Corticospinal Tract

Summary

The corticospinal tract, also known as the lateral corticospinal tract or pyramidal tract, is a neural pathway that carries motor information downwards from the brain, through the spinal cord, to eventually reach the muscles of our body below. In this way, the corticospinal tract primarily functions to voluntarily control the muscles of the limbs on the contralateral or opposite of the body.

Anatomically, this nerve tract is separated into 2 neurons: the upper motor neuron and the lower motor neuron. The upper motor neuron’s cell body resides in the primary motor cortex of the frontal lobe of the brain. This neuron sends fibers downwards through the posterior limb of the internal capsule, then through the cerebral peduncle to reach the brainstem. In the brainstem, these fibers decussate or cross to the opposite side of the body in the medulla, before continuing downwards into the spinal cord as the lateral corticospinal tract.

The upper motor neuron ends in a synapse in the anterior horns of the spinal cord. This is where the cell body of the lower motor neuron resides. The nerve fibers of the lower motor neuron then exit the spinal cord to reach the muscles they innervate. They do this by synapsing at the neuromuscular junction, which induces muscle contraction in response to nervous impulses.

Key Points

• (Lateral) Corticospinal Tract
  • Descending motor tract
    • Nerve tract carries motor information from brain to peripheral muscles
  • Function
    • Voluntary movement of contralateral limbs
  • Pathway
    • 1st order neuron (UMN): cell body in primary motor cortex
      • Descends ipsilaterally (through posterior limb of internal capsule and cerebellar peduncle)
      • Most fibers decussate at caudal medulla (pyramidal decussation)
      • Descends contralaterally in spinal cord
      • Synapses in contralateral anterior horn of spinal cord
    • 2nd order neuron (LMN): Cell body of anterior/ventral horn of spinal cord
      • Leaves spinal cord, operates as peripheral motor nerve
      • Synapses at neuromuscular junction (NMJ)
        • Innervates muscle fibers
  • Clinical notes:
    • Uncal herniations can compress the cerebellar peduncle near the level of the pons, leading to damage to the corticospinal tract and contralateral motor deficits
    • Vitamin B12 deficiency leads to subacute combined degeneration of both the dorsal column and the lateral corticospinal tracts.