Control and Coordination

CONTROL AND COORDINATION is carried out by TWO systems in animals: the NERVOUS SYSTEM, which uses electrical impulses and acts fast, and the ENDOCRINE SYSTEM, which uses chemical hormones and acts more slowly but reaches every cell. Plants have NO nervous system and no specialised tissue for conducting information — they use chemical (hormonal) control, PLUS electrochemical signals passed directly from cell to cell.

A STIMULUS is any change in the environment that a living organism responds to — heat, light, sound, smell, touch, gravity, water, chemicals.

RECEPTORS are specialised cells that DETECT a stimulus. They are usually located in the sense organs: PHOTORECEPTORS in the eye (light), PHONORECEPTORS in the ear (sound — the ear also maintains BALANCE), OLFACTORY receptors in the nose (smell), GUSTATORY receptors on the tongue (taste), and receptors in the skin for heat, cold, pain and pressure.

If receptors do not work properly (as can happen to nerve endings in people with DIABETES) the environment is no longer detected correctly and coordination fails.

A NEURON (nerve cell) is the structural and functional unit of the nervous system, and is the LONGEST cell in the body. Parts: DENDRITES — the branching tips ACQUIRE the information, and at the DENDRITIC TIP that information sets off a CHEMICAL REACTION which CREATES an ELECTRICAL IMPULSE → the impulse travels to the CELL BODY → then along the AXON (the long fibre) to its end → NERVE ENDINGS. Note the symmetry: chemical becomes electrical at the dendritic tip, and electrical becomes chemical again at the synapse.

The impulse always travels in ONE direction: dendrite → cell body → axon → nerve ending.

SYNAPSE — the tiny GAP between the nerve ending of one neuron and the dendrite of the next. The electrical impulse CANNOT jump the gap: it makes the nerve ending release a CHEMICAL, which crosses the synapse and starts a FRESH electrical impulse in the next neuron. This is why the synapse makes the impulse travel in one direction only.

The same arrangement exists where a neuron meets a MUSCLE cell — the NEUROMUSCULAR JUNCTION — and that is how a nerve impulse finally makes a muscle move.

REFLEX ACTION — a sudden, rapid, AUTOMATIC (involuntary) response to a stimulus, which is NOT under the control of the thinking part of the brain. Examples: withdrawing the hand from a hot object, blinking, sneezing, watering of the mouth at the smell of food.

REFLEX ARC — the pathway a reflex takes: RECEPTOR (skin) → SENSORY NEURON → SPINAL CORD (RELAY neuron) → MOTOR NEURON → EFFECTOR (the muscle), which contracts and produces the response.

WHY reflex arcs evolved: the thinking process of the brain is comparatively SLOW, and for an urgent stimulus like a burn that delay would cause injury. So the SPINAL CORD completes the arc directly. The brain is informed afterwards — which is why you feel the pain a moment AFTER pulling your hand away.

The BRAIN is the main coordinating centre. It has THREE regions: FORE-BRAIN, MID-BRAIN and HIND-BRAIN.

FORE-BRAIN — mainly the CEREBRUM, the THINKING part of the brain. It receives sensory impulses from all the receptors, and is the seat of memory, reasoning, learning and decisions. It has separate areas for hearing, smell, sight and touch, and a centre that gives the sensation of being FULL.

HIND-BRAIN — the CEREBELLUM controls POSTURE and BALANCE and gives PRECISION to voluntary actions (walking in a straight line, picking up a pencil, riding a bicycle). The MEDULLA controls INVOLUNTARY actions such as blood pressure, salivation and vomiting. The PONS regulates breathing (respiration).

MID-BRAIN, along with the hind-brain, controls involuntary actions — including reflex movements of the head, neck and eyes, and changes in pupil size.

VOLUNTARY actions are under the control of the thinking brain (writing, talking). INVOLUNTARY actions are not (heartbeat, breathing, digestion). REFLEX actions are involuntary AND very fast. MOST are completed by the SPINAL CORD (e.g. withdrawing the hand from a hot object); a few — blinking, changes in pupil size, salivation — are completed by the mid-brain or hind-brain. But NONE of them wait for the THINKING part of the brain.

THE NERVOUS SYSTEM HAS TWO PARTS. CENTRAL NERVOUS SYSTEM (CNS) = the BRAIN + the SPINAL CORD; it receives information from all parts of the body and integrates it. PERIPHERAL NERVOUS SYSTEM (PNS) = the nerves that connect the CNS to the rest of the body — CRANIAL NERVES arising from the brain and SPINAL NERVES arising from the spinal cord.

PROTECTION of the central nervous system: the brain sits inside a bony box, the CRANIUM (skull), and is cushioned by a fluid that absorbs shocks; the SPINAL CORD runs inside the VERTEBRAL COLUMN (backbone).

HOW NERVOUS TISSUE CAUSES MOVEMENT: muscle cells contain special PROTEINS. When the nerve impulse arrives, the arrangement of those proteins CHANGES, and the muscle cell becomes SHORTER — that is muscle contraction.

TWO LIMITATIONS of electrical impulses: (1) they can only reach cells that are CONNECTED by nervous tissue, not every cell in the body; (2) once a nerve cell has fired, it needs TIME to reset before it can fire again. This is why animals ALSO need chemical (hormonal) communication.

MOVEMENT IN PLANTS is of two kinds. (a) INDEPENDENT of growth — NASTIC movement, e.g. the touch-me-not (Mimosa pudica) folding its leaves when touched. It is FAST and TEMPORARY, and the direction does not depend on the direction of the stimulus. (b) DEPENDENT on growth — TROPIC movement, which is SLOW and PERMANENT, and IS directed by the stimulus.

HOW the touch-me-not moves without any nerves or muscles: the plant cells change the amount of WATER in them. Swollen cells become flabby, so the leaflets fold and the leaf droops. The information travels from cell to cell as an electrochemical signal.

TENDRIL — when a tendril touches a support, the side of the tendril AWAY from the support grows FASTER than the side touching it, so the tendril coils around the support. This is a growth movement and is PERMANENT.

TROPISM = a growth movement DIRECTED by the direction of the stimulus. POSITIVE = towards the stimulus; NEGATIVE = away from it.

PHOTOTROPISM — growth in response to LIGHT. Shoots bend TOWARDS light (positively phototropic); roots bend AWAY from light (negatively phototropic).

GEOTROPISM (gravitropism) — growth in response to GRAVITY. Roots grow DOWNWARD, towards gravity (positively geotropic); shoots grow UPWARD, away from it (negatively geotropic).

HYDROTROPISM — growth in response to WATER: roots turn towards a source of moisture (positively hydrotropic). CHEMOTROPISM — growth in response to a CHEMICAL; the standard example is the growth of the POLLEN TUBE down the style towards the OVULE.

PLANT HORMONES (phytohormones) — AUXIN: made at the shoot TIP, makes cells GROW LONGER; it is responsible for phototropism. GIBBERELLINS: help the STEM grow. CYTOKININS: promote CELL DIVISION, and are present in greater concentration in areas of rapid division such as fruits and seeds. ABSCISIC ACID: INHIBITS growth and causes WILTING of leaves (the only one of the four that works against growth).

PHOTOTROPISM EXPLAINED (a favourite exam question): when light falls on ONE side of a shoot, auxin diffuses AWAY from the bright side and collects on the SHADED side. Auxin makes cells grow longer, so the cells on the SHADED side become LONGER than those on the lit side. One side is now longer than the other, so the shoot BENDS TOWARDS the light.

HORMONES in animals are chemical messengers made by ENDOCRINE glands (ductless glands). They are released directly into the BLOOD, which carries them all over the body — so they can reach cells that nerves never reach. They act on specific target organs and are needed only in very small amounts.

ADRENALINE — from the ADRENAL glands, above the kidneys. The emergency 'FIGHT-or-FLIGHT' hormone: the heart beats faster, blood is diverted away from the digestive system and skin to the skeletal muscles, and the breathing rate rises.

THYROXINE — from the THYROID gland. It regulates the metabolism of CARBOHYDRATES, FATS and PROTEINS. IODINE is essential for making it, which is why iodised salt is recommended: a lack of iodine causes GOITRE (a swollen neck).

GROWTH HORMONE — from the PITUITARY gland. It controls growth of the body. Too little in childhood causes DWARFISM; too much causes GIGANTISM.

TESTOSTERONE (from the TESTES in males) and OESTROGEN (from the OVARIES in females) bring about the changes at PUBERTY. INSULIN (from the PANCREAS) controls the level of sugar in the blood — if it is not secreted in the right amount, blood sugar rises, causing DIABETES.

FEEDBACK MECHANISM — the amount of a hormone is regulated automatically. Example: when the blood sugar level RISES, the cells of the pancreas detect it and secrete MORE insulin; as the sugar level falls, insulin secretion is REDUCED. This keeps the level within a narrow range.

Direction of the nerve impulse

dendrite → cell body → axon → nerve ending → (synapse) → next cell

One-way only. At the synapse a CHEMICAL crosses the gap and starts a fresh electrical impulse.

The reflex arc

receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle)

The most-asked diagram in this chapter. The brain is bypassed; the spinal cord completes the arc.

Organisation of the nervous system

Nervous system = CNS (brain + spinal cord) + PNS (cranial nerves + spinal nerves)

A standard 1-mark question: 'Name the parts that constitute the central nervous system.'

Brain — three regions

fore-brain (cerebrum) • mid-brain • hind-brain (cerebellum, medulla, pons)

Cerebrum = thinking; cerebellum = posture, balance, precision; medulla = involuntary; pons = breathing.

Phototropism (auxin)

light on one side → auxin moves to the SHADED side → those cells grow longer → shoot bends TOWARDS the light

Learn this causal chain in order — it is worth 3 marks on its own.

Tropisms and their stimuli

photo → light • geo → gravity • hydro → water • chemo → a chemical

Shoots: +ve phototropic, −ve geotropic. Roots: −ve phototropic, +ve geotropic, +ve hydrotropic.

Plant hormones

auxin (cell elongation) • gibberellin (stem growth) • cytokinin (cell division) • abscisic acid (INHIBITS)

Three promote growth; only abscisic acid inhibits it — it also causes wilting.

Hormone → gland → effect

adrenaline → adrenal • thyroxine → thyroid • growth hormone → pituitary • insulin → pancreas

Testosterone → testes and oestrogen → ovaries bring about the changes at puberty.

Deficiency diseases

iodine ↓ → thyroxine ↓ → GOITRE; growth hormone ↓ → DWARFISM; insulin ↓ → DIABETES

Growth hormone in EXCESS causes gigantism. Iodised salt prevents goitre.

Feedback control

blood sugar ↑ → pancreas secretes MORE insulin → blood sugar ↓ → secretion REDUCED

This is how the body decides how much hormone to make; it keeps levels within a narrow range.

Nervous vs hormonal control

nerves: fast, electrical, along nervous tissue, brief. hormones: slower, chemical, via blood, reach every cell, longer-lasting

A standard 3-mark comparison — give speed, medium, route and duration.