CLASSIFICATION OF MOTOR PATHWAYS
There are
two methods to classify the motor pathways. In the first method of
classification, motor pathways are divided into pyramidal and extrapyramidal
tracts. In the second method, motor pathways are classified into lateral and
medial systems.
Pyramidal and
Extrapyramidal Pathways
Motor
pathways are classified into pyramidal and extrapyramidal tracts, depending
upon the situation of their fibers in medulla oblongata.
Pyramidal
tracts
Pyramidal
tracts are those fibers which form the pyramids in upper part of medulla.
Pyramidal tracts are
the
anterior and lateral corticospinal tracts. These tracts control the voluntary movements of the body.
Extrapyramidal
tracts
Motor
pathways other than pyramidal tracts are known as extrapyramidal tracts.
Extrapyramidal
tracts are:
1. Medial
longitudinal fasciculus
2.
Anterior and lateral vestibulospinal tracts
3.
Reticulospinal tract
4.
Tectospinal tract
5.
Reticulospinal tract
6.
Rubrospinal tract
7.
Olivospinal tract.
Extrapyramidal
tracts are concerned with the regulation of tone, posture and equilibrium.
Lateral and Medial
Motor Systems
Depending
upon the location or termination, motor pathways are divided into two
categories, namely the
lateral
system or pathway and the medial system or pathway. Lateral motor system is phylogenetically
new and medial motor system is old.
Lateral
motor system
Fibers of
this system terminate on motor neurons situated in lateral part of ventral gray
horn in spinal
cord
(directly or via interneurons) and on equivalent motor neurons of cranial nerve
nuclei in brainstem.
Components
of lateral system:
1. Lateral
corticospinal tract, which arises from different areas of cerebral cortex and
terminates
in the
alpha motor neurons situated in lateral part of ventral gray horn of spinal
cord.
2.
Rubrospinal tract, which arises from red nucleus in midbrain
3. Part of
corticobulbar tract, which arises from different areas of frontal and parietal
lobes of cerebral cortex along with corticospinal tracts. Part of corticobulbar
tract belonging to lateral motor system terminates on the nucleus of
hypoglossal nerve and motor nucleus of facial nerve. Fibers from hypoglossal nerve
innervate the muscles of tongue. Fibers from motor nucleus of facial nerve
innervate the muscles of lower part of face.
Functions
of lateral motor system:
1. Lateral
corticospinal tract activates the muscles of distal portions of limbs and
regulates the skilled
voluntary
movements
2.
Rubrospinal tract facilitates the tone in the muscles, particularly the flexor
muscles
3.
Corticobulbar fibers of lateral system are concerned with the movements of
expression in lower part of
face and
movements of tongue.
Medial
motor system
Fibers of
medial motor system terminate on motor neurons situated in the medial part of
ventral gray horn of spinal cord (via interneurons) and on equivalent motor
neurons of cranial nerve nuclei, situated in the brainstem.
Components
of medial motor system:
1.
Anterior corticospinal tract, which arises from different areas of cerebral
cortex and terminates in the
alpha
motor neurons situated in medial part of ventral gray horn of spinal cord 2.
Part of corticobulbar fibers of medial system, which arises
from different areas of frontal and parietal lobes
of cerebral cortex along with corticospinal tracts.
Fibers of corticobulbar tract belonging to medial
motor system innervate the muscles of trunk and
limbs, muscles of jaw and muscles of upper part
of face.
3. Lateral and medial
vestibulospinal tracts that arise from lateral vestibular nucleus and medial
vestibular
nucleus
4. Reticulospinal
tract, which arises from reticular formation in brainstem
5. Tectospinal tract,
which takes origin from superior colliculus of midbrain.
Functions of medial
motor system:
1. Anterior
corticospinal tract is responsible for the maintenance of posture and
equilibrium
2. Fibers of corticobulbar
tract belonging to medial motor system, innervating muscles of upper part
of trunk are involved
in the maintenance of posture and equilibrium. Fibers innervating muscles of
jaw
and face are involved
in the movements of chewing and movements of eyebrow.
3. Vestibulospinal
tract is concerned with the adjustment of position of head and body during
angular
and linear
acceleration
4. Pontine fibers of
reticulospinal tract facilitate the tone of extensor muscles and regulate the
postural
reflexes. However,
medullary fibers of this tract inhibit the tone of the muscles involved in
postural
movements.
5. Tectospinal tract
is responsible for the movement of head in response to visual and auditory
stimulus.
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