Skin

The skin is the largest organ in the body. Scattered throughout the skin are millions of tiny

sense organs containing sensory nerve endings. Each sensory nerve cell is sensitive to

either:

1. Touch e.g. fingertips, tongue, lips (blind person uses fingertips – Braille)

2. Pressure e.g. fingertips, tongue (cook tells if there are lumps in the food)

3. Pain e.g. tendons, muscles, ligaments: distributed evenly in the skin.

warning device to say if muscle is over-stretched

4. Temperature e.g. front of fore arm, back of hand: hot and cold receptors distinguish

½ºC difference. The tongue is NOT temperature sensitive – this is

why it burns.

The distribution or density of each type of sensory receptor varies.

Hair movements

There are sensory receptor cells at the base of the hair in the skin. These are stimulated

when hairs move (by touch or wind). Cats and mice avoid obstacles in the dark using their

whiskers.

nerve ending

in hair

hair follicle

sweat gland

epidermis

dermis

free nerve ending –

detects pain/cold sweat duct /

pore

capillaries

near surface

of dermis

layer of fat and

connective tissue

Tongue

The tongue is muscle. On its upper surface are taste cells grouped into taste buds. Taste is a

property of substances in solution.

There are four types of taste bud – each is specialised and sensitive to one particular taste:

1. Sweet property of sugars at tip/front of tongue

2. Sour property of acids at edge/sides of tongue

3. Salt property of strong acids at tip/front of tongue

4. Bitter property of plant alkaloids at back of tongue

The 4 tastes are grouped into different parts of the tongue. The tongue is temperature

insensitive and that is why we burn it.

Flavour is a combination of taste and smell. This is shown by not being able to taste

properly when you have a cold.

Smell: there are small smell sensory receptors high up in the nasal cavity (i.e. the nose!) –

generally these are sensitive to harmful substances (e.g. decaying food).

sweet

and salt

sour

bitter

Brain

The brain is a complex network of neurones and weighs approximately 1.5 kg. It contains

thousands of millions of neurones. Each neurone synapses with thousands of other

neurones. It needs protection because it is so important and the protection is given by the

skull bones.

Medulla oblongata positioned at the top of the spinal cord – where the spinal cord joins the

brain. It is the reflex centre for unconscious activities

(e.g. heart beat, blood pressure, blinking)

Cerebellum this is the area concerned with muscular coordination. It receives nerve

impulses from all the sense organs concerned with muscular activity

Cerebrum this is a dome shaped mass of neurones and is the largest part of the

brain. There are two cerebral hemispheres: the right and the left. In

the centre of the cerebrum is the grey matter. This is the part, which is

the most advanced in humans. It is responsible for all conscious

activity. The cerebrum receives nerve impulses from all the sense

organs and will be responsible for all the conscious activity. This

includes reasoning, decision-making, memory storage and emotions.

Hypothalamus this lies on the underneath front part of the cerebrum. It is the part of

the brain that monitors and controls the body temperature, the

concentration of the salts in the blood and the concentration of CO2 and

acid in the blood. It is linked to the pituitary gland.

Pituitary gland this controls the activity of all endocrine glands. Therefore, the

hypothalamus is the link between the nervous system and the endocrine

system in controlling and coordinating the body.

medulla oblongata

hypothalamus

pituitary gland

spinal cord

cerebellum

responsible for

eyesight

cerebrum