Synaptic connexions of alpha extensor motoneurones with ipsilateral and contralateral cutaneous nerves.

1. Synaptic responses of alpha extensor motoneurones to stimulation of cutaneous nerves of the hind limb were recorded from the lumbosacral region in decerebrate and decerebrate-spinal unanaesthetized cats. Responses to ipsi- and contralateral stimulation are compared.2. Usually a single volley in an ipsilateral cutaneous nerve led to an inhibitory post-synaptic potential while one in a contralateral cutaneous nerve led to an excitatory post-synaptic potential or to discharge of the cell, thus demonstrating the classical reciprocal innervation pattern. In some cells either ipsilateral or crossed spinal action was of opposite sign to normal.3. Tests using graded stimulation showed the range of myelinated afferents associated with crossed spinal actions to be narrower than that for ipsilateral actions; for the sural nerve the ranges are 1-6 mu and 1-9 mu respectively. A substantial proportion of the coarser myelinated afferents do not make connexion with alpha motoneurones on either side of the cord; in the sural nerve this proportion includes the range 10-17 mu.4. Central transmission times for ipsilateral and crossed-spinal actions were estimated after making systematic allowance for conduction time to the cord in the associated cutaneous afferents. Mean values for inhibitory pathways from ipsilateral cutaneous nerves were about 2 msec, while those for crossed spinal facilitatory pathways were notably greater, 3-5 msec.5. Crossed spinal actions were more profoundly depressed by anaesthesia than were corresponding ipsilateral actions.6. Interpretation of the results was aided by a separate study of the input to the spinal cord; stimulus strength is related to the diameters of the cutaneous fibres excited and to peripheral conduction time in those fibres.