Why this chapter matters for UPSC: Control and coordination bridges biology and India's most pressing health crises. India is the diabetes capital of the world (~101 million diabetics, 2023 IDF data). Neurological disorders — Parkinson's, Alzheimer's, epilepsy — are a growing burden as the population ages. Mental health (NMHP, Mental Healthcare Act 2017) and hormonal disruption from endocrine-disrupting chemicals (pesticides, BPA in plastics) are active GS3 themes. Understanding how the nervous and endocrine systems work provides the scientific foundation for all these policy discussions.

PART 1 — Quick Reference Tables

Nervous System: Divisions and Functions

Division Components Function
CNS Brain + Spinal cord Processes all information; coordination centre
PNS — Somatic Sensory + Motor nerves to skeletal muscles Voluntary movements (picking up a pen)
PNS — Autonomic — Sympathetic Nerves to internal organs "Fight or flight" — raises heart rate, BP, blood glucose
PNS — Autonomic — Parasympathetic Nerves to internal organs "Rest and digest" — slows heart, promotes digestion

Brain Regions and Functions

Region Function Clinical Relevance
Cerebrum Thinking, voluntary movement, speech, memory, sensory perception Stroke, traumatic brain injury, epilepsy
Cerebellum Balance, coordination, fine motor control Affected in alcohol intoxication
Brainstem/Medulla oblongata Breathing, heartbeat, blood pressure (involuntary) Brain death defined by brainstem failure
Hypothalamus Body temperature, hunger, thirst, sleep, links nervous + endocrine systems Obesity, thermoregulation disorders
Limbic system Emotions, memory formation, reward Depression, addiction, PTSD

Key Endocrine Glands: Hormones and Disorders

Gland Key Hormone(s) Function Deficiency Excess
Pituitary GH (Growth Hormone) Body growth Dwarfism Gigantism (children), Acromegaly (adults)
Pituitary ADH (Antidiuretic Hormone) Water reabsorption in kidney Diabetes insipidus SIADH
Thyroid Thyroxine Metabolic rate (requires iodine) Hypothyroidism, goitre, cretinism Hyperthyroidism, Graves' disease
Pancreas (Islets of Langerhans) Insulin Lowers blood glucose Type 1 diabetes Hypoglycaemia
Pancreas Glucagon Raises blood glucose — —
Adrenal medulla Adrenaline (Epinephrine) Fight-or-flight response — Phaeochromocytoma
Adrenal cortex Cortisol Stress response, anti-inflammatory Addison's disease Cushing's syndrome
Adrenal cortex Aldosterone Na+/K+ balance, BP regulation — Hyperaldosteronism
Testes Testosterone Male secondary sex characters Hypogonadism —
Ovaries Oestrogen, Progesterone Female sex chars, menstrual cycle Menopause symptoms —

Plant Hormones (Phytohormones)

Hormone Primary Effect Key Responses Application
Auxin (IAA) Cell elongation Phototropism, gravitropism, apical dominance Rooting powders; weedkillers (2,4-D)
Gibberellin Stem elongation, seed germination Bolting in plants; breaks seed dormancy Malt production; seedless fruit
Cytokinin Cell division Delays aging of leaves; lateral bud growth Cell culture media
Abscisic Acid (ABA) Growth inhibition, stress response Stomatal closure in drought; leaf fall Stress physiology research
Ethylene Fruit ripening Senescence; abscission; ripening Artificial ripening of bananas, mangoes

PART 2 — Detailed Notes

1. Neurons — The Building Blocks of the Nervous System

Key Term

Neuron (Nerve cell): The structural and functional unit of the nervous system. A typical neuron has three parts:

  • Dendrites: Short, branched projections that receive signals from other neurons or sense organs.
  • Cell body (Soma): Contains the nucleus and organelles; integrates incoming signals.
  • Axon: Long projection that transmits electrical impulse away from cell body to next neuron or effector (muscle/gland). Covered by myelin sheath in many neurons — speeds up transmission.

Synapse: The gap between the axon terminal of one neuron and the dendrite of the next. Electrical impulse cannot directly jump this gap — instead, chemicals called neurotransmitters are released.

Key neurotransmitters: Acetylcholine (motor neurons, parasympathetic), Dopamine (reward, movement), Serotonin (mood, sleep), GABA (inhibitory — calms neural activity), Noradrenaline (arousal, sympathetic).

2. Reflex Arc — Faster than Thought

The reflex arc is a neural pathway that bypasses the brain for speed:

Stimulus → Receptor → Sensory neuron → Spinal cord (control centre) → Motor neuron → Effector → Response

Example: Touching a hot surface — hand is pulled back within milliseconds. The spinal cord processes this and sends the motor signal before the brain even registers pain. The brain learns about the event slightly later.

Explainer

Why the spinal cord and not the brain? Because the time for a signal to travel to the brain and back would cost precious milliseconds — enough for tissue damage to worsen. Reflex arcs are evolutionary shortcuts for survival.

Conditioned reflexes (Pavlov's dogs) involve learned responses where a neutral stimulus becomes associated with a reflex — these do involve the brain.

3. The Endocrine System — Chemical Coordination

Key Term

Hormone: A chemical messenger secreted directly into the bloodstream by an endocrine (ductless) gland. Hormones travel to distant target organs and regulate their activity. They act slowly compared to neural signals but effects are long-lasting.

Pituitary gland (located in the brain, controlled by hypothalamus) is called the master gland because it secretes hormones that control other endocrine glands: TSH stimulates thyroid, ACTH stimulates adrenal cortex, FSH/LH stimulate gonads, GH stimulates growth.

Thyroid and Iodine Deficiency:

The thyroid gland needs iodine to synthesise thyroxine. Iodine deficiency is the leading preventable cause of intellectual disability globally. In India:

  • IDD (Iodine Deficiency Disorders) affects millions — particularly in sub-Himalayan belt and flood-prone river plains where soil iodine is leached.
  • Deficiency in pregnancy → cretinism (stunted physical and mental growth in child).
  • Deficiency in adults → goitre (enlarged thyroid gland as it tries to compensate).
  • India mandated iodisation of edible salt under the Prevention of Food Adulteration Act. The NIDDCP (National Iodine Deficiency Disorders Control Programme) monitors IDD prevalence and iodised salt coverage.

Pancreas and Diabetes — India's Epidemic:

UPSC Connect

UPSC GS3 — Health, Science & Technology:

The pancreas has dual functions — exocrine (digestive enzymes via pancreatic duct) and endocrine (hormones into blood). The Islets of Langerhans are clusters of endocrine cells:

  • Beta cells → Insulin: Released when blood glucose rises (after a meal). Insulin signals cells to absorb glucose from blood → lowers blood glucose. In Type 1 diabetes, the immune system destroys beta cells (autoimmune) → no insulin → glucose accumulates in blood → must inject insulin daily.
  • Alpha cells → Glucagon: Released when blood glucose drops → stimulates liver to release stored glycogen as glucose → raises blood glucose.

India's diabetes burden: ~101 million diabetics (ICMR-INDIAB 2023 study) — the world's largest absolute number, making India the "Diabetes Capital of the World." Type 2 diabetes (insulin resistance, lifestyle-driven) accounts for ~90% of cases. Risk factors: obesity, physical inactivity, processed food, genetic predisposition. Long-term complications: nephropathy, neuropathy, retinopathy, cardiovascular disease.

Policy response: NPCDCS (National Programme for Prevention and Control of Cancer, Diabetes, CVD and Stroke) — screening at district hospitals; NHM funds anti-diabetic drugs; PMJAY covers hospitalisation. India's target: halt the rise of NCDs per SDG 3.4.

Adrenal Glands and Stress:

The adrenal glands sit atop the kidneys. The medulla (inner part) secretes adrenaline (epinephrine) in response to sudden stress:

  • Heart rate increases, BP rises, blood glucose surges, bronchioles dilate, blood flow diverted from gut to muscles — the body prepares for physical action ("fight or flight").
  • This is why heart pounds before an exam or during danger.

Chronic stress → prolonged cortisol elevation → suppressed immunity, hypertension, metabolic disruption. India's growing urban stress burden is a public health concern linked to rising CVD and mental health disorders.

4. Plant Hormones and Tropisms

Key Term

Phototropism: Growth of a plant toward (or away from) light. Controlled by auxin — light causes auxin to migrate to the shaded side of the stem. Higher auxin concentration on the shaded side causes those cells to elongate more → stem bends toward light.

Gravitropism (Geotropism): Roots grow downward (positive gravitropism); shoots grow upward (negative gravitropism) in response to gravity.

Thigmotropism: Response to touch — climbing plants like pea tendrils wrap around supports.

Ethylene in Agriculture:

Ethylene gas is produced naturally by ripening fruits and is used commercially to accelerate ripening of bananas and mangoes during transport. Ethylene-ripened fruits are often of lower nutritional quality than vine-ripened fruits. India's FSSAI regulates the use of ethylene for artificial ripening — calcium carbide (which releases acetylene, not ethylene, and contains toxic arsenic and phosphorus impurities) is banned for food ripening.

Abscisic Acid and Drought:

ABA is the plant's stress hormone. During drought:

  1. ABA is synthesised in roots and leaves.
  2. ABA travels to guard cells of stomata.
  3. Guard cells lose water → stomata close → reduces water loss by transpiration.
  4. ABA also promotes leaf fall (abscission) in deciduous trees.

This mechanism underpins research into drought-tolerant crop varieties — manipulating ABA sensitivity is a target for climate-resilient agriculture.

5. Neurological Disorders — UPSC Context

UPSC Connect

UPSC GS3 — Health and emerging technologies:

Parkinson's disease: Caused by degeneration of dopamine-producing neurons in the substantia nigra (part of basal ganglia). Symptoms: tremors, rigidity, slow movement. Treatment: L-DOPA (dopamine precursor); deep brain stimulation (DBS). India has ~580,000 Parkinson's patients; burden rising with aging population.

Alzheimer's disease: Progressive neurodegeneration; accumulation of amyloid plaques and tau tangles. Dementia — memory loss, cognitive decline. No cure; management with acetylcholinesterase inhibitors. India has ~5.3 million dementia patients (Alzheimer's and Related Disorders Society of India — ARDSI).

Endocrine disruptors: Pesticides (organochlorines, organophosphates), industrial chemicals (PCBs), and plastics (BPA, phthalates) can mimic or block hormones → thyroid disruption, reproductive disorders, developmental problems. India's high pesticide use in agriculture and poor plastic waste management make this a major environmental health concern (GS3 — environment + health).

Mental health: India's National Mental Health Programme (NMHP) provides community-based care. Mental Healthcare Act 2017 decriminalised suicide attempts (earlier punishable under Section 309 IPC) — recognising mental illness as a health condition, not a criminal act. Neurotransmitter imbalances (serotonin, dopamine, GABA) underlie depression, schizophrenia, and anxiety disorders.

Exam Strategy

Prelims traps:

  • Pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood — hormones and nerve signals work differently; do not confuse endocrine (blood-borne, slow) with nervous (electrical, fast).
  • The pituitary is the "master gland" but it is itself controlled by the hypothalamus — a common MCQ distinction.
  • Insulin lowers blood glucose; glucagon raises it — opposite functions of same gland.
  • Type 1 diabetes = autoimmune, insulin-dependent, often childhood-onset. Type 2 = lifestyle, insulin resistance, most common, adult-onset (though rising in youth).
  • Calcium carbide is banned for fruit ripening in India; ethylene (or ethephon) is permitted.
  • ABA is the only inhibitory plant hormone in this list — others (auxin, gibberellin, cytokinin, ethylene) promote various growth responses.
  • India's diabetes figure: ~101 million (2023 IDF/ICMR data) — not "80 million" (older figure).

Previous Year Questions

Prelims:

  1. Which of the following is a function of the hormone insulin?
    (a) Stimulates the adrenal cortex to secrete cortisol
    (b) Facilitates uptake of glucose by cells, lowering blood glucose
    (c) Stimulates the thyroid to produce thyroxine
    (d) Promotes protein breakdown in muscle tissue

  2. Which of the following statements about auxin is correct?
    (a) Auxin promotes root growth toward light
    (b) Auxin accumulates on the illuminated side of a stem
    (c) Auxin accumulates on the shaded side, causing those cells to elongate more
    (d) Auxin inhibits cell division in all plant tissues

Mains:

  1. India is described as the "Diabetes Capital of the World." Examine the physiological basis of Type 2 diabetes and critically evaluate India's policy response to control the diabetes epidemic. (CSE Mains 2023, GS Paper 3, 15 marks)

  2. "Endocrine-disrupting chemicals in pesticides and plastics pose a silent public health threat." Discuss the scientific basis of this concern and suggest regulatory measures India should adopt. (CSE Mains 2022, GS Paper 3, 10 marks)