QS Study

Pyramidal tract (corticospinal and corticonuclear tracts) is a descending tract constituting pyramidal cells in the cerebral cortex and their axons extending to various motor nuclei of the cranial and spinal nerves. These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other functions.

Origin: Each pyramidal tract contains about one million fibers which originate from:

  1. The motor area of the cortex.
  2. Premotor area (area 6).
  3. The somesthetic area (areas 3, 2, 1); the motor activities of the body are represented upside down.

These tracts originate in the cerebral cortex, carrying motor fibers to the spinal cord and brain stem. They are responsible for the voluntary control of the musculature of the body and face.

Course: After formation, the track passes through the following parts of the CNS:

  1. Corona radiata.
  2. Internal capsule, occupying the genu and the anterior 2/3 of the posterior limb.
  3. Medial 2/3 of crus cerebri of the midbrain.
  4. Basilar part of the pons.
  5. Pyramid of the medulla.

Pyramidal Tract 1

Fig – The corticospinal pathway

In the lower part of the medulla, about 75% of the fibers cross to the opposite side and descend as the lateral corticospinal tract. About 20% fibers remain uncrossed and run down as the anterior corticospinal tract. The remaining 5% uncrossed fibers run with the crossed fibers.

In the spinal cord, the lateral (crossed) and anterior (uncrossed) corticospinal tract run as a pyramidal tract.

Termination:

They terminate in the motor nuclei of cranial nerves and the anterior horn cells of the spinal cord to form corticonuclear tract and corticospinal tract respectively.

Functions –

  • The pyramidal tract is concerned with the voluntary movement of the body.
  • It also actively participates in mediating superficial reflexes.

Effects of lesion of the pyramidal tract:

Lessons above the level of decussation cause contralateral paralysis while lesions below the decussation cause ipsilateral paralysis. It is an upper motor type of paralysis which is characterized by –

  1. Loss of power of voluntary movements.
  2. Spastic type of paralysis.
  3. Exaggerated tendon reflex.
  4. Superficial reflexes are lost.
  5. Planter extensor (Babinski’s sign is positive).
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