Structure and function — AQA GCSE Biology

The nervous system allows humans to detect changes in their surroundings and produce appropriate responses. Receptors detect stimuli such as light, sound, touch, temperature and chemicals. Electrical impulses travel along sensory neurones to the central nervous system, which processes the information and coordinates a response. Motor neurones then carry impulses to effectors, which are muscles or glands. Muscles contract to produce movement, while glands secrete chemicals such as hormones. This coordination allows rapid reactions, including reflexes such as withdrawing a hand from a hot object, and more complex behaviours such as catching a ball. The nervous system therefore links detection of a stimulus to a coordinated response, helping the body respond to its environment and maintain control.

When a receptor detects a stimulus, it generates an electrical impulse. This impulse travels along sensory neurones to the central nervous system, which consists of the brain and spinal cord. Sensory neurones are adapted to carry impulses rapidly: they are long, have branched endings to connect with receptors and other neurones, and are insulated by a myelin sheath that speeds up transmission. At synapses between neurones, the impulse triggers the release of a neurotransmitter that diffuses across the gap and stimulates the next neurone. The CNS processes the information and coordinates a response, which is then carried by motor neurones to effectors. This pathway allows the body to react quickly to changes in the internal and external environment.

The central nervous system, or CNS, is the body's main processing centre and consists of two linked organs: the brain and the spinal cord. The brain receives information from receptors, integrates it and issues instructions; the spinal cord carries signals between the brain and the rest of the body and also coordinates some rapid reflexes on its own. In mammals the CNS is protected by the skull and vertebral column, and by membranes and fluid. It is distinct from the peripheral nervous system, which is the network of nerves connecting receptors and effectors to the CNS. For example, touching a hot object sends impulses along sensory neurones into the spinal cord, which can trigger a response even before the brain consciously registers pain.

After receptors detect a stimulus, impulses travel to the central nervous system, where information is processed and a response is coordinated. The CNS then sends impulses along motor neurones to effectors. Effectors are the parts of the body that carry out the response, and there are two main types. A muscle effector responds by contracting, which shortens the muscle and moves a body part, as when the biceps contracts to bend the arm. A gland effector responds by secreting a chemical, such as a hormone, into the blood; for example, the adrenal glands secrete adrenaline. Coordination therefore means the CNS decides which effector should respond and ensures the response is appropriate to the stimulus.

This arrow sequence summarises the pathway of a control system. A stimulus is a change in the environment, such as a rise in blood glucose or a drop in skin temperature. A receptor detects the stimulus and generates electrical impulses. The coordinator, such as the brain, spinal cord or pancreas, receives and processes the information. The effector, a muscle or gland, then brings about a response. For example, touching something hot: temperature receptors in the skin detect the stimulus; the spinal cord coordinates a rapid reflex; the arm muscle contracts as the effector; the hand pulls away as the response. In blood glucose control, pancreatic receptors detect the change, the pancreas coordinates, and liver cells respond by storing or releasing glucose. The response restores optimum levels, usually by negative feedback.

A reflex arc is the fixed pathway taken by a nerve impulse during a rapid, automatic response that does not involve conscious thought. A receptor detects a stimulus and generates an electrical impulse in a sensory neurone, which carries it towards the central nervous system. At a synapse, the impulse causes release of a neurotransmitter that diffuses across the tiny gap and stimulates the next neurone, so the signal passes to a relay neurone inside the spinal cord or brain. The relay neurone links sensory and motor pathways, allowing the impulse to be processed and directed. A motor neurone then carries the impulse to an effector, such as a muscle or gland, which produces the response. Each structure's shape and position suit its role: long fibres conduct quickly, and the synapse ensures one-way transmission.

Reflex actions are rapid, automatic responses that do not require conscious thought. They are important because they protect the body from harm and help maintain a stable internal environment. Because the pathway through the spinal cord is short and does not involve the brain's conscious processing, the response happens very quickly, which can prevent injury; for example, withdrawing a hand from a hot surface or blinking when something approaches the eye. Reflexes also free the brain to deal with complex tasks while automatic responses continue. Some reflexes, such as those controlling heart rate, breathing and digestion, help regulate conditions inside the body. In examinations, explaining importance means linking speed and automaticity to survival or protection.

A reflex action is a fast, involuntary response that protects the body from harm. A receptor detects a stimulus, such as a sharp pinprick, and generates an electrical impulse in a sensory neurone. This impulse travels to a relay neurone in the spinal cord or an unconscious region of the brain, then to a motor neurone, which carries it to an effector such as a muscle. The muscle contracts and the hand pulls away. Because the relay neurone links directly to the motor neurone, the pathway is short and the response is rapid. The conscious part of the brain is not involved in deciding to move, so the action is automatic and cannot be prevented once triggered. This reduces damage while the brain processes the pain separately.

Nervous system data often appear as tables of reaction times, bar charts comparing groups, or line graphs showing how a response changes with a variable such as caffeine intake or age. To extract data, read the axes and units carefully, locate the required category or point, and quote the value with its unit. To interpret data, describe the overall trend, compare values between groups or conditions, and use the numbers to support each statement. For example, a bar chart might show that mean reaction time falls from 0.30 s to 0.22 s after a period of practice, indicating improved speed. You should also recognise anomalous results, describe patterns such as a plateau, and relate the pattern to nervous system function, for example faster impulse transmission or increased alertness.

Reaction time data can be presented as a table of numerical values or as a graph, and you may need to move between the two. To translate from a table to a graph, choose a sensible scale, label both axes with the variable and unit, plot each pair of values accurately, and join points with straight lines or a smooth curve as appropriate. To translate from a graph to numerical form, read the value on the vertical axis for a given value on the horizontal axis, or read the horizontal value for a given vertical value. For example, if a graph shows reaction time falling as practice time increases, you can read that at 4 minutes the reaction time is 0.24 s and record this in a table. Always keep units consistent and check that plotted points match the original data.

In this practical you investigate how a chosen factor, such as caffeine intake, background noise, exercise or time of day, affects human reaction time. A common method uses a ruler drop test: a partner holds a ruler vertically with the zero mark level with your open thumb and finger, then releases it without warning. You catch the ruler and record the distance it falls. A shorter distance indicates a faster reaction time. You should plan a hypothesis, identify the independent variable and control relevant variables such as the same hand, same partner and same starting position. Repeat readings and calculate a mean to improve reliability, and consider ethical issues such as informed consent. Present results in a table and graph, then interpret whether the factor affected reaction time.

This statement identifies the working scientifically skills developed through Required Practical 7: investigating the effect of a factor on human reaction time. AT 1 involves using appropriate apparatus to make and record measurements accurately, such as using a ruler to measure drop distance to calculate time. AT 3 involves using techniques for the observation and measurement of biological changes, such as measuring physiological response times. AT 4 involves the safe and ethical use of living organisms to measure physiological functions, ensuring human subjects are treated safely. Students should plan the method, record clear results, and interpret the evidence regarding the nervous system.