Option E.1 – Stimulus and Response

Option E.1 – Stimulus and Response

E.1.1 – Define the terms stimulus, response and reflex in the context of animal behaviour

Stimulus – A change in the environment, internal or external, that is detected by a receptor and elicits a response

Response – The activity of a cell or organism in terms of movement, hormone secretion or enzyme production as a result of a stimulus

Reflex – A rapid, unconscious response to a stimulus E.1.2 – Explain the role of receptors, sensory

E.1.2 – Explain the role of receptors, sensory neurons, relay neurons, motor neurons, synapses and effectors in the response of animals to stimuli


These detect a stimulus, and can be sensory cells or nerve endings of sensory neurons.

Sensory Neurons

These receive messages across synapses from receptors and carry them to the central nervous system (spinal cord or brain). These have a single, long nerve fibre called a dendron to bring impulses towards the cell body and a single axon to take them away.

Relay Neurons

These receive messages, across synapses, from sensory neurons, and pass them to the motor neurons that can cause the appropriate response. These may also be called interneurons, and have many short, nerve fibres

Motor Neurons

These receive messages across synapses from relay neurons and carry them to an effector. These have dendrites which bring impulses towards the cell body, and a single long nerve fibre called an axon which carries impulses away from the cell body.

The Schwann cells wrap around the axon and form the myelin sheath, which provides protection and support. The gaps between the cells are called nodes of Ranvier. This increases the speed at which the impulses are conducted. Motor neurons form part of the nerve fibres, which send rapid and precise impulses.


The synapse is the small space between neurons. Since electrical impulses cannot pass across this space, chemical messengers called neurotransmitters are used to send messages from neurons.




These carry out a response after a message from a motor neuron. These can be muscles, which respond by contracting, or glands, which respond by secreting hormones or enzymes

The brain and spine form the central nervous system, whilst the nerve fibres to the rest of the body form the peripheral nervous system.


E.1.3 – Draw and label a diagram of a reflex arc for a pain withdrawal reflex, including spinal cord and it spinal nerves, the receptor cell, sensory neuron, relay neuron, motor neuron and effectors

The reflex arc allows us to respond to a stimulus in the same way every time. The response is an immediate and unconscious one – the message is not sent to the brain, but to the spinal cord. For example, the pain withdrawal reflex, such as when we touch something hot, means that we immediately pull back from the heat source. Some reflex arcs also send messages to the brain.

E.1.4 – Explain how animal responses can be affected by natural selection, using two examples

Many inherited traits are beneficial to the survival of an animal, and will increase its chances of being passed on. It may help the organism’s ability to feed, breed and survive. This contributes to natural selection. Such characteristics are the result of genetic variation and the expression of genetic variation in the phenotype. Not only do these alter the structure and appearance of the organism, but they can also contribute to its behaviour.

Example 1 – Erinaceus europaeus (European hedgehog)

Hedgehogs are nocturnal and omnivorous. The hedgehog exhibits self-defensive behaviour by curling up into a ball to expose its spines when it senses danger. The spines create a shield for its skin, body and limbs.

Across different populations, this response can vary:

  • Raising spines
  • Periodically peering out of spines to gauge danger levels
  • Running away if possible

In areas where there are vehicles and roads, the hedgehogs that run away when faced with danger will be favoured over others. They are better able to escape danger and only use rolling up as a last resort and cannot escape. In the situation of motor vehicles, a hedgehog will receive no protection from its spines. Those that roll up in a ball will be at a disadvantage, and are not selected for in the population.

Example 2 – Sylvia atricapilla (blackcap)

This is a bird that used to breed in Germany summer, and then go to Spain and the Mediterranean in winter. Studies show that 10% of migrating blackcaps will fly to the UK

Studies show that 10% of migrating blackcaps will fly to the UK instead. To show that this behaviour was genetic, and not learned, a sample of eggs was taken from birds that flew to the UK and birds that flew to Spain. They were raised independently of their parents so that there was no opportunity for them to learn the behaviour. Once the time came to migrate, they were not able to follow their parents. The data showed that birds whose parents had flown to the UK also instinctively flew west, whilst those with parents who flew to Spain, flew south.

This shows that the direction the birds flew in was genetic, indicating that their response was the result of natural selection. The number of blackcaps flying to the UK is increasing, as the birds have a greater chance of survival due to warmer conditions.