Monday, September 12, 2005

Bringing to senses

Dr. K. Rohiniprasad

The brain has evolved from a primitive neural network in prehistoric life forms. The nervous system, which must have helped prehistoric aquatic creatures to avoid obstacles and predators and to seek food, developed five individual organs with specialised functions.

Our eyes evolved from light-sensitive projections of the brain as simple structures capable of differentiating light and darkness. The simplest eyes of jellyfish etc consist of groups of pigment cells covered with a cuticle forming a kind of lens. Some insects have compound eyes. Our ‘single’ eyes can make out minute variations of shape, colour, brightness and distance but the actual process of seeing is performed by the brain. The eyeball has a complex structure. The retina is largely composed of light-sensitive receptor cells packed closely together on the outer surface in front of a pigmented tissue layer. The optic nerve enters the eyeball, below and slightly to the inner side of a small round area of the retina. The eye merely translates the light into patterns of nerve impulses that are conveyed to the brain.

Our sense of smell is much inferior to that of dogs and other animals. In the nose the nerve of smell has several small branches terminating in the soft and thick mucous membrane in the nasal cavity. They end in tiny varicose fibres that in turn terminate in elongated epithelial cells projecting into the free surface of the nose. The seven primary odours that resemble camphor, ether and peppermint or can be called musky, floral, pungent or putrid are sensed by corresponding seven types of receptors in the olfactory-cell hairs. Substances with similar odours have molecules of similar shape and probably combine with specific chemicals within cells. This process results in the transmission of impulses through the olfactory nerve and the perception of odour by the brain.

Our ears evolved from a membranous air-filled floatation device of an ancient aquatic life form. The vibrations picked up by the nerves evolved to sense sounds in the vicinity for survival. In our ears sound waves pass through the external canal to the eardrum and its vibrations are picked up by a chain of tiny bones in the middle ear to the fluid in the inner ear. The movement stimulates in the cochlea a set of fine hair cells, which transmit signals directly to the brain via the auditory nerve. The overall pattern encodes information about sound that is interpreted by the auditory centres in the brain.

The tongue has taste buds located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. They are scattered over its surface, concentrated mostly at the back of the tongue. The mucous membrane lining these areas has tiny projections (papillae) with each of them having 200 to 300 taste buds. When a substance moistened by the salivary glands, its taste is determined by these taste buds. The back of the tongue transmits the sensation of bitterness while tip of the tongue transmits sweetness. Saltiness and sourness are transmitted from the sides of the tongue. Each taste bud has an opening at its base through which nerve fibres pass and these fibres transmit impulses directly to the brain, which senses the taste.

Touch, the least specialized of the senses, is perceived through the skin with nerve endings in all parts of the body conveying sensations to the brain via nerve fibres. Some complex form of nerve endings found in the sensitive pad of each finger, for example, have tiny swellings. All the sensory organs evolved by ‘trial and error’, as a means of survival, very long ago.


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