Understanding Coordination: Edexcel IGCSE Human Biology Explained

Coordination is the specification's broadest section, spanning the nervous system, the senses, hormones, drugs and mental health. Think of it as the body's control network, everything that detects a stimulus and produces a response.

If you have ever flinched when a ball flies towards your face, adjusted to a dark cinema, or felt your heart pound before a sprint, you have experienced coordination at work. The edexcel igcse human biology specification breaks this into three topics: the nervous system, sense organs, and drugs and mental health. These edexcel igcse human biology revision notes for the 4HB1 specification cover all three with the depth and precision the exam rewards. A thorough understanding of edexcel igcse human biology coordination prepares you for questions that span the nervous system, hormones and the senses in a single extended response.

The nervous system

Neurone types

The specification requires you to know the structure of three types of neurone:

The central nervous system

The CNS consists of the brain and the spinal cord. All coordination of responses passes through the CNS. Sensory neurones carry information in; motor neurones carry instructions out.

The brain

The specification names five areas of the brain and their functions:

• Cerebral hemispheres: The largest part of the brain. Responsible for conscious thought, memory, learning, language, and voluntary actions.
• Cerebellum: Located at the back of the brain, beneath the cerebral hemispheres. Controls balance, posture, and the coordination of voluntary movements. Think of it like this: the cerebral hemispheres decide to pick up a glass, but the cerebellum makes sure your hand moves smoothly and does not knock it over.
• Mid brain: Connects the cerebral hemispheres to the rest of the brain. Involved in visual and auditory reflexes.
• Pituitary gland: A small gland at the base of the brain. Secretes several hormones including ADH, FSH, LH, and growth hormone. Often called the "master gland" because it controls other endocrine glands.
• Hypothalamus: Located just above the pituitary gland. Monitors conditions in the blood (temperature, water content, glucose levels) and controls the pituitary gland. It is the link between the nervous system and the endocrine (hormonal) system.

Reflex arcs

A reflex arc is the pathway a nerve impulse follows during a rapid, automatic response. The sequence is:

1. A receptor detects a stimulus (e.g. a sharp object touching the skin).
2. A sensory neurone carries the impulse to the spinal cord (CNS).
3. A relay neurone in the spinal cord passes the impulse to a motor neurone.
4. A motor neurone carries the impulse to an effector (a muscle or gland).
5. The effector produces the response (e.g. the muscle contracts, pulling the hand away).

Reflexes are faster than conscious responses because the impulse goes to the spinal cord rather than the brain, cutting the pathway short.

Synapses

A synapse is the tiny gap between two neurones. Nerve impulses cannot jump across the gap as an electrical signal. Instead:

1. The impulse arrives at the end of the pre-synaptic neurone.
2. Chemical messengers called neurotransmitters are released into the synaptic cleft.
3. Neurotransmitters diffuse across the gap and bind to receptors on the post-synaptic neurone.
4. This triggers a new electrical impulse in the next neurone.

Synapses ensure impulses travel in one direction only, because neurotransmitters are released on one side and receptors are on the other.

Hormones

The specification requires knowledge of hormones from several glands:

Nervous vs hormonal systems

Sense organs

The eye

The specification requires you to explain how the eye focuses on near and distant objects, responds to changes in light intensity, and achieves stereoscopic vision.

Focusing (accommodation): The lens changes shape to focus light on the retina. For distant objects, the ciliary muscles relax, the suspensory ligaments pull tight, and the lens becomes thinner and less curved. For near objects, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more curved.

Light intensity: In bright light, the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller and reducing the amount of light entering the eye. In dim light, the radial muscles contract and the circular muscles relax, making the pupil larger to allow more light in.

Stereoscopic vision: Having two eyes facing forward gives overlapping fields of view. The brain compares the slightly different images from each eye to judge distance and depth. This is especially important for judging how far away objects are.

Eye defects

Corneal transplants can replace a damaged or diseased cornea with a healthy one from a donor.

The ear

The ear has two functions: hearing and balance.

Hearing: Sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations pass through three small bones (ossicles: hammer, anvil and stirrup) in the middle ear, which amplify the vibrations. The stirrup transmits the vibrations to the oval window, which passes them into the fluid of the cochlea in the inner ear. Hair cells in the cochlea convert the vibrations into nerve impulses, which travel along the auditory nerve to the brain.

Balance: The semicircular canals in the inner ear contain fluid and sensory hair cells. When the head moves, the fluid moves, bending the hair cells and generating nerve impulses that the brain interprets as information about the direction and speed of movement.

Noise damage: Prolonged exposure to high noise levels damages the hair cells in the cochlea. Once damaged, these cells do not regenerate, leading to permanent hearing loss. This is why ear protection is essential in noisy environments.

Drugs and mental health

A drug is a substance that alters the body's chemistry or the way the nervous system works. The specification distinguishes legal and illegal drugs and requires knowledge of specific examples.

Illegal drugs:

• Heroin: A highly addictive depressant. Binds to receptors in the brain, causing euphoria and pain relief, but leads to severe physical dependence and withdrawal symptoms.
• Cannabis: Alters mood and perception. Can impair short-term memory and concentration. Long-term use is associated with mental health problems.
• Cocaine: A stimulant that increases dopamine levels in the brain, producing feelings of alertness and confidence. Highly addictive and can cause heart problems.

Common painkillers: Paracetamol works by blocking the production of chemicals (prostaglandins) that cause pain and inflammation. It acts on the nervous system to reduce the perception of pain.

Alcohol: A depressant that slows down the nervous system. It impairs coordination, reaction time and judgement. Long-term excessive drinking damages the liver (leading to cirrhosis) and the nervous system (leading to nerve damage and brain shrinkage). Behavioural consequences include impaired decision-making, aggression, and addiction.

Mental illness

The specification requires knowledge of the causes, symptoms and treatments of several conditions:

• Schizophrenia: Symptoms include hallucinations, delusions, and disordered thinking. Caused by a combination of genetic and environmental factors, linked to imbalances in neurotransmitters (particularly dopamine). Treated with antipsychotic medications.
• Depression: Symptoms include persistent low mood, loss of interest, fatigue, and changes in sleep and appetite. Linked to low levels of serotonin. Treated with antidepressants (e.g. SSRIs), talking therapies, or a combination.
• Alzheimer's disease: A progressive neurodegenerative condition causing memory loss, confusion, and personality changes. Caused by the build-up of abnormal proteins in the brain, leading to the death of neurones. Treated with medications that temporarily slow symptom progression.
• Vascular dementia: Caused by reduced blood flow to the brain, often after strokes. Symptoms include problems with planning, reasoning, and memory. Treatment focuses on managing the underlying vascular risk factors (blood pressure, cholesterol).
• Parkinson's disease: Caused by the loss of dopamine-producing neurones in the brain. Symptoms include tremor, stiffness, and slow movement. Treated with medications that increase dopamine levels (e.g. L-DOPA).

Self-check questions

1. Name the three types of neurone and state the function of each.
2. Describe the pathway of a reflex arc, from stimulus to response.
3. Explain how a nerve impulse crosses a synapse.
4. State two differences between the nervous system and the hormonal system.
5. Describe how the eye focuses on a near object.
6. Name the structure in the ear that converts sound vibrations into nerve impulses.
7. Explain why prolonged exposure to loud noise can cause permanent hearing loss.
8. State the effect of alcohol on the nervous system and give one long-term consequence of excessive drinking.

These edexcel igcse human biology notes on coordination cover all three sub-topics the specification requires. On the Green Bridge CBT platform, you can tackle coordination edexcel igcse questions and igcse 4hb1 coordination practice sets under exam conditions. For the complete picture, explore the edexcel igcse human biology explained article series and the edexcel igcse human biology practice questions available for every section of the course.