Vestibular Foundations - Session 1: Anatomy and Physiology Part 2

The VOR can respond rapidly to head movements with a maximum velocity of 550 degrees per second and maximum frequency components of 20 Hz. This is due to its short processing time of about 15 milliseconds and a direct three-neuron pathway from the inner ear's receptors to the eye muscles.

The VOR maintains stable eye images during non-constant velocity head movements, such as acceleration or deceleration.

The optokinetic system is used during constant velocity motion to maintain stable images, particularly focusing on objects of interest in the fovea.

In a normal VOR, the eye movement is equal and opposite to the head movement, maintaining a 1:1 ratio for stable vision.

• impaired VOR comes up in the history taking as patients with this experience a retinal slip which in turn causes blurred vision and postural instability

• integrity of vestibular end organs can be assessed through functional measurement of the VOR

complementary and accurate sensory input from vestibular, vison and somatosensory systems to perceive and interpret the persons location with regard to gravity, the horizon and space?

whether we're moving and the direction we're moving in relative to surrounding.

-this info is then interpreted by the brain which sends info the visual
and postural centers so that counter or corrective action is made

• a breakdown anywhere in the system and imbalance or dizziness will be perceived

• 3 semi-circular canals
  -2 otolith organs: utricle and saccule
  -connections to the 8th nerve via the superior vestibular nerve and the inferior vestibular nerve

• to maintain balance the brain relies on equal but opposite input from the peripheral vestibular pathways, to achieve this, the left anterior SCC is paired with right posterior SCC.

left posterior SCC paired with right anterior.

the left and right horizontal SCC are paired.

This push-pull dynamic ensures accurate detection of head rotation and balance.

in a healthy VS the right sends an equal amount of inhibitory info to the brain,

if one of the vestibular organs is weak or impaired the brain sends a mismatch resulting in the sensation of dizziness

an endolymph filled swelling at the base of the SCC that houses the cuplla

a gelatinous membrane that seals teh inner ear diameter of the ampulla

stereocilium are hair cells that project into the cupula.

the kinocilium is the stereocilium that is larger than the rest

displacement of hair cells towards the kinocilium causes increased excitation

-displacement away from the kinocilium causes inhibition

-utricle: detects horizontal movements/ acceleration & head tilts (moving your head backwards and forwards (to me to you)

-saccule: detects vertical movements (e.g. jumping & riding in an elevator) (saccule=slide= up and down)

gelatinous web like structure that houses the weight and mass of calcium carbonate crystals

-its the mass and wight of the otolith that bend the hair cells within the utricle and saccule sending either excitatory or inhibitory info to the brain

the system is continuously providing a loop of info to and from the brain to maintain a sense of balance

If the head is turned to the left the acceleration of the head (1) causes fluid
movement in the horizontal SCCs(1). This fluid movement causes movement of the
cupula (1) which moves the stereocilia (1) and depolarises the sensory hair cell (1) in
the crista (1), causing a change in the firing rate along the vestibular nerves (1). As
the head accelerates to the left the firing rate in the left vestibular nerve increases (1)
and the firing rate in the right vestibular nerve decreases (1) Eyes will move to the right (1)

a patient may have normal functioning visual and vestibular structures, but to still perceive imbalance

horizontal= lateral
anterior= superior
posterior= inferior

• Brainstem; vestibular and oculomotor nuclei

-cerebellum

-cerebral cortex

-vestibulo- spinal reflex (VSR)

-vestibulo- collic reflex (VCR)

-vestibulo- ocular reflex (VOR)

responsible for detecting head movements in order to maintain balance, postural control and clear vison.

-lateral
-anterior
-posterior

The semi-circular canals detect angular changes in head velocity, such as rotational movements.

-The semi-circular canals lie orthogonal (at right angles) to each other, allowing them to detect rotation in 3 different planes

Anterior: detects rotational movement in the vertical plane (moving head up & down)

Lateral: detects horizontal rotational movements (turning head left & right)

Posterior: detects tilting of the head to the side (moving head towards shoulders)

Type I hair cells:

• flask shaped with a dense core terminal surrounding them.

-typically associated with afferent nerve fibres that are highly specialised for rapid and precise signalling.

Type II hair cells:

• cylindrical in shape with a clear core terminal.

-associated with both afferent and efferent nerve fibres, playing a role in both sensing and modulating signals.

in the cristae within the ampullae (in the SCC) and the maculae (in the utricle and saccule).

by sensing deflections of their stereocilia (small hair-like projections) and kinocilium (a larger single projection)

The typical resting firing rate is 70–100 spikes per second, allowing precise detection of increases and decreases in activity during head motion.

• Deflection toward the kinocilium excites the hair cell, increasing the firing rate.

• Deflection away from the kinocilium inhibits activity, decreasing the firing rate.

Supporting cells surround the hair cells and provide structural and metabolic support.

The basal lamina provides a foundation for both hair cells and supporting cells, maintaining their structural integrity.

• Detect changes in linear
  acceleration and gravitational
  tilts

• Arranged in different planes
  which enable them to respond
  maximally to motion in all
  directions

1) Superior Vestibular Nerve (VN) and Anterior Vestibular Artery, which supply:

• Lateral Semi-Circular Canal (SCC)

• Anterior Semi-Circular Canal (SCC)

• Utricle

2) Inferior Vestibular Nerve (VN) and Vestibulocochlear Artery, which supply:

• Posterior Semi-Circular Canal (SCC)

• Saccule

• Deflection of hair cells located in the ampulla of the semi-circular canals and
  macula of the otolith organs will result in excitatory or inhibitory responses

• Afferent neural information is transmitted along peripheral vestibular nerve

• Central processing of information

• Reflex pathways control way body responds to these movements

• Responsible for maintaining upright position and stabilisation of our bodies

• Helps maintain desired orientation

helps stabilize and reorient the head position relative to the body when there is a sudden movement or disturbance (perturbation).

Responsible for reorienting the head with respect to the body following a sudden perturbation

linear acceleration and gravitational tilts