Coordination and Response

Stimuli, responses and coordination

A stimulus is a change in the internal or external environment of an organism. A response is the reaction to that stimulus. Step into bright sunlight and the light is the stimulus while your pupils narrowing is the response. Coordination is the process that links the two, so that the activities of the body are regulated and kept in balance.

Two systems do this coordinating work: the nervous system and the endocrine system. The nervous system uses electrical impulses that travel along neurones. It is fast, precise and its effects are short-lived. The endocrine system uses hormones, chemical messengers carried in the blood. It is slower, it can act on target organs anywhere in the body, and its effects last much longer.

Both systems are essential because they suit different jobs. Pulling your hand off a hot pan has to happen almost instantly, so it uses the nervous system. Growth, puberty and the long-term control of blood glucose are slower processes, so they use hormones.

The nervous system: CNS, PNS and neurones

The mammalian nervous system is divided into two parts. The central nervous system (CNS) is the brain and the spinal cord. The peripheral nervous system (PNS) is all the nerves outside the brain and spinal cord, and it connects the CNS to the rest of the body.

The brain is the main coordinating centre: it receives information, makes decisions and sends out instructions. The spinal cord carries impulses between the brain and the rest of the body, and it also coordinates many reflex actions on its own, without waiting for the brain to decide anything.

Information travels along neurones as electrical impulses. Cambridge wants you to recognise three types of neurone in diagrams and to say which way each one carries its impulse.

• Sensory neurone: carries an impulse from a receptor (in a sense organ) towards the CNS.
• Relay neurone (also called an intermediate neurone): carries the impulse from a sensory neurone to a motor neurone, inside the CNS.
• Motor neurone: carries the impulse from the CNS to an effector - a muscle or a gland - which produces the response.
• The role of the nervous system is coordination and regulation: it collects information about stimuli and produces rapid, appropriate responses.

The reflex arc and the synapse

A reflex action is a rapid, automatic response to a stimulus that you do not have to think about. Reflexes protect the body: pulling your hand off a hot pan, blinking when something flies towards your eye, or your pupil narrowing in bright light. A reflex is automatic and rapid because the impulse never has to travel up to the brain and back - the brain is not part of the reflex pathway.

The pathway the impulse follows is the reflex arc: receptor -> sensory neurone -> relay neurone -> motor neurone -> effector. The receptor detects the stimulus, and the effector (a muscle or a gland) carries out the response. In the hand-withdrawal reflex the receptor is in the skin of the hand, the relay neurone is in the spinal cord, and the effector is a muscle in the arm.

At the end of a neurone the impulse has to cross a tiny gap to reach the next neurone. This junction is a synapse. The impulse causes vesicles in the first neurone to release neurotransmitter molecules into the synaptic gap. The molecules diffuse across the gap and bind with receptor proteins on the next neurone, which stimulates a new impulse there.

Because neurotransmitter is released only from the first neurone and the receptor proteins are found only on the next one, impulses can cross a synapse in one direction only. That is what keeps the signal travelling the right way along the reflex arc.

The eye: structure, function and the pupil reflex

A sense organ is a group of receptor cells that respond to a specific stimulus. The eye responds to light, the ear to sound, the skin to touch and temperature, and the tongue and nose to chemicals. Light enters the eye through the cornea, which refracts (bends) the light as it passes in. The iris is the coloured ring of muscle that controls how much light enters the pupil, the hole at the centre of the iris. The lens focuses the light onto the retina, the light-sensitive layer at the back of the eye that contains the light receptors. The optic nerve carries impulses from the retina to the brain. Where the optic nerve leaves the retina there are no receptors at all, and this is the blind spot.

The pupil reflex controls how much light reaches the retina. In bright light the pupil narrows so that less light enters and the retina is protected from damage; in dim light the pupil widens so that as much light as possible reaches the retina.

The iris contains two sets of muscles that work antagonistically. The circular muscles run in a ring around the pupil, and the radial muscles run outwards from the pupil like the spokes of a wheel. When one set contracts, the other must relax.

Accommodation, rods and cones

Accommodation is the eye changing the shape of its lens so that it can focus on objects at different distances. The lens is held in place by the suspensory ligaments, which are attached to the ciliary muscles.

To view a near object, the ciliary muscles contract, which slackens the suspensory ligaments. With less tension pulling on it, the lens becomes thicker (more convex), so it refracts the light more strongly and focuses the diverging rays from a close object onto the retina.

To view a distant object, the ciliary muscles relax, which pulls the suspensory ligaments tight. This tension makes the lens thinner (less convex), so it refracts the light less and focuses the parallel rays from a far object onto the retina.

The retina contains two types of light receptor. Rods are very sensitive to light and work in dim light, giving night vision, but they do not detect colour. Cones need brighter light but detect colour: there are three types, each absorbing light of a different colour. The fovea is the small area of the retina with the highest concentration of cones and no rods, which is why it gives the sharpest colour vision.

Hormones and homeostasis

A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. The endocrine glands you must know are the adrenal glands (adrenaline), the pancreas (insulin and glucagon), the testes (testosterone) and the ovaries (oestrogen). ADH, made by the pituitary gland, controls the water content of the blood by changing how much water the kidneys reabsorb.

Adrenaline is the hormone secreted in fight or flight situations, such as a fright or a near miss. It increases the breathing rate and the heart rate so that more oxygen and glucose reach the muscles, and it increases the pupil diameter so that more light enters the eye. It also raises the blood glucose concentration by causing the liver to convert glycogen back into glucose.

Homeostasis is the maintenance of a constant internal environment. It works by negative feedback: a change away from the normal set point is detected, and a response is triggered that brings the value back towards the set point. The control of blood glucose is the classic example. After a meal, blood glucose rises, the pancreas secretes insulin, and insulin causes the liver to convert the excess glucose into glycogen for storage and increases the uptake of glucose by cells, so the blood glucose falls back to normal. If it falls too low, the pancreas secretes glucagon, which makes the liver convert stored glycogen back into glucose.

Diabetes is a failure of this control. In type 1 diabetes the pancreas does not produce enough insulin, so blood glucose stays high after a meal; it is treated with insulin injections together with a controlled diet and regular exercise. In type 2 diabetes the body still produces insulin but the cells respond to it less well, which is often linked to being overweight; it is managed mainly by diet, weight loss and exercise, sometimes with medication.

• Temperature regulation: the body keeps its temperature constant by insulation (fat and hair), by sweating (which cools the skin as the sweat evaporates) and by shivering (which releases heat as the muscles contract).
• Vasodilation: the arterioles supplying the skin surface capillaries widen, so more blood flows near the surface and more heat is lost - this happens when you are too hot.
• Vasoconstriction: the same arterioles narrow, so less blood flows near the surface and less heat is lost - this happens when you are too cold.
• The brain detects the temperature of the blood and coordinates all of these responses.

Tropic responses in plants

Plants respond to stimuli by growing, and these growth responses are called tropic responses or tropisms. Phototropism is a response in which a plant part grows towards or away from the direction of the light source. Gravitropism (also called geotropism) is a response in which a plant part grows towards or away from gravity.

A shoot grows towards light (positively phototropic) and away from gravity (negatively gravitropic), which carries the leaves upwards into the light for photosynthesis. A root grows away from light (negatively phototropic) and towards gravity (positively gravitropic), which anchors the plant and takes the root down towards water and minerals.

Tropisms are controlled chemically by the plant hormone auxin. Auxin is made in the shoot tip and diffuses down through the plant. It is unequally distributed in response to light and gravity, and it stimulates cell elongation - the side with more auxin elongates more, so the organ bends.

In phototropism, light causes auxin to move to the shaded side of the shoot. The shaded side therefore has more auxin, its cells elongate more, and the shoot bends towards the light. In gravitropism, gravity causes auxin to collect on the lower side of the shoot, so the lower side elongates more and the shoot bends upwards. In the root the same high concentration of auxin inhibits cell elongation, so the lower side grows less and the root bends downwards, towards gravity.

• A simple investigation: place a shoot in a box with a single hole on one side. After a few days the shoot has grown out through the hole, towards the light.
• For gravitropism: germinate seeds on damp cotton wool in a dish. The shoots grow upwards and the roots grow downwards, whichever way the dish is turned.