Problems of postsynaptic autogenous and recurrent inhibition in the mammalian spinal cord

SUMMARY1. Effects on single lumbar y-motoneurones, mediated via fibres running in the ventral roots, were studied by micro-electrode recording in cats anaesthetized with chloralose. Graded electrical stimulation of ventral roots or of peripheral nerves was used.2. The cells were identified as y-motoneurones by antidromic stimulation and by measurement of their axonal conduction velocity. Some of the cells were classified as static or dynamic.3. The findings confirm the previously demonstrated existence of low-threshold, presumed recurrent, inhibition of both static and dynamic y-motoneurones.4. Strong evidence for the occurrence of high-threshold recurrent inhibition of y-motoneurones is also presented. 5. In addition, excitatory effects on y-cells, also mediated via fibres in the ventral roots, are described.6. The low-threshold effects from ventral root fibres are attributed to recurrent a-collateral activity and the high-threshold effects to y-collateral activity.

Section: Discussionmentioning

confidence: 99%

Recurrent actions on gamma‐motoneurones mediated via large and small ventral root fibres in the cat.

Appelberg¹,

Hulliger²,

Johansson³

et al. 1983

“…In addition to the excitatory inputs due to the voluntary motor discharge reaching Renshaw cells via motor axon collaterals, data from animal experiments suggest that inhibitory synaptic inputs might converge on Renshaw cells during voluntary contractions. These inhibitory influences can be elicited both from supraspinal centres and/or segmentally from various receptors in the limb involved in soleus contraction (for references, see Haase et al 1975). In relation to the present experiments, the results obtained by Koehler, Windhorst, Schmidt, Meyer-Lohmann & H. HULTBORN AND E. PIERROT-DESEILLIGNY Henatsch (1978) are of particular interest, since they have shown that stimulation of the capsula interna often produced an increase of the monosynaptic reflex together with a decrease of the resulting Renshaw cell activity, i.e.…”

Section: Discussionmentioning

confidence: 99%

Changes in recurrent inhibition during voluntary soleus contractions in man studied by an H‐reflex technique.

Hultborn¹,

Pierrot-Deseilligny²

1979

149

107

SUMMARY1. The recurrent inhibition, brought about by a conditioning H-reflex discharge, was estimated in human subjects by the amplitude of a test H-reflex involving only the soleus motoneurones which fired in response to the conditioning volley. The modifications of the recurrent inhibition during contraction were evaluated by comparing the amplitude of the test H-reflex to a reference H-reflex. Both reflexes experienced the excitation underlying the voluntary contraction, but only the test H-reflex was subjected to the recurrent inhibition evoked by the conditioning H-reflex discharge.2. Distinct differences were observed between the modifications of the test reflex and those of the reference H-reflex during both tonic and phasic voluntary contractions. Evidence is presented that these differences were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge.3. The changes in recurrent inhibition were studied while the subjects performed voluntary tonic contractions of various forces. The weakest contractions were accompanied by a decrease in the size of the test reflex. With greater contraction forces, there was no longer an inhibition of the test reflex, but instead a facilitation which grew continuously with increased contraction forces. The test reflex could eventually exceed the reference H-reflex amplitude at the strongest contractions. This is taken to indicate that the recurrent inhibition following the conditioning discharge was progressively decreasing, as the contraction force increased.4. During ramp contractions, whatever the contraction velocity, the time courses of the variations of the test and reference H reflexes were almost inverse. Evidence is presented that these differential time courses were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge.5. The possibility of occlusion in the recurrent pathway was considered and it was concluded that the decrease in the recurrent inhibition elicited by the conditioning discharge was essentially due to an inhibitory control (spinal and/or suprasegmental) acting on Renshaw cells. This inhibition of Renshaw cells eventually counteracts the increasing excitatory inputs (resulting from the voluntary motor

“…Renshaw cells also receive synaptic inputs from other sources than motor axon collaterals. Inhibitory or excitatory effects, which are not secondary to changes in the motoneuronal discharge, can be elicited both from segmental reflex pathways and descending tracts (for references see Haase, Cleveland & Ross, 1975;Fromm, Haase & Wolf, 1977;Koehler, Windhorst, Schmidt, Meyer-Lohmann & Henatsch, 1978). The question then arises whether these control systems converging on Renshaw cells merely cause a parallel shift in the relation between the input from motor axon collaterals and the amount of recurrent inhibition or if it also modifies the slope of this relation, i.e.…”

Section: Discussionmentioning

confidence: 99%

Input‐output relations in the pathway of recurrent inhibition to motoneurones in the cat.

Hultborn¹,

Pierrot-Deseilligny²

1979

SUMMARY1. The output from Renshaw cells caused by a phasic motor volley was investigated when these neurones were submitted to a background firing secondary to a tonic motor discharge elicited by a repetitive stimulation of muscle group I afferents. It was invariably found in individual Renshaw cells that tonic excitation produced an increase in the additional output caused by the phasic motor volley. The curves displaying this increase exhibited a significant 'jump' when the output resulting from the combined tonic and phasic motor discharges ranged between 2 and 5 spikes during the first 10 msec following the phasic volley.2. The whole pool of Renshaw cells was also considered by assessing the amount of recurrent inhibition in motoneurones following a phasic motor volley. Similarly it was found that additional recurrent inhibition elicited by a phasic motor volley was enhanced when the Renshaw cells received a tonic excitatory input.3. The Renshaw cell discharges elicited by stimulation of two different nerves were compared when a conditioning stimulus was previously applied to only one of them. The results strongly suggest that a preceding volley caused a decrease in synaptic efficacy at the terminals of the recurrent collaterals.4. The firing produced by current injected through the recording micro-electrode was investigated in one intracellularly recorded Renshaw cell. It was found that the current-frequency curve was linear for 'steady-state' firing but displayed a clearcut sigmoid shape for the earliest intervals before the final adaptation.5. It is demonstrated that this sigmoid input-output relation in individual Renshaw cells is sufficient to explain how the controls acting on these neurones may change the gain in the recurrent pathway when Renshaw cells are fired by a phasic motor discharge. When Renshaw cells are fired by a longlasting tonic motor discharge the linear input-output relation in individual cells should not cause any modifications of the gain in the recurrent pathway. A change in this gain secondary to the effects of segmental and supraspinal control systems is, however, possible if the motor discharge creates a subliminal fringe within the pool of Renshaw cells.