Equilibrium

Dr. KJG, PROFESSOR,

DEPT. OF PHYSIOLOGY,

MGMC & RI

Objectives

• Structure – nuclei, connections, functions and disorders of the vestibular apparatus.

Systems involved in Equilibrium

• The main systems that are involved in

equilibrium are:

The Vestibular System

The Visual System

The Proprioceptive System

Cerebellum

Vestibular apparatus

• The receptor organs for equilibrium are

collectively called the vestibular apparatus

• Has a role in regulation of posture &

equilibrium

• Complex sense organ which is stimulated by

– Gravity

– Rotation movements

Location & parts

• The vestibular apparatus is present in

inner ear & comprises of

• 3 semi-circular canals

• Otolith organs

– The utricle

– The saccule

Types of Equilibrium

Rotational

movement

Static equilibrium =

Maintenance of position of the

body (head) relative to force of

gravity

Dynamic equilibrium =

Maintenance of body position

(head) in response to rotational

movements

Static Equilibrium

• Receptors for static equilibrium are the

Maculae.

• Maculae present in Saccule & Utricle

(otolith organs)

Macula

● Each macula contains supporting cells and

scattered receptors called hair cells

• Each of the hair cells has a major ciliary process

called kinocilium and several smaller stereocilia

• Lying over the supporting and hair cells is the

gelatin-like otolithic membrane

• The otolithic membrane contains the otoliths

(calcium carbonate crystals) on their surface

• The base of the hair cells are innervated by

branches of the vestibular branch of VIII nerve

1- Supporting cells 4- Otolith membrane

2- Hair cell 5- nerve fibers

3- Cilia 6- Otoliths

• The Maculae detect linear

acceleration forces.

• The Maculae in the-

Saccule:

Responsible for

vertical acceleration

Utricle:

Responsible for

horizontal acceleration

Saccule

Utricle

Maculae in Saccule & utricle

STATIC EQUILIBRIUM

Gravity, linear acceleration, or linear

deceleration pull the otoliths & otolithic

membrane resulting in

bending of stereocilia towards kinocilium

leads to generation of nerve impulses carried

by vestibular branch of vestibulocochlear (VIII)

nerve

Linear Acceleration

• When the hair bends

towards the kinocilium

the hair cell depolarize

Faster stream of impulse

is sent to the brain

• When the hair bends in

the opposite direction the

hair cells hyperpolarize

Slower impulse

generation

The maculae respond to gravitational pull and help to

maintain static equilibrium

Macula in Utricule

K

Macula in Saccule

K

Functions of otolith organs

• Give information about static position of

head

• Activate appropriate mechanisms to

maintain an equilibrium during resting &

linear movements

DYNAMIC EQUILIBRIUM

Maintenance of body position (head) in

response to rotational movements

Subserved by the hair cells present in

the semicircular canals (SCC)

The 3 SCCs are oriented

at right angles to one

another

1. Anterior & posterior

semicircular canals

(positioned vertically)

3. Lateral semicircular

canal

(positioned horizontally)

SCCs are filled with

endolymph

Semicircular Canals

• Respond to angular acceleration in the

plane of the canal.

• Are functionally paired:

Ampulla

• Each SCC has a dilated end called ampulla containing receptor organ crista ampullaris (small elevation)

• Each crista contains hair cells and supporting cells covered by cupula (gelatinous material)

Cupula

Cristae

The orientation of 3 canals permit detection of

rotational acceleration (dynamic equilibrium)

Macula in SSC

K K

Left side Right side

• Rotational acceleration or deceleration

of head causes endolymph to flow over

the hairs and bend them results in

nerve impulses being carried away by

vestibular branch of vestibulo cochear

nerve

Mechanism of Stimulation in SCCs 1- Angular acceleration of head 2- Motion of fluid in opposite

direction in SCC

3- Pressure on cupula of SCC

4- Deflection of stereocilia to /

away from kinocilium 5 - ↑/↓ K+ influx into hair cells

6- Increase/decrease firing rate

K

The cristae respond to rotational movements and are

involved in dynamic equilibrium

Functions of semicircular canals

• Give information about rotation movements

of the head & activate appropriate

mechanisms to maintain an equilibrium

• Helps in visual fixation during angular

rotation of head through vestibulo – ocular

reflex

Connections of vestibular apparatus

Vestibulo cerebellum

VCT fibers

Spinal cord III, IV & VI Cranial N. nuclei

VST fibers MLB

Integrate labyrinthine reflexes

Otolith organs

Semicircular canal

Vestibular nucleus

Vestibulo-Ocular Reflex (VOR)

STIMULUS =

Head movement

Efferent = oculomotor nerves

Effector = Extra-ocular muscles

Sensory = Vestibular HC

Afferent = vestibular nerve

Center

Vestibulo-Spinal Reflex (VSR)

STIMULUS =

Gravity linear acceleration Efferent = Spinal nerves

Effector = Neck and body muscles Sensory = Vestibular HC

Afferent = vestibular nerve

Center

The Visual System

• 2nd system that plays an important role in

equilibrium

• The effect of the visual system is best

noticed when there is a conflict between the

messages that reach the brain.

• Motion sickness is a very common

equilibrium disorder in which is due to

sensory input mismatch. The main

symptoms are: Nausea, dizziness, vomiting

and pale appearance

Motion sickness

Example -

– When we read while traveling, the visual

inputs indicates that the body is fixed. But

as the vehicle is moving the vestibular

system detects movement and send

impulses that conflict with the visual

system.

– The brain receives conflicting information

which will lead to motion sickness.

Propreceptive system

• Nerve endings that respond to muscle and tendon stretch are called proprioceptors (ex. Muscle spindle). They send signals to the brain, giving it information about the posture of the body.

• The proprioceptors are available in the muscles, ligaments, tendons, and joints. It which they can detect the initiation and termination of body movement.

• Note: In a sitting situation the system that is responsible for equilibrium in the PRORIORECEPTOR

Vestibular apparatus - Summary

• Plays an important part in regulation of

posture

• Otolith organ

i) Reflexly maintains upright position of

head and

ii) Adjusts position of the body to that of

head in space or relative to the trunk

“Equilibrator of the head”

CNS

1- Cerebral cortex

2- Brainstem

3- Cerebellum

2- Vestibular

3- Proprioceptive

1- Muscle commands

1-

2-

Systems regulating Posture

Applied aspects

Disorders of vestibular apparatus

• Nystagmus

• Vertigo

• Travelling sickness, sea sickness

• Meniere’s disease : vertigo, tinnitus & hard of hearing

Nystagmus

Involuntary movement of eyeball

•Reflex which helps in visual fixation while the

body rotates

•Recorded by electronystagmograpy

• Normally, right vestibule tends to deviate the eyes to the left & vice versa.

• With normal tone two effects are cancelled & the eyes remain in mid position.

• If the right vestibule is destroyed, the normal tone exerted by the left side pushes the eye to the right. This is corrected to the central position with a quick movement. So there is nystagmus to the left (normal side)

• If the right vestibule is hyperactive, nystagmus to the right occurs

• Nystagmus also produced in cerebellar disease

Electronystagmogram

• Utilizes charge

difference

between retina

and cornea to

detect eye

movement

Tests

• Caloric test

• Barany’s test

spin student safely

for 10 turns

halt student,

closely observe eyes

Barany’s test

Thank you