The Sensory Organs

 Your senses are means of detecting and interpreting stimuli of you external environment.  These are the sense of taste, sight, hearing, touch and smell.  A sensory organ is a specialised organ that allows for the reception of a stimulus, the formation of an impulse and it’s transmission to the brain for interpretation.  The main sense organs are your eyes, ears, olfactory organs in your nose, taste buds on your tongue and your skin.

 Light enters the eye, the amount of which is controlled by the iris and is bent (refracted) slightly by the cornea.  It is then focused by the lens (controlled by the ciliary muscles) onto the retina.  On the retina, rods and cones convert the light into a nerve impulse, carrying the image to the brain.  The brain interprets the images from both our eyes forming a composite image.

Short Sight (Myopia)  A short sighted person can view close objects clearly but distant objects are out of focus.  Light rays are focussed short of the retina.  Cause: eyeball is too long or the focussing elements of the eye are too strong.  Correction: use a concave (divergent) lens to widen the angle over which the light rays have to be refracted.

Long Sight (Hyperopia)  Condition: can view distant objects clearly but close objects are out of focus,  The focal point is long of the retina i.e. is behind retina.  Cause: eyeball is too short or the focussing elements of the eye are too weak.  Correction: use a convex (convergent) lens to reduce the angle over which the light rays have to be refracted.

 The ear has two major functions:  Hearing: detection of vibrations, their frequency (pitch) and amplitude (loudness).  Balance: detection of direction of motion, acceleration and head position related to gravity.  It is divided into three sections, the outer ear, middle ear and inner ear.

 The skin is the outer layer of vertebrate animals. Its major functions are protection, temperature regulation and to act as a sense organ.  Two major layers of the skin: epidermis and dermis. Epidermis  The epidermis is the outer renewable layer of the skin.  Malpighian Layer: This is the base layer, which is constantly producing new cells by mitosis. The new cells are pushed towards the surface. The dark pigment melanin is produced here.  Granular Layer: The protein keratin accumulates in the cells giving them a granular appearance. The cells finally die.  Cornified Layer: This is the surface layer of dead keratinised cells, which is constantly being eroded.

Protection  Prevents excessive loss of water (the cornified layer of the epidermis is waterproof).  Prevents the entry of pathogens.  ‘Sebum oil’ keeps the skin intact preventing it from ‘cracking’.  Sebum from the sebaceous glands contain anti- microbial chemicals.  Melanin gives protection against the damaging UV rays of sunlight.  The dermis and adipose tissue protect against mechanical injury.

Sense Organ  The skin contains receptors for touch, pressure, pain, temperature rise and temperature decrease. The skin supplies information about a variety of external environment conditions. Vitamin D Production  Made when ultraviolet light penetrates the skin converting a chemical in the blood to vitamin D. As a result vitamin D is often called the ‘sunshine vitamin’. Excretion  The skin has about 2.5 million sweat glands.  Sweat is a dilute solution of water, sodium chloride, urea, ammonia, uric acid and lactic acid.

Energy Storage  There is a layer of fat storage adipose tissue below the dermis of the skin.  Fat is also a poor conductor of heat and so the skin acts as a heat insulator.  This fat layer also acts as a shock absorber protecting against mechanical damage. The Skin is a Homeostatic Organ!

 The olfactory region is high up in the nasal cavity about 5 cm 2 in area with about 20,000 receptors.  Stimulatory chemicals must be volatile and soluble in water.  There are 50 primary smell qualities but in combination they can produce over 3,000 different odours.  Prolonged exposure to a particular chemical causes fatigue of that sensation.

 There are four primary taste qualities: bitter, sour, salty, sweet.  Most tastes are combinations of these.  The taste receptors are collected in groups of about ten in the taste buds.  Taste buds are present on the tongue’s edges and upper surface.  Taste buds are also present on the soft palate and on the back of the pharynx.  Certain regions of the tongue are more sensate to a particular primary tastes than others.