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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the corticospinal tract.
The corticospinal tract is a major tract that carries movement-related information from
the motor cortex to the spinal cord.
The neurons that travel in the corticospinal tract are called upper motor neurons; they
form connections with neurons called lower motor neurons, which carry movement-related
impulses to muscle itself, causing it to contract.
The upper motor neurons of the corticospinal tract leave the motor cortex and descend to
the brainstem, entering the midbrain in large fiber bundles called the cerebral peduncles.
The tract continues down into the medulla where the fibers form two bundles, known as
the pyramids, which create visible ridges on the exterior surface of the brainstem.
At the base of the pyramids, about 90% of the fibers in the corticospinal tract decussate,
or cross to the other side of the brainstem in a bundle called the pyramidal decussation.
The decussating fibers will then enter the spinal cord on the opposite side of the body
from where they originated as part of the lateral corticospinal tract.
The other 10% of the fibers will continue into the spinal cord on the same side of the
body where they originated as part of the ventral or anterior corticospinal tract and
only cross over when they reach the level of the spinal cord where they will synapse
on a lower motor neuron.
It is thought that the lateral and anterior corticospinal tract fibers have slightly different
specializations, with the lateral corticospinal tract controlling the movement of more distal
muscles like those of the hands, and the anterior corticospinal tract controlling the movement
of more proximal muscles like those of the trunk.
Damage to the corticospinal tract can lead to a collection of symptoms known as upper
motor neuron syndrome, which involves symptoms like weakness or paralysis, hyperactive reflexes,
decreased motor control, and either increased or decreased muscle tone.
Over time patients may regain the ability to make crude movements but fine finger movements
like writing or typing may remain impaired, suggesting the corticospinal tract is especially
important for these types of movements.