1. Describe the structure and functions of cerebellum. Write briefly about cerebellar lesions.
2. Describe the function and disorder of cerebellum.
3. Draw a well labelled diagram of pain pathway and discuss briefly about pain.
4. What is reflex? Give its classification and add a note on Babinski sign.
5. Describe the functions of Basal Ganglia. Draw a well labelled diagram on the circuitry of basal ganglia related to Parkinson's disease. Discuss the clinical features of (unclear).

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CNS LONG ANSWER QUESTIONS

(Reference: Sembulingam's Essentials of Medical Physiology)

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Q1. Describe the Structure and Functions of Cerebellum. Write Briefly about Cerebellar Lesions.

INTRODUCTION

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STRUCTURE OF THE CEREBELLUM

A. Gross Anatomy / External Features

1. Two lateral hemispheres - one on each side
2. Vermis - a central narrow worm-like structure connecting the two hemispheres

• Posterolateral fissure - separates the flocculonodular lobe (archicerebellum) from the rest
• Primary fissure - divides the remaining portion into anterior lobe and posterior lobe

B. Cerebellar Peduncles

C. Internal Structure

1. Dentate nucleus - largest; receives input from the neocerebellum (lateral hemispheres)
2. Emboliform nucleus - receives input from intermediate zone
3. Globose nucleus - receives input from intermediate zone
4. Fastigial nucleus - receives input from the vermis and flocculonodular lobe

1. Molecular layer (outermost) - contains stellate cells, basket cells, and dendrites of Purkinje cells
2. Purkinje cell layer (middle) - single layer of Purkinje cells; these are the largest neurons in the CNS
3. Granular layer (innermost) - densely packed granule cells and Golgi cells

• Mossy fibers - from spinal cord, pontine nuclei; synapse on granule cells
• Climbing fibers - from inferior olivary nucleus; synapse directly on Purkinje cell dendrites (one-to-one powerful excitation)

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FUNCTIONS OF THE CEREBELLUM

1. Coordination of Voluntary Movements

2. Maintenance of Muscle Tone

3. Maintenance of Posture and Equilibrium

4. Regulation of Gait

5. Coordination of Eye Movements

6. Motor Learning

7. Feedback Control of Movements

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CEREBELLAR LESIONS

• Rebound phenomenon: Inability to stop a sudden movement (Holmes' rebound phenomenon - lack of check reflex)
• Decomposition of movement: Complex movements are broken down into their component parts and performed sequentially rather than smoothly
• Truncal ataxia (midline/vermis lesions): Wide-based gait, inability to stand (positive Romberg's test)

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Q2. Describe the Function and Disorder of Cerebellum

DISORDERS OF THE CEREBELLUM

A. Cerebellar Ataxia

B. Intention Tremor

C. Dysmetria (Past-Pointing)

D. Dysdiadochokinesia

E. Hypotonia

F. Scanning (Staccato) Speech

G. Nystagmus

H. Rebound Phenomenon

I. Decomposition of Movement

J. Specific Disease Conditions Involving the Cerebellum

1. Friedreich's Ataxia - autosomal recessive spinocerebellar degeneration; onset in childhood/adolescence; features include progressive cerebellar ataxia, sensory loss, and cardiomyopathy.
2. Multiple Sclerosis - demyelinating lesions affecting cerebellar pathways causing intention tremor, nystagmus, scanning speech (Charcot's triad).
3. Cerebellar tumors - medulloblastoma (midline), astrocytoma; cause truncal ataxia and raised intracranial pressure.
4. Alcoholic cerebellar degeneration - particularly affects the anterior vermis; presents with gait ataxia.
5. Posterior inferior cerebellar artery (PICA) syndrome - lateral medullary syndrome; causes ipsilateral ataxia, Horner's syndrome, crossed sensory loss.

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Q3. Draw a Well-Labelled Diagram of Pain Pathway and Discuss Briefly about Pain

DEFINITION

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TYPES OF PAIN

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PAIN RECEPTORS (NOCICEPTORS)

• Mechanical stimuli - strong pressure, cutting
• Thermal stimuli - extreme heat or cold
• Chemical stimuli - bradykinin, substance P, histamine, prostaglandins, serotonin, H+ ions, K+ ions released from damaged cells

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PAIN FIBERS

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PAIN PATHWAY (NEOSPINOTHALAMIC AND PALEOSPINOTHALAMIC)

A. Lateral Spinothalamic Tract (Neospinothalamic Tract) - Fast Pain

• Free nerve endings (A-delta fibers) → enters spinal cord via dorsal root → synapses in the dorsal horn, primarily in lamina I (marginal zone) and lamina V (substantia gelatinosa) of Rexed

• Crosses the midline immediately in the anterior white commissure
• Ascends as the lateral spinothalamic tract in the anterolateral white matter
• Synapses in the ventral posterolateral (VPL) nucleus of the thalamus

• From VPL thalamus → internal capsule → primary somatosensory cortex (post-central gyrus, areas 3, 1, 2)
• Mediates conscious localization and quality discrimination of pain

B. Paleospinothalamic Tract - Slow Pain

• C fibers → dorsal root → synapse mainly in laminae I, II (substantia gelatinosa), and V

• Crosses midline → ascends as paleospinothalamic tract (more diffuse pathway)
• Synapses in the reticular formation of the brainstem, periaqueductal gray (PAG), intralaminar thalamic nuclei

• Projects widely to the limbic system (emotional component), hypothalamus, association cortex
• Responsible for the suffering and emotional components of pain

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WELL-LABELLED DIAGRAM OF PAIN PATHWAY

    CEREBRAL CORTEX
    (Post-central gyrus S1)
           ↑
    THALAMUS
    (VPL nucleus) ← Paleospinothalamic → Reticular formation / Limbic system
           ↑
    Internal capsule
           ↑
─────────────────────────────────────────────────────────
       SPINAL CORD (Cross-section)
─────────────────────────────────────────────────────────
  Dorsal horn         →  Lateral
  Lamina I, II, V         Spinothalamic
  (synapse of 1st         Tract (crossed)
   order neuron)
           ↑
    Dorsal root ganglion
           ↑
   A-delta and C fibers
   (from nociceptors in skin/tissue)

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GATE CONTROL THEORY OF PAIN (Melzack and Wall, 1965)

• Large-diameter fibers (A-beta, touch) → stimulate inhibitory interneurons in SG → "close the gate" → inhibit pain transmission
• Small-diameter fibers (A-delta, C) → inhibit the SG interneurons → "open the gate" → allow pain to be transmitted

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ENDOGENOUS PAIN SUPPRESSION SYSTEM

1. Periaqueductal gray (PAG) matter - stimulated by opioids, stress; activates descending pain inhibitory pathways
2. Raphe nuclei (serotonergic) → descend to dorsal horn → release serotonin → inhibit pain
3. Locus coeruleus (noradrenergic) → descend and release norepinephrine → inhibit pain
4. Enkephalins (endogenous opioids) released in dorsal horn → inhibit substance P release
5. Beta-endorphins - released from pituitary; powerful analgesic action

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REFERRED PAIN

• Cardiac ischemia → pain in left arm, jaw, shoulder (T1-T4)
• Diaphragm irritation → pain in the shoulder (C3, C4)
• Appendicitis → initial periumbilical pain (T10)
• Renal colic → loin to groin pain (T11-L1)

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Q4. What is Reflex? Give Its Classification and Add a Note on Babinski Sign

DEFINITION OF REFLEX

1. Receptor - detects the stimulus
2. Afferent (sensory) nerve - carries impulses toward the spinal cord
3. Nerve center - integration in the spinal cord (or brainstem)
4. Efferent (motor) nerve - carries response impulses to the effector
5. Effector - muscle or gland that produces the response

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CLASSIFICATION OF REFLEXES

I. Based on Location of Nerve Center (Spinal vs. Supraspinal)

II. Based on Number of Synapses

III. Based on the Response (Muscle Type)

IV. Based on the Effector Side Responding

V. Clinical Classification (Muscle Reflexes)

• Monosynaptic
• Stimulus: Tap on tendon → stretches muscle spindle → Ia afferents → alpha motoneuron → muscle contracts
• Examples: Knee jerk (L3, L4), ankle jerk (S1, S2), biceps jerk (C5, C6), triceps jerk (C7, C8)
• Absent in lower motor neuron lesions; exaggerated in upper motor neuron lesions

• Polysynaptic
• Stimulus: Light stroking of skin
• Examples:
  • Abdominal reflex (T7-T12) - contraction of abdominal muscles
  • Cremasteric reflex (L1, L2) - elevation of testis
  • Plantar reflex (S1, S2)

• Pupillary light reflex
• Micturition reflex
• Defecation reflex

• Present only in disease states (pyramidal tract lesions)
• Example: Babinski sign

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IMPORTANT REFLEXES AND THEIR SPINAL CORD LEVELS

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NOTE ON BABINSKI SIGN

Definition

Normal Response (Plantar Reflex)

Positive Babinski Sign

Significance

• Present normally in infants up to 18-24 months (due to incomplete myelination of the corticospinal tract)
• Disappears as the pyramidal tract myelinates

Related Signs (Pyramidal Signs)

• Chaddock's sign - Stroke lateral malleolus and dorsum of foot
• Oppenheim's sign - Stroke downward along the shin (tibia)
• Gordon's sign - Squeeze calf muscles
• Hoffman's sign - Flicking the terminal phalanx of the middle finger; thumb flexion is positive (upper limb equivalent of Babinski)

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Q5. Describe the Functions of Basal Ganglia. Draw a Well-Labelled Diagram on the Circuitry of Basal Ganglia Related to Parkinson's Disease. Discuss the Clinical Features of Parkinson's Disease

INTRODUCTION

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COMPONENTS OF THE BASAL GANGLIA

1. Caudate nucleus - C-shaped nucleus; head in frontal lobe, tail in temporal lobe
2. Putamen - together with caudate = neostriatum (striatum) = main INPUT nucleus
3. Globus pallidus - main OUTPUT nucleus; divided into:
   • External segment (GPe)
   • Internal segment (GPi)

• Pars compacta (SNc) - dopaminergic neurons projecting to striatum (nigrostriatal pathway)
• Pars reticulata (SNr) - GABAergic; acts like GPi

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FUNCTIONS OF THE BASAL GANGLIA

1. Motor Functions

• Aid in planning, initiation, and smooth execution of voluntary movements
• Regulate the amplitude and velocity of movements
• Suppress unwanted movements (via the indirect pathway)
• Facilitate desired movements (via the direct pathway)
• Regulate postural tone and automatic associated movements (e.g., arm swing during walking)

2. Cognitive Functions

• Involved in procedural learning (habit formation and skill learning)
• Executive functions via connections with the prefrontal cortex
• Working memory and decision-making processes

3. Affective Functions

• Regulate emotional behavior through connections with the limbic system
• Involved in motivation, reward processing, and addiction (via the ventral striatum/nucleus accumbens)

4. Language and Speech

• Involved in the fluency and prosody of speech through connections with the supplementary motor area

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CIRCUITRY OF BASAL GANGLIA

A. Direct Pathway (Excitatory net output on cortex - facilitates movement)

Motor Cortex
    ↓ (Glutamate, excitatory)
STRIATUM (D1 receptors stimulated by dopamine → activates direct pathway)
    ↓ (GABA, inhibitory)
GPi / SNr (Internal globus pallidus / Substantia nigra pars reticulata)
    ↓ (GABA, inhibitory) - LESS inhibition to thalamus
THALAMUS (VL/VA)
    ↓ (Glutamate, excitatory)
MOTOR CORTEX → Movement FACILITATED

B. Indirect Pathway (Inhibitory net output on cortex - suppresses unwanted movements)

Motor Cortex
    ↓ (Glutamate, excitatory)
STRIATUM (D2 receptors stimulated by dopamine → inhibits indirect pathway)
    ↓ (GABA, inhibitory)
GPe (External globus pallidus)
    ↓ (GABA, inhibitory) - releases STN from inhibition
SUBTHALAMIC NUCLEUS (STN)
    ↓ (Glutamate, excitatory)
GPi / SNr
    ↓ (GABA, inhibitory) - MORE inhibition to thalamus
THALAMUS (suppressed)
    ↓ 
MOTOR CORTEX → Movement SUPPRESSED

C. Nigrostriatal Dopamine Connection

• D1 receptors on direct pathway neurons → dopamine is excitatory → facilitates movement
• D2 receptors on indirect pathway neurons → dopamine is inhibitory → reduces suppression → facilitates movement
• Overall: Dopamine promotes movement by activating direct and inhibiting indirect pathways

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DIAGRAM OF BASAL GANGLIA CIRCUITRY IN PARKINSON'S DISEASE

NORMAL STATE:
Cortex → Striatum → [Direct: GPi inhibited ↓ → Thalamus active ↑ → Cortex active ↑]
                    [Indirect: GPe inhibited ↓ → STN disinhibited ↑ → GPi active ↑ → Thalamus inhibited ↓]
SNc dopamine balances both pathways

PARKINSON'S DISEASE (SNc neurons degenerate → dopamine deficiency):
─────────────────────────────────────────────────────────────────────────
Direct pathway:  D1 receptors unstimulated → Striatum does NOT inhibit GPi
                 → GPi overactive → Thalamus strongly INHIBITED ↓

Indirect pathway: D2 receptors unstimulated → Striatum does NOT inhibit GPe
                  → GPe overactive → STN strongly INHIBITED
                  → GPe more inhibited → STN LESS active
                  Actually: D2 loss → indirect pathway OVERACTIVE
                  → GPi further overactive → Thalamus more INHIBITED

NET RESULT: Thalamus excessively inhibited → Motor cortex UNDERACTIVE
            → Bradykinesia, poverty of movement, rigidity
─────────────────────────────────────────────────────────────────────────

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PARKINSON'S DISEASE - CLINICAL FEATURES

Pathology

• Progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc)
• Loss of dopamine in the nigrostriatal pathway
• Symptoms appear when 60-80% of the SNc neurons have degenerated
• Microscopic hallmark: Lewy bodies (intracytoplasmic eosinophilic inclusions of alpha-synuclein) in surviving neurons

Epidemiology

• Affects 1-2% of individuals over age 65
• More common in males (1.5:1 male:female ratio)
• Second most common neurodegenerative disorder after Alzheimer's disease
• Estimated 7-10 million people worldwide

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CARDINAL FEATURES - "TRAP"

• The most recognizable feature
• "Pill-rolling tremor" - rhythmic rolling movement of fingers as if rolling a pill between thumb and forefinger
• 4-6 Hz frequency
• Present at REST, diminishes with voluntary movement, disappears during sleep
• Contrasts with cerebellar intention tremor (which increases toward target)

• Increased resistance to passive movement throughout the range of motion
• "Lead-pipe rigidity" - uniform resistance throughout movement
• "Cogwheel rigidity" - superimposed tremor on rigidity gives a ratchet-like feel
• Affects neck, trunk, and limbs
• Contributes to the stooped posture

• Akinesia: Difficulty in initiating movement (poverty of movement)
• Bradykinesia: Slowness in execution of voluntary movement
• Manifestations:
  • Reduced facial expression ("mask-like face" / hypomimia)
  • Micrographia (small handwriting)
  • Hypophonia (soft, monotonous voice)
  • Reduced arm swing while walking
  • Difficulty in fine tasks (buttoning, writing)
  • "Freezing" episodes (transient inability to move)

• Loss of balance and postural righting reflexes
• Festinant gait: Short, shuffling steps with the center of gravity moving forward, patient accelerates to prevent falling (festination)
• Propulsion (forward) and retropulsion (backward) - tendency to fall forward or backward
• "Simian posture" - stooped forward, neck flexed, arms flexed, slight knee flexion

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OTHER FEATURES

• Orthostatic hypotension
• Constipation, urinary urgency
• Excessive sweating, seborrhea ("oily skin")
• Sialorrhea (drooling) - due to reduced swallowing, not excess production

• Dementia (in later stages, ~30-40% of patients)
• Depression (most common psychiatric feature)
• Psychosis (hallucinations - often drug-induced)
• Sleep disturbances (REM sleep behavior disorder)

• Anosmia (loss of smell) - often an early pre-motor symptom
• Pain and sensory discomfort

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TREATMENT OF PARKINSON'S DISEASE

• Levodopa (L-dopa) + Carbidopa (Sinemet) - most effective; carbidopa prevents peripheral conversion, reduces side effects (nausea, cardiac arrhythmias)
• Dopamine agonists - Bromocriptine, Pramipexole, Ropinirole
• MAO-B inhibitors - Selegiline, Rasagiline - prevent dopamine breakdown
• COMT inhibitors - Entacapone - prolong L-dopa effect
• Anticholinergics - Trihexyphenidyl - mainly for tremor
• Amantadine - mild dopaminergic; helps dyskinesias

• Deep Brain Stimulation (DBS) - subthalamic nucleus or globus pallidus interna; reduces tremor and rigidity
• Pallidotomy - lesioning of GPi
• Thalamotomy - for tremor only

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1. Draw well-labelled diagram, explain neural connections in cerebellar cortex. Explain physiological basis of clinical features in cerebellar dysfunction.
2. Enumerate functions of basal ganglia & write briefly the disorders of basal ganglia.
3. Write the connections of cerebellum, functions & disorders of cerebellum.
4. Describe synaptic transmission in the nervous system. Enumerate the properties of synapse.
5. Name four functions of hypothalamus & describe the contribution of hypothalamus in regulation of food intake.
6. Describe briefly the degenerative & regenerative changes in peripheral nerve after injury.
7. Describe physiological basis of memory.
8. Describe physiological basis of Parkinson's disease.
9. Draw a well labelled diagram of pain pathway. Write about referred pain.

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CNS - LONG ANSWER QUESTIONS

Reference: Sembulingam's Essentials of Medical Physiology

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Q1. Draw a Well-Labelled Diagram, Explain Neural Connections in Cerebellar Cortex. Explain the Physiological Basis of Clinical Features Observed in Cerebellar Dysfunction.

INTRODUCTION

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STRUCTURE OF CEREBELLAR CORTEX

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FIVE TYPES OF NEURONS

1. Purkinje cells - The most important cells. They are very large neurons. Their dendrites spread into the molecular layer like a tree. Their axons are the ONLY OUTPUT from the cerebellar cortex. They send inhibitory (GABA) signals to the deep cerebellar nuclei.
2. Granule cells - The most numerous cells. They sit in the granular layer. They receive excitatory input from mossy fibers and send their axons upward into the molecular layer, where the axon splits into a "T" shape, forming parallel fibers.
3. Basket cells - In the molecular layer. They inhibit Purkinje cells by wrapping around them like a basket.
4. Stellate cells - In the molecular layer. They also inhibit Purkinje cells (from the sides).
5. Golgi cells - In the granular layer. They provide feedback inhibition to granule cells.

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AFFERENT FIBERS (INPUT TO CEREBELLAR CORTEX)

• Carry sensory and motor information from spinal cord, brainstem, and cerebral cortex
• They synapse on granule cells and also send collaterals to deep nuclei
• They are excitatory (glutamate)
• One mossy fiber excites many granule cells

• Come from the inferior olivary nucleus (in the medulla)
• They "climb" along the Purkinje cell dendrites and make very powerful one-to-one connections
• Each Purkinje cell receives input from only ONE climbing fiber, but each climbing fiber makes 300+ synaptic contacts with that Purkinje cell
• They are excitatory (glutamate) and are important in motor learning

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NEURAL CONNECTIONS IN CEREBELLAR CORTEX (CIRCUIT)

MOSSY FIBERS (from spinal cord, pons, brainstem)
        ↓ (excitatory)
GRANULE CELLS (granular layer)
        ↓ (excitatory, via PARALLEL FIBERS)
PURKINJE CELLS (Purkinje cell layer)
        ↓ (inhibitory, GABA)
DEEP CEREBELLAR NUCLEI (dentate, interpositus, fastigial)
        ↓ (excitatory output to thalamus and brainstem)
THALAMUS → MOTOR CORTEX → SMOOTH MOVEMENT

Also:
CLIMBING FIBERS (from inferior olive)
        ↓ (powerful excitation, for error detection/learning)
PURKINJE CELLS

BASKET CELLS + STELLATE CELLS → inhibit Purkinje cells (modulation)
GOLGI CELLS → inhibit Granule cells (feedback loop)

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DIAGRAM OF CEREBELLAR CORTEX NEURAL CONNECTIONS

         MOLECULAR LAYER
    ─────────────────────────────────────────
    Stellate cell──┐    Parallel fibers ─────→
    Basket cell────┼──→ [Purkinje cell dendrites]
                   │         ↓
    ───────────────────────────────────────────
         PURKINJE CELL LAYER
    [PURKINJE CELL BODIES] → axon goes to deep nuclei (inhibitory)
         ↑ (climbing fibers from inferior olive)
    ───────────────────────────────────────────
         GRANULAR LAYER
    [GRANULE CELLS] ← Mossy fibers (excitatory)
    [GOLGI CELLS] → inhibit Granule cells

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PHYSIOLOGICAL BASIS OF CLINICAL FEATURES IN CEREBELLAR DYSFUNCTION

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1. HYPOTONIA (Reduced muscle tone)

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2. ATAXIA (Incoordination of movement)

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3. INTENTION TREMOR

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4. DYSMETRIA (Past-pointing)

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5. DYSDIADOCHOKINESIA

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6. SCANNING SPEECH (Cerebellar Dysarthria)

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7. NYSTAGMUS

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8. REBOUND PHENOMENON (Holmes' rebound)

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9. DECOMPOSITION OF MOVEMENT

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Q2. Enumerate the Functions of Basal Ganglia and Write Briefly the Disorders of Basal Ganglia

INTRODUCTION

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COMPONENTS

• Caudate nucleus - head (frontal), body, tail (temporal)
• Putamen - together with caudate = striatum (main INPUT nucleus)
• Globus pallidus - divided into external (GPe) and internal (GPi) segments
• Subthalamic nucleus (STN) - in diencephalon
• Substantia nigra - in midbrain: pars compacta (dopaminergic) and pars reticulata

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FUNCTIONS OF BASAL GANGLIA

1. Control of Voluntary Movements

• Direct pathway → promotes desired movements (net excitatory effect on cortex)
• Indirect pathway → suppresses unwanted movements (net inhibitory effect on cortex)

2. Regulation of Muscle Tone

3. Programming of Complex Movements

4. Procedural Learning and Habit Formation

5. Cognitive Functions

6. Emotional and Motivational Functions

7. Regulation of Eye Movements

8. Language

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DISORDERS OF BASAL GANGLIA

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A. PARKINSON'S DISEASE (Hypokinetic disorder)

• Direct pathway is underactive → movements not facilitated
• Indirect pathway is overactive → movements are excessively suppressed
• Net result: thalamus is over-inhibited → motor cortex is underactive → poverty of movement

• T - Resting Tremor - pill-rolling, 4-6 Hz, absent during movement
• R - Rigidity - lead-pipe or cogwheel
• A - Akinesia/Bradykinesia - slowness and poverty of movement
• P - Postural instability - shuffling gait, festination

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B. HUNTINGTON'S DISEASE (Hyperkinetic disorder)

• Chorea - involuntary, jerky, dance-like writhing movements
• Dementia (cognitive decline)
• Psychiatric symptoms (depression, personality change)
• No cure; onset typically age 30-50

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C. HEMIBALLISMUS

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D. WILSON'S DISEASE (Hepatolenticular Degeneration)

• Tremor, rigidity, dysarthria (neurological)
• Liver cirrhosis
• Kayser-Fleischer rings in the cornea (copper deposits)
• Psychiatric symptoms

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E. TOURETTE SYNDROME

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Q3. Write the Connections of Cerebellum, Functions and Disorders of Cerebellum

CONNECTIONS OF THE CEREBELLUM

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A. Inferior Cerebellar Peduncle (Restiform Body)

1. Dorsal spinocerebellar tract - proprioception from ipsilateral body (unconscious)
2. Cuneocerebellar tract - proprioception from upper limb
3. Vestibulocerebellar fibers - from vestibular nuclei and otolith organs (balance)
4. Olivocerebellar fibers - from inferior olivary nucleus (climbing fibers)
5. Reticulocerebellar fibers - from reticular formation

• Cerebellovestibular fibers - from fastigial nucleus to vestibular nuclei (controls balance and eye movements)

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B. Middle Cerebellar Peduncle (Brachium Pontis) - LARGEST peduncle

• Pontocerebellar fibers - from the contralateral pontine nuclei (which receive input from the cerebral cortex via corticopontine fibers)
• This is the main route by which the cerebral cortex sends information to the cerebellum about intended movements

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C. Superior Cerebellar Peduncle (Brachium Conjunctivum)

• Dentatorubrothalamic tract - from dentate nucleus → decussates in the midbrain → red nucleus → VL/VA thalamus → motor cortex
• This is the main OUTPUT pathway of the cerebellum

• Ventral spinocerebellar tract - proprioception from lower limb
• Trigeminocerebellar fibers - from trigeminal nuclei

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SUMMARY TABLE OF CEREBELLAR CONNECTIONS

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FUNCTIONS OF THE CEREBELLUM

1. Coordination of voluntary movements - ensures movements are smooth, accurate, and well-timed (comparator function - compares intended vs. actual movement and corrects errors)
2. Maintenance of muscle tone - regulates gamma motor neurons
3. Maintenance of posture and equilibrium - via the flocculonodular lobe and vestibular connections
4. Regulation of gait - ensures rhythmic, smooth walking
5. Coordination of eye movements - via the flocculonodular lobe
6. Motor learning - storing and retrieving the memory of learned motor skills (via long-term depression at the parallel fiber-Purkinje cell synapse)
7. Planning of sequential movements - works with the motor cortex to program complex movements before they are executed

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DISORDERS OF THE CEREBELLUM

• The cerebellum controls the ipsilateral side of the body
• The cerebellar output crosses and then recrosses (double-decussation via the red nucleus), so the final effect remains ipsilateral

• Dysmetria (past-pointing)
• Rebound phenomenon
• Decomposition of movement
• Titubation (rhythmic nodding of head in vermis lesions)

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Q4. Describe Synaptic Transmission in the Nervous System. Enumerate the Properties of Synapse

INTRODUCTION

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TYPES OF SYNAPSES

A. Electrical Synapses (Gap Junctions)

• Two cells are directly connected by gap junction channels (protein tunnels called connexons)
• Electrical current (ions) flows directly from one cell to the next without any chemical messenger
• Transmission is very fast and bidirectional
• Found in: cardiac muscle, smooth muscle of uterus and bladder (allows coordinated contraction)
• Less important in the central nervous system

B. Chemical Synapses

• There is a small gap (synaptic cleft) between the presynaptic and postsynaptic membranes
• A chemical messenger (neurotransmitter) carries the signal across the gap
• Transmission is unidirectional (one-way) - from presynaptic to postsynaptic only
• This is the most common type in the nervous system

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STRUCTURAL COMPONENTS OF A CHEMICAL SYNAPSE

1. Presynaptic terminal (synaptic knob/bouton) - the swollen end of the axon that releases neurotransmitter
2. Synaptic vesicles - small bags inside the presynaptic terminal that store neurotransmitter
3. Synaptic cleft - the tiny gap (20-40 nm wide) between the two neurons
4. Postsynaptic membrane - the membrane of the receiving cell, studded with specific receptors
5. Mitochondria - abundant in the presynaptic terminal (energy needed for neurotransmitter synthesis and packaging)

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SEQUENCE OF EVENTS IN SYNAPTIC TRANSMISSION

• Excitatory Postsynaptic Potential (EPSP) - if the neurotransmitter is excitatory (e.g., glutamate, acetylcholine) → Na⁺ channels open → depolarization → may generate an action potential
• Inhibitory Postsynaptic Potential (IPSP) - if the neurotransmitter is inhibitory (e.g., GABA, glycine) → Cl⁻ channels open or K⁺ channels open → hyperpolarization → inhibits action potential

• Reuptake - taken back into the presynaptic terminal (most common, e.g., serotonin, dopamine)
• Enzymatic degradation - broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine)
• Diffusion - drifts away from the cleft

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SUMMATION AT SYNAPSES

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PROPERTIES OF A SYNAPSE

1. Unidirectional conduction - impulse travels only from presynaptic to postsynaptic (because only the presynaptic terminal has vesicles and only the postsynaptic membrane has receptors). This is the most important property.
2. Synaptic delay - There is a time delay of about 0.5 ms at each synapse (time needed for Ca²⁺ entry, vesicle fusion, diffusion of transmitter, and receptor binding). This is why reflexes with more synapses are slower.
3. Fatigue - With repeated stimulation, the synapse tires out. This is because neurotransmitter stores get depleted faster than they can be replenished.
4. Summation - Spatial and temporal summation (explained above). The synapse can add up small signals to reach threshold.
5. Facilitation - After a synapse is activated, it becomes easier to activate again for a short time. The presynaptic terminal still has elevated Ca²⁺, making the next stimulus more likely to release transmitter.
6. Post-tetanic potentiation - After a burst of rapid stimulation (tetanus), the synapse is temporarily more excitable. This is a temporary enhancement of synaptic transmission.
7. Occlusion - When two pathways share the same postsynaptic neuron, stimulating both together gives a smaller combined response than expected. This is because both stimuli use the same pool of neurons.
8. Susceptibility to drugs, hypoxia, and fatigue - Chemical synapses are much more sensitive to drugs (anesthetics, toxins, antidepressants) than axons. Hypoxia (lack of oxygen) and certain drugs (like curare, which blocks ACh receptors) can block synaptic transmission.
9. Low safety factor - A single EPSP is usually not enough to fire an action potential. Multiple EPSPs must summate (unlike the neuromuscular junction where a single impulse reliably fires the muscle).
10. After-discharge - Due to reverberating circuits (closed-loop neural circuits), the postsynaptic neuron may keep firing even after the original stimulus has stopped.

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TYPES OF SYNAPSES BASED ON LOCATION

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Q5. Name Four Functions of Hypothalamus and Describe the Contribution of Hypothalamus in Regulation of Food Intake

INTRODUCTION

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FOUR MAIN FUNCTIONS OF HYPOTHALAMUS

1. Regulation of body temperature (thermoregulation)
2. Regulation of food intake (hunger and satiety)
3. Regulation of water balance (thirst and ADH secretion)
4. Control of the pituitary gland (neuroendocrine control - releasing and inhibiting hormones)

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CONTRIBUTION OF HYPOTHALAMUS IN REGULATION OF FOOD INTAKE

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A. FEEDING CENTER (Hunger Center)

• Electrical stimulation of the lateral hypothalamus → animal eats voraciously (hyperphagia) even when not hungry
• Destruction/lesion of the lateral hypothalamus → animal completely stops eating (aphagia) → progressive weight loss and starvation
• The lateral hypothalamus operates by exciting the motor drives to search for food

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B. SATIETY CENTER (Fullness Center)

• Stimulation of VMN → animal refuses to eat even if hungry (aphagia)
• Destruction of VMN → animal eats continuously and becomes very obese (hyperphagia) - sometimes 4 times normal body weight

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C. OTHER HYPOTHALAMIC NUCLEI INVOLVED

---

D. HORMONES AND CHEMICAL SIGNALS INVOLVED

• Leptin - released from fat (adipose) cells when fat stores are full → acts on arcuate nucleus → reduces hunger
• Insulin - released from pancreas when blood glucose rises → reduces appetite
• Cholecystokinin (CCK) - released from small intestine after a meal → signals satiety
• Glucagon-like peptide-1 (GLP-1) - from intestine → reduces hunger
• Peptide YY (PYY) - from intestine after eating → reduces food intake

• Ghrelin - released from the STOMACH when it is EMPTY → powerful hunger stimulator (the "hunger hormone")
• Neuropeptide Y (NPY) - produced in the arcuate nucleus → strongly increases appetite
• Agouti-related peptide (AgRP) - from arcuate nucleus → increases food intake

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E. ARCUATE NUCLEUS - THE KEY INTEGRATING CENTER

1. POMC/CART neurons (Pro-opiomelanocortin/Cocaine- and Amphetamine-Regulated Transcript):
   • Release α-MSH (alpha-melanocyte stimulating hormone)
   • α-MSH acts on MC4R receptors → DECREASES food intake and INCREASES energy expenditure
   • These are ANOREXIGENIC neurons
2. NPY/AgRP neurons (Neuropeptide Y/Agouti-related Peptide):
   • Release NPY and AgRP
   • NPY increases food intake; AgRP BLOCKS MC4R receptors
   • These are OREXIGENIC neurons

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F. SUMMARY DIAGRAM OF FOOD INTAKE REGULATION

EMPTY STOMACH → ↑ Ghrelin → ↑ NPY/AgRP in arcuate nucleus
                                    ↓
                          LATERAL HYPOTHALAMUS activated
                                    ↓
                          FEELING OF HUNGER → EATING

AFTER EATING → stomach stretches (vagal signals) + blood glucose rises
+ gut hormones (CCK, GLP-1, PYY) released + leptin from fat stores
                                    ↓
                    POMC/CART neurons in arcuate activated
                    α-MSH released → MC4R stimulated
                                    ↓
                          VMN SATIETY CENTER activated
                                    ↓
                    LATERAL HYPOTHALAMUS (feeding center) INHIBITED
                                    ↓
                          FEELING OF FULLNESS → STOP EATING

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G. OTHER FACTORS AFFECTING FOOD INTAKE

• Blood glucose level - Low glucose (hypoglycemia) → stimulates hunger; high glucose → reduces hunger (glucostatic theory)
• Body temperature - Warmth reduces appetite; cold increases it
• Psychological/cortical factors - Sight, smell, taste of food stimulate eating via cortical connections to the hypothalamus
• Emotional state - Stress, anxiety, depression can alter appetite via limbic connections

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Q6. Describe Briefly the Degenerative and Regenerative Changes in Peripheral Nerve After Injury

INTRODUCTION

1. Degenerative changes - the nerve fiber dies and breaks down (distal to the injury)
2. Regenerative changes - the nerve attempts to grow back and reestablish connections

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TYPES OF NERVE INJURY (Seddon's Classification)

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DEGENERATIVE CHANGES AFTER NERVE INJURY

A. WALLERIAN DEGENERATION (Distal to the injury - in the distal stump)

• The axon distal to the cut is separated from the cell body (which makes the proteins and nutrients)
• The axon begins to swell

• The axon starts to break down and fragment into small pieces (granular disintegration)
• The myelin sheath also breaks down into oval droplets (ovoids)

• Schwann cells are activated. They start to proliferate (multiply) and clean up the debris
• Macrophages are recruited. They phagocytose (eat up) the myelin debris and axon fragments
• Schwann cells form bands of Büngner - these are tubes of Schwann cells that remain inside the old basement membrane (endoneural tube). These tubes act as a guide for nerve regeneration later.

---

B. RETROGRADE DEGENERATION (Proximal stump - toward the cell body)

• The axon proximal to the injury degenerates backward (retrograde direction), but only up to the nearest node of Ranvier (the last node proximal to the injury site)
• This is much less extensive than Wallerian degeneration

---

C. CHANGES IN THE CELL BODY (Chromatolysis)

• Nucleus moves to the periphery of the cell body (eccentric nucleus)
• Nissl granules (rough endoplasmic reticulum) dissolve and disappear - this is called chromatolysis (chrome = color, lysis = dissolving; Nissl granules stain blue, so they "lose their color")
• Cell body swells up
• Nucleolus increases in size (sign that the cell is working hard to make proteins for regeneration)

---

REGENERATIVE CHANGES AFTER NERVE INJURY

• The type of injury (neuropraxia heals fastest; neurotmesis is hardest)
• The distance between the injury site and the target organ
• The age of the patient (younger = better regeneration)

---

STAGES OF NERVE REGENERATION

• Chromatolysis reverses - Nissl granules reappear, nucleus returns to center
• The cell body resumes normal protein synthesis AND increases production of structural proteins (actin, tubulin) for the new axon

• The proximal axon (the part connected to the cell body) grows new sprouts from its cut end
• Multiple thin sprouts (growth cones) push forward from the proximal stump

• The sprouts enter the endoneural tubes left by the degenerating distal stump
• The Schwann cells in the bands of Büngner release neurotrophic factors (like NGF - Nerve Growth Factor) that attract and guide the growing axon tip
• The axon grows at approximately 1-4 mm per day along the Schwann cell tubes

• As the axon grows through the tubes, Schwann cells wrap around it and produce new myelin sheath
• The new myelin is initially thinner and shorter (between nodes) than the original, which is why the nerve does not conduct as fast as before

• If the growing axon reaches the target muscle or sense organ successfully, it reinnervates it
• The muscle or receptor gradually regains function

---

FACTORS DETERMINING SUCCESSFUL REGENERATION

---

CLINICAL IMPORTANCE

• Rate of regeneration: 1 mm per day (approximately)
• This is why patients wait months for recovery after nerve injury
• Example: If a nerve is cut 10 cm above the target muscle, recovery will take approximately 100 days + time for the muscle to regain function
• Neuroma formation - if the nerve cannot find the correct endoneural tube, the sprouts form a disorganized tangle called a neuroma, which is painful

---

Q7. Describe the Physiological Basis of Memory

INTRODUCTION

---

TYPES OF MEMORY

A. Based on Duration

• Duration: Fraction of a second (less than 1 second)
• Example: The brief image that persists after looking at something
• Stored in the primary sensory cortices

• Duration: Seconds to minutes (up to about 30 seconds without rehearsal)
• Capacity: Limited (7 ± 2 items)
• Example: Remembering a phone number long enough to dial it
• Location: Prefrontal cortex
• Mechanism: Maintained by reverberating circuits (neurons keep firing in a loop)

• Duration: Hours to lifetime
• Capacity: Essentially unlimited
• Requires consolidation (conversion from STM to LTM)
• Located in multiple cortical areas

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B. Based on Content (Types of Long-Term Memory)

• Memory for facts and events that can be consciously recalled and verbally described
• Depends heavily on the hippocampus

• Episodic memory - personal experiences and events (e.g., "I had pasta for lunch yesterday")
• Semantic memory - general knowledge and facts (e.g., "Paris is the capital of France")

• Memory for skills and habits that are performed automatically without conscious effort
• Does NOT require the hippocampus

• Procedural memory - motor skills and habits (e.g., riding a bicycle, typing) - involves basal ganglia and cerebellum
• Priming - prior exposure to a stimulus facilitates later response
• Conditioned reflexes - classical conditioning (e.g., Pavlov's dogs) - involves amygdala and cerebellum

---

BRAIN AREAS INVOLVED IN MEMORY

---

MEMORY CONSOLIDATION

1. Cellular/Synaptic consolidation - occurs within hours; involves changes in existing synaptic proteins (long-term potentiation)
2. Systems consolidation - occurs over weeks/years; involves gradual transfer of memory from the hippocampus to the cortex

---

CELLULAR MECHANISM - LONG-TERM POTENTIATION (LTP)

1. When a synapse is repeatedly activated, the postsynaptic neuron's response becomes stronger and faster over time.
2. NMDA receptors (N-methyl-D-aspartate receptors) are key. They are "coincidence detectors" - they only open when BOTH:
   • Glutamate is bound to them (presynaptic activation)
   • AND the postsynaptic membrane is already somewhat depolarized (postsynaptic activity) This requires both cells to be active simultaneously - which is the principle: "neurons that fire together, wire together" (Hebb's rule).
3. When NMDA receptors open, Ca²⁺ flows in to the postsynaptic cell.
4. Ca²⁺ activates intracellular enzymes (like CaMKII - calcium/calmodulin-dependent protein kinase II).
5. Short-term LTP: More AMPA receptors are inserted into the postsynaptic membrane → synapse becomes more sensitive → stronger response.
6. Long-term LTP: The genes in the nucleus are activated → new proteins are synthesized → NEW synapses are formed (structural change). This is the physical basis of long-term memory storage.

---

MOLECULAR BASIS OF MEMORY

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FORGETTING

1. Decay - synaptic changes fade if not used (use-it-or-lose-it)
2. Interference - new information interferes with old (retroactive interference) or vice versa (proactive interference)
3. Retrieval failure - the memory exists but cannot be accessed (like a file on a computer that cannot be found)

---

DISORDERS OF MEMORY

• Anterograde amnesia - cannot form NEW memories (damage to hippocampus, as in H.M.)
• Retrograde amnesia - cannot recall PAST memories (usually after head trauma)
• Alzheimer's disease - progressive loss of memory starting with recent memory; involves loss of cholinergic neurons in the nucleus basalis of Meynert and hippocampal atrophy

---

Q8. Describe the Physiological Basis of Parkinson's Disease

INTRODUCTION

---

PATHOLOGICAL BASIS (What Goes Wrong)

Degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc) → loss of dopamine in the striatum → imbalance of the basal ganglia circuits → poverty of movement

• The substantia nigra pars compacta (SNc) is a small nucleus in the midbrain that sends dopaminergic (dopamine-producing) axons to the striatum (caudate + putamen). This pathway is called the nigrostriatal pathway.
• In Parkinson's disease, these neurons progressively degenerate and die.
• Symptoms appear when 60-80% of SNc neurons have degenerated - by then, dopamine levels in the striatum are critically low.
• Microscopic hallmark: Lewy bodies - clumps of a protein called alpha-synuclein that accumulate inside the surviving neurons. These are the pathological signature of Parkinson's disease.

---

PHYSIOLOGICAL BASIS (Why the Symptoms Occur)

Normal State:

• Direct pathway: Striatum → inhibits GPi → releases (disinhibits) thalamus → thalamus activates motor cortex → movement FACILITATED
• Indirect pathway: Striatum → inhibits GPe → disinhibits STN → STN excites GPi → GPi inhibits thalamus → movement SUPPRESSED
• Dopamine from SNc balances both pathways:
  • D1 receptors (on direct pathway neurons) - dopamine ACTIVATES direct pathway
  • D2 receptors (on indirect pathway neurons) - dopamine INHIBITS indirect pathway
  • Net effect: dopamine promotes movement

In Parkinson's Disease (Dopamine Deficiency):

• No dopamine → D1 receptors not stimulated → striatum does NOT inhibit GPi
• GPi remains overactive → heavily inhibits thalamus
• Motor cortex is suppressed → LESS movement (bradykinesia, akinesia)

• No dopamine → D2 receptors not inhibited → striatum actively inhibits GPe
• GPe over-inhibited → releases STN from inhibition → STN becomes overactive
• STN over-excites GPi → GPi strongly inhibits thalamus → motor cortex further suppressed

---

DIAGRAM (Basal Ganglia in Parkinson's)

NORMAL:
Cortex → Striatum → [D1: → ↓GPi → ↑Thalamus → ↑Cortex] (Direct, excitatory)
                   [D2: → ↓GPe → ↑STN → ↑GPi → ↓Thalamus] (Indirect, inhibitory)
SNc Dopamine balances both

PARKINSON'S DISEASE (↓↓ Dopamine):
Direct pathway UNDERACTIVE → GPi overactive ↑↑
Indirect pathway OVERACTIVE → GPi overactive ↑↑ (double problem)
↓
THALAMUS excessively inhibited
↓
MOTOR CORTEX underactive
↓
Bradykinesia, Akinesia, Rigidity, Tremor

---

CLINICAL FEATURES

The TRAP Mnemonic:

• Physiological basis: Abnormal oscillatory activity in the basal ganglia-thalamocortical loop due to dopamine deficiency creates rhythmic involuntary signals.
• Features: "Pill-rolling" tremor at 4-6 Hz; present at REST; decreases during voluntary movement; disappears during sleep
• Appears first in one hand, then spreads

• Physiological basis: Loss of dopamine → imbalance of inhibitory and excitatory inputs to motor cortex → increased corticospinal output → both agonist and antagonist muscles are simultaneously contracted.
• "Lead-pipe rigidity" - uniform resistance throughout passive movement
• "Cogwheel rigidity" - jerky feel when tremor is superimposed on rigidity
• Affects all muscles: neck, trunk, limbs

• Physiological basis: Motor cortex is underactivated → cannot generate sufficient drive to initiate or execute movements.
• Akinesia = difficulty starting a movement (the car won't start)
• Bradykinesia = slowness of movement (the car moves very slowly)
• Signs: Masked face (hypomimia), soft voice (hypophonia), small handwriting (micrographia), reduced arm swing while walking, drooling (reduced swallowing)

• Physiological basis: Loss of postural reflexes due to basal ganglia dysfunction affecting the supplementary motor area, which programs automatic postural adjustments.
• Festinant gait: Short, shuffling steps with the body leaning forward; the patient accelerates involuntarily to prevent falling (looks like chasing their center of gravity)
• Simian posture: Stooped forward, neck flexed, arms at sides with slight elbow flexion, slight knee flexion
• Falls are common, especially backward (retropulsion)

---

OTHER FEATURES

• Constipation (may precede motor symptoms by years)
• Orthostatic hypotension (dizziness on standing)
• Excessive sweating (hyperhidrosis)
• Urinary urgency

• Loss of smell (anosmia) - often one of the FIRST symptoms, appearing years before motor symptoms

• REM sleep behavior disorder (patient acts out dreams - may punch or shout during sleep)
• Insomnia

• Depression (30-40% of patients)
• Dementia in later stages
• Hallucinations (often drug-induced)

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TREATMENT (Physiological Basis)

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Q9. Draw a Well-Labelled Diagram of Pain Pathway. Write About Referred Pain.

INTRODUCTION

1. Sensory component - tells WHERE the pain is and HOW INTENSE it is
2. Emotional/affective component - causes suffering and distress

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PAIN RECEPTORS (NOCICEPTORS)

• Pain receptors are free nerve endings found in almost all tissues
• They respond to mechanical, thermal, and chemical stimuli (tissue damage releases bradykinin, substance P, prostaglandins, K⁺, H⁺, histamine - these chemicals activate nociceptors)
• They show very little adaptation (don't get used to the pain) - this is a protective feature

---

PAIN FIBERS

---

PAIN PATHWAYS

A. LATERAL SPINOTHALAMIC TRACT (NEOSPINOTHALAMIC TRACT) - Fast Pain

• Free nerve endings (A-delta) in skin/tissue → travel in peripheral nerves → enter spinal cord via dorsal root → synapse in the dorsal horn (lamina I - marginal zone, and lamina V) and release glutamate and substance P

• Crosses to the opposite side immediately through the anterior white commissure
• Ascends as the lateral spinothalamic tract in the anterolateral white matter
• Goes up to the ventral posterolateral (VPL) nucleus of the thalamus
• Some fibers also go to the reticular formation and periaqueductal gray

• VPL thalamus → through internal capsule → primary somatosensory cortex (post-central gyrus) - areas 3, 1, and 2
• This pathway allows conscious localization, intensity, and quality (sharp vs. dull) discrimination of pain

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B. PALEOSPINOTHALAMIC TRACT - Slow Pain

• C fibers → dorsal root → synapse in laminae I, II (substantia gelatinosa), and V → release substance P (mainly)

• Crosses midline → ascends diffusely
• Synapses at multiple levels in the reticular formation of the brainstem, periaqueductal gray (PAG), and intralaminar nuclei of thalamus (not VPL)

• Diffuse projection to the limbic system (cingulate gyrus, insula), hypothalamus, and association cortex
• Responsible for: the emotional suffering, the unpleasantness, and the autonomic responses (elevated heart rate, blood pressure) associated with pain

---

DIAGRAM OF PAIN PATHWAY

POST-CENTRAL GYRUS               LIMBIC SYSTEM
(Somatosensory Cortex)           (Cingulate gyrus, insula)
  Localization, intensity          Suffering, emotion
        ↑                                ↑
   VPL THALAMUS                  Intralaminar thalamus
        ↑                                ↑
        └──────────────┬─────────────────┘
                INTERNAL CAPSULE
                       ↑
        ┌──────────────┴──────────────────┐
   LATERAL               PALEOSPINOTHALAMIC
SPINOTHALAMIC TRACT          TRACT
(Neospinothalamic)       (Diffuse pathway)
        ↑                        ↑
        └─────────┬──────────────┘
           ANTERIOR WHITE COMMISSURE
              (fibers cross here)
                  ↑
           DORSAL HORN
     (Lamina I, II, V - synapse)
          ↑           ↑
  A-delta fibers    C fibers
  (First pain)    (Second pain)
       ↑                ↑
  FREE NERVE ENDINGS (Nociceptors in skin, muscle, viscera)

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REFERRED PAIN

Definition

---

WHY DOES REFERRED PAIN OCCUR? (Mechanisms)

• Sensory fibers from a viscus (e.g., heart) and sensory fibers from a somatic area (e.g., left arm skin) both converge onto the SAME second-order neuron in the dorsal horn of the spinal cord
• The brain has "learned" from past experience that pain signals on that neuron usually come from the skin (because skin pain is much more common than cardiac pain)
• So the brain interprets the pain as coming from the skin even though it is actually coming from the heart
• This is called "misinterpretation" by the brain

• Pain from a viscus causes sensitization of the dorsal horn neurons in that spinal segment
• Any weak sensory input from the skin (same dermatome) is now amplified → the skin area becomes more sensitive to pain

---

DERMATOME RULE

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COMMON EXAMPLES OF REFERRED PAIN

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CLINICAL IMPORTANCE OF REFERRED PAIN

1. Understanding referred pain helps doctors diagnose the actual source of a disease even when the patient points to a different location
2. Cardiac pain referred to the arm is one of the most important clinical signs of a heart attack
3. Shoulder tip pain in a patient after abdominal surgery suggests diaphragmatic irritation (blood under the diaphragm)
4. Knowledge of dermatomes is essential to interpret referred pain correctly

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• Q10. Describe the structure & functions of cerebellum. Write briefly about cerebellar lesions.
• Q11. Briefly discuss the role of hypothalamus in thermoregulation.
• Q12. Describe the functions & disorders of cerebellum.

---

LONG ANSWER QUESTIONS — Q10, Q11, Q12

Reference: Sembulingam's Essentials of Medical Physiology

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Q10. Describe the Structure and Functions of Cerebellum. Write Briefly about Cerebellar Lesions.

INTRODUCTION

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PART 1: STRUCTURE OF THE CEREBELLUM

A. Gross (External) Anatomy

• Two lateral hemispheres - one on each side
• Vermis - a narrow, worm-like central strip connecting the two hemispheres

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B. Cerebellar Peduncles (Connections to Brainstem)

---

C. Internal Structure

• Mossy fibers - from spinal cord, pontine nuclei; synapse on granule cells; excitatory
• Climbing fibers - from inferior olivary nucleus; synapse directly on Purkinje cell dendrites in a powerful one-to-one relationship; important for motor learning

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D. Neural Circuit of the Cerebellar Cortex (Simple Explanation)

MOSSY FIBERS (from spinal cord, pons)
        ↓ (excite)
GRANULE CELLS → send PARALLEL FIBERS horizontally
        ↓ (excite)
PURKINJE CELLS (also modulated by basket/stellate cells)
        ↓ (inhibit via GABA)
DEEP CEREBELLAR NUCLEI
        ↓ (excite via glutamate)
THALAMUS → MOTOR CORTEX → SMOOTH VOLUNTARY MOVEMENT

Also:
CLIMBING FIBERS (from inferior olive)
        ↓ (powerful direct excitation for error detection)
PURKINJE CELLS

---

PART 2: FUNCTIONS OF THE CEREBELLUM

1. Coordination of Voluntary Movements

• What the motor cortex intended to do (efference copy received via corticopontocerebellar pathway)
• What is actually happening (sensory feedback via spinocerebellar pathways)

2. Maintenance of Muscle Tone

3. Maintenance of Posture and Equilibrium

4. Regulation of Gait

5. Coordination of Eye Movements

6. Motor Learning

7. Planning of Sequential Movements

---

PART 3: CEREBELLAR LESIONS

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Signs of Cerebellar Lesion - Mnemonic "DANISH"

• Inability to perform rapid, alternating movements
• Example: The patient cannot rapidly pronate and supinate the forearm, or cannot tap the fingers rapidly
• Cause: Cerebellum times the rapid switching between agonist and antagonist muscles. Without it, timing is lost.

• Incoordination of voluntary movements
• Gait ataxia: Wide-based, staggering, unsteady walk resembling an intoxicated person
• Limb ataxia: Clumsy, incoordinated limb movements
• Cause: Loss of the comparator and error-correction function

• Involuntary, rhythmic, back-and-forth movement of the eyeballs
• Most prominent on lateral gaze toward the side of the lesion
• Cause: Impaired coordination of the eye muscles by the flocculonodular lobe

• Tremor that is ABSENT at rest but appears during voluntary movement
• Becomes WORSE as the limb approaches the target
• Tested by: Finger-nose test (ask patient to touch their nose) and Heel-shin test
• Cause: Without the cerebellum, the brain cannot brake and fine-tune the movement as it nears the endpoint → oscillations occur near the target
• Contrast with Parkinson's tremor which is a RESTING tremor that DISAPPEARS during movement

• Slow, slurred, irregular, explosive speech
• Syllables are spoken with unequal force and unequal gaps between them
• Sounds like the patient is "scanning" each syllable separately
• Cause: Incoordination of the many muscles involved in speech production

• Reduced resistance to passive movement of the limbs
• Muscles feel floppy and soft
• Pendular knee jerk - the leg swings like a pendulum after the knee jerk (normally the cerebellum damps this oscillation)
• Cause: Loss of cerebellar control of gamma motor neurons reduces muscle spindle sensitivity → reduced reflex tone

---

Additional Signs

• The patient misjudges distance and overshoots or undershoots a target
• Hypermetria = overshooting; Hypometria = undershooting
• Tested by: Finger-nose test, finger-finger test

• When the patient holds an arm in flexion against the examiner's resistance and the resistance is suddenly removed, the arm flies up and may hit the face
• Normal: The cerebellum activates the antagonist (triceps) to stop the limb
• In cerebellar lesion: This braking response is absent → unchecked rebound

• Complex movements that normally happen simultaneously and smoothly are broken down into individual jerky components executed one after another

• A rhythmic nodding or bobbing of the head or trunk (seen in midline/vermis lesions)

---

Vermis Lesion vs. Hemisphere Lesion

---

Common Causes of Cerebellar Lesions

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Q11. Briefly Discuss the Role of Hypothalamus in Thermoregulation

INTRODUCTION

---

WHERE IN THE HYPOTHALAMUS?

---

HOW DOES THE HYPOTHALAMUS DETECT TEMPERATURE?

• Warm-sensitive neurons in the preoptic/anterior hypothalamus detect when blood temperature rises above normal
• Cold-sensitive neurons detect when blood temperature falls below normal
• The blood flowing through the hypothalamus is continuously monitored

• Warm receptors in the skin - respond to temperatures above ~30°C
• Cold receptors in the skin - respond to temperatures between 10-35°C (most active around 25°C)
• These send signals via sensory nerves to the hypothalamus to give advance warning before core temperature actually changes

---

RESPONSES TO HIGH BODY TEMPERATURE (Heat loss mechanisms)

1. Sweating (Most important in humans)

• The hypothalamus activates the sweat glands via the sympathetic cholinergic nerves
• Sweat evaporates from the skin surface → takes away large amounts of heat (latent heat of evaporation)
• One litre of sweat evaporation removes approximately 580 kcal of heat
• This is the MOST effective heat-loss mechanism in humans

2. Vasodilatation of Skin Blood Vessels

• Hypothalamus causes the blood vessels in the skin to dilate (widen)
• More warm blood from the core flows to the skin surface
• Heat is then lost by radiation and conduction from the skin
• This is why the skin looks flushed (red) when a person is hot

3. Reduction in Heat Production

• Metabolic rate is reduced
• Muscle tone and shivering stop
• Thyroid hormone secretion may decrease (reduces overall metabolic rate)

4. Behavioral Responses

• Person seeks cool environment, removes clothing, drinks cold water
• Assumes a spread-out posture to maximize skin surface area exposed to air

---

RESPONSES TO LOW BODY TEMPERATURE (Heat production and conservation mechanisms)

1. Shivering (Most important heat production mechanism)

• Hypothalamus activates the primary motor cortex and motor pathways → causes rapid, rhythmic, involuntary contractions of skeletal muscles
• These contractions produce no useful work but generate a large amount of heat
• Can increase heat production by 2-5 times the basal metabolic rate

2. Vasoconstriction of Skin Blood Vessels

• Hypothalamus causes skin blood vessels to constrict (narrow) via sympathetic adrenergic nerves
• Blood is shunted away from the skin surface to the core
• This reduces heat loss (skin acts as an insulator)
• The skin looks pale and cold; lips may turn blue (cyanosis)

3. Increased Metabolic Rate (Non-shivering thermogenesis)

• Hypothalamus signals the anterior pituitary → releases TSH → thyroid releases thyroxine → raises overall metabolic rate of all cells
• In newborns and some animals: brown adipose tissue (BAT) generates heat by uncoupling oxidative phosphorylation (non-shivering thermogenesis via uncoupling protein-1/thermogenin)
• Epinephrine from the adrenal medulla also increases cellular metabolism

4. Piloerection

• Tiny muscles at the base of hair follicles (arrector pili) contract → hairs stand up
• In animals with thick fur, this traps a layer of insulating air → reduces heat loss
• In humans, this produces "goosebumps" and has little practical insulating effect

5. Behavioral Responses

• Person seeks warm environment, adds clothing, curls up (reduces surface area)
• Huddling (in animals/humans together)

---

SET POINT OF TEMPERATURE

• If temperature is above the set point → heat loss mechanisms activated
• If temperature is below the set point → heat production and conservation mechanisms activated

---

FEVER (PYREXIA)

1. Infection/inflammation → macrophages/monocytes release pyrogens (fever-causing substances)
2. Exogenous pyrogens - bacterial lipopolysaccharide (LPS), toxins
3. These stimulate immune cells to release endogenous pyrogens - mainly:
   • Interleukin-1 (IL-1) - most important
   • Interleukin-6 (IL-6)
   • Tumor Necrosis Factor-alpha (TNF-α)
4. These endogenous pyrogens reach the hypothalamus (preoptic area) via the bloodstream
5. They stimulate the synthesis of prostaglandin E2 (PGE2) in the hypothalamus
6. PGE2 raises the hypothalamic set point to, say, 39°C
7. Now the body "thinks" 37°C is too cold → activates heat production mechanisms (shivering, vasoconstriction) until the new set point of 39°C is reached
8. The person feels cold and shivers even though the body temperature is actually rising → this is called a "chill" or rigor

• They inhibit the enzyme cyclooxygenase (COX) → block prostaglandin synthesis → set point drops back to normal → heat loss mechanisms activate (sweating, vasodilatation) → fever comes down

---

HEAT STROKE vs. HEAT EXHAUSTION

---

HYPOTHERMIA

• Below 35°C: Shivering, confusion
• Below 32°C: Shivering stops (muscles cannot produce enough heat)
• Below 28°C: Cardiac arrhythmias, loss of consciousness
• Below 20°C: Death

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SUMMARY TABLE

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Q12. Describe the Functions and Disorders of Cerebellum

INTRODUCTION

---

FUNCTIONS OF THE CEREBELLUM

1. Coordination of Voluntary Movements (Comparator Function)

• What was intended (the motor cortex's plan)
• What is actually happening (sensory feedback)

---

2. Maintenance of Muscle Tone

• Cerebellum → stimulates reticulospinal and vestibulospinal tracts → maintain baseline gamma motor neuron activity → muscle spindles remain sensitive → appropriate reflex tone in muscles

---

3. Maintenance of Posture and Equilibrium

• Receives input from the vestibular nuclei (which get signals from the semicircular canals and otolith organs of the inner ear)
• Also receives visual input
• Sends output to the fastigial nucleus → vestibular nuclei → spinal cord (via vestibulospinal tract) → adjusts axial and limb muscle tone to maintain balance

---

4. Regulation of Gait

• Left and right limb movements must be alternated rhythmically
• The cerebellum times the swing phase and stance phase of each step
• It ensures smooth, rhythmic, appropriately paced walking

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5. Coordination of Eye Movements

• Conjugate gaze - both eyes move together
• Vestibulo-ocular reflex (VOR) - keeps the visual image stable on the retina when the head moves (if you shake your head while reading, the eyes compensate automatically - this is VOR, controlled by cerebellum)

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6. Motor Learning

• First time you do a skill (writing, playing piano, riding a bike): many errors, requires conscious effort
• With practice: the cerebellum stores the timing patterns and gradually makes the movement automatic and error-free
• Mechanism: Long-term depression (LTD) at the parallel fiber-Purkinje cell synapse, guided by climbing fiber inputs (which act as error signals)

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7. Planning and Programming of Movements

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8. Contribution to Cognitive Functions

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DISORDERS OF THE CEREBELLUM

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D - Dysdiadochokinesia

• What it is: Inability to perform rapid, alternating movements
• Test: Ask the patient to rapidly tap the back and front of one hand alternately on the thigh (rapid pronation-supination), or to tap the foot rapidly
• Physiological basis: The cerebellum is responsible for timing the quick ON/OFF switching between agonist and antagonist muscle groups. When damaged, this timing is lost → movements become slow, irregular, and clumsily paced.
• Where: Both hemisphere and vermis lesions

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A - Ataxia

• What it is: Loss of coordination of movement
• Gait ataxia: Wide-based, staggering, unsteady walk - looks like a drunk person walking. Patient tends to fall to the side of the lesion.
• Truncal ataxia: Body sways and cannot sit or stand steadily (seen in vermis/midline lesions)
• Limb ataxia: Arms or legs move in an uncoordinated, clumsy way
• Physiological basis: Loss of the comparator function - no real-time error correction of movements
• Important: Unlike sensory ataxia, cerebellar ataxia is NOT worsened by closing the eyes (Romberg's sign is NEGATIVE in pure cerebellar ataxia)

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N - Nystagmus

• What it is: Involuntary, rhythmic oscillation (back-and-forth movement) of the eyeballs
• Type: Horizontal nystagmus is most common; most prominent on gaze toward the side of the lesion
• Physiological basis: The flocculonodular lobe normally coordinates eye muscle contraction to hold the eyes steady in a fixed position (gaze-holding). When this is lost, the eyes drift and then jerk back → nystagmus
• Test: Ask the patient to follow a pen to the left and right without moving the head.

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I - Intention Tremor

• What it is: A tremor that appears DURING voluntary movement and gets WORSE as the limb approaches the target. It is ABSENT at rest.
• Test: Finger-nose test (ask patient to touch their nose then the examiner's finger, alternating back and forth) - the finger oscillates and overshoots near the target
• Physiological basis: Normally, as the limb approaches its target, the cerebellum applies a "braking" signal to slow and stop the limb precisely. Without the cerebellum, there is no braking → the limb overshoots → the brain over-corrects → it undershoots → oscillations (tremor) develop
• KEY CONTRAST: Parkinson's tremor is a RESTING tremor (present at rest, disappears with movement). Cerebellar tremor is an INTENTION tremor (absent at rest, appears with movement).

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S - Scanning Speech (Cerebellar Dysarthria)

• What it is: Abnormal speech with slow, slurred, irregular rhythm and explosive bursts of syllables
• The patient seems to "scan" each syllable separately with unequal emphasis and irregular pauses between words
• Physiological basis: Speech requires precise timing and coordination of many muscles (tongue, lips, soft palate, larynx, diaphragm). The cerebellum normally coordinates all these. When damaged, this coordination is lost → irregular, poorly controlled speech
• Distinguish from: Dysphonia (weak voice from muscle weakness), Dysphasia (language problem from cortical damage)

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H - Hypotonia

• What it is: Reduced muscle tone; muscles feel soft and floppy when passively moved
• Pendular knee jerk: Instead of the normal single kick and stop after a knee tap, the leg swings back and forth like a pendulum (3-4 swings). Normally, the cerebellum damps this oscillation.
• Weak grip: The patient may have a weak, floppy handshake
• Physiological basis: Loss of cerebellar drive to gamma motor neurons → decreased muscle spindle sensitivity → reduced myotatic reflex tone

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Additional Signs:

• Inability to judge the correct range and force of movement
• Patient overshoots (hypermetria) or undershoots (hypometria) when reaching for objects

• Test: Ask patient to flex the forearm hard against resistance → suddenly remove resistance → the forearm flies upward and may hit the face
• Normally: Cerebellum activates the antagonist muscle (triceps) to check the movement
• In cerebellar lesion: No checking response → uncontrolled rebound

• Complex, multi-joint movements break down into clumsy sequential single-joint movements

• Ask patient to point at a target with eyes open, then close eyes and return to pointing position → patient consistently deviates to the side of the lesion

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CAUSES OF CEREBELLAR DISORDERS

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SUMMARY TABLE: CEREBELLAR LOBE AND CORRESPONDING SYNDROME

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• Title page with document name and reference (Sembulingam)
• Index table listing all 12 questions at a glance
• 12 fully written sections (one per question), each with:
  • Color-coded headings (dark blue H1, medium blue H2, green H3)
  • Formatted tables for every set of comparisons, classifications, and mnemonics
  • Bullet-point explanations in easy language
  • Page breaks between questions so each one starts fresh
• Quick Revision Card at the end - a single-page summary of all mnemonics and key facts (DANISH, TRAP, Gate Control theory, etc.)
• Running header and page-numbered footer on every page