Function of extrapyramidal tracts
Pyramidal
tracts are concerned with voluntary movements of the body. Fibers of the pyramidal tracts transmit motor
impulses from motor area of cerebral cortex to the anterior motor neurons of the spinal cord. These two tracts are responsible for fine, skilled movements.
Effects
of lesion
Lesion in
the neurons of motor cortex and the fibers of pyramidal tracts is called the
upper motor neuron
lesion. In human
beings, pure pyramidal tract lesions do not occur.
Lesion of pyramidal fibers occurs most commonly in stroke (cardiovascular accident) due to hemorrhage and thrombosis. During such lesions, many extrapyramidal fibers are also damaged along with pyramidal fibers. Because of this reason, neurologists often
consider the lesion as upper motor neuron lesion and not as
pyramidal tract lesion.
Following are the effects
of lesion:
1. Voluntary
movements
Voluntary movements of the
body are very much affected. Initially, there is loss of
voluntary movements
in the
extremities. Later, it involves the other parts of the body like hip and
shoulder.
2. Muscle
tone
Muscle
tone is increased leading to spasticity. Muscles are also paralyzed. This type of paralysis of muscles is called
the spastic paralysis. The spasticity is due to
the failure of inhibitory impulses from upper motor neurons, particularly the neurons of extrapyramidal system to reach the γ-motor neurons in spinal cord. However,
hypotonia occurs in pure pyramidal tract lesion, which is
very rare. In monkeys, sectioning of pyramidal tract fibers alone results in hypotonia.
3. Reflexes
All the
superficial reflexes are lost and the deep reflexes are exaggerated. Abnormal
plantar reflex called
Babinski
sign is present (Babinski sign positive).
Effects of Lesion at
Different Levels
Cerebral cortex
Lesion of
pyramidal tract fibers in cerebral cortex causes hypertonia, spasticity and contralateral monoplegia (para lysis of one limb) or contralateral hemiplegia (para lysis of one side of the body).
Internal
capsule
Lesion of
pyramidal tract fibers at posterior limb of internal capsule results in
contralateral hemiplegia.
Brainstem
Lesion at
brainstem involves not only pyramidal tract fibers but also other structures
such as VI and VII
cranial
nerve nuclei. So the lesion results in contralateral hemiparesis
(weakness of muscles in one side of the body) along with VI and VII cranial nerve palsies.
Spinal
cord
Unilateral
lesion of lateral corticospinal fibers at upper cervical segment causes
ipsilateral hemiplegia and
bilateral lesion causes quadriplegia
(paralysis of all four limbs) and paralysis of
respiratory muscles.
Bilateral
lesion of these fibers in thoracic and lumbar segments results in paraplegia
(paralysis of both lower limbs) without
paralysis of respiratory muscles.
In spinal cord
Fibers which cross the midline and
form pyramidal decussation descend through posterior part of lateral
white column of spinal
cord. This bundle of crossed fibers is called the crossed
pyramidal tract or
lateral
corticospinal tract or indirect corticospinal
tract. Remaining 20% of fibers do not cross to the opposite
side but descend down through the
anterior white column of the spinal cord. This bundle of
uncrossed
fibers is called the uncrossed
pyramidal tract or
anterior
corticospinal tract or
direct
corticospinal
tract. This
tract is well marked in cervical region. Since, the fibers of this tract
terminate in different
segments of spinal cord, this tract
usually gets thinner while descending through the successive segments of spinal
cord. Fibers of this tract are absent mostly below the
mid thoracic level. Before termination, majority of the
fibers of this anterior corticospinal tract cross to the
opposite side at different levels of spinal cord.
0 Comments