DECEREBRATE CONTROL OF REFLEXES TO PRIMARY AFFERENTS

Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated predominantly via presynaptic GABA_A receptors on afferent terminals. We investigated whether ionotropic nicotinic acetylcholine receptors (nAChRs) also provide modulatory actions, focusing on myelinated afferent excitability in in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked synaptic transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs), whereas concurrently recorded dorsal root potentials (DRPs) were used as an indirect measure of PAD. Changes in afferent membrane excitability were simultaneously measured as direct current (DC)-shifts in membrane polarization recorded in dorsal roots or peripheral nerves. The broad nAChR antagonist d-tubocurarine (d-TC) selectively and strongly depressed Aδ-evoked synaptic EFPs (36% of control) coincident with similarly depressed A-fiber DRP (43% of control), whereas afferent electrical excitability remained unchanged. In comparison, acetylcholine (ACh) and the nAChR agonists, epibatidine and nicotine, reduced afferent excitability by generating coincident depolarizing DC-shifts in peripheral axons and intraspinally. Progressive depolarization corresponded temporally with the emergence of spontaneous axonal spiking and reductions in the DRP and all afferent-evoked synaptic actions (31%-37% of control). Loss of evoked response was long-lasting, independent of DC repolarization, and likely due to mechanisms initiated by spontaneous C-fiber activity. DC-shifts were blocked with d-TC but not GABA_A receptor blockers and retained after tetrodotoxin block of voltage-gated Na⁺ channels. Notably, actions tested were comparable between three mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by central Aδ-afferent actions, and by transient extrasynaptic axonal activation of high-threshold primary afferents.NEW & NOTEWORTHY Primary afferents express many nicotinic ACh receptor (nAChR) subtypes but whether activation is linked to presynaptic inhibition, facilitation, or more complex and selective activity modulation is unknown. Recordings of afferent-evoked responses in the lumbar spinal cord identified two nAChR-mediated modulatory actions: 1) selective control of Aδ afferent transmission and 2) robust changes in axonal excitability initiated via extrasynaptic shifts in DC polarization. This work broadens the diversity of presynaptic modulation of primary afferents by nAChRs.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

Plasticity in ascending long propriospinal and descending supraspinal pathways in chronic cervical spinal cord injured rats

The high clinical relevance of models of incomplete cervical spinal cord injury (SCI) creates a need to address the spontaneous neuroplasticity that underlies changes in functional activity that occur over time after SCI. There is accumulating evidence supporting long projecting propriospinal neurons as suitable targets for therapeutic intervention after SCI, but focus has remained primarily oriented toward study of descending pathways. Long ascending axons from propriospinal neurons at lower thoracic and lumbar levels that form inter-enlargement pathways are involved in forelimb-hindlimb coordination during locomotion and are capable of modulating cervical motor output. We used non-invasive magnetic stimulation to assess how a unilateral cervical (C5) spinal contusion might affect transmission in intact, long ascending propriospinal pathways, and influence spinal cord plasticity. Our results show that transmission is facilitated in this pathway on the ipsilesional side as early as 1 week post-SCI. We also probed for descending magnetic motor evoked potentials (MMEPs) and found them absent or greatly reduced on the ipsilesional side as expected. The frequency-dependent depression (FDD) of the H-reflex recorded from the forelimb triceps brachii was bilaterally decreased although H_max/M_max was increased only on the ipsilesional side. Behaviorally, stepping recovered, but there were deficits in forelimb–hindlimb coordination as detected by BBB and CatWalk measures. Importantly, epicenter sparing correlated to the amplitude of the MMEPs and locomotor recovery but it was not significantly associated with the inter-enlargement or segmental H-reflex. In summary, our results indicate that complex plasticity occurs after a C5 hemicontusion injury, leading to differential changes in ascending vs. descending pathways, ipsi- vs. contralesional sides even though the lesion was unilateral as well as cervical vs. lumbar local spinal networks.

Local and diffuse mechanisms of primary afferent depolarization and presynaptic inhibition in the rat spinal cord

Two types of dorsal root potential (DRP) were found in the spinal cord of urethane-anaesthetized rats. Local DRPs with short latency-to-onset were evoked on roots close to the point of entry of an afferent volley. Diffuse DRPs with a longer latency-to-onset were seen on more distant roots up to 17 segments from the volley entry zone. The switch to long latency-to-onset occurred abruptly as a function of distance along the cord and could not be explained by conduction delays within the dorsal columns. Long-latency DRPs were also present and superimposed on the short-latency DRPs on nearby roots. Both local and diffuse DRPs were evoked by light mechanical stimuli: von Frey hair thresholds were Collapse

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MESH Headings

• Action Potentials/physiology
• Animals
• Electrophysiology
• Evoked Potentials/physiology
• GABA Antagonists/pharmacology
• Male
• Neural Inhibition/physiology
• Neurons, Afferent/physiology
• Picrotoxin/pharmacology
• Presynaptic Terminals/physiology
• Rats
• Rats, Sprague-Dawley
• Spinal Cord/physiology
• Spinal Nerve Roots/physiology
• Sural Nerve/drug effects
• Sural Nerve/physiology
• Synaptic Transmission/physiology

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Grants

• Wellcome Trust
• 052639 Wellcome Trust

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Affiliation(s)

Malcolm Lidierth King's College London, Hodgkin Building, Guy's Hospital Campus, London SE1 1UL, UK.

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Dopamine and the spinal cord in restless legs syndrome: Does spinal cord physiology reveal a basis for augmentation?

The pathophysiology of restless legs syndrome (RLS) is incompletely understood. L-DOPA, as the precursor of dopamine, as well as dopamine agonists, plays an essential role in the treatment of RLS leading to the assumption of a key role of dopamine function in the pathophysiology of RLS. Periodic limb movements in sleep are a key feature of RLS. They are generated in the spinal cord. Here we review RLS phenomenology on the basis of known dopaminergic influence on spinal control, which has been studied a great deal in recent decades in animals. In particular, we propose that the differential effects of l-DOPA and opioids on early and late flexor reflexes may be linked to the phenomenon of augmentation.

Tonic differential supraspinal modulation of PAD and PAH of segmental and ascending intraspinal collaterals of single group I muscle afferents in the cat spinal cord

We compared in the anesthetized cat the effects of reversible spinalization by cold block on primary afferent depolarization (PAD) and primary afferent hyperpolarization (PAH) elicited in pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pairs ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. The results indicate that the segmental and ascending collaterals of individual afferents are subjected to a tonic PAD of descending origin affecting in a differential manner the excitatory and inhibitory actions of cutaneous and joint afferents on the pathways mediating the PAD of group I fibers. The PAD-mediating networks appear to function as distributed systems whose output will be determined by the balance of the segmental and supraspinal influences received at that moment. It is suggested that the descending differential modulation of PAD enables the intraspinal arborizations of the muscle afferents to function as dynamic systems, in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs, and funneled to specific targets according to the motor tasks to be performed.

Central control of information transmission through the intraspinal arborizations of sensory fibers examined 100 years after Ramón y Cajal

About 100 years ago, Santiago Ramón y Cajal reported that sensory fibers entering the spinal cord have ascending and descending branches, and that each of them sends collaterals to the gray matter where they have profuse ramifications. To him this was a fundamental discovery and proposed that the intraspinal branches of the sensory fibers were "centripetal conductors by which sensory excitation is propagated to the various neurons in the gray matter". In addition, he assumed that "conduction of excitation within the intraspinal arborizations of the afferent fibers would be proportional to the diameters of the conductors", and that excitation would preferentially flow through the coarsest branches. The invariability of some elementary reflexes such as the knee jerk would be the result of a long history of plastic adaptations and natural selection of the safest neuronal organizations. There is now evidence suggesting that in the adult cat, the intraspinal branches of sensory fibers are not hard wired routes that diverge excitation to spinal neurons in an invariable manner, but rather dynamic pathways where excitation flow can be centrally addressed to reach specific neuronal targets. This central control of information flow is achieved by means of specific sets of GABAergic interneurons that produce primary afferent depolarization (PAD) via axo-axonic synapses and reduce transmitter release (presynaptic inhibition). The PAD produced by single, or by small groups of GABAergic interneurons in group I muscle afferents, can remain confined to some sets of intraspinal arborizations of the afferent fibers and not spread to nearby collaterals. In muscle spindle afferents this local character of PAD allows cutaneous and descending inputs to differentially inhibit the PAD in segmental and ascending collaterals of individual fibers, which may be an effective way to decouple the information flow arising from common sensory inputs. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Reflex pathways from group II muscle afferents. 3. Secondary spindle afferents and the FRA: a new hypothesis

A hypothesis is forwarded regarding the role of secondary spindle afferents and the FRA (flexor reflex afferents) in motor control. The hypothesis is based on evidence (cf. Lundberg et al. 1987a, b) summarized in 9 introductory paragraphs. Group II excitation. It is postulated that subsets of excitatory group II interneurones (transmitting disynaptic group II excitation to motoneurones) may be used by the brain to mediate motor commands. It is assumed that the brain selects subsets of interneurones with convergence of secondary afferents from muscles whose activity is required for the movement. During movements depending on coactivation of static gamma-motoneurones impulses in secondary afferents may servo-control transmission to alpha-motoneurones at an interneuronal level. The large group II unitary EPSPs in interneurones are taken to indicate that, given an adequate interneuronal excitability, impulses in single secondary afferents may fire the interneurone and produce EPSPs in motoneurones; interneuronal transmission would then be equivalent to that in a monosynaptic pathway but with impulses from different muscles combining into one line. It is postulated that impulses in the FRA are evoked by the active movements and that the role of the multisensory convergence from the FRA onto the group II interneurones is to provide the high background excitability which allows the secondary spindle afferents to operate as outlined above. The working hypothesis is put forward that a movement governed by the excitatory group II interneurones is initiated by descending activation of these interneurones, but is maintained in a later phase by the combined effect of FRA activity evoked by the movement and by spindle secondaries activated by descending activation of static gamma-motoneurones. As in the original "follow up length servo" hypothesis (Rossi 1927; Merton 1953), we assume that a movement at least in a certain phase can be governed from the brain solely or mainly via static gamma-motoneurones. However, our hypothesis implies that the excitatory group II reflex connexions have a strength which does not allow transmission to motoneurones at rest and that the increase in the gain of transmission during an active movement is supplied by the movement itself. Group II inhibition. It is suggested that the inhibitory reflex pathways like the excitatory ones have subsets of interneurones with limited group II convergence. When higher centres utilize a subset of excitatory group II interneurones to evoke a given movement, there may mobilize inhibitory subsets to inhibit muscles not required in the movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Integration of posture and locomotion in acute decerebrate cats and in awake, freely moving cats

For the past 10 years, our group has been engaged in the study of posture and locomotion in decerebrate cats and in freely moving awake cats. Our initial objective was to analyse the neuronal mechanisms of locomotion from a viewpoint of "postural control". Therefore, in this review, I have focussed my attention on two major control aspects of the brain stem; one related to the interaction of posture and locomotion; and the other to initiation of locomotion. It is apparent that elucidation of the second aspect is feasible. In Fig. 15, I have summarized all the neuronal structures that have been functionally identified as being actively involved in the regulation of posture and locomotion. Obviously, contribution of the cerebello-cerebral pathways and the basal ganglia to both the postural and to the locomotor control cannot be elucidated in decerebrate preparations (Fig. 15A). These contributions can, to a certain degree, be elucidated in intact awake cats (Fig. 15B). Although it is difficult to directly compare the results obtained in the decerebrate cats with those obtained in intact cats, it has been encouraging that the selective activation of certain neuronal structures within the brain stem allowed us to evoke comparable postural and locomotor changes in both groups of preparations. It can be expected that the knowledge obtained from studies of the cerebello-cerebral pathways and of the basal ganglia, together with those that can be further obtained from studies of the brain stem, should result in the elucidation of the two major control aspects of the brain stem described above. In this sense, both the models of the decerebrate locomotor preparation and the freely moving, awake cat seem to provide an unique opportunity to study the nature and the sources of command signals that set the postural and the locomotor synergies into a single functional synergy, and even to approach elucidation of the intriguing question as to how and where volitional control signals for initiating and/or halting locomotion are organized. More than 70 years have passed since the pioneer studies by Sherrington (1906) and by Graham Brown (1911, 1914) on postural control and on locomotion, as exemplified by "decerebrate reflex standing, and stepping" and the "central rhythmic generator" relating to locomotion. Based on these discoveries, Shik, Severin and Orlovsky (1966) have made a splendid breakthrough in a wide area of investigation relating to locomotion.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute effects of dexamethasone on normal and on posttraumatic spinal cord polysynaptic reflex activity and axonal conduction

The effects of a single intravenous injection of a high dose of dexamethasone (4 mg/kg) on polysynaptic reflex activity and axonal conduction were measured for 5 hours in the intact and in the compression-injured L-7 spinal cord segment of high spinal cats. The segment was injured by a transient compression of preset degree and duration. In the uninjured preparation, dexamethasone administration significantly reduced polysynaptic reflex size for 2 hours. Axonal conduction was unaltered. One group of injured animals was given dexamethasone 30 minutes after trauma, whereas another was not treated. The acute posttraumatic changes in both parameters did not differ significantly in treated and untreated animals. Histopathologically, differences in the amount of segmental edema and hemorrhage between untreated and treated animals were not significant.

Actions on gamma-motoneurones elicited by electrical stimulation of joint afferent fibres in the hind limb of the cat

Effects on seventy-one single lumbar gamma-motoneurones, evoked by graded electrical stimulation of fibres running in the posterior articular nerve of the ipsilateral knee joint (p.a.n.), were studied by micro-electrode recording in twenty-one cats anaesthetized with alpha-chloralose. Sixty-seven of the gamma-cells were classified indirectly as dynamic (thirty-seven) or static (thirty) using the method of mesencephalic stimulation (cf. Appelberg, Hulliger, Johansson & Sojka, 1982). A high general responsiveness (i.e. number of cells with effect/number of cells tested) was found for the whole sample of gamma-cells (91.9% for dynamic and 93.3% for static cells). The thresholds for the effects were related to the stimulation intensity at which the early negative cord dorsum potential appeared (T). For all subpopulations of gamma-cells (dynamic and static, flexor and extensor cells) excitatory as well as inhibitory effects were observed at 0.9-1.1 T, probably corresponding to 1.1-1.4 times the threshold for evoking a compound action potential in p.a.n. (cf. Discussion). In addition, a considerable number of high-threshold effects were found. Some cells were influenced only from low-threshold joint afferents, some only from high-threshold joint afferents and some cells were influenced from both low- and high-threshold joint afferents. No statistically significant differences in thresholds were found between dynamic and static cells. Among flexor gamma-cells excitatory effects were found to predominate, while for extensor gamma-cells excitation and inhibition occurred with about equal frequency. The shortest latencies for excitatory effects on dynamic gamma-motoneurones were compatible with a trisynaptic pathway, while the routes for excitation of static units and for inhibition of both types of gamma-cells seemed to be longer. The possible functional significance of the findings is discussed. The findings seem to support the idea, as suggested by Freeman & Wyke (1967b), that the joint receptors may contribute to the 'co-ordination of muscle tone in posture and movement' via the gamma-loop. It is furthermore suggested that the latter mechanism may serve to regulate joint stiffness and joint stability.

Concussive head injury producing suppression of sensory transmission within the lumbar spinal cord in cats

This study examines the effects of concussive levels of a fluid-percussion head injury on sensory transmission within the lumbar spinal cord of the cat. Primary afferent depolarization (PAD) was suppressed for 2 to 5 minutes following injury, as assessed by dorsal root potentials and augmentation of antidromic dorsal root potentials, both evoked by stimulation of adjacent dorsal roots. Polysynaptic reflex discharges in ventral root potentials evoked by dorsal root stimulation were also profoundly suppressed during this same period, even when spontaneous and monosynaptic reflex discharges were facilitated. Changes in PAD produced by injury were abolished by spinal cord transection, but were not affected by midpontine transection. These findings suggest that concussive head injury can produce suppression of segmental sensory transmission by neurally mediated processes involving the bulbar brain stem. Recordings of dorsal root resting potentials, antidromic dorsal root potentials, and reductions of antidromic dorsal root potentials induced by tetanic root stimulation indicated that depressed segmental sensory function produced by injury was due to suppression of postsynaptic interneuronal transmission rather than to excitability changes in primary afferent fibers. Somatosensory cortical potentials evoked by dorsal root stimulation were profoundly depressed at the same time as segmental sensory transmission was suppressed, suggesting that suppressed segmental sensory transmission may also contribute to suppression of ascending sensory transmission. It is hypothesized that transmission failure of interneuronal systems in the initial period following insult may be a general response occurring in wide areas of the central nervous system, and not restricted to areas to which mechanical stress is directly applied. This response pattern may result from indiscriminate activation of interconnected excitatory and inhibitory elements of interneuronal systems.

Depression of afferent-induced primary afferent depolarization at the lumbar spinal cord following concussive head injury

Afferent-induced primary afferent depolarization (PAD) was depressed for 2-5 min following concussive head injury in the cat, as assessed by dorsal root potentials and augmentation of antidromic dorsal root potentials, both evoked by stimulation of adjacent dorsal roots. These changes in PAD were abolished by spinal cord transection but not affected by midpontine transection. Spontaneous dorsal root potentials, resting amplitudes of antidromic dorsal root potentials and reductions of antidromic dorsal root potentials following tetanic root stimulation were not substantially altered by injury. These findings suggest that concussive head injury depresses spinal interneuronal transmission by neurally mediated processes involving the bulbar brainstem.

Electrophysiological characteristics of lumbosacral evoked potentials in patients with established spinal cord injury

Surface electrodes positioned over the S1 and T12 vertebrae and referenced to T6 were used to record spinal potentials evoked by unilateral stimulation of the posterior tibial nerve at the knee. Data were collected on 24 patients who received spinal cord injuries 2 months to 31 years previously. The recording sites were below the level of spinal injury. The lumbosacral evoked potentials (LSEPs) were compared with the results of measurements obtained from 19 neurologically healthy subjects. Additional data were collected on each patient to characterize segmental reflex responses and preservation of sensory and motor functions associated with the L5 through S2 segments of the spinal cord. Assuming that the LSEP reflects the activity of spinal cord interneurons, the results demonstrate a degree of spinal cord dysfunction caudal to the area of injury in a substantial number of the patients with spinal cord injury which we studied.

Distribution of bilateral dorsal root potentials evoked by volleys in afferents entering lumbar and sacral segments of the spinal cord

Ipsilateral dorsal root potentials evoked by volleys entering lumbar and sacral segments of the cord are largest at the level of entry of afferent volleys. Contralateral potentials resulting from stimulation of any of these nerves attain maximum amplitude in lower sacral and upper caudal segments.

Changes in segmental reflexes following chronic spinal cord hemisection in the cat. II. Conditioned monosynaptic test reflexes

In a companion paper (Hultborn & Malmsten 1983) it was described that ventral root discharges to stimulation of peripheral nerves became larger on the side of a chronic spinal hemisection (left) than on the other side. In the present paper, based on the same experiments, conditioning of monosynaptic test reflexes was used to study changes of both excitatory and inhibitory effects on specified motoneuronal pools. Conditioning stimulation was given to IA afferents (reciprocal Ia inhibition, presynaptic inhibition of Ia fibers), high threshold muscle afferents, low and high threshold cutaneous afferents and motor axons (recurrent inhibition). A comparison of the efficacy of conditioning stimuli on the two sides showed that facilitatory effects were larger on the side of hemisection in a clear majority of cases. Inhibition was almost always either more efficient on the side of hemisection or equally efficient on the two sides. In control cats, facilitatory effects tended to be larger on the right side, while the results for inhibitory conditioning generally showed no clear side-bias. The increase in facilitatory effects after lesions may contribute to symptoms of spasticity.

Differential decerebrate control of depolarization in the central terminals of cutaneous afferents in the sacral cord

Presynaptic depolarization of cutaneous afferents has been investigated in the sacral cord of decerebrate cats before and after spinal cord transection. In the decerebrate state the central terminals of caudal femoral cutaneous nerve are depolarized by ipsilateral volleys entering the cord via sacral and lumbar dorsal roots. A significant increase of depolarization occurring after severing the cord indicates that there is tonic decerebrate inhibition of presynaptic depolarization in terminals of caudal femoral cutaneous nerve. In contrast to this finding, presynaptic depolarization evoked in the central terminals of the pudendal nerve by ipsilateral volleys entering the cord through sacral and lumbar dorsal roots is not subjected to decerebrate inhibitory control. It is suggested that differential inhibitory control of depolarization in the central terminals of cutaneous nerves in the sacral cord is related to the intraspinal course of their fibres, to differences in the receptor types involved, and to the location of their innervation fields. In more than half of the decerebrate preparations stimulation of the central terminals of cutaneous afferents through microelectrodes evokes antidromic spikes appearing simultaneously in ipsi- and contralateral nerves. The time course of bilateral excitability changes is similar on both sides of the cord. It is assumed that presynaptic effects are transmitted to the contralateral side by collaterals of ipsilateral cutaneous afferents.

Presynaptic depolarization of unmyelinated primary afferent fibers in the spinal cord of the cat

Low intensity (1-20 micro A) intraspinal stimulation produces in the sural nerve of the anesthetized cat short latency responses (3-4 ms) due to antidromic activation of fibers conducting in the A range (43-65 m/s). With higher stimulus intensities (up to 400 micro A) late responses (120-250 ms latency) may also be recorded. Simultaneous recording from two sites in the sural nerve shows that the peripheral processes of the fibers generating the late responses have a conduction velocity between 0.8-1.3 m/s. Collision between antidromic and orthodromic responses further indicates that these fibers have a peripheral threshold 20-25 times that of the A fibers. The late responses were largest when the intraspinal stimulating electrode was located in the dorsal horn, in the region corresponding to Laminae II and III of Rexed. The above observations suggest that the late responses are due to population responses of C fibers which are antidromically activated in the dorsal horn. The excitability of the C fiber terminals is increased by conditioning stimuli applied to other cutaneous afferents with a time course resembling that of the excitability increase of the A fibers on the same nerve. It is suggested that the effectiveness of synaptic transmission from C fibers to second order cells may be modulated presynaptically. In the decerebrate cat the antidromic responses of C fibers are reduced, but not abolished, by reversible spinalization produced by cooling or by sectioning the thoracic spinal cord. This suggests in addition that in the decerebrate preparation the presynaptic effectiveness of the C fiber (presumably nociceptive) input may be tonically decreased by supraspinal influences.

Inhibitory actions from low and high threshold cutaneous afferents on groups II and III muscle afferent pathways in the spinal cat

The inhibitory effects caused by volleys in cutaneous afferents on the transmission through some polysynaptic segmental pathways activated by high threshold muscle afferents were studied in chloralose anesthetized, spinal cats. Pathways studied were groups II and III to motoneurones as well as group II to primary afferents. The results suggested that two different mechanisms were involved. One mechanism, with a very slow time course (duration more than 400 ms), is suggested to be an example of presynaptic inhibition between different primary afferent systems. This mechanism required high threshold (greater than or equal to 1.6T) conditioning shocks, and appeared simultaneously with the component II dorsal root potential being evoked by the cutaneous afferent volley. The other mechanism, with a faster time course (duration always below 300 ms), was dependent upon low threshold (less than or equal to 1.5T) cutaneous conditioning volleys. This inhibitory interaction could not be ascribed to the same presynaptic mechanism, but is suggested to be an example of postsynaptic inhibition at an interneuronal level. The presumed disynaptic excitatory pathway from group II muscle afferents to flexor motoneurones was not inhibited by cutaneous conditioning shocks, but could on the contrary be facilitated by activity in low threshold cutaneous afferents, probably at the only interneurone involved in this group II pathway.

Reciprocal inhibition during the tonic stretch reflex in the decerebrate cat

1. The aim of this study was to investigate post-synaptic reciprocal Ia inhibition during the stretch reflex; particularly the extent to which an increased Ia excitation of the Ia inhibitory interneurones will be counteracted by recurrent inhibition from motor axon collaterals. For this purpose we investigated depression of monosynaptic test reflexes antagonist flexors (reciprocal inhibition) during static stretch of quadriceps or triceps surae in unanaesthetized decerebrate cats. 3. With increasing stretch of the extensor muscle there was first a linear augmentation of reciprocal inhibition, but along with the stretch reflex in the extensor a plateau appeared in the inhibition of the flexors, although the extensor stretch reflex (judged by the e.m.g.) increased with further stretching. Within the range of stretching of triceps surae which gave increased stretch reflexes the plateau in the reciprocal inhibition was usually maintained, while during stretching of quadriceps a second phase of augmenting reciprocal inhibition often appeared. Stretch beyond the level which increased the stretch reflex activity gave augmenting reciprocal inhibition both in case of quadriceps and triceps surae. 3. Excitability measurements from central terminals of Ia afferents revealed that the increasing reciprocal inhibition during increasing stretch reflex activity in quadriceps was associated with a primary afferent depolarization in knee flexor Ia afferents; there was no corresponding effect in ankle flexor Ia afferents during stretch reflexes in triceps surae. 4. The primary afferent depolarization evoked in knee flexor Ia afferents by electrical nerve stimulation was then compared with the presynaptic inhibition of knee flexor monosynaptic test reflexes produced by the same stimuli. The results suggest that the second phase of increasing reciprocal inhibition in knee flexors is due to presynaptic inhibition and accordingly that the depth of post-synaptic reciprocal inhibition remains constant at different degrees of stretch reflex activity in both knee and ankle extensors. 5. It is postulated that during increasing stretch reflex activity the increment in Ia excitation and recurrent inhibitio; on to the Ia inhibitory interneurones almost exactly balance each other. It is suggested that recurrent inhibition of Ia inhibitory interneurones may serve as a segmental autoregulatory mechanism to keep 'alpha-gamma-linked reciprocal inhibition' at a constant depth during different levels of agonist activity.

Analysis of the circuitry responsible for primary afferent depolarization in the trigeminal spinal nucleus caudalis of cats

Depth analysis was performed on the field potential evoked by stimulation of the infraorbital nerve in the trigeminal spinal nucleus caudalis and the subjacent lateral reticular formation of cats. It was shown by dye marking of the recording positions that each subnucleus of the nucleus caudalis (subnucleus marginalis, gelatinosus and magnocellularis) and the reticular formation could be differentiated from one another by the characteristics of the peripherally evoked field potentials. Responses of neurons were extracellularly recorded in the subnuclei gelatinosus and magnocellularis of the nucleus caudalis and in the reticular formation to stimulation of the trigeminal sensory branches (the frontal, infraorbital and lingual nerves), the nucleus ventralis posteromedialis of the thalamus and the cerebral cortex. The properties of the neurons were studied in relation to their thresholds, latencies, receptive fields (sensory branches effective for spike generation) and frequency-following capacities. These responses were then compared with properties of the PAD induced in the fibers terminating in the nucleus caudalis by similar peripheral and central stimulation. It was found that the neurons in the subnucleus magnocellularis were the most likely candidates for the interneurons mediating the peripherally evoked disynaptic PAD in the trigeminal nerve fibers terminating in the nucleus caudalis.

Tonic inhibitory influence of a supraspinal monoaminergic system on presynaptic inhibition of an extensor monosynaptic reflex

Presynaptic inhibition of the extensor (quadriceps, QUAD) monosynaptic reflex (MSR) in unanaesthetized decerebrate cats was antagonized by imipramine hydrochloride (2-5 mg/kg), 5-hydroxytryptophan (75 mg/kg) and a specific 5-hydroxytryptamine (5-HT) neuronal uptake blocker, fluoxetine hydrochloride (Lilly 110140, 0.25-6 mg/kg). These effects of imipramine and fluoxetine were partially reversed by the 5-HT antagonist, cyproheptadine hydrochloride (5 mg/kg), and completely reversed by the application of a thoracic cold block which prevents supraspinal inputs to the caudal spinal cord. Imipramine, however, failed to antagonize this inhibition in animals pretreated with either DL-p-chlorophenylalanine (p-CPA, 300 mg/kg i.p. for 2 consecutive days) or DL-a-methyl-p-tyrosine methyl ester hydrochloride (a-MPt, 125 mg/kg i.p. 16 and 4 h prior to the experiment). Cyproheptadine (2.5--5 mg/kg); phenoxybenzamine hydrochloride (2.5-5 mg/kg) and a cold block enhanced the inhibition of this extensor MSR but a cold block failed to alter the inhibition in animals pretreated with p-CPA or a-MPT. Presynaptic inhibition of the flexor (posterior biceps-semitendinosus, PBST) MSR was however not blocked by imipramine, fluoxetine or a cold block nor enhanced by cyproheptadine or phenoxybenzamine. The effects of the drugs tested and a cold block on the excitability of the QUAD group Ia afferents were reciprocal to those on the MSR during presynaptic inhibition. The results of this study indicate that descending tonically active systems (1) involving 5-HT and noradrenaline, antagonize presynaptic inhibition of the QUAD but not the PBST-MSR, (2) decrease the excitability of the QUAD Ia afferents and (3) increase the excitability of QUAD motoneurones.

The rubro-bulbospinal path. A descending system known to influence dynamic fusimotor neurones and its interaction with distal cutaneous afferents in the control of flexor reflex afferent pathways

The inhibitory effect of electrical stimulation in the near-rubral region on polysynaptic segmental as well as ascending pathways activated by the flexor reflex afferents (FRA) in hind limb nerves was studied in chloralose anaesthetized cats. The effective stimulating region totally coincided with the one from which a D zone climbing fibre response may be elicited in the contralateral cerebellar cortex. The descending path was dependent upon an intact dorsolateral spinal funiculus, where also a characteristic volley could be recorded with a surface electrode on short train central stimulation. The suppressive action on the transmission through the FRA pathways was evoked in the absence of a lower lumbar dorsal root potential, and it was concluded that the effect was exerted by postsynaptic inhibition. It was suggested that this descending path, the effects of which resemble those elicited from the dorsal reticulospinal system, is identical to the rubro-bulbospinal path, previously known to influence dynamic fusimotor neurones. The transmission through the FRA pathways was also suppressed by conditioning stimulation of ipsilateral, low threshold distal cutaneous afferents. The time course of this effect was the same as that with central conditioning stimulation. Facilitatory interaction was revealed with double conditioning and it was suggested that the descending path and the distal cutaneous afferents converge upon a common group of interneurones, which postsynaptically inhibit an early (possibly the first one) interneurone in the FRA pathways. As low threshold distal cutaneous afferents supply the primary peripheral input via climbing fibres to the cerebello-cortical D zone, it was concluded that the different stimuli (central or peripheral) which activate a common group of inferior olivary neurones destined for the D zone also activate a common group of segmental inhibitory interneurones. The results are discussed in relation to current concepts of segmental motor control, and it is suggested that the mechanisms studied could be involved in the regulation of stepping.

Medical modification of sensation

The authors describe the sensory examinations of 3 patients who had undergone cervical rhizotomy alone and in combination with trigeminal tractotomy and section of the nervus intermedius, the glossopharyngeal nerve, and the upper portion of the vagus nerve. Following administration of L-dopa there was an increase in their pain and a decrease in the area of clinically anesthetic or analgesic skin. When methyldopa was given, the subjective and objective changes were the opposite of those elicited by L-dopa. These observations support the existence of a wider dorsal root cutaneous distribution than is usually accepted as well as significant control of cutaneous sensation by suprasegmental areas of the central nervous system. Part of the suprasegmental bias supplied to the area in the spinal cord that processes sensory information apparently occurs by way of an aminergic descending reticulospinal tract. These findings are discussed in terms of attempts totally to denervate restricted cutaneous areas of the body for treatment of pain-producing states.

Facilitation from contralateral primary afferents of interneuronal transmission in the Ia inhibitory pathway to motoneurones

The action of volleys in contralateral primary afferents on transmission in the Ia inhibitory pathways to motoneurones was investigated with intracellular recording from motoneurones. Ia IPSPs in flexor as well as most extensor motoneurones were regularly facilitated by volleys in contralateral high threshold muscle, cutaneous and joint afferents in spinal cats under chloralose anaesthesia. In decerebrate cats with a low pontine lesion transmission in Ia inhibitory pathways was not facilitated but rather depressed by volleys in these afferents. The recurrent effects from motor axon collaterals were investigated on inhibitory transmission from different contralateral afferents to motoneurones. Previous investigations have shown that the interneurones mediating the reciprocal Ia inhibition receive recurrent inhibition via motor axon collaterals and Renshaw cells. Now a strong positive correlation was revealed between recurrent depression of IPSPs evoked from different contralateral afferents and facilitation of Ia IPSPs by the same afferent volleys. These results suggest that the recurrent depression of IPSPs from different contralateral primary afferents depends on their excitatory convergence onto the Ia inhibitory interneurones, which then partly mediate the IPSP evoked in the motoneurone from these afferents.

Properties and distribution of peripherally evoked presynaptic hyperpolarization in cat lumbar spinal cord

1. The action of peripheral nerve volleys on the polarization of presynaptic terminals of inactive sensory fibres in cat lumbar spinal cord has been investigated by recording (a) the dorsal root potential (DRP), (b) intracellular changes in polarization of single preterminal axons (PAD or PAH), and (c) changes in excitability of populations of preterminal axons.2. Presynaptic hyperpolarization (positive DRP-PAH) can be evoked by stimulation of muscle group III afferents as well as by volleys in cutaneous Abeta, Adelta and C afferents. These volleys can also produce presynaptic depolarization (negative DRP-PAD).3. The positive DRP is observed in the decerebrate state and increases in amplitude following spinalization.4. Picrotoxin blocks the positive DRP at the same dosages required to block the negative DRP. Test negative DRPs are depressed during a conditioning positive DRP. These results are used to support earlier suggestions that the positive DRP results from inhibition of interneurones mediating the negative DRP.5. Trains of group III stimuli at 20/sec evoke a steady positive DRP. Trains of the same intensity at 200/sec evoke a phasic negative DRP. This frequency dependence is observed for PAD and PAH in single sensory axons.6. The DRPs recorded from different dorsal root filaments in response to a given stimulus vary widely in the ratio of negative to positive DRP.7. Intracellular recording from single axons reveals that the same stimuli evoke widely varying ratios of PAD and PAH.8. Stimulation of FRA evokes PAH > PAD in PBST group I afferents, PAD > PAH in sural A fibres and intermediate effects in G-S group I units.9. It is suggested that activation of flexor reflex afferents may selectively potentiate the synaptic efficacy of large muscle afferents mediating the flexor reflex rather than large skin afferents or large afferents from extensor muscles.