Coordination, Nervous and Endocrine Systems
The nervous system
Coordination allows organisms to respond to stimuli. The nervous system uses fast electrical impulses along neurones; the endocrine system uses slower, longer-lasting hormones carried in the blood. Know the reflex arc: stimulus, receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, effector, response. A common trap is omitting the relay neurone or confusing sensory with motor neurone direction.
The eye and the synapse
Know the parts of the eye: cornea, iris, pupil, lens, retina, optic nerve. In bright light the pupil constricts and the iris muscles adjust. At a synapse, a chemical neurotransmitter diffuses across a small gap to carry the impulse between neurones. Questions often ask why a reflex is faster than a voluntary response: the pathway is short and does not involve conscious thought.
Hormones
Learn key examples: insulin and glucagon control blood glucose, adrenaline prepares the body for action, testosterone and oestrogen control sexual development, and plant hormones such as auxin cause phototropism. A trap is saying nerves and hormones act at the same speed or for the same duration.
Worked example
A person touches a hot pan and withdraws the hand before feeling pain. The impulse passes through the spinal cord reflex arc, producing a response before the brain registers the pain. Rule of thumb: reflexes bypass conscious decision-making.
Practical note
Reaction-time experiments need repeated trials and a control to give reliable, fair results.