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Nociceptive neurons in the superficial dorsal horn of cat lumbar spinal cord and their primary afferent inputs



Nociceptive neurons in the superficial dorsal horn of cat lumbar spinal cord and their primary afferent inputs



Experimental Brain Research 58(1): 171-182



The morphology, background activity and responses to stimulation of primary afferent inputs of small neurons in the superficial dorsal horn which could only be excited from the skin by noxious stimulation were investigated by intracellular recording and ionophoresis of HRP [horseradish peroxidase]. Neurons which gave similar responses to afferent stimulation were morphologically heterogeneous with respect to dendritic tree geometry and axonal projection, but were located around the lamina I/II border. Cutaneous excitatory receptive fields responding to noxious stimulation were generally small; most neurons had more extensive inhibitory fields responding to inocuous mechanical stimulation, in many cases overlapping the excitatory fields. Stimulation of the excitatory field resulted in depolarization of the neuron and increased action potential firing, and stimulation of the inhibitory field resulted in hyperpolarization. Electrical stimulation of peripheral nerves revealed the existence of converging excitatory inputs carried by different fiber groups, and all neurons received an inhibitory input activated at low threshold. Excitatory responses were short-lived and occurred consistently in response to repeated stimulation. Central delay measurements gave evidence of a number of A.delta. monosynaptic inputs but only 1 A.beta. monosynaptic input; inhibitory inputs along A.beta. fibers were polysynaptic. The constant latency and regularity of the C response suggested monosynaptic connections. Low intensity stimulation of inhibitory inputs elicited a short-lived i.p.s.p. which increased in amplitude with increasing stimulus strength until it disappeared into a more prolonged hyperpolarization. This was associated with inhibition of background action potentials, and increased in duration with increasing stimulus strength up to C levels, indicating an A.delta. and C component. The level of excitability of these neurons depends on the relative amounts of concurrent noxious and innocuous stimulation, and the resultant output, which is conveyed mainly to other neurons within the spinal cord, could modulate reflex action at the spinal level as well as affecting components of ascending sensory pathways.

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Accession: 005989222

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PMID: 2985419


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