Physiology of

External, Middle and Inner Ear

Dr. Karishma R PandeyAssistant Professor

Dept. of Basic And Clinical PhysiologyBPKIHS

1) Structure and Function of External , middle and inner ear

2) Organ of corti

3) Auditory transduction

4) Impedence matching

5) Cochlear amplification

Learning objectives

The external ear funnels sound waves

External auditory meatus

Tympanic membrane(eardrum).

External Ear

Air-filled cavity in the temporal bone

Opens via the auditory (eustachian) tube into the nasopharynx and through the nasopharynx to the exterior.

Middle Ear

Three auditory ossicles:malleus, incus, and stapes

Two small skeletal muscles: tensor tympani and stapedius

The tube is usu. closed, opens during swallowing, chewing, and yawning.

Thus keeps the air pressure on the two sides of the eardrum equalized.

Inner EarThe inner ear (labyrinth) is made up of two parts, one within the other.

Bony labyrinth is a series of channels in the petrous portion of the temporal bone

Inside these channels, surrounded by a fluid called perilymph, is the membranous labyrinth.

It is filled with a K+-rich fluid called endolymph

No communication

Throughout its length, the basilar membrane and Reissner's membrane divide it into three chambers or scalae

Bony walls of the scala vestibuli are rigid, Reissner's membrane is flexible

Cochlea

Coiled tube 35 mm long Makes a two and three quarter turns.

Organ of CortiLocated on the basilar membrane,contains the hair cells (auditory receptors).

four rows of hair cells :three rows -- outer hair cellsone row -- inner hair cells

Processes of the hair cells pierce the tough, membrane-like reticular lamina that is supported by the pillar cells or rods of Corti

Thin, viscous, but elastic tectorial membranein which the tips of the hairs of the outer hair cells are embedded.

Modiolus, the bony core around which the cochlea is woundwithin it spiral ganglion is located comprises of cell bodies of the sensory neurons around the bases of the hair cells

Axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve.

Stereocilia, are present in all hair cells

Inner hair cells -- primary sensory cells --generate action potentials in the auditory nerves, They are stimulated by the fluid movements

Outer hair cells on depolarization becomes short and hyperpolarization becomes longIncreases the amplitude and clarity of sounds.

Resting membrane potential of the hair cells = –60 mV

Stereocilia are pushed toward the kinocilium = –50 mV.

Processes is pushed in the opposite direction = hyperpolarized.

Hair processes provide a mechanism for generating changes in membrane potential proportional to the direction and distance the hair moves.

The ionic composition of endolymph is same as intracellular fluid of hair = so no diffusion of ions

When transduction channels open large potential difference of 150mV exist between the endolymph (80 mV) and the hair cell interior is -70 mV that drives the cation into the steriocilia

The depolarization causes voltage gated Ca channels at the base of the hair cells to open and allow more influx of Ca which in turn causes release of neurotransmitter

The hair cell depolarization also opens up voltage sensitive K channels at the base of hair cells, thus K ions diffuse out into perilymph which has low conc of K and thus restores the resting potential of the hair cells. Ca pump restores the Ca ions inside the cell

K+ that enters hair cells via the mechanically sensitive cation channels is recycled

It enters supporting cells then to other supporting cells by way of tight junctions eventually reaches the stria vascularis secreted back into the endolymph

Scala media is electrically positive by 85 mv relative to the scala vestibuli and scala tympani.

Impedence Matching

brought about 2 ways1. Force exerted by the sound on the tympanic membrane is

concentrated on a much smaller area of the stapes of the footplate—it result in pressure gain

2. The mechanical advantage of the ossicle is higher than 1 which produces further pressure gain

Cochlear Amplification

Outer hair cells amplify the sound

Depolarization of OHC= causes them to contract which pulls the Basilar membrane and thus amplifies its movement