The Vestibular System

Question 1

Describe the components of the vestibular apparatus

Answer 1

The Vestibular Apparatus has 2 components: 1 within Bony labyrinth Vestibule & 1 within Cochlea!

Utricle + Saccule membrane sacs are within Vestibule & house the Otolith system

Semicircular ducts in Semicircular canals house the Membranous Labyrinth components

Question 2

The ampulla is part of the vestibular apparatus. What is this?

Answer 2

The Ampullae is where the semi-circular ducts attach to the Utricle.
Hair cells are in all 3 ampullae, as well as on the edge of the Utricle + Saccule

Question 3

Label this vestibular system

Answer 3

Question 4

Describe the otolith system
2 things otolith system detects, what does the wall of the labyrinth support, what is found in the ECF + what projects into the ICF, what r the hair cells associated with

Answer 4

Otolith system detects Linear head movements + Gravity

The thick membrane wall of the labyrinth supports hair cells which release Glut

Hair cell bodies are found in ECF (Perilymph), whilst the Stereocilia project up into ICF Endolymph.

The hair cells are associated w otolith (a Gelatinous membrane)- base of the Stereocilia embedded in this!

Question 5

What is the otolith weighed down by and what do these provide?

Answer 5

The otolith is weighted down by otoconia crystals – these provide inertia (the tendency to remain in constant stillness or motion / resist changes in movement velocity)

Question 6

Label this, what is it?

Answer 6

Question 7

What happens to the stereocilia when we move versus when we are still?
What is the rmp of hair cells?

Answer 7

When we’re still, the stereocilia in the otolith=static and straight. RMP is ~-40mV, v few open K channels=small leak of glutamate= minimal afferent activity
If the head moves, otolith inertia means it will lag slightly during the first part of movement. Stereocilia tilt.

Question 8

Explain how the direction of tilting of the stereocilia determines the response.

Answer 8

If they tilt towards the tallest stereocilium, mechanically gated K channels open→ hair cell depolarises→ more glutamate release and afferent activity.
If they tilt towards the shortest stereocilium, all channels close→hair cell hyperpolarises, no glutamate release, no afferent

Question 9

Explain how the otoliths system follows the law of inertia.

Answer 9

If we keep at constant velocity, the otolith will catch up and return to its neutral position. This means it only responds to head acceleration, not constant movement.
When the head stops moving, the otolith continues to move for a while after (due to the inertia).
This tilts the stereocilia in the opp direction

Question 10

how do stereocilia respond to gravity?

Answer 10

If we tilt our head, the otolith and stereocilia will sag towards gravity.
If the stereocilia tilt towards the tallest stereocilium, ion channels open → hair cell depolarises→ glutamate release at the synapse w its afferent

Each movement activates a unique group of hair cells - The associated afferent pattern tells us the movement direction or gravity!

Question 11

how do otolith organs provide Postural control?

Answer 11

Otolith organs provide postural control via Lateral Vestibulospinal tract - sends afferents to antigravity muscles of Leg + Trunk
Signals along this tract help us stay upright + brace us against external movements (e.g Wobbling on a train)

Question 12

Otolith system responds to linear movements of the head. What responds to rotation?
where r hair cells found?

Answer 12

The semicircular canals respond to head rotation
The hair cells are found within the ampullae, which is closed off from the semi-circular duct by the cupula (a gelatinous membrane)
The stereocilia are also attached to the cupula
Again, the hair cells release glutamate!

Question 13

Describe the mechanisms that occur when we turn our head to the left.

Answer 13

When we turn our head left, the LEFT horizontal canal rotates. The endolymph lags behind + presses against the cupula.
This causes stereocilia in the semicircular canal to lean to the tallest tip, opening K channels→ hair cell depolarisation→ more glut + afferent activity

In right horizontal semicircular canal= a mirror image: stereocilia in the right canal tilt to the shortest, closing K+ channels→ hyperpolarisation→ no afferent activity on the right

Question 14

How are the semicircular canals arranged and what are the targets for afferent signals?

Answer 14

Within the inner ear, the semicircular canals sit at right angles to each other= can cover all possible axes of rotation
Afferent signals from the semicircular canals travel via the medial vestibulospinal tract - targets= neck and shoulder muscles, bilaterally

Compensate for externally induced movements in regards to Head Stability + Visually guided movements

Question 15

What are smooth pursuit eye movements?

Answer 15

Smooth pursuit: focuses on + follows an object, which must be fixed in the Fovea
Spatial input from Parietal cortex→ Frontal eye fields→Pontine nuclei→ Vestibular nuclei via the Cerebellum
Vestibular apparatus feeds into the vestibular nuclei w info on our head movements, so we can make eye movements to follow the object
The vestibular nuclei →cranial nerve nuclei that supply the extraocular muscles

Question 16

What is the vestibulo ocular reflex? Use an example of your head turning left to explain this.

Answer 16

At rest, the left semi-circular canal fires steady APs. When head turns left while focusing on an object ahead: left horizontal canal increases firing
These afferents travel→ left vestibular nucleus ipsilaterally→ contralateral abducens nucleus → contracts lateral rectus→ abducting the right eye

Simultaneously - Medial Longitudinal Fasciculus →(left) oculomotor nucleus→contracts the Left Medial Rectus
Result=both eyes move right when head moves left!

Question 17

What type of axons are involved in the vestibulo ocular reflex?
What does this increase vulnerability to?

Answer 17

Since these pathways need to be very fast to ensure the eyes + head move w no lag, the axons involved are heavily myelinated, making them vulnerable to MS; eye movements of affected patients are uncontrolled

Question 18

What is the importance of cerebellar movements in the vestibulo occular reflex

Answer 18

Certain things can throw eye movements out of sync w the moving object= retinal slip:
Retinal ganglion cells detect this + send error signal to accessory optic system - activate olivary + pontine nuclei. These adj inhibition to match excitation from the vestibular system

Olivary→climbing fibres→ purkinje→vestibular nuclei
Pontine→granule cells→parallel fibres→purkinje→vestibular n

This happens if hair cells are lost (Meniere’s, age) + if an acoustic neuroma/MS inhibits excitatory signals from the vestibular→cranial nerve nuclei

Question 19

What happens in vestibular dysfunction? Describe symptoms of this

Answer 19

Turning head left= increased output from left, decreased output from the right horizontal canal
The difference between the 2 outputs= sensation of turning left and causes the eyes to turn to the right to compensate for turning

BUT if right canal is dysfunctional, the signal from it becomes weaker at all times. There is a difference in output, giving sensation of turning left and causing the eyes to drift right, even when still =vertigo!
In vertigo hay nystagmus (flicking of eyes), nausea & vomiting

Question 20

Describe vehicle associated motion sickness

Answer 20

We’re tilting at the same time as the vehicle, so our visual system says we’re stable, but our vestibular system senses motion.
This disagreement between visual and vestibular inputs causes motion sickness

Question 21

How are funfair rides and zero gravity associated with motion sickness?

Answer 21

On funfair rides, our vestibular system is massively overstimulated, which mimics the effect of certain toxins - activating poison defence system (vomiting)

The otolith system is calibrated to deal with gravity, so if we take gravity away, we get strange vestibular outputs that mimic poisoning, inducing vomiting

Question 22

Relate vestibular inputs to the the vomiting pathway

Answer 22

Vestibular/cerebellar dysfunction + overstimulation cause aberrant vestibular inputs to vestibular nuclei
These activate our poison-defence system, aka the NTS, coordinating nausea and vomiting (+ associated features, e.g. vasopressin release)

Question 23

What is the significance of motion sickness history?
3 things history of motion sickness can predict? + 4 grps of ppl who are susceptible toit…

Answer 23

A history of motion sickness can predict:

Pregnancy sickness
Anticipatory emesis
Postop. emesis

Some people are especially susceptible, eg those with:

Family history
Asian origin
Females
Migraine sufferers

Question 24

Treating nausea and vomiting depends on the cause. Compare the different treatments for nausea?

Answer 24

vagal afferents + area postrema send inputs to the NTS using 5-HT3 receptors
This means 5-HT3 antagonists treat nausea.

BUT, inputs from vestibular organs + eyes to vestibular nuclei & then NTS use Ach and Histamine So we can treat motion sickness w muscarinic blockers + H1 antagonists.

Alt to this include herbal treatments, eg ginger

Question 25

explain Vestibular - Cerebellum diagram

Answer 25

Vestibular system→vestibular nuclei→activates CN III, IV, VI
+ also activates Granule cells→ activate Purkinje cells→inhibit Vestibular nuclei !
Vestibular nuclei output depends on vestibular apparatus excitation vs cerebellar Inhibition
e.g If excitation gets weaker, inhibition is also weakened to correct the movement!