OxfordAQA International GCSE Biology: Nervous system, reflexes and animal behaviour (9201)

This guide is aligned to the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. It covers sections 3.4.1 (the human nervous system) and 3.4.6 (behaviour). The qualification is untiered, so everything here is for every candidate. Find the course on the OxfordAQA IGCSE Biology hub and tick off each statement on the printable checklist.

Companion pages: revision notes and practice questions. To find your weak spots first, take one of the free 10-minute diagnostics.

What this unit covers

Section	What you must be able to do
3.4.1 a, b	Say what the nervous system does. Describe how information passes along neurones as impulses to the CNS (brain and spinal cord)
3.4.1 c, d, e	Describe a reflex action, including sensory, relay and motor neurones, synapses and the two kinds of effector
3.4.1 (analysis)	Analyse a given example as stimulus, receptor, coordinator, effector, response
3.4.6 a	Explain courtship and four mating strategies
3.4.6 b	Explain why parental care can be a successful strategy, and how it can put parents at risk
3.4.6 c, d	Distinguish innate behaviour, imprinting, habituation, classical conditioning and operant conditioning. Describe how conditioning trains sniffer dogs and police horses
3.4.6 e	Describe sound, chemical and visual signals used in communication

3.4.1 The human nervous system

Why you need one (a, b)

The nervous system lets you react to your surroundings and coordinate your behaviour. A change in your surroundings is a stimulus. A receptor is a cell that detects a stimulus. Receptors send information along nerve cells called neurones as impulses. The impulses travel to the central nervous system (CNS), which is the brain and spinal cord. The brain coordinates the response.

An impulse is an electrical signal in the neurone. Do not say that “a message of pain” or “a nerve” travels. Name the cell (neurone) and the signal (impulse).

Reflex actions (c)

A reflex action is automatic and rapid. You do not decide to do it. It often involves three types of neurone:

Neurone	Where it carries impulses
Sensory neurone	From a receptor to the CNS
Relay neurone	Within the CNS, linking sensory and motor neurones
Motor neurone	From the CNS to an effector

Reflexes are fast because few neurones are involved and the response does not wait for conscious thought. Many reflexes protect the body from harm, such as pulling a hand away from something hot.

The pain-withdrawal reflex (d, e)

Follow the pathway in order:

1. A pain receptor in the skin detects the stimulus and starts impulses.
2. Impulses pass along a sensory neurone to the CNS.
3. At a synapse between the sensory neurone and a relay neurone, a chemical is released. It causes an impulse to be sent along the relay neurone.
4. A chemical is released at the synapse between the relay neurone and the motor neurone. This causes impulses to be sent along the motor neurone.
5. The motor neurone carries impulses to the effector.
6. The effector responds.

A synapse is a junction between two neurones. The signal crosses it as a chemical, not as an electrical impulse. A chemical is released at the end of one neurone and the next neurone then sends its own impulse.

An effector is either a muscle or a gland. A muscle responds by contracting. A gland responds by releasing (secreting) chemical substances. In the pain-withdrawal reflex the effector is a muscle in the arm, which contracts and pulls the hand away.

Worked example 1: analysing a reflex

A person touches a hot saucepan handle and snatches their hand back before feeling pain.

Use the model stimulus, receptor, coordinator, effector, response.

Stage	Answer
Stimulus	The heat from the handle
Receptor	Temperature or pain receptors in the skin of the hand
Coordinator	The CNS (spinal cord, through a relay neurone)
Effector	A muscle in the arm
Response	The muscle contracts and the hand is pulled away

The person only feels pain afterwards because impulses also travel up to the brain, which takes a little longer than the spinal reflex. This is why the hand moves first.

Worked example 2: a gland as the effector

A hungry person smells frying onions and their mouth begins to water.

• Stimulus: chemicals from the onions in the air.
• Receptor: smell receptors in the nose.
• Coordinator: the CNS.
• Effector: the salivary glands.
• Response: the glands secrete saliva.

Notice that a gland does not contract. It releases a substance. If a question gives you a response that is secretion, the effector is a gland. If the response is movement, the effector is a muscle.

Worked example 3: speed of a reflex

A pathway from a finger to the spinal cord and back to the arm muscle is 1.2 m long in total. Impulses travel at 60 m/s in the neurones. Ignore the delay at synapses.

time = distance / speed = 1.2 / 60 = 0.020 s, which is 20 milliseconds.

A reflex arc in the spinal cord is much shorter than a path that goes through the brain and back, so the response is faster. The synapses add a small delay, so a pathway with fewer synapses is quicker.

3.4.6 Behaviour

Mate choice, courtship and mating strategies (a)

Sexual reproduction needs two animals to find each other and choose a suitable mate. Many species use courtship behaviour: displays, calls, dances or gifts. Courtship can advertise the quality of the individual. A male with a bright, large display is showing that he is healthy enough to grow and maintain it. The choosing animal can then pick a mate whose genes are likely to give offspring a better chance of survival.

Animals follow different mating strategies:

Strategy	Example (typical)
A mate for life	Albatrosses
Several mates over a lifetime	Birds that choose a new partner in later years
A mate for a breeding season	Emperor penguins, for one breeding season
Several mates over one breeding season	Male red deer, which mate with several females during the rut

Exam answers should name the strategy and link it to an advantage. A mate for life can mean both parents help raise young and the pair works well together. Several mates in one season can mean one individual passes on more copies of his genes.

Parental care (b)

Some animals have special behaviours for rearing young: feeding, guarding, keeping warm, or building a nest. Parental care can be a successful evolutionary strategy for two reasons:

• It increases the chance that offspring survive.
• It increases the chance that the parents’ genes are passed on by the offspring.

Worked example 4: interpreting survival data

In a fish species, 40 eggs were left unguarded and 40 eggs were guarded by a parent. After the hatching period, 2 unguarded eggs and 26 guarded eggs had hatched and survived.

• Unguarded: 2 / 40 x 100 = 5.0 %.
• Guarded: 26 / 40 x 100 = 65 %.
• Conclusion: guarding raised survival from 5.0 % to 65 %, so more offspring survive to carry the parent’s genes.

Risks to parents. Parental care costs energy and can be dangerous. A parent may stop feeding while it guards a nest, use up its energy reserves, be attacked by a predator while defending young, or be easier for predators to find. Parents that are weakened may produce fewer offspring in later breeding seasons. Be ready to give two different risks, linked to a named kind of care.

Types of behaviour (c)

Type	What it is	Example
Innate	Inborn. It does not need to be learned and appears fully formed	Newly hatched sea turtles heading towards the sea; spiders spinning webs
Imprinting	Learned very early in life. Young animals form an attachment to the first suitable moving object they see	Goslings following the first large moving object they see after hatching
Habituation	Learning to ignore a repeated stimulus that brings neither reward nor harm	Town pigeons ignoring passing traffic
Classical conditioning	Learning to link a neutral stimulus with an unrelated stimulus that already causes a response	A dog salivating at a bell it has learned to link with food
Operant conditioning	Learning by trial and error. Behaviour followed by reward is repeated. Behaviour followed by punishment is not	A rat pressing a lever to receive food

The specification writes “classic conditioning”. Both names mean the same thing.

Imprinting happens during a short sensitive period soon after birth or hatching. The young animal then follows the imprinted object. This helps it stay close to a parent and be fed and protected.

Worked example 5: habituation data

A pond snail withdraws into its shell when tapped. It was tapped every 10 seconds, and the time it stayed withdrawn was recorded.

Tap number	1	5	10	15	20
Time withdrawn (s)	42	31	18	9	4

Percentage decrease from tap 1 to tap 20 = (42 - 4) / 42 x 100 = 90.5 %.

The snail has stopped treating the harmless tap as a threat. That is habituation. It saves energy and lets the snail keep feeding. If a different, stronger stimulus was used, the snail would be likely to withdraw again.

Worked example 6: classical conditioning data

Before training, a dog produces no saliva to a buzzer. The buzzer is sounded every time food is given. After training the buzzer is sounded alone and drops of saliva in one minute are counted.

Number of pairings	0	5	10	15	20
Drops of saliva to buzzer alone	0	2	6	11	12

• Food is the stimulus that causes saliva naturally. The buzzer is the neutral stimulus.
• After 15 pairings the dog produces 11 drops to the buzzer alone: it has linked the sound with food.
• The gain from 15 to 20 pairings is only 1 drop, so the learning is levelling off.

Using conditioning to train animals (d)

Humans use conditioning to train captive animals.

• Sniffer dogs are trained to detect a scent, such as a drug or explosive. The dog is rewarded (with food or a toy) each time it indicates the target scent. This is operant conditioning: rewarded behaviour is repeated.
• Police horses are trained to stay calm near crowds, flags, loud noises and sirens. Repeated, safe exposure makes them stop reacting to these stimuli, which is habituation. Rewards for calm behaviour add operant conditioning.

Communication (e)

Animals use signals to communicate with each other. The types you need to describe:

Signal	Description	Example
Sound	Calls and songs carry over distance, even through dense habitat or at night	Bird song to defend a territory; alarm calls; whale song
Chemical	Scent substances released into the environment, which last after the animal has gone	Ants laying a trail to food; moths attracting a mate; scent marking of territory
Visual	Colour, posture, movement or light that another animal can see	A peacock’s tail display; a dog’s body posture; flashes of light from fireflies

Sound works in the dark but can attract predators. Chemical signals last but change slowly. Visual signals are quick but need light and a clear line of sight.

Common errors

• Writing that the brain is the coordinator in every reflex. In a spinal reflex the spinal cord coordinates the response.
• Saying the relay neurone is outside the CNS. It is inside the CNS.
• Describing a synapse as a gap that an electrical impulse jumps. A chemical crosses it.
• Saying that glands contract. Muscles contract. Glands secrete.
• Mixing habituation (learning to ignore) with classical conditioning (learning to link two stimuli).
• Calling imprinting “any learned attachment”. It happens in a short early period.
• Listing only benefits of parental care when the question asks for risks.

Next steps

Use the revision notes, then test yourself on the practice set.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA: sections 3.4.1 and 3.4.6.