Postsynaptic responses of lamina II neurons of the spinal cord of the cat to activation of primary afferent input

Abstract

Development of a complex response evokedin vivo in the neurons of lamina II of the spinal cord gray matter in cats by single electrical stimulation of primary afferents was simulated using mathematical models of these neurons, including the electrically excitable soma and axon and passive equivalent nonuniform dendrite. The intracellular response consisted of an excitatory postsynaptic potential (EPSP) with an action potential (AP) followed by a two-component hyperpolarization determined by the afterprocesses of hyperpolarization. The “fast” early hyperpolarization component appeared at the threshold stimulation of the most fast-conducting fibers; with an increase in the stimulation intensity it became superimposed on a “slow” later component. The direction of the early component changed after the hyperpolarizing shift of the membrane potential by 10 to 20 mV with respect to the resting level of −60÷−70 mV. The later component was abolished but not reversed even by the 50-mV shifts (to the −120-mV level). Simulation experiments showed that observedin vivo hyperpolarization-induced modification of the complex response is determined principally by a local interaction of electrotonus with synatic processes and does not depend on the behavior of the usual potential-activated sodium and potassium conductances in the soma. Inhibitory chloride synapses located on the soma and close to it represent the main source of fast early hyperpolarization, while distal dendritic potassium synapses are responsible for its late phase.