Control & Coordination

Every living thing must notice what is happening around it and react in the right way. This ability to detect changes and respond to them in an organised, purposeful way is called control and coordination. It keeps all the parts of the body working together smoothly so the organism stays alive and safe.

A stimulus is any change in the surroundings (or inside the body) that an organism can detect and react to. The reaction it produces is called the response. For example, when you touch a hot object, the heat is the stimulus and quickly pulling your hand away is the response.

In animals, control and coordination are carried out by two systems working together:

• The nervous system is fast. It uses nerve cells (neurons) to carry electrical impulses, along with chemicals at the junctions between neurons, so messages travel very quickly but their effect is short-lived.
• The endocrine (hormonal) system is slower. Glands release chemical messengers called hormones directly into the blood, which carries them to the parts of the body where they act. Their action starts slowly but usually lasts longer.

In plants, there is no nervous system and there are no muscles. Plants coordinate only chemically, using plant hormones, and they respond mainly through movements — for example, a shoot bending towards light or a root growing downwards.

The nervous system is made of nerve cells called neurons. A neuron is the structural and functional unit of the nervous system — the basic building block that receives, carries and passes on information in the body.

A neuron has these main parts:

• Dendrites — the branched tips that receive the information (the stimulus) coming from other cells.
• Cell body — the wider central part that contains the nucleus and cytoplasm.
• Axon — a single long fibre that carries the impulse away from the cell body.
• Nerve endings — the fine tips at the far end of the axon that hand the message on to the next cell.

Here is how a message travels. When a stimulus acts on a dendrite tip, it sets off a chemical change that becomes an electrical impulse. This impulse moves in one direction only: from the dendrite, to the cell body, then along the axon, and finally to the end of the axon.

The point where the nerve ending of one neuron meets the next neuron (or a muscle cell) is a tiny gap called a synapse. When the impulse reaches the end of the axon, it causes the release of tiny amounts of chemicals into this gap. These chemicals cross the synapse and start a fresh electrical impulse in the next cell.

So remember the two languages of a nerve message: within a single neuron the signal is an electrical impulse, while across a synapse it becomes a chemical signal that passes the message from one cell to the next.

A reflex action is a sudden, automatic and very fast response to a stimulus. It happens on its own, without any conscious thinking. Because you do not decide to do it, we call it an involuntary action.

Common examples are quickly pulling your hand away the moment you touch a hot object, and blinking your eyes when something suddenly comes near them. The body reacts to protect itself before you have even finished feeling the danger.

The pathway that a reflex signal follows is called the reflex arc. Trace it in order:

1. Stimulus — for example, the heat of a hot object.
2. Receptor — a sense organ such as the skin detects the stimulus.
3. Sensory (afferent) neuron — carries the message from the receptor towards the spinal cord.
4. Spinal cord — a relay neuron (interneuron) here connects the incoming signal to the outgoing signal.
5. Motor (efferent) neuron — carries the command away from the spinal cord to the muscle.
6. Effector — a muscle that carries out the action.
7. Response — the muscle contracts and you pull your hand away.

Notice that the signal is processed in the spinal cord and does not wait to travel up to the brain and back. This makes the response much faster, which matters when you must move away from danger at once. The message about what happened does still reach the brain, so you feel the pain and become aware of it — but the protective action has already taken place by then.

The brain is the main coordinating centre of the body. The brain and the spinal cord together form the central nervous system (CNS). The nerves that spread out to every part of the body make up the peripheral nervous system. Messages pass between the CNS and the rest of the body through these nerves.

The brain has three main parts — the forebrain, the midbrain and the hindbrain — and each has its own job.

• Forebrain: its main part is the cerebrum, the thinking and main coordinating part of the brain. It controls voluntary actions and is the seat of intelligence and memory. It also contains the centres that receive sensations — sight, hearing, smell, taste and touch.
• Midbrain: connects the different parts of the brain and controls some reflexes.
• Hindbrain: made up of the cerebellum, the medulla and the pons.
  • The cerebellum controls posture and balance and the precision (coordination) of voluntary movements, for example walking in a straight line or picking up a pencil.
  • The medulla controls many involuntary actions, such as heartbeat, breathing, blood pressure and vomiting.
  • The pons is also a part of the hindbrain.

The brain is a delicate organ, so the body protects it carefully. It sits inside a strong bony box called the skull (cranium). Within this box it is wrapped in protective membranes called meninges, and it floats in a fluid called cerebrospinal fluid, which cushions it against shocks and jerks.

Nervous tissue does not do the actual work of moving your body. Its job is to carry information as electrical impulses. The real movement is carried out by muscles. So the pathway ends when a nerve impulse is delivered to a muscle.

When a nerve impulse (nervous, electrical stimulation) reaches a muscle cell, the cell responds by changing its shape. Muscle cells contain special proteins. In response to the electrical impulse, these proteins change both their shape and their arrangement inside the cell. This new arrangement gives the muscle cell a shorter form, so the muscle shortens, or contracts, and pulls the body part to move it.

Because muscles are the parts that finally carry out the response (the action), they are called effectors.

Sending messages as electrical impulses is fast and effective, but it has two important limitations:

• An electrical impulse can reach only those cells that are connected by nervous tissue — not every cell of the body.
• After a cell sends (transmits) an impulse, it needs a short time to reset before it can generate and send a fresh impulse. So a cell cannot keep firing impulses continuously.

Because of these limits, animals also use chemical communication. Chemical messengers called hormones are released into the blood, which carries them to all cells of the body — even cells that no nerve reaches. This chemical (hormonal) system works alongside the nervous system, and it is what we look at next.

Plants have no nervous system, no nerve cells and no muscles, yet they still respond to stimuli like light, gravity, water and touch. They coordinate their responses using chemical messengers called plant hormones, and they act by movements of two main kinds.

1. Movement dependent on growth (tropism): This is a slow, directional growth of a plant part towards or away from a stimulus. The direction in which the part grows depends on the direction of the stimulus.

• Phototropism — response to light. A shoot grows towards light (positive phototropism), while a root grows away from light (negative phototropism).
• Geotropism (gravitropism) — response to gravity. Roots grow downward, towards gravity (positive); shoots grow upward, away from gravity (negative).
• Hydrotropism — response to water. Roots grow towards water.
• Chemotropism — response to a chemical. For example, a pollen tube grows towards the ovule during fertilisation.

Phototropism is controlled by the hormone auxin, which is made at the tip of the shoot. When light falls on one side, auxin moves to the shady side of the shoot. There the extra auxin makes the cells grow longer, so the shaded side lengthens more than the lit side and the shoot bends towards the light.

2. Movement independent of growth: This is an immediate movement that is not caused by growth. A good example is the sensitive plant (Mimosa pudica, the touch-me-not), whose leaves fold up quickly when touched. Here the cells change shape by gaining or losing water, which changes their swelling (turgidity). Information about the touch travels from cell to cell as an electrical-chemical signal — remember, plants have no specialised nerve cells to carry impulses.

Plants have no nervous system and no muscles, yet they still respond to their surroundings. They control their growth and responses using chemical compounds called plant hormones (also called phytohormones). A hormone is made in one part of the plant and can move to other parts, where it coordinates growth, development and reactions to the environment.

There are four plant hormones you must know, and each has its own main job:

• Auxin — made at the tip of the shoot. It helps cells grow longer, that is, it promotes cell enlargement or elongation. Auxin is responsible for phototropism, the bending of a shoot towards light.
• Gibberellins — help in the growth of the stem.
• Cytokinins — promote cell division. They are present in greater amounts in areas where cells divide rapidly, such as in fruits and seeds.
• Abscisic acid (ABA) — an inhibitor. It slows down or inhibits growth. Its effects include the wilting of leaves and the closing of stomata, which help the plant respond to stress.

A simple way to remember this: three of the hormones (auxin, gibberellins and cytokinins) mostly promote growth, while abscisic acid inhibits it. This balance lets the plant grow when conditions are good and slow down when it is under stress, such as during a shortage of water.

Phototropism is worth understanding closely, as it is a common exam question. When light falls on a shoot from one side, the auxin made at the shoot tip moves away from the light towards the shaded side of the shoot. The extra auxin on the shaded side makes the cells there grow longer than the cells on the brighter side. Because one side grows more than the other, the shoot bends towards the light.

In animals, tasks are controlled and coordinated not only by nerves but also by chemicals called hormones. Hormones are chemical messengers. They are made by special glands called endocrine glands, also known as ductless glands because they have no tubes (ducts). These glands pour their hormones straight into the blood, which carries them to faraway target organs where they act. Hormones are needed only in very small (tiny) amounts to do their work.

Together these glands make up the endocrine system. Here are the main glands you must know, with the hormone each one makes and what it does:

• Adrenal gland → makes adrenaline, the 'fight-or-flight' hormone. When you feel fear or anger, it is released into the blood. It speeds up the heartbeat and breathing rate and raises the blood pressure, so the body is ready to act quickly.
• Pancreas → makes insulin, which controls (lowers) the blood sugar level. If the pancreas makes too little insulin, the blood sugar stays high — this disease is called diabetes.
• Thyroid gland → makes thyroxine, which regulates the metabolism of carbohydrates, fats and proteins. The gland needs iodine to make thyroxine, so a lack of iodine in the diet causes the deficiency disease goitre (a swollen neck).
• Pituitary gland → makes growth hormone, which controls the growth of the body. Too little of it in childhood causes dwarfism (very short height), while too much causes gigantism (very tall height).
• Testes (in males) → make testosterone, and ovaries (in females) → make oestrogen. These hormones bring about the changes called secondary sexual characters at puberty.

The body must release just the right amount of each hormone, because too much or too little is harmful. This is managed by a feedback mechanism. For example, when the sugar level in the blood rises, the cells of the pancreas detect it and release more insulin to bring the sugar level down. As the blood sugar falls, less insulin is released.