Vestibulo-ocular reflex and nystagmus

Definitions & Key takeaways

The vestibulo-ocular reflex is a reflex that helps stabilize the visual field during head movements. It occurs in response to head movement and leads to the eyes moving in the opposite direction to maintain a steady gaze. Physiological vestibular nystagmus is a normal and expected type of nystagmus that is part of the vestibulo-ocular reflex.

The receptors of the vestibulo-ocular reflex pathway, are the hair cells inside the structures that make up the vestibular apparatus which detect the direction of movement. Then, they pass the message through the vestibulo-cochlear nerve, which then triggers the firing of the vestibular, abducens, and oculomotor nuclei. The efferent pathway is represented by the abducens and the oculomotor nerve, which cause the extrinsic eye muscles to contract or relax to adjust the direction of gaze.

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Introduction0:00–0:22

Vestibulo- refers to the vestibular apparatus which is in the inner ear, that helps maintain our balance, and ocular refers to the eyes.
So the vestibulo-ocular reflex is a reflex that starts in the vestibular apparatus, in response to head movement, and ends with movement of the eyes.

Inner ear0:22–1:48

The inner ear, sometimes called the labyrinth, is a marvelous bit of engineering. On the outside, it has a tough bony shell - the bony labyrinth; and inside the bony labyrinth, there’s the membranous labyrinth.
The bony and membranous labyrinth help form all three parts of the inner ear. The first is the vestibule, which is like a hallway that leads up to two other parts or rooms - the cochlea, towards the front of our head, that deals with hearing, and a second room towards the back of our head, that contains three semicircular canals - an anterior, a posterior and a lateral one - which play a role in balance.
Along with the semicircular canals, there are also two other balance-related structures in the vestibule - the utricle and the saccule.
Together, the semicircular canals, the utricle, and the saccule make up the vestibular apparatus, and each of these structures have special balance receptors called hair cells - which function like motion sensors, picking up different kinds of movement.
Hair cells in the semicircular canals detect changes in our dynamic equilibrium, like when we rotate our head, while those in the utricle and saccule detect changes in our head position in relation to horizontal or vertical acceleration - like when we feel pushed towards the back of the seat in a speeding car, or when we go up or down an elevator.
Now let’s switch gears and look at the eyes - pun intended! The eyes normally focus on an image that’s projected on the fovea - which is the central part of the retina.

Eyes1:48–2:34

But, our head is almost always moving - from bopping ever so slightly when we walk, to turning when we want to take a sneak peek at a cute person walking by.
So if our eyes weren’t moving along with our head, the image would bop up and down with every step we take. On the outside of each eye, there are 6 extrinsic eye muscles - a superior and an inferior oblique muscle, a superior and an inferior rectus muscle, as well as a lateral and a medial rectus.
These muscles contract or relax in order to adjust the direction of our gaze. The vestibulo-ocular reflex has five components: a receptor that receives the stimulus, an afferent pathway, a central nervous system relay, an efferent pathway, and an effector that responds with a reaction.

Vestibulo-ocular reflex2:34–6:17

So, let’s use an example - let’s say that you turn your head slightly to the right. This is our stimulus.
This stimulus is picked up by receptors - which are the hair cells in the left and right semicircular canals. Now, each of the semicircular canals respond to different kinds of rotation, but in our example, simply rotating our head to the right engages the lateral semicircular canal in the left and right inner ear.
So, let’s look at just the right side. The right lateral semicircular canal ends with a dilated portion called the ampulla - which is lined by hair cells which are balance receptors.
These hair cells get their name because they have projections called cilia on top of them - kinda like a haircut. These projections are embedded in a gelatinous substance called the cupula.
When we turn our head to the right, this sets in motion a fluid called endolymph that can be found inside the membranous duct.
The endolymph moves in the opposite direction of movement - so to the left. And the movement of the endolymph deflects the gel-like cupula as well as the cilia on the hair cells to the left.
When the cilia bend in that way, it triggers the hair cells to generate an action potential. This action potential travels through to nerve fibers that synapse with the hair cells.
These nerves converge to form the vestibular branch of the vestibulo-cochlear, or eighth cranial nerve. This is our afferent pathway, that travels all the way to the part of the central nervous system called the brainstem - more specifically, to the right vestibular nuclei in the pons.
This makes up the central nervous system relay. Now, for the efferent pathway, things get interesting.
From the right vestibular nucleus, nerve fibers cross over to the left abducens nucleus. Some fibers originating in the left abducens nucleus make up the sixth, or abducens, cranial nerve, which makes the left lateral rectus muscle contract causing the left eye to move to the left.
However, some other fibers from the left abducens nucleus act as interneurons and go to the right oculomotor nucleus in the midbrain - so we’re crossing back over the right side of the brainstem.
The right oculomotor nucleus innervates 4 of the 6 extrinsic eye muscles, including the right medial rectus. So the right oculomotor nucleus fires signals into the right oculomotor nerve, telling the right medial rectus muscle to contract.
The left lateral and right medial rectus muscles are the effectors in our scenario, and they complete the reflex by moving the eyes leftwards.
Of course, we have two sets of semicircular canals: one on the right, and one on the left. Head rotation to the right also makes the endolymph inside the left semicircular canal move towards the left.
However, in this case, the endolymph is moving away from the ampulla, so this means no action potentials are generated in the left hair cells, so the neural pathway of the left vestibular nerve is inhibited.
So for the vestibulo-ocular reflex to function properly, when rotating the head to the right, it’s not enough for just the hair cells on the right side to generate an action potential that makes the left lateral rectus and right medial rectus contract - hair cells on the left also have to not generate an action potential, therefore inhibiting the contraction of the right lateral rectus and left medial rectus.

Nystagmus6:17–6:59

So the result of the vestibulo-ocular reflex is a physiological form of nystagmus - meaning, involuntary eye movement. Normally, physiological nystagmus has two phases: the first phase, called the slow phase is a smooth movement of the eyes in the opposite direction of movement.
The second phase is called the fast phase, and it’s when the eyes quickly “jump” back in the same direction of movement.
The direction in which the eyes move during the fast phase defines the direction of the nystagmus - so even though the initial response to moving our head to the right is a slow movement of the eyes towards the left, when the eyes have moved as far left as they can, this leads to a right nystagmus.
In practice, there are two tests can be performed in order to determine whether or not the vestibular apparatus is working properly.

Bárány test6:59–7:50

The first is the Bárány test, which uses a Bárány spinning chair in which the person is spun on the chair for about 10 turns.
Normally, during rotation to the right, the fast component of nystagmus will be to the right - so we call it a right rotatory nystagmus - and the person tends to fall to the right.
Upon stopping, there will be a left postrotatory nystagmus, and the person tends to fall to the left. Then we spin them the other way around - so during rotation to the left, the fast component of nystagmus will be to the left - which makes it a left rotatory nystagmus - and the person tends to fall to the left.
Upon stopping, there will be a right postrotatory nystagmus, and the person tends to fall to the right. The second test is the caloric test - where “caloric” stands for “temperature”, because this test involves putting water in the person’s ears that’s either colder or hotter than body temperature, which causes endolymph to move.

Caloric test7:50–8:20

Normally, when putting cold water in a person’s ear, the nystagmus will be towards the opposite ear, whereas with hot water, nystagmus will be towards the same side.
You can remember this more easily by using COWS - “Cold - Opposite, Warm - Same”. Alright, as a quick recap: the vestibulo-ocular reflex occurs in response to head movement, and leads to the eyes moving in the opposite direction in order to maintain a steady gaze.

Review8:20–8:58

The receptors of this pathway are the hair cells inside the structures that make up the vestibular apparatus, that detect the direction of movement.
Then, they pass the message through the vestibulo-cochlear nerve, which then triggers the firing of the vestibular, abducens, and oculomotor nuclei.
The efferent pathway is represented by the abducens and the oculomotor nerve, which cause the extrinsic eye muscles to contract or relax in order to adjust the direction of gaze.