Presynaptic Inhibition in the Spinal Cord

103

1. The possibility was investigated that, in man, some of the descending command for tonic voluntary wrist extension is transmitted to extensor motoneurones over a nonmonosynaptic pathway. 2. Stimulation of the cutaneous superficial radial nerve at 3 times perceptual threshold depressed the electromyogram (EMG) of extensor carpi radialis (ECR) and the discharge of single ECR motor units, both with a mean central delay of 4-2 ms. Such stimuli depressed the response to transcranial magnetic stimulation of the motor cortex, but had little effect on the H reflex. 3. The possibility that the relative sparing of the H reflex was due to an alteration in transmission of the afferent volley for the H reflex was excluded. 4. The central latency of the cutaneous-induced depression of the discharge of single motor units in biceps brachii (C5-C6) was shorter by about 1 ms than that of the more caudal wrist and finger extensor motor units. This suggests that the locus for the cutaneousinduced effects was spinal but above the cervical enlargement. 5. The pattern of EMG depression (evoked by superficial radial but not palmar stimuli, in wrist extensors but not wrist flexors) is that previously described for the presumed propriospinal system of human subjects. 6. It is concluded that a significant component of the voluntary command for tonic wrist extension reaches the relevant motoneurone pool via a non-monosynaptic pathway. It is suggested that the interposed neurones could be C3-C4 propriospinal neurones.

Section: Effects Of Sr Stimulation On the Transmission Of The Afferensupporting

confidence: 76%

Non‐monosynaptic transmission of the cortical command for voluntary movement in man.

Burke

Graciès

Mazevet

et al. 1994

103

“…(2) the relationship between activity of these cells and dorsal root potentials (DRPs) which have been reported to correspond to presynaptic inhibition of primary afferent fibres (Wall, 1958(Wall, , 1962(Wall, , 1964Eccles, 1964); (3) the modifications in cell discharges and DRPs produced by the administration of various drugs.…”

Section: Introductionmentioning

confidence: 99%

Spinal interneurones involved in presynaptic controls of supraspinal origin†

Besson¹,

Rivot²

1973

SUMMARY1. Interneurones presenting heterotopic and heterosensory convergence have been identified in laminae VI-VII of the lumbar dorsal horn in the cat. Stimulation of the hind limbs sometimes induced a bimodal response, but we considered only the late convergent discharge.2. The fact that response latencies are longer to hind limb than to forelimb stimulation at this level suggests the intervention of a supraspinal loop in the activation of spinal convergent units. This hypothesis is supported by the relationship between the excitability of supraspinal structures and the discharge intensity of convergent cells as well as by the absence of long latency responses in the spinal preparation.3. Electrophysiological and pharmacological evidence discloses a strong relationship between convergent unit discharges and the occurrence of dorsal root potentials to cortical, heterosegmental and heterosensory stimulation.4. It is suggested that convergent units receive information of supraspinal origin and exert control over sensory input to the cord via primary afferent depolarization.

“…A conditioning afferent volley can produce PAD in a myelinated fibre, irrespective of whether this particular fibre is activated by the stimulus or not. It is now well established that PAD is the electrophysiological correlate of presynaptic inhibition in the spinal cord (for reviews see Eccles, 1964;Schmidt, 1971). This type of inhibition reveals a functional organization with regard to the modality of the interacting afferent fibres, i.e.…”

Section: Introductionmentioning

confidence: 99%

Presynaptic depolarization of myelinated afferent fibres evoked by stimulation of cutaneous C fibres

Jänig¹,

Zimmermann²

1971

SUMMARY1. The origin of the dorsal root potential observed following a volley in cutaneous unmyelinated or C fibres (C-DRP) has been investigated by several assays.2. Measurements of the spatial decrement of the C-DRP along the dorsal roots indicate that mainly the thick myelinated primary afferent fibres contribute to this C-DRP.3. The spatial configuration of the intraspinal potential field during the course of the C-DRP is similar to that of the field correlated to the A-DRP, i.e. the DRP evoked by a volley in the A fibres of the same cutaneous nerve. It is concluded therefrom that both A-and C-DRPs signal presynaptic depolarizations of afferent fibres in the same regions of the dorsal horn.4. Intrafibre recordings reveal that C input produces primary afferent depolarization (C-PAD) mainly in cutaneous and articular afferents, but not in muscle Group Ia afferents.5. These findings and direct measurements of the interactions between A-and C-DRPs indicate that the pathways involved in A-and C-PAD exhibit convergence.6. C-DRPs were also recorded from contralateral rootlets. 7. Some functional aspects of the PAD produced by an afferent volley in C fibres are discussed.