---

🧠 Nervous System (तंत्रिका तंत्र) - Complete Notes

---

1. Nervous System क्या है?

कार्य (Functions):

---

2. तंत्रिका तंत्र का वर्गीकरण (Classification)

NERVOUS SYSTEM
├── CNS (Central Nervous System)
│   ├── Brain (मस्तिष्क)
│   └── Spinal Cord (मेरु रज्जु)
│
├── PNS (Peripheral Nervous System)
│   ├── Cranial Nerves - 12 जोड़ी
│   └── Spinal Nerves - 31 जोड़ी
│
└── ANS (Autonomic Nervous System)
    ├── Sympathetic NS ("Fight or Flight")
    └── Parasympathetic NS ("Rest & Digest")

---

3. Neuron (न्यूरॉन)

संरचना (Structure):

न्यूरॉन के प्रकार (Types):

• Sensory (Afferent) - Receptor से CNS की ओर
• Motor (Efferent) - CNS से Effector (muscle/gland) की ओर
• Interneuron - CNS के अंदर connection - सबसे अधिक संख्या में

1. Dendritic zone - local potential changes integrate होती हैं
2. Initial segment - Action potential generate होती है
3. Axon - signal propagate होती है
4. Terminal buttons - Neurotransmitter release होता है

---

4. Neuroglia (Glial Cells)

याद रखें: CNS में myelin = Oligodendrocytes | PNS में myelin = Schwann Cells

---

5. Nerve Impulse (तंत्रिका आवेग)

Resting Membrane Potential (विश्राम अवस्था)

• Normal value: -70 mV (अंदर negative, बाहर positive)
• K⁺ अधिक अंदर, Na⁺ अधिक बाहर
• Na⁺/K⁺ ATPase pump: 3 Na⁺ बाहर, 2 K⁺ अंदर

Action Potential (क्रिया विभव)

Saltatory Conduction (Salt-टू-Salt Jump)

• Myelinated fibers में AP Nodes of Ranvier से Nodes of Ranvier तक "jump" करती है
• यह बहुत तेज होती है - continuous conduction से कई गुना तेज
• Energy efficient भी होती है

---

6. Synapse (सिनेप्स)

Presynaptic neuron → Synaptic cleft (20-40 nm) → Postsynaptic neuron

प्रकार:

• Chemical Synapse - Neurotransmitters के through (सबसे common)
• Electrical Synapse - Gap junctions के through (तेज, दोनों दिशाओं में)

Signal Transfer:

1. AP presynaptic terminal पर आती है
2. Ca²⁺ channels खुलते हैं
3. Ca²⁺ vesicles को membrane से fuse करवाता है
4. Neurotransmitter synaptic cleft में release होता है
5. Postsynaptic receptors से bind होता है

प्रमुख Neurotransmitters:

---

7. Brain (मस्तिष्क)

Brain (औसत weight ≈ 1400 gram)
├── Cerebrum (Forebrain का सबसे बड़ा भाग)
│   ├── Frontal Lobe - Motor, personality, speech (Broca's area)
│   ├── Parietal Lobe - Sensory processing, spatial awareness
│   ├── Temporal Lobe - Hearing, memory (Wernicke's area)
│   └── Occipital Lobe - Vision
│
├── Diencephalon
│   ├── Thalamus - Sensory relay station "Gateway to cerebral cortex"
│   └── Hypothalamus - Homeostasis, hunger, thirst, temperature, hormones, ANS
│
├── Brainstem
│   ├── Midbrain (Mesencephalon) - Vision, hearing reflexes, eye movement
│   ├── Pons - Breathing rhythm, facial expressions, sleep
│   └── Medulla Oblongata - Vital centers: Heart rate, breathing, BP, vomiting
│
└── Cerebellum - Coordination, balance, fine motor control

---

8. Ventricles और CSF (Cerebrospinal Fluid)

CSF क्या है?

Ventricles (4):

Lateral Ventricles (2) → 3rd Ventricle → Cerebral Aqueduct → 4th Ventricle → Subarachnoid space

CSF की विशेषताएं:

CSF के कार्य:

1. Brain को cushion/protect करता है (shock absorber)
2. Brain का buoyancy - effective weight घटाता है (1400g → ~50g)
3. Waste removal - metabolic waste products हटाता है
4. Nutrients पहुंचाता है
5. Intracranial pressure regulate करता है

---

9. Meninges (मस्तिष्क की झिल्लियाँ)

Brain/Spinal Cord
    ↑
Pia Mater (सबसे अंदर - brain से चिपकी)
    ↑
Arachnoid Mater (बीच में - spider web जैसी)
    [Subarachnoid space = CSF यहाँ होती है]
    ↑
Dura Mater (सबसे बाहर - tough, fibrous)
    ↑
Skull/Vertebral canal

Meningitis = Meninges की inflammation - bacteria/virus से। CSF examination से diagnose।

---

10. Spinal Cord (मेरु रज्जु)

संरचना:

• Length: ~45 cm (adult)
• Location: Foramen magnum से L1-L2 तक
• Grey Matter (H-shaped, अंदर) - cell bodies
• White Matter (बाहर) - myelinated axons के tracts

Grey Matter के horns:

• Dorsal (Posterior) Horn - Sensory neurons
• Ventral (Anterior) Horn - Motor neurons
• Lateral Horn - ANS neurons (T1-L2, S2-S4)

Ascending Tracts (Sensory - ऊपर जाते हैं):

• Spinothalamic tract - Pain, temperature, crude touch
• Dorsal columns - Fine touch, proprioception, vibration

Descending Tracts (Motor - नीचे जाते हैं):

• Corticospinal (Pyramidal) tract - Voluntary movement

Reflex Arc:

Stimulus → Sensory receptor → Afferent nerve → Spinal cord
    → Integration center → Efferent nerve → Effector → Response

• Reflex arc में brain की जरूरत नहीं - spinal cord level पर complete

---

11. Cranial Nerves (कपाल तंत्रिकाएं) - 12 जोड़ी

याद करने की Trick:

Mixed nerves: V, VII, IX, X - इन्हें याद रखें

---

12. Spinal Nerves (मेरुरज्जु तंत्रिकाएं) - 31 जोड़ी

• Brachial plexus (C5-T1) - Arm की nerves
• Lumbosacral plexus (L1-S3) - Leg की nerves

---

13. Autonomic Nervous System (ANS)

Sympathetic बनाम Parasympathetic:

---

14. Reflex Action

Reflex Arc के 5 components:

1. Receptor → 2. Afferent nerve → 3. Nerve center (spinal cord) → 4. Efferent nerve → 5. Effector

उदाहरण:

---

15. सामान्य रोग (Clinical Disorders)

---

पढ़ने का Quick Revision क्रम:

NS Introduction → Classification (CNS/PNS/ANS) → Neuron structure → 
Neuroglia → Nerve Impulse (RMP/AP) → Synapse & NTs → Brain parts → 
Meninges → CSF → Spinal Cord → Cranial Nerves (12) → Spinal Nerves (31) → 
ANS (Symp vs Parasymp) → Reflex Arc → Clinical Disorders

---

---

THE NERVOUS SYSTEM — Complete Long Answer Notes

---

1. WHAT IS THE NERVOUS SYSTEM?

Functions of the Nervous System

---

2. CLASSIFICATION OF THE NERVOUS SYSTEM

NERVOUS SYSTEM
│
├── CENTRAL NERVOUS SYSTEM (CNS)
│   ├── Brain
│   └── Spinal Cord
│
├── PERIPHERAL NERVOUS SYSTEM (PNS)
│   ├── Afferent (Sensory) Division
│   ├── Efferent (Motor) Division
│   │   ├── Somatic NS — voluntary, skeletal muscle
│   │   └── Autonomic NS — involuntary, viscera
│   ├── Cranial Nerves (12 pairs)
│   └── Spinal Nerves (31 pairs)
│
└── AUTONOMIC NERVOUS SYSTEM (ANS) [subdivision of PNS]
    ├── Sympathetic Division ("Fight or Flight")
    └── Parasympathetic Division ("Rest and Digest")

---

3. NEURON — THE STRUCTURAL AND FUNCTIONAL UNIT

Structure of a Neuron

"A typical neuron also has a long fibrous axon that originates from a thickened area of the cell body (axon hillock). The axon divides into presynaptic terminals, each ending in a number of synaptic knobs that contain granules or vesicles in which synaptic transmitters released by the nerves are stored." - Ganong's Review of Medical Physiology

Four Functional Zones of a Neuron

1. Dendritic zone - local potential changes generated by synaptic connections are integrated
2. Initial segment - propagated action potentials are generated
3. Axon - conducts the action potential to distant targets
4. Terminal buttons - release neurotransmitters at the synapse

Classification of Neurons

---

4. NEUROGLIA (GLIAL CELLS)

Central Nervous System Glia

Peripheral Nervous System Glia

Key Distinction: CNS myelin = Oligodendrocytes | PNS myelin = Schwann cells. This is why MS (CNS demyelination) and Guillain-Barre syndrome (PNS demyelination) are separate diseases.

---

5. NERVE IMPULSE (ACTION POTENTIAL)

Resting Membrane Potential (RMP)

• Na⁺/K⁺-ATPase Pump: Actively pumps 3 Na⁺ out and 2 K⁺ in (net outward positive charge)
• Selective permeability: Membrane at rest is more permeable to K⁺ than Na⁺ (K⁺ leaks out)
• Large intracellular anions (proteins, phosphates) that cannot cross the membrane

• Na⁺ : High outside (145 mEq/L), Low inside (12 mEq/L)
• K⁺ : High inside (140 mEq/L), Low outside (4 mEq/L)
• Cl⁻ : High outside (100 mEq/L)

Action Potential

"An action potential is triggered when a stimulus depolarizes the membrane to a threshold level, usually around -55 mV. At this time, the membrane suddenly becomes permeable to sodium ions, allowing rapid diffusion of positively charged sodium ions to the interior of the axon." - Guyton & Hall

The All-or-None Law

Saltatory Conduction

• Conduction velocity: up to 120 m/s in large myelinated fibers vs. < 2 m/s in unmyelinated fibers
• Extremely energy-efficient - ionic exchange occurs only at nodes
• Demyelination (as in Multiple Sclerosis) dramatically slows or blocks conduction

---

6. SYNAPSE

Structure of a Chemical Synapse

Presynaptic Neuron (Axon Terminal)
    - Synaptic vesicles (contain neurotransmitters)
    - Active zone (site of vesicle docking)
    - Voltage-gated Ca²⁺ channels
            ↓
Synaptic Cleft (~20-40 nm gap)
            ↓
Postsynaptic Membrane
    - Neurotransmitter receptors (ionotropic or metabotropic)
    - Postsynaptic density (scaffold proteins)

Mechanism of Synaptic Transmission

1. Action potential reaches the presynaptic terminal
2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters the terminal
3. Ca²⁺ triggers vesicle fusion with the presynaptic membrane (exocytosis)
4. Neurotransmitter is released into the synaptic cleft
5. Neurotransmitter binds to receptors on the postsynaptic membrane
6. Postsynaptic response occurs (excitation or inhibition)
7. Neurotransmitter is removed by: reuptake, enzymatic degradation, or diffusion

Types of Synapses

Excitatory vs. Inhibitory Synapses

• Excitatory synapse: Releases glutamate, acetylcholine, or serotonin → opens Na⁺ or Ca²⁺ channels → depolarization → EPSP (Excitatory Post-Synaptic Potential) → action potential
• Inhibitory synapse: Releases GABA or glycine → opens Cl⁻ channels → hyperpolarization → IPSP (Inhibitory Post-Synaptic Potential) → prevents action potential

"The ultimate generation of a nerve impulse in a postsynaptic neuron depends on the summation of excitatory and inhibitory impulses reaching that neuron. This allows precise regulation of the reaction of a postsynaptic neuron." - Ross & Pawlina Histology

Receptor Types

Major Neurotransmitters

---

7. THE BRAIN

A. Cerebrum

• Primary Motor Cortex (precentral gyrus, frontal lobe) - voluntary movement
• Primary Somatosensory Cortex (postcentral gyrus, parietal lobe) - sensory perception
• Broca's Area (left inferior frontal gyrus) - speech production; damage → Broca's (expressive) aphasia
• Wernicke's Area (left superior temporal gyrus) - language comprehension; damage → Wernicke's (receptive) aphasia

B. Diencephalon

• Located at the center of the brain; paired egg-shaped structures flanking the 3rd ventricle
• The "relay station" and "gateway" to the cerebral cortex
• All sensory information (except smell/olfaction) passes through the thalamus before reaching the cortex
• Key nuclei: Lateral Geniculate (vision), Medial Geniculate (hearing), VPL/VPM (somatosensation)

• Located below the thalamus; small but enormously important
• Controls the pituitary gland (master endocrine gland) via releasing/inhibiting hormones
• Regulates: body temperature, hunger, thirst, circadian rhythms, sleep-wake cycles, sexual behavior, emotional responses
• Contains centers for the ANS
• Key nuclei: suprachiasmatic (circadian), arcuate (hunger), supraoptic/paraventricular (ADH, oxytocin)

C. Brainstem

• Contains the substantia nigra (dopamine-producing; degenerated in Parkinson's disease)
• Controls eye movements (CN III, CN IV nuclei), pupillary reflex
• Superior colliculi - visual reflexes | Inferior colliculi - auditory reflexes
• Reticular formation begins here

• Means "bridge" - connects midbrain to medulla; connects cerebellum to brainstem
• Contains nuclei for CN V, VI, VII, VIII
• Pneumotaxic and apneustic centers - regulate breathing rhythm
• Important in sleep regulation (REM sleep)

• Most caudal part; continuous with spinal cord at foramen magnum
• Contains vital centers:
  • Cardiac center - heart rate and force
  • Respiratory center - rhythmic breathing (dorsal and ventral respiratory groups)
  • Vasomotor center - blood pressure via vascular tone
• Reflex centers: vomiting, swallowing, coughing, sneezing, hiccupping
• Contains nuclei for CN IX, X, XI, XII
• Site of pyramidal decussation - corticospinal tracts cross here

D. Cerebellum

• Located posterior to the brainstem in the posterior cranial fossa
• Two hemispheres + central vermis; highly folded cortex (folia)
• Functions: coordination of movement, balance, posture, motor learning, fine-tuning of voluntary movements
• Does NOT initiate movement - it compares intended vs. actual movement and corrects errors
• Damage (cerebellar lesions) → Ataxia (uncoordinated gait), dysmetria (past-pointing), intention tremor, nystagmus, dysdiadochokinesia

---

8. VENTRICLES AND CEREBROSPINAL FLUID (CSF)

The Ventricular System

Two Lateral Ventricles (one in each cerebral hemisphere)
         ↓ (through Interventricular Foramen of Monro)
     Third Ventricle (diencephalon)
         ↓ (through Cerebral Aqueduct of Sylvius)
     Fourth Ventricle (between pons/medulla and cerebellum)
         ↓ (through Foramina of Magendie and Luschka)
     Subarachnoid Space → Circulation around brain and spinal cord
         ↓ (absorbed at)
     Arachnoid Villi/Granulations into venous sinuses

CSF - Cerebrospinal Fluid

• Very low protein (~0.35 mg/mL vs. 70 mg/mL in plasma)
• No RBCs (normally)
• Glucose ~60% of plasma glucose
• Na⁺, Cl⁻ slightly higher; K⁺ lower than plasma

Functions of CSF

1. Mechanical protection (Cushioning): Acts as a shock absorber for brain and spinal cord against physical trauma
2. Buoyancy: The brain effectively weighs only ~50g in CSF versus 1,400g in air - prevents crushing of inferior brain under its own weight
3. Waste removal: Removes metabolic waste products from the CNS extracellular space
4. Nutrient delivery: Carries nutrients (glucose, amino acids) to neural tissue
5. Intracranial pressure regulation: Maintains constant chemical environment for neurons
6. Immune surveillance: Contains immunoglobulins and immune cells

Clinical Application: CSF examination via lumbar puncture is used to diagnose meningitis (↑ WBC, ↑ protein, ↓ glucose in bacterial meningitis), subarachnoid hemorrhage (xanthochromia = yellow CSF), and multiple sclerosis (oligoclonal bands).

---

9. MENINGES

SKULL / VERTEBRAL CANAL
        ↓
[Epidural Space - fat + epidural veins in spine]
        ↓
DURA MATER (tough, fibrous outer layer)
        ↓
[Subdural Space - potential space]
        ↓
ARACHNOID MATER (thin, avascular, spider-web like)
        ↓
[SUBARACHNOID SPACE - contains CSF + blood vessels]
        ↓
PIA MATER (delicate, closely adherent to brain surface)
        ↓
BRAIN / SPINAL CORD

• Epidural hematoma: Blood in epidural space (usually arterial - middle meningeal artery rupture); lens-shaped on CT
• Subdural hematoma: Blood in subdural space (venous - bridging vein rupture); crescent-shaped on CT
• Subarachnoid hemorrhage: Blood in subarachnoid space (berry aneurysm rupture); "worst headache of life"
• Meningitis: Inflammation of meninges; signs include nuchal rigidity, Kernig's sign, Brudzinski's sign

---

10. SPINAL CORD

Structure

• Length: ~45 cm; diameter ~1-1.5 cm
• Cervical and lumbar enlargements: Areas of increased thickness supplying the limbs

WHITE MATTER (outer ring) - myelinated axon tracts
        ┌────────────────────┐
        │    Posterior       │  ← Dorsal columns (sensory tracts)
        │    White matter    │
WHITE   │  ┌──────────────┐  │  WHITE
MATTER  │  │ GREY MATTER  │  │  MATTER
(Lateral│  │  H-SHAPED    │  │  (Lateral
columns)│  │              │  │  columns)
        │  │  Posterior   │  │
        │  │  horn (sens) │  │
        │  │  Lateral horn│  │  ← ANS neurons (T1-L2, S2-S4)
        │  │  Anterior    │  │
        │  │  horn (motor)│  │
        │  └──────────────┘  │
        │    Anterior        │
        │    White matter    │  ← Corticospinal tract (motor)
        └────────────────────┘

• Dorsal (Posterior) Horn - Contains sensory neuron cell bodies and interneurons; receives afferent input
• Ventral (Anterior) Horn - Contains lower motor neuron (LMN) cell bodies; sends motor commands to muscles
• Lateral Horn (T1-L2 and S2-S4 only) - Contains preganglionic ANS neurons

Major Spinal Cord Tracts

Reflex Arc

STIMULUS
    ↓
1. RECEPTOR (sensory nerve ending detects stimulus)
    ↓
2. AFFERENT (SENSORY) NERVE (carries impulse to spinal cord)
    ↓
3. INTEGRATION CENTER (spinal cord interneurons process input)
    ↓
4. EFFERENT (MOTOR) NERVE (carries command to effector)
    ↓
5. EFFECTOR (muscle contracts or gland secretes = RESPONSE)

The key clinical significance of spinal reflexes is that they continue even if higher centers are damaged. Testing reflexes maps the integrity of specific spinal cord segments.

---

11. CRANIAL NERVES (12 Pairs)

Mnemonic to Remember the Names:

The 12 Cranial Nerves:

Mixed nerves (Sensory + Motor): CN V, VII, IX, X

---

12. SPINAL NERVES (31 Pairs)

Major Nerve Plexuses:

---

13. AUTONOMIC NERVOUS SYSTEM (ANS)

Organization

• Preganglionic neuron: Cell body in CNS; myelinated axon
• Postganglionic neuron: Cell body in peripheral ganglion; unmyelinated axon → effector organ

Sympathetic vs. Parasympathetic - Complete Comparison

"The sympathetic and parasympathetic divisions of the ANS often supply the same organs. In these cases, the actions of the two are usually antagonistic. For example, sympathetic stimulation increases the rate of cardiac muscle contractions, whereas parasympathetic stimulation reduces the rate." - Ross & Pawlina Histology

---

14. REFLEX ACTION

Importance of Reflexes

• Faster than voluntary responses (no conscious cortical processing needed)
• Protective function (withdrawal from pain)
• Maintain posture and balance (stretch reflexes)
• Diagnostic tools (testing reflexes reveals neurological deficits at specific spinal levels)

Types of Reflexes

The Stretch Reflex (Myotatic Reflex) in Detail

1. Muscle spindles (intrafusal fibers) detect stretch
2. Ia afferent neurons carry signal to spinal cord
3. Synapse directly on alpha motor neurons (monosynaptic)
4. Motor neuron fires → muscle contracts (resists further stretch)
5. Simultaneously, Ia inhibitory interneurons inhibit the antagonist muscle (reciprocal inhibition)

---

15. CLINICAL DISORDERS OF THE NERVOUS SYSTEM

1. Stroke (Cerebrovascular Accident - CVA)

2. Meningitis

3. Parkinson's Disease

4. Alzheimer's Disease

5. Epilepsy

6. Multiple Sclerosis (MS)

---

QUICK REVISION SUMMARY

NS = CNS (Brain + Spinal Cord) + PNS (Cranial 12 + Spinal 31) + ANS (Symp + Parasymp)

NEURON: Cell body → Axon hillock → Axon → Terminal buttons
        Dendrites receive; Axons transmit
        Types: Sensory (Afferent) | Motor (Efferent) | Interneuron (99.9%)

GLIA:   CNS: Astrocyte, Oligodendrocyte (myelin), Microglia (immune), Ependymal (CSF)
        PNS: Schwann cells (myelin), Satellite cells

AP:     RMP = -70 mV → Threshold -55 mV → Na⁺ in (depolarization to +35 mV)
        → Na⁺ channels close, K⁺ out (repolarization) → Hyperpolarization → Rest

SYNAPSE: Ca²⁺ → Vesicle fusion → NT release → Receptor binding
         Excitatory (Na⁺ in) = EPSP | Inhibitory (Cl⁻ in) = IPSP

BRAIN:  Cerebrum (thinking) > Cerebellum (coordination) > Brainstem (vital)
        Thalamus (relay) | Hypothalamus (homeostasis)

CSF:    Made by Choroid Plexus (ependymal cells) | 500mL/day | 150mL total
        Absorbed by Arachnoid granulations | Lateral→3rd→Aqueduct→4th→Subarachnoid

MENINGES (outside→in): Dura → Arachnoid → Pia | CSF in Subarachnoid space

CRANIAL NERVES: 12 pairs | Sensory: I, II, VIII | Motor: III, IV, VI, XI, XII
                Mixed: V, VII, IX, X | Longest = Vagus (X)

SPINAL NERVES: 31 pairs = C8 + T12 + L5 + S5 + Co1

ANS:    Sympathetic = T1-L2, Norepinephrine postganglionic, "Fight or Flight"
        Parasympathetic = Craniosacral (CN III/VII/IX/X + S2-S4), ACh, "Rest & Digest"

DISORDERS:
Stroke = "Time is Brain" | Meningitis = Fever+Headache+Stiff neck
Parkinson = Dopamine↓, TRAP triad | Alzheimer = ACh↓, amyloid plaques
Epilepsy = GABA↓/Glutamate↑ | MS = CNS demyelination (oligodendrocytes)

---