Physiological basis of motor effects of a transient stimulus to cerebral cortex

This contribution includes a selective review of previously published material, findings from some new experiments, and discussion of some relationships between animal and recent human data. The major questions are: What descends from the cerebral cortex after a brief surface stimulus? What explains the various components of the corticofugal discharge? What are the motor consequences of the corticofugal discharge, and what are the effects of lesions on both? The focus is on the corticospinal system, which through its monosynaptic connection with alpha motoneurons of distal muscles accounts for the short latency movements after a transient cortical stimulus. The pyramidal and lateral corticospinal tract response in monkey or cat to a surface stimulus applied to area 4 is a direct (D) wave conducted in fast axons followed by several indirect (I) waves with a period of greater than 1 ms. Although computer summing reveals, at increasing amplitudes, D and I waves in recordings from nuchal skin, vertebra, and surface of the spinal cord, "killed end" recording is essential to reveal the true extent of I relative to D waves. The D wave might result from excitation of: the initial segment (IS), i.e., the classical spike trigger zone; the first or deeper nodes in white matter; or arborizations of the axon collaterals in gray matter. Under different circumstances, each of these modes of excitation can be effective. Thus, with threshold stimulation through separated bipolar electrodes, intracellular recording from pyramidal tract (PT) and uninvaded motor cortical neurons shows that D activation usually occurs when the membrane potential immediately before the stimulus is relatively depolarized, implying excitation of the IS region, i.e., close to the site of synaptic transfer. A monopolar surface (+) stimulus at the appropriate focus usually generates a D wave at weaker intensity than does a surface (-) stimulus. However, if a little above threshold, stimuli of either polarity generate both D and I waves, but the ratio of D:I amplitude is usually greater with surface (+) stimulation. A theoretical estimate of the depth of excitation by a surface (+) stimulus was consistent with threshold excitation occurring at the first node. Slow PT neurons are excited by surface stimulation, but trivially contribute to population PT or corticospinal recordings. Intracellular recording from PT neurons identifies a monosynaptic excitatory postsynaptic potential as the cause of the first I wave, the period between successive I waves reflecting single delays for synaptic discharge.(ABSTRACT TRUNCATED AT 400 WORDS)