pyramidal tract

n. A major pathway of the central nervous system, originating in the sensorimotor areas of the cerebral cortex and generally descending through the brainstem to the spinal cord. The fibers of the pyramidal tract transmit motor impulses that control voluntary movement.

pyram′idal tract`

n. any of the bundles of motor fibers extending from the cerebral hemispheres into the medulla and spinal cord.

Thesaurus

Noun

pyramidal tract - any of the important motor nerves on each side of the central nervous system that run from the sensorimotor areas of the cortex through the brainstem to motor neurons of the cranial nerve nuclei and the ventral root of the spinal cordcorticospinal tract, pyramidal motor systemefferent, efferent nerve, motor nerve - a nerve that conveys impulses toward or to muscles or glandscentral nervous system, CNS, systema nervosum centrale - the portion of the vertebrate nervous system consisting of the brain and spinal cordbasal ganglion - any of several masses of subcortical grey matter at the base of each cerebral hemisphere that seem to be involved in the regulation of voluntary movement

Pyramidal Tract

(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.

REFERENCES

Obshchaia 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

pyramidal tract

py·ram·i·dal tract

[TA] an imprecisely defined term, generally used to describe axons from the cerebral cortex that enter the pyramid thereby representing all corticospinal fibers rostral to the pyramidal decussation; when the term is used to describe the lateral corticospinal tract, this entity should be designated as the crossed pyramidal tract. See: corticospinal tract.

pyramidal tract

py·ram·i·dal tract

(pir-am'i-dăl trakt) [TA] A massive bundle of fibers originating from pyramidal cells in the precentral motor and premotor area and in the postcentral gyrus. Fibers from these cortical regions descend through the internal capsule, the middle third of the crus cerebri, and the ventral part of the pons to emerge on the ventral surface of the medulla oblongata as the pyramis. Continuing caudally, most of the fibers cross to the opposite side in the pyramidal decussation and descend in the spinal cord as the lateral pyramidal tract, which distributes its fibers to interneurons of the spinal gray matter. Interruption of the pyramidal tract at or below its cortical origin causes impairment of movement in the opposite body-half, especially severe in the arm and leg and characterized by muscular weakness, spasticity and hyperreflexia, and a loss of discrete finger and hand movements. Babinski sign is associated with this condition of hemiplegia.

pyramidal tract

The great inverted pyramid of motor nerve fibres descending from the motor cortex of the cerebrum through the internal capsule and down into the brainstem where the fibre bundles on each side cross to the other side. This is why a STROKE on the right side causes paralysis on the left side of the body.

pyramidal tract

Related to pyramidal tract: pyramidal tract signs

noun

Synonyms for pyramidal tract

noun any of the important motor nerves on each side of the central nervous system that run from the sensorimotor areas of the cortex through the brainstem to motor neurons of the cranial nerve nuclei and the ventral root of the spinal cord

Synonyms

corticospinal tract
pyramidal motor system

Related Words

efferent
efferent nerve
motor nerve
central nervous system
CNS
systema nervosum centrale
basal ganglion