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Day 191: Know your spinal cord – The Vestibulospinal tract

cranial nerves

No, we’re not moving away from the spine, but we are talking about the cranial nerves.

It’s day thirty-four in our spinal cord series. As usual, if you’re new here welcome and you can find each and every post in our series in the handy neuroanatomy category! All the posts are in reverse chronological order and while we don’t technically have a specific order, you should probably start with the medullary pyramids and work your way forward. If you’re here, then you probably are interested in the vestibulospinal tract, something we haven’t covered yet, but fear not, we are going to do that now.

If you’re familiar with the human body, you may already know of the vestibular system. If you’re not the vestibular system is responsible for providing our brain with information about motion, head position, and spatial orientation. So you may guess that the vestibulospinal tract has something to do with this vestibular system, you would be correct. The vestibular nuclei (located with the pons and medulla in the brainstem) receive information from the vestibulocochlear nerve (the eighth cranial nerve, which relays sound and equilibrium information from the inner ear to the brain) shown below.

8th cranial nerve

The vestibulospinal tract is considered part of the vestibular system. The vestibular system uses the information (motion, head position, etc.) to maintain hand eye coordination, balance, and correct posture. However, it also has a role in the conscious realization of spatial orientation and motion. When the vestibular sensory neurons detect small movements of the body, the vestibulospinal tract commands motor signals to specific muscles to counteract these movements and re-stabilize the body. It is one of the three ways we maintain our balance (the other two are: visual input and proprioceptive feedback). Below is an image showing the route the vestibulospinal tract takes in the brainstem.

vestibulospinal tract brain stem

You may guess based on the function, but this tract is an upper motor neuron tract. As is the case with most tracts, even though it is small, there are two sub-pathways. The medial vestibulospinal tract and the lateral vestibulospinal tract, both of which do different things.

The medial vestibulospinal tract projects bilaterally from the medial vestibular nucleus located within the medial longitudinal fasciculus to the ventral horns of the upper cervical cord (this would be about vertebra C6). This tract helps stabilize the head and innervates neck muscles (so you don’t have a floppy head).

The lateral vestibulospinal tract provides excitatory signals to interneurons, which then relay the signals to motor neurons that innervate the muscles responsible for resisting gravity (aptly referred to as antigravity muscles). They are the extensor muscles in the legs that help maintain upright and balanced posture.

Typically when we see an image of the tract, they are grouped together, but if you recall, lateral would be toward the side while medial would be toward the middle, so we have a good idea where the tracts split even when the image shows them together. Below is such an image, I’ve labeled the vestibulospinal tract, however there was no distinction between the lateral and medial tracts, so I did not try to seperate them myself.

vestibulospinal tract

Location of the vestibulospinal tract in the spinal cord

A typical person  will sway from side to side when the eyes are closed. This is the result of the vestibulospinal reflex working correctly. When an individual sways to the left side, the left lateral vestibulospinal tract is activated to bring the body back to midline. Generally damage to the vestibulospinal system results in impaired balance (ataxia) and postural instability. Patients with bilateral or unilateral vestibular system damage will likely regain postural stability over weeks and months through a process called vestibular compensation. This process is likely related to a greater reliance on other sensory information (like the visual and proprioceptive systems we mentioned above).

That pretty much wraps up the vestibulospinal tract! Tomorrow I think we can cover the tectospinal tract and the spinotectal tract, because they are easy to confuse because of the name.

Until next time, don’t stop learning!

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