Detecting and responding to change

To survive, an organism must detect changes (stimuli) and respond. In the nervous system, receptor cells detect a stimulus and information travels as electrical impulses along neurones.

• Sense organs contain groups of receptor cells that respond to a specific stimulus: light (eyes), sound (ears), touch, temperature and chemicals (taste, smell).
• The central nervous system (CNS) = the brain and spinal cord. It coordinates the response.
• Sensory neurones carry impulses from receptors to the CNS; motor neurones carry impulses from the CNS to effectors.
• Effectors are muscles (which contract) or glands (which secrete hormones).

The pathway: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.

Reflex actions & the reflex arc

Reflex actions are fast and automatic — they don't involve the conscious part of the brain, and many are protective (e.g. the withdrawal reflex, blinking, and the pupil narrowing in bright light).

Synapse: where two neurones meet there is a tiny gap. The impulse crosses by releasing a chemical (neurotransmitter) that diffuses across and triggers a new impulse in the next neurone.

Put the arc in order

How fast is a nervous response?

Nervous responses are very fast because impulses travel as electricity along neurones. Reaction time is the time between a stimulus and a response. It can be affected by tiredness, caffeine, age and distractions.

Practical (SP4.1): the ruler-drop test measures reaction time — a partner drops a ruler and you catch it; the distance it falls before you catch it shows how quickly you reacted.

Your turn — impulse speed

The eye — a sense organ for light

The eye is a sense organ full of light receptor cells. Light is focused onto the retina, which sends impulses along the optic nerve to the brain.

• Cornea — transparent front; refracts (focuses) light entering the eye.
• Iris — coloured ring; controls the size of the pupil (how much light enters).
• Pupil — the hole that light passes through.
• Lens — focuses light onto the retina (changes shape to focus near/far).
• Retina — layer of light receptor cells at the back.
• Optic nerve — carries impulses from the retina to the brain.

Which part does the job?

The brain & studying it

The brain coordinates complex behaviour. You need three main regions:

• Cerebral hemispheres — the large folded top; control conscious thought, memory, language and intelligence.
• Cerebellum — at the back; controls coordination and balance of movement.
• Medulla — at the base; controls heart rate and breathing.

Studying the brain is hard: it is delicate and complex. Scientists use MRI scans and electrical stimulation, or study patients with brain damage — but this raises ethical issues, and damage to the CNS (as in Parkinson's disease or multiple sclerosis) is very difficult to treat.

Hormones — the slower messengers

The endocrine system uses hormones — chemical messengers made by glands, carried in the blood to target organs. Compared with nerves, hormonal responses are slower to start but longer-lasting.

• Pituitary — the "master gland"; releases hormones that control other glands (e.g. TSH, FSH, LH).
• Thyroid — releases thyroxine, which controls metabolic rate.
• Adrenals — release adrenaline ('fight or flight').
• Pancreas — releases insulin and glucagon to control blood glucose.
• Ovaries / testes — release oestrogen / testosterone and control reproduction.

Nervous or hormonal?

Tap a feature, then tap the system it describes. (Some are true of both.)

Adrenaline, thyroxine & negative feedback

Adrenaline is released by the adrenal glands in fear or stress ('fight or flight'). It raises heart rate and breathing rate and diverts blood to the muscles, ready for action. The liver later converts it to a less active compound.

Thyroxine (from the thyroid) controls metabolic rate and is a classic example of negative feedback: when thyroxine is low, the pituitary releases TSH (triggered by TRH) to make the thyroid release more; when it is high, TSH release is reduced. This keeps the level steady.

Match each gland to its hormone

Tap a gland on the left, then the hormone it releases on the right.

Reproductive hormones & the menstrual cycle

Four hormones control the menstrual cycle:

• FSH (from pituitary) — matures an egg in the ovary and stimulates oestrogen.
• Oestrogen (from ovary) — repairs the uterus lining; a peak triggers LH.
• LH (from pituitary) — triggers ovulation (release of the egg) at around day 14.
• Progesterone (from ovary) — maintains the uterus lining; when it falls, the lining is shed.

Applications: hormonal contraceptives (e.g. the pill) use oestrogen and/or progesterone to stop FSH and prevent egg maturation; fertility treatments like IVF use FSH and LH to stimulate egg release. You should be able to evaluate hormonal versus non-hormonal methods.

Homeostasis — keeping conditions constant

Homeostasis is the maintenance of a constant internal environment, so that cells and enzymes work at their optimum. The body controls things like blood glucose, temperature and water balance, mostly using negative feedback.

Over the next few screens we'll look at three examples: blood glucose, body temperature, and the kidney controlling water.

Controlling blood glucose

The pancreas monitors and controls blood glucose using two hormones:

• When blood glucose rises (e.g. after a meal), the pancreas releases insulin. Insulin makes the liver convert glucose into insoluble glycogen for storage — so blood glucose falls.
• When blood glucose falls too low, the pancreas releases glucagon, which makes the liver convert glycogen back into glucose — so blood glucose rises.

Diabetes: in Type 1, the pancreas doesn't produce enough insulin (treated with insulin injections). In Type 2, the body's cells stop responding properly to insulin (linked to obesity; managed by diet and exercise).

After a sugary meal…

Controlling body temperature

Human core temperature is held near 37 °C — the optimum for enzymes. The skin helps control it:

• Too hot: blood vessels near the skin surface widen (vasodilation) so more heat is lost; sweat is produced and evaporates, taking heat away; hairs lie flat.
• Too cold: blood vessels narrow (vasoconstriction) to reduce heat loss; sweating stops; hairs stand up (erector muscles) trapping air; shivering generates heat by muscle contraction.

Watch out: in vasodilation the vessels don't move to the surface — they simply change diameter. Wider vessels near the skin lose more heat; narrower ones conserve it.

The kidney — water balance & waste

The kidneys filter the blood to remove waste (mainly urea and excess salts) and control the water balance of the body. The waste leaves as urine.

• Filtration: blood is filtered under pressure in the glomerulus (capillary knot) into the Bowman's capsule — water, glucose, salts and urea pass into the tubule.
• Selective reabsorption: useful substances are taken back into the blood along the tubule — all the glucose, some salts and much of the water.
• What's left (urea, excess salts and water) forms urine, which passes to the bladder.

Excretory system: renal arteries bring blood to the kidneys; ureters carry urine to the bladder, which stores it before it leaves via the urethra. Practical SP4.3 is dissection of a mammalian kidney.

ADH & controlling water

Water content is controlled by the hormone ADH (antidiuretic hormone) using negative feedback. ADH increases the permeability of the collecting-duct walls to water.

• When you are short of water (blood too concentrated): more ADH is released → collecting ducts become more permeable → more water is reabsorbed → a small volume of concentrated urine.
• When you have too much water (blood too dilute): less ADH is released → less water reabsorbed → a large volume of dilute urine.

Remember: more ADH → more water back into the blood → more concentrated urine. It's the opposite when you're over-hydrated.

You've been sweating on a hot day…

Plant hormones & tropisms

Plants also respond to their environment, using hormones. Auxins control growth and cause tropisms — growth responses to a direction.

• Phototropism: shoots grow towards light (positive phototropism), so leaves get more light for photosynthesis.
• Gravitropism (geotropism): roots grow towards gravity (downwards), anchoring the plant and reaching water.

Uses of plant hormones: auxins in weedkillers and rooting powders; gibberellins to start seeds germinating; ethene to ripen fruit.

Your turn — mean reaction time

The big ideas to know

You've covered all four parts of Eduqas Topic 4 — nervous coordination, hormonal control, homeostasis and plant hormones. Press Finish to see your score.