Learning outcomes:
describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems
understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves
understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses
describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object
describe the structure and function of the eye as a receptor
understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity
describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation
understand the sources, roles and effects of the following hormones: ADH, adrenaline, insulin, testosterone, progesterone and oestrogen.
Homeostasis: the maintenance of a constant internal environment

All organisms try and maintain a constant internal environment. This is called homeostasis. Examples of homeostasis include the regulation of water levels (see above) and the regulation of body temperature (see below).

Humans have two systems which carry out homeostasis;

Nervous System – immediate responses to stimuli (sec - hours)
Endocrine System – long term responses to stimuli (hours - months)

Both systems respond to stimuli (i.e. events that change the internal environment). Both systems have a detector (which detects the stimulus) and an effector, which carries out a response to correct the effect of the stimulus. The message from detector to effector is carried either via an electrical nerve impulse or as a hormone, depending which homeostatic system is being used.

Coordination in Humans:

Nerves & the Nervous system:

The nervous system consists of the brain and the spinal cord.
Sense organs (e.g. pain receptors in skin, or photoreceptors in the eye) are linked to the brain via nerves.
Stimulation of the sense organs results in an electrical signal (a nerve impulse) being sent along the nerve to the brain. Nerve impulses are very quick (~120m/s), allowing rapid responses to the stimulus. Some sense organs are not connected directly to the brain. This is a defense mechanism allowing almost instant responses to threatening or dangerous stimuli (e.g. pain). These instant responses are controlled by nerves in the spine, rather than the brain and are called reflexes

A reflex arc:

1. A stimulus is detected by a receptor
2. The receptor initiates a nerve impulse in the sensory nerve
3. The sensory nerve (which runs from the receptor to the spine) passes the message onto an interneurone in the spine
4. The interneurone passes the message on the a motor nerve
5. The motor nerve (which runs from the spine to a muscle in the same limb as the receptor) passes the message onto the effector muscle
6. The effector muscle carries out the response.

The entire process (stimulus to response) happens in less than a second and does not involve the brain. The purpose of the interneurone is to inform the brain of what has happened.

Reflexes in the eye:
Structure Function

Cornea - Refracts (bends) light entering the eye.
Iris - Controls the amount of light entering the eye by adjusting the size of the pupil.
Pupil - Hole which allows light into the eye.
Lens - Allows fine focusing by changing shape.
Ciliary muscle - Changes the shape of the lens by altering the tension on the suspensory ligaments.
Retina - Contains light-sensitive rod and cone cells which convert light energy into a nerve impulse (i.e. transduce energy).
Fovea - Area where most light is focused, very sensitive to colour (most cones here).
Optic nerve - Transmits nerve impulses to the brain, where they are interpreted.
Sclera - Outer protective layer of eye
Choroid - Contains blood vessels

Light is detected by photoreceptors in the eye. These receptors form the retina (the inner lining of the eye). There are two types of photoreceptor;

- Rods, which see only in black & white
- Cones, which see in either red, blue or green (3 types of cone)

There are two types of reflex you need to know about in the eye;

1. Responding to different light levels
2. Focusing the eye

Responding to different light levels:

In the dark

1. Photoreceptors detect
2. Reflex occurs
3. Muscles in the Iris are the effectors
- Radial muscles in Iris contract
- Circulatory muscles in Iris relax
4. Pupil diameter opens
5. More light enters the eye

In the light

1. Photoreceptors detect
2. Reflex occurs
3. Muscles in the Iris are the effectors
- Radial muscles in Iris relax
- Circulatory muscles in Iris contract
4. Pupil diameter closes
5. Less light enters the eye

Focusing the eye:
Near Object
1. Incoming light is divergent
2. Ciliary muscles contract
3. Suspensory ligaments are loose
4. Lens becomes fat
5. Light is refracted more
Light converges on the retina

Distant Object
1. Incoming light is parallel
2. Ciliary muscles relax
3. Suspensory ligaments are tight
4. Lens is pulled thin
5. Light is refracted less
Light converges on the retina

Controlling Skin temperature:


Too hot
When you are hot the following happen (controlled by reflexes);
1. Hairs on skin lie flat (less insulating air trapped)
2. Sweating starts
3. Blood is diverted close to the surface of the skin (more heat radiation)

Too cold
When you are cold the following happen (controlled by reflexes);
1. Hairs on skin stand up (more insulating air trapped)
2. Sweating stops
3. Shivering starts, so muscles respire more, producing more heat
4. Blood is diverted away from the surface of the skin (less heat radiation)

How is blood diverted?

Arterioles in the skin can open and close in response to nerve messages.

Vasoconstriction – arteriole closes
Vasodilation – arteriole opens


The net effect is to open arterioles under the surface of the skin when hot and close them when cold.

Random Hormones you need to know:

It might be worth your while looking these up in more detail…

Control of blood glucose;

1. You eat a meal. It is digested and glucose is absorbed into the blood stream.
2. Blood glucose level rises
3. Pancreas detects
4. Pancreas releases insulin into bloodstream
5. Insulin travels all over the body
6. Only the cells in the liver have receptors for insulin, so only they respond to the hormone.
7. The liver cells (they’re called hepatocytes) take up the glucose out of the blood stream.
8. The glucose if converted into glycogen, which is stored inside liver cells.
9. Blood glucose level falls back to normal.

The hormone glucagon does exactly the opposite to insulin. Glucagon is released when blood glucose levels fall too low.

Hyperglycaemia: blood glucose level is dangerously high (causes coma and can be fatal)
Hypoglycaemia: blood glucose level is dangerously high (causes coma and can be fatal)

Type I Diabetes: a disease in which people cannot make insulin

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