about ways we can promote PN plasticity and manipulate the 'hostile' micro-environment that limits regeneration in the damaged spinal cord. The second is to study the cellular/intrinsic properties of PNs to better understand their function in both the normal and injured spinal cord. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Thoracolumbar multisegmental motor responses in the upper and lower limbs in healthy subjects

STUDY DESIGN

Experimental design.

OBJECTIVES

This descriptive study presents muscular responses from both the upper and the lower extremities during T11-12 segmental stimulation.

SETTING

Neuro Lab of the Texas Woman's University (School of Physical Therapy, TX, USA).

METHODS

A total of 13 healthy subjects were electrically stimulated using surface electrodes. In trial 1, signals were recorded from the flexor hallucis brevis, soleus, vastus medialis and gluteus medius in the lower right extremity. In trial 2, responses were recorded from the abductor digiti minimi, abductor pollicis brevis (APB), flexor carpi radialis and biceps brachii in the right upper extremity. In trial 3, stimulation was carried out and signals were recorded for both the upper and the lower extremities simultaneously, using different muscle combinations. Five traces per muscle were averaged for each step of the testing. Amplitude and deflection latency were the measured parameters and were compared using descriptive statistics.

RESULTS

Results showed signal amplitudes ranging from 85 to 821 μV in the upper extremity and from 582 to 3927 μV in the lower extremity, with the largest signal recorded in the soleus muscle and the APB. Response latency varies between 5.5 and 14 ms in the upper limbs and between 7.7 and 27 ms in the lower limbs and was comparable in bilateral recording.

CONCLUSION

These muscular responses seem to be elicited from electrical stimulation of motor nuclei in lower limb muscles or from pathways to those nuclei in upper limb muscles, and could be useful in testing patients with spinal disorders.

SPP1 expression in spinal motor neurons of the macaque monkey

In the macaque cerebral cortex, the SPP1 (secreted phosphoprotein 1) gene is mainly expressed in corticospinal neurons. In this study, we found that SPP1 was principally expressed in motor neurons in lamina IX of the macaque spinal cord. The expression level varied among different spinal segments and correlated positively with neuron size. The expression was weak in Errγ-positive neurons, presumably gamma motor neurons, and in neurons in sacral Onuf's nucleus. These results suggest that SPP1 is a molecular characteristic of spinal motor neurons and is preferentially expressed in neurons with high conduction velocities.

Organization and neurochemical properties of intersegmental interneurons in the lumbar enlargement of the adult rat

Intersegmental interneurons with relatively short axons perform an important role in the coordination of limb movement but surprisingly little is known about their organization and how they contribute to neuronal networks in the adult rat. We undertook a series of anatomical tract-tracing studies to label cell bodies and axons of intersegmental neurons in the lumbar cord and characterized their neurochemical properties by using immunocytochemistry. The b-subunit of cholera toxin was injected into L1 or L3 segments of seven rats in the vicinity of lateral or medial motor nuclei. In L5 lumbar segments, cells were found to be concentrated in contralateral lamina VIII, and in ipsilateral lamina VII and laminae V-VI following injections into the lateral and medial motor nuclei respectively. About 25% of labelled cells contained calbindin or calretinin or a combination of both. Calbindin positive cells were mainly distributed within the ipsilateral side of the L5 segment, especially within the ipsilateral dorsal horn whereas there was a concentration of calretinin cells in contralateral lamina VIII. A small population of cells around the central canal were cholinergic. We also examined axon terminals that projected from L1/3 to the L5 contralateral lateral motor nucleus. The majority of these axons were excitatory (75%) and made direct contacts with motoneurons. However, most inhibitory axons in L5 contained a mixture of GABA and glycine (20%) and about 22% of the total population of axons contained calbindin. In contrast, 19% of all intra-segmental axons in the L3 contralateral lateral motor nucleus were found to be purely glycinergic and 17% contained a mixture of GABA and glycine. This study shows that short range interneurons form extensive ipsi- and contralateral projections within the lumbar enlargement and that many of them contain calcium binding proteins. Those projecting contralaterally to motor nuclei are predominantly excitatory.

Lack of axonal sprouting of spared propriospinal fibers caudal to spinal contusion injury is attributed to chronic axonopathy

We have previously shown that a small percentage of long descending propriospinal tract (LDPT) axons are spared, whereas few short thoracic propriospinal (TPS) fibers survive 2 weeks following severe (50 mm weight drop) low thoracic spinal cord contusion injury (SCI). Here, we extended those findings to a moderate (25 mm weight drop) T9 SCI and assessed the effects of this lesion severity on propriospinal tract fibers at different time periods after injury. We anterogradely labeled fibers with fluororuby (FR) or WGA-HRP to determine their location and number 2, 4, 6, and 16 weeks post-SCI. Findings were compared with non-injured controls. At chronic time points, surviving FR-labeled LDPT fibers rostral to the injury remained as reactive endings or as putative regenerative sprouts. Caudal to the injury, spared LDPT fibers ran along a rim of lateral and ventral white matter, and ended as small abnormal-appearing putative terminal boutons or reactive endings within the intermediate gray matter of lumbosacral cord, with little axonal arborization and no evidence of injury-induced sprouting. One striking difference in the WGA-HRP experimental operates was the increased density of labeling of spared axons within the white matter caudal to the injury compared to controls. This labeling pattern was reminiscent of the labeling found after axotomy in studies by others, and raises a question as to contusion injury-induced impaired axonal transport. We hypothesize that axonal sprouting of axons after partial spinal cord injury seen in previous investigations was not found in the present investigation because of the additional pathological effects of contusion injury, similar to what is observed after traumatic brain injury.

Propriospinal neurons are sufficient for bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord

We recently showed that propriospinal neurons contribute to bulbospinal activation of locomotor networks in the in vitro neonatal rat brainstem-spinal cord preparation. In the present study, we examined whether propriospinal neurons alone, in the absence of long direct bulbospinal transmission to the lumbar cord, can successfully mediate brainstem activation of the locomotor network. In the presence of staggered bilateral spinal cord hemisections, the brainstem was stimulated electrically while recording from lumbar ventral roots. The rostral hemisection was located between C1 and T3 and the contralateral caudal hemisection was located between T5 and mid-L1. Locomotor-like activity was evoked in 27% of the preparations, which included experiments with staggered hemisections placed only two segments apart. There was no relation between the likelihood of developing locomotor-like activity and the distance separating the two hemisections or specific level of the hemisections. In some experiments, where brainstem stimulation alone was ineffective, neurochemical excitation of propriospinal neurons (using 5-HT and NMDA) at concentrations subthreshold for producing locomotor-like activity, promoted locomotor-like activity in conjunction with brainstem stimulation. In other experiments, involving neither brainstem stimulation nor cord hemisections, the excitability of propriospinal neurons in the cervical and/or thoracic region was selectively enhanced by bath application of 5-HT and NMDA or elevation of bath K(+) concentration. These manipulations produced locomotor-like activity in the lumbar region. In total, the results suggest that propriospinal neurons are sufficient for transmission of descending locomotor command signals. This observation has implications for regeneration strategies aimed at restoration of locomotor function after spinal cord injury.

Spinal cord stimulation modulates intraspinal colorectal visceroreceptive transmission in rats

Previous studies have shown that spinal cord stimulation (SCS) of upper lumbar segments decreases visceromotor responses to mechanical stimuli in a sensitized rat colon and reduces symptoms of irritable bowel syndrome in patients. SCS applied to the upper cervical spinal dorsal column reduces pain of chronic refractory angina. Further, chemical stimulation of C1-C2 propriospinal neurons in rats modulates the responses of lumbosacral spinal neurons to colorectal distension. The present study was designed to compare the effects of upper cervical and lumbar SCS on activity of lumbosacral neurons receiving noxious colorectal input. Extracellular potentials of L6-S2 spinal neurons were recorded in pentobarbital anesthetized, paralyzed and ventilated male rats. SCS (50 Hz, 0.2 ms) at low intensity (90% of motor threshold) was applied to the dorsal column of upper cervical (C1-C2) or upper lumbar (L2-L3) ipsilateral spinal segments. Colorectal distension (CRD, 20 mmHg, 40 mmHg, 60 mmHg, 20s) was produced by air inflation of a latex balloon. Results showed that SCS applied to L2-L3 and C1-C2 segments significantly reduced the excitatory responses to noxious CRD from 417.6+/-68.0 to 296.3+/-53.6 imp (P<0.05, n=24) and from 336.2+/-64.5 to 225.0+/-73.3 imp (P<0.05, n=18), respectively. Effects of L2-L3 and C1-C2 SCS lasted 10.2+/-1.9 and 8.0+/-0.9 min after offset of CRD. Effects of SCS were observed on spinal neurons with either high or low-threshold excitatory responses to CRD. However, L2-L3 or C1-C2 SCS did not significantly affect inhibitory neuronal responses to CRD. C1-C2 SCS-induced effects were abolished by cutting the C7-C8 dorsal column but not by spinal transection at cervicomedullary junction. These data demonstrated that upper cervical or lumbar SCS modulated responses of lumbosacral spinal neurons to noxious mechanical stimulation of the colon, thereby, proved two loci for a potential therapeutic effect of SCS in patients with irritable bowel syndrome and other colonic disorders.

Long ascending propriospinal projections from lumbosacral to upper cervical spinal cord in the rat

The retrograde tracer cholera toxin beta-subunit (CTB) was used to trace long ascending propriospinal projections from neurons in the lumbosacral spinal cord to the upper cervical (C3) gray matter in adult male Sprague-Dawley rats. Following large 0.5 microl CTB injections restricted mainly to the upper cervical ventral horn (n=5), there were many lumbosacral CTB-positive neurons (14-17/section) in the intermediate gray and ventral horn (dorsal lamina VIII, medial VII extending into X) contralaterally, with fewer at corresponding ipsilateral locations. Labeled cells (4-8/section) were also observed in contralateral laminae IV-VI and the lateral spinal nucleus, with fewer ipsilaterally. Few labeled cells (<2/section) were observed in superficial laminae I-II. Smaller (0.15 microl) microinjections of CTB restricted to the upper cervical ventral gray matter labeled cells in contralateral laminae VII-VIII (approximately 6-9/section) with fewer ipsilaterally. There were relatively fewer (<2/section) in the intermediate dorsal horn and very few (<1/section) in lamina I. Larger (0.5 microl) CTB injections encompassing the C3 dorsal and ventral gray matter on one side labeled significantly more CTB-positive neurons (>6/section) in contralateral lamina I compared to ventral horn injections. These results suggest direct projections from ventromedially located neurons of lumbar and sacral segments to the contralateral ventral gray matter of upper cervical segments, as well as from neurons in the intermediate but not superficial dorsal horn. They further suggest that some lumbosacral superficial dorsal horn neurons project to the upper cervical dorsal horn. These propriospinal projections may be involved in coordinating head and neck movements during locomotion or stimulus-evoked motor responses.

Magnetically evoked inter-enlargement response: an assessment of ascending propriospinal fibers following spinal cord injury

The aim of the present study was to characterize a novel electrophysiological assessment, the magnetically evoked interenlargement response (MIER), by defining the anatomical location of the fast conducting (large myelinated) ascending axons that mediate the response and the relationship between the response and locomotor function following experimental spinal cord injury. Electromyographic (EMG) responses were recorded from the triceps muscles following magnetic stimulation of one hip. Short-latency (approximately 6 ms) EMGs were recorded from triceps muscles in normal controls and following different laceration injuries (dorsal, lateral or dorsal and lateral hemisections) or a 150-kilodyne (kd) contusion injury at the T9 level. The amplitude of the triceps MIER was significantly correlated to the area of spared white matter in the lateral funiculus and to hindlimb function during open field locomotion (r2 = 0.55). Following a complete lateral hemisection, MIERs were present in the triceps bilaterally following stimulation of either hip. Responses could also be recorded from the masseter muscles indicating that the influence of this pathway extends beyond the spinal cord. Anatomical evidence of a bilaterally distributed propriospinal pathway was found when biotinylated dextran amine (BDA) was injected into the lateral white matter on one side of the spinal cord at T9. BDA-labeled axons with varicosities were found bilaterally in the intermediate and ventral gray matter of the caudal region of the cervical enlargement. These observations suggest that MIERs may be useful to quantitatively assess neurotransmission and functional recovery over time after experimental spinal cord injury.

Propriospinal neurons contribute to bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord

This study examines whether propriospinal transmission contributes to descending propagation of the brainstem locomotor command signal in the in vitro neonatal rat spinal cord. Using double bath partitions, synaptic transmission was suppressed in the cervicothoracic region while monitoring locomotor-like activity on lumbar ventral roots evoked by either chemical or electrical stimulation of the brainstem. Locomotor-like activity induced by electrical stimulation was more stable (cycle period coefficient of variation (CV) 11.7 +/- 6.1%) than the rhythm induced by chemical stimulation (CV 31.3 +/- 6.4%). Ca(2+)-free bath solution, elevated Mg(2+) ion concentration, excitatory amino acid receptor antagonists (AP5 and/or CNQX), and the muscarinic receptor antagonist, atropine, were used in attempts to block synaptic transmission. Each of these manipulations, except muscarinic receptor blockade, was capable of blocking locomotor-like activity induced by brainstem stimulation. However, locomotor-like activity induced by higher intensity electrical stimulation of the brainstem (1.2-5 times threshold) was relatively refractory to synaptic suppression using AP5 and CNQX, and Ca(2+)-free solution was more effective if combined with high Mg(2+) (15 mm) or EGTA. Enhancement of neuronal excitation in the cervicothoracic region, using Mg(2+)-free bath solution, facilitated brainstem activation of locomotor-like activity in the lumbar cord, consistent with a propriospinal mechanism of locomotor signal propagation. Blockade of brainstem-induced locomotor-like activity was related to the number of cervicothoracic segments exposed to synaptic suppression, being most effective if five or more segments were included. These results provide direct evidence that propriospinal pathways contribute to bulbospinal activation of the locomotor network in the in vitro neonatal rat brainstem-spinal cord preparation, and suggest that a propriospinal system is recruited in parallel with long direct projections that activate the locomotor network.

Persistent monoarthritis of the temporomandibular joint region enhances nocifensive behavior and lumbar spinal Fos expression after noxious stimulation to the hindpaw in rats

Effects of persistent temporomandibular joint (TMJ) inflammation on nociceptive responses of remote bodily areas of the rat were investigated. Monoarthritis of the TMJ region was evoked by the injection of complete Freund's adjuvant (CFA) into the left TMJ region. Rats without injection of CFA into the TMJ region served as controls (non-CFA group). Time spent on licking behavior evoked by the injection of formalin into the left hindpaw and withdrawal thresholds of mechanical stimulation to both sides of the hindpaw were measured during TMJ inflammation for 3 weeks. Furthermore, expression of Fos protein in the lumbar dorsal horn was immunohistochemically investigated following the injection of formalin into the hindpaw during TMJ inflammation. Formalin-evoked nocifensive behavioral activities were significantly enhanced at 10 and 14 days after CFA injection in the late phase, while the withdrawal threshold to mechanical stimulation was significantly decreased bilaterally at 8, 10 and 14 days after CFA injection. Both formalin-evoked licking behavior and mechanical withdrawal thresholds to bilateral hindpaw at 21 days after CFA injection were similar to those in the non-CFA group. The number of Fos-positive neurons in the lumbar dorsal horn ipsilateral to the formalin injection at 1 and 7 days after CFA injection into the TMJ were similar to those in the non-CFA group; however, those were significantly increased in the laminae I-II and V-VI of the lumbar dorsal horn at 14 days after CFA injection. TMJ inflammation for 7 and 14 days alone produced a small number of Fos-expressing neurons in the lumbar dorsal horn. These results provide evidence that persistent unilateral inflammation of the TMJ region causes an increase in behavioral hyperalgesia of the hindpaw, which is attributed to the modulation of neural activities, in part, in the lumbar dorsal horn, likely mediated by supraspinal neural mechanisms.

Differential vulnerability of propriospinal tract neurons to spinal cord contusion injury

The propriospinal system is important in mediating reflex control and in coordination during locomotion. Propriospinal neurons (PNs) present varied patterns of projections with ascending and/or descending fibers. Following spinal cord contusion injury (SCI) in the rat, certain supraspinal pathways, such as the corticospinal tract, appear to be completely abolished, whereas others, such as the rubrospinal and vestibuospinal tracts, are only partially damaged. The amount of damage to propriospinal axons following different severities of SCI is not fully known. In the present study retrograde and anterograde tracing techniques were used to assess the projection patterns of propriospinal neurons in order to determine how this system is affected following SCI. Our findings reveal that PNs have differential vulnerabilities to SCI. While short thoracic propriospinal axons are severely damaged after injury, 5-7% of long descending propriospinal tract (LDPT) projections survive following 50 and 12.5-mm weight drop contusion lesions, respectively, albeit with a reduced intensity of retrograde label. Even though the axons of short thoracic propriospinal cells are damaged, their cell bodies of origin remain intact 2 weeks after injury, indicating that they have not undergone postaxotomy retrograde cell death at this time point. Thus, short PNs may constitute a very attractive population of cells to study regenerative approaches, whereas LDPT neurons with spared axons could be targeted with therapeutic interventions, seeking to enhance recovery of function following incomplete lesions to the spinal cord.

Descending modulation of thoracic visceroreceptive transmission by C1-C2 spinal neurons

Extracellular potentials of single T3 neurons were recorded in pentobarbital anesthetized male rats. Thoracic esophageal distension (ED, 0.3-0.4 ml, 20 s) and intrapericardial injection of bradykinin (BK, 10(-5) M, 0.2 ml, 1 min) were used as noxious visceral stimuli. Chemical activation of C1-C2 neurons with glutamate pledgets (1 M, 1-3 min) decreased background activity and/or excitatory responses of 26/35 (74%) neurons to ED and 34/44 (77%) neurons to BK. After spinal transection at rostral C1 in five animals, glutamate at C1-C2 still significantly reduced excitatory responses of five neurons to BK. Data showed that intraspinal descending modulation of C1-C2 neurons primarily produced descending inhibition of excitatory responses of thoracic spinal neurons to noxious visceral stimuli.

Premotor nitric oxide synthase immunoreactive pathway connecting lumbar segments with the ventral motor nucleus of the cervical enlargement in the dog

In this study we investigate the occurrence and origin of punctate nitric oxide synthase immunoreactivity in the neuropil of the ventral motor nucleus in C7-Th1 segments of the dog spine, which are supposed to be the terminal field of an ascending premotor propriospinal nitric oxide synthase-immunoreactive pathway. As the first step, nitric oxide synthase immunohistochemistry was used to distinguish nitric oxide synthase-immunoreactive staining of the ventral motor nucleus. Dense, punctate nitric oxide synthase immunoreactivity was found on control sections in the neuropil of the ventral motor nucleus. After hemisection at Th10-11, axotomy-induced retrograde changes consisting in a strong upregulation of nitric oxide synthase-containing neurons were found mostly unilaterally in lamina VIII, the medial part of lamina VII and in the pericentral region in all segments of the lumbosacral enlargement. Concurrently, a strong depletion of the punctate nitric oxide synthase immunopositivity in the neuropil of the ventral motor nucleus ipsilaterally with the hemisection was detected, thus revealing that an uncrossed ascending premotor propriospinal pathway containing a fairly high number of nitric oxide synthase-immunoreactive fibers terminates in the ventral motor nucleus. Application of the retrograde fluorescent tracer Fluorogold injected into the ventral motor nucleus and analysis of alternate sections processed for nitric oxide synthase immunocytochemistry revealed the presence of Fluorogold-labeled and nitric oxide synthase-immunoreactive axons in the ventrolateral funiculus and in the lateral and medial portions of the ventral column throughout the thoracic and upper lumbar segments. A noticeable number of Fluorogold-labeled and nitric oxide synthase-immunoreactive somata detected on consecutive sections were found in the lumbosacral enlargement, mainly in laminae VIII-IX, the medial part of lamina VII and in the pericentral region (lamina X), ipsilaterally with the injection of Fluorogold into the ventral motor nucleus. In summary, the present study provides evidence for a hitherto unknown ascending premotor propriospinal nitric oxide synthase-immunoreactive pathway connecting the lumbosacral enlargement with the motoneurons of the ventral motor nucleus in the dog.

Somatic and visceral afferents to the 'vasodepressor region' of the caudal midline medulla in the rat

Previous research has found that the integrity of a restricted region of the caudal midline medulla (including caudal portions of nucleus raphé obscurus and nucleus raphé pallidus) was critical for vasodepression (hypotension, bradycardia, decreased cardiac contractility) evoked either by haemorrhage or deep pain. In this anatomical tracing study we found that the vasodepressor part of the caudal midline medulla (CMM) receives inputs arising from spinal cord, spinal trigeminal nucleus (SpV) and nucleus of the solitary tract (NTS). Specifically: (i) a spinal-CMM projection arises from neurons of the deep dorsal horn, medial ventral horn and lamina X at all spinal segmental levels, with approximately 60% of the projection originating from the upper cervical spinal cord (C1-C4); (ii) a SpV-CMM projection arises primarily from neurons at the transition between subnucleus caudalis and subnucleus interpolaris; (iii) a NTS-CMM projection arises primarily from neurons in ventrolateral and medial subnuclei. In combination, the specific spinal, SpV and NTS regions which project to the CMM receive the complete range of somatic and visceral afferents known to trigger vasodepression. The role(s) of each specific projection is discussed.

Localization and distribution patterns of nicotinamide adenine dinucleotide phosphate diaphorase exhibiting axons in the white matter of the spinal cord of the rabbit

The funicular distribution of nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-exhibiting axons was examined in the white matter of the rabbit spinal cord by using horizontal, parasaggital, and transverse sections. Four morphologically distinct kinds of NADPHd-exhibiting axons (2.5-3.5 microm in diameter) were identified in the sulcomarginal fasciculus as a part of the ventral column in the cervical and upper thoracic segments and in the long propriospinal bundle of the ventral column in Th3-L3 segments. Varicose NADPHd-exhibiting axons of the sympathetic preganglionic neurons, characterized by widely spaced varicosities, were found in the ventral column of Th2-L3 segments. A third kind of NADPHd-positive ultrafine axons, 0.3-0.5 microm in diameter with numerous varicosities mostly spherical in shape, was identified in large number within Lissauer's tract. The last group of NADPHd-exhibiting axons (1.0-1.5 microm in diameter) occurred in the Lissauer tract. Most of these axons were traceable for considerable distances and generated varicosities varying in shape from spherical to elliptical forms. The majority of NADPHd-exhibiting axons identified in the cuneate and gracile fascicles were concentrated in the deep portion of the dorsal column. An extremely reduced number of NADPHd-exhibiting axons, confirmed by a computer-assisted image-processing system, was found in the dorsal half of the gracile fascicle. Axonal NADPHd positivity could not be detected in a wide area of the lateral column consistent with the location of the dorsal spinoccrebellar tract. Numerous, mostly thin NADPHd-positive axonal profiles were detected in the dorsolateral funiculus in all the segments studied and in a juxtagriscal portion of the lateral column as far as the cervical and lumbar enlargements. A massive occurrence of axonal NADPHd positivity was detected in the juxtagriseal layer of the ventral column all along the rostrocaudal axis of the spinal cord. The prominent NADPHd-exhibiting bundles containing thick, smooth, nonvaricose axons were identified in the mediobasal and central portion of the ventral column. First, the sulcomarginal fasciculus was found in the basal and medial portion of the ventral column in all cervical and upper thoracic segments. Second, more caudally, a long propriospinal bundle displaying prominent NADPHd positivity was localized in the central portion of the ventral column throughout the Th3-L3 segments.

Spinal inhibitory effects of cardiopulmonary afferent inputs in monkeys: neuronal processing in high cervical segments

Noxious stimulation of spinal afferents inhibits primate spinothalamic tract (STT) neurons in segments distant from the region of afferent entry. Inhibitory effects of cardiopulmonary sympathetic afferent (CPSA) stimulation remain after C(1) transection but disappear with spinal transection between C(3) and C(7). We hypothesized that spinal inhibitory effects produced by CPSA stimulation are processed by neurons in C(1)-C(3) segments. One purpose of this study in anesthetized monkeys was to determine whether chemical activation of high cervical neurons reduced sacral STT cell responses to colorectal distension (CRD) and urinary bladder distension (UBD). First, effects and interactions of pelvic and cardiopulmonary visceral afferent inputs were determined in 10 monkeys on extracellular activity of sacral STT neurons recorded in deep dorsal horn. CRD and UBD increased activity in 95 and 91% of sacral STT neurons, respectively. CPSA and cardiopulmonary vagal stimulation decreased activity in 84 and 56% of STT neurons, respectively. CPSA stimulation decreased CRD-evoked activity in six of eight sacral STT neurons and decreased UBD-evoked activity in five of eight STT neurons tested. Excitatory amino acid application at C2 segment decreased CRD-evoked responses in 7 of 10 sacral STT neurons and decreased UBD-evoked responses in 9 of 12 STT neurons. The second purpose of this study was to examine responses of C(1)-C(3) descending propriospinal neurons to stimulation of cardiopulmonary afferent fibers. If C(1)-C(3) neurons process CPSA input to suppress STT transmission, then CPSA stimulation should excite C(1)-C(3) neurons with descending projections. Effects of thoracic vagus nerve stimulation also were examined. Vagal stimulation inhibits STT neurons in segments below C(3) but excites C(1)-C(3) STT neurons; we theorized that vagal inhibition of sensory transmission might relay in high cervical segments and, therefore, excite C(1)-C(3) descending propriospinal neurons. Extracellular discharge rate was recorded for C(1)-C(3) neurons antidromically activated from thoracic or lumbar spinal cord in 24 monkeys. CPSA stimulation increased activity of 16 of 45 neurons and inhibited one cell. Thoracic vagus stimulation increased activity of 20 of 43 neurons and inhibited one cell; stimulation of abdominal vagus fibers did not affect activity of six of six cells that were excited by thoracic vagal input. Mechanical stimulation of somatic fields excited 30 of 41 neurons tested. All neurons activated by visceral input received convergent somatic input from noxious pinch of somatic receptive fields that generally included the neck and upper body; 11 C(1)-C(3) propriospinal neurons did not respond to any afferent input examined. Results of these studies were consistent with the idea that modulation of spinal nociceptive transmission might involve neuronal connections in high cervical segments.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

A correlative physiological and morphological analysis of monosynaptically connected propriospinal axon-motoneuron pairs in the lumbar spinal cord of frogs

Intracellular stimulation of single propriospinal axons evoked excitatory postsynaptic potentials (EPSPs) in lumbar motoneurons. Mean EPSP amplitudes differed by two orders of magnitude when measured in different connections. After analyzing the distribution of mean amplitudes of 47 single-fiber EPSPs, two populations of responses could be defined: (1) those with mean amplitudes between 0.1 and 1.2 mV (mean+/-S.D.: 0.48+/-0.30 mV, 34 pairs), which is in the range of values typical for single-fiber EPSPs evoked by stimulation of supraspinal fibers and primary muscle afferents, (2) those with mean amplitudes between 1.6 and 8 mV (4.2+/-2.0 mV, 13 pairs). Both populations of responses had similarly short latencies and rise times and responded similarly to paired-pulse stimulation, consistent with monosynaptic transmission. However, the high-efficacy connections had significantly smaller coefficients of variation of EPSPs, as well as increased quantal content and quantal size. Tetanic stimulation gradually depressed the amplitude of large EPSPs by 81-86%, but did not affect small EPSPs. Recovery of large EPSPs was exponential with a time constant of 3-5.6 min. During post-tetanic depression the amplitude ratio between the test and conditioned EPSPs evoked by paired-pulse stimulation was not changed but the coefficient of variation was increased, suggesting that the depression was due to depletion of synaptic vesicles available for release.Intracellular labeling of seven electrophysiologically studied propriospinal axon-motoneuron pairs revealed that the number of axon varicosities establishing close appositions with dendrites of the labeled motoneuron was higher for connections where large-amplitude EPSPs were recorded. These varicosities were more often located on proximal dendrites of motoneurons than those of low-efficacy connections. In addition, the number of boutons in highly effective connections was several times lower than the maximal number of available quanta estimated from physiological data, implying that the large EPSPs may be generated by multivesicular release from presynaptic boutons. We conclude that the efficacy and related mode of use-dependent modulation of propriospinal connections is determined by a number of factors, including the number and position of synaptic contacts and the number of active zones or vesicles available for release.

Neurones in the cervical enlargement of the cat spinal cord antidromically activated from sacral segments and the inferior cerebellar peduncle

The electrophysiological investigation of neurones located in the cervical enlargement of the spinal cord was performed in eight-chloralose anaesthetized cats. Neurones were recorded intracellularly or extracellularly and identified by antidromic stimulation. The main purpose of the study was to test whether these neurones give off collateral branches ascending to the inferior cerebellar peduncle and descending to the sacral segments (S1/S2). Recordings were made from 78 neurones located in medial and central parts of Rexed's laminae VII and VIII of C6/C7 segments. Four subpopulations could be distinguished from their patterns of propriospinal or supraspinal projections: (a) ascending/descending neurones with axons ascending to RB and descending to S1/S2 (23%); (b) ascending/descending neurones projecting to RB and the level of Th13 (14%); (c) propriospinal neurones descending to Th13 (15%); (d) propriospinal neurones descending to S1/S2 (48%). Within these groups, ipsilateral, contralateral and bilateral descending projections were observed. The mean axonal conduction velocities for descending and ascending collaterals of bidirectional neurones were 59 and 39 m/s, respectively. Results suggest that parallel transmission of information to supraspinal and spinal centres plays an important role in the process of movement coordination.

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Studies utilizing the expression of Fos protein as a marker of neuronal activation have revealed that pain of deep somatic or visceral origin selectively activates the ventrolateral periaqueductal gray (vlPAG). Previous anatomical tracing studies revealed that spinal afferents to the vlPAG arose from the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus at all spinal segmental levels, with approximately 50% of vlPAG-projecting spinal neurons found within the upper cervical spinal cord. This study utilized detection of Fos protein to determine the specific populations of vlPAG-projecting spinal neurons activated by noxious deep somatic or noxious visceral stimulation. Pain of cardiac or peritoneal (i.e., visceral) origin activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the thoracic cord, whereas pain of hindlimb (i.e., deep somatic) origin activated neurons in the same laminar regions but in the lumbosacral cord. Each of these deep noxious manipulations also activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the upper cervical spinal cord. In a second set of experiments, the combination of retrograde tracing and Fos immunohistochemistry revealed that vlPAG-projecting spinal neurons activated by deep somatic pain were located in both the upper cervical and lumbosacral cord, whereas those activated by visceral pain were restricted to the thoracic spinal cord. Thus pain arising from visceral versus deep somatic body regions influences neural activity within the vlPAG via distinct spinal pathways. The findings also highlight the potential significance of the upper cervical cord in integrating pain arising from deep structures throughout the body.

Fictive locomotion and scratching inhibit dorsal horn neurons receiving thin fiber afferent input

In decerebrate paralyzed cats, we examined the effects of two central motor commands (fictive locomotion and scratching) on the discharge of dorsal horn neurons receiving input from group III and IV tibial nerve afferents. We recorded the impulse activity of 74 dorsal horn neurons, each of which received group III input from the tibial nerve. Electrical stimulation of the mesencephalic locomotor region (MLR), which evoked fictive static contraction or fictive locomotion, inhibited the discharge of 44 of the 64 dorsal horn neurons tested. The mean depth from the dorsal surface of the spinal cord of the 44 neurons whose discharge was inhibited by MLR stimulation was 1.77 +/- 0.04 mm. Fictive scratching, evoked by topical application of bicuculline to the cervical spinal cord and irritation of the ear, inhibited the discharge of 22 of the 29 dorsal horn neurons tested. Fourteen of the twenty-two neurons whose discharge was inhibited by fictive scratching were found to be inhibited by MLR stimulation as well. The mean depth from the dorsal surface of the cord of the 22 neurons whose discharge was inhibited by fictive scratching was 1.77 +/- 0.06 mm. Stimulation of the MLR or the elicitation of fictive scratching had no effect on the activity of 22 dorsal horn neurons receiving input from group III and IV tibial nerve afferents. The mean depth from the dorsal surface of the cord was 1.17 +/- 0.07 mm, a value that was significantly (P < 0.05) less than that for the neurons whose discharge was inhibited by either MLR stimulation or fictive scratching. We conclude that centrally evoked motor commands can inhibit the discharge of dorsal horn neurons receiving thin fiber input from the periphery.

Chemical activation of cervical cell bodies: effects on responses to colorectal distension in lumbosacral spinal cord of rats

We have shown that stimulation of cardiopulmonary sympathetic afferent fibers activates relays in upper cervical segments to suppress activity of lumbosacral spinal cells. The purpose of this study was to determine if chemical excitation (glutamate) of upper cervical cell bodies changes the spontaneous activity and evoked responses of lumbosacral spinal cells to colorectal distension (CRD). Extracellular potentials were recorded in pentobarbital-anesthetized male rats. CRD (80 mmHg) was produced by inflating a balloon inserted in the descending colon and rectum. A total of 135 cells in the lumbosacral segments (L(6)-S(2)) were activated by CRD. Seventy-five percent (95/126) of tested cells received convergent somatic input from the scrotum, perianal region, hindlimb, and tail; 99/135 (73%) cells were excited or excited/inhibited by CRD; and 36 (27%) cells were inhibited or inhibited/excited by CRD. A glutamate (1 M) pledget placed on the surface of C(1)-C(2) segments decreased spontaneous activity and excitatory CRD responses of 33/56 cells and increased spontaneous activity of 13/19 cells inhibited by CRD. Glutamate applied to C(6)-C(7) segments decreased activity of 10/18 cells excited by CRD, and 9 of these also were inhibited by glutamate at C(1)-C(2) segments. Glutamate at C(6)-C(7) increased activity of 4/6 cells inhibited by CRD and excited by glutamate at C(1)-C(2) segments. After transection at rostral C(1) segment, glutamate at C(1)-C(2) still reduced excitatory responses of 7/10 cells. Further, inhibitory effects of C(6)-C(7) glutamate on excitatory responses to CRD still occurred after rostral C(1) transection but were abolished after a rostral C(6) transection in 4/4 cells. These data showed that C(1)-C(2) cells activated with glutamate primarily produced inhibition of evoked responses to visceral stimulation of lumbosacral spinal cells. Inhibition resulting from activation of cells in C(6)-C(7) segments required connections in the upper cervical segments. These results provide evidence that upper cervical cells integrate information that modulates activity of distant spinal neurons responding to visceral input.

Propriospinal neurons in the C1-C2 spinal segments project to the L5-S1 segments of the rat spinal cord

Physiological studies indicate that neurons in the upper cervical spinal cord have descending projections to the lumbosacral spinal cord and mediate inhibition of dorsal horn neurons activated from afferent input. In the present study, retrograde tracing techniques were used to examine the distribution of propriospinal neurons in C1-C2 spinal segments that project to lumbosacral spinal segments. Fluorogold or horseradish peroxidase were injected unilaterally or bilaterally into the L5-S1 spinal segments. After 2-4 days, rats were perfused with fixative and C1-C2 spinal segments were processed for retrograde labeling. Numerous neurons were found in the C1-C2 segments. In unilaterally and bilaterally injected rats, retrogradely labeled neurons were located on both the ipsilateral and contralateral sides. Retrogradely labeled neurons were located in the following locations: lateral cervical and spinal nuclei, nucleus proprius, ventral horn and the central gray region (area X). These studies demonstrate a descending projection from C1-C2 segments to the lower lumbar and sacral spinal cord. We hypothesize that many of these C1-C2 propriospinal neurons are important in modulating responses of spinal neurons at lower segmental levels to various peripheral stimuli.

Noxious heat-evoked fos-like immunoreactivity in the rat lumbar dorsal horn is inhibited by glutamate microinjections in the upper cervical spinal cord

Microinjections of glutamate into the upper cervical spinal cord significantly reduced (to 57% of control) the total number of neurons demonstrating noxious heat-evoked fos-like immunoreactivity in the lumbar spinal cord. Neurons in the upper cervical spinal cord, with descending propriospinal projections to the lumbar spinal cord, therefore, produce inhibitory effects on dorsal horn neurons in the lumbar spinal cord that receive nociceptive input from cutaneous thermal nociceptors.

Lumbar collaterals of neurons of the C6 segment projecting to sacral segments of the cat spinal cord

Electrophysiological investigations of neurons of the C6 segment of the spinal cord were made in alpha-chloralose anesthetized animals. It was established in the experiments that a part of long descending propriospinal neurons originating in the sixth cervical segment (C6) that projected to sacral segments (S1/S2) gave off collateral branches at the level of the fourth lumbar segment (L4). Several types of neurons were distinguished according to the ipsilateral, contralateral or bilateral course of axons at the thoracic level as well as their lumbar or sacral projections. The cell bodies of 58 identified neurons were distributed in Rexed's laminae VII and VIII of the gray matter. Axons descended in lateral funiculi and their conduction velocities varied from 50 to 85 m/s. The existence of collaterals to various segments of the lumbosacral enlargement indicates that the same information conveyed by long descending propriospinal neurons can reach separate motor centers controlling various muscles of the hindlimbs.