Coordination and control: nervous system, sense organs and hormones
What this domain tests
Neurone structure, the reflex arc, brain regions, the eye and accommodation, and hormones with their feedback control. Questions regularly compare nervous with hormonal control and ask you to correct defective vision.
Core rules
- A neurone has a cell body, dendrites, an axon and a myelin sheath. Impulses are electrical along the neurone and chemical across the synapse, where neurotransmitter diffuses across the gap.
- Reflex arc: receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, effector. It bypasses conscious decision-making, so it is fast and protective; the brain is informed a moment later.
- Cerebrum: intelligence, memory, conscious sensation. Cerebellum: balance and coordination. Medulla: heart rate, breathing and other involuntary functions.
- Eye: the cornea does most refraction and the lens fine-focuses. For near objects the ciliary muscles contract, suspensory ligaments slacken and the lens becomes thicker. Rods work in dim light; cones give colour and detail.
- Myopia (short sight): the image forms in front of the retina, corrected with a concave lens. Hyperopia: image behind the retina, corrected with a convex lens.
- Hormones are chemicals carried in blood to target organs. Insulin lowers blood glucose, glucagon raises it, adrenaline prepares the body for action, thyroxine sets metabolic rate, testosterone and oestrogen drive secondary sexual characteristics.
Worked example
A girl touches a hot pan and pulls her hand away before feeling pain. Heat receptors in the skin generate an impulse that travels along a sensory neurone to a relay neurone in the spinal cord, then along a motor neurone to the biceps, which contracts and withdraws the hand. Because the pathway is completed in the spinal cord, the response happens before the impulse reaches the cerebrum, where pain is registered. This is a reflex action, and it is faster than a voluntary response.
Traps that cost marks
Saying impulses jump the synapse electrically. Confusing the cerebellum with the cerebrum. Writing that the lens does all the focusing. Reversing concave and convex lenses for myopia and hyperopia.