EXTRAPYRAMIDAL TRACTS

EXTRAPYRAMIDAL TRACTS

Descending tracts of spinal cord other than pyramidal tracts are called extrapyramidal tracts.

MEDIAL LONGITUDINAL FASCICULUS

Situation

Medial longitudinal fasciculus descends through posterior part of anterior white column of the spinal cord.

Origin

Actually, this tract is the extension of medial longitudinal fasciculus of brainstem. Fibers of this tract take origin from four different areas in brainstem:

i. Vestibular nuclei

ii. Reticular formation

iii. Superior colliculus

iv. Interstitial cells of Cajal.

Course

After entering the spinal cord from the brainstem, the fibers of medial longitudinal fasciculus descend through posterior part of anterior white column of the same side. In the spinal cord, this tract is well defined only in upper cervical segments. Below this level, the fibers run along with the fibers of anterior vestibulospinal tract.

Extent

Fibers of this tract extend up to the upper cervical segments of spinal cord.

Termination

Fibers of this tract terminate in anterior motor neurons of the spinal cord along with fibers of anterior vestibulospinal tract either directly or through internuncial neurons.

Function

Medial longitudinal fasciculus helps in the coordination of reflex ocular movements and the integration of ocular and neck movements.

Effects of Lesion

Reflex ocular movements and reflex neck movements are affected in the lesion of this tract.

ANTERIOR VESTIBULOSPINAL TRACT

Situation

Anterior vestibulospinal tract is situated in the anterior white column, along the periphery of spinal cord lateral to tectospinal tract.

Origin

Fibers of this tract arise from medial vestibular nucleus in medulla oblongata. In fact, anterior vestibulospinal tract is the extension of medial longitudinal fasciculus. Most of the fibers are uncrossed.

Extent

Fibers run up to thoracic segments of spinal cord.

Course

Fibers of this tract run down from medulla into the anterior column of spinal cord along the periphery. All

the fibers are uncrossed

Termination

Along with fibers of lateral vestibulospinal tract, the fibers of this tract terminate in anterior motor neurons

directly or through internuncial neurons.

Function

Function of this tract is explained along with the function of lateral vestibulospinal tract.

LATERAL VESTIBULOSPINAL TRACT

Situation

Lateral vestibulospinal tract occupies the anterior part of lateral white column of spinal cord.

Origin

Fibers of this tract take origin from the lateral vestibular nucleus in medulla. This nucleus is also

called Deiter nucleus.

Extent

Fibers of this tract are present throughout the spinal cord.

Course

From Deiter nucleus, most of the fibers descend directly through lateral column. Very few fibers cross to

the opposite side before descending.

Termination

Fibers of this tract terminate in the anterior motor neuron, either directly or via internuncial neurons.

Functions

Vestibular nuclei receive impulses concerned with muscle tone and posture from vestibular apparatus

and cerebellum. Vestibular nuclei in turn convey the impulses to different parts of the body through the

anterior and lateral vestibulospinal tracts. Vestibulospinal tracts are concerned with adjustment of position of head and body during angular and linear acceleration.

Effect of Lesion

Adjustment of head and body becomes difficult during acceleration when the vestibulospinal tracts are

affected by lesion.

RETICULOSPINAL TRACT

Situation

Reticulospinal tract is situated in the anterior white column, posterior to anterior vestibulospinal tract.

Origin

Fibers of this tract arise from the reticular formation of pons and medulla. Pontine reticular fibers are

uncrossed (direct) and descend in medial part of anterior column. Fibers from medullary reticular

formation are predominantly uncrossed and only few fibers are crossed. These fibers descend in lateral part of anterior column and to some extend in the anterior part of lateral column.

Extent

Fibers of reticulospinal tract extend up to thoracic segments.

Termination

Fibers of reticulospinal tract terminate in gamma motor neurons of anterior gray horn through the internuncial neuron.

Functions

Reticulospinal tract is concerned with control of movements and maintenance of muscle tone,

respiration and diameter of blood vessels. Pontine and medullary fibers have opposite effects on these

functions,

TECTOSPINAL TRACT

Situation

Tectospinal tract is situated in the anterior white column of spinal cord.

Origin

Nerve fibers of this tract arise from superior colliculus of midbrain.

Extent

Tectospinal tract extends only up to lower cervical segments.

Course

After taking origin from superior colliculus, the fibers cross the midline in dorsal tegmental decussation

and descend in anterior column.

Termination

Fibers of tectospinal tract terminate in the anterior motor neurons of spinal cord, directly or via internuncial

neurons.

Function

Tectospinal tract is responsible for the movement of head in response to visual and auditory stimuli.

RUBROSPINAL TRACT

Situation

Rubrospinal tract is situated in the lateral white column of spinal cord.

Origin

Fibers of this tract arise from large cells (nucleus magnocellularis) of red nucleus in midbrain.

Extent

Nerve fibers of this tract appear in the spinal cord only up to thoracic segments.

Course

After arising from the red nucleus, the fibers cross the midline in ventral tegmental decussation and descend into spinal cord through the reticular formation of pons and medulla.

Termination

Fibers of rubrospinal tract end in the anterior motor neurons of the spinal cord via internuncial neurons.

Function

Rubrospinal tract exhibits facilitatory influence upon flexor muscle tone.

OLIVOSPINAL TRACT

Situation

Olivospinal tract is present in lateral white column of spinal cord.

Origin

The nerve fibers of the olivospinal tract take origin from the inferior olivary nucleus, which is present in the medulla oblongata.

Termination

Fibers of this tract terminate in the anterior motor neurons of spinal cord.

Function

Functions of the olivospinal tract are not known clearly. It is believed that this tract is involved in reflex movements arising from the proprioceptors.

 

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