Corticospinal tract
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| Brain: Corticospinal tract | ||
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| Deep dissection of brain-stem. Lateral view. ("pyramidal tract" visible in red, and "pyramidal decussation" labeled at lower right.) | ||
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| Diagram of the principal fasciculi of the spinal cord. | ||
| Gray's | subject #185 759 | |
| NeuroNames | ancil-373 | |
| MeSH | Pyramidal+Tracts | |
The corticospinal or pyramidal tract is a massive collection of axons that travel between the cerebral cortex of the brain and the spinal cord.
The corticospinal tract mostly contains motor axons. It actually consists of two separate tracts in the spinal cord: the lateral corticospinal tract and the medial corticospinal tract. An understanding of these tracts leads to an understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.
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[edit] The motor pathway
The corticospinal tract originates from cells in layer V of the motor cortex. The neuronal cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons (cortico-pontine tract), medulla oblongata (cortico-bulbar tract); the bulk of these fibers, however, extend all the way down to the spinal cord (corticospinal tract).
- Most of the cortico-spinal fibers (about 90%) cross over to the contralateral side in the medulla oblongata (pyramidal decussation). Those that cross in the medulla oblongata travel in the lateral corticospinal tract.
- The remainder of them (10%) cross over at the level that they exit the spinal cord, and these travel in the medial corticospinal tract.
Despite which of these two tracts it travels in, the axon of a neuron which is part of this tract will synapse with another neuron in the ventral horn. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
There is a common misconception that there is a precise somatotopic organisation of the motor cortex. The experiments performed by Penfield in the 1930's involved the stimulation of small areas of the motor cortex and mapping movements. With the information derived from over 400 neurosurgical patients, Penfield created a precise somatotopic map of the different body parts in the primary motor cortex with the leg area located medially (close to the midline), and the head and face area located laterally on the convex side of the cerebral hemisphere (motor homunculus). This map was somewhat fitting, as it showed a similar pattern of over-representation to the somatotopic maps of the somatosensory cortex. However, upon re-examination of the results obtained by Penfield, as well as new intracortical microstimulation mapping, a different picture was obtained.
Penfield's results show that stimulation of many different areas of the motor cortex would cause contraction of a particular muscle (nearly 50% of the cortex caused leg stimulation despite its relative under-representation). Now, there is evidence that the stimulation of even individual motor neurons causes stimulation, as well as inhibition, of spinal motor neurons leading to different muscles. It seems likely that groups of closely linked neurons in the cortex actually control co-ordinated muscle contractions and relaxations that lead to a specific movement. So, whilst the overall map given by Penfield's motor homunculus is correct, the principle behind the organisation of the motor cortex appears to be different.
The motor axons move closer together as they travel down through the cerebral white matter, and form part of the posterior limb of the internal capsule.
The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids") on the ventral surface of medulla oblongata - this is where the name pyramidal tract comes from.
After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the medulla oblongata travel down the separate ventral corticospinal tract, and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.
The motor neuron cell bodies in the motor cortex, together with their axons that travel down the brain stem and spinal cord, are referred to as upper motor neuron. In the spinal cord, these axons connect (most of them via interneurons, but to a lesser extent also via direct synapses) with the lower motor neurons (LMNs), located in the ventral horn of the spinal cord. In the brain stem, the lower motor neurons are located in the motor cranial nerve nuclei (occulomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory, hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior roots of the spinal nerves respectively, end-up at the neuromuscular plate and provide motor innervation for voluntary muscles.
[edit] Sensory pathways
- Spinothalamic tract
- Spinocerebellar tract
- Visual pathway
- Auditory pathway
- Gustatory pathway
- Olfactory system
- Posterior column pathway
[edit] Corticospinal tract damage
see upper motor neuron.
[edit] Extrapyramidal motor pathways
These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and muscle tone. See extrapyramidal motor system.
[edit] Additional images
 Dissection of brain-stem. Lateral view. |
 Superficial dissection of brain-stem. Ventral view. |
