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The execution of voluntary movements is carried out by the descending corticospinal tract, which arise from motor cortex. The motor cortex is present in the precentral gyrus which includes the Brodmann’s area 4 (Fig. 4.57). It contains the representation of the opposite half of the body called motor homunculus. The body is represented upside down with leg at the medial surface and face towards the lateral surface of the hemisphere (Fig. 4.58). Facial area is bilaterally represented in the cortex and the rest

Fig 4.56: Two way communication between motor cortex and lateral cerebellar hemispheres. The connections help in planning and programming of sequential movements VLN : Ventrolateral nucleus

of muscles in the opposite side of the body. From motor cortex, the fibers going to corticospinal tract constitute 30%.

Supplementary motor area (Fig. 4.57)

It is situated in the superior surface of the premotor area 6. It is concerned with motor planning and execution of complex voluntary movements. It also helps in the coordination of voluntary movements.

Premotor cortex

It is present in front of area 4 (Fig. 4.59). It includes areas 6, 8 and 44. Premotor areas receive major input from somesthetic areas of sensory cortex. It gives fibers (30%) to the corticospinal tract. This projection helps to regulate posture during voluntary movements, as it influences the medial descending pathways.

Somatosensory cortex and posterior parietal cortex (Fig. 4.59)

The corticospinal tract also receives fibers from sensory cortex (areas 3, 1 and 2) and sensory asso-ciation areas (5 and 7), situated in the posterior

Fig. 4.57: Lateral view of cerebral hemisphere showing motor cortex situated in the precentral gyrus Area 4 is the motor area; area 6 premotor area.

Supplementary motor area is situated in the superior border of area 6

Figs 4.58A and B: (A) Motor sequence of the contralateral half of the body in the motor cortex, (B) Motor sequence and Motor homunculus in the motor cortex

The motor cortex shows upside down representation of opposite half of the body. The region of the body that is involved in motor activity has a greater representation in the cortex

is generally unilateral. The part of the body, which is more involved in voluntary activity has a wider representation in cortex. This gives a disproportionate body representation, with the hand, feet, lips, having a wider representation and trunk and proximal part of limbs with a smaller representation in the motor cortex.

Electrical stimulation of an area in the motor homunculus causes contraction of discrete group

A B

parietal cortex. The projections from these areas to the corticospinal tract constitute 40% and help to execute learned sequence of motor activity.

Pyramidal tract

The descending motor tract,which arises from Betz cells of motor cortex, includes fibers from area 4, (30%), premotor area and supplementary motor area (30%), somatosensory and posterior parietal cortex (40%). The fibers after leaving the cortex, enter the internal capsule and occupy the genu and anterior two third of posterior limb. The fibers from internal capsule descend down in the brain stem and give projections to the opposite cranial nerve nuclei to form corticobulbar tract.

In the medulla, 80% of fibers cross over to the opposite side which forms a pyramid like appearance and hence the name pyramidal tract.

The crossed fibers descend as lateral cortico-spinal tract (Fig. 4.60). The uncrossed 20% of fibers descend as ventral corticospinal tract which also becomes crossed to the opposite side of the spinal cord at the termination. Lateral cortico spinal tract ends directly on the motor neuron in the spinal cord whereas, the ventral corticospinal tract ends

on the motor neuron through an interneuron. The motor tract, which descends from motor cortex and end on spinal motor neurons forms the upper motor neuron. The anterior horn cell containing the spinal motor nerve which supplies muscle is called lower motor neuron.

Functions of pyramidal tract

The lateral corticospinal tract is well developed in humans and controls the motor activity of muscles of distal limbs such as digits. These muscles are involved in the execution of smooth, skilled and purposeful voluntary movements.

The ventral corticospinal tract is phylogeneti-cally older and is involved in the control of motor activity of proximal group of musculature in limbs.

Effects of lesion of corticospinal tract

The most common occurrence is a vascular lesion in the internal capsule which results in hemiplegia. It is the paralysis of one half of the body. The paralysis will be in the opposite side of the body in hemiplegia. The extensors of the lower limbs and flexors of the forelimbs show

Fig. 4.59: Diagram of cerebral cortex to show motor cortex and its relation with the premotor, supplementary motor, somatosensory areas and posterior parietal cortex. The fibers for corticospinal tract arise from all these regions

Fig. 4.60: Corticospinal tracts (Pyramidal tract)

spastic paralysis. The superficial reflexes are lost.

The abnormal plantar reflex namely the Babinski’s sign appears. The ankle clonus will also be present.

Effects of lesion of lower motor neuron Injury or damage to spinal motor neuron causes flaccid paralysis of muscles in the affected segment of spinal cord. The superficial reflexes are also lost. The atrophy of muscles and abnormal electrical potentials, which are charac-teristics of lower motor neuron lesion are observed. The abnormal electrical potentials such as fasciculation and fibrillation potentials occur due to denervation hypersensitivity of the affected muscles. These changes also cause reaction of degeneration in the electrical excitation of the muscle. The Babinski’s sign and ankle clonus are not present in lower motor neuron lesion.

Lower Motor Neuron Paralysis

Caused by lesion of anterior horn cells in the spinal cord and cranial nerve nuclei or the nerves arising from them

• Flaccid paralysis

• Hypotonia

• Atrophy of muscles

• Loss of superficial and deep reflexes

• Fibrillation and fasciculations from the affected muscles

Upper Motor Neuron Paralysis

Caused by the lesion of descending motor tract

• Spastic paralysis

• Hypertonia (clasp knife rigidity)

• Exaggeration of deep reflexes

• Loss of superficial reflexes

• Positive Babinski’s sign

• No muscle atrophy

Overall motor organization and control Motor action involves execution of simple reflex to a more complex type of motor sequence of movements, organized and controlled by spinal cord, brain stem, basal ganglia, cerebellum and

motor cortex. In humans, the neocortex is highly developed and hence with greater enceph-alization, it is possible to produce a complex motor sequence of movements. To produce a voluntary movement, the idea is formed first in the association areas of frontal and parietal cortices. The motor activity that is executed is always in response to a sensory stimulus. The idea which is generated in the association areas is converted into motor planning and program in the supplementary motor cortex, basal ganglia and cerebellum. The motor action, which is to be performed is communicated to the motor cortex.

This area executes the intended movement through the corticospinal tract. As the motor action is being executed, there are other regions of the brain like basal ganglia, cerebellum, reticular formation, vestibular nucleus and red nucleus, which help to maintain the background muscle tone and posture. The neocerebellum, through its feedback connection with the cerebral cortex (cerebro-cerebellar-cerebral circuit), is able to produce motor activity which is coordinated and error free.

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