Homeostasis and the Nervous System

This lesson introduces the concept of homeostasis and explores the structure and function of the human nervous system. Understanding how the body maintains a stable internal environment is central to AQA GCSE Biology and provides the foundation for the rest of the Homeostasis and Response topic.

What Is Homeostasis?

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

The conditions that need to be controlled in the human body include:

Condition	Why It Must Be Controlled	What Happens If It Is Not
Body temperature	Enzymes work best at 37 degrees C	Enzymes denature at high temperatures; reactions slow at low temperatures
Blood glucose concentration	Cells need a constant supply of glucose for respiration	Too much or too little glucose can be dangerous or fatal
Water balance	Cells need the correct concentration of water for reactions	Cells may swell and burst (lysis) or shrink and die (crenation)
pH levels	Enzymes are sensitive to pH changes	Incorrect pH causes enzymes to denature
Carbon dioxide levels	CO2 is a waste product that can lower blood pH	Excess CO2 makes blood too acidic, harming enzymes

Exam Tip: Homeostasis literally means "staying the same". In the exam, always emphasise that it is about maintaining optimum conditions for enzyme action and cell function, not just keeping things constant.

Automatic Control Systems

All homeostatic control systems involve three key components working together:

Receptors — these detect stimuli (changes in the environment). For example, temperature receptors in the skin detect a drop in external temperature.
Coordination centres — these receive and process information from receptors, then organise a response. The brain, spinal cord, and pancreas are all coordination centres.
Effectors — these carry out the response to restore optimum conditions. Effectors are muscles or glands.

flowchart LR
    A[Stimulus] --> B[Receptor]
    B --> C[Coordination Centre]
    C --> D[Effector]
    D --> E[Response]
    E -->|Negative feedback| A

Negative Feedback

Most homeostatic mechanisms use negative feedback. This is a process where the response produced by the effector works to reverse (counteract) the original change, returning the condition to its normal (set) level.

For example:

If body temperature rises above 37 degrees C, mechanisms act to cool the body down.
If body temperature falls below 37 degrees C, mechanisms act to warm the body up.
Once the normal level is restored, the corrective mechanisms are switched off.

flowchart TD
    A[Normal level] --> B{Change detected}
    B -->|Level rises| C[Receptor detects increase]
    C --> D[Coordination centre processes]
    D --> E[Effector acts to decrease level]
    E --> A
    B -->|Level falls| F[Receptor detects decrease]
    F --> G[Coordination centre processes]
    G --> H[Effector acts to increase level]
    H --> A

Exam Tip: Negative feedback is called "negative" because the response is in the opposite direction to the change. Do not confuse this with something bad happening.

The Nervous System

The nervous system enables humans to react to their surroundings and coordinate their behaviour. It uses electrical impulses to communicate rapidly.

Structure of the Nervous System

Component	Role
Central nervous system (CNS)	The brain and spinal cord — processes information and coordinates responses
Peripheral nervous system (PNS)	All the nerves outside the CNS that connect receptors and effectors to the CNS
Sensory neurones	Carry impulses from receptors to the CNS
Relay neurones	Found within the CNS; connect sensory and motor neurones
Motor neurones	Carry impulses from the CNS to effectors (muscles or glands)

Receptors and Stimuli

Receptors are specialised cells that detect changes in the environment called stimuli. Different receptors detect different stimuli:

Receptor Location	Stimulus Detected
Eyes	Light
Ears	Sound and changes in position (balance)
Nose	Chemicals (smell)
Tongue	Chemicals (taste)
Skin	Touch, pressure, temperature, pain

Neurones — Structure and Function

Neurones (nerve cells) are specialised cells that carry electrical impulses. They are elongated to cover long distances and have adaptations for rapid signal transmission.

Types of Neurone

Feature	Sensory Neurone	Relay Neurone	Motor Neurone
Direction of impulse	Receptor to CNS	Within the CNS	CNS to effector
Cell body position	In the middle of the axon	In the CNS	At one end
Axon length	Long	Short	Long
Dendrites	At one end	Multiple short branches	Short dendrites at cell body end

Key Structural Features

Axon — the long, thin fibre that carries the electrical impulse along the neurone.
Myelin sheath — a fatty insulating layer that surrounds the axon and speeds up impulse transmission.
Dendrites — thin branching extensions that receive impulses from other neurones or receptors.
Synaptic knob (nerve ending) — the end of the axon where it meets another neurone or an effector.

Synapses

A synapse is the junction between two neurones. There is a tiny gap (the synaptic cleft) between them, so the electrical impulse cannot simply jump across. Instead, chemical transmission occurs:

An electrical impulse arrives at the end of the first neurone (the pre-synaptic neurone).
This triggers the release of chemical molecules called neurotransmitters into the synaptic cleft.
The neurotransmitters diffuse across the cleft.
They bind to receptors on the membrane of the second neurone (the post-synaptic neurone).
This stimulates a new electrical impulse in the second neurone.
The neurotransmitter is then broken down or reabsorbed so the synapse is ready for the next impulse.

flowchart LR
    A[Electrical impulse arrives] --> B[Neurotransmitter released into cleft]
    B --> C[Neurotransmitter diffuses across gap]
    C --> D[Binds to receptors on post-synaptic neurone]
    D --> E[New electrical impulse generated]
    E --> F[Neurotransmitter removed from cleft]

Exam Tip: Many students confuse receptor cells (which detect stimuli) with receptor molecules on neurones (which bind neurotransmitters at synapses). Make sure you specify which type of receptor you mean in your answers.

Effectors

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

Muscles — contract to produce movement (e.g. pulling your hand away from a hot object).
Glands — secrete hormones or other chemicals (e.g. the adrenal gland releases adrenaline).

The full pathway from stimulus to response is:

Stimulus → Receptor → Sensory neurone → CNS (relay neurone) → Motor neurone → Effector → Response

Comparing Nervous and Endocrine Systems

The body uses two communication systems: the nervous system and the endocrine (hormonal) system. Although both coordinate body functions, they work in very different ways.

Feature	Nervous System	Endocrine System
Signal type	Electrical impulses	Chemical hormones
Transmission	Along neurones	In the blood
Speed	Very fast (milliseconds)	Slower (seconds to minutes)
Duration of effect	Short-lived	Longer-lasting
Area affected	Precise, localised target	Widespread, general target

Summary

Homeostasis is the maintenance of a stable internal environment, including body temperature, blood glucose, and water balance.
Homeostatic control systems consist of receptors, coordination centres, and effectors.
Negative feedback is the key mechanism: the response counteracts the original change.
The nervous system uses electrical impulses for fast, short-lived responses.
The nervous system is divided into the CNS (brain and spinal cord) and the PNS (peripheral nerves).
Neurones carry electrical impulses; there are three types: sensory, relay, and motor.
Synapses use chemical neurotransmitters to transmit signals between neurones.
Effectors (muscles and glands) carry out the response.

Exam Tip: When describing a nervous pathway in an exam, always use the correct sequence: stimulus, receptor, sensory neurone, relay neurone (in CNS), motor neurone, effector, response. Missing a step will lose you marks.