1. Introduction — What is Control and Coordination?

Control and coordination are fundamental processes by which organisms maintain internal order and respond appropriately to external and internal stimuli. Control refers to regulation (maintaining internal conditions), while coordination links body parts so that activities occur in a harmonised manner.

In animals, control and coordination are achieved mainly through two systems:

• Nervous system — fast, electrical signals (nerve impulses) transmitted via neurons; suitable for rapid, short-term, precise responses (e.g., withdrawal from a hot object).
• Endocrine (hormonal) system — chemical messengers (hormones) secreted into blood; slower, long-lasting effects (e.g., growth, metabolism).

Plants lack nervous systems but display coordination through chemical regulators called plant hormones (phytohormones) and signal transduction mechanisms.

2. Nervous System — Overview

The nervous system detects internal and external changes (stimuli), processes information and generates appropriate responses. Structurally it is divided into:

• Central Nervous System (CNS) — brain and spinal cord; the main processing centres.
• Peripheral Nervous System (PNS) — nerves that connect CNS to limbs and organs; includes sensory (afferent) and motor (efferent) nerves.

Functionally, the nervous system operates through neurons that transmit nerve impulses — rapid electrochemical signals along specialised cell membranes.

3. Neuron — Structure and Types

Neuron (nerve cell) is the basic structural and functional unit of the nervous system. Key parts:

• Cell body (soma): Contains nucleus and organelles; metabolic centre.
• Dendrites: Short branched fibres that receive signals from other neurons or receptors.
• Axon: Long fibre that transmits impulses away from the cell body; many axons are covered by myelin sheath (Schwann cells) which speeds conduction.
• Synapse: Junction between two neurons (axon terminal of one and dendrite of another) where neurotransmitters mediate signal transfer.

Types of neurons: sensory (afferent), motor (efferent) and interneurons (relay neurons).

4. How a Nerve Impulse Travels (Signal Transmission)

A nerve impulse is an electrochemical wave resulting from ion movements across the neuron's membrane. Important points:

1. Resting potential: The neuron at rest maintains a membrane potential (~−70 mV) due to unequal distribution of Na⁺ and K⁺ ions.
2. Depolarisation: Stimulus opens Na⁺ channels; Na⁺ influx causes membrane potential to become less negative (action potential triggered).
3. Repolarisation: K⁺ channels open; K⁺ efflux restores potential, and Na⁺/K⁺ pumps re-establish ion gradients.
4. Propagation: Action potentials travel along axon; myelinated axons conduct via saltatory conduction (jumping between nodes of Ranvier), increasing speed.
5. Synaptic transmission: At synapse, action potential triggers neurotransmitter release (e.g., acetylcholine) into synaptic cleft; transmitter binds receptors on post-synaptic membrane to continue signal.

Exam tip: Know the sequence: stimulus → receptor → sensory neuron → CNS → motor neuron → effector response (muscle or gland).

5. Human Brain and Spinal Cord

The brain is the control centre; key regions and functions:

• Cerebrum: Largest part — responsible for voluntary actions, intelligence, memory, learning and sensory perception. Divided into lobes (frontal, parietal, temporal, occipital).
• Cerebellum: Coordinates muscular movements and balance.
• Medulla oblongata (brainstem): Controls involuntary actions — heartbeat, breathing, swallowing.
• Hypothalamus: Regulates homeostasis, body temperature, hunger and thirst; links nervous and endocrine systems.

Spinal cord transmits information between brain and body and coordinates reflex actions.

6. Reflex Action and Reflex Arc

Reflex actions are rapid, automatic responses to stimuli that protect the body (e.g., withdrawing hand from hot object). They do not require conscious thought.

Reflex arc (pathway of a reflex):

1. Receptor (detects stimulus)
2. Sensory neuron (to spinal cord)
3. Relay neuron (interneuron) in spinal cord
4. Motor neuron (from spinal cord)
5. Effector (muscle/gland responds)

Reflexes are adaptive: they minimise damage by bypassing higher brain centres, allowing immediate response.

7. Sense Organs — Eye and Ear (Structure & Function)

Eye (vision) — main parts and functions:

• Cornea: Transparent front layer that refracts (bends) light.
• Iris & pupil: Iris controls pupil size, regulating light entry.
• Lens: Focuses light on retina by changing curvature (accommodation).
• Retina: Light-sensitive layer containing photoreceptor cells (rods for low-light, cones for colour); signals pass via optic nerve to brain.
• Optic nerve: Carries visual information to brain.

Ear (hearing & balance) — three regions:

• Outer ear: Pinna and ear canal collect sound waves; eardrum (tympanic membrane) vibrates.
• Middle ear: Ossicles (malleus, incus, stapes) amplify vibrations; Eustachian tube equalises pressure.
• Inner ear: Cochlea converts vibrations into nerve impulses (hearing); semicircular canals help maintain balance.

8. Endocrine System — Hormones and Glands

Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream; they act on distant target organs and regulate processes like growth, metabolism, reproduction and stress responses.

Major endocrine glands and key hormones:

• Pituitary gland (master gland): Produces growth hormone (GH), stimulates other endocrine glands (TSH, ACTH, LH, FSH, ADH).
• Thyroid gland: Produces thyroxine — regulates metabolism, growth and development.
• Adrenal glands: Produce adrenaline (fight-or-flight) and corticosteroids — stress response and metabolism.
• Pancreas (islets of Langerhans): Produces insulin (lowers blood glucose) and glucagon (raises blood glucose) — vital for glucose homeostasis.
• Gonads (ovaries/testes): Produce sex hormones (oestrogen, progesterone, testosterone) controlling secondary sexual characteristics and reproduction.

Negative feedback is a central concept: hormone secretion is often regulated by feedback signals. For example, low blood glucose stimulates glucagon release; high blood glucose stimulates insulin release — this keeps levels within narrow limits.

9. Plant Hormones and Coordination in Plants

Plants coordinate growth and responses using chemical regulators called plant hormones or phytohormones. Major plant hormones and roles:

• Auxins (e.g., indole-3-acetic acid): Promote cell elongation, apical dominance and root initiation; involved in phototropism and gravitropism by asymmetric distribution.
• Gibberellins: Stimulate stem elongation, seed germination and flowering in some species.
• Cytokinins: Promote cell division and delay leaf senescence; work in balance with auxins to control organ formation.
• Abscisic acid (ABA): Promotes stomatal closure under water stress and induces seed dormancy (conserving resources).
• Ethylene: A gaseous hormone that promotes fruit ripening and leaf abscission.

Coordination examples:

• Phototropism: Shoots bend toward light due to auxin redistribution — cells on the shaded side elongate more.
• Gravitropism: Roots grow downward (positive gravitropism) and shoots grow upward (negative gravitropism) via hormone-mediated growth responses.

10. Nervous vs Endocrine System — Key Differences

11. Exam-Focused Revision Notes & Quick Tips

• Memorise the definitions: neuron, synapse, reflex arc, hormone, phototropism, gravitropism.
• Learn the structure and function of a neuron (label a diagram) and the sequence of events in a reflex arc.
• Practice labelling the brain regions and write one key function for each (cerebrum, cerebellum, medulla).
• Know hormone sources and one primary function (e.g., insulin — lowers blood glucose; thyroxine — regulates metabolic rate).
• Understand examples of coordination in plants (phototropism, gravitropism, stomatal movement) and relate them to hormone action.
• Remember the photos of eye and ear parts; practise simple diagrams and short notes (neat labelling helps score marks).
• Compare nervous and endocrine systems with a two-line point for each feature; examiners like concise tables.

12. Practice Questions (Short)

1. Define a reflex action and give one example.
2. State the function of the cerebellum.
3. What is the role of insulin and where is it produced?
4. Describe phototropism in one sentence.
5. Differentiate between endocrine and nervous control in two points.

(Attempt answers in one or two sentences each — then check your notes above for accuracy.)