[Electrophysiological identification of neurons of the substantia innominata]

Recently in senile dementia of Alzheimer type, neuronal loss of cholinergic neurons in the substantia innominata is described. However these findings are based on morphological studies and, so far, these neurons have not been identified physiologically. In this study we tried to record these neurons electrophysiologically by antidromic activation. Cats were anesthetized with sodium pentobarbital and paralyzed with gallamine triethiodide. Three arreys of 3 or 4 silver-ball stimulating electrodes were fixed to the surface of the cerebral cortex. Each electrode was stimulated by rectangular pulses of 1 msec duration. Recording electrodes were glass micropipettes filled with 2 M NaCl saturated with fast green FCF and inserted into the ipsilateral substantia innominata stereotaxically. Twenty-eight neurons were constituted of mainly negative component; suggesting that they were recorded from cell bodies, not from axons. Also they responded in an all-or-none manner and showed constant latencies when stimulus intensities were at threshold level. When paired stimuli were applied, the latency of the action potential to the second stimulus was equal to that of the first one. These neurons were, therefore, considered to be activated antidromically. These neurons had axons of very slow conduction velocities. They are divided into two groups according to conduction velocities. The first group had mean conduction velocity of 2.3 m/sec and they responded to rather high frequency stimuli. The second group had mean conduction velocity of 1.0 m/sec and neurons belonging to this group showed quite long refractory periods. Based on conduction velocity analysis, the former is thought to include neurons with myelinated axons and the latter those with unmyelinated ones.(ABSTRACT TRUNCATED AT 250 WORDS)