Sense Organs: Overview
Sense organs detect all types of changes in the environment and send appropriate signals to the CNS, where all the inputs are processed and analysed. Signals are then sent to different centers of the brain. Sense organs are also called receptors.
We smell things with our nose, taste with our tongue, hear with our ear, and see objects with our eye. The nose contains mucus-coated receptors specialized for receiving the sense of smell, called olfactory receptors, which open into the olfactory bulb, an extension of the brain.
Both the nose and the tongue detect dissolved chemicals; the chemical sense of gustation (taste) and olfaction (smell) are functionally similar and interrelated. The tongue detects taste through taste buds, which carry gustatory receptors.
The Eye
Eyes are the sense organs of vision. They are sensitive detectors of light, hence called photoreceptors. The eye is situated in a deep, protective bony cavity of the skull called the orbit, or eye socket. It is about 2.5 cm in diameter and weighs about 7 gm.
Protection of the Eye
- Eyebrows are present on the supraorbital region.
- Eyelashes: some glands open at the base of the follicle of the eyelashes, called Meibomian glands (modified sebaceous glands), which secrete oil that lubricates the eyeball and reduces friction during movement.
- There are three eyelids, of which 2 are functional eyelids, and one is the reduced, vestigial nictitating membrane.
- Adipose tissue surrounds the eyeball.
- The lacrimal (tear) gland is present on the outer side of the eye, just inside the orbit. It is almond-shaped and secretes its product in the form of tears (containing lysozyme), by way of a duct.
- Glands are present on the eyelid edges: the gland of Zeis and the gland of Moll. Infection of the gland of Zeis by bacteria causes the disease called sty (stye).
Structure of the Human Eye
In the human eye, the wall of the eyeball consists of 3 layers:
- Outermost layer: consists of the sclera and the cornea.
- Middle layer: consists of the choroid, the ciliary body, and the iris.
- Innermost layer: the retina. It has photoreceptor cells called rods and cones.
Outermost Layer
- The sclera is an opaque, white, outermost covering, composed of dense connective tissue. It maintains the shape of the eyeball.
- The cornea is a thin, transparent, front part of the sclera, which lacks blood vessels but is rich in nerve endings. The cornea allows light to pass into the eye.
- The conjunctiva protects the cornea and also secretes oil and mucus that moisten and lubricate the eye.
Middle (Vascular) Layer
- The choroid is present beneath the sclera and contains numerous blood vessels that nourish the retina.
- It becomes thick in the anterior part, forming the ciliary body, which holds the lens in position.
- The iris is the visible, coloured portion of the eye, a pigmented circle attached to the ciliary body in front of the lens.
- The diameter of the pupil is regulated by the muscle fibres of the iris.
Inner (Nervous) Layer: Retina
The inner layer is the retina, and it contains different layers of cells, from inside to outside: the layer of ganglion cells, the bipolar cell layer, and the photoreceptor and pigment cell layer.
The photoreceptor cells are of two types, rods and cones. Rods contain a photosensitive pigment called rhodopsin. Cones are of three types, possessing characteristic photopigments (collectively called iodopsin, or photopsins) that respond to red, green, and blue light.
Daylight (photopic) vision and colour vision are functions of cones, while dim-light (scotopic) vision is a function of rods.
- The space between the cornea and the lens is called the aqueous chamber, filled with aqueous humour. It helps maintain the shape of the front part of the eye and also provides nutrition to the cornea and the lens.
- The space between the lens and the retina is called the vitreous chamber, filled with a transparent, jelly-like fluid called vitreous humour.
- The optic nerve carries fibres of sensory neurons and leaves the eyeball from the back, at a point called the blind spot or optic disc, which lacks rods and cones.
- At the posterior pole of the eye, just opposite the lens, there is a yellowish, pigmented spot called the macula lutea, with a central pit called the fovea centralis.
- The fovea centralis is the part of the retina where only cones are densely packed, and it is the point where resolution is greatest (where the sharpest image is formed).