Ears - Organs of Hearing | Biology | NEET

Ears ( Organs of Hearing )

The ears perform two sensory functions , hearing and maintenance of body balance. Anatomically , the ear can be divided into three major sections called the outer ear , the middle ear and the inner ear

Outer ear comprises two parts , pinna and external auditory canal . Pinna is an oval , somewhat funnel shaped , skin - covered flap of elastic cartilage and muscles . It collects sound waves and directs them into the external auditory canal , a tubular passage supported by cartilage in exterior part and by bone in its inner part . It has ceruminous glands , that are modified sweat glands and secrete ear wax .

Middle ear is formed of an air filled cavity called tympanic cavity in the temporal bone that opens via auditory ( Eustachian ) tube into the nasopharynx . The tympanic membrane of an external ear separates the tympanic cavity from external auditory canal . There are also present ear ossicles which are a flexible chain of three small bones : the malleus , the incus and the stapes. The function of the ossicles is to transmit and amplify sound waves across the tympanic cavity from the tympanic membrane to the oval window ( fenestra ovalis ) . Stapes is the smallest bone in the body.

Inner ear is a delicate , irregular organ called membranous labyrinth surrounded by bony labyrinth and separated from it by a narrow perilymphatic space . This space contains a watery fluid called perilymph . Thus , membranous labyrinth floats in perilymph .

The coiled portion of the labyrinth is called cochlea . It is the main hearing organ . Internally , it consists of three fluid filled chambers or canals . The upper scala vestibuli , lower scala tympani and the middle chamber , called scala media.

Both scala vestibuli lower scala tympani are filled with perilymph and scala media is filled with endolymph .

Scala media bears an upper membrane , the Reissner's membrane , and lower membrane , basilar membrane . On the basilar membrane a sensory ridge , the organ of Corti is present . Organ of Corti consists of hair cells ( phonoreceptors ) , which bear ' hair ' at the free surface and have synaptic contacts with the dendrites of neurons at the bases . Above the rows of hair cells a thin elastic membrane called tectorial membrane is present .

The inner ear also contains a complex system called vestibular apparatus . The vestibular apparatus is responsible for maintenance of balance of the body and posture.

Mechanism of hearing

The outer ear receives sound waves and directs them to the tympanic membrane . The tympanic membrane vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles to the oval window . The vibrations are passed through the oval window on to the fluid of the cochlea , where they generate waves in the lymphs which induce ripple in the basilar membrane . These movements of the basilar membrane bend the hair cells , pressing them against the tectorial membrane . As a result , nerve impulses are generated that are transmitted by the auditory nerves to the auditory cortex of the brain , where impulses are analysed and sound is recognised.

MANISH MEVADA

M.Sc, M.Phil, B.Ed