Transformation of cortical motor signals in spinal cord

Abstract

A review of the neuronal organization of the corticospinal (pyramidal) system is presented. Parameters of pyramidal tract fibers are described; special attention is paid to the existence of pyramidal neurons with fast- and slow-conducting axons which have different locations of somata over the sensorimotor cortex, different locations of terminals on spinal neurons, and different parameters of impulse activity. Two types of pyramidal descending action are considered: direct action of corticifugal signals upon motor units, and control of sensory flow to motor units and higher brain centers. Corticifugal motor signals produce predominantly excitatory postsynaptic potentials in flexor and inhibitory ones--in extensor motoneurons. The following types of connections between pyramidal fibers and motoneurons are distinguished: 1) monosynaptic connections (only in primates), 2) connections via propriospinal interneurons localized in the external basilar region and specialized for transmission of pyramidal fiber activity only, and 3) connections via interneurons of the internal basilar region and intermediate zone which transmit both segmental afferent and pyramidal activity. Evidence is presented that the activity of fast-conducting pyramidal fibers via the first two types of connections creates phasic synaptic processes in motoneurons while slow-conducting fibers via the third type render the tonic action. Properties of corresponding interneuronal groups are specialized for transmission of phasic and tonic signals. The types of pyramidal control of sensory flow considered are: 1) postsynaptic facilitation and inhibition of interneurons of segmental reflex arcs, 2) postsynaptic facilitation and inhibition of transmission via ascending pathways, 3) presynaptic inhibition of primary afferent activity, and 4) postsynaptic influences on γ-motoneurons.