Motor coordination is one of the most essential functions of the central nervous
system. In vertebrates it is executed mainly by the brain centers. By way of
descending synaptic projections these centers govern the spinal motoneurons and
the movements of the animal as a whole "as a single machine" (SHERRINGTON,
1906). In contrast, reflex movements may be accomplished at segmental levels.
The progressive evolution of the supraspinal motor control may be considered
as the main content of the process of encephalization. SHERRINGTON (1906, 1940),
SPERRY (1969), and BERNSTEIN (1966) pointed out that central motor mechanisms
are subject to considerable changes in the course of phylogenefic development
and may be considered as the key factor in the evolution of the brain. The
comparison of neuronal and synaptic organization of supraspinal control in
different vertebrates may give considerable insight into the nature of the neuronal
circuitry subserving the central regulation of motor activity and the trends of its
evolution.