The Nervous System

This lesson covers the structure and function of the nervous system as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the components of the nervous system, the types of neurones, how stimuli are detected and how the body produces a response.

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Why Do We Need a Nervous System?

The environment around us is constantly changing. Organisms need to detect changes and respond to them in order to survive. The nervous system allows the body to:

• Detect stimuli (changes in the environment).
• Process information and make decisions.
• Coordinate rapid responses.

The nervous system uses electrical impulses to transmit information, making responses very fast (typically occurring in milliseconds).

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Components of the Nervous System

The human nervous system consists of two main parts:

1. Central Nervous System (CNS)

The CNS is made up of the brain and the spinal cord. It acts as the coordination centre — it receives information from receptors, processes it and sends instructions to effectors.

2. Peripheral Nervous System (PNS)

The PNS consists of all the nerves that connect the CNS to the rest of the body. These nerves carry impulses between the sense organs, the CNS and the effectors.

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 Stimulus–Response Pathway

The nervous system follows a clear pathway from detecting a stimulus to producing a response:

Step	Component	Function
1	Stimulus	A change in the environment (e.g. a hot surface, a loud noise, light)
2	Receptor	Detects the stimulus and generates an electrical impulse (e.g. temperature receptors in skin)
3	Sensory neurone	Carries the electrical impulse from the receptor to the CNS
4	CNS (coordinator)	Processes the information and decides on a response
5	Motor neurone	Carries the electrical impulse from the CNS to the effector
6	Effector	Carries out the response — either a muscle (contracts) or a gland (secretes a hormone or chemical)

Exam Tip: Learn the full pathway: Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector → Response. This sequence comes up frequently in exam questions.

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Receptors

Receptors are specialised cells or groups of cells that detect stimuli. Different receptors detect different types of stimulus:

Receptor Location	Stimulus Detected
Eyes (retina)	Light
Ears	Sound and position/balance
Nose	Chemicals (smell)
Tongue	Chemicals (taste)
Skin	Temperature, pressure, pain, touch

Receptors convert the stimulus into an electrical impulse that can be transmitted along neurones.

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Types of Neurone

There are three main types of neurone in the nervous system:

Sensory Neurones

• Carry impulses from receptors to the CNS.
• Have a long axon and a cell body located off to one side (in the middle of the neurone).

Relay Neurones (Interneurones)

• Found entirely within the CNS (brain and spinal cord).
• Connect sensory neurones to motor neurones.
• Have short axons and many connections.

Motor Neurones

• Carry impulses from the CNS to effectors (muscles or glands).
• Have a long axon, a cell body at one end and many dendrites for receiving signals.

Feature	Sensory Neurone	Relay Neurone	Motor Neurone
Direction	Receptor → CNS	Within CNS	CNS → Effector
Cell body position	Middle of the neurone	Within CNS	At one end
Axon length	Long	Short	Long
Location	PNS	CNS only	PNS

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Effectors

Effectors are the parts of the body that carry out a response. There are two types:

• Muscles — respond by contracting (e.g. pulling hand away from a hot object, running away from danger).
• Glands — respond by secreting chemicals or hormones (e.g. the adrenal gland releasing adrenaline, salivary glands producing saliva).

Exam Tip: Always remember that effectors include both muscles and glands. Many students only mention muscles — do not forget glands.

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Electrical Impulses and Speed

Nerve impulses travel as electrical signals along neurones. Key features:

• Impulses travel very quickly — typical speeds range from 1 m/s to over 100 m/s.
• The myelin sheath (a fatty insulating layer surrounding some axons) speeds up transmission by allowing the impulse to "jump" between gaps in the sheath.
• Nerves are bundles of many neurones enclosed in a connective tissue sheath.

Exam Tip: Be precise with terminology. A neurone is a single nerve cell. A nerve is a bundle of many neurones grouped together. Do not use these terms interchangeably.

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Summary

• The nervous system detects stimuli and coordinates rapid responses using electrical impulses.
• The CNS (brain and spinal cord) is the coordination centre; the PNS connects the CNS to the rest of the body.
• The stimulus–response pathway is: Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector → Response.
• Receptors detect stimuli; effectors (muscles or glands) carry out responses.
• There are three types of neurone: sensory (receptor to CNS), relay (within CNS) and motor (CNS to effector).
• A neurone is a single nerve cell; a nerve is a bundle of many neurones.

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Detailed Neurone Structure

Although the Combined Science specification focuses on function, understanding structure makes the function clearer.