The Corticospinal Tract Is the Primary Efferent Path From the Cortex

Traditionally, the descending motor tract originating in the cerebral cortex has been called the pyramidal tract because it traverses the medullary pyramids on its way to the spinal cord (Fig. 5.15). This path is the corticospinal tract. All other descending motor tracts emanating from the brain-stem were generally grouped together as the extrapyramidal system. Cells in Brodmann's area 4 (MI) contribute 30% of the corticospinal fibers,- area 6 (MII) is the origin of 30% of the fibers,- and the parietal lobe, especially Brodmann's areas 1, 2, and 3, supplies 40%. In primates, 10 to 20% of corticospinal fibers ends directly on motor neurons, the others end on interneurons associated with motor neurons.

From the cerebral cortex, the corticospinal tract axons descend through the brain along a path located between the basal ganglia and the thalamus, known as the internal capsule. They then continue along the ventral brainstem as the cerebral peduncles and on through the pyramids of the medulla. Most of the corticospinal axons cross the midline in the medullary pyramids, thus, the motor cortex in each hemisphere controls the muscles on the contralateral side of the body. After crossing in the medulla, the corticospinal axons descend in the dorsal lateral columns of the spinal cord and terminate in lateral motor pools that control distal muscles of the limbs. A smaller group of axons do not cross in the medulla and descend in the ventral spinal columns. These axons terminate in the motor pools and adjacent intermediate zones that control the axial and proximal musculature.

The corticospinal tract is estimated to contain about 1 million axons at the level of the medullary pyramid. The largest-diameter, heavily myelinated axons are between 9 and 20 |xm in diameter, but that population accounts for only a small fraction of the total. Most corticospinal axons are small, 1 to 4 |xm in diameter, and half are unmyelinated.

Primary motor

Primary motor

Corticospinal Tract

^FGUREIHIH^^ The corticospinal tract. Axons arising from cortical neurons, including the primary motor area, descend through the internal capsule, decussate in the medulla, travel in the lateral area of the spinal cord as the lateral corticospinal tract, and terminate on motor neurons and interneu rons in the ventral horn areas of the spinal cord. Note the upper and lower motor neuron designations.

^FGUREIHIH^^ The corticospinal tract. Axons arising from cortical neurons, including the primary motor area, descend through the internal capsule, decussate in the medulla, travel in the lateral area of the spinal cord as the lateral corticospinal tract, and terminate on motor neurons and interneu rons in the ventral horn areas of the spinal cord. Note the upper and lower motor neuron designations.

In addition to the direct corticospinal tract, there are other indirect pathways by which cortical fibers influence motor function. Some cortical efferent fibers project to the reticular formation, then to the spinal cord via the reticu-lospinal tract, others project to the red nucleus, then to the spinal cord via the rubrospinal tract. Despite the fact that these pathways involve intermediate neurons on the way to the cord, volleys relayed through the reticular formation can reach the spinal cord motor circuitry at the same time as, or earlier than, some volleys along the corticospinal tract.

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Responses

  • mulu simon
    Does the indirect pathway internal capsule?
    7 years ago
  • april
    Where do pyramidal tracts cross?
    7 years ago

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