Coordination and Homeostasis

14.1 Mammalian nervous system

The nervous system (the brain, spinal cord and nerves) coordinates and regulates body functions.

• The central nervous system (CNS) is the brain and spinal cord.
• The peripheral nervous system (PNS) is the nerves outside the brain and spinal cord.

Electrical impulses travel along neurones (nerve cells).

Types of neurone

The reflex arc

The path is called a reflex arc:

$\text{stimulus} \to \text{receptor} \to \text{sensory neurone} \to \text{relay neurone} \to \text{motor neurone} \to \text{effector} \to \text{response}$

Example: touching a hot object. The receptors in the skin detect the heat; the impulse passes along a sensory neurone to the spinal cord, then through a relay neurone to a motor neurone, which carries the impulse to a muscle (the effector) that contracts to pull the hand away. Because few neurones are involved and the brain is not involved first, the reaction is very fast, and it protects the body from harm.

Synapses

There is a tiny gap (the synaptic gap) between the neurones. The first neurone's end contains vesicles that hold neurotransmitter molecules, and the next neurone has receptor proteins on its membrane.

Events at a synapse.

1. An impulse arrives and stimulates the release of neurotransmitter molecules from the vesicles into the synaptic gap.
2. The neurotransmitter molecules diffuse across the gap and bind to receptor proteins on the next neurone.
3. This stimulates an impulse in the next neurone.

Synapses ensure that impulses travel in one direction only, because only the first neurone releases neurotransmitter, and only the second has receptors.

14.2 Mammalian sense organs

Sense organs are groups of receptor cells that respond to specific stimuli: light (eye), sound (ear), touch, temperature (skin) and chemicals (nose and tongue).

The eye

The pupil reflex

The iris has two sets of muscles that act antagonistically (in opposite ways): circular muscles and radial muscles.

Accommodation

Accommodation is changing the shape of the lens to focus on objects at different distances.

14.3 Mammalian hormones

Adrenaline

Adrenaline is made by the adrenal glands in situations of fear, stress or excitement (for example before a race, or when frightened). It increases the heart rate (so more blood carries oxygen and glucose to the muscles) and it increases the blood glucose concentration (the liver changes glycogen into glucose), so that more energy is available for muscles in a "fight or flight" response.

Nervous and hormonal control compared

14.4 Homeostasis

Examples: body temperature, blood glucose concentration, water content.

If the level goes above the set point, a response makes it fall. If it goes below, a response makes it rise. This keeps the level close to the set point.

14.5 Temperature control

Mammals keep their body temperature constant at about $37\,^\circ\text{C}$, so that their enzymes work at the optimum.

The skin

• Hairs and fatty tissue insulate the body (the fat under the skin reduces heat loss).
• Hair erector muscles raise the hairs.
• Sweat glands produce sweat.
• Temperature receptors in the skin detect the temperature of the surroundings, and receptors in the brain detect the temperature of the blood.
• Blood vessels (arterioles supplying capillaries) can change in diameter.

The hypothalamus in the brain is the control centre. It monitors the temperature of the blood and receives impulses from the temperature receptors in the skin, and it sends impulses to the effectors.

Responses

14.6 Blood glucose control

The blood glucose concentration must be kept near its set point, because cells need glucose for respiration, and if it is too high or too low, cells can be damaged (for example by osmotic effects).

• After a meal, blood glucose rises. The pancreas secretes insulin, which makes the liver convert glucose into glycogen for storage (and cells take up more glucose), so the blood glucose falls.
• If blood glucose falls (for example between meals or in exercise), the pancreas secretes glucagon, which makes the liver convert glycogen into glucose and release it into the blood, so the blood glucose rises.

Diabetes (Type 1)

In Type 1 diabetes the pancreas does not make enough insulin. The blood glucose concentration rises (increased blood glucose) and glucose appears in the urine. It is treated by injecting insulin (usually with meals), together with attention to diet.

The pancreas secretes insulin. Insulin makes the liver change glucose into glycogen (and cells take up glucose), so the blood glucose concentration falls back to the set point. This is negative feedback.