five senses provide sensory input when they react to stimuli by
transmitting an action potential sent to the Central Nervous System.
These are classified into five types of sensory receptors, the
mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors and
electroreceptors. Mechanoreceptors detect and respond to hearing,
balance and stretching. Photoreceptors are, in one word, eyes.
Chemoreceptors are associated with smell, taste and the digestive and
circulatory systems. Thermoreceptors detect and respond to heat and
cold. Electroreceptors detect and respond to electricity.
The ear converts sound waves into vibrations that vibrate a liquid in the inner ear. These in turn make hairs, also in the inner ear, vibrate. A sensory dendrite then creates action potentials defined by these vibrations which are sent along the auditory nerve to the brain. Loud sounds can cause damage to these hair cells by creating large vibrations that tear them.
These hairs also serve the purpose of detecting orientation and gravity. They are situated along three planes and respond to fluctuations of the liquid mentioned earlier. Hence we are able to maintain our balance and sense if we are falling or not.
Have you ever wondered why things seem brighter in the corner of your eye than in the centre or that they seem more colourful in the centre than in the corner? Well, there are two types of photoreceptor cells that work together to produce our vision. Rods, found more towards the edge of the eye, detect light intensity while cones, found further in the centre, detect colour. This explains the phenomena above.
When light enters the eye, it causes a chemical to decompose (relax, this does no harm) and action potentials defined by these chemical reactions are created and sent to the occipital lobe (there are four lobes of the cerebrum, mentioned in the brain.) via the optic nerve.
[©Martin and Ambrose 2001]