Control and Coordination

What this chapter is about

This chapter explains how living organisms detect changes in their environment and respond to them in a controlled manner. It covers two main systems that help animals coordinate their body functions: the nervous system and the endocrine system. Plants also show responses to stimuli, though they lack a nervous system.

A Class 10 student studies this topic to understand how our body maintains order among billions of cells working together. You learn how nerve impulses travel, how hormones regulate body processes, and how plants bend towards light or respond to gravity. This builds on earlier knowledge of cells and tissues from previous classes.

After studying this chapter, you should be able to describe the structure and function of neurons, explain reflex actions, identify major parts of the human brain and their roles, list important hormones and their effects, and describe how plants coordinate their movements without nerves.

Key ideas

• Stimulus and response: A stimulus is any change in the environment that an organism detects; a response is the action taken by the organism in reaction to that stimulus.
• Neurons are the functional units of the nervous system: Each neuron has a cell body, dendrites that receive information, and an axon that carries impulses away to other neurons or muscles.
• Reflex arc provides quick, automatic responses: In a reflex action, the impulse travels from receptor to sensory neuron, then to relay neuron in the spinal cord, then to motor neuron, and finally to effector organ, bypassing the brain for speed.
• The brain has three main regions: The forebrain (cerebrum) handles thinking, memory and voluntary actions; the midbrain controls some reflexes; the hindbrain (cerebellum and medulla) manages balance, posture and involuntary functions like breathing.
• Hormones are chemical messengers secreted by endocrine glands: They travel through blood and act on specific target organs to regulate growth, metabolism, reproduction and other processes.
• Plants show tropic movements in response to stimuli: Phototropism is growth towards or away from light; geotropism is response to gravity; hydrotropism is response to water.
• Plant hormones like auxin control growth: Auxin accumulates on the shaded side of a stem, causing cells there to elongate more, so the plant bends towards light.

Formulas and facts to remember

1. Neuron structure: Dendrite → Cell body → Axon → Axon terminal (synapse).
2. Reflex arc pathway: Receptor → Sensory neuron → Relay neuron (in spinal cord) → Motor neuron → Effector.
3. Synapse: The small gap between two neurons where chemical signals (neurotransmitters) pass the impulse from one neuron to the next.
4. Parts of the brain: Cerebrum (thinking, memory), Cerebellum (balance, coordination), Medulla oblongata (breathing, heartbeat).
5. Major endocrine glands and hormones: Pituitary (growth hormone), Thyroid (thyroxine for metabolism), Pancreas (insulin for blood sugar), Adrenal (adrenaline for emergency response), Testes (testosterone), Ovaries (oestrogen).
6. Insulin regulates blood glucose: It lowers blood sugar by helping cells absorb glucose; its deficiency causes diabetes.
7. Iodine is needed to make thyroxine: Deficiency of iodine causes goitre, an enlargement of the thyroid gland.
8. Auxin, gibberellin and cytokinin are plant hormones: Auxin promotes cell elongation; gibberellin promotes stem growth; cytokinin promotes cell division.

Worked examples

Example 1: Identifying parts of a reflex arc

Problem: Rahul accidentally touches a hot vessel. His hand jerks away before he feels the pain. Trace the path of the nerve impulse in this reflex action.

Solution:

• Receptor: Heat receptors in the skin of Rahul's finger detect the high temperature.
• Sensory neuron: Carries the impulse from the receptor to the spinal cord.
• Relay neuron: Located in the spinal cord, it processes the signal and passes it on.
• Motor neuron: Carries the impulse from the spinal cord to the muscles of the arm.
• Effector: The arm muscles contract, pulling the hand away from the hot vessel.

This entire process takes a fraction of a second because the brain is bypassed, making the response automatic and protective.

Example 2: Hormone deficiency problem

Problem: A 12-year-old girl in a hilly region has developed swelling in her neck. The doctor says her thyroid gland is enlarged. What deficiency might be causing this, and what hormone is affected?

Solution:

• The swelling in the neck region indicates goitre, which is enlargement of the thyroid gland.
• The thyroid gland produces thyroxine, which requires iodine for its synthesis.
• In hilly regions, soil and water often lack sufficient iodine, leading to iodine deficiency.
• Without enough iodine, the thyroid cannot produce adequate thyroxine, so it enlarges while trying to compensate.
• The doctor would likely recommend iodised salt in her diet to correct this deficiency.

Example 3: Plant tropism

Problem: A potted plant is kept near a window for two weeks. The stem bends towards the window while the roots grow straight downward. Explain both observations.

Solution:

• The stem shows positive phototropism: it grows towards the source of light (the window).
• Auxin hormone accumulates on the shaded side of the stem, causing cells there to elongate more than cells on the lit side. This unequal growth makes the stem bend towards light.
• The roots show positive geotropism: they grow in the direction of gravity, which is downward.
• Roots are less sensitive to light but highly sensitive to gravity, ensuring they grow into the soil to absorb water and minerals.

Common mistakes

• Thinking reflex actions involve the brain → Reflex arcs pass through the spinal cord, not the brain, which is why they are fast and involuntary.
• Confusing nerves and neurons → A neuron is a single nerve cell; a nerve is a bundle of many neuron fibres wrapped together.
• Believing hormones act instantly like nerve impulses → Hormones travel through blood and take longer to act, but their effects last longer than nerve impulses.
• Assuming all plant movements are tropisms → Tropisms involve growth; nastic movements (like leaf folding in touch-me-not) are non-directional and do not depend on stimulus direction.
• Mixing up insulin and glucagon → Insulin lowers blood sugar; glucagon (also from pancreas) raises it. Diabetes results from insulin problems.

Quick revision

• Nervous system uses electrical impulses; endocrine system uses chemical hormones carried by blood.
• Reflex arc: Receptor → Sensory neuron → Relay neuron → Motor neuron → Effector.
• Cerebrum thinks, cerebellum balances, medulla controls involuntary actions.
• Insulin from pancreas controls blood sugar; thyroxine from thyroid controls metabolism.
• Auxin causes phototropism by making the shaded side of stems grow faster.
• Iodine deficiency causes goitre; growth hormone deficiency causes dwarfism.