Projection of individual pyramidal tract neurons to lumbar motor nuclei of the monkey

Abstract

The projection of individual pyramidal tract (PT) neurons from the hindlimb area in the precentral gyrus of the cerebral cortex to the lumbar spinal cord was studied in the monkey by systematically searching for sites within identified regions of the spinal gray from which the PT neurons could be antidromically activated by local stimulation. All investigated neurons belonged to the fast conducting fraction of PT neurons. The following results were obtained. 1. Each PT neuron could be activated from more than one region of the spinal gray matter, including identified spinal motor nuclei and areas dorsomedial to these nuclei, but do not the intermediate nucleus or regions dorsal to it. "Passage areas" and "termination areas" were defined. 2. Half of the PT neurons with termination areas within motor nuclei had these areas in more than one nucleus. There were thus strong suggestions for synaptic contacts of some PT neurons with motoneurons of more than one muscle. 3. Four groups of three or four neurons were recorded simultaneously by the same cortical electrode. Comparisons of passage and termination areas within groups revealed both similarities and differences in projections of neighboring neurons. Every neuron was activated from some region(s) where others of the group were not. Common passage areas, or passage and termination areas, for two or three neurons of a group within at least one motor nucleus were found for all groups. Termination areas in the same motor nucleus have been found for the majority of the neurons of only one group. These common projection areas are compatible with, but not prove, that group of adjacent PT neurons has common target cells in the spinal cord.

Muscle representation in the macaque motor cortex: An anatomical perspective

“…Thus, the CM cells we have labeled may have connections with other muscles in addition to the one we injected. On the other hand, Asanuma et al (29) found that groups of PTNs that were simultaneously recorded from the same electrode sent at least one axon to a common motor nucleus in the spinal cord. They argued that this innervation pattern made it possible to construct focal, cortical efferent zones related to a single muscle even when the neurons in these zones branched widely.…”

Functional properties of monkey motor cortex neurones receiving afferent input from the hand and fingers

“…Variation in the size of motor fields for area 4 neurones is to be expected from the work of Fetz and colleagues who have shown both with operant conditioning and cross-correlation techniques that the discharge of a single cortical neurone may result in the co-activation of just one or of several different forelimb muscles (Fetz & Finnochio, 1975;Fetz, Cheney & German, 1976). The size of motor fields for corticospinal neurones probably depends on the degree to which their axons branch within the spinal cord (Shinoda, Zarzecki & Asanuma, 1979;Asanuma, Zarzecki, Jankowska, Hongo & Marcus, 1979).…”

Actions of single sensory fibres on cat dorsal column nuclei neurones: vibratory signalling in a one‐to‐one linkage.

“…The single-fibre e.p.s.p.s. evoked in motoneurones either by individual group I a fibres or by individual corticospinal tract neurones are small, necessitating elaborate spatial summation to generate motor output (Burke & Nelson, 1966;Kuno & Miyahara, 1969;Mendell & Henneman, 1971;Watt, Stauffer, Taylor, Reinking & Stuart, 1976;Asanuma, Zarzecki, Jankowska, Hongo & Marcus, 1979;Kirkwood & Sears, 1981;Redman & Walmsley, 1982). Although this form of synaptic organization may be optimal for achieving smooth, co-ordinated muscular contractions it may reflect a fundamental difference in synaptic organization between sensory and motor nuclei.…”