Monosynaptic Excitation of Alpha Motoneurones from Supraspinal Structures in the Cat

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Abstract
The origin of descending fibres evoking monosynaptic excitatory postsynaptic potentials (EPSPs) in motoneurones was studied.Lumbar flexor and extensor motoneurones were recorded from intracellularly.The ipsilateral ventral and lateral funicles were stimulated at thoracic level in cats with and without chronic lesions of the ipsilateral cervical cord or vestibular complex.In other experiments the ipsilateral Deiters' nucleus was stimulated.Valleys in decending ipsilateral fibres evoke a monosynaptic EPSP in the majority of the flexor and extensor motoneurones with an amplitude corresponding to about 25 per cent of the homonymous group Ia EPSP.The effect is almost entirely of supraspinal origin.Most extensor motoneurones receive this action from fibres originating in the ipsilateral Deiters' nucleus.Flexor, and probably some species of extensor motoneurones, receive the action from a supraspinal structure outside the ipsilateral vestibular complex.The results are discussed in relation to the cerebellar influence on the Deiters' nucleus and the cerebellar inhibition of extensor rigidity.