The nervous system allows the body to detect changes in the environment and respond to them rapidly. It consists of the brain, spinal cord, and a network of nerves that carry electrical signals called impulses throughout the body.
What is the nervous system?
The human nervous system has two main divisions:
- Central nervous system (CNS) — the brain and spinal cord. This is where information is processed and decisions are made.
- Peripheral nervous system (PNS) — all the nerves that branch out from the CNS to the rest of the body, carrying signals to and from organs, muscles, and sense organs.
The nervous system works using electrical impulses that travel along specialised cells called neurones.
Neurones: the cells of the nervous system
A neurone is a nerve cell adapted to carry electrical impulses rapidly over long distances. There are three main types:
| Neurone type | Function | Location |
|---|---|---|
| Sensory neurone | Carries impulses FROM receptors TO the CNS | In sense organs and skin |
| Relay neurone | Connects sensory and motor neurones within the CNS | In the brain and spinal cord |
| Motor neurone | Carries impulses FROM the CNS TO muscles and glands | Between CNS and effectors |
A typical neurone has:
- A cell body (containing the nucleus)
- Dendrites — short branching extensions that receive signals from other neurones
- An axon — a long fibre that carries the impulse away from the cell body
- A myelin sheath in many neurones — a fatty insulating layer that speeds up signal transmission
The pathway: stimulus → response
Every nervous system response follows this sequence:
Stimulus → Receptor → Sensory neurone → CNS (relay neurone) → Motor neurone → Effector → Response
- Stimulus: a detectable change in the environment (e.g. a bright light, a hot surface, a loud sound).
- Receptor: a specialised cell or organ that detects the stimulus (e.g. rod and cone cells in the eye detect light; thermoreceptors in the skin detect heat).
- Effector: a muscle (which contracts) or a gland (which secretes a substance) that produces the response.
Example: You touch a hot pan.
Stimulus = heat → Receptor = thermoreceptors in fingertip → CNS processes signal → Motor neurone → Effector = muscles in arm → Response = hand pulled away.
The reflex arc
A reflex is a rapid, automatic, involuntary response to a stimulus. Reflexes do not involve conscious thought — they happen faster than a voluntary action because the signal does not need to travel all the way to the brain to be processed. Instead, it is processed in the spinal cord.
The pathway for a simple spinal reflex is:
Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector
The knee-jerk reflex: a worked example
When a doctor taps just below the kneecap (the patellar tendon), the lower leg kicks outward automatically. This is the patellar reflex:
- The tap stretches the tendon → stimulus
- Stretch receptors in the muscle detect the stretch → receptor detects stimulus
- Impulse travels along a sensory neurone to the spinal cord
- A relay neurone in the spinal cord connects to a motor neurone (no brain involvement needed)
- Impulse travels along a motor neurone to the quadriceps muscle in the thigh
- The quadriceps contracts → the leg kicks → response
The whole sequence takes less than 50 milliseconds — faster than the brain can react.
The withdrawal reflex: touching a pin
- Pin pricks the finger → stimulus (pain)
- Pain receptors in the fingertip → receptor
- Impulse travels via sensory neurone to spinal cord
- Relay neurone in spinal cord connects to motor neurone (AND sends a signal up to the brain so you feel the pain — but the withdrawal happens before conscious awareness)
- Motor neurone carries impulse to biceps muscle
- Biceps contracts, arm pulls away → response
Why are reflexes important?
Reflexes protect the body from harm faster than conscious thought allows. The pupil reflex (the iris contracting to reduce pupil size in bright light) protects the retina. The coughing and sneezing reflexes expel irritants from the airways. Swallowing and blinking are also partly reflex actions.
Because reflex responses bypass the conscious brain, they are described as involuntary — you cannot easily choose not to do them.
Voluntary actions vs reflex actions
| Feature | Voluntary action | Reflex action |
|---|---|---|
| Conscious control? | Yes | No |
| Speed | Slower | Very fast |
| Processing in | Brain (cerebrum) | Spinal cord (usually) |
| Example | Picking up a pen | Pulling hand away from heat |
| Can it be prevented? | Yes | Difficult; some can be suppressed with training |
The brain and CNS functions
Different regions of the brain have different functions:
- Cerebrum — conscious thought, language, memory, voluntary movement
- Cerebellum — coordination of movement and balance
- Medulla oblongata — controls automatic functions: breathing rate, heart rate, blood pressure
- Hypothalamus — regulates body temperature and links the nervous system to the endocrine (hormonal) system
According to the Department for Education's KS3 Science Programme of Study, pupils should be taught about the structure and functions of the human nervous system, including how it coordinates the body's response to stimuli. BBC Bitesize KS3 Biology covers the neurone pathway, reflex arc, and the distinction between voluntary and involuntary responses as core content for Year 8 and Year 9 (see bbc.co.uk/bitesize/subjects/z9ddmp3).
Frequently asked questions
What is the difference between a voluntary action and a reflex action?
A voluntary action is a deliberate, conscious response controlled by the cerebrum in the brain — for example, deciding to wave at a friend. A reflex action is an automatic, unconscious response that does not require the brain to initiate it — the signal is processed in the spinal cord and the response occurs before you are even aware of the stimulus.
Why do reflexes bypass the brain?
In a spinal reflex, the sensory neurone connects to a relay neurone in the spinal cord, which connects directly to a motor neurone. This shorter pathway means the response is much faster than waiting for a signal to travel up to the brain and back. A signal also travels up to the brain in parallel, which is why you feel pain a fraction of a second after you have already moved your hand — the reflex came first.
What is a synapse?
A synapse is the tiny gap between one neurone and the next. Electrical impulses cannot jump across the gap, so when an impulse reaches the end of a neurone, chemical signals called neurotransmitters are released. They diffuse across the synapse and bind to receptors on the next neurone, triggering a new electrical impulse. Synapses allow the direction of signals to be controlled and allow signals to be modified.
How is the nervous system different from the endocrine system?
Both systems coordinate responses, but they work differently. The nervous system uses electrical impulses and gives rapid, short-lived, precise responses. The endocrine system uses hormones carried in the blood and gives slower, longer-lasting, widespread responses. For example, the nervous system controls the immediate withdrawal reflex from heat, while the endocrine system controls growth and puberty over months or years.
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