a) The nervous system enables humans to react to their surroundings and coordinate their behaviour

b) Cells called receptors detect stimuli (changes in the environment).

Receptors and the stimuli they detect include:

■ receptors in the eyes that are sensitive to light

■ receptors in the ears that are sensitive to sound

■ receptors in the ears that are sensitive to changes in position and enable us to keep our balance

■ receptors on the tongue and in the nose that are sensitive to chemicals and enable us to taste and to smell

■ receptors in the skin that are sensitive to touch, pressure, pain and temperature changes.

c) Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.

d) Information from receptors passes along cells (neurones) in nerves to the brain. The brain coordinates the response. Reflex actions are automatic and rapid. They often involve sensory, relay and motor neurones.

e) Candidates should understand the role of receptors, sensory neurones, motor neurones, relay neurones, synapses and effectors in simple reflex actions. In a simple reflex action:

■ impulses from a receptor pass along a sensory neurone to the central nervous system
■ at a junction (synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released that causes an impulse to be sent along a relay neurone
■ a chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system, causing impulses to be sent along a motor neurone to the organ (the effector) that brings about the response
■ the effector is either a muscle or a gland, a muscle responds by contracting and a gland responds by releasing (secreting) chemical substances.