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LESSON 48

Central vestibular pathways

Year 1
Spring semester
Anatomy II (Neuroanatomy)
Central auditory and vestibular pathways
Duration: 11:39

Balance information from the inner ear gathers in the vestibular nuclei of the brainstem and is distributed to the eyes, cerebellum, spinal cord and cortex. In this lesson we look at the four vestibular nuclei, the medial longitudinal fasciculus (MLF), the pathway to the eye muscles, the role of the cerebellum and the vestibulospinal tracts.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn where balance information from the inner ear arrives in the brainstem and how it is distributed.
We will look at the vestibular nuclei, the pathway to the eye muscles, the cerebellum and the tracts descending to the spinal cord.
Information from the semicircular canals and the otolith organs is carried along the vestibular nerve.
The cell bodies of these fibers lie in Scarpa's ganglion, the vestibular ganglion, in the internal auditory canal.
The vestibular nerve reaches the brainstem together with the cochlear nerve, through the internal auditory canal.
The nerve enters the brainstem at the level where the pons meets the medulla oblongata.
Most fibers end in the vestibular nuclei, while some go directly to the cerebellum.
So from the very first stop, balance information is processed both in the nuclei and in the cerebellum.
The vestibular nuclei lie in the brainstem, in the floor of the fourth ventricle.
There are four main nuclei on each side: the superior, medial, lateral and inferior nuclei.
The superior nucleus connects mainly to the nuclei that control eye movements and helps keep gaze steady.
The medial nucleus is the largest; it takes part both in eye movements and in stabilising the head through the neck muscles.
The lateral nucleus, also known as Deiters' nucleus, acts on trunk and leg muscles through a tract descending to the spinal cord.
The inferior nucleus is connected with the cerebellum and the reticular formation and integrates different inputs.
This division is schematic; in reality there are dense connections between the nuclei, and their roles overlap.
The vestibular nuclei do not receive input only from the inner ear; they are a multisensory integration center.
Visual motion information from the eyes also reaches these nuclei, for example the signal created when the surroundings slide past.
Position information from the neck muscles and joints reports where the head is relative to the body.
The cerebellum constantly sends regulating signals to these nuclei.
The nuclei on the two sides are also linked to each other by connections called commissural fibers.
Thanks to these multiple inputs, the brain judges head motion without depending on a single source.
The right and left vestibular nerves fire continuously, at similar rates, even when the head is still.
When the head turns to one side, activity on that side increases and activity on the other side decreases.
Through the commissural fibers, the nuclei compare the two sides and derive the motion from this difference.
If the labyrinth or nerve on one side suddenly loses function, the resting activity from that side drops.
The brain reads this imbalance as if the head were constantly turning; severe spinning vertigo and nystagmus appear.
Over time the cerebellum and the nuclei reduce this imbalance; this process is called central compensation.
The main pathway linking the vestibular nuclei to the eye muscle nuclei is the medial longitudinal fasciculus.
Known as the MLF, it is a fiber bundle running on each side near the midline of the brainstem.
The MLF links the three cranial nerve nuclei that control eye movements: the oculomotor, trochlear and abducens nuclei.
The oculomotor nucleus gives rise to the third cranial nerve, the trochlear to the fourth, and the abducens to the sixth.
In this way, the muscles of both eyes contract together, as a matched pair, in response to head movement.
Descending fibers within the MLF go to the neck muscles and help coordinate head and eye movements.
Each eye is moved by six extraocular muscles.
The lateral rectus turns the eye outward, and the medial rectus turns it inward.
The superior rectus moves the eye up, and the inferior rectus moves it down.
The superior and inferior oblique muscles also rotate the eye around its own axis; this movement is called torsion.
The sixth nerve drives the lateral rectus and the fourth nerve the superior oblique; the other four muscles are driven by the third nerve.
Each semicircular canal connects to the muscle pairs that produce eye movement in its own plane.
Let us follow a horizontal example: when the head turns left, the left horizontal semicircular canal is excited.
The left vestibular nuclei excite the abducens nucleus on the opposite side, the right.
The right abducens nucleus contracts the right lateral rectus and turns the right eye to the right.
Interneurons from the same nucleus cross the midline, ascend in the MLF and contract the left medial rectus.
So both eyes turn together, opposite to the head, to the right, and the target stays in place.
The opposing muscles relax at the same time; this reciprocal arrangement keeps the movement smooth.
When the MLF is damaged, a picture called internuclear ophthalmoplegia, or INO, appears.
On side gaze, the eye on the side of the lesion cannot turn inward, towards the nose, or turns slowly.
The eye on the opposite side shows nystagmus as it turns outward.
When looking at something near, both eyes often still turn inward, which shows that the muscle and the third nerve are intact.
This finding can be seen in conditions affecting the brainstem, such as multiple sclerosis and stroke; evaluation is for the physician.
The cerebellum is the tuning center of the vestibular system; the part involved in this work is called the vestibulocerebellum.
The flocculus and paraflocculus adjust the gain of the vestibulo-ocular reflex, or VOR, and help keep gaze steady.
When the head moves along with a target we are tracking, the VOR must be suppressed; this is largely done by the flocculus.
The nodulus and uvula take part in briefly storing rotation information, called velocity storage, and in otolith processing.
Purkinje cells in the cerebellar cortex send direct inhibitory fibers to the vestibular nuclei.
That is why dizziness and eye movement disorders are common in cerebellar diseases.
Knowing the central vestibular pathways helps us tell peripheral findings from central ones.
Peripheral nystagmus usually decreases with visual fixation, while nystagmus from the cerebellum or brainstem may not.
The direction of peripheral nystagmus does not change with gaze; with central causes it may change direction with gaze.
In acute peripheral loss, the head impulse test is often abnormal, because the VOR pathway is affected in the labyrinth or nerve.
Pure downbeat or pure torsional nystagmus should bring brainstem or cerebellar causes to mind.
These patterns are guides; they do not make a diagnosis, and such findings are passed on to the physician.
Two main tracts descend from the vestibular nuclei to the spinal cord; these are called the vestibulospinal tracts.
The lateral vestibulospinal tract starts in the lateral nucleus, Deiters' nucleus.
This tract descends on the same side along the whole length of the spinal cord.
It supports upright posture by exciting the antigravity muscles of the trunk and legs, which work against gravity.
The medial vestibulospinal tract comes mainly from the medial nucleus, descends on both sides and ends in the neck region.
By controlling the neck muscles, it helps stabilise the head during head movement; this is called the vestibulocollic reflex.
We will see how these tracts are used in posture in detail in the lesson on balance.
Pathways from the vestibular nuclei also reach the cerebral cortex via the thalamus.
Unlike hearing, there is no single primary vestibular cortex; multisensory areas in the insula and parietal region process it.
Our conscious perception that we are moving or turning depends on this cortical processing.
The nuclei are also linked to the autonomic nervous system, which is why severe vertigo can come with nausea, vomiting and sweating.
The same connections also help adjust blood pressure quickly when we stand up.
A clinical tip: observe nystagmus both with fixation and with video goggles, with fixation removed.
Peripheral nystagmus grows clearer without fixation; if nystagmus does not decrease with fixation, question the central pathways.
Also note any lack of coordination in eye movements, such as one eye failing to turn inward.
Stating these findings separately in the report speeds up the physician's evaluation.
The vestibular nerve reaches the four vestibular nuclei in the brainstem; these nuclei gather information from many senses.
The medial longitudinal fasciculus links the nuclei to the nuclei of the third, fourth and sixth cranial nerves.
The cerebellum tunes the gain of the VOR, suppresses it when needed, and regulates the nuclei.
The lateral and medial vestibulospinal tracts support upright posture and head stabilisation, respectively.
Test yourself: which is the main pathway linking the vestibular nuclei to the eye muscle nuclei?
The correct answer is B: the medial longitudinal fasciculus.
The lateral lemniscus is part of the auditory pathway, and the lateral vestibulospinal tract descends to the spinal cord.
In the next lesson we will learn how auditory nerve fibers code the frequency and intensity of a sound.
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