Nervous system and sense organs
Neuron and impulse
Dendrites receive, the cell body integrates, the axon conducts and the myelin sheath speeds conduction; the synaptic knob releases a neurotransmitter such as acetylcholine into the synapse. The impulse is electrical within a neuron and chemical across the gap.
Organisation
The CNS is the brain plus spinal cord; the PNS has 12 pairs of cranial and 31 pairs of spinal nerves. The autonomic system has a sympathetic division for emergency and a parasympathetic division for rest and digestion. A reflex action is a quick involuntary response through a reflex arc, protecting the body before the brain registers pain. Cerebrum: thinking, memory, voluntary movement; cerebellum: balance and coordination; medulla: heartbeat, breathing, blood pressure; hypothalamus: temperature, hunger, thirst and control of the pituitary.
Eye
Light passes cornea, aqueous humour, pupil (iris), lens (adjusted by ciliary muscles) to the retina - rods for dim light, cones for colour and detail - then the optic nerve to the brain. Myopia: distant objects blur, image in front of the retina, concave lens. Hypermetropia: near objects blur, image behind the retina, convex lens. Cataract is a clouded lens; glaucoma is raised pressure.
Ear
Pinna, ear canal, tympanic membrane, malleus, incus, stapes, then cochlea for hearing and auditory nerve; semicircular canals give balance; the Eustachian tube equalises pressure. Taste buds and olfactory cells work together.
Worked example
A 12-year-old cannot read the blackboard but reads a book easily. Distant blur with clear near vision is myopia; the image falls in front of the retina, so a concave (diverging) lens is prescribed.
Marks-losing traps
Rods do not see colour; the lens, not the pupil, focuses; the Eustachian tube equalises pressure rather than detecting sound.