Monday, August 10, 2020

Neural Control and Coordination - Notes | Class 11 | Part 6: Sense Organs - Ear, Nose, Tongue, Skin

21. NEURAL CONTROL AND CO-ORDINATION

SENSORY RECEPTION & PROCESSING (SENSE ORGANS)


EAR (STATO-ACOUSTIC ORGAN)

It is the organ for hearing & balancing.

It has 3 divisions: External ear, middle ear & inner ear.


External ear 

Consists of pinna (ear lobe) & auditory meatus (ear canal).

At the opening of ear canal, hairs are seen.

Ear canal and skin of pinna contains ceruminous glands (modified sweat glands). They secrete wax (cerumen).

Wax and hairs protects ears from foreign objects.

Ear canal ends in tympanic membrane (Tympanum or ear drum). It is a semi-transparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside.

Middle ear 

Consists of tympanic cavity and ear ossicles.

Tympanic cavity is an air filled space that separates the external and inner ear portions.

An auditory tube (Eustachian canal) connects middle ear to the pharynx. It maintains an equal pressure on either side of the eardrum.

Ear ossicles include 3 small bones namely Malleus, Incus and stapes. Malleus is attached to tympanum.

Stapes is the smallest bone of the body. It is attached to membrane of oval window (fenestra ovalis) of inner ear.

Inner ear 

It consists of bony labyrinth & membranous labyrinth.

Bony labyrinth is a cavity filled with perilymph.

The membranous labyrinth consists of cochlea and Vestibular apparatus.

Cochlea (organ of hearing):

It is a coiled structure having 3 canals:
  • Upper scala vestibula
  • Middle scala media
  • Lower scala tympani
Scala vestibula & scala media are separated by Reissner’s membrane.

Scala media and scala tympani are separated by basilar membrane.

S. vestibula & S. tympani are filled with perilymph and scala media is filled with endolymph.

Resting on the basilar membrane and projecting into scala media is complex receptor organ called Organ of Corti. It consists of row of sensory hair cells. The hairs (stereo cilia) of these cells project upwards and lie in contact with tectorial membrane, which projects above them.


Vestibular apparatus:

It consists of 3 semicircular canals and otolith organ.

2 semicircular canals are vertical and one is horizontal. One end of each canal has a bulging called ampulla. Inside it is a lump called crista ampullaris. Long cilia of cells of crista are grouped together in a bundle (cupula).

Otolith organ consists of utricle and saccule.

Utricle & Saccule have a projecting ridge called macula.

Crista and Macula are specific receptors in vestibular apparatus. They contain sensory hair cells. They are responsible equilibrium & posture of body.

Mechanism of hearing 

Pinna collects sound waves → waves reach the tympanic membrane via ear canal → tympanic membrane vibrates → vibrations transmit to ear ossicles & oval windowperilymph in the vestibular canal vibrates → vibrations reach the scala tympani and force the basilar membrane to vibrate → hair endings of sensory hair cells press against tectorial membrane → sensory hair cells are excited → auditory nerve carries impulses to auditory centre of the brain → hearing.

NOSE


Organ of smell (olfaction).

It contains mucus-coated receptors (olfactory receptors) made up of olfactory epithelium. 

They receive sense of smell. It contains 3 kinds of cells.

The neurons of olfactory epithelium extend from the outside environment directly into a pair of broad bean-sized organs, called olfactory bulb. These are extensions of the brain’s limbic system.

TONGUE


Organ of taste (gustation).

4 primary tastes are sweet, salt, sour and bitter.

Taste buds (Gustatoreceptors + supporting cells) are seen around the bases of taste papillae.

Nose & tongue are chemoreceptors (detect dissolved chemicals). 

Senses of taste & smell are functionally similar and interrelated. The brain integrates different input from taste buds and a complex flavour is perceived.

SKIN (Cutaneous receptors)


Largest sense organ.

It contains receptors for heat, cold, touch, pain & pressure.
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