THE EAR AND EQUILIBRIUM LECTURER IN CHARGE BAMIDELE

THE EAR AND EQUILIBRIUM LECTURER IN CHARGE : BAMIDELE O. 1

INTRODUCTION • The sense of equilibrium is provided by structures in the internal ear. • This structures include: vestibular apparatus and semicircular canals. • Movement of the head cause the fluid in this structures to bend stereocilia of the sensory hair cells. • This bending result to the generation of action potentials. • The action potentials generated is transmitted to cerebellum, brain stem and spinal cord to maintain balance. 2

VESIBULAR APPARATUS AND EQUILIBRIUM • The sense of equilibrium which provides orientation with respect to gravity is as a result of the function of an organ known as vestibular apparatus. • Vestibular apparatus consists of two parts: 1. The otolith organs which is made up of the utricle and the saccule. 2. The semicircular canals The vestibular apparatus is located within the membranous labyrinth which is a tubular structure filled with a fluid similar in composition to intracellular fluid. The fluid is called endolymph. 3

VESIBULAR APPARATUS AND EQUILIBRIUM CONT'D • The membranous labyrinth is located within bony cavity in the skull called bony labyrinth. • Between the membranous and bony labyrinths is a fluid called perilymph. • Perilymph is similar in composition to the cerebrospinal fluid. • The utricle and saccule of the vestibular apparatus provide information about linear acceleration i. e changes in velocity when travelling horizontally or vertically. 4

Vestibule contains utricle and saccule • Each contains a macula • Senses static equilibrium and linear acceleration of the head (not rotational movements) • Tips of hairs imbedded in otolithic membrane (calcium carbonate “stones”) • Vestibular nerve branches of VIII (Vestibulocochlear n. ) 5

VESIBULAR APPARATUS AND EQUILIBRIUM CONT'D • We experience a sense of acceleration and deceleration when riding in a car or when skipping rope. • The receptors for equilibrium are modifield epithelial cells known as hair cells. • Each hair cells contains 20 -50 hairlike extensions (stereocilia). • The stereocilia consist of filaments of protein surrounded by part of the cell membrane. • One larger extension with the structure of true cilium is known as the kinocilium. 6

GENERATION OF ACTION POTENTIALS IN THE VESTIBULAR APPARATUS • Bending of stereocilia in the direction of the kinocilium depressed the cell membrane and makes it becomes depolarized and as a result of this sypnatic transmitter is released from the hair cell to stimulates the dendrites of sensory neuron that are part of the vestibulocochlear nerve (VIII). • But when the stereocilia are bent in the opposite direction, the membrane of the hair cell becomes hyperpolarized and less sypnatic transmitter is released. • The frequency of action potentials in the sensory neuron that innervate the hair cell relay messages about movement that cause the stereocilia to bend. 7

UTRICLE AND SACCULE • The otolith organs comprise utricle and saccule. Each contains macula which is a patch of specialized epithelium. • The macula consists of hair and supporting cells. The hair cells project into the membranous labyrinth with their extensions embedded in a gelatinous otolith membrane. • The otolithic membrane is made up of microscopic crystals of Ca. CO 3. (stone). The stone results in higher inertia. • The utricle is more sensitive to horizontal acceleration while saccule is more sensitive to vertical acceleration. • During forward acceleration, the otolithic membrane lags behind the hair cells so that the hairs of the utricle are push backward. 8

UTRICLE AND SACCULE CONT'D • The inertia of the otolithic membrane also causes the hairs of the saccule to be pushed backward when a person descends quickly in an elevator. • These effects and the opposite ones that take place when a person accelerates backward or upward generate a changed pattern of action potentials in sensory nerve fibres that allows us to maintain equilibrium with respect to gravity during linear acceleration. 9

SEMICIRCULAR CANALS • A sense of rotational or angular acceleration is provided by the semicircular canals. • Semicircular canal are oriented in three planes like the faces of a cube. • The canals are arranged in three different planes nearly at right angles to each other. • It helps us maintain balance when turning the head, spinning or tumbling. • Each canal contains an inner extension of the membranous labyrinth known as semicircular duct. 10

SEMICIRCULAR CANALS CONT'D • At base of each duct is an enlarged swelling called ampulla which houses a small crest: crista ampulla. • Hairs project into jellylike cupula which has a higher density than the surrounding endolymph & basilar cells synapse with fibers of vestibular nerve. 11

SEMICIRCULAR CANALS CONT'D • The endolymph of the semicircular canals serves a function similar to that of the otolithic membrane by provinding inertia so that the hair cells will be bent in a direction opposite to that of angular acceleration. • When the head rotates to the right, the endolymph causes the cupula to be bent to the left thereby stimulating the hair cells. • Hair cells in the semicircular canal are stimulated as follows: - anterior semicircular canal when doing a somersault - posterior semicircular canal when performing a cartwheel - lateral semicircular canal when spinning around the long axis of the body. 12

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EQUILIBRIUM PATHWAY • Via vestibular nerve branch of VIII (Vestibulocochlear n. ) to the brain stem • These fibres transmit impulses to the cerebellum and to the vestibular nuclei of the medulla oblongata. • The vestibular nuclei send fibres to the occulomotor center of the brain stem and to the spinal cord. • Neurons in the occulomotor center regulates the movement of eyes while spinal cord control the movements of the head, neck and limbs. • Movements of the eyes and the body produced by the pathway s serve to maintain balance and track the field visual field during rotation. 15

Neural pathway involved in the maintenance of equilibrium and balance 16

CLINICAL CORELLATES • Nystagmus: is a condition of involuntary (or voluntary, in some cases) eye movement , acquired in infancy or later in life, that may result in reduced or limited vision. Due to the involuntary movement of the eye, it has been called "dancing eyes"(Wikipedia) • People experiencing nystagmus may feel that they or the room are spinning. The loss of equilibrium that result is called vertigo • Vertigo is a natural response of the vestibular apparatus to spinning. It may be caused by anything altering the firring rate of one of the vestibule cochlear nerves e. g viral infection (vestibular neuritis). • Vertigo is accompanied by dizziness, pallor, sweating and nausea. 17

Thank you for your attention Stay safe 18