The Nervous System

Overview

The nervous system allows the body to detect stimuli (changes in the environment) and coordinate responses. It is made up of the brain, spinal cord (together forming the central nervous system, CNS) and the peripheral nervous system (all other nerves).

Stimulus → Receptor → Coordinator (CNS) → Effector → Response

Receptors

Receptors are specialised cells that detect a specific type of stimulus and convert it to a nerve impulse:

• Photoreceptors in the eye (retina) — detect light
• Thermoreceptors in the skin — detect temperature change
• Mechanoreceptors in the skin — detect pressure and touch
• Chemoreceptors in the nose and tongue — detect chemicals (smell and taste)
• Auditory receptors in the ear — detect sound vibrations

Neurones (Nerve Cells)

Neurones carry electrical impulses (nerve signals) through the nervous system. There are three main types:

Structure of a motor neurone:

• Cell body (nucleus, cytoplasm)
• Dendrites — short branches receiving impulses from other neurones
• Axon — long fibre that carries the impulse away from the cell body
• Myelin sheath — fatty insulating layer around the axon; speeds up impulse conduction (impulse 'jumps' between nodes of Ranvier)
• Synaptic knob — end of axon that releases neurotransmitters

Synapses

A synapse is the tiny gap between two neurones (or between a neurone and an effector).

How a synapse works:

1. Electrical impulse arrives at the synaptic knob of the pre-synaptic neurone
2. Triggers release of neurotransmitter (chemical) from vesicles into the synapse gap
3. Neurotransmitter diffuses across the gap and binds to receptor proteins on the post-synaptic membrane
4. This triggers a new electrical impulse in the post-synaptic neurone
5. Neurotransmitter is then broken down and recycled (or reabsorbed)

Key functions of synapses:

• Ensure impulses travel in one direction only (receptors only on one side)
• Allow integration of signals (multiple impulses needed to trigger a response)
• Drugs (e.g., beta-blockers, painkillers, recreational drugs) can interfere with synaptic transmission

Reflex Arcs

A reflex action is a rapid, involuntary (automatic) response to a stimulus. It bypasses the conscious brain — the spinal cord acts as the coordinator.

Why reflexes are important: Very fast (no time wasted sending signal to the brain first) → protect the body from harm.

The reflex arc: Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response

Example — withdrawing hand from a pin:

1. Pin prick (stimulus) detected by pain receptors in fingertip
2. Sensory neurone carries impulse to spinal cord
3. Relay neurone in spinal cord links to motor neurone
4. Motor neurone sends impulse to bicep muscle (effector)
5. Muscle contracts → hand pulls away (response) (Brain receives information about the event afterwards — that's why you feel pain slightly after the reflex)

The Brain

The brain controls complex, voluntary behaviour. Key regions:

• Cerebral cortex — conscious thought, language, memory, personality
• Cerebellum — balance and coordination of movement
• Medulla oblongata — controls unconscious processes (heart rate, breathing rate)

WJEC note: Higher Tier papers may ask about specific brain regions and their functions.