Control and Coordination
Touch a hot stove by accident and your hand is already pulling back before the word "hot" has even formed in your mind. A tendril of a pea plant, with no muscles and no brain at all, slowly wraps itself around a fence wire over a few hours. A person under sudden fright finds their heart pounding and breathing quickening, all without deciding any of it. Every one of these is control and coordination — the biological systems that detect a change in the surroundings, a stimulus, and produce an appropriate, timely response. This chapter builds that story in two connected halves. In animals, coordination rests on two systems working together: the nervous system, a network of neurons carrying information as fast electrical impulses, and the endocrine system, ductless glands that release slower-acting chemical messengers called hormones into the blood. We follow a nerve impulse from a receptor, across a synapse, into the spinal cord and, for urgent situations, straight back out to a muscle through a shortcut called the reflex arc — without troubling the brain's thinking centres first. We then map the brain's three regions and see why each is built for a different job, and how the skull and vertebral column protect this delicate tissue. In plants, which have neither nerves nor muscles, coordination is achieved entirely by chemical means: an immediate, growth-independent response using changes in cell water content (the sensitive plant), and slower, growth-dependent directional movements called tropisms, steered by plant hormones such as auxin. We then return to animals to see how hormones such as thyroxin, insulin, adrenaline and the sex hormones regulate growth, metabolism, stress response and puberty, fine-tuned by feedback loops. By the end, you should be able to trace any stimulus-to-response pathway in the chapter, nerve or hormone, plant or animal, and explain why the body needed two different systems in the first place.
- Control and coordination are the functions by which organisms detect a stimulus and produce a matching response, using nervous tissue (fast, electrical) and hormones (slower, chemical) in animals, and only chemical means in plants.
- A neuron has a cell body, branching dendrites that receive information, and a long axon carrying an electrical impulse to its tip, where a synapse converts the signal into a chemical one for the next cell.
- A reflex action is a rapid, automatic response wired through a reflex arc — receptor to sensory neuron to spinal cord to motor neuron to effector — so the response begins before the brain finishes processing the event.
- The brain has three regions: forebrain (thinking, sensory interpretation, voluntary action, hunger centre), midbrain and hindbrain, where the cerebellum governs posture and balance and the medulla governs involuntary actions like heartbeat and blood pressure.
- The brain sits inside the skull's bony box cushioned by fluid, and the spinal cord runs inside the vertebral column — both need mechanical protection.
- Muscle movement happens when a nerve impulse triggers special muscle proteins to change shape and shorten the fibre.
- Plants show an immediate response independent of growth (the sensitive plant folding on touch, via changing cell water content), and a slower, growth-dependent movement called tropism — bending toward or away from light (phototropism), gravity (geotropism), water (hydrotropism) or chemicals (chemotropism).
- Plant hormones coordinate growth: auxin promotes elongation and steers tropisms, gibberellins promote stem growth, cytokinins promote cell division, and abscisic acid inhibits growth and causes wilting.
- In animals, endocrine glands secrete hormones directly into the blood; growth hormone, thyroxin, insulin, adrenaline and the sex hormones each act on specific target organs, regulated by feedback mechanisms that raise or lower secretion as conditions change.
- Key idea: living organisms detect a stimulus and respond through nervous tissue, which carries fast electrical impulses along a receptor to effector pathway (with a reflex arc as the fastest, brain-independent shortcut), and through hormones, slower chemical messengers regulated by feedback loops, while plants achieve the same control and coordination using only chemical signals and either turgor-driven immediate movements or hormone-driven tropisms.
Control and coordination in animals runs on two cooperating systems, a fast electrical nervous system built from neurons and synapses, with the reflex arc as its emergency shortcut through the spinal cord, and a slower chemical endocrine system of hormone-secreting glands regulated by feedback, while plants achieve the same detect-and-respond behaviour with no nerves or muscles at all, using turgor-driven immediate movements and hormone-steered growth movements called tropisms.
What this chapter covers
The NCERT chapter builds outward from a single question — how does a living organism detect a change around it and respond appropriately — and answers it separately for animals and plants.
- Animals — nervous system (6.1): the structure of a neuron and how an electrical impulse travels through it and crosses a synapse; reflex actions and the reflex arc; the human brain's three regions, forebrain, midbrain and hindbrain, and their separate jobs; how the skull and vertebral column protect the brain and spinal cord; and how a nerve impulse finally makes a muscle contract.
- Coordination in plants (6.2): the immediate, growth-independent response of the sensitive plant to touch; growth-dependent directional movements called tropisms, including phototropism, geotropism, hydrotropism and chemotropism; and the plant hormones — auxin, gibberellins, cytokinins and abscisic acid — that steer this growth.
- Hormones in animals (6.3): the endocrine system as a second, chemical means of control and coordination, using adrenaline as the opening example; the major human endocrine glands and their hormones — pituitary, thyroid, adrenal, pancreas, testes and ovaries — and the feedback mechanisms that keep hormone levels precisely regulated.
The core ideas
The neuron: how a nerve cell carries information
Every piece of information from the environment — heat, taste, smell, light, touch, sound — is first picked up by the specialised tips of nerve cells called receptors, usually clustered in sense organs such as the inner ear, nose, tongue, eye and skin. Gustatory receptors on the tongue detect taste, olfactory receptors in the nose detect smell. A single nerve cell, or neuron, is built for this job of picking up and passing on information: a cell body with a nucleus, branching dendrites that receive a stimulus and start a chemical reaction generating an electrical impulse, and a single long axon that carries this impulse away toward its end. At the tip of the axon the electrical impulse cannot simply jump into the next cell; it triggers the release of chemicals that cross a narrow gap called the synapse, starting a fresh electrical impulse in the dendrite of the next neuron. This chemical relay at every junction is deliberate: it lets impulses cross a synapse in only one direction, and lets many neurons combine or filter information at each junction. The same kind of synapse also connects the last neuron in a chain to a muscle cell or gland — a neuromuscular junction — finally delivering the message to the tissue that will act on it.
Reflex actions and the reflex arc
Some situations, such as touching a flame, are too urgent to wait for conscious thought. If the signal had to travel all the way to the brain's thinking regions, be processed there, and then travel back to the muscles, the delay could be long enough to cause real injury. The body's solution is the reflex action: a rapid, automatic, involuntary response to a stimulus that does not wait for the brain to finish thinking about it. The shortcut that makes this possible is the reflex arc: rather than routing the input nerve all the way to the brain and back, it is connected to an output nerve at the first place the two meet, the spinal cord. A stimulus is detected by a receptor, and the impulse travels along a sensory neuron into the spinal cord; there, without waiting for the brain, the signal is passed (often through a connecting relay neuron) directly to a motor neuron, which carries the outgoing impulse to an effector — usually a muscle — producing the response. The information is also sent onward to the brain, so we do become aware of what happened, but the response itself has already begun before that awareness registers. Because a signal simply travels a fixed distance at a fixed speed along a nerve fibre, the time it takes is given by the same relation used for any steady motion:
where: is the time taken, is the length of the nerve pathway from receptor to effector, and is the conduction speed of the impulse along that fibre. This relation says nothing about how the impulse is generated, only about how long it takes to travel once it exists, and it is the tool the first derivation and the third worked example use to separate "time spent moving along a nerve" from "time spent at synapses" in a real reflex.
The human brain: three regions, three kinds of jobs
Reflex actions are not the whole story of the nervous system — thinking, deciding and remembering also happen, needing far more complex circuitry than a simple arc through the spinal cord. This complexity is concentrated in the brain, which with the spinal cord makes up the central nervous system (CNS). The peripheral nervous system (PNS) — cranial nerves from the brain and spinal nerves from the spinal cord — carries messages between the CNS and the rest of the body. The brain has three major regions. The forebrain is the main thinking part: separate sensory areas for hearing, smell, sight and other senses, association areas that interpret this information by combining it with what is already stored in the brain, and motor areas that instruct voluntary muscles once a decision is made. It also has specific centres for particular sensations, such as a hunger centre — quite different from ordinary sensory input like sight or sound. The midbrain and hindbrain together handle actions we do not consciously think about. The medulla, part of the hindbrain, controls involuntary actions such as blood pressure, salivation and vomiting, along with breathing and heartbeat. The cerebellum, also in the hindbrain, is responsible for precision of voluntary actions and for maintaining posture and balance — the reason we can walk in a straight line or ride a bicycle without consciously calculating every muscle contraction. Because the brain is so delicate and essential, it is carefully protected: it sits inside the bony box of the skull, cushioned further by a fluid-filled membrane that absorbs shocks. The spinal cord is protected in turn by the bony vertebral column (the backbone), felt as a hard, bumpy structure down the middle of the back.
How a nerve impulse finally moves a muscle
Once a nerve impulse reaches a muscle at the neuromuscular junction, the muscle fibre must change shape to produce movement. Muscle cells contain special proteins that change both shape and arrangement in response to the arriving impulse; this rearrangement shortens the fibre. This is the final, mechanical step that converts an electrical-then-chemical signal into visible movement — voluntary (clapping), involuntary (the heart beating) or reflex (withdrawing a hand from a flame).
Coordination in plants: immediate response without nerves or muscles
Plants have neither nervous tissue nor muscles, yet they clearly detect and respond to stimuli. Touch the leaves of the sensitive plant (chhui-mui, of the Mimosa family) and they fold and droop within seconds — a movement with no growth involved at all. Since the point touched is often different from the point that moves, information about the touch must be communicated across the plant, using electrical-chemical signals passed cell to cell, but with no specialised conducting tissue equivalent to a nerve. The movement itself happens because specific cells rapidly change the amount of water they hold, swelling or shrinking and so changing shape — an entirely different mechanism from the sliding muscle proteins used by animals.
Movement due to growth: tropisms and plant hormones
Most plant movement is slower and tied to growth. A pea plant's tendril, on touching a support, grows more slowly on the side in contact with the object than on the side away from it, and this uneven growth curls the tendril around the support. More generally, plants bend in a particular direction because they grow faster on one side than the other, a directional growth response called tropism. A tropism can be positive (growth toward the stimulus) or negative (growth away from it). Shoots show positive phototropism, bending toward light, while roots show negative phototropism. Roots show positive geotropism, growing downward with gravity, while shoots show negative geotropism, growing upward and away from the earth. Hydrotropism is directional growth toward water, and chemotropism is growth toward or away from a chemical, as when a pollen tube grows toward the chemical signals released by an ovule during fertilisation. These directional growth movements are steered by plant hormones, chemical substances synthesised in one part of the plant that diffuse to another part to act — analogous in principle to animal hormones, though chemically different. Auxin, synthesised at the shoot tip, promotes cell elongation; when light falls unevenly on a shoot, auxin diffuses toward the shaded side, making those cells elongate faster than the lit side, so the shoot bends toward the light — worked through step by step in the first derivation below. Gibberellins, like auxin, also promote stem growth. Cytokinins promote cell division and occur in greater concentration wherever cells divide rapidly, as in fruits and seeds. Not every plant hormone promotes growth: abscisic acid inhibits growth and causes wilting, acting as the plant's stress and "stop growing" signal.
Hormones in animals: a second, chemical control system
Electrical impulses along nerves are fast, but they can only reach cells connected by nervous tissue, and a nerve cell needs time to reset before firing again — so nerves cannot carry a signal meant to reach every cell and act persistently. When a squirrel is startled and must prepare either to fight or flee, many tissues — heart, lungs, muscles, digestive system — need to change activity together, and a slower but wide-reaching chemical signal does that job better than nerves alone. This chemical route is the endocrine system: ductless glands that secrete hormones directly into the blood, which carries them to specific target organs anywhere in the body.
Adrenaline, secreted by the adrenal glands, is the example NCERT opens with. Released into the blood in a stressful situation, adrenaline makes the heart beat faster (supplying more oxygen to muscles), narrows the blood vessels supplying the digestive system and skin (diverting blood to the skeletal muscles), and increases breathing rate — changes that prepare the whole body for urgent physical action, without anyone consciously ordering each response.
Beyond adrenaline, several human hormones illustrate coordinated growth and regulation. The thyroid gland needs iodine to make thyroxin, which regulates carbohydrate, protein and fat metabolism for proper growth; iodine deficiency can lead to goitre, a visible swelling of the neck. Growth hormone, from the pituitary gland, regulates overall body growth; too little in childhood leads to dwarfism, too much to gigantism. Testosterone (testes) and oestrogen (ovaries) drive the physical changes of puberty. Insulin, from the pancreas, regulates blood sugar; insufficient secretion lets blood sugar rise, a condition called diabetes, sometimes treated with insulin injections. Because such precise quantities matter, hormone release is regulated by feedback mechanisms: rising blood sugar is detected by the pancreas, which releases more insulin; as blood sugar falls, insulin secretion is reduced again. The hypothalamus also links the nervous and endocrine systems — when growth hormone is low, it releases a factor that stimulates the pituitary to release more. The third derivation below works through this feedback loop step by step.
The goal is to reconstruct, structure by structure and in the correct order, exactly what happens between the instant a finger touches a flame and the instant the hand is pulled away, and to show why this pathway is faster than any route that requires conscious thought. Board and competitive questions on this chapter routinely ask students to sequence the components of a reflex arc, and mixing up sensory and motor neurons, or believing the brain must act first, are the two errors this derivation is built to prevent.
Step 1. The stimulus, heat, is detected by a heat-sensitive receptor in the skin of the finger — the specialised tip of a sensory neuron's dendrite — and detecting the heat sets off a chemical reaction that creates an electrical impulse right at that tip.
Step 2. The impulse travels along the dendrite to the cell body of the sensory neuron, and then along its axon toward the spinal cord. Sensory neurons are the input wires of the reflex arc: they always carry information from a receptor toward the central nervous system, never the other way.
Step 3. Inside the spinal cord, the incoming sensory impulse is passed, typically through a short connecting relay neuron, directly across a synapse to a motor neuron, without first travelling up to the brain's thinking regions. This hand-off inside the spinal cord is the reflex arc itself — the shortcut NCERT identifies as the reason reflex arcs evolved as fast, efficient responses, and remain faster than thought even in animals with complex brains.
Step 4. The motor neuron carries the outgoing impulse from the spinal cord to the effector, the muscles of the arm. At the neuromuscular junction the impulse triggers the muscle's contractile proteins to rearrange and shorten the fibre, and the arm muscle contracts, pulling the hand away from the flame.
Step 5. Independently of this fast loop, the same sensory information is relayed upward to the brain, so a person becomes consciously aware of the heat and pain. This awareness travels a longer path, so it always arrives after the withdrawal response has begun — exactly why the hand is already moving before the pain is consciously felt.
Go deeper · Derivation B · 1 minWhy a shoot bends toward light: the auxin reasoning chain
The goal is to explain, cause by cause, why a growing shoot curves toward a light source that falls on it from one side only, using nothing beyond the definitions of auxin and cell elongation already introduced in the chapter. This is the standard "explain phototropism" reasoning demanded in board exams, and stepping through it explicitly avoids the shortcut answer "the plant grows toward light" that skips the mechanism.
Step 1. Light falls unevenly on the shoot tip, illuminating the side facing the source strongly while leaving the opposite side in shade. The shoot tip is where the plant hormone auxin is synthesised.
Step 2. Auxin, once synthesised, diffuses away from its site of production. Under one-sided light, more auxin diffuses laterally toward the shaded side of the shoot than stays on the illuminated side, so an uneven, side-to-side concentration builds up across the tip.
Step 3. Auxin's specific effect on plant cells is to promote elongation. Because the shaded side now carries a higher concentration of auxin than the lit side, its cells elongate more, over the same span of time, than the cells on the illuminated side.
Step 4. When one side of a growing stem elongates faster than the other, the stem does not grow straight; it curves toward the side that grew less — the illuminated side. The visible result is a shoot bending toward light, even though no single cell "moved"; every cell simply grew, and grew unevenly.
Step 5. This same asymmetric-growth logic, run with gravity instead of light, explains geotropism in roots and shoots, and with a support surface instead of light explains how a pea tendril curls around whatever it touches — uneven growth rates on two sides of an organ, not movement of the whole organ, produce the bending.
Go deeper · Derivation C · 1 minThe blood-sugar feedback loop that regulates insulin
The goal is to show precisely how a hormone's release is kept in the right range without any conscious control, using insulin and blood glucose as the chapter's own worked example of a feedback mechanism. This loop is the model for how NCERT expects every hormone-regulation question to be answered: a cycle in which a change is detected, a hormone response is triggered, and the response itself removes the original change.
Step 1. After a meal, glucose from digested food enters the bloodstream, and the blood sugar level rises above its normal range — the stimulus that must be detected.
Step 2. Specialised cells of the pancreas continuously monitor blood sugar level. When they detect a rise, they respond by secreting more of the hormone insulin directly into the blood.
Step 3. Insulin travels through the blood to its target tissues and increases the rate at which cells take up and use glucose. As a direct consequence, blood sugar falls back toward its normal range.
Step 4. The pancreas cells, still monitoring blood sugar, now detect that the level has fallen; insulin secretion is reduced again, and the system has returned to its starting point, ready for the next rise.
Step 5. This is a negative feedback loop: the hormone's effect (lowering blood sugar) switches off the stimulus that triggered it (high blood sugar), which is why insulin secretion tracks blood sugar up and down rather than climbing without limit. When this loop fails — the pancreas cannot secrete enough insulin, or cells stop responding to it properly — blood sugar stays abnormally high, the condition called diabetes, sometimes managed by insulin injections substituting for the missing response.
Worked examples
Board level - sequencing the reflex arc for a pinprick
A student accidentally steps on a drawing pin. Arrange, in the correct order, the structures involved in the reflex response of lifting the foot: (i) motor neuron, (ii) effector (leg muscle), (iii) receptor in the skin of the foot, (iv) spinal cord, (v) sensory neuron. Also state where conscious awareness of the pain is registered.
Step 1. Identify the very first structure in any reflex pathway: it must be the one that directly detects the stimulus. Here the stimulus is the sharp pressure of the pin, detected by a pain/pressure receptor in the skin of the foot, so receptor comes first.
Step 2. The impulse generated at the receptor must travel toward the central nervous system before anything else can happen, and the neuron that always carries signals inward, from receptor to CNS, is the sensory neuron.
Step 3. The sensory neuron's impulse reaches the spinal cord, where the reflex arc connection is made and the signal is switched over to the outgoing side of the pathway.
Step 4. From the spinal cord, the outgoing impulse is carried by the motor neuron, which always carries signals outward from the CNS to an effector.
Step 5. The motor neuron delivers the impulse to the effector, the leg muscle, which contracts and lifts the foot off the pin. Conscious awareness of the pain is registered afterward, in the forebrain, once the same sensory information (relayed separately) reaches the brain. Answer: receptor, then sensory neuron, then spinal cord, then motor neuron, then effector (leg muscle) — with pain becoming consciously felt only afterward, in the forebrain.
Board level - matching hormone, gland and deficiency disorder
A patient shows a visibly swollen neck and sluggishness, and the doctor attributes it to a dietary deficiency. (a) Name the hormone and the gland involved. (b) Name the dietary deficiency responsible. (c) Name the disorder. (d) Contrast this briefly with what happens if growth hormone is deficient in childhood.
Step 1. A swollen neck points directly to the thyroid gland, which sits in the neck; the hormone it secretes is thyroxin, which regulates carbohydrate, protein and fat metabolism for proper growth.
Step 2. Thyroxin synthesis specifically requires iodine as a raw material. If the diet lacks iodine (for example, if iodised salt is not used), the thyroid cannot make enough thyroxin.
Step 3. The thyroid gland enlarges as it tries to compensate for low hormone output, producing the visible swelling in the neck; this disorder is called goitre, and it is accompanied by symptoms of low metabolism such as sluggishness.
Step 4. Growth hormone deficiency is a different gland (pituitary) and a different disorder: instead of a swollen neck, insufficient growth hormone in childhood leads to poor overall growth and dwarfism, with no particular effect on metabolism the way thyroxin deficiency has. Answer: thyroxin from the thyroid gland, caused by iodine deficiency, is goitre; growth hormone deficiency from the pituitary instead causes dwarfism, a different gland, hormone and disorder entirely.
Board HOTS / foundation JEE-NEET - timing a reflex: how much of the delay is nerve conduction
A doctor measures a patient's simple reflex response time (stimulus to visible muscle response) at milliseconds. The nerve pathway, from receptor in the foot to the effector muscle via the spinal cord, is about metre long, and typical myelinated human nerve fibres conduct impulses at about metres per second. (a) Calculate the time used purely in carrying the impulse along the nerves. (b) Estimate how much of the total ms is used up elsewhere, and name what causes that extra delay.
Step 1. Apply from the core-ideas relation: .
Step 2. Compare this to the total measured reflex time: the nerves account for only ms of the ms total, a fifth of the whole response — nerve conduction along the axons is not the slow step.
Step 3. Subtract to find the remaining delay: is used up elsewhere — at the receptor (converting the stimulus into an impulse), at the synapse(s) in the spinal cord (a chemical hand-off always takes longer than simple conduction), and at the neuromuscular junction as the muscle mechanically contracts.
Step 4. State the conclusion: even though the reflex arc is the fastest pathway the body has, most of a reflex's response time is spent at the junctions, the synapses, not moving along the fibres. Answer: nerve conduction takes about ms; the remaining ms is synaptic and receptor/effector delay. Timing note: about two minutes. The only working step is ; attributing the leftover time to synaptic delay rather than conduction is what separates a HOTS-level answer from a plain calculation, and is exactly the "explain the number" follow-up foundation NEET biology favours.
Common traps
Students confuse reflex action with involuntary action: both happen without conscious control, but a reflex action is a rapid, one-off response wired through a fixed reflex arc (a knee jerk, pulling a hand from a flame), while involuntary actions such as heartbeat, breathing and digestion are continuous, ongoing processes controlled mainly by the midbrain and hindbrain, not single-event arcs. A related mix-up is assuming the brain plays no role in a reflex; NCERT is explicit that sensory information also travels on to the brain, so we become aware of the stimulus just after the reflex response has already started. Students also reverse sensory and motor neurons: sensory neurons always carry signals toward the CNS, motor neurons always carry signals away from it toward an effector. In the brain, the three regions are frequently mismatched with their functions: thinking, sensory interpretation and voluntary movement belong to the forebrain; balance, posture and precision of movement belong to the cerebellum (hindbrain); involuntary actions such as blood pressure, salivation, vomiting, heartbeat and breathing are controlled chiefly by the medulla (hindbrain), not the forebrain. Do not describe the cerebellum as controlling involuntary organs — its job is movement and balance, not running the heart or lungs. In the plant section, students often mix up the direction of tropism: shoots are positively phototropic but negatively geotropic (grow upward, away from gravity), while roots are the reverse. Another frequent error is treating auxin as accumulating on the lit side; it is the shaded side that receives more auxin and elongates faster, producing the bend toward light. Students also sometimes describe the sensitive plant's leaf-folding as a growth response; NCERT is explicit that this immediate movement involves no growth, only a rapid change in cell water content. In the hormone section, the commonest error is mismatching hormone, gland and effect: thyroxin (thyroid, metabolism, iodine-dependent, deficiency causes goitre) versus growth hormone (pituitary, body growth, deficiency causes dwarfism), or forgetting insulin (pancreas) regulates blood sugar, not growth. Students also treat hormone action as instantaneous like a nerve impulse; it is deliberately slower and longer-lasting because it must reach every cell through the bloodstream, not just cells wired into a nervous pathway.
You should be able to…
- Draw and label a neuron, and state the direction information travels through it.
- Explain what happens at a synapse, and why chemical transmission there is useful.
- Sequence the five components of a reflex arc, and explain why the response begins before the brain is involved.
- Match each brain region to its function: forebrain (thinking, senses, voluntary action), cerebellum (posture, balance), medulla (involuntary actions).
- Explain how the brain and spinal cord are physically protected from injury.
- Describe how a nerve impulse ultimately makes a muscle fibre contract.
- Distinguish the sensitive plant's immediate response from a tropism.
- Define phototropism, geotropism, hydrotropism and chemotropism, and state whether shoots and roots respond positively or negatively.
- Explain, using auxin, why a shoot bends toward light, step by step.
- Name four human hormones with their gland, target and effect, and explain feedback using the insulin-blood sugar example.
Ask the tutor: Want to work through a few more reflex-arc or tropism scenarios out loud until the sequencing becomes automatic?
Why does my hand pull away from something hot before I actually feel the pain?
Because the withdrawal is a reflex action, wired through a reflex arc running from the receptor in your skin to a motor neuron in your spinal cord and back out to your arm muscle, without waiting for the brain to finish processing anything. The same signal is also sent up to your brain, which is how you eventually feel the pain, but that longer path takes more time than the short spinal loop. So the muscle contraction is already underway before the pain reaches conscious awareness — the reflex arc is simply the faster route.
If reflex actions and involuntary actions both happen without me thinking about them, what makes them different?
A reflex action is a single, rapid, all-or-nothing response to a sudden stimulus, wired through a fixed reflex arc — a knee jerk, blinking at a flash, pulling back from a pin. It is triggered, it happens, and it is over. An involuntary action is an ongoing process that runs continuously whether or not a stimulus just occurred, such as your heart beating or food moving through your gut. These are controlled mainly by the medulla, and you cannot bring most of them under conscious control even if you try, whereas a reflex like blinking can occasionally be suppressed for a moment.
Plants have no nervous system and no muscles, so how can they detect touch or bend toward light?
Plants replace both missing systems with chemistry. Instead of nerves, they pass information cell to cell using electrical-chemical signals, with no specialised conducting tissue. Instead of muscles, the sensitive plant's leaves move fast because specific cells rapidly change how much water they hold, swelling or shrinking. Slower tropisms happen through uneven growth: a hormone such as auxin builds up unevenly across a shoot or root, and the side with more auxin elongates faster, so the organ curves. No nerve impulse and no muscle contraction are needed anywhere — only diffusing chemicals and unequal cell growth.
Why does the body need both nerves and hormones instead of just one system?
Because they solve different problems. Nervous signals are fast electrical impulses, but they only reach cells wired into that pathway, and a nerve needs a short reset before firing again, so nerves cannot maintain a steady, body-wide signal. Hormones travel more slowly through the blood, but blood reaches almost every cell, so a hormone like adrenaline can change the heart, blood vessels, lungs and muscles all at once, and keep acting as long as it remains in the blood. Urgent, split-second responses call for nerves; broad, sustained, body-wide changes call for hormones — together they cover the full range the body needs.