The nervous system & reflex arc

The central nervous system (CNS) is the brain and spinal cord. It receives information from receptors and coordinates a response by effectors (muscles or glands).

• Reflexes are fast, automatic and protective — they do not involve conscious thought.
• In a reflex arc the signal passes through a relay neurone in the spinal cord.
• The tiny gap between two neurones is a synapse, crossed by chemical neurotransmitters.

Why reflexes are fast: they skip the conscious brain, so the response happens before you have time to think — protecting you from harm.

Order of a reflex

The brain and the eye

Different regions of the brain control different functions:

• Cerebral cortex — consciousness, memory, language.
• Cerebellum — balance and coordinating movement.
• Medulla — unconscious activities like heart rate and breathing.

In the eye, light is focused onto the retina by the cornea and lens. Accommodation is changing the lens shape to focus on near or far objects.

• Near object: ciliary muscles contract, lens becomes fatter (more rounded).
• Far object: ciliary muscles relax, lens becomes thinner.
• The iris controls how much light enters through the pupil (a reflex).

Focusing on a near object

Hormones & the endocrine system

Hormones are chemical messengers made by glands and carried in the blood to target organs. Compared with nerves, hormonal responses are usually slower but longer-lasting.

• Pituitary gland — the "master gland"; releases hormones that control other glands.
• Pancreas — releases insulin and glucagon to control blood glucose.
• Thyroid — releases thyroxine (controls metabolic rate).
• Adrenal glands — release adrenaline for "fight or flight".

Nerves vs hormones: nervous responses are fast and short-lived and use neurones; hormonal responses are slower and longer-lasting and travel in the blood.

Match each gland or system to its role

Tap a statement, then its matching answer.

Homeostasis & blood glucose

Homeostasis is keeping internal conditions (temperature, water, blood glucose) stable using negative feedback.

Controlling blood glucose (the pancreas):

• Blood glucose too high → pancreas releases insulin → liver and muscle cells take up glucose and store it as glycogen → glucose falls.
• Blood glucose too low → pancreas releases glucagon → liver breaks glycogen back into glucose → glucose rises.

Type 1 diabetes: the pancreas makes too little insulin — treated with insulin injections. Type 2: body cells stop responding to insulin — managed by diet and exercise.

Blood glucose too high

Nerves vs hormones

Plant hormones & responses

Plants respond to their environment using hormones. Auxin controls growth responses called tropisms:

• Phototropism — shoots grow towards light (positive), roots away from it.
• Gravitropism (geotropism) — roots grow towards gravity (down), shoots away (up).
• Auxin gathers on the shaded/lower side; in shoots it makes those cells elongate more, so the shoot bends towards the light.

Uses of plant hormones: rooting powders (auxins), weedkillers, ripening fruit and seedless fruit — a common exam application.

Your turn — reaction time

Sort each into its group

Tap each response, then tap whether it is nervous, hormonal, or a plant response.

The big ideas to know

Press Finish to see your score.