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SENSES, SENSITIVITY, STIMULI, and SKIN

The theme for the next few units is CO-ORDINATION. This is a process of control of the functioning of all an individual's organs and systems, so that it operates harmoniously. In most animals, it is generally brought about by 2 means: the nervous system, and the action of "chemical messengers" called hormones.
Sensitivity (also known as irritability) is one of the 7 characteristics of life, but it is especially important in animals, because they need to react quickly to a variety of different information. It is defined as the responsiveness of organisms to changes in their immediate environment. These changes - also called stimuli - might originate either outside the organism, or inside it.
Skin is a remarkable organ. It has a large surface area, and acts as an interface between the vulnerable cells inside the body and the non-living environment.
As well as having a role in keeping body temperature stable, and keeping out infecting micro-organisms, skin is also a sense organ.

The human body has a number of other sense organs which mostly detect changes in the external environment, as well as sensors which monitor the internal environment, and play a part in homeostasis.
Each of these sense organs provides information which is passed as an input into the nervous system and processed in some way, perhaps by the brain, resulting in a sensation . It may also produce a response which is an output (see later).

Complete the table below, about the body's sense organs.

Sense organ	Sense/senses:	Sensitive to (factors)
> `skin`	> `touch`	> `touch` - `pressure, heat, cold`
> `tongue`	> `taste`	> `chemicals in food`
> `nose`	> `smell`	> `chemicals in air`
> `ears`	> `hearing` `& balance`	> `sounds` `& gravity/movement`
> `eyes`	> `sight`	> `light`

Give an example of "monitoring the internal environment":
hunger/thirst

List below the different stimuli to which different nerve endings in the skin respond and show the nerve endings responsible: (The diagrams have been marked with letters).

sections through skin

a> heavy pressure
b> light pressure
c> pain
d> heat and cold
e> pulling hairs

In what way do the functions of nerve endings detecting pressure deep inside the skin differ from ones near the surface?
> nearer the surface - light pressure (fine sensation)
deeper- heavy pressure (coarse sensation/ pain!)

On the main entry points for substances taken into the body, skin has additional sensory and protective functions.
These are provided by the taste buds of the tongue and the olfactory organs of the nose (nasal cavities).

What substances are usually entering at each of these "entry points"?
> food & drink
> air (and dust, etc)
How many different tastes are said to separately detectable?
> five - but we used to say just 4, and nowadays we say even more!
i.e.?
> sweet, sour, salty, bitter, umami

How many different smells are said to separately detectable?
> 3000

In what way can the taste buds be said to have a protective function?
> warns of poisons and bad foods

In what way can the sensory part of the nose be said to have a protective function?
> smell can warn of danger/ be used for hunting

In what way do the taste buds and the olfactory organs operate similarly?
> both detect "chemicals" (must be dissolved)

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