The nervous system (B3.1)

The nervous system is examined on every OCR Gateway A Biology paper. Expect a reflex arc diagram, a question on synaptic transmission, and a 6-marker comparing nervous and hormonal coordination.

Organisation of the nervous system

• Central nervous system (CNS) = brain + spinal cord. Processes information and coordinates responses.
• Peripheral nervous system (PNS) = all nerves outside the CNS. Carries signals to and from the CNS.

Types of neurone

Neurone	Direction	Function
Sensory neurone	Receptor → CNS	Carries impulses from receptors to CNS
Relay (interneurone)	Within CNS	Connects sensory to motor neurones
Motor neurone	CNS → Effector	Carries impulses from CNS to muscles or glands

Neurone structure

• Cell body — contains nucleus.
• Axon — long fibre that carries impulses. Covered by myelin sheath (made by Schwann cells) in myelinated neurones — the myelin insulates and speeds up conduction (saltatory conduction).
• Dendrites — short branches receiving impulses from other neurones.
• Synaptic knobs — at the ends of the axon, release neurotransmitters.

The reflex arc

A reflex is an automatic, rapid response to a stimulus. It does NOT require conscious thought and does NOT pass through the brain first — impulse goes through the spinal cord.

Sequence:

Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response

Example — touching a hot plate:

1. Heat receptors in skin detect the stimulus.
2. Sensory neurone carries impulse to spinal cord.
3. Relay neurone in spinal cord connects to motor neurone.
4. Motor neurone carries impulse to bicep (effector).
5. Muscle contracts — hand withdrawn.

Meanwhile, a separate pathway sends a signal to the brain so you become aware of the pain AFTER the reflex has already fired.

Why reflexes are important

• Faster than voluntary actions (bypasses the brain's processing time).
• Protect the body from damage (no time to think; respond first).

Synaptic transmission

At the gap between two neurones (the synapse):

1. An impulse arrives at the synaptic knob.
2. Neurotransmitters (e.g. acetylcholine) are released from vesicles into the synaptic cleft.
3. Neurotransmitters diffuse across the gap.
4. They bind to receptor proteins on the postsynaptic membrane.
5. An impulse is triggered in the next neurone (or the effector is activated).
6. The neurotransmitter is broken down by enzymes and the components are recycled.

⚠ Key point: impulses travel in ONE direction across a synapse (neurotransmitters only released from the pre-synaptic side; receptors only on the post-synaptic side).

Effectors

Muscles — contract to produce movement. Glands — secrete hormones or enzymes (e.g. salivary glands secrete amylase).

Nervous vs hormonal control

Feature	Nervous	Hormonal
Speed	Very fast (ms)	Slower (seconds to minutes)
Type of signal	Electrical impulse	Chemical (hormone in blood)
Duration	Short-lived	Longer-lasting
Target	Specific effector	Specific cells with receptors
Transmission	Along neurones	Via bloodstream

Common Gateway-paper mistakes

1. Drawing the reflex arc in the wrong order (stimulus → receptor, not the other way round).
2. Forgetting relay neurones in the spinal cord — putting sensory directly to motor.
3. Saying impulses pass through the brain in a reflex — they don't (except for some cranial reflexes).
4. Confusing the synapse gap with the neurone — impulses are electrical within neurones, chemical ACROSS synapses.
5. Not naming the neurotransmitter (acetylcholine is the most common safe answer at GCSE).