Voluntary Muscular Control and The Corticospinal Tract
Last week we looked at an ascending track, the lateral spinothalamic tract.
This week we’ll be looking at a descending track.
If you remember, an ascending track is a neural pathway that sends information from different parts of the body to the brain; the electric signals go “up” to the brain.
A descending track is a neural pathway that sends information from the brain down to different parts of the body.
The Pathway for Voluntary Motor Activity
Simply put, the corticospinal tract connects your brain to your limbs. Every time your motor cortex wants to move your hand or your feet, it sends a signal down the corticospinal tract.
You might remember that last week we said that most ascending pathway had 3 “neuronal levels.” Well, the corticospinal tract has one neuronal level that travels all the way down from the motor cortex to the anterior horn of the spinal cord, and one neuronal level where motor neurons from the anterior horn of the spinal cord connect to the muscles.
At first glance the corticospinal tract appears to be fairly simple!
Let’s look at it in more details.
The Corticospinal Tract
The cell bodies of the neurons making up this tract are located in the primary motor area of the cerebral cortex. The axons will travel throught the brain to get to the brainstem so that they may travel down the spinal cord.
The axons will travel down through the crus cerebri and to the pons. At this point all the fibers that used to be somewhat “bundled” together “debundle” as they travel through the pons.
After they’ve travelled through the pons, all the axons “re-bundle” together and keep on traveling down through the pyramids (at the level of the upper and middle medulla ).
Once they reach the lower medulla, something strange is going to happen.
Up to now, all the fibers from the right motor cortex travelled down on the right side (of the crus cerebri, pons, pyramids,…..) and all the fibers from the left motor cortex travelled on the left side.
But at the level of the lower medulla, about 75% of the fibers traveling on the right side will “cross-over” (what we call “decussate”) to the left side, and 75% of the fibers on the left side cross-over to the right side before traveling down the spinal cord.
- In other words, after the lower medulla, 75% of the fibers will travel down on the contralateral side, and about 15% will travel down on the ipsilateral side.
The 75% of the fibers that decussate will travel down through what is called the lateral corticospinal tract, and the 15% that do not decussate will travel down through what is called the anterior corticospinal tract.
The area where the fibers decussate is called the decussation of the pyramids.
The axons will travel down their respective tracts until they reach their appropriate spinal level. There, they will synapse on new motor neurons in the anterior horn of the spinal cord.
And this marks the end of the corticospinal tract 🙂
Minimum To Remember
- The corticospinal tract is the major descending pathway involved in voluntary motor movement .
- One neuronal segment travels all the way from the motor cortex down to the anterior horn of the spinal cord.
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