Function of Descending tracts of spinal cord

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.