Anatomy and physiology of the peripheral and central auditory system

Lecture 2(B)

Dr. Ghulam SaqulainM.B.B.S., D.L.O, F.C.P.S

Head of Department of E.N.TCapital Hospital

INNER EAR

It exists within the temporal bone (petrous portion).

It is a complex structure. It is located in a bony cavity called the BONY LABYRINTH

It is filled with a fluid called PERILYMPH, which is similar to CSF.

Bony labyrinth:

Series of cavities & canals Contain –Perilymph

1.Vestibule (centre)2.Cochlea (anterior)3.Semicircular canals(post)

Ant.

Post.

Within the bony labyrinth is the MEMBRANOUS LABYRINTH, filled with ENDOLYMPH.

Membranous Labyrinth has two parts: Vestibular Labyrinth:

Semicircular Canals 3 Lateral, posterior and superior

(Crista ampularis) Angular acceleration

Utricle and Saccule Utricle 5 openings of semicircular canals

(Macula) Linear acceleration

Cochlea

Central axisModiolus

Cochear canalruns 2 1/2*turns.

modiolus

Cochlear Duct: Scala vestibuli,Scala

tympani, Scala Media Organ of Corti

Openings -Fenestra vestibuli Closed by ? -Fenestra Cochleae Closed by ?

Hair Cells (Inner and outer) Transduction of Mechanical

Energy to Electrical Energy

Cochlear canalDivided intoDivided into SCALA VESTIBULI SCALA TYMPANIby by Spiral lamina& & Basilar membrane

COCHLEAR DUCTwithin C.Canalwithin C.Canal((roof-Vesti membr.roof-Vesti membr.Floor-spiral lam.& basilar membr.)Floor-spiral lam.& basilar membr.)

Receptor Organ(Hearing)ORGAN OF CORTI...

COCHLEA

Organ of Corti..

Receptor organs (Hearing) Organ of Corti

Specialised org.of Hearing

within Cochlear duct on the basilar membr.

C.S. Cochlear duct

Scala vesti.

Scala tympani

Basilar membr.

Spiral lamina

contains Hair cells impulses carried by VIII N.

Receptors for Balancing

1. Macula (thickening)

in the walls of Saccule Utricle

2. Crest in Ampullae of Semicircular ducts

Basilar Membrane

Forms the floor of scala media 35 mm long Auditory nerve endings are located on the BM Organ of corti resides on the BM

Width: Base (0.1 mm) = narrow & stiff Apex (0.5 mm) = broad/wide & flaccid opposite to cochlear duct’s width Reacts more to vibrations of IE than do most of the other structures

Hair Cells (HCs)

Hair cells lays down on the fibrous BM 3-5 rows of 12,000 to 15,000 parallel outer hair cells (OHCs) One row of 3,000 inner hair cells (IHCs)

On top of each hair cell are hair-like projections called “Stereocilia” Stereocilia on the top of OHCs are embedded in the Tectorial Membrane

“gelatinous flap”

The direction in which stereocilia are bent during stimulation:

If cilia bend in one direction the nerve cells are stimulated If cilia bend in the other way the nerve impulses are inhabited If cilia bend to the side no stimulation at all

Hair Cells (HCs)

Each IHC is supplied by 20 nerve fiber, each nerve fiber only contacting only one hair cell

Each OHC is supplied by many different nerve fibers and a given nerve fiber goes to several OHCs

Nerve fibers exits cochlea and extends centrally toward the modiolus

Cell bodies of nerve fibers group together to form the Spiral Ganglion

Nerve fibers pass the modiolus to form the cochlear branch of the auditory nerve (VIII Cranial nerve)

Cochlear Fluids

1) Perilymph: Found in Scala vestibuli and Scala tympani High in its concentration of sodium and low in potassium Exhibits a small positive direct current (DC) potential

1) Endolymph: Found in Scala media High in its concentration of potassium and low in sodium Exhibits a strong positive DC potential

Reminder of cochlear structures exhibits a negative DC potentials

When OW is moved by the stapes, the annular ligament around the footplate stretches a displaces the perilymph at the basal turn of the cochlea

A sound wave is propagated toward the apex of the cochlea Because the IE fluids are incompressible, they are inwardly

displaced as the OW, the RW membrane must yield moving into the ME

Sound vibrations introduced to the SV are conducted into the cochlear duct by yielding of Reissner membrane

Endolymph is then disturbed, so the vibration continues and the BM is displaced also resulting in the release of RW membrane

Sounds introduced to the IE cause a wavelike motion that is always moving from the base to the apex of the cochlea

BM reacts more to vibration of IE than do most of the other structures

Physiology of the Cochlea Mechanism of sound analysis by inner ear (cochlea) and brain stem(auditory pathway )

Inside the cochlea are special neurons called HAIR CELLS; their axons form CN VIII.

The stapes is attached to the OVAL WINDOW, and vibrations cause the perilymph to vibrate; the hair cells here transmit this vibration.

Therefore the HAIR CELLS in this region are receptors for HEARING.

Stereocilia on tips of OHCs are embedded in the tectorial membrane

When BM moves up and down in response to fluids displacement, HCs are sheared (twisted) in a complex manner

BM and tectorial membrane move in opposite directions as they move up and down

The size of electrical response of the cochlea is directly related to the extent to which HCs or the ciliary projections at their tops are sheared

When the cilia are sheared, a chemical is released at the base of the HCs

Here in the inner ear the sound (mechanical energy) is converted into electrical energy (action potential) which travels through the auditory nerve the brain

The brain perceives this action potential as a sound along with its characteristics.

COCHLEAR MICROPHONIC

Resemblance between Cochlea and microphone in their function HOW? Cochlea converts sound waves into an energy form useful to the

auditory nerve Microphone converts the sound pressure coming from a speaker’s

mouth into an alternating electrical current

→ This action is called: Cochlear Microphonic (CM)

A result of changes in polarization caused by the bending back and forth of hair cells cilia

For every up and down cycle of BM, there is one in and out cycle of stererocilia of the OHCs causing them to become alternatively depolarized and hyperpolarized

CM can be measured by placing needle electrode over the RW or within the cochlea

AUDITORY NEURONS

Afferent (sensory) neurons About 30,000 Carries impulses from the cochlea to the central auditory

nervous system (CANS) They have cell bodies in the spiral ganglion in the modiolus Efferent (motor) neurons About 1,800 Carries impulses from the CANS back to the cochlea The act of conveying information between neurons is called:

“Neurotransmission”

THE ACTION POTENTIAL

A change in the electrical potential occurring on the surface of each neuron after they are being stimulated by HCs

Increases in the intensity of the auditory input signal to the cochlea result in increased electrical output from HCs

This stimulation causes increased electrical activity in the neuron