An analysis of the cortical response to antidromic pyramidal tract stimulation in the cat

Abstract

The cortical response to medullary pyramidal tract stimulation in the cat has been recorded with vertically oriented bipolar electrodes. The true response uncontaminated by the dromic response due to current spread to the medullary medial lemniscus was obtained by lemniscus section in the midbrain. The antidromic potential is practically confined to the Betz cell containing area about the cruciate sulcus, particularly anterior sigmoid gyrus. The pattern consists of a positive spike and wave which are assigned to soma and proximal apical dendrite discharge in the cells of origin of two major groups of pyramidal fibers. Following and probably underlying the positive wave is a slow negative wave assigned to antidromic conduction in apical dendrites of both cell groups. No axon-soma delay was recorded. When the lemniscus is intact there is a prominent specific response in posterior sigmoid with delayed cortico-cortical and possibly thalamo-cortical projection to anterior sigmoid. Strychnine spike driving in both sigmoid gyri is possible only when the afferent path is functioning. The lemniscus specific response has a prolonged but incomplete depression phase in its excitability cycle, assigned largely to the thalamic relay. The true antidromic response has a rapid recovery after-discharge. At short intervals there is prominent summation of the negative wave; this is inferred to be further evidence for the graded response character of apical dendrites. With repetitive stimulation this summation can maintain a DC potential shift. The most prominent effect of both strychnine and veratrine on the antidromic response is to depress the dendritic potential.