Corticospinal tract | Corticobulbar tract | Extrapyramidal Motor System | Vestibular System | Cerebellum
Pyramidal Motor System Corticospinal tract
Motor system animation
- The pyramidal motor system controls all of our voluntary movements. Pathological processes which damage the pyramidal motor system are extremely important causes of disability and suffering.
- The pyramidal system is a two neuron system consisting of upper motor neurons in the Primary Motor Cortex and lower motor neurons in the anterior horn of the spinal cord. Each of these neurons have extremely long axons. The upper motor neuron axon extends all the way from the brain down to the spinal cord, a distance 1-3 feet or more, and the lower motor neuron axon extends from the spinal cord to the skeletal muscles of the arms or legs, a distance 4-5 feet in very tall people.
- The upper motor neurons reside in the precentral gyrus of the frontal lobe also called the "motor strip". These upper motor neurons are arranged in a stereotypical fashion. Neurons which control movements of the face and mouth are located near the Sylvian or lateral fissue and neurons which control the muscles of the thighs and legs are located near the medial longitudinal fissure and within the central sulcus.
- Many textbooks illustrate this phenomenon graphically using a distorted human figure called Homunculus. The homunculus has very large face and mouth because there are many upper motor which innervate these parts of the body.
- The axons which extend from these upper motor neurons traverse the deep matter and coalesce to form the Internal Capsule. They then descend through the midbrain in the Cerebral Peduncle.
- The axons of the corticospinal tract the condense to form the pyramids. The axons are arranged in a very orderly fashion. Axons which will innervate the muscles of the face are located medially. This tract is known as the corticobulbar tract. The axons which form the corticobulbar tract exit at their appropriate levels to synapse with their lower motor neurons in the cranial nerve nuclei. Link to cranial nerves.
- The axons which will innervate the legs are located laterally within the cerebral peduncle. Thus the name "lateral corticospinal tract". These axons then aggegate to form the pyramids in the medulla. Thence the name "pyramidal tract".
- 80 - 90 % of corticospinal tract axons cross to the other side in the distal medulla. This is a very distinctive area on cross section known as the Pryamidal Decussation. The pyramidal decussation separates the medulla,above, from the spinal cord, below.
- Injuries to the upper motor neurons in the cortex or to their axons before they enter the pyramidal decussation result in spastic paralysis of the opposite side of the body. Injuries to the pyramidal tract below the decussation or to the lower motor neurons in the spinal cord cause paralysis on the same side of the body.
- The Ventral Corticospinal tract contains the axons from the 10-20% of neurons whose axons did not cross to the other side at the pyramidal decussation.
- The upper motor neuron axons then synapse on lower motor neurons in the Anterior horn of the spinal cord. The axons of these lower motor neurons then exit the spinal cord via the Ventral root. Damage to LMN's causes flaccid paralysis.
- The ventral root then joins the dorsal root to form the spinal nerve, which finally innervates the skeletal muscle.
- Upper motor neurons which innervate the muscles of the face and head are located near the lateral fissure of the brain. Their axons coalesce to form the corticobulbar tract. These axons then descend within the Genu of the internal capsule to the medial part of the cerebral peduncle. The upper motor neuron axons then synapse on Lower motor neurons of the cranial nerve nuclei which are located in midbrain, pons and medulla. Link to cranial nerves.
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last update Jan 21, 2002