The nervous system and the reflex arc (order is everything)
A reflex action is a fast, automatic (involuntary) response that protects the body. Examiners want the reflex arc in the correct sequence, and marks are awarded for the order: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord/CNS) → motor neurone → effector → response.
Use precise terms: the receptor detects the stimulus, the sensory neurone carries the impulse to the CNS, the relay neurone connects within the spinal cord, the motor neurone carries the impulse to the effector (a muscle or gland), which produces the response. The gap between neurones is a synapse, where a chemical (neurotransmitter) diffuses across (Extended). A reflex is fast and does not involve conscious thought initially, which is why it protects against danger. The classic example is pulling your hand off a hot object. Missing a step or reversing the order loses sequence marks.
The eye: structure, function and the iris reflex
Know the eye's parts and functions: the cornea refracts (bends) light; the iris controls how much light enters by changing pupil size; the lens focuses light onto the retina; the retina contains light receptors; the optic nerve carries impulses to the brain.
The most-examined process is the pupil reflex. In bright light, the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller to protect the retina. In dim light, the radial muscles contract and circular muscles relax, making the pupil larger to let in more light. (Extended only) Accommodation: to focus on near objects the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker/more rounded; for distant objects the reverse. Students often swap circular and radial muscles. Learn 'circular contract = constrict in bright light'.
Hormones and nervous control compared
A hormone is a chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more target organs. Adrenaline (from the adrenal glands) prepares the body for action: it increases heart rate and breathing rate, raises blood glucose, and widens pupils. The 'fight or flight' response (Extended).
The comparison is a reliable exam question:
| Feature | Nervous | Hormonal |
|---|---|---|
| Transmission | Electrical impulses along neurones | Chemicals in the blood |
| Speed | Fast | Slower |
| Duration | Short-lived | Longer-lasting |
| Area affected | Localised/precise | Can be widespread |
The key marks are 'travels in the blood' for hormones and 'fast but short-lived' versus 'slower but longer-lasting'. Saying hormones 'travel along nerves' is a clear error.
Homeostasis and negative feedback (Extended)
Define homeostasis as the maintenance of a constant internal environment. The mechanism is negative feedback: when a factor moves away from its set point, a change is triggered that reverses it back towards normal. The general pattern examiners want: a change is detected by a receptor, a corrective mechanism (effector) acts to return the factor to normal, then conditions are restored.
(Extended only) Blood glucose control: when glucose rises, the pancreas releases insulin, which causes the liver to convert excess glucose to glycogen, lowering glucose. When glucose falls, the pancreas releases glucagon, causing glycogen to be converted back to glucose. Type 1 diabetes is the inability to produce enough insulin. Students confuse insulin/glucagon and glucose/glycogen. Spell each precisely. Always frame the answer as 'a change triggers a response that reverses the change' to score the negative-feedback mark.
Temperature regulation and tropisms
(Extended for detail) Thermoregulation uses the skin. When too hot: vasodilation (arterioles widen so more blood flows near the skin surface, losing heat by radiation) and increased sweating (sweat evaporates, taking heat away). When too cold: vasoconstriction (arterioles narrow, reducing heat loss), shivering (muscle contraction releases heat from respiration), and hairs stand up to trap air. The classic confusion is vasodilation versus vasoconstriction. Remember 'dilation = wider = lose heat'.
For plants, tropisms are growth responses: phototropism (growth towards light) and gravitropism/geotropism (response to gravity). (Extended only) Auxin is a plant hormone made at shoot tips; it accumulates on the shaded side, causing those cells to elongate more, so the shoot bends towards the light. Saying the plant 'wants' to grow to light loses marks. Describe it as a growth response controlled by auxin. To practise the feedback and tropism answers, take a free trial.