---

NERVOUS SYSTEM QUESTION BANK — COMPLETE ANSWERS

---

Q1. Draw a well-labelled diagram and explain neural connections in the cerebellar cortex. Explain the physiological basis of clinical features observed in cerebellar dysfunction. (LQ)

A. Structure of the Cerebellar Cortex

Layers (from inside out):

1. Granular Layer (innermost) - granule cells, Golgi cells, glomeruli
2. Purkinje Cell Layer (middle) - Purkinje cells only
3. Molecular Layer (outermost) - stellate cells, basket cells, parallel fibers

Five Cell Types:

Key: 4 of 5 cell types are inhibitory; only granule cells are excitatory.

B. Diagram of Cerebellar Cortical Connections

MOLECULAR LAYER
┌──────────────────────────────────────────────────────┐
│  Parallel fibers (axons of granule cells)            │
│  ─────────────────────────────────────────────→      │
│      │           │          │         │              │
│   Basket       Stellate   Purkinje   Purkinje        │
│   cell         cell       dendrite   dendrite        │
│  (inhibits     (inhibits  (excitied  (inhibited by   │
│   Purkinje)    Purkinje)  by Glu)    basket/stellate)│
└──────────────────────────────────────────────────────┘
PURKINJE CELL LAYER
┌──────────────────────────────────────────────────────┐
│        ⬭ Purkinje cell body (output = GABA)          │
└──────────────────────────────────────────────────────┘
GRANULAR LAYER
┌──────────────────────────────────────────────────────┐
│  Granule cell ──→ (axon rises to molecular layer)    │
│  Glomerulus: Mossy fiber + Granule dendrites         │
│  Golgi cell (inhibits granule cell = feedback)       │
└──────────────────────────────────────────────────────┘
         ↑                          ↑
    MOSSY FIBERS               CLIMBING FIBERS
  (from pontine nuclei,       (from inferior olive)
   vestibular, spinal         Direct to Purkinje
   cord; excite granule       dendrites; 1 per
   cells via glomeruli)       Purkinje cell

C. Two Afferent Input Systems

• Origin: Contralateral inferior olivary nucleus of medulla
• Synapse: Wind directly onto Purkinje cell dendrites (one climbing fiber per Purkinje cell)
• Effect: Single impulse → complex spike (prolonged depolarization)
• Role: Motor error teaching/learning - encodes error signals; drives cerebellar motor learning
• Neurotransmitter: Glutamate (AMPA receptors)

• Origin: Pontine nuclei (from cerebral cortex), vestibular apparatus, spinal cord
• Synapse: On granule cells in glomeruli; granule cell axons → parallel fibers → Purkinje dendrites
• Effect: Produce simple spikes in Purkinje cells
• One mossy fiber activates ~1 million parallel fibers across many Purkinje cells

D. Circuit Logic

Direct input:
Mossy/Climbing fiber collaterals → Deep Cerebellar Nucleus (excite)

Via Cortex (delayed):
Mossy fiber → Granule cell → Parallel fibers → Purkinje cell
Climbing fiber → directly onto Purkinje cell
Purkinje cell axon → INHIBITS Deep Cerebellar Nucleus (GABA)

Result: Deep nucleus receives fast excitation then delayed inhibition
= "Timing mechanism" that damps and refines movements

---

E. Physiological Basis of Cerebellar Dysfunction — DANISH

• Vermis → truncal/gait ataxia
• Cerebellar hemispheres → ipsilateral limb incoordination (double crossing of pathways)
• Flocculonodular lobe → nystagmus, vestibular symptoms

---

Q2. Clinical Features in Upper and Lower Motor Neuron Lesions (SQ)

Anatomical Basis

• UMN: Neurons from motor cortex → anterior horn (via corticospinal tract) or cranial nerve nuclei (corticobulbar)
• LMN: Alpha motor neurons in anterior horn → skeletal muscle via peripheral nerve

Comparison Table

• Upper limb: Extensors weaker (flexion posture)
• Lower limb: Flexors weaker (extension posture)
• Lower face affected (contralateral); upper face spared (bilateral cortical supply)

---

Q3. Tendon Reflexes (SQ)

Definition

Reflex Arc

Hammer tap → Tendon stretch → Muscle stretch
→ Muscle spindle (Ia primary endings) activated
→ Type Ia afferent fibers (fastest: 70–120 m/s) → Dorsal root
→ DIRECTLY synapse on alpha motor neuron (monosynaptic)
→ Motor nerve → Extrafusal fibers contract → Limb jerks

Types of Stretch Reflex

• Dynamic: By rapid stretch (Ia nuclear bag endings) → brisk, phasic
• Static: By sustained stretch (Ia + Group II nuclear chain endings) → tonic; maintains posture

Gamma Motor System

• Co-activated with alpha neurons → keeps spindle taut during voluntary contraction → maintains damping → prevents oscillation

Grading of DTRs

Common Reflexes and Cord Levels

Clinical Significance

• ↑ DTRs → UMN lesion, hyperthyroidism
• ↓ / absent DTRs → LMN lesion, peripheral neuropathy, cerebellar disease (pendular reflexes)
• Asymmetry always significant

---

Q4. Clasp-Knife Rigidity is Seen in Upper Motor Neuron Lesion (PB)

Definition

1. Initial free movement
2. Sudden catch with rapid increase in resistance
3. Then sudden give way (collapse of resistance)

• Resembles the opening of a jackknife.
• It is velocity-dependent (only occurs with fast passive stretch).

Physiological Basis

• UMN lesion damages descending inhibitory pathways (dorsal reticulospinal)
• Disinhibition of gamma motor neurons → increased fusimotor drive → sensitized muscle spindles → exaggerated Ia afferent firing
• Exaggerated stretch reflex produces the strong muscle contraction (resistance)

• As stretch continues, muscle tension rises sharply
• Golgi Tendon Organs (GTOs) in tendons are activated by high tension
• GTOs → Ib afferent fibers → inhibitory interneurons in spinal cord → hyperpolarize alpha motor neurons (autogenic inhibition / inverse stretch reflex)
• Alpha motor neurons suddenly inhibited → muscle suddenly relaxes

Summary

Contrast with Other Rigidities

• Lead-pipe rigidity (Parkinson's): Velocity-independent; equal throughout range; no sudden release
• Cogwheel rigidity (Parkinson's): Lead-pipe + superimposed tremor; ratchet-like feel

---

Q5. Enumerate the Functions of Basal Ganglia and Write Briefly the Disorders of Basal Ganglia (LQ)

Components of Basal Ganglia

• Striatum = Caudate nucleus + Putamen (input structure)
• Globus pallidus = Internal (GPi) + External (GPe) (output + relay)
• Substantia nigra = Pars compacta (SNc) + Pars reticulata (SNr) (output)
• Subthalamic nucleus (STN)

Functions

Direct and Indirect Pathways

Disorders of Basal Ganglia

1. Parkinson's Disease (Hypokinetic)

• Lesion: Loss of dopaminergic neurons in SNc
• Mechanism: ↓ Direct pathway + ↑ Indirect pathway → excessive thalamic inhibition
• Features: Resting tremor (pill-rolling, 4–6 Hz), bradykinesia, lead-pipe/cogwheel rigidity, postural instability, shuffling gait, masked facies, micrographia
• Treatment: L-DOPA + carbidopa, dopamine agonists, MAO-B inhibitors, deep brain stimulation

2. Huntington's Disease (Hyperkinetic)

• Lesion: Loss of striatal neurons (especially indirect pathway neurons, GABAergic)
• Mechanism: ↓ GPe inhibition → ↓ STN activation → ↓ GPi activity → thalamus disinhibited → excess movement
• Features: Chorea (writhing, involuntary), dementia, psychiatric disturbance; autosomal dominant (CAG repeat expansion on chromosome 4)

3. Hemiballismus

• Lesion: Contralateral subthalamic nucleus (STN), usually lacunar infarct
• Mechanism: ↓ STN activity → ↓ GPi activation → thalamic disinhibition
• Features: Wild, flinging, ballistic involuntary movements of proximal limbs on one side

4. Athetosis

• Slow, writhing, worm-like involuntary movements of distal limbs; associated with striatal lesions (neonatal asphyxia)

5. Chorea

• Rapid, irregular, purposeless involuntary movements; rheumatic fever (Sydenham's chorea) or Huntington's

---

Q6. Define Pain. Draw a Diagram of the Pain Pathway (SQ)

Definition (IASP)

Types of Pain Fibers

Pain Pathway Diagram

PAIN STIMULUS (nociceptors in skin/viscera)
        ↓
Primary afferent (Aδ or C fibers)
        ↓
Dorsal Root Ganglion → enters via dorsal horn
        ↓
┌─────────────────────────────────────┐
│         SPINAL CORD DORSAL HORN     │
│  Aδ → Lamina I (Marginalis)         │
│  C  → Lamina II–III (Sub. Gelatinosa)│
└──────────────┬──────────────────────┘
               ↓ (crosses via anterior white commissure)
    ANTEROLATERAL SPINOTHALAMIC TRACT (contralateral)
               ↓
    ┌──────────────────────────────────────┐
    │ NEOSPINOTHALAMIC (Aδ, fast pain)     │
    │ → VPL thalamus → Somatosensory cortex│
    │ → Precise localization, quality      │
    └──────────────────────────────────────┘
    ┌──────────────────────────────────────┐
    │ PALEOSPINOTHALAMIC (C, slow pain)    │
    │ → PAG, Reticular formation,          │
    │   Intralaminar thalamus              │
    │ → Suffering, arousal, affect         │
    └──────────────────────────────────────┘

• Fast pain (Aδ): Glutamate
• Slow pain (C): Substance P + Glutamate

---

Q7. Sleep Cycle (SQ)

Definition

Two Types

1. NREM (Non-REM/Slow Wave Sleep) — restful, restorative
2. REM (Rapid Eye Movement/Paradoxical Sleep) — active, dreaming

NREM Sleep — Four Stages

REM Sleep

• EEG: Low voltage, high frequency (like wakefulness) — hence "paradoxical sleep"
• Features: Vivid dreaming, rapid eye movements, complete muscle atonia, irregular HR/RR, loss of thermoregulation, ↑ brain metabolism 20%
• Makes up ~25% of total sleep in young adults

Sleep Architecture

• Pattern: Stage 1 → 2 → 3 → 4 → back to 2 → REM
• Cycle repeats every ~90 minutes (4–6 cycles/night)
• More slow-wave sleep early in night; more REM in second half of night

Neurobiology

---

Q8. Functions of Hypothalamus (SQ)

Overview

Key Nuclei and Functions

Enumerated Functions

1. Cardiovascular: Controls HR, BP (posterior → ↑; anterior → ↓)
2. Thermoregulation: Anterior = heat loss (sweating, vasodilation); Posterior = heat conservation (shivering, vasoconstriction)
3. Food intake: Lateral = hunger; VMN = satiety
4. Water balance/Osmolality: ADH secretion (supraoptic); thirst (lateral)
5. Endocrine control: Releasing/inhibiting hormones → anterior pituitary via portal blood (TRH, CRH, GHRH, GnRH, Somatostatin, Dopamine)
6. Uterine contraction and milk ejection: Oxytocin
7. Circadian rhythms: SCN as pacemaker
8. Sleep-wake cycle: Preoptic area (sleep); posterior area (wakefulness)
9. Emotional behavior/Rage: Periventricular = punishment; medial forebrain bundle = reward
10. Sexual behavior: Anterior and posterior nuclei

---

Q9. NREM Sleep (SN)

Definition

Four Stages of NREM

Physiological Changes in NREM (especially Stage 3–4)

• ↓ Blood pressure (10–20%)
• ↓ Heart rate
• ↓ Respiratory rate
• ↓ Basal metabolic rate by 10–30%
• ↓ Body temperature
• ↓ Muscle tone (but not absent — unlike REM)
• ↑ Growth hormone secretion (most GH released during Stage 4)
• ↑ Gastric acid secretion
• Dreams may occur but are not usually remembered

Role in Memory

• NREM sleep is important for declarative memory consolidation (replaying hippocampal memories during slow oscillations → transfer to neocortex)

Neurobiology

• Serotonergic (raphe nuclei) neurons → promote NREM
• GABA/Adenosine build up (sleep pressure) → promote NREM
• Ventrolateral preoptic nucleus (VLPO) — key NREM-ON area; inhibits waking centers

---

Q10. Write the Connections of the Cerebellum, Functions and Disorders of Cerebellum (LQ)

A. Functional Divisions

B. Cerebellar Peduncles

C. Afferent Connections

• Dorsal spinocerebellar tract (from Clarke's nucleus) — unconscious proprioception, ipsilateral lower limb
• Cuneocerebellar tract — proprioception from upper limb/neck
• Olivocerebellar tract (= climbing fibers from inferior olive) — somatosensory error signals
• Vestibulocerebellar — from vestibular receptors
• Reticulocerebellar — from lateral reticular nucleus

• Pontocerebellar tract — cortical information via pontine nuclei (mossy fibers)

• Ventral spinocerebellar tract — proprioception from below T6
• Trigeminocerebellar — facial sensation

D. Deep Cerebellar Nuclei and Efferent Connections

• Dentatothalamic tract → VL nucleus of thalamus → motor cortex
• Dentatorubral tract → red nucleus → rubrospinal tract

E. Functions of the Cerebellum

1. Controls rate, range, direction, and force of movement (synergy)
2. Coordinates agonist/antagonist muscle activation timing
3. Error correction — compares intended vs. actual movement; sends corrective signals
4. Posture and equilibrium maintenance
5. Motor learning — climbing fibers modify Purkinje cell activity over time
6. Eye movement control — smooth pursuit and saccades

F. Disorders of the Cerebellum (DANISH)

1. Ataxia — incoordination of gait and limb movements
2. Dysmetria — finger-nose test: overshooting/undershooting
3. Intention tremor — worsens approaching target; absent at rest
4. Dysdiadochokinesia — failure of rapid alternating movements
5. Dysarthria — scanning, staccato speech
6. Hypotonia — pendular reflexes; decreased fusimotor activity
7. Nystagmus — impaired ocular pursuit
8. Rebound phenomenon — positive Holmes rebound
9. Decomposition of movement
10. Truncal titubation (vermis lesion)

---

Q11. Describe the Process of Sympathetic Transmission. Enumerate the Properties of Synapse (LQ)

A. Sympathetic Transmission

• Division: Thoracolumbar (T1–L3)
• Preganglionic neurons: Myelinated (B fibers); cell bodies in intermediolateral horn of spinal cord
• Postganglionic neurons: Unmyelinated (C fibers); cell bodies in paravertebral or prevertebral ganglia

1. AP in preganglionic neuron → reaches ganglionic synapse
2. Depolarization → Ca²⁺ influx → exocytosis of ACh from preganglionic terminal
3. ACh diffuses across synaptic cleft → binds nicotinic N2 receptors on postganglionic neuron
4. Na⁺ influx → depolarization → AP in postganglionic neuron

1. AP reaches adrenergic varicosities of postganglionic fiber
2. Depolarization → Ca²⁺ influx → exocytosis of NE (from small dense-core vesicles) + ATP (co-transmitter)
3. ATP acts first → fast response via purinergic receptors
4. NE acts second → sustained response via α1/β adrenoreceptors
5. With intense stimulation: Neuropeptide Y released from large dense-core vesicles → prolonged modulation

• Reuptake into presynaptic terminal (primary, via NET transporter)
• Enzymatic breakdown: MAO (monoamine oxidase) intraneuronally; COMT extraneuronally
• Diffusion away from cleft

• Postganglionic sympathetic fibers to eccrine sweat glands are cholinergic (ACh → muscarinic receptors)

• Modified sympathetic ganglion; preganglionic ACh → chromaffin cells secrete 80% epinephrine + 20% NE directly into blood

---

B. Properties of Synapse

---

Q12. Name Four Functions of Hypothalamus and Describe Hypothalamus in Regulation of Food Intake (LQ)

Four Functions of Hypothalamus

1. Thermoregulation — anterior area (heat loss); posterior area (heat conservation)
2. Control of food intake — lateral (hunger center); VMN (satiety)
3. Water balance — ADH secretion (supraoptic); thirst (lateral)
4. Pituitary control — releasing/inhibiting hormones → anterior pituitary axis

Regulation of Food Intake

Two Main Centers

Hypothalamic Obesity

• Bilateral VMN lesions → satiety center destroyed → LH unopposed → continuous hunger
• Results in morbid obesity (demonstrated in rats: VMN-lesioned rats eat continuously)
• Mechanism: ↑ vagal tone → ↑ insulin secretion → ↑ lipogenesis

Hormonal Regulation — Arcuate Nucleus

Key Arcuate Nucleus Neurons

• NPY/AgRP neurons: Orexigenic — increase appetite (stimulated by ghrelin, fasting)
• POMC/CART neurons: Anorexigenic — decrease appetite (stimulated by leptin, insulin)

---

Q13. Describe Briefly Degenerative and Regenerative Changes in Peripheral Nerve After Injury (LQ)

Classification of Nerve Injury (Sunderland)

1. Neurapraxia (Grade I): Local conduction block; no structural damage; full recovery
2. Axonotmesis (Grade II): Axon severed; endoneurial tube intact; Wallerian degeneration; good recovery
3. Neurotmesis (Grade III–V): Partial to complete disruption; poor recovery without repair

---

A. Degenerative Changes — Wallerian Degeneration

• Axon degenerates back to first node of Ranvier proximal to injury
• Chromatolysis in cell body:
  • Dispersion/dissolution of Nissl substance (RER)
  • Nucleus moves to periphery (eccentric)
  • Cell body swells
  • Gene expression shifts from maintenance → regeneration mode
  • Very proximal injury may → cell body apoptosis

[Cell body] → [Axon] → [Injury site] → [Distal axon]
Chromatolysis            ← retrograde        Wallerian degeneration
(Cell body reaction)        changes          Schwann cell + macrophage
                                             phagocytosis → Bands of Büngner

---

B. Regenerative Changes

1. Schwann cell dedifferentiation → upregulate adhesion molecules (laminin, fibronectin), neurotrophins (NGF, BDNF, NT-3)
2. Bands of Büngner form — Schwann cells align along original basement membrane → provide a cellular "highway" for axon regrowth
3. Growth cone forms at tip of regenerating axon — bears filopodia that sample environment
4. Neurotropism — growth cone guided by:
   • Attractants: NGF, laminin, netrin
   • Repellants: semaphorins, ephrins
5. Axon grows at 1–3 mm/day (~1 inch/month)

• Proximal-to-distal regeneration: Main mechanism after complete axon division
• Collateral sprouting: From intact adjacent axons at nodes of Ranvier → reinnervate nearby denervated fibers; faster (3–6 months)

• Wallerian degeneration complete: ~1 week
• Regeneration begins: ~3–4 weeks after injury
• Full recovery: 6–24 months (depending on distance)
• After 4 months without reaching target: tube collapses → impedes recovery
• After 2 years: irreversible muscle fibrosis

• Best: Endoneurium intact (axonotmesis)
• Worst: Complete disruption (neurotmesis) → misdirected growth → neuroma

---

Q14. Babinski's Sign (SQ)

Description

• Stimulus: Firm stroking of lateral plantar surface of foot from heel to ball
• Normal response (flexor plantar): Downward (plantar) flexion of all toes
• Positive Babinski sign (extensor plantar): Dorsiflexion (extension) of big toe + fanning/abduction of other toes

Clinical Significance

• Indicates UMN lesion at any level of the corticospinal tract
• Normal in infants (<2 years) due to incomplete myelination
• Described by Joseph Babinski in 1896

Physiological Mechanism

1. Intact UMN provides tonic inhibition of spinal flexor reflex programs
2. UMN lesion → loss of this inhibition → disinhibition of spinal circuits
3. Noxious plantar stimulus → activates withdrawal/flexor synergy (triple flexion: hip flexion, knee flexion, ankle dorsiflexion)
4. Dorsiflexion of big toe = physiological flexor response of the hallux (it withdraws from plantar surface)
5. In infants (before corticospinal myelination), this primitive reflex is normally present

Variants with Same Significance

• Chaddock's sign: Stroke lateral malleolus
• Oppenheim's sign: Pressure along tibial shin
• Gordon's sign: Squeeze the calf

---

Q15. Name the Pyramidal and Extrapyramidal Tracts. Explain the Origin, Course and Termination of Corticospinal Tract (LQ)

A. Pyramidal Tracts

1. Corticospinal tract (to spinal cord)
2. Corticobulbar tract (to cranial nerve nuclei in brainstem)

B. Extrapyramidal Tracts (do not pass through pyramids)

1. Rubrospinal tract
2. Medial reticulospinal tract
3. Lateral reticulospinal tract
4. Lateral vestibulospinal tract
5. Medial vestibulospinal tract
6. Tectospinal tract
7. Interstitiospinal tract

C. Corticospinal Tract — Origin, Course, Termination

ORIGIN (Cerebral Cortex)
├── ~30% Primary motor cortex (Area 4) — Betz cells
├── ~30% Premotor + SMA (Area 6)
└── ~40% Somatosensory cortex (Areas 3, 1, 2)

         ↓ (converge in corona radiata)
         
POSTERIOR LIMB OF INTERNAL CAPSULE
(between caudate nucleus medially and putamen laterally)

         ↓
         
CRUS CEREBRI / BASIS PEDUNCULI (midbrain)
(middle three-fifths of cerebral peduncle)

         ↓
         
BASIS PONTIS
(fibers scattered among pontine nuclei)

         ↓
         
MEDULLARY PYRAMIDS
(form the pyramids — hence "pyramidal tract")

         ↓ AT PYRAMIDAL DECUSSATION (lower medulla):
         
┌────────────────────────────────────────────┐
│ ~90% CROSS → LATERAL CORTICOSPINAL TRACT   │
│ (in lateral funiculus of spinal cord)       │
│                                             │
│ ~10% DO NOT CROSS → ANTERIOR CORTICOSPINAL │
│ TRACT (in anterior funiculus)               │
│ (most eventually cross segmentally)         │
└────────────────────────────────────────────┘

TERMINATION:
├── Lateral CST → Interneurons in intermediate gray (Rexed laminae IV–VII)
│                  + Direct monosynaptic contact with alpha motor neurons
│                    (especially for fine hand/finger control)
└── Anterior CST → Bilateral axial/proximal muscles

• Betz cells: ~34,000; 16 μm diameter; 70 m/sec conduction velocity; only 3% of total fibers
• Total fibers in each tract: >1 million
• Posterior limb of IC: Somatotopic — arm fibers anterior, leg fibers posterior
• At midbrain: upper limb fibers medial, lower limb fibers lateral in crus cerebri
• Lateral CST: controls contralateral limb movements (fine, distal)
• Anterior CST: controls bilateral postural (axial) muscles

---

Q16. Synaptic Transmission (SQ)

Types of Synapses

Steps in Chemical Synaptic Transmission

1. Action potential arrives at presynaptic terminal
2. Depolarization → opens voltage-gated Ca²⁺ channels
3. Ca²⁺ influx → triggers vesicle-membrane fusion (SNARE proteins)
4. Exocytosis of neurotransmitter (quantal release) into synaptic cleft
5. NT diffuses across cleft (~20–40 nm)
6. NT binds postsynaptic receptors:
   • Ionotropic (e.g., AMPA, NMDA, GABA-A): Direct channel opening → fast (ms)
   • Metabotropic (e.g., GABA-B, mAChR): G-protein → second messenger → slow (s–min)
7. EPSP or IPSP generated
   • EPSP: Na⁺ influx → depolarization (e.g., glutamate)
   • IPSP: Cl⁻ influx or K⁺ efflux → hyperpolarization (e.g., GABA, glycine)
8. Summation of EPSPs to reach threshold → AP generated
9. Termination of NT action:
   • Enzymatic degradation (AChE breaks down ACh)
   • Reuptake (catecholamines, glutamate via transporters)
   • Diffusion

Important Properties

• Unidirectional; synaptic delay ~0.5–1 ms; summation (temporal + spatial); fatigue; facilitation; plasticity

---

Q17. Describe Physiological Basis of Memory (LQ)

Definition

Classification

Role of Hippocampus

• Essential for forming new declarative/explicit long-term memories
• Bilateral removal (patient H.M.) → anterograde amnesia (cannot form new declarative memories); procedural memory intact
• Hippocampus "decides" what to store based on emotional salience and reinforcement
• After long-term consolidation, memories shift to neocortex (hippocampus no longer needed)

Long-Term Potentiation (LTP) — Cellular Basis of Memory

1. Tetanic stimulation → repeated presynaptic glutamate release
2. Glutamate acts on AMPA receptors → initial depolarization
3. Depolarization removes Mg²⁺ block from NMDA receptor channel
4. NMDA receptor opens → Ca²⁺ enters postsynaptic neuron
5. Ca²⁺ → activates CaMKII (Ca²⁺/calmodulin-dependent protein kinase II)
6. → Phosphorylates AMPA receptors (↑ conductance) + inserts new AMPA receptors into membrane
7. Retrograde signal (NO) → presynaptic terminal → ↑ glutamate release long-term
8. Gene expression changes → new synapse formation (structural changes)

• Input specificity: Only active synapses potentiated
• Associativity: Weak inputs potentiated if co-active with strong ones (Hebbian learning: "neurons that fire together, wire together")
• Cooperativity: Requires simultaneous multiple inputs
• Persistence: Lasts days to weeks

Memory Consolidation

• Synaptic consolidation: LTP changes (minutes–hours)
• Systems consolidation: Transfer from hippocampus → neocortex (weeks–years) via sleep-dependent replay
• NREM sleep → sharp-wave ripples in hippocampus → replay of daily events → long-term cortical storage

Retrieval and Forgetting

• Retrieval: Re-activation of distributed neocortical engram
• Long-term depression (LTD): Counterbalances LTP; allows updating and forgetting → cognitive flexibility

---

Q18. Describe Physiological Basis of Parkinson's Disease (LQ)

Definition

Normal vs. Parkinsonian Circuit

• SNc dopamine → D1 receptors in striatum → excites direct pathway → facilitates movement
• SNc dopamine → D2 receptors in striatum → inhibits indirect pathway → prevents excess movement suppression
• Net: Balanced facilitation and suppression of movement

• ↓ D1 stimulation → Direct pathway under-active (less thalamic disinhibition)
• ↓ D2 inhibition → Indirect pathway over-active → STN is hyperactive → GPi/SNr hyperactive → excess thalamic inhibition
• Both changes → reduced thalamocortical excitation → hypokinesia

Pathological Hallmarks

• Lewy bodies: Eosinophilic intraneuronal inclusions containing alpha-synuclein; marker of degeneration
• Braak staging: Pathology begins in brainstem (olfactory bulb, dorsal vagal nucleus) → SNc → neocortex (6 stages)
• Symptoms appear after >60–80% of SNc neurons lost

Clinical Features — Four Cardinals

Resting Tremor — Physiological Basis (also see Q37)

• Not from direct striatal dopamine loss
• Results from abnormal oscillatory synchrony in the basal ganglia-thalamo-cortical circuit
• Hyperactive STN → GPi rhythmic burst firing → thalamic VL nucleus oscillations → rhythmic activation of motor cortex → tremor at 4–6 Hz
• Disappears with movement (voluntary movement overrides oscillation)

Treatment Rationale

---

Q19. Draw a Well-Labelled Diagram of the Pain Pathway. Write Briefly About Referred Pain (LQ)

Pain Pathway Diagram

                 PAIN STIMULUS
                      ↓
          Free nerve endings (nociceptors)
                      ↓
    ┌─────────────────────────────────────────────┐
    │       DORSAL ROOT GANGLION                  │
    │   Aδ fibers (myelinated, fast 6–30 m/s)     │
    │   C fibers (unmyelinated, slow 0.5–2 m/s)   │
    └───────────────────┬─────────────────────────┘
                        ↓
    ┌─────────────────────────────────────────────┐
    │       SPINAL CORD DORSAL HORN               │
    │  Aδ → LAMINA I (Lamina Marginalis)           │
    │  C  → LAMINA II–III (Substantia Gelatinosa)  │
    │  Both → Lamina V (spinothalamic neurons)     │
    └───────────────────┬─────────────────────────┘
                        ↓
            ANTERIOR WHITE COMMISSURE (cross)
                        ↓
    ┌──────────────────────────────────────────────────┐
    │  ANTEROLATERAL SPINOTHALAMIC TRACT               │
    │  (Contralateral ascent)                          │
    │                                                  │
    │  NEOSPINOTHALAMIC            PALEOSPINOTHALAMIC  │
    │  (Aδ, fast pain)             (C, slow pain)      │
    │        ↓                           ↓             │
    │  VPL nucleus of thalamus    PAG, Reticular       │
    │        ↓                    formation, Hypo-     │
    │  Somatosensory cortex       thalamus, Intralami- │
    │  (S1, S2)                   nar thalamus         │
    │  → Precise localization     → Suffering, arousal │
    │    quality, intensity         autonomic, affect  │
    └──────────────────────────────────────────────────┘

Referred Pain

• Visceral afferent pain fibers and somatic (skin) afferent pain fibers converge onto the same second-order neurons in the dorsal horn of the spinal cord
• The brain has no way to distinguish the source
• It misinterprets the visceral signal as coming from the somatic area that shares those dorsal horn neurons
• Pain is projected to the dermatome of the same embryonic origin as the organ

---

Q20. Saltatory Conduction (SQ)

Definition

Structural Basis

• Myelin sheath: Multiple membrane layers from Schwann cells (PNS) or oligodendrocytes (CNS) — acts as electrical insulator
• Nodes of Ranvier: Short (1–2 μm) unmyelinated gaps every 0.2–2 mm
• Voltage-gated Na⁺ channels: Highly concentrated at nodes; essentially absent under myelin
• Internodes: Electrically insulated → prevent current leakage

Mechanism

Node 1 (active AP) → Na⁺ influx
       ↓
Local current flows INSIDE axon (along internodal length)
Myelin PREVENTS current leakage to outside
       ↓
Current reaches Node 2 (depolarized to threshold)
Node 2 fires → AP generated
       ↓
Repeats → Node 3, 4, 5 ...

Advantages of Saltatory Conduction

Clinical Significance

• Multiple sclerosis: Autoimmune CNS demyelination → slowed/blocked saltatory conduction → motor, sensory, visual deficits; plaques on MRI
• Guillain-Barré syndrome: Peripheral nerve demyelination → flaccid weakness
• Nerve conduction velocity (NCV) studies detect demyelination (slowed conduction)

---

Q21. Synaptic Delay (SQ)

Definition

• Minimum synaptic delay: ~0.5 ms per synapse at chemical synapses
• Typical CNS synaptic delay: ~1 ms

Causes of Synaptic Delay (Sequential Steps)

Significance

1. Determines monosynaptic vs. polysynaptic reflexes:
   • Knee jerk central delay = 0.6–0.9 ms → only one synapse (monosynaptic)
   • Longer central delays → multiple interneurons (polysynaptic)
2. Slows multineuronal pathways: More synapses = more total delay; influences timing of complex circuits
3. One-way transmission: Delay is specific to chemical synapses (not electrical); helps explain unidirectionality
4. Limits maximum transmission speed through reflex arcs — exploited in clinical neurophysiology to localize lesions

---

Q22. Describe the Structure and Functions of Cerebellum. Write Briefly About Cerebellar Lesions (LQ)

Structure

• 2 hemispheres + midline vermis; separated from cerebral cortex by tentorium cerebelli
• Surface folded into folia; deep fissures separate lobules
• Anterior lobe + Posterior lobe + Flocculonodular lobe

• Vestibulocerebellum (flocculonodular) → balance/eye movements
• Spinocerebellum (vermis + paravermal zone) → gait, limb coordination
• Pontocerebellum (lateral hemisphere) → fine voluntary movement, motor planning

Functions (see Q10 for detail)

1. Coordinates synergistic action of muscles for smooth movement
2. Controls rate, range, direction, force of movement
3. Maintains posture and equilibrium
4. Damps oscillations and prevents overshoot
5. Motor learning (via climbing fiber-induced LTD of Purkinje cells)
6. Eye movement coordination

Cerebellar Lesions

• Cerebellar lesions produce ipsilateral deficits (pathways double-cross)
• No weakness, no change in consciousness
• Cardinal features: DANISH (see Q1)

---

Q23. Motor Speech Centre (SQ)

Location

• Broca's area: Inferior frontal gyrus, left hemisphere (dominant in 95% of right-handed people)
• Specifically: Pars triangularis (BA45) + Pars opercularis (BA44) of inferior frontal gyrus
• Immediately anterior to primary motor cortex; superior to Sylvian fissure
• Premotor area — controls the sequence and articulation of speech sounds

Function

• Coordinates the planning and execution of speech motor programs
• Sequences muscle movements of lips, tongue, larynx, pharynx, jaw for articulate speech
• Integrates language formulation (from Wernicke's area via arcuate fasciculus) into motor output

Broca's Aphasia (Expressive/Motor Aphasia)

• Patient can think of words but cannot produce them fluently
• Associated with right hemiplegia (motor cortex adjacent)
• Caused by MCA (superior division) occlusion

Language Circuit

Wernicke's area (posterior superior temporal) → 
Arcuate fasciculus → 
Broca's area (inferior frontal) → 
Motor cortex → 
Speech muscles

---

Q24. Positive Babinski Sign (PB)

Clinical Presentation

What It Indicates

Why It Occurs

1. The intact corticospinal tract normally tonically suppresses primitive spinal flexor reflex programs
2. With UMN damage, this inhibition is lost
3. Noxious plantar stimulation → activates the withdrawal (triple flexion) reflex
4. Component of this: dorsiflexion of the great toe (which from the toe's perspective is "withdrawal" from the stimulus on the plantar surface)
5. This primitive reflex is normal in infants (before myelination complete at ~2 years) — its presence in adults = pathological UMN lesion

---

Q25. Renshaw Cell Inhibition (SQ)

Definition

Circuit

Alpha Motor Neuron fires
        ↓
    Recurrent AXON COLLATERAL (branches off before exiting cord)
        ↓
RENSHAW CELL (excited by ACh via nicotinic receptors)
        ↓ (releases GLYCINE)
Alpha Motor Neuron INHIBITED (hyperpolarized)
+ Adjacent synergist motor neurons inhibited
+ Ia inhibitory interneurons modulated

• Motor neuron → Renshaw cell: Acetylcholine (nicotinic receptors)
• Renshaw cell → motor neuron: Glycine (inhibitory)

Functional Significance

1. Negative feedback / Rate limitation: Prevents runaway firing of motor neurons; sets a ceiling on firing rate
2. Stabilizes and smooths motor neuron discharge → smooth muscle contractions
3. Regulates motor unit synergies: Coordinates activity among groups of motor neurons
4. Modulates reciprocal inhibition: By inhibiting Ia interneurons, Renshaw cells can reduce antagonist inhibition → allows co-contraction for joint stability

Clinical Relevance

• Tetanus toxin (C. tetani): Blocks glycine + GABA release from Renshaw cells → removal of inhibition → uncontrolled motor neuron firing → spastic paralysis, lockjaw (trismus)
• Strychnine: Blocks glycine receptors → same mechanism → tetanic spasms

---

Q26. Enumerate the Functions of Hypothalamus. Briefly Discuss the Role of Hypothalamus in Thermoregulation (LQ)

Functions of Hypothalamus (see Q8 for detail)

1. Thermoregulation
2. Food intake regulation (hunger/satiety)
3. Water balance (ADH secretion, thirst)
4. Cardiovascular regulation
5. Anterior pituitary control (releasing/inhibiting hormones)
6. Posterior pituitary hormone synthesis (oxytocin, ADH)
7. Circadian rhythm (SCN)
8. Sleep-wake regulation
9. Emotional behavior and rage
10. Sexual behavior
11. Autonomic nervous system regulation

Thermoregulation in Detail

• Acts like a thermostat with a set-point of 37°C
• Receives afferents from:
  • Peripheral thermoreceptors (skin — cold and warm receptors)
  • Central thermoreceptors (neurons in preoptic area sensitive to local blood temperature)

When Core Temperature Falls Below Set-Point (Cold Response):

When Core Temperature Rises Above Set-Point (Heat Response):

Fever Mechanism

• Pyrogens (bacteria, viruses) → macrophages/monocytes → IL-1, IL-6, TNF-α → anterior hypothalamus → ↑ prostaglandin E2 (PGE2) production
• PGE2 raises the set-point (e.g., from 37°C to 39°C)
• Body perceives 37°C as too cold → activates all heat-producing mechanisms → fever
• Antipyretics (aspirin, paracetamol): Inhibit cyclooxygenase (COX) → ↓ PGE2 → restore normal set-point

---

Q27. Synaptic Plasticity (SQ)

Definition

Forms of Synaptic Plasticity

1. Long-Term Potentiation (LTP)

• Persistent increase in synaptic strength following high-frequency (tetanic) stimulation
• Best studied: Hippocampal Schaffer collateral synapses (CA3→CA1)
• Requires NMDA receptor activation (coincidence detector: needs simultaneous presynaptic glutamate + postsynaptic depolarization to displace Mg²⁺)
• Mechanism: Ca²⁺ influx → CaMKII activation → AMPA receptor phosphorylation + insertion → ↑ synaptic strength
• Properties: Input-specific, associative, cooperative, persistent
• Significance: Cellular model of learning and memory (Hebb's rule)

2. Long-Term Depression (LTD)

• Persistent decrease in synaptic strength following low-frequency stimulation
• Requires smaller Ca²⁺ rise than LTP → activates calcineurin (phosphatase) → AMPA receptor dephosphorylation + internalization
• Critical in cerebellum for motor learning (parallel fiber–Purkinje cell synapse weakened by simultaneous activation with climbing fiber)
• Significance: Prevents synaptic saturation; allows memory updating and forgetting

3. Short-Term Plasticity

• Facilitation: Residual Ca²⁺ after first AP → ↑ release from next AP (paired-pulse facilitation)
• Depression: Vesicle depletion with rapid stimulation
• Post-tetanic potentiation: Enhanced transmission for seconds–minutes after tetanus

Summary

---

Q28. Describe the Functions and Disorders of Cerebellum (LQ)

Functions

1. Synergy of movement — coordinating agonist/antagonist/synergist activation; controlling rate, range, force, direction
2. Equilibrium and posture — vestibulocerebellum coordinates balance
3. Error detection and correction — compares "intended" motor command (efference copy) with actual movement feedback; sends corrections via dentate/interpositus nuclei
4. Motor learning — climbing fibers signal motor errors → modify Purkinje cell sensitivity over time → optimize movement
5. Fine voluntary movements — dentate nucleus → VL thalamus → motor cortex (activated before voluntary movement — anticipatory)
6. Eye movement control — smooth pursuit, saccades (flocculonodular, vermis)
7. Regulation of tone — via spinocerebellar connections

Disorders (see Q1 and Q22 for full table)

• Sudden onset; hypotonia prominent

• DANISH (Dysmetria, Ataxia, Nystagmus, Intention tremor, Slurred speech, Hypotonia)

• Finger-nose test: Demonstrates dysmetria and intention tremor
• Heel-shin test: Lower limb ataxia
• Romberg test: Negative in cerebellar disease (patient can stand with eyes open but shows wide-based ataxia); Romberg POSITIVE = proprioceptive/vestibular problem
• Rapid alternating movements: Dysdiadochokinesia
• Tandem gait: Wide-based, cannot walk heel-to-toe

---

Q29. Basal Ganglia (SQ)

Components

• Striatum (caudate + putamen) — input
• Globus pallidus interna (GPi) and externa (GPe)
• Substantia nigra pars compacta (SNc) + pars reticulata (SNr) — output
• Subthalamic nucleus (STN)

Connections

• Input: Cerebral cortex (all areas) → striatum (glutamate); SNc → striatum (dopamine)
• Output: GPi/SNr → thalamus (VL/VA) → motor cortex (GABA, inhibitory)

Direct vs. Indirect Pathway

• Direct (D1): Cortex → Striatum → GPi/SNr (inhibit) → Thalamus (released) → Motor cortex ↑ — facilitates movement
• Indirect (D2): Cortex → Striatum → GPe → STN → GPi/SNr (activate) → Thalamus (inhibited) → Motor cortex ↓ — suppresses movement
• Dopamine: Facilitates direct; inhibits indirect → promotes movement

Functions

1. Planning and initiation of voluntary movement
2. Inhibition of unwanted/competing movements
3. Procedural learning (habit formation)
4. Cognitive (dorsal caudate) and limbic (ventral striatum) functions
5. Eye movement control (SNr → superior colliculus)

---

Q30. Huntington's Disease (SQ)

Genetics

• Autosomal dominant; chromosome 4p; CAG trinucleotide repeat expansion in HTT gene
• Normal: <35 repeats; Disease: >36 repeats
• Anticipation: Earlier onset/worse severity in successive generations
• Onset typically: 35–50 years

Pathophysiology

• Neurodegeneration of striatum (especially indirect pathway GABAergic neurons) + cortex
• Loss of Striatum → GPe inhibition → GPe more active → inhibits STN → STN less active → GPi less active → Thalamus disinhibited → excess/involuntary movements (chorea)

Clinical Features — Three Domains

• Chorea: Rapid, irregular, involuntary writhing movements; begins distally, progresses proximally
• Oculomotor abnormalities (saccadic pursuit)
• Dysarthria, dysphagia
• Late stage: Rigidity and bradykinesia replace chorea

• Executive dysfunction, slowed thinking (bradyphrenia)
• Memory impairment; poor procedural learning
• Eventual dementia

• Depression, irritability, apathy (most common)
• Anxiety, OCD features, disinhibition, psychosis (late)

Course and Management

• Gradually progressive; death 15–20 years after onset
• No disease-modifying therapy; symptomatic:
  • Tetrabenazine (VMAT2 inhibitor) — reduces chorea
  • Antipsychotics — chorea + psychiatric symptoms
  • Antidepressants, physiotherapy, speech therapy

---

Q31. Physiological Significance of Emotion (SQ)

Definition

Limbic System — Anatomical Basis

Physiological Significance

1. Survival and Adaptive Behavior

• Fear (amygdala) → SNS "fight or flight" activation → ↑ HR, ↑ BP, pupil dilation, bronchodilation, ↑ blood glucose
• Anger/Rage (lateral hypothalamus) → prepares for attack/defense
• Both evolved as survival mechanisms

2. Motivation and Goal-Directed Behavior

• Reward pathway (dopaminergic mesolimbic system: VTA → nucleus accumbens) → pleasure/satisfaction → reinforces beneficial behaviors (feeding, reproduction, social bonding)
• Punishment centers (PAG, periventricular hypothalamus) → aversion → avoid dangerous situations

3. Memory Enhancement

• Emotional events are remembered better — amygdala enhances hippocampal consolidation (via norepinephrine during arousal)
• Fear conditioning (amygdala-dependent) helps avoid repeat threats

4. Communication and Social Behavior

• Facial expressions, vocal tone, body language — all mediated by limbic-motor connections
• Empathy, social bonding, parental behavior — hypothalamic/cingulate mediation

5. Autonomic Regulation

• Emotions drive ANS responses via hypothalamus-SNS/PNS axis (heart rate, BP, digestion, etc.)

Amygdala — Key Emotion Center

• Processes fear and threat appraisal
• Bilateral lesions → Klüver-Bucy syndrome (placidity, hypersexuality, hyperorality, visual agnosia)
• Stimulation → fear, rage, defensive response

---

Q32. Effect of Sympathetic Stimulation of CVS (SQ)

Overview

Adrenoreceptors in CVS

Effects on Each Cardiac Parameter

Net Cardiovascular Effect of Sympathetic Activation

---

Q33. Motor Aphasia (SQ)

Key Points

• Location: Broca's area (BA44 + 45); inferior frontal gyrus; left dominant hemisphere
• Lesion: Superior division MCA occlusion
• Features: Non-fluent, effortful, telegraphic speech; comprehension relatively preserved; repetition impaired; right hemiplegia often present
• Mechanism: Loss of motor speech program planning → cannot coordinate articulation muscles into fluent speech despite preserved language comprehension and desire to speak

---

Q34. Placebos as Pain Relievers (SQ)

Definition

Evidence

• Placebo injections for post-operative pain → significant analgesia
• This effect is blocked by naloxone (opioid receptor antagonist) → proves mediation by endogenous opioids (endorphins)

Mechanism

EXPECTATION / BELIEF
        ↓
Prefrontal cortex + Anterior cingulate cortex
        ↓ (descending activation)
PERIAQUEDUCTAL GRAY (PAG)
        ↓
Nucleus Raphe Magnus (serotonin)
        ↓
DORSAL HORN of spinal cord
        ↓ (enkephalin + serotonin release)
Presynaptic inhibition of Aδ/C pain fibers
+ Postsynaptic hyperpolarization
        ↓
REDUCED PAIN TRANSMISSION → Analgesia

Endogenous Opioid System

Clinical Significance

• Placebo effect is a real neurochemical event, not imagination
• Explains why: strong therapeutic relationships improve outcomes; acupuncture may have partial effect via endorphins; context of care matters
• Nocebo effect: Negative expectation → enhanced pain (reverse of placebo)

---

Q35. Hypothalamic Obesity (SN)

Definition

Mechanism

1. VMN = Satiety center — when active, signals fullness → stops eating
2. Lateral hypothalamus (LH) = Hunger center — normally kept in check by VMN
3. VMN lesion → LH unopposed → continuous hunger → hyperphagia (voracious eating)
4. Result: Massive caloric intake → morbid obesity

Additional Mechanisms

• ↑ Vagal tone → ↑ insulin secretion → ↑ lipogenesis
• ↓ Sympathetic tone → ↓ lipolysis

Clinical Features

• Rapid weight gain (can be dramatic in hypothalamic tumor patients)
• Continuous eating, no satiety
• Often associated with other hypothalamic dysfunction (diabetes insipidus, amenorrhea, visual problems if due to tumor)

Causes of Hypothalamic Obesity in Clinical Practice

• Craniopharyngioma (most common tumor causing this)
• Traumatic brain injury
• Surgical damage
• Inflammatory/infiltrative disease

Animal Model

---

Q36. Cerebral Edema During Slow Ascent (SQ)

Altitude-Related Brain Syndromes

1. Acute Mountain Sickness (AMS) — benign, headache, nausea, insomnia
2. High-Altitude Cerebral Edema (HACE) — severe brain swelling, potentially fatal

Mechanism During Slow Ascent

Ascent to high altitude
        ↓
Hypobaric hypoxia (↓ PaO₂)
        ↓
↓ O₂ delivery to brain
        ↓
Cerebral VASODILATION (hypoxic vasodilation)
        ↓
↑ Cerebral blood flow + ↑ capillary pressure
        ↓
If autoregulation overwhelmed:
        ↓
CAPILLARY LEAKAGE (vasogenic edema)
+ VEGF upregulation → ↑ vascular permeability
+ Na⁺/K⁺-ATPase impairment → cytotoxic edema component
        ↓
↑ Intracranial pressure
        ↓
Brain herniation → death

• Slow ascent allows partial acclimatization: ↑ ventilation → ↑ PaO₂; ↑ erythropoietin → ↑ RBCs; ↑ 2,3-DPG (rightward O₂-Hb curve shift)
• However, if ascent exceeds acclimatization capacity → HACE can still develop
• Faster ascent = less acclimatization time = higher risk

Clinical Features of HACE

• Ataxia (first sign — cerebellar type)
• Severe headache, photophobia
• Confusion, disorientation
• Drowsiness → coma → death (if untreated)

Treatment

1. Immediate descent (most important)
2. Supplemental O₂ (reverses hypoxic vasodilation)
3. Dexamethasone (↓ vasogenic edema)
4. Acetazolamide (carbonic anhydrase inhibitor → ↑ ventilation → ↑ PaO₂; reduces CSF production)
5. Portable hyperbaric chamber (Gamow bag) if available

---

Q37. Resting Tremors in Basal Ganglia Dysfunction (PB)

Clinical Setting

Definition of Resting Tremor

• Tremor occurring at rest, absent during voluntary movement, absent during sleep
• Frequency: 4–6 Hz
• Character: Pill-rolling (thumb and forefinger opposing), sometimes "coin-counting" motion
• Typically asymmetric (worse on one side, at least initially)

Physiological Basis — Mechanism

• Loss of SNc dopaminergic neurons → ↓ dopamine in striatum
• Direct pathway under-active (↓ D1 stimulation)
• Indirect pathway over-active (↓ D2 inhibition) → STN becomes hyperactive

• Hyperactive STN generates rhythmic burst firing (oscillations at 4–6 Hz)
• This oscillatory activity propagates through GPi → thalamus (VL nucleus)
• Thalamus produces rhythmic bursting → activates motor cortex rhythmically
• Motor cortex drives rhythmic muscle activation at 4–6 Hz → tremor

• Voluntary movement commands from cortex override the oscillatory basal ganglia output
• The corticospinal tract's voluntary signal dominates → oscillatory tremor temporarily suppressed during goal-directed movement
• This distinguishes it from cerebellar intention tremor (which is ABSENT at rest, PRESENT during movement)

Key Comparison

Treatment

• L-DOPA + carbidopa: Restores dopamine → reduces STN hyperactivity → reduces oscillations
• Anticholinergics: Reduce relative ACh excess in striatum → particularly effective for tremor
• Beta-blockers: Can also dampen some oscillatory activity
• Deep brain stimulation (STN or GPi): Disrupts abnormal oscillatory firing → markedly reduces tremor

---