09 Control and Coordination - part 13

Ear :

I. External ear :

1.Ear pinna :

2. Auditory canal :

Ends at the ear drum.
Transfers the sound waves to the ear drum.
There are very fine hair and wax secreting sebaceous glands in the skin of pinna and auditory canal.

3. Tympanic membrane :

Is a delicate, membranous structure.
Transmits the sound waves to the middle ear.
Formed of connective tissues covered with skin on the outside and mucous membrane on the inside.

II. Middle ear :

On receiving the vibrations from the tympanic membrane, the ear ossicles amplify the vibrations and transfer these to the cochlea.
A short eustachian tube connects the middle ear to the pharynx.
It equalises air pressure on both sides of the ear drum.

III. Internal ear :

1. Labyrinth :

Bony labyrinth Filled with perilymph.
Membranous labyrinth Filled with endolymph .
Coiled portion of the labyrinth is cochlea.
Cochlea contains fluid filled three chambers separated by Reissner’s membrane and basilar membrane.
Upper chamber towards vestibul is called scala vestibuli and the bottom chamber scala tympani are filled with perilymph.
Middle chamber is the scala media. It is filled with endolymph .

Organ of Corti :

Pea sized structure located on basilar membrane (floor of scala media).
The organ of corti has a sensory epithelium over the basilar membrane.
Sensory epithelium is in contact with a gelatinous tectorial membrane.
Sensory cells have sensory hair on their free end so also called hair cell.
In between the rows of hair cells are present supporting cells.
Hair cells have long stiff microvilli called stereocillia on their apical surfaces.

Tectorial membrane :

Above these stereocellia, is a jelly like membrane called tectorial membrane.
This organ acts as a transducer, converting sound vibrations into nerve impulses.

Inner Ear and the mechanism of balance :

2. vestibular apparatus :

Besides the cochlea, the inner ear also has the vestibular apparatus which is composed of three semi-circular canals and the utriculo saccular region with the otolith organ.
All three semi-circular canals lie in different planes at right angle to each other.
These canals are filled with endolymph.
The base of each of the canal has an ampulla in which there is a sensory spot called crista.
Cristae help in maintaining equilibrium.
The vestibule has two sensory spots-

Macula consist of hair cells and supporting cells.
Tips of the hair and cilium project into a thick gelataneous sheath otolithic membrane.
Within this membrane minute particle otoliths or otoconia are secreted.
These are made of CaCO3 and protein.
Macula and crista are the receptors sensitive to the position of the head with respect to gravity.
Three semicircular canals are arranged such away that the movement in any plane can be detected by these cells and the balance and posture of the body is maintained.
Receptors for dynamic balance lie in the cristae of ampullae while for static/linear balance these are in the maculae of utriculus and sacculus.

Mechanism of Hearing :

Pinna of the ear receives the sound waves and directs them to eardrum.
Eardrum vibrates and these vibrations are amplified and transmitted through the ear ossicles to the endolymph inside cochlea.
This generates, wave in the endolymph.
These waves induce ripples in the basilar membrane.
These movements in the basilar membrane cause the hair cells to press against tectorial membrane.
This generates nerve impulse in the afferent neurons.
Impulse is sent to the brain via the auditory nerve.
Auditory cortex of the brain decodes the sound.

Yogy's biology