NEURAL CONTROL AND COORDINATION

 

Middle ear :

• Middle ear contains three ear ossicles called Malleus (hammer), Incus (anvil) and stapes (stirrup).
• The Malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea.
• The ear ossicles amplify the sound waves comes from the tympanic membrane.
• A Eustachian tube connects the middle ear cavity with the pharynx.
• Eustachian tube helps in equalizing the pressures on either sides of the ear drum.

Internal ear :

• The fluid filled internal ear is called labyrinth consists of two parts, the bony and membranous labyrinth.
• The bony labyrinth is a series of channels, inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph.
• The membranous labyrinth is filled by a fluid called endolymph.
• The labyrinth consists of two portions –
  • The coiled portion called cochlea.
  • The complex above the cochlea called vestibular apparatus.

Cochlea:

• The coiled portion of the labyrinth is called cochlea.
• The membrane constituting cochlea are-
  • The reissner’s membrane
  • The basilar membrane.
  
  
• Reissner’s and basilar membrane divide the surrounding perilymph into an upper scala vestibuli and lower scala tympani.
• The space within cochlea called scala media is filled with endolymph.
• At the base of the cochlea, the scala vestibule ends at the oval window (fenestra ovalis), while scala tympani terminate at the round window (fenestra rotundus) which opens into the middle ear.
• The organ of corti is a structure located on the basilar membrane which contains hair cells that act as auditory receptors.
• The basal end of hair cells is in close contact with the afferent nerve fibres.
• Hair cells contain stereo cilia projected from the apical part of each hair cell.
• Hair cells covered by a thin elastic membrane called tectorial membrane.

Vestibular apparatus:

• Vestibular apparatus located above the cochlea.
• Vestibular apparatus consists of –
  • Three semi-circular canals
  • Otolith organ consisting saccule and utricle.
  
  
• Each semicircular canal lies in a different plane at right angles to each other.
• Membranous semi-circular canals are suspended in the perilymph of bony canal.
• The base of canals is swollen and is called ampulla, which contain a projecting ridge called crista ampullaris with hair cells.
• The saccule and utricle contain a projecting ridge called macula.
• Crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body and posture.

Mechanism of hearing:

• The external ear receives sound waves and directs them to the ear drum.
• Sound waves are amplified by the ear ossicles and send it to the oval window in the middle ear.
• The vibration of the oval window creates waves in the perilymph of scala vestibuli.
• The waves in perilymph induce a ripple in the basilar membrane.
• Movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane.
• As a result nerve impulses are generated in the associated afferent neuron.
• These impulses are transported to the auditory cortex of the brain where the impulses are analysed and the sound is recognized.

 

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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