Recurrent conditioning in the cat spinal cord. Differential effect of meprobamate on recurrent facilitation and inhibition

Radiofrequency cordotomy for the relief of spasticity in decerebrate cats

The effectiveness of radiofrequency (RF) cordotomy of segmental motoneurone pools of the lumbosacral cord in reducing spasticity of decerebrate cats is evaluated. The need for a new form of therapy for clinical spasticity is based upon the limitations of contemporary methods, including surgical and pharmacological techniques. In man, spasticity of spinal origin may be treated effectively by intrathecal administration of hyperbaric phenol solutions. The advantages and disadvantages are described. Difficulty in controlling the lesion is emphasized. Tension and EMG-length curves of the spastic triceps surae muscle in acute and chronic animals show that RF lesions (fixed amperage and duration) of the segmental motoneurone pools reduces myotatic reflex activity in accordance with the number of segments cordotomized. Clinical examination including cinematography and electromyography complement the physiological interpretation. RF lesions of the internuncial pool induce spontaneous EMG discharges. This finding is related to similar observations of EMG discharges and alterations in muscle tone after asphyxiation of the spinal cord.

Neuronal pathway of the recurrent facilitation of motoneurones

1. The recurrent facilitation of motoneurones is a disinhibition, i.e. a release of the motoneurones from a sustained hyperpolarization evoked by tonically active inhibitory interneurones. Only two groups of interneurones are known to receive recurrent inhibition from motor axon collaterals via Renshaw cells; the interneurones mediating the reciprocal Ia inhibition and the Renshaw cells themselves. The properties of these two groups of neurones were studied to determine if they could produce the tonic inhibition of motoneurones removed during recurrent facilitation.2. It was found that the tonic firing of Ia inhibitory interneurones is sensitive to anaesthetics to the same degree as is recurrent facilitation. The range of frequencies of tonic discharges of Renshaw cells appeared to be similarly low in unanaesthetized and anaesthetized preparations although in individual cells the discharge rates were decreased by anaesthesia.3. The recurrent inhibition of Ia interneurones inhibiting a given group of motoneurones and the recurrent facilitation of the same group of motoneurones were, as a rule, evoked from the same nerves, although in some cats the origin of the recurrent facilitation was somewhat wider. In contrast no evidence could be found that the Renshaw cells which inhibit a functional group of motoneurones are inhibited by volleys in the nerves from which recurrent facilitation is regularly evoked.4. It was concluded that the recurrent facilitation is caused mainly by inhibition of the tonic activity of Ia inhibitory interneurones and that it is thus a manifestation of the recurrent control of Ia reciprocal inhibition of motoneurones.

Relative contribution from different nerves to recurrent depression of Ia IPSPs in motoneurones

1. The pattern of depression of Ia IPSPs by volleys in recurrent motor axon collaterals was investigated in motoneurones supplying hind-limb muscles in the cat. The test IPSPs were evoked by stimulation of dorsal roots and the conditioning antidromic volleys by stimulation of motor fibres in different peripheral muscle nerves.2. In all motor nuclei investigated the strongest depression of Ia IPSPs is evoked from motor fibres to muscles whose Ia afferents produce the IPSPs. For example, the Ia IPSP from the knee extensor recorded in motoneurones to a knee flexor is most effectively depressed by antidromic stimulation of motor fibres to the knee extensor.3. The origin of recurrent inhibition of alpha-motoneurones and of Ia inhibitory interneurones with the same Ia input display a striking similarity. This suggests that the same population of Renshaw cells mediates effects to motoneurones and to Ia inhibitory interneurones.4. The functional significance of impulses in motor axon collaterals was discussed and it was suggested that they have an important role in the control of the excitatory as well as inhibitory Ia actions to motoneurones. The recurrent inhibition may limit the Ia effects to excitation of homonymous motoneurones, which would provide optimal conditions for control of individual muscles via the gamma-loop.

Recurrent inhibition from motor axon collaterals of transmission in the Ia inhibitory pathway to motoneurones

1. The effects of impulses in recurrent motor axon collaterals on reflex transmission from different types of primary afferents to motoneurones were investigated in the cat by conditioning of PSPs evoked in motoneurones.2. IPSPs evoked by volleys in large muscle spindle (Ia) afferents were effectively decreased when preceded by an antidromic stimulation of ventral roots. Some IPSPs from group II muscle afferents and low threshold cutaneous afferents were also slightly depressed, while other PSPs were unaffected.3. The depression of the IPSPs could be evoked by antidromic volleys, which produced neither conductance changes in the motoneurones nor depolarization of Ia afferent terminals.4. The effect on the Ia IPSPs is most likely due to post-synaptic inhibition of the Ia inhibitory interneurones, evoked through alpha-motor axon collaterals and Renshaw cells. The depression of some IPSPs from flexor reflex afferents is explained by a convergence of excitatory effects from these afferents on the Ia inhibitory interneurones.5. The results indicate a selective recurrent control from motor axon collaterals of the interneurones in the reciprocal Ia inhibitory pathway to motoneurones.