The Nervous System — Structure and Function

This lesson covers the structure and function of the human nervous system as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand how the nervous system detects stimuli, transmits electrical impulses, and coordinates rapid responses.

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Overview of the Nervous System

The nervous system enables humans to react to their surroundings and coordinate their behaviour. It does this by transmitting electrical signals (nerve impulses) extremely quickly along specialised cells called neurones.

The nervous system is divided into two main parts:

graph TD
    A["Nervous System"] --> B["Central Nervous System (CNS)"]
    A --> C["Peripheral Nervous System (PNS)"]
    B --> D["Brain"]
    B --> E["Spinal Cord"]
    C --> F["Sensory Neurones"]
    C --> G["Motor Neurones"]

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The Central Nervous System (CNS)

The CNS consists of the brain and the spinal cord. It acts as the body's coordination centre:

• It receives information from receptors via sensory neurones.
• It processes and integrates the information.
• It sends instructions to effectors via motor neurones.

Exam Tip: In the AQA Trilogy spec (8464), you are not required to know the detailed structure of the brain, but you must know that the CNS comprises the brain and spinal cord and that it coordinates responses.

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Types of Neurone

There are three types of neurone you must know:

Neurone	Function	Direction of Impulse
Sensory neurone	Carries impulses from receptors to the CNS	Receptor → CNS
Relay neurone	Connects sensory and motor neurones within the CNS	Within the CNS
Motor neurone	Carries impulses from the CNS to effectors	CNS → Effector

Neurone Structure

All neurones share the following features:

• Cell body — contains the nucleus and most of the cytoplasm.
• Dendrites — short, branching fibres that receive impulses from other neurones or receptors.
• Axon — a long fibre that carries electrical impulses away from the cell body.
• Myelin sheath — an insulating fatty layer around some axons that speeds up impulse transmission.

flowchart LR
    A["Dendrites"] --> B["Cell Body\n(nucleus)"]
    B --> C["Axon\n(myelin sheath)"]
    C --> D["Axon Terminal\n(synaptic knob)"]

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Receptors

Receptors are specialised cells or groups of cells that detect changes in the environment (stimuli). Different receptors detect different stimuli:

Receptor Type	Location	Stimulus Detected
Photoreceptors	Eyes (retina)	Light
Chemoreceptors	Tongue, nose	Chemicals (taste and smell)
Mechanoreceptors	Ears, skin	Sound, pressure, touch
Thermoreceptors	Skin, hypothalamus	Temperature
Pressure receptors	Skin	Touch and pressure

Exam Tip: Do not confuse cell-surface receptors (protein molecules on cell membranes that bind to specific chemicals, e.g. hormones or neurotransmitters) with receptor cells (specialised cells that detect stimuli). Both appear in the specification.

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Effectors

Effectors carry out a response once they receive impulses from the CNS. There are two types:

1. Muscles — contract to produce movement.
2. Glands — secrete (release) chemical substances such as hormones or enzymes.

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Synapses

A synapse is the junction between two neurones. There is a tiny gap (the synaptic cleft) between them. Electrical impulses cannot jump across this gap, so a chemical process transmits the signal:

1. An electrical impulse arrives at the end of the pre-synaptic neurone (the synaptic knob).
2. This triggers the release of neurotransmitter molecules from vesicles into the synaptic cleft.
3. The neurotransmitter molecules diffuse across the synaptic cleft.
4. They bind to specific receptors on the membrane of the post-synaptic neurone.
5. This triggers a new electrical impulse in the post-synaptic neurone.

sequenceDiagram
    participant Pre as Pre-synaptic Neurone
    participant Cleft as Synaptic Cleft
    participant Post as Post-synaptic Neurone
    Pre->>Cleft: Releases neurotransmitter
    Note over Cleft: Neurotransmitter diffuses across gap
    Cleft->>Post: Binds to receptors
    Note over Post: New electrical impulse triggered

Why Do Synapses Use Chemicals?

• Neurotransmitters ensure impulses travel in one direction only (pre-synaptic → post-synaptic), because vesicles are only on the pre-synaptic side and receptors only on the post-synaptic side.
• Synapses allow impulses to be directed along specific pathways.

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The Pathway from Stimulus to Response

The full pathway for a conscious (voluntary) nervous response is:

$\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Sensory neurone} \rightarrow \text{CNS (relay neurone)} \rightarrow \text{Motor neurone} \rightarrow \text{Effector} \rightarrow \text{Response}$Stimulus→Receptor→Sensory neurone→CNS (relay neurone)→Motor neurone→Effector→Response

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Common Mistakes

• Saying impulses "travel through" a synapse — impulses do not cross the gap. Neurotransmitter chemicals diffuse across and trigger a new impulse.
• Confusing sensory and motor neurones — remember: Sensory = Sense organ to CNS; Motor = CNS to Muscle/gland.
• Forgetting that relay neurones are in the CNS — they are not in the peripheral nervous system.

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Summary

• The nervous system consists of the CNS (brain + spinal cord) and the peripheral nervous system (sensory and motor neurones).
• Sensory neurones carry impulses from receptors to the CNS; motor neurones carry impulses from the CNS to effectors.
• Relay neurones connect sensory and motor neurones within the CNS.
• Synapses transmit signals between neurones using neurotransmitter chemicals.
• The nervous system produces fast, short-lived responses.
• This content is assessed in AQA GCSE Combined Science Trilogy (8464), Paper 1: Biology.

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Extended Examples and Exam Technique

Extended Worked Example — Describing a Complete Nervous Pathway

Describe how information about a loud noise reaches the brain and produces a response of turning the head. [6 marks]

Model answer: