Biology · Both
The Nervous System
How the nervous system detects stimuli and coordinates responses, including reflex arcs and synapses.
The nervous system allows rapid communication. Sensory receptors detect changes; the CNS processes information; effectors (muscles or glands) respond.
Structure
• **CNS** – brain and spinal cord
• **Sensory neurone** – carries impulses from receptor to CNS
• **Motor neurone** – carries impulses from CNS to effector
• **Relay neurone** – connects neurones in the CNS
Neurones have a cell body, dendrites and a long axon (often with myelin sheath for faster transmission).
Synapses
A **synapse** is the gap between two neurones.
When an impulse arrives, a chemical **neurotransmitter** is released, diffuses across the gap and triggers an impulse in the next neurone.
This is slower than electrical transmission along the axon but allows one-way signalling and integration.
Reflex arc
A **reflex** is a rapid, automatic response that protects the body.
Pathway: stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response
The brain is not required for the immediate reflex (though it may become aware afterwards).
Worked example
Why are reflexes faster than considered responses?
Solution
They involve a short pathway through the spinal cord and do not wait for conscious processing in the brain.
Practice questions
Try these without looking at the answers first.
Q1.What does the CNS consist of?
Show answer
Brain and spinal cord
Q2.What chemical crosses a synapse?
Show answer
Neurotransmitter
Q3.Order of a reflex arc after the receptor?
Show answer
Sensory neurone → relay → motor → effector