A level-readyAll subjects · Biology · WBI15 (Unit 5)
Neurones, reflex arcs and the nervous system
The nervous system detects stimuli and makes effectors respond, often in milliseconds. Reflexes are the fastest responses of all.
Types of neurone
- Sensory neurones carry impulses from receptors to the CNS. They have a long dendron and a cell body just outside the spinal cord (in the dorsal root ganglion).
- Relay neurones are short neurones in the CNS that link sensory and motor neurones.
- Motor neurones carry impulses from the CNS to effectors (muscles or glands). The cell body is in the CNS and a long axon runs to the effector.
Many neurones are myelinated: Schwann cells wrap around the axon, forming a fatty myelin sheath that insulates it. Gaps between Schwann cells are nodes of Ranvier.
Stimulus to response and the reflex arc
Stimulus → receptor → sensory neurone → CNS (coordinator) → motor neurone → effector → response
A reflex is a rapid, automatic (involuntary) response that protects the body, for example withdrawing a hand from a hot object:
1. Receptors in the skin are stimulated.
2. A sensory neurone carries impulses into the spinal cord through the dorsal root.
3. A relay neurone in the grey matter passes the impulse to a motor neurone.
4. The motor neurone leaves through the ventral root to the biceps, which contracts.
Only three neurones and two synapses are involved, so the response is fast. The spinal cord has central grey matter (cell bodies, unmyelinated) surrounded by white matter (myelinated axons).
Organisation and comparison with hormones
- Central nervous system (CNS): brain and spinal cord.
- Peripheral nervous system (PNS): all other neurones, sensory and motor. Motor neurones form the somatic system (voluntary, to skeletal muscle) and the autonomic system (involuntary, to glands, cardiac and smooth muscle), which has sympathetic and parasympathetic divisions.
| Nervous | Hormonal |
|---|---|
| Electrical impulses along neurones (and chemicals at synapses) | Chemicals carried in the blood |
| Very fast | Slower |
| Short-lived response | Often long-lasting response |
| Localised (specific effectors) | Widespread (any cell with receptors) |
Worked example
Explain why a reflex arc gives a faster response than a voluntary action.
- The impulse passes through only a few neurones and synapses (sensory, relay, motor).
- It goes through the spinal cord without needing processing by the brain's conscious areas.
Answer: Fewer synapses and no conscious decision make the reflex faster.
Check yourself
- Which neurone carries impulses from the CNS to an effector?
- Sensory neurone
- Relay neurone
- Bipolar neurone in the retina
- Motor neurone
Show answer
Motor neurone. Motor neurones go to muscles and glands.
- What do Schwann cells form?
- The synaptic cleft
- The myelin sheath around axons
- Neurotransmitters
- The cell body of motor neurones
Show answer
The myelin sheath around axons. Schwann cells wrap round the axon, insulating it.
- Where are relay neurones found?
- In the skin
- In skeletal muscle
- In the blood
- In the central nervous system
Show answer
In the central nervous system. They link sensory and motor neurones in the CNS.
- What does the grey matter of the spinal cord mainly contain?
- Cell bodies and unmyelinated neurones
- Myelinated axons only
- Blood cells
- Schwann cells only
Show answer
Cell bodies and unmyelinated neurones. White matter is myelinated axons; grey matter has cell bodies.
- Which is a feature of a reflex action?
- It needs conscious thought
- It is controlled only by hormones
- It is rapid and involuntary
- It always involves the cerebrum deciding
Show answer
It is rapid and involuntary. Reflexes are automatic and protective.
- Which part of the nervous system controls heart rate without conscious thought?
- Autonomic nervous system
- Somatic nervous system
- Sensory neurones only
- The cerebrum alone
Show answer
Autonomic nervous system. Autonomic = automatic control of internal organs.
- Which statement compares hormonal with nervous coordination correctly?
- Hormonal responses are faster and more localised
- Hormones travel along neurones
- Hormonal responses are slower and often longer-lasting
- Nervous responses are always widespread
Show answer
Hormonal responses are slower and often longer-lasting. Hormones travel in blood, reaching many tissues.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.