Excitatory and inhibitory propriospinal pathways from lumbo-sacral to cervical segments in the cat

Exercise dependent increase in axon regeneration into peripheral nerve grafts by propriospinal but not sensory neurons after spinal cord injury is associated with modulation of regeneration-associated genes

Insufficient regeneration of central nervous system (CNS) axons contributes to persisting neurological dysfunction after spinal cord injury (SCI). Peripheral nerve grafts (PNGs) support regeneration by thousands of injured intraspinal axons and help them bypass some of the extracellular barriers that form after SCI. However this number represents but a small portion of the total number of axons that are injured. Here we tested if rhythmic sensory stimulation during cycling exercise would boost the intrinsic regenerative state of neurons to enhance axon regeneration into PNGs after a lower thoracic (T12) spinal transection of adult rats. Using True Blue retrograde tracing, we show that 4 weeks of cycling improves regeneration into a PNG from lumbar interneurons but not by primary sensory neurons. The majority of neurons that regenerate their axon are within 5 mm of the lesion and their number increased 70% with exercise. Importantly propriospinal neurons in more distant regions (5-20 mm from the lesion) that routinely exhibit very limited regeneration responded to exercise by increasing the number of regenerating neurons by 900%. There was no exercise-associated increase in regeneration from sensory neurons. Analyses using fluorescent in situ hybridization showed that this increase in regenerative response is associated with changes in levels of mRNAs encoding the regeneration associated genes (RAGs) GAP43, β-actin and Neuritin. While propriospinal neurons showed increased mRNA levels in response to SCI alone and then to grafting and exercise, sensory neurons did not respond to SCI, but there was a response to the presence of a PNG. Thus, exercise is a non-invasive approach to modulate gene expression in injured neurons leading to an increase in regeneration. This sets the stage for future studies to test whether exercise will promote axon outgrowth beyond the PNG and reconnection with spinal cord neurons, thereby demonstrating a potential clinical application of this combined therapeutic intervention.

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.

The development of mammalian motor systems: the opossum Monodelphis domestica as a model

The opossum Monodelphis domestica is a marsupial born considerably immature 14-15 days after conception. It is possible to study postnatally, in this species, almost the entire development of its motor behaviors as well as of the nerve centers involved in their control. The lumbosacral spinal cord of the newborn comprises a thick ventricular zone containing mitotic figures, an intermediate zone of small and undifferentiated cells, and a thin marginal zone. The hindlimbs are little more than embryonic buds. The presumptive bones consist of cartilageneous or mesenchymal condensations and the presumptive muscles of immature myofibers mixed and surrounded with mesenchyme. Cholinergic fibers from lumbosacral motoneurons are already seen among the myofibers, but most of hindlimb motor innervation develops postnatally. The long descending and ascending projection systems connecting the lumbosacral enlargement to the cervical cord and the encephalon also form largely postnatally, but lateral vestibular and medullary reticular axons are present in the lumbosacral cord at birth. Synaptogenesis in the lumbosacral enlargement occurs largely postnatally, according to a general outside-in gradient, and the earliest evidence for it is on lateral motoneurons. Myelinogenesis therein is even later. These observations on neural development are correlated with observations on the development of simple reflex behaviors and locomotion.

Long-lasting depression of soleus motoneurons excitability following repetitive magnetic stimuli of the spinal cord in multiple sclerosis patients

The effect of repetitive magnetic stimulation at the spinal level on the soleus H-reflex amplitude was evaluated in II MS patients with lower limb spasticity and in nine healthy subjects. In MS patients stimulation with a train of 16 stimuli at 25 Hz induced a decrease in amplitude to 61.2 +/- 25.7% of the unconditioned H-reflex amplitude at interstimulus interval (ISI) of 10-1000 ms (P < 0.01). The amount of decrease in H-reflex amplitude was highly dependent on the stimulation intensity and the placement of the coil, and to a lesser extent influenced by the stimulation frequency. No decrease in motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation was seen following trains of 16 stimuli at mid-thoracic in contrast to the post-stimulation depression in H-reflex amplitude which could imply that mechanisms acting at presynaptic level were involved. In response to repetitive magnetic stimuli for 5 min, a long-lasting decrease in H-reflex amplitude to a level of about 70% of the pre-stimulation H-reflex amplitude occurred in MS patients (P < 0.01). A similar although not significant decrease was observed in healthy subjects. We propose that long-lasting depression of the soleus H-reflex amplitude after repetitive magnetic stimuli is due to long-term depression of the synaptic transmission.

The development of the long descending propriospinal projections in the opossum, Monodelphis domestica

The origin of the long descending propriospinal (LDP) projections have been studied in adult and developing opossums, Monodelphis domestica. This species has been chosen because of the considerable immaturity of the hindlimbs at birth, the postnatal appearance of their motility and the late development of coordination between them and the forelimbs. Neuroanatomical tracing has indicated that some LDP projections form postnatally. The ones present at birth arise from the regions of the cord where they are the most numerous in the adult opossum, presumptive laminae VII and VIII of the brachial enlargement. Subsequently, LDP projections arise from neurons located in adjacent laminae (IV to VI and IX and X) and at more rostral cervical levels. The origin of LDP projections in the adult opossums generally matches that reported for other mammals. These long propriospinal projections are in place well before the behavioral appearance of coordination between the hindlimbs and the forelimbs, but the timing of their synaptogenesis is not yet known.

Cutaneus trunci muscle reflex of the guinea pig

The cutaneus trunci muscle reflex in guinea pigs was studied with a combination of video analysis, electromyography, lesioning, and light microscopy. The muscle forms a bilateral, subdermal sheet over much of the trunk. Local contractions of the dorsal part of the muscle are produced in response to brief tactile or electrical stimulation of the skin and consist of a twitch centered 1-2 cm rostral of the stimulus site. The reflex receptive field covers most of the thoracic and lumbar dorsal surface. The sensory information is carried via segmental dorsal cutaneous nerves. Receptive fields of adjacent nerves overlap and form rectangular areas perpendicular to the midline, at thoracic levels. Motor innervation projects through the lateral thoracic nerves of the brachial plexus. The motoneurons are located near the cervical thoracic junction (C7-T1). Lesions of the lower thoracic cord indicate that ascending sensory information is carried to the motor nuclei via the ventral half of the lateral funiculus. This pathway conveys information primarily from ipsilateral skin. There is a weaker input from contralateral skin, crossing at segmental levels. Electromyographic responses to brief electrical stimulation of lower thoracic skin occur usually as 10-12 msec bursts at latencies of 10-20 msec, and do not readily habituate or fatigue at stimulus frequencies below 10 Hz. The reflex persists under light pentobarbital anesthesia. This combination of characteristics makes the reflex useful for a variety of physiological and pathophysiological studies.

The effects of 4-aminopyridine on the spinal cord: rhythmic discharges recorded from the peripheral nerves

The effects of an intravenous injection (20 mg/kg) of 4-aminopyridine (4-AP) were initially investigated in acute low spinal cats (Th 13), in which L-DOPA had induced fictive locomotion after paralysis. 4-AP first accelerated the locomotor rhythm and could also change markedly the pattern of activation of some muscle nerves. Shortly after, the locomotor activity was replaced by synchronous rhythmic discharges (2.5-8.5 Hz) in flexor and extensor muscle nerves of the same limb girdle. Similar rhythmic activity was recorded after 4-AP alone (5-20 mg/kg) in the acute decerebrate spinal cat. Whilst the mean rate of the rhythmic activity could differ in the two limb girdles, discharges generated in one girdle appeared to be strongly influenced by those generated in the other. After a complete section of the spinal cord (Th13), the activity persisted in both the rostral and caudal segments although the interactions between the two disappeared. The persistence of the rhythmic activity caudal to the section underscores its spinal origin. In the chronic spinal rat, such rhythmic activity could still be induced in the lumbo-sacral cord despite degeneration of descending pathways. It appears that large doses of 4-AP exert potent effects on the spinal cord which can override other patterns of activity and synchronize the electrical activity of many neuronal elements.

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

The distribution spinal neurons with long descending axons was studied in the cat by means of retrograde transport of horseradish peroxidase. Labeled neurons appeared bilaterally in the cervical and the thoracic cord following injections in the lumbosacral cord. In some cases hemisections were made rostrally and contralaterally to the injections in an attempt to determine whether or not the axons crossed. Neurons with uncrossed descending axons were located in laminae I, V, VII and VIII. Lamina I neurons were present in all the spinal segments. In lamina V labeled neurons were distributed mainly laterally in the cervical cord but medially and laterally in the thoracic cord. In the upper cervical and the thoracic cord laminae VII and VIII neurons were distributed very densely along the lateral cord, accounting for 30 and 40 of the total labeled neurons, respectively. In the cervical enlargement they were located in the middle part of lamina VII and in lamina VIII, accounting for about 25% of the total labeled neurons. Neurons with crossed descending axons were found in laminae V, VII and VIII, in the medial part of lamina VII including the intermediomedial nucleus of the thoracic levels and close to the central canal. Lamina V neurons were very small in number. The largest collections of labeled neurons were present in the medial part of laminae VII and VIII. They accounted for about 45% to 55% and 37% of the total in the cervical and the thoracic cord. These neurons may function as the long spinal reflex paths for forelimb-hindlimb synergies and the intercalated paths between the supraspinal descending tracts and the spinal motor centers.