Ear

B J E C T I V ES

• Describe the anatomy and the structures of organs of equilibrium.
• List the major events in the physiology of equilibrium.
• Identify the vestibular receptor, and describe how they function.
Describe vestibular functions.
• Describe the vestibular pathway.
Vestibular dysfunction.
Equilibrium

Dancing with music

dynamic equilibrium
Organ of equilibrium
dynamic: crista ampularis (hair cells + cupula) of the semicircular canals
nerve : vestibular nerve
static: maculae

Equilibrium:

• The Sensory organ
• is vestibular apparatus which is encased in a system of bony tubes & chambers located in the petrous portion of the temporal bone.
• bony labyrinth :
• within this system are membranous tubes & chambers the membranous labyrinth is the functional part of the vestibular apparatus

Vestibular functions

• Gives subjective sensation to motion & spatial orientation along with visual, proprioceptive and cutaneous (exteroceptive) , inputs:
• Vestibular input to regions of the nervous system controlling eye movements:
• a. It helps stabilize the eye in space during head movements and
• b. reduced movement of the image of a fixed object on the retina (vestibulo-ocular reflex VOR)
• Responding to gravity & acceleration this maintains:
• body posture by labyrinthine reflexes
• Equilibrium by vestibulocerebellar connections

• Vestibular apparatus

• Vestibular portion of the labyrinth (filled with endolymph), it consists of:

• Vestibule: composed of utricle and Saccule, Its responsible for linear movements acceleration and deceleration
2.The 3 semicircular canals :
Responsible for
rotational movements.
•

• Vestibular receptors

• Macula (otolith organ) in
Utricle ,oriented in horizontal plane it Responds to:
• changes in head position fore & aft lift
• linear acceleration in horizontal plane
• running
• Saccule oriented in vertical plane Responds to:
• changes in head position lateral lift
• linear acceleration in vertical plane jumping down.
Crista ampularis
• in each of the expanded ends (Ampulla) of the 3 semicircular canal
• detect angular/rotational acceleration

Detection of linear acceleration

• When the body is suddenly thrust forward-that is, when the body accelerates-
• the statoconia, which have greater mass inertia than the surrounding fluid, fall backward on the hair cell cilia, and information of dysequilibrium is sent into the nervous centers, causing the person to feel as though he or she were falling backward.
• This automatically causes the person to lean forward until the resulting anterior shift of the statoconia exactly equals the tendency for the statoconia to fall backward because of the acceleration.
• At this point, the nervous system senses a state of proper equilibrium and leans the body forward no farther.
• Thus, the maculae operate to maintain equilibrium during linear acceleration in exactly the same manner as they operate during static equilibrium

Detection of rotational acceleration

-






when stereocilia bend towards kinocilium –> stimulation -when sterocilia bend away from kinocilium –> inhibition
• Rotational acceleration in the plane of a given semicircular canal stimulates its crista.
• The endolymph, because of its inertia, is displaced in a direction opposite to the direction of rotation. The fluid pushes on the cupula, deforming it. This bends the processes of the hair cells.

• When a constant speed of rotation is reached, the fluid spins at the same rate as the body and the cupula swings back into the upright position.
• When rotation is stopped, deceleration produces displacement of the endolymph in the direction of the rotation, and the cupula is deformed in a direction opposite to that during acceleration. It returns to mid position in 25 to 30 s.
• Movement of the cupula in one direction commonly causes an increase in the firing rate of single nerve fibers from the crista, whereas movement in the opposite direction commonly inhibits neural activity

• Vestibular connections

• Vestibular pathways

• Vestibular dysfunction
• Impairment
• loss of equilibrium & postural adjustments
• Absence of nystagmus on vestibular stimulation
• caloric test:
• setting up convection currents in endolymph of the lateral semicircular canal (made vertical) by instilling water hotter/cooler than body temperature into external auditory canal
• Overstimulation
• motion sicknss
• giddiness
• nausea
• vomiting
• irritative lesions in vestibular pathways
• vestibular neuronitis