Nervous System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Nervous System — Key Facts for NEET PG

• CNS: Brain + Spinal cord — processing and integration
• PNS: Cranial + Spinal nerves — communication with body
• Neuron: Cell body (soma), dendrites (receive), axon (conduct away)
• Resting Membrane Potential: -70 mV (K⁺ leak channels maintain)
• ⚡ Exam tip: Action potential propagation: Myelinated = saltatory (node-to-node); Unmyelinated = continuous; Velocity: 120 m/s (large myelinated) vs 0.5-2 m/s (unmyelinated)

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nervous System — NEET PG Study Guide

Organization

Central Nervous System (CNS):

• Brain: Cerebrum, cerebellum, brainstem
• Spinal cord: 31 segments, gray matter inside, white matter outside

Peripheral Nervous System (PNS):

• Somatic: Voluntary (skeletal muscle)
• Autonomic: Involuntary (smooth muscle, cardiac, glands)
  • Sympathetic (fight or flight)
  • Parasympathetic (rest and digest)
  • Enteric (gut)

Neuron Structure

Cell Body (Soma):

• Nissl bodies (RER) for protein synthesis
• Neurofibrils for structural support
• Centrioles for cell division (not in mature neurons)
• Satellite cells in ganglia

Dendrites:

• Receive signals (receptive zone)
• Dendritic spines (receive excitatory synapses)
• Many branches for synaptic inputs

Axon:

• Single, originates from axon hillock
• Axoplasm (cytoplasm) with neurofibrils, vesicles
• Axolemma (axonal membrane)
• Axon terminals (synaptic terminals)

Nerve Fibers

Myelinated Fibers (Type A and B):

• Schwann cells (PNS) or Oligodendrocytes (CNS)
• Nodes of Ranvier every 1-2 mm
• Saltatory conduction (jumping between nodes)
• Faster: 20-120 m/s

Unmyelinated Fibers (Type C):

• Slowest: 0.5-2 m/s
• All PNS autonomic fibers, sensory fibers for pain/temperature

NCE Exam Pattern

Common question types:

1. Brain structures and functions
2. Spinal cord tracts
3. Cranial nerve names and functions
4. Autonomic nervous system
5. Synaptic transmission

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Nervous System — Comprehensive NEET PG Notes

Detailed Theory

1. Resting Membrane Potential

Value: -70 mV (inside negative relative to outside)

Mechanism:

• Na⁺/K⁺ ATPase: 3 Na⁺ out, 2 K⁺ in (uses 1 ATP per cycle)
• K⁺ leak channels: High permeability to K⁺, K⁺ leaves cell
• Na⁺ leak channels: Small inward leak
• Donnan equilibrium: Cl⁻ is permeable, enters to balance K⁺

Key Points:

• Ion gradients established by pumps
• Resting potential maintained by leak channels
• Nernst equation: E = (RT/zF)ln([ion]out/[ion]in)
• Goldman equation for multiple ions

2. Action Potential

Phases:

1. Resting: -70 mV, Na⁺ and K⁺ channels closed
2. Depolarization: Na⁺ channels open, Na⁺ influx, reaches +30 mV
3. Repolarization: Na⁺ channels inactivate, K⁺ channels open, K⁺ efflux
4. Hyperpolarization: K⁺ channels stay open briefly, goes below -70 mV

Threshold: -55 mV (when enough Na⁺ channels open to self-sustain)

All-or-None: Suprathreshold stimulus → Full AP; Subthreshold → No AP

Refractory Period:

• Absolute: Na⁺ channels inactivated, cannot fire again (~1 ms)
• Relative: Some Na⁺ channels recovering, can fire with stronger stimulus

Propagation:

• Unmyelinated: Continuous propagation in both directions
• Myelinated: Saltatory conduction, faster (120 m/s)

3. Synaptic Transmission

Types of Synapses:

Chemical Synapse (most common):

• Presynaptic terminal: Vesicles with neurotransmitter
• Synaptic cleft: 20-40 nm gap
• Postsynaptic membrane: Receptors
• One-way transmission

Electrical Synapse (gap junctions):

• Direct cytoplasmic connection
• Bidirectional
• Found in cardiac muscle, some neurons

Synaptic Transmission Steps:

1. AP arrives at presynaptic terminal
2. Voltage-gated Ca²⁺ channels open
3. Ca²⁺ enters, triggers vesicle fusion
4. NT released by exocytosis
5. NT diffuses across cleft
6. Binds postsynaptic receptors
7. Postsynaptic potential (EPSP or IPSP)
8. NT degraded/reuptaken

Neurotransmitters:

Acetylcholine:

• NMJ: Nicotinic receptors (ionotropic)
• CNS, autonomic: Muscarinic receptors (metabotropic)
• Effects: Excitatory (muscle) or inhibitory (heart)
• Degradation: Acetylcholinesterase

Catecholamines:

• Dopamine, Norepinephrine, Epinephrine
• Biosynthesis: Tyrosine → DOPA → Dopamine → NE → E
• Reuptake by DAT, NET, SERT

Other NTs: Serotonin (5-HT), GABA (inhibitory), Glutamate (excitatory), Glycine (inhibitory)

4. Central Nervous System

Meninges:

• Dura mater: Tough outer layer (periosteal and meningeal layers in brain)
• Arachnoid mater: Spider-web projections (arachnoid granulations for CSF absorption)
• Pia mater: Delicate, closely adherent to brain surface
• Subarachnoid space: Contains CSF

Ventricles:

• Lateral ventricles (I, II): Largest, in cerebral hemispheres
• Third ventricle: Between thalami
• Cerebral aqueduct: Midbrain
• Fourth ventricle: Pons/medulla
• CSF filled, continuous with central canal of cord

Blood-Brain Barrier:

• Tight junctions between endothelial cells
• Basement membrane, astrocyte foot processes
• Selective permeability (glucose, amino acids via transporters)
• Breaks down in inflammation, some tumors

5. Brain Structures

Cerebral Cortex:

• Frontal lobe: Motor cortex (precentral gyrus), Broca’s area (speech production), PFC (executive function)
• Parietal lobe: Sensory cortex (postcentral gyrus), somatosensory association
• Temporal lobe: Auditory cortex, Wernicke’s area (speech comprehension), hippocampus
• Occipital lobe: Visual cortex (calcarine sulcus)

Basal Ganglia:

• Caudate nucleus, putamen (striatum), globus pallidus, substantia nigra, subthalamic nucleus
• Motor planning, habit formation
• Disorders: Parkinson’s (nigrostriatal dopamine depletion), Huntington’s (striatal degeneration)

Thalamus:

• Relay station for sensory information (except olfaction)
• Medial geniculate body: Auditory
• Lateral geniculate body: Visual
• Ventral posterolateral: Somatosensory

Hypothalamus:

• Homeostatic control center
• Controls pituitary (infundibulum)
• Regulates: Temperature, hunger, thirst, circadian rhythms, autonomic functions
• Mammillary bodies: Part of Papez circuit (memory)

Cerebellum:

• Coordination, balance, motor learning
• Purkinje cells, deep nuclei
• Three peduncles connect to brainstem
• Lesions: Ataxia, dysmetria, intention tremor

Brainstem:

• Midbrain: Superior/inferior colliculi, cerebral peduncles, red nucleus, substantia nigra
• Pons: Pontine nuclei, respiratory centers, connecting fibers
• Medulla: Vital centers (cardiac, respiratory, vasomotor), olive, vagal nuclei

6. Spinal Cord

Structure:

• 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
• Ends at L1-L2 (conus medullaris)
• Cauda equina: Spinal nerve roots below L2
• Gray matter: H-shaped, butterfly shape (posterior horns, anterior horns, lateral horns)
• White matter: Ascending and descending tracts

Tracts (Descending):

• Corticospinal (lateral): Voluntary motor control, 90% decussate at pyramidal decussation
• Rubrospinal: Red nucleus, fine motor control
• Reticulospinal: Posture, balance
• Vestibulospinal: Extensor tone, balance

Tracts (Ascending):

• Dorsal column-medial lemniscus: Fine touch, vibration, proprioception (fasciculus gracilis, cuneatus)
• Spinothalamic (anterolateral): Pain, temperature, crude touch (crosses at spinal level)
• Spinocerebellar: Unconscious proprioception to cerebellum (flexture, coordination)

Cross-Section Knowledge:

• Posterior horn: Sensory neurons
• Lateral horn: Preganglionic autonomic (T1-L2)
• Anterior horn: Motor neurons (alpha, gamma motor neurons)
• Dorsal column: Medial (gracilis) and lateral (cuneatus)

7. Cranial Nerves (12 Pairs)

Nerve	Name	Type	Primary Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil, accommodation
IV	Trochlear	Motor	Eye movement (superior oblique)
V	Trigeminal	Mixed	Face sensation, mastication
VI	Abducens	Motor	Eye movement (lateral rectus)
VII	Facial	Mixed	Face expression, taste (ant 2/3), lacrimation
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Mixed	Taste (post 1/3), swallowing, carotid
X	Vagus	Mixed	Parasympathetic to thoracic/abdominal viscera
XI	Accessory	Motor	SCM, trapezius
XII	Hypoglossal	Motor	Tongue movement

Mnemonic: “Oh Oh Oh To Touch And Feel Very Green Vegetables AH!” (sensory, sensory, sensory, motor, mixed, motor, mixed, sensory, mixed, mixed, motor, motor)

8. Autonomic Nervous System

Sympathetic (Thoracolumbar):

• Origin: T1-L2 (lateral horn)
• Short preganglionic, long postganglionic
• Neurotransmitter: ACh (preganglionic), NE (postganglionic)
• Fight or flight: ↑ HR, ↑ BP, ↑瞳孔, ↓ digestion
• Adrenal medulla: Modified sympathetic ganglion, secretes Epi/NE

Parasympathetic (Craniosacral):

• Origin: CN III, VII, IX, X; S2-S4
• Long preganglionic, short postganglionic
• Neurotransmitter: ACh (both preganglionic and postganglionic)
• Rest and digest: ↓ HR, ↓ BP, ↑ digestion, ↑ salivation

Enteric Nervous System:

• GI tract intrinsic control
• Myenteric (Auerbach): Peristalsis
• Submucosal (Meissner): Secretion, absorption
• Connected to ANS

ANS Receptors:

• Nicotinic ACh receptors (all ganglionic, NMJ)
• Muscarinic receptors: M2 (heart), M3 (glands, smooth muscle)
• Adrenergic: α1 (vasoconstriction), α2 (presynaptic inhibition), β1 (heart), β2 (bronchodilation)

9. Reflexes

Components: Receptor → Afferent (sensory) → Integration center → Efferent (motor) → Effector

Monosynaptic Reflex: 2 neurons, 1 synapse (muscle stretch reflex)

• Example: Patellar reflex (L2-L4, quadriceps)

Polysynaptic Reflex: Interneurons present

• Example: Withdrawal reflex (flexor reflex)

Common Reflexes:

• Patellar: L2-L4 (quadriceps)
• Achilles: S1-S2 (gastrocnemius)
• Biceps: C5-C6
• Triceps: C6-C8
• Plantar: Babinski sign (upgoing = abnormal in adults)

10. Clinical Correlations

Stroke (CVA):

• Ischemic (thrombotic/embolic) or hemorrhagic
• Middle cerebral artery: Most common
• Symptoms: Contralateral weakness, facial droop, aphasia (dominant hemisphere)
• “Time is brain” — thrombolysis within window

Parkinson’s Disease:

• Substantia nigra dopaminergic neuron loss
• Bradykinesia, resting tremor, rigidity, postural instability
• Lewy bodies (alpha-synuclein)

Multiple Sclerosis:

• CNS demyelination
• Optic neuritis, diplopia, weakness, sensory loss
• Relapsing-remitting course

Guillain-Barré Syndrome:

• Acute inflammatory demyelinating polyradiculoneuropathy
• Ascending paralysis, areflexia
• Post-infectious (Campylobacter)

Practice Questions for NEET PG

1. Explain the mechanism of action potential generation and propagation in a neuron.
2. Describe the structure and function of the major brain regions.
3. Trace the pathway of the dorsal column-medial lemniscus system for fine touch.
4. Compare the sympathetic and parasympathetic nervous systems.
5. Discuss the blood-brain barrier and its clinical significance.

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