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 (e.g. pulling your hand away from a hot object).
• 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

How can you move before you think?

The brain and the eye

Different regions of the brain control different functions:

• Cerebrum (its outer layer is the 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, suspensory ligaments slacken, lens becomes fatter (more rounded).
• Far object: ciliary muscles relax, suspensory ligaments pull tight, lens becomes thinner.
• The iris controls how much light enters through the pupil (a reflex).

Focusing on a near object

What keeps your blood glucose steady?

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.

Hormones & the menstrual cycle

The menstrual cycle lasts about 28 days. Hormones control it:

• FSH (pituitary) — makes an egg mature in an ovary.
• LH (pituitary) — triggers ovulation, the release of the egg (about day 14).
• Oestrogen (ovaries) — rebuilds the uterus lining.
• Progesterone (ovaries) — keeps the lining ready for a fertilised egg.
• Testosterone (testes) — the main male hormone; controls sperm production.

Contraception: hormonal methods (pill, implant, injection) stop eggs maturing or being released; non-hormonal methods (condoms, diaphragm) stop sperm reaching the egg. Hormonal methods are more effective when used correctly; condoms also protect against STIs.

Higher tier — how they interact: FSH stimulates oestrogen; oestrogen inhibits FSH and triggers an LH surge; LH causes ovulation; progesterone inhibits FSH and LH. Infertility: FSH and LH can be given to make eggs mature; in IVF the eggs are collected, fertilised in the lab and embryos placed in the uterus.

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

What keeps your blood glucose steady?

Controlling body temperature

A constant internal environment lets enzymes, and so metabolic reactions, work at the right rate. Receptors in the skin detect external temperature; the brain responds:

• Too hot: sweating — sweat evaporates and cools the skin; vasodilation — blood vessels near the skin widen, so more heat is lost.
• Too cold: shivering — muscles contract, releasing heat from respiration; vasoconstriction — blood vessels near the skin narrow, so less heat is lost.

Watch out: in vasodilation the blood vessels widen — they don't move closer to the skin surface.

Water balance & the kidneys

If body fluids change concentration, water moves into or out of cells by osmosis:

• Fluid with a higher water potential than the cell → water enters → animal cells swell and may burst (lysis).
• Lower water potential → water leaves → cells shrink. Equal → no net change.

The kidneys keep the balance by changing the amount and concentration of urine. Each kidney tubule (nephron): Bowman's capsule (filters the blood) → proximal convoluted tubule (glucose reabsorbed) → loop of Henlé → collecting duct (water reabsorbed).

Higher tier — ADH: when the blood is too concentrated (sweating, dehydration, salty food), the pituitary releases more ADH and you feel thirsty. ADH makes the collecting ducts more permeable, so more water is reabsorbed → less, more concentrated urine. Too much water → less ADH → lots of dilute urine. This is negative feedback.

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

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