Control and coordination

A stimulus is a change that an organism notices. Heat, light, sound, touch, smell, and gravity are examples. A response is what the organism does after noticing the change.

For example, when you touch something hot:

Your skin notices the heat. A message travels through nerves. Your body tells your hand muscles to move. Your hand pulls away. This protects you from being burned.

Animals use two main systems for control and coordination. The nervous system gives quick messages, while hormones give chemical messages that are usually slower but can affect many parts of the body.

The nervous system

The nervous system is the body’s fast messaging system. It is made of the brain, spinal cord, and nerves.

The brain is like the control room. The spinal cord is like a thick main communication cable running down your back. Nerves connect the brain and spinal cord to the rest of the body.

The basic cell of the nervous system is called a neuron. You can imagine a neuron as a tiny wire with branches.

The branches called dendrites receive information. The information travels through the cell body and then along the axon. At the end of the axon, the message must cross a tiny gap to reach the next neuron. This gap is called a synapse.

The message travels along the neuron as an electrical impulse. When it reaches the end of the axon, chemicals are released. These chemicals cross the synapse and start a new electrical impulse in the next neuron.

The process is similar to passing a message from one person to another. One person speaks, the message crosses the small space between people, and the next person hears it.

A neuron can send messages to another neuron, a muscle cell, or a gland. When a muscle receives the message, it may contract and cause movement. When a gland receives the message, it may release a substance.

Receptors and sense organs

A receptor is something that notices a stimulus. Receptors are found in sense organs.

The eyes contain receptors for light. The ears contain receptors for sound. The nose contains receptors for smell. The tongue contains gustatory receptors for taste. The skin contains receptors for touch, pressure, heat, and cold.

When you smell an incense stick, smell molecules enter your nose. Olfactory receptors detect these molecules. They create nerve impulses. The impulses travel through nerves to the brain. The brain recognises the smell as incense.

Taste and smell work together. If you block your nose while eating sugar, the sugar still tastes sweet, but food may seem less flavourful. During a cold, the nose may be blocked, so smell molecules cannot reach the smell receptors easily. This makes food taste less interesting.

Reflex actions

A reflex action is a very quick action that happens without waiting for conscious thinking.

Suppose you touch a hot pan. Special receptors in your skin detect the heat. A nerve carries the message to the spinal cord. The spinal cord quickly sends a message to the arm muscles. Your hand moves away.

The pathway followed by this message is called a reflex arc.

The steps are:

Stimulus: your hand touches heat.

Receptor: your skin detects the heat.

Sensory nerve: carries the message to the spinal cord.

Spinal cord: quickly processes the message.

Motor nerve: carries an instruction to the muscle.

Muscle: contracts and pulls your hand away.

Your brain receives the information too. You feel the pain and understand what happened, but the hand has already moved. This makes sense because thinking carefully takes longer. If you had to think, “This is hot, I may get burned, I should move my hand,” your hand could be injured before moving.

Other reflex actions include blinking when something comes close to your eye, coughing when something enters your throat, and quickly moving away from danger.

Reflex actions and voluntary actions

A reflex action is automatic and very fast. Pulling your hand away from a flame is a reflex action.

A voluntary action is done with conscious decision. Walking, writing, picking up a pencil, and moving a chair are voluntary actions.

When you walk, your brain decides to move. It sends instructions to several muscles. Your muscles must work in the correct order and with the correct amount of force.

Involuntary actions

An involuntary action happens without conscious control. Your heart beats, your lungs breathe, and your digestive system works even when you are asleep.

You do not need to remember to make your heart beat. The body controls it automatically.

Reflex actions and involuntary actions are both automatic, but they are usually different kinds of responses. A reflex action is generally a quick response to a particular stimulus, such as pulling away from heat. An involuntary action is an ongoing body activity, such as heartbeat or digestion.

The human brain

The brain is the main coordinating centre of the body. It receives messages, understands them, stores information, makes decisions, and sends instructions.

The document divides the brain into three main regions: the forebrain, midbrain, and hindbrain.

The forebrain is the main thinking part. It receives information from the sense organs. It helps you see, hear, smell, remember, think, feel, and decide.

Different areas of the forebrain deal with different kinds of information. One area may receive information from the eyes. Another may receive information from the ears. Association areas combine new information with memories and other information.

For example, when you see a dog, your eyes send information to the brain. Your brain connects this sight with memories. It may decide whether the dog is friendly or dangerous. It then tells your muscles what to do.

The forebrain also contains a centre connected with hunger. The feeling of being full is controlled by a particular area of the forebrain.

The midbrain helps control some involuntary responses, such as changes in the size of the pupil.

The hindbrain contains the cerebellum and medulla.

The cerebellum helps with balance, posture, and accurate movement. It helps you walk in a straight line, ride a bicycle, stand properly, and pick up objects smoothly.

The medulla controls important involuntary activities. These include heartbeat, breathing, blood pressure, salivation, and vomiting.

Protection of the brain and spinal cord

The brain is delicate, so the body protects it carefully.

The skull is a hard bony box around the brain. The brain is also surrounded by a fluid-filled covering. This fluid acts like a cushion and helps absorb shocks.

The spinal cord runs through the backbone. The vertebral column, or backbone, is made of bones that protect the spinal cord.

If the spinal cord is injured, messages may not travel properly between the brain and parts of the body below the injury. This can affect sensation and movement.

How muscles create movement

Nerves send instructions, but muscles perform the final movement.

A muscle cell contains special proteins. When a nerve impulse reaches the muscle, these proteins change their shape and position. The muscle fibre becomes shorter. When many muscle fibres shorten together, the whole muscle contracts.

When a muscle contracts, it pulls on a bone and produces movement.

Muscles that you can control consciously are called voluntary muscles. Muscles that work automatically, such as many muscles in the digestive system, are involuntary muscles.

Coordination in plants

Plants do not have a brain, nerves, or muscles like animals. They still respond to their surroundings.

A plant can respond to light, touch, gravity, water, and chemicals. These responses may happen quickly or slowly.

The sensitive plant, also called Mimosa or the touch-me-not plant, folds its leaves when touched. This movement does not happen because the plant grows. It happens because cells in the leaves quickly gain or lose water.

When the amount of water inside plant cells changes, the cells swell or shrink. This changes the shape of the leaf, causing it to fold and droop.

The plant must send information from the touched part to other parts of the leaf. It uses electrical and chemical signals, but it does not have special nerve tissue like animals.

Plant movements caused by growth

Some plant movements happen because different parts grow at different speeds.

A pea plant has thin structures called tendrils. A tendril can touch a stick or fence. The side touching the support grows more slowly. The opposite side grows faster. This unequal growth makes the tendril bend around the support.

The tendril then holds the plant up. It is similar to one side of a soft ribbon becoming longer than the other side. The ribbon bends because the two sides are different lengths.

Plants also grow in particular directions. These directional growth movements are called tropisms.

Phototropism

Phototropism is growth in response to light.

Shoots usually grow towards light. This is called positive phototropism.

Roots usually grow away from light. This is called negative phototropism.

Shoots need light for photosynthesis. Photosynthesis is the process by which green plants make food. Growing towards light helps the plant receive more light.

The hormone auxin helps the shoot bend towards light. Auxin is made near the shoot tip. When light comes from one side, auxin moves to the shaded side.

Auxin makes cells on the shaded side grow longer. Because one side grows more than the other, the shoot bends towards the light.

The shoot does not move like an animal walking. New growth on one side makes it appear to bend.

Geotropism

Geotropism is growth in response to gravity.

Roots grow downwards, towards the pull of gravity. This is positive geotropism. Growing down helps roots anchor the plant and find water and minerals in the soil.

Shoots grow upwards, away from gravity. This is negative geotropism. Growing upwards helps the plant reach light and air.

Hydrotropism

Hydrotropism is growth in response to water.

Roots generally grow towards areas with more water. This helps the plant absorb the water it needs.

A simple experiment can demonstrate hydrotropism. Put moist soil on one side of a container and dry soil on the other side. Place a germinating seed between them. After some time, the root should grow towards the moist soil.

The root is not thinking about water. Chemical changes inside the plant guide its growth.

Chemotropism

Chemotropism is growth in response to chemicals.

A pollen tube is an example. After pollen reaches the stigma of a flower, it grows downwards towards the ovule. Chemical substances from the ovule help guide the pollen tube.

This allows the male reproductive cell to reach the female reproductive cell.

Plant hormones

Plant hormones are chemical messengers made in one part of a plant. They travel to another part and control growth or other activities.

Auxins help cells grow longer. They are important in bending towards light and in the growth of tendrils.

Gibberellins help the stem grow longer.

Cytokinins help cells divide. They are found in higher amounts in areas where many cells are being made, such as fruits and seeds.

Abscisic acid slows or stops growth. It can also cause leaves to wilt. It helps the plant respond when conditions are difficult, such as during a lack of water.

Plant hormones usually act slowly compared with nerve impulses. A nerve message can travel quickly, while a plant may need hours or days to show a growth response.

Electrical and chemical messages

Electrical impulses are useful when a very fast message is needed. They can travel quickly through connected cells.

However, electrical messages have limits. They can reach only cells connected by suitable pathways. Also, a nerve cell needs time to reset before sending another impulse.

Chemical messages solve some of these problems. A chemical can spread through blood or plant tissues and affect many cells. Chemical communication is slower, but its effects can last longer and reach a wider area.

Animals and plants use both kinds of communication in different ways.

Hormones in animals

Animal hormones are chemical messengers produced by endocrine glands. These glands release hormones directly into the blood.

The blood carries the hormones throughout the body. However, a hormone affects only cells that have the correct receptors for it. These cells are called target cells.

You can think of a hormone as a letter and its receptor as the correct letterbox. The blood carries the letter everywhere, but only the right letterbox can receive it.

Adrenaline

Adrenaline is produced by the adrenal glands. It is released when an animal feels frightened, threatened, or very excited.

It prepares the body to fight or run away. This is sometimes called the “fight-or-flight” response.

Adrenaline makes the heart beat faster. This sends more blood and oxygen to the muscles.

It increases the breathing rate. The lungs take in more oxygen, and the blood carries that oxygen to the muscles.

It reduces blood flow to the digestive system and skin. More blood is directed to the skeletal muscles, which are more important during an emergency.

These changes happen together so the body can react quickly.

Thyroxine and iodine

The thyroid gland produces thyroxine.

Thyroxine helps control metabolism. Metabolism means all the chemical activities that happen inside the body, including how food is changed into energy.

The thyroid gland needs iodine to make thyroxine. If a person does not get enough iodine, the thyroid gland may become enlarged. This condition is called goitre, and it can cause swelling in the neck.

Iodised salt contains added iodine. Eating iodised salt helps provide the iodine needed to make thyroxine.

Growth hormone

The pituitary gland produces growth hormone.

Growth hormone helps the body grow and develop. It affects many organs and tissues.

If a child produces too little growth hormone, the child may become much shorter than usual. This condition is called dwarfism.

Hormones must be released in the correct amount. Too little or too much can cause problems.

Testosterone and oestrogen

During puberty, the body changes from a child’s body towards an adult body.

Testosterone is mainly produced by the testes. It helps cause male body changes during puberty, including development of male reproductive organs and other physical changes.

Oestrogen is mainly produced by the ovaries. It helps cause female body changes during puberty and helps regulate the menstrual cycle.

These hormones help the body grow and develop in an organised way. For example, fingers grow from the hands rather than appearing randomly on the face.

Insulin

Insulin is produced by the pancreas.

After you eat, food is broken down and some of it becomes glucose, a type of sugar. Glucose enters the blood.

If blood glucose becomes too high, the pancreas releases insulin. Insulin helps body cells take in glucose and helps lower the amount of sugar in the blood.

If the body does not produce enough insulin, blood sugar can remain too high. This condition is associated with diabetes. Some people need insulin injections to replace the insulin their bodies do not make properly.

Feedback mechanisms

The body needs to control hormone release carefully. It does this through feedback mechanisms.

Imagine a room with a heater. If the room becomes too cold, the heater turns on. When the room becomes warm enough, the heater turns down or off. The body uses a similar system.

When blood sugar rises, the pancreas releases more insulin. Insulin lowers the blood sugar. When the sugar level falls, the pancreas releases less insulin.

This keeps the blood sugar from becoming too high or too low.

The hypothalamus and hormone control

The hypothalamus is a part of the brain that helps control hormone release.

For example, if the amount of growth hormone in the body is low, the hypothalamus releases a growth hormone releasing factor. This tells the pituitary gland to release more growth hormone.

The hypothalamus and pituitary gland therefore help regulate several other hormone systems.

Nervous control compared with hormonal control

Nervous control uses electrical impulses travelling through neurons. It is very fast and usually affects a specific place for a short time. Pulling your hand away from a flame is an example.

Hormonal control uses chemicals carried by blood or other body fluids. It is slower and may affect many parts of the body for a longer time. Adrenaline preparing the whole body for danger is an example.

Both systems work together. The nervous system can notice a danger quickly, while hormones can prepare many organs for the response.

The main idea of the document is that living organisms must detect changes, send information, and produce suitable responses. Animals mainly use nerves, the brain, spinal cord, muscles, and hormones. Plants use electrical and chemical signals, growth, changes in water content, and plant hormones. This coordination allows organisms to survive, grow, protect themselves, and respond to their environment.