Control and Coordination
How living organisms detect changes in their environment and respond in a controlled, purposeful way — animals through the nervous system and muscles, plants through electrochemical signals and directional growth.
This is high-yield NCERT science: Prelims loves matching brain parts to functions (medulla, cerebellum, fore-brain), neuron structure and the direction of the nerve impulse, the spinal-cord location of the reflex arc, and types of tropism. For Mains it underpins GS-III general science awareness, helping you reason about reflexes, neuro-disorders and plant responses when they surface in current affairs. A clear grip here also speeds up the rest of Class 10–12 biology.
Understand the chapter
What Control and Coordination Means
Living organisms show movement as a controlled response to environmental change, often to use that change to their advantage. Each kind of stimulus evokes a specific, appropriate response — we whisper to a friend but jump from a bus — so response must be matched to the trigger. In multicellular animals, specialised nervous and muscular tissues provide this control and coordination; plants achieve it without any nerves or muscles.
- Stimulus = a change in the environment; response = an appropriate, controlled movement
- Growth-dependent movement (a germinating seedling) vs growth-independent movement (a running cat, touch-me-not folding)
- Animals use nervous + muscular tissue; plants use electrochemical signalling + directional growth
- The response made depends on the event triggering it — control implies recognition then correct action
The Neuron and the Nerve Impulse
Information from the environment is detected by receptors at the dendritic tips of nerve cells, usually located in sense organs. The stimulus sets off a chemical reaction that creates an electrical impulse, which travels from the dendrite to the cell body and along the axon to its end. There, the impulse triggers release of chemicals that cross the synapse to start a fresh impulse in the next neuron's dendrite, or to stimulate a muscle or gland.
- Receptors: gustatory (taste) and olfactory (smell), housed in sense organs like tongue, nose, inner ear
- Impulse path within a neuron: dendrite → cell body → axon → axon endings
- Synapse = the gap where an electrical impulse is carried across as a chemical signal
- Neuromuscular junction = synapse delivering the impulse from a neuron to a muscle cell
Reflex Action and the Reflex Arc
A reflex is a sudden, automatic response to a stimulus made without conscious thought, such as pulling the hand back from a flame. Conscious thinking through the brain involves many neurons and is too slow for such urgent, dangerous situations. So the sensory (input) nerve and motor (output) nerve are linked at the point they first meet — the spinal cord — forming a reflex arc, while the information still travels onward to the brain.
- Reflex arc is formed in the spinal cord for a fast, automatic response
- Evolved because brain 'thinking' is not fast enough; many animals have little or no thinking brain
- Reflex arcs stay more efficient for quick responses even after complex brains evolved
- Examples: withdrawing hand from flame, mouth watering, change in pupil size
The Human Brain: CNS and PNS
The brain and spinal cord together form the central nervous system (CNS), which receives and integrates information from the whole body. The peripheral nervous system (PNS) — cranial nerves arising from the brain and spinal nerves from the spinal cord — connects the CNS to all other parts. The brain has three major regions: fore-brain, mid-brain and hind-brain, each integrating different inputs and outputs.
- Fore-brain: main thinking part — sensory, association and motor areas, plus the hunger centre
- Mid-brain and hind-brain: control involuntary actions
- Medulla (hind-brain): blood pressure, salivation, vomiting
- Cerebellum (hind-brain): precision of voluntary actions, posture and balance
Protection of Nervous Tissue and How Muscles Act
The delicate brain sits inside a bony box (the skull) and is further cushioned by a fluid-filled balloon that absorbs shock; the spinal cord is protected by the vertebral column or backbone. When a nerve impulse reaches a muscle, the muscle fibre responds. Muscle cells contain special proteins that change their shape and arrangement in response to the electrical impulse, giving the cell a shorter form — and this shortening produces movement.
- Brain protection: bony skull + fluid (cerebrospinal) cushion
- Spinal cord protection: vertebral column / backbone
- Muscle movement: special proteins change shape and arrangement → cell shortens
- Voluntary muscles (under thinking control) vs involuntary muscles differ in this control
Coordination in Plants: Movement Without Nerves
Plants have neither a nervous system nor muscles, yet they respond to stimuli. They show two movement types: one independent of growth (the touch-me-not folding) and one dependent on growth (directional movement). In the sensitive plant the movement happens at a point different from the point of touch, so the information is conveyed cell-to-cell by electrical-chemical means — but without any specialised conducting tissue. Plant cells change shape not by special proteins but by changing the amount of water in them, swelling or shrinking.
- Two movements: growth-independent (touch response) and growth-dependent (directional)
- Chhui-mui / touch-me-not = the sensitive plant of the Mimosa family
- Plants use electrochemical signalling but lack specialised nervous tissue
- Plant cell shape changes through gain/loss of water (turgor change)
Tropic Movements (Tropisms)
Growth-dependent movements directed by an environmental trigger are tropic movements, and can be towards the stimulus or away from it. In phototropism, shoots bend towards light while roots bend away. In geotropism, roots grow downward and shoots upward in response to gravity. Hydrotropism (water) and chemotropism (chemicals) are others — the growth of pollen tubes towards the ovule is the classic example of chemotropism.
- Phototropism: shoot towards light (positive), root away from light (negative)
- Geotropism: root grows down (positive), shoot grows up (negative)
- Hydrotropism = growth towards water; Chemotropism = growth towards chemicals
- Tendrils (pea plant): the side touching a support grows slower, so the tendril coils around it
Key terms
- Neuron
- The structural and functional unit of the nervous system, specialised to conduct information as electrical impulses.
- Dendrite
- The branched receiving tip of a neuron where a stimulus/information is first acquired.
- Axon
- The long fibre of a neuron along which the electrical impulse travels to its end.
- Synapse
- The microscopic gap between two neurons (or neuron and muscle) where the impulse is carried across as a chemical signal.
- Receptor
- Specialised nerve-cell tips in sense organs that detect specific stimuli — e.g., gustatory (taste), olfactory (smell).
- Reflex arc
- The nerve pathway (receptor → spinal cord → effector) that produces a fast, automatic reflex action.
- Central Nervous System (CNS)
- The brain and spinal cord, which receive, integrate and process information.
- Peripheral Nervous System (PNS)
- Cranial nerves (from the brain) and spinal nerves (from the spinal cord) connecting the CNS to the body.
- Tropism
- A directional growth movement of a plant part towards or away from an external stimulus.
Must-know facts exam-ready
- Neuron is the structural and functional unit of the nervous system; the impulse travels dendrite → cell body → axon → synapse.
- At a synapse the electrical impulse is converted into a chemical signal that crosses the gap to the next neuron, muscle or gland.
- Gustatory receptors detect taste and olfactory receptors detect smell; receptors lie in the sense organs.
- Reflex arcs are formed in the spinal cord (not the brain), giving quick automatic responses, while the signal also reaches the brain.
- CNS = brain + spinal cord; PNS = cranial nerves (from brain) + spinal nerves (from spinal cord).
- The brain has three regions: fore-brain, mid-brain and hind-brain.
- Fore-brain is the main thinking part and contains the hunger centre, along with sensory, association and motor areas.
- Medulla (in the hind-brain) controls involuntary actions such as blood pressure, salivation and vomiting.
- Cerebellum (in the hind-brain) maintains posture, balance/equilibrium and the precision of voluntary movements.
- Brain is protected by the bony skull and a fluid (cerebrospinal) cushion; the spinal cord by the vertebral column.
- Muscle cells contract because special proteins change their shape and arrangement, shortening the cell.
- Phototropism: shoot bends towards light, root away; geotropism: root grows down, shoot up; pollen-tube growth to the ovule = chemotropism.
Memory tricks remember it for good
Traps to avoid
- Reflex arcs are formed in the SPINAL CORD, not the brain — but the information still travels onward to the brain, so the brain isn't entirely bypassed.
- Cerebellum (posture, balance, precision) vs Medulla (involuntary actions like BP, salivation, vomiting): both lie in the hind-brain but do different jobs.
- Phototropism and geotropism point oppositely for shoots and roots — shoot is positive to light but negative to gravity; root is negative to light but positive to gravity.
- The touch-me-not response is NOT due to growth and NOT due to muscle proteins — plant cells change shape by gaining/losing water; tendril coiling, by contrast, IS growth-dependent.
- At a synapse the signal crosses as a CHEMICAL, not as a continuous electric current, and transmission is one-way.
- Cranial and spinal nerves belong to the PNS, not the CNS; the CNS is only the brain and spinal cord. The hunger centre is in the fore-brain, not the medulla.
Exam focus
🧠 Prelims angles
- Match-the-following on brain parts and functions: medulla (BP/salivation/vomiting), cerebellum (balance/posture), fore-brain (thinking/hunger).
- Neuron structure and the direction of the impulse (dendrite → axon → synapse), plus what happens at the synapse.
- Location of the reflex arc (spinal cord) and the difference between a reflex action and a voluntary action.
- Types of tropism — phototropism, geotropism, hydrotropism, chemotropism — and the positive/negative direction for shoots vs roots.
- Receptors: gustatory vs olfactory, and the sense organs that house them.
- Composition of CNS vs PNS (cranial nerves, spinal nerves).
✍️ Mains angles GS-III
- How does the body achieve rapid yet controlled responses — compare reflex action with voluntary action.Contrast the reflex-arc pathway (spinal cord, automatic, fast) against brain-mediated voluntary action (fore-brain, conscious, slower); note reflexes evolved for speed.
- Plants coordinate responses without a nervous system or muscles — discuss the mechanisms involved.Explain cell-to-cell electrochemical signalling, water/turgor-driven movement (Mimosa) and growth-based tropic movements, contrasting them with animal nerve-muscle coordination.
- Foundations of neuroscience as a base for understanding modern brain/neuro-technology.Link CNS–PNS organisation and neuron-synapse signalling to applications like reflex testing, brain-injury protection (skull/CSF) and emerging neurotech awareness.
Last-minute revision tick as you recall
- Neuron = unit of nervous system; impulse: dendrite → cell body → axon → synapse.
- Synapse converts an electrical impulse into a one-way chemical signal.
- Reflex arc is formed in the spinal cord for fast, automatic responses.
- CNS = brain + spinal cord; PNS = cranial + spinal nerves.
- Fore-brain = thinking + hunger centre; medulla = BP, salivation, vomiting; cerebellum = balance & posture.
- Brain protected by skull + fluid cushion; spinal cord by the vertebral column.
- Muscles shorten when special proteins change shape and arrangement.
- Mimosa folds by water change (no growth); tendrils coil by growth.
- Phototropism: shoot to light, root away; geotropism: root down, shoot up; pollen tube to ovule = chemotropism.
Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.