Your body has two master control systems: the fast electrical nervous system and the slower chemical endocrine system. Together they sense the world, decide, and act — pulling your hand from a flame, racing your heart before an exam, or releasing insulin after lunch. For NDA Biology, these two systems are a near-guaranteed source of marks, so let us make them effortless.
Why Control and Coordination Matters
Coordination means different organs of the body working together as a single unit so that a stimulus produces a proper, timely response. Without it, your heart, lungs and muscles would act independently and chaos would follow. Every time you read this sentence, dozens of organs are silently being coordinated — your eyes focus, your neck holds steady, your heart paces blood to your brain.
In animals, coordination is achieved by two systems that constantly talk to each other:
- Nervous system — uses electrical impulses; response is very fast but short-lived.
- Endocrine system — uses chemical messengers called hormones; response is slower but long-lasting.
These two systems are not rivals; they are partners. The nervous system handles instant emergencies, while the endocrine system manages slow processes like growth, puberty and metabolism over months and years. The hypothalamus in the brain is the key bridge that links them.
Nerves are like a phone call (instant, brief). Hormones are like a posted letter (slower, but the effect lingers). NDA loves to test this exact contrast.
A stimulus is any change in the environment to which the body reacts — light, heat, sound, smell, touch or pain. The reaction the body produces is called a response. Receptors detect the stimulus, the coordinating system processes it, and effectors (muscles or glands) carry out the response.
The Neuron: Basic Unit of the Nervous System
The neuron (nerve cell) is the structural and functional unit of the nervous system. It is the longest cell in the human body. A neuron has three main parts:
- Dendrites — short branched fibres that receive information.
- Cell body (cyton) — contains the nucleus; processes the information.
- Axon — a long fibre that carries the impulse away from the cell body to the next neuron or muscle.
The axon is often covered by a fatty myelin sheath that insulates it and speeds up the impulse. The gap between two neurons is the synapse.
How does an impulse actually travel? When a neuron is stimulated, a tiny electrical change sweeps along the membrane of the axon. This travelling electrical disturbance is the nerve impulse. It moves in only one direction within a single neuron because the synapse acts like a one-way valve.
Based on function, neurons are of three types: sensory neurons carry impulses from receptors to the CNS, motor neurons carry impulses from the CNS to effectors, and relay (inter) neurons connect the two inside the brain and spinal cord. Bundles of many axons wrapped together form a nerve.
Direction of impulse in one neuron: Dendrite → Cell body → Axon → Synapse. At the synapse, the electrical signal is converted into a chemical signal (neurotransmitter) to cross the gap.
Reflex Action and the Reflex Arc
A reflex action is a sudden, automatic, involuntary response to a stimulus — like jerking your hand off a hot plate before you even feel the pain consciously. It protects the body from harm by saving precious time. If the message had to travel all the way up to the conscious brain, be processed, and travel back down, those extra milliseconds could mean a serious burn or injury.
The pathway travelled by the impulse during a reflex is the reflex arc. Crucially, the message does not wait to travel all the way to the brain — the spinal cord takes the decision and triggers the response immediately. The brain is informed in parallel, which is why you feel the pain a moment after your hand has already moved.
Reflex arc pathway: Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector (muscle).
- Receptor — detects the stimulus (e.g. skin).
- Effector — the muscle or gland that responds.
Examples of reflexes commonly asked: knee-jerk, blinking, sneezing, coughing, watering of the mouth, and pulling the hand away from heat. The brain is informed after the action is done.
The Human Brain and Its Parts
The brain is the body's central command centre, protected inside the skull (cranium) and cushioned by cerebrospinal fluid within three membranes called meninges. It has three major regions.
Forebrain (Cerebrum)
The cerebrum is the largest part. It is the seat of intelligence, memory, thinking, reasoning, will power and voluntary actions. It also receives sensory information from all the sense organs, interprets it, and decides on voluntary responses. Its surface is heavily folded into ridges and grooves, which greatly increases the area available for these higher functions.
Midbrain
A small portion that connects the forebrain and hindbrain and controls some reflexes of the eyes and ears.
Hindbrain
- Cerebellum — controls balance, posture and coordination of voluntary muscle movements (e.g. walking straight, riding a bicycle).
- Medulla oblongata — controls involuntary actions like heartbeat, breathing, blood pressure, vomiting and sneezing.
- Pons — relays signals and helps regulate breathing.
Students mix up cerebrum and cerebellum. Remember: cerebrum = thinking; cerebellum = balance. And the medulla controls the life-supporting involuntary functions.
Divisions of the Nervous System
The human nervous system is broadly divided into two parts:
- Central Nervous System (CNS) — the brain and the spinal cord. It is the main coordinating centre.
- Peripheral Nervous System (PNS) — all the nerves arising from the brain (cranial nerves) and spinal cord (spinal nerves) that connect the CNS to the rest of the body. Think of the CNS as the head office that makes decisions and the PNS as the network of wires that carries instructions out and brings information back in.
The PNS includes the autonomic nervous system, which controls involuntary actions and has two opposing parts:
- Sympathetic — the “fight or flight” system; speeds up heartbeat, widens pupils.
- Parasympathetic — the “rest and digest” system; slows heartbeat, aids digestion.
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves — a popular one-mark fact in NDA GS.
The Endocrine System: Chemical Coordination
The endocrine system is made of ductless glands that pour their secretions, called hormones, directly into the blood. The blood then carries hormones to distant target organs, where they fit into specific receptors like a key into a lock. Only the correct target cells respond, even though the hormone travels through the entire body.
Hormone levels are controlled by a smart self-regulating mechanism called feedback control. For example, when blood sugar rises after a meal, insulin is released to bring it down; once sugar falls, insulin secretion is reduced. This keeps the internal environment stable — a state called homeostasis.
Two terms you must separate clearly:
- Endocrine (ductless) glands — secrete hormones into blood (e.g. thyroid, pituitary).
- Exocrine glands — secrete through ducts onto a surface (e.g. sweat glands, salivary glands).
Hormones are required in very small amounts, are chemical messengers, and are not stored — they are made as needed. The pituitary gland is called the master gland because it controls many other glands.
Major Endocrine Glands and Their Hormones
Learn these gland-hormone-function trios; NDA repeatedly tests them.
- Pituitary (base of brain) — secretes growth hormone (GH); controls growth and other glands.
- Thyroid (neck) — secretes thyroxine (needs iodine); controls metabolism.
- Parathyroid — secretes parathormone; regulates blood calcium.
- Pancreas — islets of Langerhans secrete insulin and glucagon; control blood sugar.
- Adrenal (above kidneys) — secretes adrenaline, the “emergency hormone”.
- Testes (male) — secrete testosterone.
- Ovaries (female) — secrete oestrogen and progesterone.
The pancreas is a dual gland — endocrine (insulin/glucagon) and exocrine (digestive enzymes). This dual nature is a favourite trick question.
Important Hormones You Must Know
Insulin and Glucagon
Insulin lowers blood sugar by helping cells absorb glucose. Glucagon raises blood sugar. If insulin is deficient, blood sugar rises — this disease is diabetes mellitus.
Thyroxine
Controls the rate of metabolism. Iodine is essential to make it; lack of iodine causes goitre (swelling of the neck).
Adrenaline
Released during fear, anger or stress. It increases heartbeat, breathing rate and blood sugar, diverts blood to the muscles, and widens the pupils, preparing the body for “fight or flight”. This is why your heart pounds before an exam or an interview.
Sex Hormones
Testosterone in males and oestrogen in females control the development of secondary sexual characters at puberty — such as deepening of voice and facial hair in boys, and body changes in girls. Progesterone maintains pregnancy in females.
Diabetes insipidus (caused by deficiency of vasopressin/ADH from the pituitary) is different from diabetes mellitus (caused by lack of insulin). Do not confuse the two.
Deficiency of growth hormone in childhood causes dwarfism; excess causes gigantism; excess in adults causes acromegaly.
Coordination in Plants (Quick Look)
Plants have no nervous system, yet they respond to stimuli through plant hormones (phytohormones) and movements.
Tropic movements are growth movements directed by a stimulus:
- Phototropism — growth towards light (shoots).
- Geotropism — growth towards gravity (roots grow down).
- Hydrotropism — growth towards water.
- Chemotropism — growth towards a chemical.
Main plant hormones: Auxin (cell elongation, bending towards light), Gibberellin (stem growth), Cytokinin (cell division), Abscisic acid (inhibits growth, closes stomata), and Ethylene (ripening of fruits).
Auxin explains how a shoot bends towards light: the hormone collects more on the shaded side, making those cells grow longer, so the stem curves towards the brighter side. Note that abscisic acid is often called the stress hormone of plants because it helps them survive drought by closing stomata and slowing growth.
The folding of leaves in the touch-me-not (Mimosa) plant is a nastic movement — it is not a growth movement and is not directional.
Worked Example: Tracing a Reflex
A person steps on a sharp pin and instantly lifts the foot. Identify the components of this reflex arc in correct order.
Notice the order: receptor → sensory neuron → spinal cord → motor neuron → effector. The conscious feeling of pain reaches the brain only after the protective action is complete.
Previous-Year Style Practice
Q. Which of the following glands is known as the “master gland” of the human body?
(a) Thyroid (b) Pancreas (c) Pituitary (d) Adrenal
Answer: (c) Pituitary. It is called the master gland because its secretions control the activity of several other endocrine glands such as the thyroid, adrenals and gonads.
Other high-frequency one-liners: thyroxine needs iodine; insulin comes from the pancreas; the medulla oblongata controls heartbeat and breathing; the cerebellum controls balance.
Quick Revision
- Neuron = basic unit; impulse goes dendrite → cell body → axon → synapse.
- Reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector.
- Cerebrum = thinking; cerebellum = balance; medulla = heartbeat & breathing.
- CNS = brain + spinal cord; PNS = cranial + spinal nerves (12 + 31 pairs).
- Pituitary = master gland; thyroxine needs iodine; insulin lowers blood sugar.
- Plants respond by tropisms and hormones like auxin; nerves are absent.
Revise the gland-hormone-disease links until they are automatic — they alone can fetch you several marks in NDA Biology. A smart strategy is to draw a single chart with three columns (gland, hormone, function/disorder) and read it once every day in the last week before the exam.
Frequently asked questions
What is the difference between the nervous and endocrine systems?
The nervous system uses electrical impulses for fast, short-lived responses, while the endocrine system uses chemical hormones carried by blood for slower but longer-lasting responses. Both work together to coordinate the body.
Why is the pituitary gland called the master gland?
Because it secretes hormones that control and regulate the activity of many other endocrine glands, including the thyroid, adrenal glands and gonads. It also produces growth hormone.
Which part of the brain controls heartbeat and breathing?
The medulla oblongata in the hindbrain controls involuntary vital functions such as heartbeat, breathing, blood pressure, vomiting and sneezing.
What is the difference between reflex action and a voluntary action?
A reflex action is automatic and involuntary, controlled by the spinal cord for quick protection, whereas a voluntary action is consciously controlled by the cerebrum of the brain.
Which hormone controls blood sugar and which gland secretes it?
Insulin lowers blood sugar and glucagon raises it; both are secreted by the islets of Langerhans in the pancreas. A deficiency of insulin causes diabetes mellitus.
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