1.1.5 Structure and Function of the Heart · 1. Structure and Functions in Living Organisms: Part 2 | Edexcel International GCSE Science Double Award (Modular): Biology Unit 2

Section 1 of 9Why we need a nervous system

Organisation of the Human Nervous System

Central nervous system and peripheral nervous system

Synapse: vesicles, neurotransmitters, receptor molecules

Synapse: vesicles with neurotransmitters, receptors

Organisation of the central and peripheral nervous systems

The nervous system allows humans to react to their surroundings and to coordinate their behaviour.

Why we need a nervous system

Stimulus

A change in the environment is called a stimulus.

Receptor

It is detected by a receptor, information is sent along nerve cells to the central nervous system (CNS),

and the CNS coordinates a response carried out by an effector.

Muscles

Effectors are either muscles, which contract, or glands, which secrete hormones or other chemicals.

The CNS consists of the brain and spinal cord. The nerves leading to and from it make up the peripheral nervous system. The route followed by information is: stimulus → receptor → sensory neurone → CNS (coordinator) → motor neurone → effector → response.

Section 2 of 9Receptors

Receptors are groups of specialised cells that detect particular stimuli. Different receptors are sensitive to different types of energy or chemical.

Receptor location	Stimulus detected
Eye (retina)	Light intensity and colour
Ear	Sound and changes in position of the head (balance)
Tongue	Chemicals in food (taste)
Nose	Chemicals in the air (smell)
Skin	Touch, pressure, pain, temperature

Receptors convert the energy of the stimulus into electrical impulses. These impulses travel along neurones, the cells that make up nerves.

Section 3 of 9Neurones and their adaptations

Sensory, relay and motor neurones with receptor cell and muscle

There are three types of neurone, each with a different job.

Sensory neurones

Sensory neurones carry impulses from receptors to the CNS.

Relay neurones

Relay neurones are found inside the CNS and connect sensory neurones to motor neurones.

Motor neurones

Motor neurones carry impulses from the CNS to effectors.

A neurone has a long thin axon that carries the impulse over long distances, and the axon is often covered by a fatty myelin sheath that insulates it and speeds up transmission.

Dendrites

At the ends there are branched dendrites that connect with other neurones.

Neurones are long so that a single cell can reach from the spinal cord to a finger or toe.

Section 4 of 9Synapses

Neurones do not touch each other. The tiny gap between two neurones is a synapse. The impulse is carried across it by chemicals, not electricity.

The electrical impulse arrives at the end of the first neurone.

Neurotransmitter

This triggers the release of neurotransmitter molecules into the gap.

Diffuses

The neurotransmitter diffuses across the synapse.

Receptor molecules

It binds to receptor molecules on the membrane of the next neurone.

This starts a new electrical impulse in the second neurone.

Because neurotransmitter is released from only one side, synapses make sure impulses travel in one direction only. Diffusion across the gap takes a little time, which is part of the reaction time. Some drugs and poisons work by interfering with synapses.

Section 5 of 9Coordination and links

Table comparing nervous and endocrine coordination

Responses in the skin when body temperature is too low

Nervous vs endocrine coordination comparison

Coordination and links

The coordinator for a response can be the brain or spinal cord.

Nervous responses are fast, short-lived and usually act on a precise part of the body,
which is different from hormonal responses,
which are slower and longer lasting.
The nervous system works with the endocrine system to maintain a steady internal environment.

When you touch a hot object, receptors in your skin are the first part of the pathway; the effector is a muscle in your arm.

If the brain and the spinal cord are damaged, impulses cannot pass, which can lead to paralysis.

Link this topic to reflex actions (5.1.2), the brain (5.1.4),

and the control of body temperature (5.1.6),
where the same stimulus-receptor-coordinator-effector pathway is used.

Section 6 of 9Worked pathway: catching a falling object

Retina Signal Pathway — Step-by-Step Simulation

How a nerve impulse travels along a neuron

Consider a student who sees a pen roll off a desk and catches it.

1

Light from the pen is the stimulus.

2

Light receptors in the retina detect it and send impulses along the optic nerve (sensory neurones) to the brain.

3

In the brain, many neurones and synapses process the information, estimate the position of the pen and decide on a response.

4

Impulses then travel down the spinal cord and out along motor neurones to the muscles of the arm and hand, the effectors,

which contract to catch the pen.

This is a voluntary, coordinated response, so more synapses are involved and it takes longer than a reflex.

Worked pathway: catching a falling object

Notice that every response, however complex, can be described using the same chain: stimulus, receptor, sensory neurone, coordinator, motor neurone, effector, response.

In exam answers, always name the actual structures from the context rather than writing generic words,

and remember that the route along any neurone is electrical while the route across a synapse is chemical.

Section 7 of 9Exam-style questions

Synaptic Transmission — Real Step-by-Step Mechanism Simulation

Chemical Synapse Transmission — Animated

Describe the pathway of a nerve impulse from a receptor to an effector. [4]4 marks
Explain how an impulse crosses a synapse. [4]4 marks
Give two ways in which a motor neurone is adapted to its function. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Stimulus	A change in the environment detected by a receptor.
Receptor	A cell or group of cells that detects a stimulus.
Effector	A muscle or gland that carries out a response.
Central nervous system	The brain and spinal cord.
Synapse	The gap between two neurones across which neurotransmitter diffuses.
Neurotransmitter	A chemical released at a synapse that triggers an impulse in the next neurone.
Myelin sheath	Fatty insulating layer around some axons that speeds up impulses.
Sensory neurone	A neurone carrying impulses from receptors to the CNS.

Section 9 of 9Exam technique

1

Saying the CNS includes all nerves. The CNS is only the brain and spinal cord.

2

Writing that the impulse 'jumps' across the synapse as electricity. A chemical neurotransmitter diffuses across.

3

Confusing sensory and motor neuronessensory goes to the CNS, motor goes from it.

4

Calling receptors and effectors the same thing. Receptors detect stimuli; effectors produce responses.

5

Saying muscles are coordinators. Muscles and glands are effectors.