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The corticospinal tract contains both sensory and motor axons. About 85% of the axons cross over in the medulla (at a point known as the pyramidal decussation). This means that movement of one side of the body, is controlled by the opposite side of the brain.
The motor pathway
The motor part of the corticospinal tract starts in the primary motor cortex (located mostly on the precentral gyrus ). Neuronal cell bodies are in the grey matter of this area; they have long axons, some of which will extend down the whole spinal cord.
Each neuron (and its axon) controls a different part of the body. The axons arranged the same general direction give a fibrous appearance, and the direction can be traced in a gross specimen. The fibres move closer together as they travel through the white matter of the brain, and form part of the posterior limb of the internal capsule.
The motor fibres continue down into the brainstem, through the midbrain and pons. At the medulla, the bundle of corticospinal axons is visible as two column-like structures on the ventral surface called pyramids (this is where the name pyramidal tract comes from).
The left and right sides swap over, more or less completely, at the level of the closed medulla. (This is the aforementioned decussation). After the decussation the axons travel down the spinal cord as the lateral corticospinal tract. Fibres that do not cross over go down the separate ventral corticospinal tract.
These motor neurons in the cortex, with their axons that travel down the spinal cord, are the upper motor neurons of the body. In the spinal cord these synapse with the lower motor neurons (LMNs), present in the ventral horn of the spinal cord. (The axons of LMNs then travel out of the spinal cord to form spinal nerves and innervate muscle.)
The sensory pathways
Even though there are sensory fibres in the corticospinal tract, sensation is best discussed as separate systems.
Corticospinal tract damage
Damage to the parts of CST may be best discussed at upper motor neuron .
Extrapyramidal motor pathways
These are motor pathways that lie outside the corticospinal tract. The extrapyramidal system includes:
- Vestibulospinal tract
- Reticulospinal tracts
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