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1. Intra- and extracellular recordings were made from cells of the spinocervical tract in the lumbosacral spinal cord. A convergence of monosynaptic excitatory post-synaptic potentials (EPSPs) and disynaptic inhibitory post-synaptic potentials (IPSPs) was a general pattern of effects from the low threshold cutaneous fibres. Unitary IPSPs, probably mediated via the same disynaptic path, were evoked by light touch of hairs, which was also the adequate stimulus for exciting the cells. The receptive field for unitary IPSPs was closely related to the excitatory receptive field but was eccentric, not of a surround type.2. EPSPs, IPSPs, or both, were evoked from the flexor reflex afferents in the great majority of neurones. Disynaptic IPSPs may be evoked from the interosseous nerve. No effects were produced by volleys in group I muscle afferents.3. It is suggested, on the basis of the spatial organization of the excitatory and inhibitory receptive skin fields, that the spinocervical tract may give information regarding the direction of tactile stimuli.

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.

1. Unilateral section of the nerves to an extensor muscle of the head, biventer cervicis, leads to an ataxia of the hind legs in cats.2. In chloralose anaesthetized cats, shocks to the central end of the cut biventer cervicis muscle nerves leads to direct discharges over lumbosacral ventral roots, to a facilitation of monosynaptic reflexes in flexor and extensor muscles and to a prolonged inhibition of spino-bulbo-spinal reflexes. Such effects are not unique, but follow stimulation of skin and muscle nerves of the forepaw.3. Supratentorial pathways are involved in descending spinal interactions in the chloralose anaesthetized cat, but are more critical for interactions taking origin in biventer cervicis nerve than in forepaw nerves.4. In non-anaesthetized spinal cats showing interactions from forepaw nerves, no interactions could be found taking origin in biventer cervicis nerves. Descending interactions from forepaw nerves could be reduced or abolished by anaesthetic doses of chloralose.5. The rich pathway from biventer cervicis nerve to the superior colliculus does not appear to be involved in the descending interactions reported here as neither stimulation nor ablation of the superior colliculi has any effect on descending interactions.