ZIMSEC O Level · Biology · Form 4
The nervous system
Stimulus → receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector (muscle or gland) → response.
- The central nervous system is the brain and spinal cord.
- A reflex action is fast and automatic, e.g. pulling your hand from a hot plate. The reflex arc goes through the spinal cord, without thinking.
- Synapses are gaps between neurones crossed by chemical transmitters.
The eye
The cornea and lens focus light on the retina. The iris controls pupil size: in bright light the pupil narrows. Accommodation: the lens becomes fatter for near objects.
Hormones
Chemical messengers carried in the blood from endocrine glands to target organs. Slower but longer-lasting than nerves.
- Adrenaline (adrenal glands): “fight or flight” – faster heart rate and breathing.
- Insulin (pancreas); testosterone (testes); oestrogen (ovaries).
Plant responses
Tropisms are growth responses to a stimulus: shoots grow towards light (positive phototropism) and roots grow down with gravity (positive geotropism). Auxin controls this – in shoots it collects on the shaded side and causes more growth there.
Check yourself
- Give the correct order of parts in a reflex arc.
- Name the hormone that prepares the body for action.
- Why does a shoot bend towards light?
Show answers
- Receptor, sensory neurone, relay neurone, motor neurone, effector.
- Adrenaline.
- Auxin moves to the shaded side, cells there elongate more, so the shoot bends towards the light.
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