⚡ NCERT Line-by-Line • Class 10, 11 & NEET 2026 Master Deck

Human Nervous System: Master Biology Encyclopedia

Comprehensive histology of neurons, ionic biophysics of resting and action potentials, chemical synaptic delay, human brain ventricles, CSF circulation, spinal cord reflex arc, cranial nerve mnemonics, ANS antagonism, and clinical pathology.

Anatomical Map of Central & Peripheral Nervous Divisions (NEET Core Standard)

01 Neural vs. Endocrine System: Comparative Coordination

The human body maintains internal homeostasis through two complementary, integrated regulatory networks: the Neural System and the Endocrine System.

Feature	Neural Coordination Network	Endocrine System (Hormonal)
Signal Transmission	Electrochemical impulses propagated along axon membranes and across synapses.	Chemical messengers (hormones) circulating systemically through blood and lymph.
Speed of Response	Rapid / Instantaneous (measured in milliseconds, up to 120 m/s).	Slow and Latent (takes minutes, hours, or days to alter cellular transcription).
Duration of Action	Short-lived / Transient (terminates as soon as impulses cease).	Prolonged / Persistent (remains active until degraded by liver or excreted by kidneys).
Target Specificity	Highly localized point-to-point wiring (innervates discrete muscle fibers or glands).	Widespread systemic diffusion (acts on all tissues possessing specific target receptors).

02 Histology of a Neuron & Structural Classifications

Neurons are excitable cells of ectodermal origin that have lost the capacity for cell division (lack centrosomes/centrioles in adult state).

🧬 1. Cyton (Soma / Perikaryon)

Nissl's Granules: Distinct basophilic granules composed of Rough Endoplasmic Reticulum (RER) and free ribosomes; primary site of protein and neurotransmitter synthesis.
Neurofibrils: Intracellular intermediate protein filaments that run throughout the soma and axon; provide structural support and facilitate internal axoplasmic transport.
Axon Hillock: The conical junction between cyton and axon. Devoid of Nissl's granules. Possesses the lowest threshold for excitation; primary site where action potentials are generated.

⚡ 2. Axon & Myelin Envelopes

Axolemma & Axoplasm: Long single efferent cylindrical process. Axoplasm contains mitochondria and neurofibrils, but completely lacks Nissl granules and Golgi complexes.
Schwann Cells (PNS): Wrap around axons to produce the lipoprotein Myelin Sheath. Outermost nucleated cytoplasmic layer is the Neurilemma (enables peripheral nerve regeneration).
Oligodendrocytes (CNS): Form myelin sheaths around central axons (brain and spinal cord). Lack a neurilemma (regeneration does not occur in CNS).
Nodes of Ranvier: Unmyelinated periodic gaps (~1 μm) between adjacent Schwann cells where axolemma is exposed to ECF.

🌳 3. Dendrites & Telodendria

Dendrites: Multiple, short, tapering, highly branched afferent processes that receive stimuli and conduct impulses centripetally (toward the cyton). Contain Nissl's granules.
Telodendria (Axon Terminals): Distal branched arborizations of the axon that conduct impulses centrifugally (away from the cyton).
Synaptic Knobs (Terminal Boutons): Bulbous terminal swellings packed with mitochondria and membrane-bound synaptic vesicles storing neurotransmitters (e.g., Acetylcholine).

🔬 Morphological Classification of Neurons (NCERT Standard):

Neuron Class	Anatomical Structural Plan	Definitive Human Locations (NEET High-Yield)
Non-Polar / Apolar	Multiple uniform processes without distinction into axon and dendrites.	Primitive nervous networks (e.g., Hydra, Cnidarians).
Unipolar	Cell body with a single process acting as an axon (no dendrites).	Found predominantly in the early embryonic stage.
Bipolar	One axon and one dendrite emerging from opposite poles of the soma.	Retina of the eye, Olfactory epithelium of nasal chamber, and inner ear cochlea.
Multipolar	One long axon and two or more dendrites branching from soma.	Cerebral cortex, spinal cord motor neurons (most common type in vertebrates).
Pseudounipolar	Single process emerges from soma and bifurcates into a central and peripheral branch.	Dorsal Root Ganglia (DRG) of spinal nerves.

03 Electrophysiology: Resting Membrane & Action Potential

The transmission of a nerve impulse is an electrochemical event driven by selective ionic permeability across voltage-gated channels and active transporter pumps in the axolemma.

Phase	Membrane Voltage	Ionic Permeability Status	Molecular Mechanism & State
1. Resting Potential (Polarized)	-70 mV (Negative inside)	Axolemma is nearly impermeable to Na⁺, but freely permeable to K⁺ and completely impermeable to large intracellular negatively charged proteins.	Resting K⁺ leak channels remain open; K⁺ diffuses out down its chemical gradient. Maintained actively by the Na⁺/K⁺ ATPase pump.
2. Depolarization (Action Potential)	-70 mV → +30 mV (Overshoot reversal)	Stimulus reaches threshold (-55 mV). Rapid opening of voltage-gated Na⁺ channels leads to an explosive influx of Na⁺ down electrical and concentration gradients.	Inner surface becomes positively charged (+30 mV). This polarity reversal along the axolemma constitutes the Action Potential (Nerve Impulse).
3. Repolarization	+30 mV → -70 mV	Voltage-gated Na⁺ channels close/inactivate within 0.5 ms. Voltage-gated K⁺ channels open, driving rapid efflux of K⁺ out of the axoplasm.	Restores negative resting electrical polarity inside the membrane, completing repolarization.
4. Hyperpolarization	-70 mV → -90 mV	Voltage-gated K⁺ channels are sluggish and slow to close, allowing excess K⁺ efflux beyond baseline.	Membrane potential temporarily dips below resting level (-90 mV) before settling back to -70 mV.

The Active Na⁺/K⁺ ATPase Electrogenic Pump: 3 Na⁺ (Axoplasm) + 2 K⁺ (ECF) + 1 ATP → 3 Na⁺ (Pumped Out) + 2 K⁺ (Pumped In) + ADP + Pi Translocates 3 positive charges outward for every 2 positive charges inward, directly maintaining resting electronegativity inside.

⚡ Saltatory Conduction (Myelinated Fibers)

The myelin sheath possesses high electrical resistance and acts as an insulator, preventing ion leakage.
Voltage-gated Na⁺ channels are clustered at high density at the Nodes of Ranvier.
Action potentials do not travel continuously; they jump from one Node of Ranvier to the next.
Speed Comparison: Up to 120 meters/sec in myelinated fibers vs. only 0.5–2 meters/sec in unmyelinated fibers. Conserves significant cellular ATP!

04 Synaptic Transmission: Electrical vs. Chemical Mechanics

🔌 Electrical Synapse (Fast / Direct)

Presynaptic and postsynaptic membranes are separated by an ultra-narrow gap of only ~0.2 to 2 nm.
Cytoplasms are physically linked via tubular protein channels called Gap Junctions (Connexons).
Ionic current flows directly from one neuron to the next with virtually zero synaptic delay.
Impulse transmission can be bidirectional. Rare in human CNS; found in cardiac intercalated discs, pulp of tooth, and retina.

🧪 Chemical Synapse (Unidirectional / Controlled)

Separated by a fluid-filled gap of 10 to 20 nm called the Synaptic Cleft.
Impulse transmission relies on chemical messengers called Neurotransmitters.
Strictly unidirectional (presynaptic knob to postsynaptic receptor membrane).
Exhibits a measurable synaptic delay (~0.5 milliseconds) and is susceptible to synaptic fatigue due to neurotransmitter depletion.

🔬 Step-by-Step Chemical Transmission Mechanism:

• Step 1: Action potential arrives at axon terminal → depolarizes the presynaptic knob membrane.
• Step 2: Depolarization opens voltage-gated Calcium channels (Ca²⁺) → Ca²⁺ rushes into the presynaptic terminal from ECF.
• Step 3: Intracellular Ca²⁺ influx causes synaptic vesicles to mobilize, dock, and fuse with the presynaptic membrane.
• Step 4: Neurotransmitter (e.g., Acetylcholine) is released into the synaptic cleft by exocytosis.
• Step 5: Neurotransmitter diffuses across the cleft and binds specifically to ligand-gated receptors on the postsynaptic membrane.
• Step 6: Receptor binding alters postsynaptic permeability:
• Excitatory (EPSP): Opens Na⁺ channels → Na⁺ influx causes depolarization (Acetylcholine, Noradrenaline, Glutamate).
• Inhibitory (IPSP): Opens Cl⁻ or K⁺ channels → Cl⁻ influx or K⁺ efflux causes hyperpolarization (GABA, Glycine, Dopamine).
• Step 7 (Enzyme Clearance): Acetylcholine is rapidly hydrolyzed into acetate and choline by Acetylcholinesterase (AChE) to prevent continuous firing.

05 The Human Brain: Meningeal Envelopes & Anatomical Divisions

The human adult brain weighs ~1.2 to 1.4 kg (contains ~100 billion neurons). Enclosed within the bony cranium and shielded by three protective connective tissue coats called Cranial Meninges:

🛡️ Cranial Meninges (Outer → Inner)

Dura Mater: Outermost, thick, fibrous, non-vascular, tough collagenous layer attached to inner cranial bone.
Arachnoid Mater: Middle, very thin, delicate, spider web-like avascular connective tissue layer.
Pia Mater: Innermost, highly vascular, transparent membrane in direct physical contact with brain surface (dips into sulci).
Subarachnoid Space: Anatomical gap between arachnoid and pia mater; filled with circulating Cerebrospinal Fluid (CSF).

🧠 1. Forebrain (Prosencephalon)

Cerebrum: Largest part (~80% of brain mass). Divided into two hemispheres by a deep longitudinal fissure, connected internally by the Corpus Callosum (a broad transverse curved tract of myelinated nerve fibers — mammalian hallmark!).
Cerebral Cortex: Outer layer of gray matter (unmyelinated cell bodies) folded into elevations (Gyri) and depressions (Sulci).
Basal Ganglia: Deep subcortical nuclei (caudate, putamen, globus pallidus) regulating subconscious motor planning.

🧭 2. Diencephalon & Limbic System

Thalamus: Major relay station; coordinates all sensory inputs (except olfaction/smell!) and directs them to specific cortical areas.
Hypothalamus: Located at base of thalamus. Master autonomic controller: thermoregulation ("body thermostat"), hunger, satiety, thirst, osmolarity, biological clock, and control of pituitary hormones.
Limbic System ("Emotional Brain"): Ring formed by Amygdala (rage, fear, aggression), Hippocampus (converts short-term to long-term memory), Hypothalamus, and Olfactory bulb.

🗺️ Functional Mapping of Cerebral Lobes:

1. Frontal Lobe: Primary motor cortex (voluntary muscle movements); Broca's Motor Speech Area (translation of thoughts into speech); prefrontal cortex for reasoning, logic, personality, planning, and judgment.

2. Parietal Lobe: Somatosensory cortex (processes touch, pain, temperature, pressure sensations); gustatory center (taste perception).

3. Temporal Lobe: Primary auditory cortex (hearing); Wernicke's Sensory Speech Area (comprehension and understanding of spoken/written words); olfactory cortex (smell).

4. Occipital Lobe: Visual sensory and association cortex (decoding images, color, and optical depth).

👁️ Midbrain (Mesencephalon)

Located between diencephalon (thalamus/hypothalamus) and pons.
Traversed by a narrow canal called the Cerebral Aqueduct (Aqueduct of Sylvius / Iter).
Corpora Quadrigemina: Dorsal surface bears 4 rounded optical/auditory swelling lobes:
• Superior Colliculi (Upper 2): Visual reflex integration center (pupillary and ocular tracking).
• Inferior Colliculi (Lower 2): Auditory reflex integration center (head-turning to sound).
Crura Cerebri (Cerebral Peduncles): Ventral paired tracts of nerve fibers connecting forebrain to hindbrain.

⚖️ Hindbrain (Rhombencephalon)

Pons Varolii: Bridge of transverse nerve fibers connecting lobes of cerebellum and cerebrum. Contains the Pneumotaxic Center (limits inspiratory phase).
Cerebellum ("Little Brain"): Highly convoluted surface with outer cortex of Purkinje cells and inner branching white matter tree called Arbor Vitae. Coordinates voluntary muscular activities, equilibrium, posture, and muscle tone. First affected by alcohol consumption.
Medulla Oblongata: Conical extension terminating at foramen magnum. Contains vital autonomic centers: Cardiac center, Respiratory rhythm center, Vasomotor center, and non-vital reflex centers (vomiting, swallowing, coughing, sneezing).

⚠️ Brainstem Definition Trap: The Brainstem consists strictly of: Midbrain + Pons + Medulla Oblongata. The Cerebellum is NOT a part of the brainstem!

06 Ventricles of the Brain & CSF Circulation

The interior of the brain contains 4 interconnected hollow fluid-filled chambers called Ventricles, lined by simple ciliated cuboidal neuroglial cells called Ependyma.

Lateral Ventricles (1 & 2) → Foramen of Monro → 3rd Ventricle (Diocoel) → Aqueduct of Sylvius → 4th Ventricle (Metacoel) → Foramen of Magendie & Luschka → Subarachnoid Space

💧 Cerebrospinal Fluid (CSF) Secretion

Synthesized continuously by vascular tufts called Choroid Plexuses (located in roofs of lateral, 3rd, and 4th ventricles).
Total adult volume: ~120 to 150 mL (re-synthesized and turned over 3 to 4 times a day; daily production ~500 mL).
Reabsorbed into venous blood through finger-like Arachnoid Villi (Granulations) projecting into the superior sagittal dural venous sinus.

🛡️ Crucial Physiological Functions of CSF

Mechanical Shock Absorber: Acts as a liquid cushion, protecting brain against mechanical concussion and gravitational forces.
Buoyancy: Reduces effective brain weight from ~1400 g down to ~50 g, preventing compression of basal cranial nerves.
Homeostasis & Waste Clearance: Delivers micronutrients and removes metabolic catabolites from central neuroglia.

07 Spinal Cord Architecture & The Reflex Arc

🦴 Spinal Cord Histology

Cylindrical structure (~45 cm in length) running from foramen magnum to lower border of L1-L2 vertebra, tapering into Conus Medullaris and fibrous Filum Terminale.
Inverted Tissue Organization: Unlike the brain, the spinal cord contains Internal Gray Matter (H-shaped butterfly) surrounded by External White Matter (Funiculi/Columns).
Gray Matter Horns:
• Dorsal (Posterior) Horn: Receives sensory afferent fibers.
• Ventral (Anterior) Horn: Houses motor cell bodies of efferent fibers.
• Lateral Horn: Houses sympathetic preganglionic neurons (T1–L2).
Central Canal runs through the center, filled with circulating CSF.

⚡ Reflex Arc: 5 Cardinal Components

A reflex action is an immediate, automatic, involuntary motor response to a sensory stimulus without conscious cortical intervention.

1. Sensory Receptor: Detects stimulus (e.g., thermal, nociceptive, spindle stretch).
2. Sensory (Afferent) Neuron: Conducts impulses from receptor to spinal cord; cell body lies in Dorsal Root Ganglion (DRG).
3. Integration Center: Interneuron within spinal cord gray matter.
4. Motor (Efferent) Neuron: Conducts impulse from ventral horn to peripheral effector.
5. Effector Organ: Muscle (contracts) or gland (secretes).

Monosynaptic Reflex: Direct synapse between sensory and motor neuron with no interneuron (e.g., Knee-jerk reflex).

08 Peripheral Nervous System: 12 Pairs of Cranial & 31 Pairs of Spinal Nerves

No.	Cranial Nerve	Nature	Innervation & Core Physiological Function
I	Olfactory	Sensory	Olfactory mucosa of nasal cavity → Smell perception.
II	Optic	Sensory	Retina of eye → Vision perception.
III	Oculomotor	Motor	Superior, inferior, medial recti & inferior oblique eye muscles; ciliary muscle & pupillary sphincter.
IV	Trochlear (Pathetic)	Motor	Superior oblique eye muscle. Smallest cranial nerve.
V	Trigeminal (Dentist's Nerve)	Mixed	Largest cranial nerve. 3 divisions (Ophthalmic, Maxillary, Mandibular). Sensation of face, teeth, and muscles of mastication (chewing).
VI	Abducens	Motor	Lateral rectus eye muscle (abducts eyeball outward).
VII	Facial	Mixed	Taste from anterior 2/3 of tongue; controls muscles of facial expression; stimulates submandibular and sublingual salivary glands.
VIII	Vestibulocochlear (Auditory)	Sensory	Cochlea (hearing) and semicircular canals/vestibule (equilibrium and body balance).
IX	Glossopharyngeal	Mixed	Taste from posterior 1/3 of tongue; swallowing movements; parotid gland secretion.
X	Vagus (Pneumogastric)	Mixed	Longest cranial nerve. Widespread parasympathetic innervation: heart (slows rate), lungs (constricts bronchioles), GI tract (stimulates peristalsis and acid secretion).
XI	Spinal Accessory	Motor	Sternocleidomastoid and Trapezius muscles → Head turning and shoulder shrugging.
XII	Hypoglossal	Motor	Intrinsic and extrinsic muscles of tongue → Tongue movements for speech and swallowing.

🧠 Cranial Nerve Classification Shortcuts

Purely Sensory (3 Nerves): Nerve I, II, VIII (1, 2, 8).
Purely Motor (5 Nerves): Nerve III, IV, VI, XI, XII (3, 4, 6, 11, 12).
Mixed Nerves (4 Nerves): Nerve V, VII, IX, X (5, 7, 9, 10).
Mnemonic for Nature: "Some Say Marry Money, But My Brother Says Big Brains Matter More" (S=Sensory, M=Motor, B=Both/Mixed).

🦴 31 Pairs of Spinal Nerves (All are MIXED)

Cervical (C): 8 Pairs (C1 emerges above atlas; C8 emerges below C7).
Thoracic (T): 12 Pairs (T1 to T12).
Lumbar (L): 5 Pairs (L1 to L5).
Sacral (S): 5 Pairs (S1 to S5).
Coccygeal (Co): 1 Pair (Co1).
Total: 8 + 12 + 5 + 5 + 1 = 31 Pairs (62 nerves in total). Every spinal nerve carries both motor and sensory fibers.

09 Autonomic Nervous System: Sympathetic vs. Parasympathetic Antagonism

The ANS innervates involuntary viscera, smooth muscles, and glands. Subdivided into two physiologically antagonistic divisions:

🔥 Sympathetic ("Fight or Flight")

Outflow: Thoracolumbar Outflow (lateral horns of spinal cord segments T1 to L2/L3).
Ganglia: Paravertebral sympathetic chain ganglia located close to the spinal cord.
Fiber Lengths: Short Preganglionic fibers; Long Postganglionic fibers.
Neurotransmitters:
• Preganglionic: Acetylcholine (ACh).
• Postganglionic: Noradrenaline (Norepinephrine) — Adrenergic.
Prepares the body to confront emergency, danger, exercise, and extreme stress.

🌿 Parasympathetic ("Rest and Digest")

Outflow: Craniosacral Outflow (Cranial nerves III, VII, IX, X + Sacral spinal segments S2, S3, S4).
Ganglia: Terminal ganglia located on or directly inside effector organs.
Fiber Lengths: Long Preganglionic fibers; Very Short Postganglionic fibers.
Neurotransmitters:
• Both Preganglionic and Postganglionic release Acetylcholine (ACh) — Cholinergic.
Conserves and restores energy, lowers metabolic demand, and promotes vegetative functions.

⚖️ Master Organ-by-Organ Antagonistic Response Table:

Target Effector Organ	Sympathetic Action (Stress)	Parasympathetic Action (Rest)
Heart (SA Node & Myocardium)	Increases heart rate and force of contraction (Tachycardia).	Decreases heart rate and force of contraction (Bradycardia via Vagus).
Pupil of Eye (Iris Muscles)	Dilates pupil (Mydriasis — contracts radial pupillary dilator muscles).	Constricts pupil (Miosis — contracts circular pupillary sphincter muscles).
Bronchi & Bronchioles	Dilates bronchial lumen (bronchodilation → maximizes airflow).	Constricts bronchial lumen (bronchoconstriction) & stimulates mucus secretion.
Gastrointestinal Peristalsis	Inhibits motility, decreases glandular secretion; contracts sphincters.	Stimulates peristalsis, increases acid/enzyme secretion; relaxes sphincters.
Salivary Glands	Inhibits secretion; produces small amount of thick, viscous saliva.	Stimulates copious, watery salivary secretion rich in digestive enzymes.
Urinary Bladder	Relaxes Detrusor muscle; contracts Internal Urethral Sphincter (Prevents Urination).	Contracts Detrusor muscle; relaxes Internal Urethral Sphincter (Promotes Urination).
Adrenal Medulla	Stimulates direct release of Adrenaline & Noradrenaline into blood.	No parasympathetic innervation!
Arrector Pili Muscles	Contracts → causes goosebumps (Piloerection).	No parasympathetic innervation!

10 Neurological Disorders & Clinical Pathology

Disorder	Pathological Etiology & Neurochemical Basis	Core Clinical Manifestations
Parkinson’s Disease	Progressive degeneration of dopaminergic neurons in the Substantia Nigra of the midbrain and basal ganglia, causing dopamine deficiency.	Involuntary resting tremors, "pill-rolling" hand movements, muscle rigidity, mask-like face, stooped posture, and shuffling gait. Treated with L-Dopa.
Alzheimer’s Disease	Accumulation of extracellular Beta-amyloid plaques and intracellular neurofibrillary tangles of hyperphosphorylated Tau protein; loss of cholinergic neurons in cerebral cortex and hippocampus.	Progressive irreversible memory loss (amnesia), cognitive deterioration, language breakdown, disorientation, and behavioral changes.
Multiple Sclerosis (MS)	Chronic autoimmune disorder where the immune system destroys the Myelin Sheath of CNS axons (oligodendrocytes attacked).	Formation of scar plaques (scleroses); disrupts saltatory conduction → muscle weakness, visual disturbances, uncoordinated movements, paralysis.
Epilepsy	Abnormal, paroxysmal, excessive synchronous electrical discharges of cerebral cortical neurons.	Recurrent spontaneous seizures, convulsions, sensory hallucinations, and temporary loss of consciousness.
Poliomyelitis (Polio)	Infection by enterovirus (Poliovirus) that selectively infects and destroys the motor neurons in the ventral horn of the spinal cord.	Loss of voluntary motor innervation → flaccid asymmetric muscular paralysis, limb wasting, and skeletal deformities.
Meningitis	Bacterial (e.g., Neisseria meningitidis, Streptococcus pneumoniae) or viral inflammation of the cranial and spinal meninges.	Severe headache, high fever, neck stiffness (nuchal rigidity), photophobia, and positive Kernig's/Brudzinski's signs.

11 15 High-Yield NEET Traps & Memory Accelerators

1. Nissl's Granules Location: Present in Cyton and Dendrites, but strictly ABSENT in Axon and Axon Hillock.

2. Myelin Builders: Schwann cells myelinate PNS nerves; Oligodendrocytes myelinate the CNS (brain and spinal cord).

3. Centrosomes in Neurons: Differentiated adult neurons lack centrosomes/centrioles and are permanently arrested in G₀ phase (cannot divide).

4. Na⁺/K⁺ Pump Ratio: Actively pumps 3 Na⁺ OUT for every 2 K⁺ pumped IN, consuming 1 molecule of ATP.

5. Bipolar Neuron Locations: Exclusively located in the Retina of the eye, Olfactory epithelium, and inner ear cochlea.

6. Corpus Callosum: A broad curved band of myelinated transverse nerve fibers connecting the two cerebral hemispheres — found only in mammals!

7. Brainstem Anatomy: Brainstem consists of Midbrain + Pons + Medulla. The Cerebellum is NOT part of the brainstem.

8. Thermostat of the Body: Center for thermoregulation, hunger, thirst, and satiety resides in the Hypothalamus.

9. Arbor Vitae: The "Tree of Life" is the characteristic branching white matter pattern found inside the Cerebellum.

10. Smallest vs. Largest Cranial Nerves: Trochlear (IV) is the smallest cranial nerve; Trigeminal (V) is the largest cranial nerve; Vagus (X) is the longest.

11. Spinal Cord Matter: Gray matter is internal (H-shaped), and white matter is external (the exact reverse of the cerebrum!).

12. Alcohol and Cerebellum: Alcohol intoxication depresses cerebellar function, leading to loss of muscle coordination and slurred speech.

13. Reflex Arc Afferent Ganglion: The cell body of a sensory neuron in a reflex arc is located in the Dorsal Root Ganglion (DRG).

14. Inhibitory Neurotransmitters: GABA (gamma-aminobutyric acid) and Glycine open Cl⁻ channels, causing hyperpolarization (IPSP).

15. PNS Outflow: Sympathetic system is Thoracolumbar (T1–L2); Parasympathetic system is Craniosacral (CN III, VII, IX, X + S2–S4).

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Complete Nervous System Notes for NEET 2027 – Structure, Functions, Brain, Reflex Arc & Autonomic System Explained