3.1.1Organism Level Systems

The Nervous System: Structure — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

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📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Why organisms need a nervous system

To survive, an organism must detect changes in its surroundings and in its own body, and respond to them.

Why organisms need a nervous system

Stimulus

A change that can be detected is called a stimulus.

Nervous system

Animals such as humans coordinate their responses with the nervous system, which passes information as fast electrical impulses.

Impulses travel along nerve cells to bring about responses within a fraction of a second,

which is much faster than the chemical messages of the hormonal system.

The nervous system has two main parts. The central nervous system (CNS) consists of the brain and spinal cord and is where information is processed and decisions are made. The peripheral nervous system consists of the nerves that connect the CNS to the rest of the body.

Part 2 of 7The pathway from stimulus to response

A response follows the same sequence:

Stimulus (change in the environment) then receptor (detects the stimulus) then sensory neurone (carries impulses to the CNS) then coordinator (the CNS, which processes the information) then motor neurone (carries impulses from the CNS) then effector (a muscle or gland) then response.

Receptors are cells or organs that are sensitive to a particular type of stimulus.

The pathway from stimulus to response

Examples include light receptors in the retina of the eye, sound receptors in the cochlea of the ear, taste receptors on the tongue, smell receptors in the nose, pressure and touch receptors in the skin,

and temperature receptors in the skin.

Effectors are muscles, which respond by contracting, and glands, which respond by secreting a hormone or enzyme.

Part 3 of 7Neurones: structure and function

Neurones are specialised cells for carrying impulses. Features common to most types are a long axon, which carries the impulse over distance, and a cell body containing the nucleus. Many axons are surrounded by a fatty myelin sheath that insulates the axon and speeds up the impulse. Neurones have branched endings so they can connect with other cells.

Type	Carries impulses	Notes
Sensory neurone	From receptor to CNS	Long dendron and axon; cell body branches off the middle
Relay neurone	Within the CNS	Short, connects sensory and motor neurones
Motor neurone	From CNS to effector	Long axon; cell body at the CNS end

Part 4 of 7Synapses

Neurones do not touch each other.

Synapse

Between them is a tiny gap called a synapse.

An impulse arriving at the end of one neurone triggers it to release chemical transmitter molecules into the gap.

These molecules diffuse across the gap and bind to receptor molecules on the next neurone, which starts a new impulse in it.

Because the transmitter is released only from one side, impulses at synapses travel in one direction only.

Synapses add some delay, but this is tiny.

Many drugs, such as some painkillers and antidepressants, act at synapses by changing the amount of transmitter present or the way it binds.

Part 5 of 7Reflex arcs

A reflex is a fast, automatic response that does not involve conscious thought.

Reflexes protect the body from harm: for example, pulling a hand away from a hot object, or the pupil narrowing in bright light.

A reflex arc passes information along a short pathway: receptor, sensory neurone, relay neurone in the spinal cord (or brain), motor neurone,

and effector.

Because the brain is not required to decide, the response is quicker than a voluntary one.

The relay neurone usually also sends information to the brain, so that you become aware of what happened.

Voluntary responses involve the brain choosing what to do, so they are slower but more flexible.

Part 6 of 7How an impulse is carried and why speed matters

An impulse is a tiny electrical signal that passes along the membrane of the axon.

How an impulse is carried and why speed matters

Myelin insulates the axon, so in myelinated neurones the impulse can jump quickly between gaps, travelling at up to about 100 metres per second.

This speed matters for reflexes: a hand that touches a hot object must be withdrawn before it is seriously burned.

Motor neurones that serve the limbs may be a metre long,

which is why a long single cell with an insulated axon is such a good adaptation for rapid communication over a distance.

The nervous system and the hormonal system work together but differ.

How an impulse is carried and why speed matters

Nervous responses are fast, short-lived and act on a precise target; hormonal responses are slower, last longer and may affect several organs.

When a question asks you to compare the two, give one point for speed, one for duration and one for the way the message is carried (electrical impulse and neurotransmitter, versus hormone in the blood).

Diseases such as multiple sclerosis, in which myelin is damaged, show the importance of the structure: impulses slow down or fail,

and muscles do not respond properly.

Part 7 of 7Exam-style questions

Describe the pathway taken by a nerve impulse when you touch a hot pan and pull your hand away. [4]4 marks
Explain why impulses can only pass in one direction across a synapse. [2]2 marks
Give two differences between a reflex action and a voluntary action. [2]2 marks

Key terms

Term	Meaning
Stimulus	A change in the environment that can be detected.
Receptor	A cell or organ that detects a stimulus.
Effector	A muscle or gland that carries out a response.
Neurone	A nerve cell specialised to carry electrical impulses.
Synapse	The gap between two neurones, crossed by chemical transmitter.
Reflex	A rapid automatic response to a stimulus.
Central nervous system	The brain and spinal cord.
Myelin sheath	Fatty insulating layer around some axons.

Avoid these mistakes

Saying that nerves and neurones are the same thing; a nerve is a bundle of many neurones.

Thinking impulses jump across the synapse as electricity; a chemical transmitter diffuses across.

Missing the relay neurone from the reflex arc, or the effector at the end.

Saying that a reflex "does not involve the nervous system"; it does, but without conscious thought.

Writing that the brain is a receptor; the brain is part of the CNS (the coordinator).

Last updated: 20 October 2026