Class 10 › Science › Chapter 6
Control and Coordination
The nervous system and reflex action, the human brain, coordination in plants through hormones and tropisms, and the endocrine system in animals.
1. Need for Control and Coordination
- Living organisms detect changes in the environment and respond with appropriate, controlled movement. This needs control and coordination.
- In animals it is done by the nervous system (electrical impulses, with muscles) and hormones (chemical). In plants it is done by hormones (no nervous system or muscles).
- Movement may be growth-related (a seedling pushing up) or independent of growth (a cat running).
2. Nervous System and the Neuron
- Receptors: specialised nerve-cell tips in sense organs that detect information. Examples: gustatory (taste), olfactory (smell), also in the ear and skin.
- Neuron: the structural and functional unit of nervous tissue, specialised to conduct electrical impulses.
| Part of neuron | Function |
|---|---|
| Dendrite (tip) | Acquires information, starting a chemical reaction that creates an electrical impulse |
| Cell body | Impulse travels from dendrite to the cell body |
| Axon | Impulse travels along it to its end |
| Axon end | Impulse is converted to a chemical signal (chemicals released) |
Path of a nerve impulse
Dendrite → cell body → axon → end of axon releases chemicals → chemicals cross the synapse (the gap) → start an impulse in the dendrite of the next neuron.
A similar synapse delivers impulses to muscle cells or glands (neuromuscular junction).
- Smell and taste: food tastes duller with a blocked nose because smell contributes to the sense of taste (as in a cold).
3. Reflex Action and Reflex Arc
Reflex action: a sudden, automatic response to a stimulus without thinking (pulling the hand away from a flame, pupil size changing in bright light).
Reflex arc: the pathway of input nerve and output nerve connected at the spinal cord, giving a quick response without waiting for the brain.
Step 1
Receptor
Detects the stimulus (heat) in the skin.
Step 2
Sensory neuron
Carries the impulse to the spinal cord.
Step 3
Spinal cord
Relay neuron connects input to output nerve; the reflex arc is formed here.
Step 4
Motor neuron → effector
Carries the impulse to the muscle, which moves the hand.
- Thinking in the brain is too slow for urgent situations, so reflex arcs evolved. The information also reaches the brain later.
- Reflex arcs are formed in the spinal cord, where nerves from all over the body meet on their way to the brain.
4. The Human Brain
- Central nervous system (CNS): brain + spinal cord (receive and integrate information). Peripheral nervous system (PNS): cranial nerves (from the brain) and spinal nerves (from the spinal cord) connect the CNS to the body.
| Region | Function |
|---|---|
| Fore-brain | Main thinking part. Sensory areas (hearing, smell, sight), association areas, and motor areas controlling voluntary muscles. Also a hunger centre (feeling of being full). |
| Mid-brain | Involuntary actions, with the hind-brain; involved in reflex control of pupil size. |
| Hind-brain: medulla | Controls involuntary actions: blood pressure, salivation, vomiting. |
| Hind-brain: cerebellum | Precision of voluntary actions; maintains posture and balance (walking in a straight line, riding a cycle, picking a pencil). |
| Type of action | Controlled by | Example |
|---|---|---|
| Reflex | Spinal cord (arc) | Withdrawing hand from heat |
| Voluntary | Fore-brain | Writing, moving a chair, clapping |
| Involuntary | Mid-brain, hind-brain (medulla) | Heartbeat, breathing, digestion, salivation |
- Spinal cord injury disrupts signals between the brain and the body parts below the injury (reflex arcs below the injury still work locally).
5. Protection of Nervous Tissue and Muscle Action
Brain
Protected by
A bony box (skull) and a fluid-filled balloon inside, which absorbs shocks.
Spinal cord
Protected by
The vertebral column (backbone).
- How muscle moves: muscle cells have special proteins that change shape and arrangement when a nerve impulse arrives, so the cell shortens (contracts).
6. Coordination in Plants
Independent of growth
Sensitive plant (touch-me-not)
Leaves fold and droop on touch. The plant uses electrical-chemical signals from cell to cell (no specialised tissue). Cells change shape by changing their water content (swell or shrink), not by muscle proteins. Movement occurs at a point different from the touch.
Dependent on growth
Tropic movements
Directional growth in response to light, gravity, water or chemicals. Slow.
- Tendrils (pea plant): the side away from the support grows faster than the side touching it, so the tendril coils around the support.
- Sunflower movement with day and night is slow; sensitive-plant movement is very fast.
7. Tropic Movements
| Tropism | Stimulus | Response |
|---|---|---|
| Phototropism | Light | Shoots bend towards light; roots bend away |
| Geotropism | Gravity | Roots grow down (towards gravity); shoots grow up |
| Hydrotropism | Water | Roots grow towards water |
| Chemotropism | Chemicals | Pollen tube grows towards the ovule |
- A tropic movement can be towards (positive) or away from (negative) the stimulus.
- Activity 6.2: bean shoots bend towards the window light and roots away; after turning the flask, the old parts stay but new growth changes direction.
8. Plant Hormones: Chemical Coordination
Hormones: chemical compounds made in one part of the organism that diffuse to another part to coordinate growth, development and responses.
| Hormone | Role |
|---|---|
| Auxin | Made at the shoot tip; makes cells grow longer |
| Gibberellins | Help growth of the stem |
| Cytokinins | Promote cell division; high in fruits and seeds |
| Abscisic acid | Inhibits growth; causes wilting of leaves |
How phototropism occurs
Light comes from one side. Auxin (made at the shoot tip) diffuses towards the shady side.
Higher auxin makes cells on the shady side grow longer than those on the lit side.
The unequal growth bends the shoot towards light.
- Auxin also explains tendril coiling: auxin at the side away from the support speeds up growth there, so the tendril circles the support.
9. Hormones in Animals (Endocrine System)
- Electrical impulses are fast but reach only connected cells, and a cell needs time to reset before sending another impulse. Chemical (hormonal) signals are slower but reach all cells and act steadily. Hormones are secreted by endocrine glands directly into the blood.
- Adrenaline (adrenal glands) prepares the body for fight or flight: heart beats faster, breathing rate increases, blood to skin and digestive system is reduced and diverted to skeletal muscles.
| Hormone | Gland | Function / deficiency |
|---|---|---|
| Growth hormone | Pituitary | Regulates growth; deficiency in childhood causes dwarfism |
| Thyroxin | Thyroid | Regulates carbohydrate, protein and fat metabolism. Needs iodine; deficiency causes goitre (swollen neck) |
| Insulin | Pancreas | Regulates blood sugar; too little causes diabetes (treated with insulin injections) |
| Testosterone | Testes | Male changes at puberty |
| Oestrogen | Ovaries | Development of female sex organs, regulates the menstrual cycle, female changes at puberty |
| Adrenaline | Adrenal | Emergency response (fight or flight) |
| Releasing hormones | Hypothalamus | Stimulate the pituitary to release hormones (e.g. growth hormone releasing factor) |
Feedback mechanism: the timing and amount of hormone released is regulated. Example: if blood sugar rises, pancreatic cells make more insulin; as sugar falls, insulin secretion is reduced.
- Why iodised salt: iodine is needed to make thyroxin; prevents goitre.
10. Quick Revision: Nervous vs Hormonal Coordination
| Feature | Nervous | Hormonal |
|---|---|---|
| Signal | Electrical impulse (then chemical at synapse) | Chemical, through blood |
| Speed | Very fast | Slower |
| Reach | Only cells connected by nerves | All body cells |
| Duration | Short-lived | Steady and persistent |
| Compare | Sensitive plant | Movement in our legs |
|---|---|---|
| Signal | Electrical-chemical, cell to cell, no specialised tissue | Nerve impulse through neurons |
| How cells move | Change in water content (swell/shrink) | Muscle proteins change shape (shortening) |
Common mistakes:
• Saying reflex arcs go through the brain first; they are formed in the spinal cord.
• Mixing up dendrite (receives) and axon (sends); impulse flows dendrite → cell body → axon.
• Saying involuntary and reflex actions are the same: involuntary actions (heartbeat) are controlled by medulla/mid-brain; reflexes by the spinal cord.
• Saying auxin moves to the lit side; it moves to the shady side.
• Mixing roots and shoots in phototropism and geotropism.
• Confusing which hormone comes from which gland (insulin: pancreas, thyroxin: thyroid).
• Forgetting that abscisic acid inhibits growth while auxin, gibberellins and cytokinins promote it.
• Saying reflex arcs go through the brain first; they are formed in the spinal cord.
• Mixing up dendrite (receives) and axon (sends); impulse flows dendrite → cell body → axon.
• Saying involuntary and reflex actions are the same: involuntary actions (heartbeat) are controlled by medulla/mid-brain; reflexes by the spinal cord.
• Saying auxin moves to the lit side; it moves to the shady side.
• Mixing roots and shoots in phototropism and geotropism.
• Confusing which hormone comes from which gland (insulin: pancreas, thyroxin: thyroid).
• Forgetting that abscisic acid inhibits growth while auxin, gibberellins and cytokinins promote it.