10.3 Control & Coordination
Key Takeaways
- In animals, control and coordination are carried out by the nervous system (electrical signals) and the endocrine system (chemical hormones); in plants, only hormones coordinate responses.
- The neuron is the basic unit of the nervous system; a reflex action is an involuntary, rapid response to a stimulus that does not involve the brain, routed through the spinal cord.
- The human brain has three main parts: forebrain (cerebrum, thalamus, hypothalamus), midbrain and hindbrain (cerebellum, pons, medulla).
- Plant hormones control growth and movement: auxin promotes cell elongation, gibberellins promote stem growth, cytokinins promote cell division, abscisic acid inhibits growth and closes stomata.
- Key endocrine glands and hormones: thyroid (thyroxine — needs iodine), pancreas (insulin — lowers blood sugar), adrenal (adrenaline — fight-or-flight), pituitary (growth hormone — master gland).
10.3 Control & Coordination
Why It Matters
A living organism must detect changes in its surroundings (stimuli) and respond to them so that it can survive. The processes that make this happen are called control and coordination. In animals, two systems work together: the nervous system (fast, electrical signals) and the endocrine system (slower, chemical hormones). In plants (which have no nervous system), only hormones do the job.
Nervous System: The Neuron
The basic structural and functional unit of the nervous system is the neuron (nerve cell). A neuron has three main parts:
- Dendrites — short, branched fibres that receive the stimulus and carry impulses TOWARDS the cell body.
- Cell body (cyton) — contains the nucleus and is the metabolic centre.
- Axon — a long fibre that carries the nerve impulse AWAY from the cell body. The axon ending branches and connects to the dendrites of the next neuron (or to a muscle or gland) at a junction called a synapse. Many axons are covered by a fatty myelin sheath that acts like insulation and speeds up the impulse.
A nerve impulse is an electrical signal that travels along the neuron. At a synapse, the impulse is converted into a chemical signal (a neurotransmitter), crosses the tiny gap, and starts a new electrical impulse in the next neuron.
Reflex Action and Reflex Arc
A reflex action is a quick, involuntary, automatic response to a stimulus that does NOT involve the brain. Examples: pulling your hand away from a hot object, the knee-jerk, blinking when something moves towards the eye, salivation at the sight of food.
The pathway taken by the nerve impulse during a reflex action is called the reflex arc:
- Stimulus (hot object) detected by receptors in the skin.
- Sensory neuron carries the impulse to the spinal cord.
- The spinal cord passes the impulse to a motor neuron via a small region called the relay centre (or directly, in a two-neuron reflex).
- Motor neuron carries the impulse to the effector (muscle in the hand), which contracts to pull the hand away.
- The information is then sent to the brain, which is why you FEEL the pain after you have already moved your hand.
Reflex arcs evolved because they let animals respond to danger very quickly — useful when the slower brain signal would arrive too late.
The Human Brain
The brain is the highest coordinating centre of the body. It is protected by the skull and by three membranes called meninges. The space between the brain and the meninges is filled with cerebrospinal fluid, which cushions the brain from shocks.
The brain has three main regions:
| Part | Region | Main Functions |
|---|---|---|
| Forebrain | Cerebrum | Thinking, memory, reasoning, voluntary actions, intelligence |
| Forebrain | Thalamus | Relays sensory and motor signals; sorts information |
| Forebrain | Hypothalamus | Controls body temperature, hunger, thirst, pituitary secretion |
| Midbrain | Midbrain | Controls visual and auditory reflexes; muscle tone |
| Hindbrain | Cerebellum | Maintains posture and balance; coordinates voluntary movements |
| Hindbrain | Pons | Regulates breathing |
| Hindbrain | Medulla oblongata | Controls involuntary activities: heartbeat, breathing, blood pressure, salivation, vomiting |
The spinal cord starts at the base of the brain and runs through the vertebral column. It carries nerve impulses between the brain and the rest of the body and is the centre for reflex actions.
The central nervous system (CNS) = brain + spinal cord. The peripheral nervous system (PNS) = all the nerves that branch out from the CNS to the body.
Coordination in Plants
Plants have no nervous system, so they coordinate using plant hormones (phytohormones), which are chemicals that control growth, development and responses to stimuli.
| Hormone | Main Action |
|---|---|
| Auxin | Promotes cell elongation (growth); synthesized at shoot tips; bends the shoot towards light (phototropism) by elongating cells on the shaded side |
| Gibberellins | Promote stem growth; help seeds germinate and fruits elongate |
| Cytokinins | Promote cell division; delay leaf ageing (senescence); useful in tissue culture |
| Abscisic acid (ABA) | Inhibits growth; causes leaves/fruit to fall (abscission); closes stomata during water stress |
| Ethylene | Promotes fruit ripening; a gaseous hormone |
Tropisms are directional growth movements in response to a stimulus:
- Phototropism — growth towards (shoots) or away from (roots) light.
- Geotropism / Gravitropism — growth in response to gravity (roots grow down, shoots up).
- Hydrotropism — growth of roots towards water.
- Chemotropism — growth in response to chemicals (e.g., pollen tube growing towards the ovule).
The classic experiment is Darwin's coleoptile tip experiment, later refined by Boysen-Jensen and Went, which showed that an auxin-like signal from the shoot tip causes the bending — when the tip is cut off, the shoot stops bending towards light.
Plants also show nastic movements (non-directional, e.g., the closing of leaflets of Mimosa pudica (touch-me-not) when touched, using electrical impulses that travel along the pulvinus, similar to animal nerve signals). Leaves of Mimosa pudica droop on touch to frighten away grazing animals.
Endocrine System in Humans
The endocrine glands secrete hormones directly into the blood (not through ducts), so they are called ductless glands. Hormones are chemical messengers that act on specific target organs.
| Gland | Hormone(s) | Function |
|---|---|---|
| Thyroid | Thyroxine | Regulates carbohydrate, protein and fat metabolism; needs iodine (lack of iodine causes goitre, a swollen neck) |
| Pancreas | Insulin, Glucagon | Insulin LOWERS blood sugar (helps cells take up glucose); glucagon raises it; lack of insulin → diabetes |
| Adrenal | Adrenaline (epinephrine) | 'Fight-or-flight' hormone; increases heart rate, breathing, blood supply to muscles in emergencies |
| Pituitary | Growth hormone and many others | 'Master gland' — controls other glands; excess growth hormone in childhood → gigantism, too little → dwarfism |
| Testes (male) | Testosterone | Development of male secondary sexual characteristics |
| Ovaries (female) | Estrogen, Progesterone | Development of female secondary sexual characteristics; regulates the menstrual cycle and maintains pregnancy |
Because hormones are released into the blood, they act more slowly than nerve impulses, but their effects last longer. The two systems complement each other: nerves give fast, short-lived responses; hormones give slower, sustained responses.
Exam Traps
- Reflex action does NOT involve the brain — only the spinal cord. The pain is felt later because the message reaches the brain afterwards.
- Iodine and thyroxine: lack of iodine in the diet causes goitre, not lack of thyroxine directly. Thyroxine is made FROM iodine.
- Insulin lowers blood sugar (does not raise it). Lack of insulin causes diabetes; diabetics may need insulin injections.
- Adrenaline is secreted by the adrenal gland, NOT by the pituitary. Adrenaline is the 'fight-or-flight' hormone.
- Medulla oblongata controls involuntary activities (breathing, heartbeat); the cerebellum controls balance and posture. Don't confuse them.
When you touch a hot plate and pull your hand away instantly, which part of the nervous system is primarily responsible for this quick response?
Which plant hormone is mainly responsible for the bending of a shoot towards light (phototropism)?
A person whose thyroid gland is not producing enough thyroxine may develop a swollen neck (goitre). Which nutrient deficiency in the diet is most directly linked to this condition?