Descending
Tracts
INTRODUCTION
● white matter of the spinal cord surrounds the central ‘H-shaped mass of grey matter,and mainly consists of nerve fibres, the large proportion of them being myelinated, give it a white appearance’.
● In each half of the spinal cord, the white matter is divided into three parts called white columns or funiculi
○ Posterior white column
○ Lateral white column
○ Anterior white column
TRACTS OF THE SPINAL CORD
● The tracts are defined as collections of nerve fibres within the central nervous system, which have same origin, course and termination.
● The tracts are named after the names of masses of grey matter connected by them. In naming usually the first term denotes the origin and second the
termination of tract.
● The tracts are classified into three types:
○ Descending
○ Ascending
○ Intersegmental
Descending tracts
● The descending tracts conduct the impulses to the spinal cord from the brain.
● They are of two types:
○ Pyramidal tract
■ corticospinal tract
○ Extrapyramidal tracts
■ Rubrospinal tract
■ Reticulospinal tract
■ Vestibulospinal tract
■ Tectospinal tract
■ Hypothalamospinal tract
Corticospinal tract(pyramidal tract)
Origin: pyramidal cells (of Betz) of the motor area of the cerebral cortex.
-primary motor cortex (area 4) -premotor cortex area (area 6)
Course: Fibres pass through -corona radiata
-internal capsule
-crus cerebri of cerebral peduncles -ventral part of pons and pyramids of medulla oblongata
-majority cross at decussation of medulla
● Crossed fibres descend in the lateral white column of the spinal cord as the lateral corticospinal tract.
● The uncrossed fibres descend in the anterior white column of the spinal cord as the anterior corticospinal tract.
● Termination:
Terminate by synapsing with interneurons,which in turn project to the motor neurons of the
anterior horn.
Location:
Lateral corticospinal tract:Lateral white column of spinal cord Anterior corticospinal tract:Anterior white column of spinal cord
Functions:
● Contraction of individual or small group of muscles particularly those which moves hand ,fingers ,feet and toes
● controls motor activity and is
○ facilitatory to flexors
○ inhibitory to extensors
Rubrospinal tract
Origin:red nucleus (nucleus ruber) located in the midbrain
Course:Immediately they cross in the ventral
tegmental decussation and descend in the lateral white column of the spinal cord, ventral to the lateral corticospinal tract.
Termination:anterior horn cells of the spinal cord.
Location: Lateral white column
Function:Unconscious coordination of movements(controls muscle tone and synergy)
Reticulospinal tract
Origin:
Lateral reticulospinal tract:Reticular formation in midbrain ,pons and medulla.
Medial reticulospinal tract:Reticular formation in the medulla
Location:
Lateral reticulospinal tract:Lateral white column Medial reticulospinal tract:Anterior white column
Termination:Anterior horn cells of spinal cord
Functions:
Lateral reticulospinal tract:Mainly responsible for the facilitatory influence on the motor neuron to
the skeletal muscles.
Medial reticulospinal tract:Mainly responsible for the inhibitory influence on the motor neuron to
the skeletal muscles.
Tectospinal tract
Origin:Superior colliculus of midbrain.
Course: It crosses ventral to the periaqueductal grey matter in the dorsal tegmental
decussation and descends in the medial part of the anterior white column of
the spinal cord .
Location:Anterior white column Termination:
Cranial nerve nuclei in medulla and anterior horn cells of upper spinal segment
Functions:
Controls movements of head ,neck and arm in response to visual stimuli.
Vestibulospinal tract
Origin:Vestibular nucleus
Course:
● vestibular nuclear complex gives rise to
the lateral and medial vestibulospinal tracts
● Lateral descends ipsilaterally
● Medial descends in the medial longitudinal fasciculus into the anterior white column
● Medial has both crossed and uncrossed fibres
Location:Anterior white column
Termination:Anterior horn cells of the spinal cord
Function:Unconscious maintenance of posture and balance.
Hypothalamospinal tract:
Origin:
projects from the hypothalamus.
Course:
descends in the lateral white column of spinal cord on the medial side of the lateral corticospinal tract.
Termination:
terminates by synapsing with lateral horn cells of Tl to L2 spinal segments responsible for sympathetic outflow; and with lateral horn cells of S2, S3, and S4 spinal segments responsible for parasympathetic outflow.
Applied
● The lesions of pyramidal tract above the level of decussation (i.e. upper motor
neuron (UMN) lesions) result in the loss of voluntary movements in the opposite half of the
body below the level of the lesion.
● The muscles are not actually paralyzed but the control of upper motor neurons and the lower motor neurons (LMN) supplying muscles is lost.
● As a result LMNs become hyperactive and the tone of muscles is increased leading to spastic paralysis.
