21. NEURAL CONTROL AND CO-ORDINATION
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.
Two eyes are located in sockets of the skull called orbits.
The adult human eyeball is nearly spherical.
Eyeball has three layers: Sclera, Choroid & Retina.
- The external layer formed of a dense connective tissue.
- Anterior transparent portion of sclera is called cornea.
- 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 visible pigmented and opaque portion of the eye called the iris.
- 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.
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).
- 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.
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 → 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 memory and experience → vision.
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