IGCSE Biology Coordination & drugs notes

The nervous system coordinates and regulates body functions using electrical impulses that travel along neurones — long cells built to carry signals quickly. It has two divisions: the central nervous system (CNS) — the brain and spinal cord, where decisions are made — and the peripheral nervous system (PNS), the nerves outside the brain and spinal cord that connect the CNS to receptors and effectors.

Three neurone types to identify in diagrams: sensory neurones carry impulses from receptors towards the CNS (their cell body sits in a side swelling, just outside the spinal cord); relay neurones are short cells entirely inside the CNS, linking sensory to motor neurones; motor neurones carry impulses from the CNS out to effectors (muscles and glands), with long fibres and the cell body inside the CNS.

A reflex action is automatic and rapid — it integrates and coordinates a stimulus with the response of effectors (muscles and glands) without conscious thought. Reflexes protect the body: withdrawing a hand from heat, blinking, knee-jerk. The pathway is the simple reflex arc: receptor → sensory neurone → relay neurone → motor neurone → effector.

A labelled diagram to picture: a finger touching a pin. The receptor in the skin detects the stimulus; a sensory neurone carries the impulse to the spinal cord; a relay neurone passes it across the cord; a motor neurone carries the impulse out to an effector — an arm muscle, which contracts and pulls the hand away. The impulse does not wait for the brain: that is why reflexes are faster than voluntary actions. (You become aware of the pain slightly later, via a branch to the brain.)

A synapse is a junction between two neurones — the neurones do not touch; a tiny gap separates them. At Core you need the idea; Supplement adds the machinery: the ending of the first neurone contains vesicles filled with neurotransmitter molecules; the gap itself is the synaptic gap; the second neurone’s membrane carries receptor proteins.

Supplement — the events, in order: an impulse arrives and stimulates release of neurotransmitter molecules from vesicles into the synaptic gap; the molecules diffuse across the gap; they bind with receptor proteins on the next neurone; and an impulse is then stimulated in the next neurone. Because receptors sit only on the receiving side, synapses ensure impulses travel in one direction only — the transmitter is only released from one side and can only bind on the other.

A sense organ is a group of receptor cells responding to specific stimuli — light (eye), sound (ear), touch, temperature and chemicals (taste and smell). Each receptor, suitably stimulated, sends impulses along sensory neurones to the brain.

A labelled diagram to picture the eye in section: the tough outer coat bulging at the front into the transparent cornea; the coloured iris surrounding the black pupil (a hole, not a structure); behind them the lens, held by suspensory ligaments; the light-sensitive retina lining the back; and the optic nerve leaving towards the brain — creating the blind spot where it exits, an area with no receptors.

Core — the pupil reflex: in bright light the pupil constricts (gets smaller), letting less light in to protect the retina; in dim light the pupil dilates (gets wider), letting more light in for vision. Supplement — the muscles: the iris has circular and radial muscles working antagonistically. In bright light the circular muscles contract and the radial muscles relax, shrinking the pupil; in dim light the radial muscles contract and the circular muscles relax, widening it. Antagonistic means one contracting while the other relaxes — like biceps and triceps.

Supplement — accommodation (focusing): to view a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens springs into a fatter, more curved shape, refracting the strongly diverging rays on to the retina. For a distant object, the ciliary muscles relax, the suspensory ligaments are pulled taut, and the lens is pulled into a thin, flatter shape. The eye is working hardest for near objects — reading tires the ciliary muscles.

Supplement — rods and cones: the retina’s receptors are of two kinds. Rods are spread across the retina but absent from the fovea; they are more sensitive to dim light and give night vision (in black and white). Cones — of three different kinds, absorbing different colours of light — give colour vision and work best in bright light; they are packed into the fovea, the point of sharpest vision. The blind spot has neither.

A hormone is a chemical substance produced by a gland, carried by the blood, which alters the activity of one or more specific target organs. The glands that secrete them are endocrine glands — ductless, releasing hormones straight into the bloodstream. The pairs to memorise: adrenal glands → adrenaline; pancreas → insulin; testes → testosterone; ovaries → oestrogen.

Adrenaline is the ‘fight or flight’ hormone, secreted in situations of fear or excitement. Its effects prepare the body for action: increased breathing rate, increased heart rate and increased pupil diameter — more oxygen, faster blood flow, more light into the eyes. Supplement: adrenaline also controls metabolic activity by increasing blood glucose concentration (releasing glucose for respiration) alongside the increased heart rate.

Homeostasis is the maintenance of a constant internal environment — keeping conditions such as blood glucose and body temperature steady despite changes outside. At Core, one fact anchors the glucose half: insulin decreases blood glucose concentration.

Supplement — negative feedback: the body defends each condition around a set point. If a level rises above the set point, a correction brings it down; if it falls below, the correction brings it up. The response is always opposite to the change — that opposition is why it is called *negative* feedback — and it keeps the level fluctuating narrowly around the set point.

Supplement — blood glucose control: after a sugary meal, blood glucose rises; the pancreas secretes insulin, and the liver takes up glucose and stores it as glycogen, lowering blood glucose back to the set point. Between meals, glucose falls; the pancreas secretes glucagon, and the liver converts glycogen back into glucose, raising the level. Insulin lowers, glucagon raises — two hormones, one set point, the liver as store.

Supplement — Type 1 diabetes: the pancreas cannot produce enough (or any) insulin, so blood glucose rises dangerously after meals. Treatment: regular insulin injections (typically before meals), a controlled diet limiting refined sugars, and monitoring blood glucose so doses match need. With treatment, people with Type 1 diabetes keep their glucose near the set point.

Supplement. Mammals maintain a constant internal body temperature; the brain (specifically its hypothalamus region, with receptors in the skin feeding it information) acts as the control centre. Its toolkit: insulation (fatty tissue and hair trap air), sweating (evaporating sweat removes heat), shivering (rapid muscle contraction generates heat) and vasodilation/vasoconstriction of skin blood vessels.

A labelled diagram to picture the skin in section: hairs with hair erector muscles, sweat glands and ducts, receptors and sensory neurones, blood vessels in the dermis and a layer of fatty tissue beneath. When hot: vasodilation — arterioles supplying skin surface capillaries widen, so more blood flows near the surface and more heat is lost by radiation; sweat is produced, and its evaporation cools the skin; hairs lie flat (little insulating air trapped). When cold: vasoconstriction narrows the arterioles, so less blood reaches the surface and less heat is lost; sweating stops; hairs are pulled erect to trap insulating air; shivering generates heat.

Plants respond to direction too. Gravitropism is a response in which parts of a plant grow towards or away from gravity — roots grow *towards* gravity (downward), shoots grow *away*. Phototropism is a response in which parts grow towards or away from the direction of light — shoots grow *towards* light. Both can be investigated by growing seedlings on their side, or with light from one direction only, and comparing shoot and root curvature with control seedlings.

Supplement — the mechanism (chemical control of plant growth): auxin is made in the shoot tip and diffuses down through the plant. When a shoot is lit from one side, auxin becomes unequally distributed — it accumulates on the shaded side — and since auxin stimulates cell elongation, the shaded-side cells elongate faster, bending the shoot towards the light. In a horizontal shoot, auxin gathers on the lower side and elongates it, bending the shoot back upwards, away from gravity.

A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Some are medicines (like antibiotics), others are recreational — all fit the definition. Antibiotics are drugs used to treat bacterial infections: they kill the bacteria (or stop them reproducing) without damaging the patient’s own cells too much.

Two hard limits: antibiotics kill bacteria but do not affect viruses — viruses lack the structures antibiotics attack and hide inside host cells — so a doctor will not prescribe them for a cold or flu. And some bacteria are resistant to antibiotics, which reduces the drugs’ effectiveness: resistant strains survive treatment and spread.

Supplement — limiting resistance: resistance spreads because antibiotics kill the non-resistant bacteria, leaving resistant ones alive to reproduce with no competition. Using antibiotics only when essential — not for viral infections, not as a precaution “just in case”, and completing the prescribed course — means resistant bacteria are not given the selective advantage of a drug-filled body to multiply in, so the development and spread of resistant strains such as MRSA is limited.