Skip to content
Get started
Biology 061014.1

The nervous system and reflex actions

Neurones, synapses and the reflex arc.

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 14.1.1Describe the human nervous system and the reflex arc.

5 minute read

The nervous system and reflex actions

The nervous system coordinates fast responses using electrical impulses.

Structure

  • Central nervous system (CNS) — brain and spinal cord, where decisions are made.
  • Peripheral nervous system — the nerves connecting the CNS to the rest of the body.

Three neurone types: sensory (receptor → CNS), relay (within the CNS), and motor (CNS → effector).

The reflex arc

A reflex action is a rapid, automatic response that does not involve conscious thought — which is exactly what makes it fast enough to protect you. The pathway, which must be learned in order: stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response Touching a hot object: heat is the stimulus, receptors in the skin detect it, the impulse travels to the spinal cord, and a motor neurone makes the arm muscle (the effector) contract to pull the hand away. The brain registers the pain afterwards — the hand is already moving.

Synapses

Neurones do not physically touch. At a synapse, the impulse arriving triggers release of a neurotransmitter, which diffuses across the tiny gap and starts a new impulse in the next neurone. Because the transmitter is released on one side and detected on the other, synapses make impulses travel in one direction only.

Think of it like this

A reflex is a spinal shortcut — like a receptionist who redirects an urgent call immediately instead of waiting for the manager. The manager (brain) is told, but only after the action has been taken.

Worked examples

Method, step by step

Describe the pathway of a reflex action when a person steps on a sharp object.

  1. 1The sharp object is the stimulus, detected by pain receptors in the skin of the foot.
  2. 2A sensory neurone carries the impulse to a relay neurone in the spinal cord.
  3. 3A motor neurone carries the impulse to the leg muscle.
  4. 4The muscle (effector) contracts, lifting the foot away.

Stimulus → receptor in skin → sensory neurone → relay neurone in spinal cord → motor neurone → leg muscle (effector) → foot lifts away.

Common misconceptions

  • Thinking the brain controls reflexes. The spinal cord handles the reflex; the brain is informed afterwards, which is why the response is so fast.
  • Saying neurones touch each other. They are separated by a synaptic gap crossed chemically by a neurotransmitter.
  • Mixing up receptor and effector. The receptor detects the stimulus; the effector (muscle or gland) carries out the response.

In the exam

  • Write the reflex arc as an ordered sequence with arrows. Marks are given for the correct order, and a jumbled list loses them.
  • Always name the effector specifically — "muscle contracts" or "gland secretes" — rather than just writing "the body reacts".