Human senses B: Smell, taste and
touch
Smell
Our sense of smell warns us about approaching danger
(e.g. fire) and possible food poisoning ("off fish! Phew!").
It also aids digestion (yummy odours stimulate salivation) and can heighten
sexual awareness.
The smell receptors of the nose are sensitive to chemicals present
in a gaseous state. As the chemicals dissolve in the mucous lining of
the nasal passage, they stimulate sensory receptors which send electrical
impulses via nerve cells in the olfactory nerve to the brain. In the
brain these electrical messages are interpreted so that we can distinguish
the type and strength of the smell .
Taste
The tongue is the organ of taste having receptors which are sensitive
to chemicals in a liquid or soluble form.
There
are separate taste buds for each of the four different pure taste sensations:
sweet, salty, sour, and bitter.
These different taste buds are concentrated in specific areas of the
tongue. Any foods with unusual or "bad" tastes can be spat
out before swallowing, preventing spoiled or poisoned food entering
and harming our digestive system.
The sense we call taste is usually a combination of taste and smell.
Pure sugar would be tasted on the tongue alone, but more complex tastes
are mainly sensed by the nose. Many foods taste very bland when the
nose is blocked with a head cold.
When different chemicals stimulate the nerve endings in the taste buds
and smell receptors, electrical impulses pass to the brain which recognises
the various combinations of tastes in relation to prior experience e.g.
coffee, banana or peach.
People's sensitivity to detecting tastes reflects their overall number
of taste buds, though recognition of minor taste variations (such as
in wine-tasting) involves learning too!
Touch
The skin is the organ of touch. In the underlying dermis layer
are located many different touch receptors sensitive to heat, cold,
touch, pressure and pain.
The distribution of receptors varies, with the fingertips having an
abundance of touch and pressure receptors. This is of great survival
benefit as it aids maximum sensitivity during delicate and intricate
hand movements, which are a truly human feature, allowing the use of
tools and writing. Similarly, it is advantageous to have fewer receptors
on the soles of the feet so that walking on rough surfaces is not too
painful.
"Pain" is the body's warning mechanism of danger and possible
damage. Messages from the various sensory receptors travel to the brain
which registers the location and extent of the painful stimulus.
Local anaesthetics work by dulling the nerve cells in a particular
area. For example, a dental anaesthetic allows a dentist to drill painlessly
in preparation for a filling. The impulses cannot get through to the
brain so no pain is registered or felt.
Some pain-relief tablets work by raising the threshold of pain in
the pain centre of the brain: a much stronger pain stimulus is needed
before the brain "feels " it. Other medications can reduce
transmission of signals to the brain.
In some cases of severe, prolonged pain such as in chronic "tennis
elbow", the nerves may need to be cut to prevent impulses getting
through at all.
|