SENSE ORGANS
These are the organs that detect the changes in the environment and convey the information to the CNS. It includes eye, ear, nose, tongue & skin.
EYE
Two eyes are located in sockets of the skull called orbits.
The adult human eyeball is nearly a spherical structure.
Eyeball has three layers: Sclera, Choroid & Retina.
a. Sclera
- The external layer formed of a dense connective tissue.
- Anterior transparent portion of sclera is called cornea.
b. Choroid
- Bluish middle layer. Contains many blood vessels.
- Choroid is thin over posterior two-thirds of the eyeball, but it is thick in the anterior part to form ciliary body.
- Ciliary body continues forward to form a pigmented and opaque structure called the iris .It is the visible coloured portion of the eye.
- Iris has a central opening called pupil. The diameter of the pupil is regulated by the muscle fibres of iris. This helps to regulate the amount of light entering the eye.
The eyeball contains a transparent crystalline lens. It is held in place by ligaments attached to the ciliary body.
c. Retina
- Inner layer. It contains 3 layers of cells – from inner to outer – ganglion cells, bipolar cells & photoreceptor cells.
- Photoreceptor cells are 2 types: rods and cones. They contain photosensitive proteins (photopigments).
- Photopigments are formed of opsin (a protein) and retinal (an aldehyde of vitamin A).
Cone cells:
Function: Daylight (photopic) vision & colour vision.
There are 3 types of cones containing photopigments (photopsin) that respond to red, green and blue lights.
The sensations of different colours are produced by combinations of these cones and their photopigments.
When the cones are stimulated equally, a sensation of white light is produced.
• Function: Twilight (scotopic) vision.
They contain a purplish-red protein called rhodopsin (visual purple). It contains a derivative of Vitamin A.
Rod cells:
- Slightly above the posterior pole of the eyeball, there is a region where optic nerves leave the eye and retinal blood vessels enter it. Here, photoreceptor cells are absent. It is called blind spot.
- Lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central pit (fovea).
- The fovea is a thinned-out portion of the retina where only the cones are densely packed. It is the point of greatest visual acuity (resolution).
The space between the cornea and lens is called aqueous chamber. It contains aqueous humor (thin watery fluid).
- The space between the lens and retina is called vitreous chamber. It contains vitreous humor (a transparent gel).
Mechanism of vision
Light reflected from an object → enters the eye through cornea & lens → focus on retina → dissociation of retinal from opsin → changes in membrane permeability → generates potential differences (impulse) in photoreceptor cells (rods & cones) → generates action potentials in ganglion cells through bipolar cells → impulses are transmitted by optic nerves to brain (visual cortex) → impulses are analyzed and the image is recognized based on earlier memory and experience → vision.
EAR
1.It is the organ for hearing & balancing.
2.It has 3 divisions: External ear, middle ear & inner ear.
External ear-
1. Consists of pinna (ear lobe) & auditory meatus (ear canal).
2.At the opening of ear canal, hairs are seen.
3. Ear canal and skin of pinna contains ceruminous glands (modified sweat glands). They secrete wax (cerumen).
4.Wax and hairs protects ears from foreign objects.
5.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-
1.Consists of tympanic cavity and ear ossicles.
2.Tympanic cavity is an air filled space that separates the external and inner ear portions.
3.An auditory tube (Eustachian canal) connects middle ear to the pharynx. It maintains an equal pressure on either side of the eardrum.
4.Ear ossicles include 3 small bones namely Malleus, Incus and stapes. Malleus is attached to tympanum.
Inner ear-
1. It consists of bony labyrinth & membranous labyrinth.
2. Bony labyrinth is a cavity filled with perilymph.
Cochlea (organ of hearing):
It is a coiled structure having 3 canals - upper scala vestibula, middle scala media and lower scala tympani.
Scala vestibula and 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 pass through ear canal → reach the tympanic membrane → tympanic membrane vibrates → vibrations transmit to ear ossicles & oval window → perilymph 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 .
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.
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.