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 cm2 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.