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(also called pyramidal system), a group of nerve structures that participate in the complex and delicate coordination of motor activity. The pyramidal tract does not exist in lower vertebrates; it is found only in mammals, where it forms the efferent portion of the motor analyzer, and it is most developed in man.
The pyramidal tract starts chiefly with the pyramidal neuror.s of the sensorimotor cortex of the brain. The long outgrowths, or fibers, of these neurons form direct, descending pathways to the motor-reflex centers of the spinal cord; cortical information is transmitted over these pathways. The fibers of the pyramidal tract descend directly to the spinal cord; they also form branches, or collaterals, to the nuclei of the cranial nerves. They intersect and cross over to opposite sides of the body, mostly in the medulla oblongata but in part in the spinal cord. They then proceed as part of the anterior and lateral horns of the spinal cord, forming synapses in each segment of the spinal cord and transmitting impulses from the cerebral cortex to its motor neurons, either directly or through intercalary neurons that are called interneurons.
The pyramidal tract in man contains about 1 million nerve fibers, which are divided largely into thick, rapidly conducting fibers that are responsible for rapid phase movements and thin, slowly conducting fibers that are responsible for tonic contrac tions of muscles. The thick fibers are about 16 microns (μ) in diameter with a conduction rate as fast as 80 m/sec, whereas the thin fibers are about 4 μ in diameter with a conduction rate that ranges from 7 to 25 m/sec.
Injuries to the pyramidal tract are manifested by paralysis, paresis, and pathological reflex activity. These disturbances may be compensated for by an intensification of activity in other descending systems, namely, those that belong to the extrapyramidal tract.
REFERENCESObshchaia i chastnaia fiziologiia nervnoi sistemy. Leningrad, 1969. Pages 338–61. (A physiological handbook.)
Kostiuk, P. G. Struktura i funktsiia niskhodiashchikh sistem spinnogo mozga. Leningrad, 1973.
A. V. POGREBKOVA