The brain, eye defects, the ear and nervous disorders

Parts of the brain

Part	Function
Cerebrum	Conscious thought, memory, intelligence, voluntary movement and interpreting sensations
Corpus callosum	Band of nerve fibres connecting the left and right cerebral hemispheres
Cerebellum	Balance, muscle tone and coordination of voluntary movements
Medulla oblongata	Controls involuntary actions: heart rate, breathing rate, diameter of blood vessels, swallowing, coughing
Hypothalamus	Controls homeostasis (temperature, water balance) and the pituitary gland

The brain and spinal cord are protected by the skull and vertebrae, by three membranes called meninges, and by cerebrospinal fluid, which cushions them against shock.

The autonomic nervous system

The autonomic nervous system controls involuntary actions. It has two parts with opposite effects:

Organ	Sympathetic (“fight or flight”)	Parasympathetic (“rest and digest”)
Heart	Rate increases	Rate decreases
Pupils	Dilate	Constrict
Bronchioles	Dilate	Constrict
Digestion	Slowed	Stimulated

The eye: pupil reflex and defects

Pupillary mechanism: in bright light the circular muscles of the iris contract and the radial muscles relax, so the pupil constricts and less light enters, protecting the retina. In dim light the radial muscles contract and the circular muscles relax, so the pupil dilates.

Defect	Cause	Correction
Short-sightedness (myopia)	Eyeball too long or lens too convex; image of distant objects forms in front of the retina	Concave (diverging) lens
Long-sightedness (hypermetropia)	Eyeball too short or lens too flat; image of near objects forms behind the retina	Convex (converging) lens
Astigmatism	Uneven curvature of the cornea or lens, so part of the image is blurred	Cylindrical lenses
Cataract	Lens becomes cloudy	Surgery: the lens is replaced with an artificial lens

The ear: balance and hearing problems

The semicircular canals detect rotational movement of the head; the utriculus and sacculus detect the position of the head relative to gravity. Impulses travel to the cerebellum, which adjusts muscles to keep balance.

Middle-ear infection — fluid builds up behind the eardrum. Grommets (small tubes) inserted in the eardrum drain the fluid and equalise pressure.
Deafness — hearing aids amplify sound; a cochlear implant stimulates the auditory nerve directly when the cochlea is damaged.

Nervous system disorders and drugs

Alzheimer’s disease — progressive loss of brain cells, causing memory loss, confusion and personality change; there is no cure.
Multiple sclerosis — damage to the myelin sheath slows or blocks impulses.
Drugs — stimulants increase activity at synapses; depressants decrease it. Repeated use can cause tolerance and dependence, and damage the brain.

✏️ Worked example: explaining a defect

1A learner can read a book but cannot see the board clearly.

2Distant objects are blurred, so the learner is short-sighted: light from distant objects focuses in front of the retina.

3A concave lens spreads the light rays out slightly so that they focus on the retina.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (4 marks)

State the function of each of the following: (a) cerebellum, (b) medulla oblongata, (c) corpus callosum, (d) hypothalamus.

Show memo

(a) Balance and coordination of voluntary movement ✓
(b) Controls involuntary actions such as heart rate and breathing ✓
(c) Connects the two cerebral hemispheres ✓
(d) Controls homeostasis and the pituitary gland ✓

Question 2 · (4 marks)

Describe how the pupil changes when a person walks from a dark room into bright sunlight, and explain why this is important.

Show memo

Circular muscles of the iris contract ✓ and radial muscles relax ✓, so the pupil constricts ✓. Less light enters the eye, which protects the retina from damage ✓.

Question 3 · (4 marks)

A person cannot see near objects clearly. (a) Name the defect. (b) Explain its cause. (c) State how it is corrected.

Show memo

(a) Long-sightedness (hypermetropia) ✓
(b) Eyeball too short or lens not convex enough ✓, so light from near objects focuses behind the retina ✓
(c) Convex (converging) lens ✓

Question 4 · (3 marks)

Explain why a child with repeated middle-ear infections may be fitted with grommets.

Show memo

Fluid builds up in the middle ear and reduces hearing ✓. Grommets are small tubes placed in the eardrum ✓ that drain the fluid and equalise the pressure, so hearing improves ✓.