Touch something hot and your hand jerks back before you even think about it. Behind that split-second action is an astonishing communication network. This topic explores how organisms detect changes and respond – through fast electrical nerve signals, slower chemical hormones, and the steady balancing act of homeostasis that keeps your internal world stable. Even plants get involved, bending towards light to survive.
The human nervous system is made of the central nervous system (brain and spinal cord) and the peripheral nerves. Messages travel as electrical impulses along three types of neurone: sensory (receptor to CNS), relay (within the CNS) and motor (CNS to effector).
An effector is a muscle that contracts or a gland that secretes. Because the impulse passes through the spinal cord rather than waiting for the brain, the response is extremely fast.
A synapse is a tiny gap between two neurones. When an impulse arrives, the neurone releases a chemical (neurotransmitter) that diffuses across the gap and triggers an impulse in the next neurone. Synapses ensure impulses travel in one direction only.
The eye is a sense organ that detects light. Key parts work together to focus an image on the light-sensitive retina.
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more target organs. Compared with nerves, hormonal responses are slower but longer-lasting.
Temperature control involves sweating and the widening of skin blood vessels (vasodilation) to lose heat when too hot.
Plants respond to their environment by growing in particular directions. A tropism is a growth response towards or away from a stimulus.
These responses are controlled by the plant hormone auxin. Auxin spreads to the shaded side of a shoot, making those cells grow longer, so the shoot bends towards the light.
Practise exam-style questions on this topic.