---

NERVOUS SYSTEM - CNS PHYSIOLOGY

Complete University-Style MBBS Answers

---

1. CONDITIONED AND UNCONDITIONED REFLEXES (2 marks)

• Examples: withdrawal reflex, knee jerk, salivation on food in mouth, pupillary light reflex.

• Pavlov's experiment: Bell (neutral stimulus) repeatedly paired with food (natural/unconditioned stimulus) -> eventually bell alone causes salivation.
• Requires an intact cerebral cortex.
• Can be extinguished if reinforcement is withdrawn.

---

2. PUPILLARY REFLEX (2 marks)

1. Direct Light Reflex: Light in one eye -> pupil of same eye constricts.
2. Consensual (Indirect) Reflex: Light in one eye -> pupil of opposite eye also constricts.

• Fixed dilated pupils: CN III palsy, herniation
• Argyll Robertson pupil: syphilis - accommodates but doesn't react to light
• Horner's syndrome: miosis + ptosis + anhidrosis

---

3. ACCOMMODATION AND LIGHT REFLEX (2 marks)

1. Convergence - both eyes turn medially (medial rectus, CN III)
2. Increased lens curvature - ciliary muscle contracts -> suspensory ligaments relax -> lens becomes more convex -> increased refractive power
3. Pupillary constriction - reduces spherical aberration, increases depth of focus

---

4. WITHDRAWAL REFLEX (3 marks)

• Receptor: Free nerve endings (nociceptors)
• Afferent: A-delta and C fibers
• Center: Spinal cord (multiple segments)
• Efferent: Alpha motor neurons to flexor muscles
• Response: Flexion of stimulated limb + crossed extension of opposite limb

1. Noxious stimulus activates nociceptors
2. Impulses travel via dorsal root to spinal cord
3. Interneurons activate ipsilateral flexors + inhibit extensors (reciprocal inhibition)
4. Crossed extension reflex: contralateral extensors activated + flexors inhibited (to support body weight)

• Polysynaptic (many interneurons)
• After-discharge (response outlasts stimulus)
• Irradiation (spreads to more segments with stronger stimulus)
• Ipsilateral flexion + contralateral extension

---

5. AXON REFLEX (3 marks)

• A noxious stimulus activates one branch of a bifurcating C-fiber nociceptor
• Impulse travels antidromically (backward) up one branch and then orthodromically down the other branch
• Terminal releases Substance P and CGRP (Calcitonin Gene-Related Peptide)
• These mediators cause: vasodilation, increased vascular permeability, mast cell degranulation

1. Red line (30 sec): local vasodilation - direct capillary response
2. Flare (red halo): axon reflex - vasodilation via Substance P/CGRP
3. Wheal (swelling): increased capillary permeability - histamine from mast cells

• Component of neurogenic inflammation
• Involved in pain sensitization (hyperalgesia)
• Absent in peripheral neuropathy - used diagnostically (absent flare = postganglionic sympathetic/sensory denervation)

---

6. STRETCH REFLEX AND INVERSE STRETCH REFLEX (2 marks)

• Receptor: Muscle spindle (Ia afferents)
• Stimulus: Muscle stretch
• Response: Contraction of the same (homonymous) muscle
• Example: Knee jerk - patellar tendon tap stretches quadriceps -> Ia afferents -> monosynaptic -> alpha motor neurons -> quadriceps contracts
• Function: Maintains muscle tone; resists passive stretch; posture maintenance

• Receptor: Golgi Tendon Organ (GTO) - located at musculotendinous junction, activated by muscle tension (especially active contraction)
• Afferent: Ib fibers
• Pathway: Ib afferents -> inhibitory interneuron (disynaptic) -> inhibition of homonymous alpha motor neuron
• Response: Relaxation of the same muscle when tension becomes excessive
• Function: Protects muscle and tendon from damage due to excessive tension; "clasp-knife" rigidity in UMN lesions

---

7. POSTURAL REFLEXES - MECHANISM OF STANDING IN HUMAN BEINGS (8 marks)

Definition of Posture:

Postural Reflexes:

A. Static Reflexes (Maintained postures)

• Stretch reflex: Maintains muscle tone; resists gravity
• Positive supporting reaction: Weight bearing on limb -> flexors + extensors co-contract -> limb becomes rigid pillar of support

• Crossed extension reflex: When one limb flexes, opposite extends to bear weight

• Tonic neck reflexes (TNR): Changes in head position alter limb tone
  • Symmetrical TNR: Neck flexion -> arm flexors + leg extensors increase tone
  • Asymmetrical TNR: Head turns right -> right arm extensor tone increases, left flexor tone increases
• Tonic labyrinthine reflexes: Otolith organs respond to head position in space; supine -> maximum extensor tone; prone -> minimum extensor tone

B. Righting Reflexes (Restore normal posture):

• Labyrinthine righting reflex: Labyrinth signals head deviation -> head corrects to upright
• Neck righting reflex: Head moves -> neck afferents -> body follows
• Body righting reflex on body: Part of body on surface -> segmental rotation to right position

C. Statokinetic Reflexes (Moving states):

• Otolith reactions (linear acceleration)
• Semicircular canal reactions (angular acceleration)

Centers Controlling Posture:

1. Spinal cord: Stretch reflex, crossed extension (basic tone)
2. Medulla (Decerebrate level): Tonic labyrinthine + tonic neck reflexes operative
3. Midbrain: Righting reflexes present; fighting reactions
4. Cerebellum: Coordinates postural adjustments, equilibrium
5. Basal ganglia: Background muscle tone, postural set
6. Cerebral cortex: Voluntary postural control, anticipatory adjustments

Mechanism of Standing in Human Beings:

• Support against gravity (anti-gravity muscles: extensors of lower limb)
• Balance (equilibrium about center of gravity)
• Coordination between multiple systems

• Proprioceptors (muscle spindles, GTOs, joint receptors) - sense limb position and loading
• Vestibular apparatus (otoliths) - sense head position and linear acceleration
• Visual system - visual cues for spatial orientation
• Plantar cutaneous receptors - pressure under feet

• Gravity pulls body down -> stretch of antigravity muscles (quadriceps, gastrocnemius, paraspinals)
• Ia afferents from muscle spindles -> stretch reflex -> tonic contraction of extensors
• This forms the basic postural tone

• Gamma motor neurons maintain spindle sensitivity during muscle contraction (alpha-gamma co-activation)
• Ensures continuous Ia afferent feedback even when muscle contracts
• Allows fine-tuning of postural tone

• Lateral vestibulospinal tract: activates ipsilateral extensor motor neurons (anti-gravity)
• Medial vestibulospinal tract: controls neck and trunk muscles
• Otolith organs detect tilt -> correct posture by adjusting extensor tone

• Pontine reticulospinal (medial): facilitates extensor tone
• Medullary reticulospinal (lateral): inhibits extensor tone
• Balance between these determines posture

• Receives proprioceptive + vestibular inputs
• Compares intended vs actual posture (comparator function)
• Adjusts via fastigial nucleus -> vestibulospinal and reticulospinal tracts
• Anterior lobe: spinocerebellum - controls ongoing movement and posture

• Normal standing involves oscillation (sway) around center of gravity
• If forward sway: gastrocnemius activates (ankle strategy)
• If larger perturbation: hip strategy (hip flexors/extensors)
• If very large: stepping strategy

• Located anterior to S2 vertebra (in pelvis), maintained over base of support (feet)
• Line of gravity must fall within base of support to maintain balance

• Soleus/Gastrocnemius (prevent forward fall)
• Quadriceps (prevent knee buckling)
• Hip abductors (gluteus medius - prevent lateral sway)
• Trunk extensors (maintain upright spine)

---

8. PAIN (8 marks)

Definition:

Types of Pain:

1. Fast (acute/sharp) pain - A-delta fibers, sharp, well-localized
2. Slow (chronic/burning) pain - C fibers, poorly localized, burning/aching
3. Somatic pain - from skin, muscles, joints
4. Visceral pain - from internal organs
5. Referred pain - felt at a site distant from origin
6. Neuropathic pain - from nervous system damage

Receptors (Nociceptors):

• Free nerve endings throughout skin, periosteum, arterial walls, joint surfaces, falx, tentorium
• Types:
  • Mechanical nociceptors (respond to strong pressure)
  • Thermal nociceptors (extremes of temperature)
  • Polymodal nociceptors (respond to all noxious stimuli) - C fibers

Chemical Mediators (Sensitizers):

• Bradykinin - most potent algogenic substance
• Prostaglandins - sensitize nociceptors (PGE2, PGI2)
• Substance P - released from C fiber terminals
• 5-HT (serotonin) - from platelets
• Histamine - from mast cells
• K+ ions - elevated in damaged tissue
• H+ ions (low pH)
• ATP

Afferent Fibers:

Double Pain Sensation:

1. First pain (fast, sharp) - A-delta fibers - felt immediately
2. Second pain (slow, burning) - C fibers - felt 0.5-1 second later

Central Pathways:

1. Nociceptors -> A-delta/C fibers -> Dorsal horn (substantia gelatinosa - Laminae I, II, V)
2. Synapse with second-order neurons -> cross midline in anterior commissure
3. Ascend in Anterolateral system (Spinothalamic tract):
   • Neospinothalamic tract (A-delta): VPL of thalamus -> somatosensory cortex (S1, S2) - conscious localized pain
   • Paleospinothalamic tract (C fibers): intralaminar, medial thalamus -> hypothalamus, limbic system - affective-motivational component
4. Also: Spinoreticular, spinomesencephalic tracts

Modulation (Gate Control Theory - Melzack & Wall, 1965):

• Large diameter A-beta fibers (touch) activate inhibitory interneurons (SG cells) in dorsal horn
• This closes the "gate" to C fiber input
• Explains why rubbing an injured area relieves pain

Endogenous Pain Control (Descending Inhibition):

• PAG (Periaqueductal Gray) -> Nucleus Raphe Magnus -> Dorsal horn
• Neurotransmitters: Enkephalins, Beta-endorphin, Dynorphin (endogenous opioids), Serotonin, Norepinephrine
• Activated by: stress, exercise, acupuncture, opioid drugs

Hyperalgesia:

• Primary hyperalgesia: Increased sensitivity at injury site (peripheral sensitization - lowered nociceptor threshold due to inflammatory mediators)
• Secondary hyperalgesia: Increased sensitivity in surrounding tissue (central sensitization - NMDA receptor activation, wind-up phenomenon)

---

9. PATHWAY OF PAIN FROM SKIN TO SENSORY CORTEX + STRESS ANALGESIA (8 marks)

Complete Pain Pathway:

• Nociceptors in skin (free nerve endings)
• A-delta fibers (myelinated, fast) and C fibers (unmyelinated, slow)
• Cell bodies in Dorsal Root Ganglion (DRG)
• Enter spinal cord via dorsal root -> Lissauer's tract (dorsolateral fasciculus) -> travel 1-2 segments up/down

• Synapse in Dorsal Horn (Rexed laminae):
  • Lamina I (marginal zone): A-delta and C fibers
  • Lamina II (substantia gelatinosa): C fibers, modulation
  • Lamina V: Wide Dynamic Range (WDR) neurons - both nociceptive and non-nociceptive input
• Neurotransmitters released at 1st synapse: Glutamate (fast, AMPA receptors), Substance P (slow, NK1 receptors)
• Cross midline via anterior white commissure -> ascend contralaterally

• Synapse in VPL (Ventral Posterolateral) nucleus of thalamus

• Synapse in intralaminar nuclei (centromedian, parafascicular) and medial thalamus
• Also: periaqueductal gray, hypothalamus, amygdala

• From VPL -> Primary Somatosensory Cortex (S1) - areas 3, 1, 2 (postcentral gyrus)
  • Conscious awareness, localization, discrimination of pain
• From medial thalamus -> Secondary Somatosensory Cortex (S2), Anterior Cingulate Cortex, Insula
  • Affective (emotional/suffering) component of pain
  • Motivational aspects

Stress Analgesia:

• Stress activates Hypothalamus -> Pituitary -> beta-endorphin release (HPA axis)
• Stress also activates PAG (Periaqueductal Gray matter) in midbrain - rich in mu-opioid receptors
• Pathway: PAG -> Nucleus Raphe Magnus (NRM) -> Dorsal horn via dorsolateral funiculus
• NRM releases Serotonin -> activates enkephalinergic interneurons in dorsal horn
• Enkephalins inhibit pain transmission:
  • Presynaptically: block Ca2+ channels on C fiber terminals -> reduce Substance P + Glutamate release
  • Postsynaptically: open K+ channels -> hyperpolarize 2nd order neuron
• Blocked by naloxone (opioid antagonist) = opioid-mediated

• Not reversed by naloxone
• Involves: norepinephrine (descending noradrenergic system from locus coeruleus), cannabinoids, serotonin
• Occurs with prolonged stress

• Stress -> HPA axis -> cortisol + beta-endorphin (from POMC cleavage in anterior pituitary)
• Stress -> Sympathoadrenal axis -> epinephrine -> activates adrenergic receptors on nociceptors (inhibitory)
• Catecholamines activate alpha-2 adrenergic receptors in dorsal horn -> inhibit pain transmission

• Soldiers in battle not feeling bullet wounds until after fight
• Athletes continuing to play despite fractures
• Acupuncture analgesia (partly opioid-mediated)

---

10. REFERRED PAIN (3 marks)

• Cardiac ischemia -> left arm, jaw, epigastrium
• Appendicitis -> initially periumbilical
• Diaphragmatic irritation -> shoulder tip (C3,4,5 - phrenic nerve)
• Ureteric colic -> groin/testis
• Liver/Gallbladder -> right shoulder

• Visceral afferents and somatic afferents converge onto the same second-order neurons in the dorsal horn
• The brain cannot distinguish between the two sources
• Since somatic pain is more common, brain "projects" pain to the somatic (skin) area
• E.g., cardiac C fibers and T1-T4 somatic fibers both converge on spinothalamic neurons

• Visceral pain sensitizes (facilitates) WDR neurons in dorsal horn
• Normal tactile input from skin is then perceived as painful

• Branches of visceral afferents have collaterals to skin
• Less accepted

• May mislead diagnosis (cardiac pain felt in arm)
• Hyperalgesia at referred site (secondary hyperalgesia)
• Understanding dermatomes helps map visceral disease location

---

11. FAST AND SLOW PAIN (2 marks)

---

12. SOMATIC AND VISCERAL PAIN (2 marks)

---

13. SUPERFICIAL AND DEEP PAIN (2 marks)

---

14. WHY SOLDIERS DO NOT FEEL PAIN DURING WAR (3 marks)

• Extreme stress activates HPA axis -> ACTH + beta-endorphin released from anterior pituitary (both from POMC)
• Beta-endorphin acts on mu-opioid receptors in PAG, limbic system -> analgesia
• PAG activation -> descending inhibition via NRM -> enkephalins in dorsal horn -> suppress pain

• Fight-or-flight response -> massive catecholamine (epinephrine, NE) release
• These act on alpha-2 adrenergic receptors in dorsal horn -> inhibit pain
• Also activate descending noradrenergic systems

• High motivation and focused attention on survival suppresses pain perception
• Frontal cortex -> PAG -> descending modulation
• "Relief of being alive" - positive emotional state itself reduces pain (limbic-PAG connections)

• Attentional mechanisms: anterior cingulate cortex (ACC) diverts attention away from pain
• Active combat provides extreme cognitive distraction

• Cannabinoid system (endocannabinoids - anandamide, 2-AG) activated by stress
• Serotonin system activation

---

15. SLEEP DISORDERS (3 marks)

• Most common sleep disorder
• Difficulty initiating or maintaining sleep
• Types: Transient (days), Short-term (weeks), Chronic (>1 month)
• Causes: stress, anxiety, depression, medications, caffeine
• Treatment: sleep hygiene, CBT, short-term benzodiazepines/zolpidem

• Repetitive upper airway collapse during sleep
• Features: loud snoring, apneic episodes, daytime somnolence
• Risk: obesity, short neck, micrognathia
• Consequences: hypoxia, hypercapnia, pulmonary hypertension, cor pulmonale
• ECG during sleep: bradycardia during apnea, tachycardia after arousal
• Treatment: CPAP, weight loss, surgery

• Uncontrollable sleep attacks during the day
• Tetrad: excessive daytime sleepiness, cataplexy, sleep paralysis, hypnagogic hallucinations
• Cause: loss of hypocretin (orexin) neurons in hypothalamus
• Treatment: modafinil, methylphenidate, sodium oxybate

• Occurs in slow wave sleep (Stage 3-4 NREM)
• More common in children
• No recall of episode

• Loss of normal muscle atonia during REM
• Act out dreams - potentially violent
• Associated with Parkinson's disease, Lewy body dementia
• Treatment: clonazepam, melatonin

• Unpleasant leg sensations at rest -> irresistible urge to move
• Worse at night; relieved by movement
• Treatment: dopamine agonists (pramipexole)

---

16. REM SLEEP / NON-REM SLEEP (4 marks)

Non-REM (NREM) Sleep - 4 Stages:

• Decreased metabolic rate, HR, BP, RR, temperature
• Growth hormone secretion (peak in SWS)
• Restorative sleep - physically
• Somnambulism and night terrors occur here
• No/little dreaming (if dreams, vague and thought-like)

REM Sleep:

• Dreams - vivid, emotional, narrative dreams
• Rapid Eye Movements (phasic REM)
• Erections (penile/clitoral) - regardless of dream content
• Atonia - complete skeletal muscle paralysis (except diaphragm, extraocular muscles)
• Memory consolidation - important for learning

• Variable HR, BP, RR (irregular - phasic)
• Increased CBF and brain oxygen consumption
• Poikilothermy (temperature regulation lost)
• Increased penile/clitoral tumescence
• Increased ACh activity; decreased NE, 5-HT (aminergic neurons silent)

• Each cycle: ~90 minutes
• 4-6 cycles per night
• NREM dominates early night; REM dominates late night/early morning
• First REM period: ~10-20 min; later REM periods progressively longer (up to 60 min)

• REM deprivation: irritability, difficulty concentrating, hallucinations; "REM rebound" on recovery
• SWS deprivation: fatigue, muscle pain, decreased GH release

---

17. ECG CHANGES DURING SLEEP (4 marks)

During NREM Sleep:

• Gradual slowing of heart rate (vagal dominance)
• Slight decrease in BP
• Regular sinus rhythm
• ECG: normal sinus rhythm, slight HR decrease

• Lowest heart rate of night
• Lowest BP (dipping - ~10-20% BP reduction)
• Predominantly parasympathetic
• ECG: Sinus bradycardia, may show sinus arrhythmia (HR varies with respiration)
• Highest R-R interval variability (HRV)

During REM Sleep:

• Moderate HR, regular
• Low NE activity (locus coeruleus silent)

• Irregular HR - tachycardia and bradycardia alternating
• BP surges - corresponding to eye movement bursts
• Increased sympathetic activity in phasic REM
• ECG changes:
  • Marked sinus arrhythmia
  • Occasional sinus tachycardia (up to 100-120 bpm)
  • Brief bradycardia episodes
  • Potential ST changes (in patients with CAD - increased risk of ischemia)
  • Increased QT variability

Pathological Changes:

• OSA: Cyclical bradycardia (apnea) -> tachycardia (arousal); may cause nocturnal AF, ventricular ectopics
• Cardiac arrhythmias most common in early morning hours (predominantly REM) - circadian risk
• Nocturnal angina: ST depression during REM sleep in CAD patients
• Sudden cardiac death risk: Peaks in early morning hours - related to REM sympathetic surges

• Highest in NREM (SWS) - parasympathetic dominance
• Lowest in REM - mixed sympathetic/parasympathetic
• HRV analysis used to assess sleep quality and autonomic function

---

18. SLOW WAVE SLEEP (3 marks)

• Deepest stage of sleep - hardest to arouse
• Minimum heart rate and blood pressure of 24-hour period
• Minimum metabolic rate (BMR 15-20% below waking)
• Minimum core body temperature
• Minimum respiratory rate
• Maximum Growth Hormone secretion (pulsatile, first third of night)
• Maximum Prolactin secretion

1. Physical restoration - tissue repair, muscle growth (via GH)
2. Immune system enhancement - cytokine release, T-cell activity
3. Memory consolidation - declarative/explicit memories (hippocampal-neocortical dialogue)
4. Energy restoration - glycogen replenishment in brain (astrocytes)

• Somnambulism (sleepwalking)
• Sleep terrors (pavor nocturnus)
• Enuresis (bedwetting)
• Sleep talking (somniloquy)
• No or vague dream recall

• Markedly decreases with age (elderly have little SWS)
• Reduction correlates with cognitive decline

---

19. PARADOXICAL SLEEP (3 marks)

• EEG appears awake (activated, desynchronized)
• Brain metabolism and blood flow are HIGHER than in some waking states
• Yet: skeletal muscles are completely paralyzed (atonia), arousal threshold is high
• So: brain is highly active, but body is immobile

1. EEG: Low amplitude, mixed frequency waves (like waking)
2. Rapid Eye Movements: Bursts of conjugate horizontal/vertical eye movements (from FEF and superior colliculus)
3. Muscle Atonia: Inhibition of alpha motor neurons via glycinergic neurons in medullary reticular formation (sublateral dorsal nucleus). Protects from acting out dreams.
4. Dreaming: Vivid, narrative, emotional, sometimes bizarre dreams
5. Pontine-Geniculate-Occipital (PGO) waves: Bursts originating in pons -> LGN -> occipital cortex; precede eye movements; possibly trigger dreams
6. Neurochemistry: Cholinergic activity HIGH (from pedunculopontine nucleus, laterodorsal tegmental nucleus); Aminergic (NE, 5-HT) activity SILENT

---

20. POSTURE - ROLE OF DIFFERENT PARTS OF BRAIN IN REGULATION OF POSTURE (8 marks)

Definition:

Brain Regions and their Role:

1. SPINAL CORD:

• Stretch reflex (monosynaptic) - maintains basic tone in anti-gravity muscles
• Crossed extension reflex - alternate limb support during walking
• Gamma motor system - maintains spindle sensitivity during movement
• Performs: basic reflex postural adjustments
• Spinal animal can maintain some tone but no true standing

2. MEDULLA OBLONGATA:

• Contains reticular formation (medial reticular formation = pontine/medullary)
• Lateral vestibular nucleus (Deiters' nucleus) -> lateral vestibulospinal tract -> ipsilateral extensor facilitation
• Tonic labyrinthine reflexes (from otolith organs, integrated at medullary/midbrain level)
• Tonic neck reflexes (from neck proprioceptors - cervical spine joints)
• Decerebrate animal (section between red nucleus and vestibular nucleus): decerebrate rigidity - exaggerated extensor tone in all limbs

3. MIDBRAIN:

• Red nucleus -> rubrospinal tract -> facilitates flexors
• Superior colliculus -> visual orientation and postural adjustment to visual stimuli
• Righting reflexes integrated here:
  • Labyrinthine righting reflex (otolith -> corrects head position)
  • Neck righting on body
  • Body on head righting
  • Optical righting reflex (via visual cortex)
• Decorticate animal: righting reflexes present; can right itself if placed on side

4. CEREBELLUM:

• Receives: vestibular input, proprioceptive input (spinocerebellar tracts), visual input
• Flocculonodular lobe (Vestibulocerebellum):
  • Equilibrium and balance
  • Projects to: Fastigial nucleus -> vestibular nuclei -> vestibulospinal tracts
  • Lesion: truncal ataxia, wide-based gait, inability to stand (astasia)
• Anterior lobe (Spinocerebellum):
  • Receives spinal proprioception (DSCT, VSCT)
  • Regulates ongoing movement and muscle tone
  • Projects via globose/emboliform nuclei -> red nucleus -> rubrospinal; also to reticular formation
  • Lesion: hypotonia, dysmetria, intention tremor
• Acts as comparator: Compares intended (from motor cortex via corticopontocerebellar) with actual position (from proprioceptors) -> error correction signal

5. BASAL GANGLIA:

• Input: Striatum (caudate + putamen) receive cortical input
• Output: Via globus pallidus -> thalamus -> motor cortex
• Functions:
  • Scaling and initiation of voluntary movements
  • Setting background postural tone
  • Procedural learning (habit formation)
  • Inhibits unwanted movements
• Direct pathway (striatum -> GPi, SNr): facilitates movement
• Indirect pathway (striatum -> GPe -> STN -> GPi): inhibits movement
• Parkinson's disease: loss of substantia nigra dopamine -> overactive indirect pathway -> reduced movement, increased rigidity, flexed posture

6. CEREBRAL CORTEX (Motor Cortex, Area 4):

• Primary Motor Cortex (M1): Voluntary postural adjustments; corticospinal tract controls distal movements
• Premotor Cortex (Area 6): Postural set, anticipatory postural adjustments (APAs) before voluntary movement
• Supplementary Motor Area (SMA): Internally generated movements, bilateral coordination
• Posterior Parietal Cortex (Areas 5, 7): Sensory-motor integration for posture
• Cortex allows: anticipatory postural adjustments, adapting posture to context, learning new postural strategies

7. VESTIBULAR SYSTEM:

• Semicircular canals: Detect angular acceleration -> corrective postural responses
• Utricle/Saccule (otolith organs): Detect head tilt and linear acceleration
• Projects to: Vestibular nuclei -> Lateral vestibulospinal tract (excites ipsilateral extensors) + Medial vestibulospinal (controls neck/trunk)

Integration:

• Posture requires integration of sensory information (proprioceptive, vestibular, visual) processed at multiple levels
• Higher centers modulate lower ones
• Cerebellum acts as major coordinator/comparator
• Loss of any level leads to characteristic postural abnormalities

---

21. TESTS TO DETECT CEREBELLAR DYSFUNCTION (3 marks)

1. Finger-Nose Test: Patient touches own nose then examiner's finger alternately; looks for past-pointing (dysmetria) and intention tremor
2. Heel-Shin Test: Heel of one foot placed on opposite knee, slid down shin; dysmetria and ataxia detected
3. Rapid Alternating Movements (Dysdiadochokinesia): Rapid pronation-supination of hands - irregular, clumsy in cerebellar disease
4. Romberg's Test: Stand with feet together, eyes open then closed; cerebellar lesion: falls with eyes OPEN (proprioception normal, but cerebellar coordination lost); sensory ataxia: falls only with eyes closed
5. Gait: Broad-based, staggering (truncal) or unsteady (ataxic)
6. Rebound Phenomenon (Holmes): Resisted arm flexion suddenly released; normal = arm stopped by triceps; cerebellum = overshoots (defect of check reflex)
7. Nystagmus: Horizontal nystagmus toward side of lesion

---

22. BROWN-SEQUARD SYNDROME (3 marks)

1. UMN paralysis below lesion
2. Loss of proprioception, vibration, discriminative touch below lesion
3. LMN weakness AT the level of the lesion

1. Loss of pain and temperature (1-2 levels below lesion) - because spinothalamic crosses within 2 segments

• Right: UMN paralysis below T10, loss of vibration/proprioception below T10
• Left: Loss of pain/temperature below T11-12

---

23. ROLE OF CEREBELLUM IN VOLUNTARY MOVEMENT + TESTS FOR CEREBELLAR DYSFUNCTION (4+4 = 8 marks)

Part A: Role of Cerebellum in Voluntary Movement (4 marks)

• 3 functional divisions:
  1. Vestibulocerebellum (flocculonodular lobe): balance, eye movements
  2. Spinocerebellum (anterior lobe + vermis): limb/trunk coordination, tone
  3. Cerebrocerebellum (Pontocerebellum) (lateral posterior lobe): planning and timing of skilled movements

1. Planning phase: Motor cortex (M1, PM, SMA) sends "efference copy" of motor command -> via corticopontine fibers -> pontine nuclei -> contralateral cerebellar cortex (via mossy fibers)
2. Cerebellar processing: Purkinje cells receive:
   • Mossy fiber input (from pontine nuclei - motor plan)
   • Climbing fiber input (from inferior olive - error signal)
   • Parallel fiber input (from granule cells) Purkinje cells are GABAergic -> inhibit deep cerebellar nuclei
3. Feedback loop:
   • Spinocerebellar tracts (DSCT, VSCT) carry proprioceptive feedback during movement
   • Cerebellum compares INTENDED movement (from cortex) with ACTUAL movement (from proprioceptors)
   • Error signal computed -> correction via thalamus -> motor cortex
4. Output:
   • Deep cerebellar nuclei (dentate, interposed, fastigial)
   • Dentate nucleus -> VL thalamus -> motor cortex (via dentatothalamocortical tract) - for limb coordination
   • Fastigial nucleus -> vestibulospinal + reticulospinal - for posture/balance

1. Timing - precise timing of muscle activation sequences
2. Coordination - synergistic muscle activation
3. Scaling - appropriate force and range of movement
4. Error correction - real-time adjustment via feedback
5. Smooth execution - prevents oscillations/tremors
6. Learning - motor learning (via climbing fiber-Purkinje cell plasticity = long-term depression, LTD)

• Dysdiadochokinesia
• Ataxia
• Nystagmus
• Intention tremor
• Scanning speech (dysarthria)
• Hypotonia

Part B: Tests for Cerebellar Dysfunction (4 marks)

• Patient alternately touches own nose and examiner's moving finger
• Dysmetria (past-pointing): inaccurate movement beyond target
• Intention tremor: tremor increases as finger approaches target (opposite of Parkinsonian resting tremor)

• Supine: heel of one foot placed on opposite knee, slid down shin
• Cerebellar: ataxic, wobbly, overshoots knee (dysmetria)

• Rapid alternating supination-pronation of hands on thigh
• Alternatively: rapid finger tapping, rapid hand opening-closing
• Cerebellar: slow, irregular, clumsy rhythm

• Stand with feet together, arms at sides
• Eyes open then closed
• Cerebellar ataxia: positive Romberg not required - falls with eyes OPEN (unlike sensory ataxia where only falls with eyes closed)
• Note: True positive Romberg = only falls with eyes closed = dorsal column/sensory deficit

• Patient flexes elbow against resistance; examiner suddenly releases resistance
• Normal: triceps quickly contracts to stop the arm
• Cerebellar: arm overshoots due to delayed check (dysmetria)

• Ask patient to walk normally -> wide-based, staggering, ataxic gait
• Tandem gait (heel-to-toe): very sensitive for cerebellar dysfunction
• Falls to side of lesion in unilateral cerebellar lesion

• Jerk nystagmus with fast phase away from examiner
• Gaze-evoked nystagmus toward side of lesion
• Vertical nystagmus suggests vestibulocerebellum lesion

• Normal knee jerk dampens after 1-2 swings
• Cerebellar: pendular (oscillating multiple times) due to hypotonia and loss of damping by Golgi tendon organ circuit

• Scanning speech: slow, monotonous, broken into syllables
• Dysarthria: slurred, ataxic speech

• Macrographia (large, irregular writing)
• Unable to draw straight line or spiral neatly

---

24. FEED BACKWARD AND FEED FORWARD INHIBITION (2 marks)

• Also called "negative feedback" or Renshaw cell inhibition
• A neuron excites an inhibitory interneuron (Renshaw cell) which circles back to inhibit the same neuron
• Mechanism: Alpha motor neuron fires -> collateral branch -> excites Renshaw cell -> Renshaw cell releases glycine -> inhibits the SAME alpha motor neuron
• Function: Limits excessive firing of motor neurons; prevents sustained tetanic contraction; sets upper limit of firing frequency; smooths muscle contraction

• Also called lateral inhibition or recurrent inhibition of neighboring cells
• An excited neuron inhibits its neighbors via inhibitory interneurons
• Mechanism: Active neuron fires -> excites inhibitory interneuron -> inhibits adjacent neurons
• Function: Sharpens signal (improves contrast/discrimination); prevents spread of excitation; focuses the active pathway ("spotlight effect")
• Example: In somatosensory cortex - enhances two-point discrimination; in cerebellum - basket and stellate cells provide feed-forward inhibition of Purkinje cells

---

25. NEURONAL CIRCUIT OF CEREBELLUM (4 marks)

Inputs to Cerebellum:

1. Mossy fibers: From spinal cord (spinocerebellar), vestibular nuclei, pontine nuclei (corticopontocerebellar) -> synapse on granule cells
2. Climbing fibers: From inferior olivary nucleus ONLY -> synapse directly on Purkinje cell dendrites (powerful 1:1 connection, acts as error signal)

Cerebellar Cortex Layers:

Circuit:

1. Mossy fiber -> excites Granule cell (glutamate)
2. Granule cell axon ascends to molecular layer, bifurcates -> Parallel fiber (runs perpendicular to Purkinje cell dendrites)
3. Parallel fiber -> weakly excites Purkinje cell (glutamate)
4. Parallel fiber also excites Basket cell + Stellate cell (inhibitory interneurons)
5. Basket + Stellate cells -> INHIBIT neighboring Purkinje cells (GABA) [Feedforward inhibition]
6. Climbing fiber (from inferior olive) -> powerfully excites Purkinje cell (one climbing fiber per Purkinje cell, ~100 synapses)
7. Purkinje cell -> output to Deep Cerebellar Nuclei (DCN) - GABA = INHIBITORY
8. Mossy fiber collaterals also directly excite DCN (excitatory backup)
9. Golgi cells in granular layer -> inhibit granule cells (GABA) [feedback inhibition of mossy fiber input]

Deep Cerebellar Nuclei Output:

• Fastigial -> vestibulospinal, reticulospinal
• Interposed (globose + emboliform) -> rubrospinal
• Dentate -> VL thalamus -> motor cortex

Plasticity (Motor Learning):

• Repeated activation of climbing fiber (error signal) while parallel fiber active -> Long-term Depression (LTD) at parallel fiber-Purkinje synapse
• Result: Purkinje cell LESS responsive to parallel fiber -> less inhibition of DCN -> motor program modified
• This is the cellular basis of cerebellar motor learning

---

26. ALPHA-GAMMA LINKAGE (CO-ACTIVATION) (3 marks)

• Alpha (α) motor neurons: Innervate extrafusal muscle fibers (the main working fibers)
• Gamma (γ) motor neurons: Innervate intrafusal fibers of muscle spindle (specifically the polar contractile ends)
• Muscle spindle: Detects muscle length and change in length via Ia (annulospiral) and II (flower spray) afferents

• When alpha motor neurons fire -> extrafusal fibers contract -> muscle shortens
• If spindle (intrafusal) was not also shortened -> Ia afferents would go SILENT (spindle "unloaded")
• This would break the feedback loop for further contraction and tone regulation

• When voluntary movement is initiated, BOTH alpha AND gamma motor neurons are activated simultaneously (co-activation)
• This is because: Descending motor commands (corticospinal, reticulospinal, vestibulospinal) activate both alpha and gamma neurons
• Gamma activation contracts the intrafusal fiber poles -> keeps spindle taut -> maintains Ia afferent firing even as muscle shortens
• Result: Spindle remains sensitive throughout the contraction range; continuous proprioceptive feedback maintained

1. Maintains spindle sensitivity during voluntary contraction
2. Ensures muscle tone is regulated throughout movement
3. Provides servo-assistance - if extra load applied, spindle detects and reflexly increases contraction
4. Gamma loop: Higher centers set gamma gain -> determines muscle stiffness and resistance to stretch
5. Fusimotor (gamma) system is independently controlled for fine motor tasks

• γd: innervate nuclear bag fibers -> regulate velocity sensitivity (Ia dynamic response)
• γs: innervate nuclear chain fibers -> regulate length sensitivity (static response, II fibers)

---

27. PRESYNAPTIC INHIBITION AND PRESYNAPTIC FACILITATION (2 marks)

• An inhibitory axon synapses on the terminal bouton of a presynaptic axon (axo-axonic synapse)
• Does NOT hyperpolarize the postsynaptic neuron directly
• Mechanism: Inhibitory neurotransmitter (usually GABA) acts on presynaptic terminal -> opens Cl- or K+ channels -> partial depolarization (depolarization block) or reduces Ca2+ influx -> less neurotransmitter released -> less excitation of post-synaptic neuron
• Mediator: GABA-B receptors on presynaptic terminal (inhibit adenylyl cyclase, reduce Ca2+ entry)
• Advantage over postsynaptic inhibition: Selectively inhibits ONE input to a postsynaptic neuron without affecting excitability to other inputs; allows selective gating of afferent information
• Example: Ia afferents to alpha motor neurons are presynaptically inhibited during voluntary movement (prevents stretch reflex from opposing voluntary contraction)

• A facilitatory synapse on a presynaptic terminal enhances neurotransmitter release
• Mechanism: Reduces K+ conductance -> action potential is prolonged -> more Ca2+ enters -> more NT released
• Less commonly discussed but important in sensitization phenomena

---

28. CEREBELLUM AS A COMPARATOR (3 marks)

1. Intended movement signal:
   • Motor cortex sends motor command to muscles
   • Simultaneously sends a copy (efference copy/corollary discharge) via corticopontocerebellar fibers -> pontine nuclei -> mossy fibers -> cerebellar cortex
   • This represents "what the brain WANTS to happen"
2. Actual movement signal:
   • Proprioceptors (muscle spindles, GTOs), vestibular apparatus send feedback
   • Reaches cerebellum via spinocerebellar tracts (DSCT, VSCT), vestibulocerebellar fibers
   • This represents "what is ACTUALLY happening"
3. Comparison:
   • Cerebellum computes the error (difference between intended and actual)
   • If movement is accurate: no significant output change
   • If error detected: cerebellum generates corrective signal -> via DCN -> VL thalamus -> motor cortex -> adjusted motor output
4. Internal Forward Model:
   • Cerebellum builds internal models of musculoskeletal dynamics
   • Predicts sensory consequences of motor commands
   • Allows predictive (feed-forward) corrections without waiting for feedback delay

• Climbing fibers from inferior olive carry error signals (mismatch between predicted and actual)
• Induce Long-term Depression (LTD) at parallel fiber-Purkinje synapses -> adjusts future motor programs
• Cerebellar timing: precisely times muscle activation sequences (millisecond precision)

---

29. BABINSKI SIGN AND ITS SIGNIFICANCE (3 marks)

• Test: Firm stroke along lateral sole of foot from heel to ball, then across metatarsal heads
• Normal (negative Babinski) = Flexor plantar response: Downward (plantar flexion) curling of toes, especially big toe

• Big toe dorsiflexes (extends) upward + fanning/abduction of other toes
• Also: Sometimes withdrawal at hip and knee (triple flexion response)

• Infants under 18 months - corticospinal tract not yet myelinated -> positive Babinski is NORMAL

• Indicates Upper Motor Neuron (UMN) lesion = damage to corticospinal tract at any level
• Causes: Stroke, brain tumor, MS, spinal cord injury, ALS, meningitis

• Normally, corticospinal tract SUPPRESSES the extensor plantar response via interneurons in spinal cord
• When corticospinal inhibition is removed -> primitive/phylogenetically older withdrawal reflex reemerges
• The response is actually a fragment of the flexion-withdrawal reflex (dorsiflexion of big toe is the equivalent of toe extension in quadrupeds - actually brings toe away from sole)

• Oppenheim sign: Firm pressure on tibia -> same response
• Gordon sign: Squeezing calf -> same response
• Chaddock sign: Stroking lateral ankle

---

30. CEREBELLAR FUNCTION TESTS (3 marks)

1. Finger-nose test - Dysmetria, intention tremor
2. Heel-shin test - Lower limb dysmetria
3. Dysdiadochokinesia - Rapid alternating movements (supination/pronation)
4. Romberg test - Falls with eyes open = cerebellar; eyes closed only = sensory ataxia
5. Rebound phenomenon (Holmes) - Loss of check reflex
6. Tandem gait - Heel-to-toe walking (most sensitive for cerebellar gait)
7. Nystagmus - Gaze-evoked, horizontal toward lesion side
8. Pendular knee jerk - Multiple oscillations (hypotonia)
9. Speech - Scanning dysarthria
10. Drawing test - Spiral drawing test

---

31. STRUCTURE AND FUNCTION OF MUSCLE SPINDLE (3 marks)

1. Nuclear Bag fibers (2 per spindle):
   • Larger, nuclei clustered in equatorial bag region
   • Type 1 (dynamic): detect rate of change of length (velocity)
   • Type 2 (static): detect steady state length
2. Nuclear Chain fibers (3-5 per spindle):
   • Smaller, nuclei in single row (chain)
   • Detect static length

• Afferent:
  • Ia (primary) annulospiral endings: Wrap around equatorial region of BOTH bag and chain fibers; respond to both velocity AND static length (dynamic + static)
  • II (secondary) flower-spray endings: On nuclear chain (and static bag 2); respond to STATIC length only
• Efferent (Fusimotor):
  • Gamma-dynamic (γd): To nuclear bag1 fibers -> increases dynamic sensitivity of Ia
  • Gamma-static (γs): To bag2 and chain fibers -> increases static sensitivity

1. Length detection: Signals absolute muscle length (static response of II fibers)
2. Velocity detection: Signals rate of stretch (dynamic response of Ia fibers)
3. Stretch reflex: Ia afferents -> monosynaptic -> alpha motor neurons -> contract same muscle (resist stretch)
4. Muscle tone: Continuous Ia input maintains background alpha motor neuron activity
5. Alpha-gamma co-activation: Maintains spindle sensitivity during voluntary contraction
6. Servo-assistance: Gamma system allows higher centers to set muscle stiffness

---

32. MEMORY - MOLECULAR BASIS + ALZHEIMER'S DISEASE (4+4 = 8 marks)

Part A: Memory and Molecular Basis (4 marks)

• Short-term (Working) memory: Seconds-minutes; hippocampus not needed; PFC; capacity ~7 items (Miller's Law)
• Long-term memory: Days-years; requires consolidation; hippocampus essential

• Repeated activation -> Ca2+ accumulation in presynaptic terminal
• More neurotransmitter released with each successive stimulus
• Reverberating circuits (re-entrant loops in neuronal networks) maintain activity

1. Repeated high-frequency stimulation of afferents (tetanic stimulation)
2. Glutamate acts on AMPA receptors + NMDA receptors
3. NMDA receptors blocked by Mg2+ at resting membrane potential
4. Repeated stimulation -> AMPA-mediated depolarization -> removes Mg2+ block from NMDA
5. NMDA receptor opens -> Ca2+ influx into postsynaptic neuron
6. Ca2+ activates CaM Kinase II (CaMKII) + PKC (protein kinase C)
7. Early LTP (short-term): Phosphorylation of existing AMPA receptors -> increased sensitivity; insertion of new AMPA receptors into synapse
8. Late LTP (long-term): Activation of PKA -> CREB (cAMP Response Element Binding protein) phosphorylation -> gene transcription -> new protein synthesis -> structural changes (dendritic spine growth, new synapses = synaptogenesis)

• CREB - master transcription factor for long-term memory
• BDNF - Brain-derived neurotrophic factor; promotes synaptic strengthening
• Arc/Arg3.1 - immediate early gene; stabilizes AMPA receptors
• CaMKII - "molecular memory switch" (remains active after Ca2+ transient)

• Critical for consolidation of declarative memory (transfer from short-term to long-term)
• Papez circuit: Hippocampus -> fornix -> mammillary bodies -> anterior thalamus -> cingulate gyrus -> entorhinal cortex -> hippocampus
• H.M. (famous patient): bilateral hippocampectomy -> anterograde amnesia (couldn't form new declarative memories); old memories and implicit memory intact

Part B: Alzheimer's Disease (4 marks)

• APP (Amyloid Precursor Protein) normally cleaved by alpha-secretase (non-amyloidogenic)
• In AD: abnormal processing by beta-secretase (BACE1) + gamma-secretase
• Produces Aβ1-42 (amyloid beta 42) -> aggregates -> oligomers (most toxic) -> fibrils -> plaques
• Aβ oligomers impair LTP, synaptic transmission, and cause neuronal death
• Extracellular deposits (senile plaques)

• Tau protein normally stabilizes microtubules in axons
• In AD: hyperphosphorylation of tau -> tau detaches from microtubules -> aggregates -> paired helical filaments (PHF) -> intracellular tangles
• NFTs disrupt axonal transport -> neuronal death
• Progress: Entorhinal cortex -> hippocampus -> neocortex (Braak staging)

• Nucleus basalis of Meynert (basal forebrain) - major cholinergic supply to cortex
• Profound loss in AD -> reduced ACh -> memory impairment
• Cholinergic hypothesis of AD memory (basis of treatment)

• Activated microglia, astrocytes, complement system around plaques

• APOE ε4 allele: strongest genetic risk factor for sporadic AD
• Familial AD: Mutations in APP, PSEN1 (Presenilin 1), PSEN2 -> increased Aβ42 production
• Down syndrome (Trisomy 21): APP gene on chromosome 21; invariably develop AD pathology by age 40

• Insidious onset; early short-term memory loss (hippocampal)
• Language difficulty (anomia)
• Visuospatial deficits
• Executive dysfunction
• Later: personality change, agitation, hallucinations, loss of ADLs

• Cholinesterase inhibitors (donepezil, rivastigmine, galantamine): Inhibit AChE -> more ACh at synapse -> symptomatic
• Memantine: NMDA receptor antagonist -> reduces excitotoxicity; moderate-severe AD
• Lecanemab, Donanemab (anti-amyloid monoclonal antibodies) - recently approved, slow progression in early AD

---

33. EXPLICIT AND IMPLICIT MEMORY (3 marks)

---

34. EPSP AND IPSP (3 marks)

• A graded depolarization of the postsynaptic membrane produced by an excitatory neurotransmitter
• Mechanism: Excitatory NT (glutamate, ACh) opens ligand-gated cation channels (Na+/K+ mixed, or Ca2+) -> net inward current -> membrane depolarizes (toward threshold ~-55 mV from resting -70 mV)
• Amplitude: Small (~0.5-1 mV); sub-threshold individually
• Summation:
  • Temporal: Multiple EPSPs from same input in quick succession summate
  • Spatial: EPSPs from multiple inputs at same time summate
• Combined summation can reach threshold -> action potential generated at axon hillock

• A graded hyperpolarization (or stabilization) of postsynaptic membrane produced by an inhibitory neurotransmitter
• Mechanism 1: GABA-A receptor -> opens Cl- channels -> Cl- influx -> hyperpolarization (membrane moves away from threshold; if ECl ~ -70 mV, may just "clamp" membrane, still inhibitory)
• Mechanism 2: Glycine -> opens Cl- channels (in spinal cord)
• Mechanism 3: GABA-B, K+ receptors -> K+ efflux -> hyperpolarization
• Amplitude: ~1 mV hyperpolarization
• IPSP makes it HARDER for EPSPs to reach threshold

---

35. WALLERIAN DEGENERATION (2 marks)

• Axoplasm becomes granular; mitochondria swell

• Schwann cells proliferate and phagocytose myelin debris
• Macrophages recruited - clear cellular debris

• Schwann cells form bands of Bungner (cylindrical tubes) along original nerve course
• These guide regenerating axons

• Cell body swells, Nissl substance disperses (central chromatolysis), nucleus migrates to periphery

• Axon sprouts from proximal stump (~1-4 mm/day)
• Sprouts grow along Bands of Bungner
• New myelin formed by Schwann cells (thinner, shorter internodes)
• Function may return (months to years depending on distance)

---

36. INHIBITORY NEUROTRANSMITTERS (3 marks)

• Most important inhibitory NT in brain (supratentorial)
• Synthesized from glutamate by glutamic acid decarboxylase (GAD)
• GABA-A: Ionotropic, Cl- channel -> rapid hyperpolarization (fast IPSP)
• GABA-B: Metabotropic, K+ channel or Ca2+ channel inhibition -> slow IPSP
• GABA-C: In retina
• Drugs: Benzodiazepines (enhance GABA-A), barbiturates, alcohol (enhance GABA-A), vigabatrin (inhibit GABA-T)

• Primary inhibitory NT in spinal cord and brainstem
• Acts on glycine receptor -> Cl- influx -> hyperpolarization
• Renshaw cells use glycine to inhibit alpha motor neurons (recurrent inhibition)
• Strychnine: Glycine receptor antagonist -> convulsions, tetanic spasm
• Deficiency: Hyperekplexia (startle disease)

• Retrograde messengers - released from postsynaptic neuron
• Act on CB1 receptors on presynaptic terminal -> inhibit NT release
• Inhibitory modulators of pain, appetite, mood

• Inhibitory in basal ganglia circuits (indirect pathway via D2 receptors)

• Inhibitory in pain modulation (descending inhibition)

• Inhibitory in pain pathways; act on mu, delta, kappa opioid receptors -> inhibit pain transmission (presynaptically reduce NT release; postsynaptically open K+ channels)

---

37. INHIBITION OF SYNAPSE (4 marks)

• Inhibitory axon synapses on the SOMA or dendrites of postsynaptic neuron
• Released GABA/Glycine -> IPSP (hyperpolarization)
• Makes it harder to reach threshold
• Two subtypes:
  • Recurrent (feedback) inhibition: Active neuron -> Renshaw cell -> back-inhibition of same neuron (glycine)
  • Lateral (feed-forward) inhibition: Excited neuron -> inhibitory interneuron -> inhibit neighboring neurons (GABA)

• Inhibitory axon synapses on the presynaptic terminal (axo-axonic synapse)
• Released GABA (GABA-B) -> reduces Ca2+ influx -> less NT released from presynaptic terminal
• Does NOT produce IPSP in postsynaptic cell; selectively gates one input
• Particularly important in sensory pathways (gate control of pain, spinal cord afferents)

• GABA-B, metabotropic glutamate receptors -> activate K+ channels -> K+ efflux -> hyperpolarization
• Slow, prolonged inhibition

• Removal of excitatory input to a neuron (not active inhibition, but decreased excitation)
• Produces apparent inhibition of that neuron

• If ECl is at resting potential (-70 mV), opening Cl- channels does NOT hyperpolarize
• But: increased Cl- conductance reduces input resistance (shunts the membrane)
• Any EPSP current is now dissipated -> EPSP amplitude reduced -> inhibition despite no voltage change
• Called "shunting inhibition"

• Presynaptic autoreceptors (e.g., D2, alpha-2 adrenergic, GABA-B) respond to the NT released
• Negative feedback: excessive NT release -> activate autoreceptor -> reduce further release

---

38. BODY RESPONSE TO HUNGER (3 marks)

1. Ghrelin:
   • "Hunger hormone" - secreted by oxyntic cells of gastric fundus when stomach is empty
   • Rises before meals, falls after eating
   • Acts on hypothalamic arcuate nucleus - activates NPY/AgRP neurons (orexigenic)
   • Stimulates food intake and GH release
2. Low blood glucose (hypoglycemia):
   • Glucose-sensitive neurons in hypothalamus (ventromedial nucleus = "satiety center") become less active
   • Lateral hypothalamus (LH, "feeding center") increases activity
3. Low leptin / Reduced leptin signaling:
   • Adipose tissue leptin falls during fasting/weight loss
   • Arcuate NPY/AgRP neurons disinhibited -> increase appetite

• Lateral Hypothalamus (LH): "Feeding/Hunger center"
  • Lesion -> Aphagia (refusal to eat) -> starvation
  • Contains: orexin/hypocretin neurons (also regulate arousal)
  • Receives: ghrelin, NPY/AgRP signals -> activates food-seeking behavior
• Ventromedial Hypothalamus (VMH): "Satiety center"
  • Lesion -> Hyperphagia -> obesity
  • Inhibits feeding

• NPY/AgRP neurons (orexigenic): stimulated by ghrelin, low leptin -> increase appetite, decrease energy expenditure
• POMC/CART neurons (anorexigenic): stimulated by leptin, insulin -> decrease appetite (alpha-MSH from POMC acts on MC4R)

• Increased gastric contractions ("hunger pangs")
• Elevated gastric acid secretion
• Increased cortisol (promotes gluconeogenesis)
• Increased growth hormone (lipolysis)
• Decreased insulin, increased glucagon
• Sympathetic activation (mild)
• Food-seeking behavior (behavioral response)

---

39. BASAL GANGLIA CONNECTIONS + PARKINSON'S DISEASE (4+3 = 7 marks)

Part A: Connections of Basal Ganglia (4 marks)

• Striatum (Caudate + Putamen) - main input structure
• Globus Pallidus (GP internus - GPi; GP externus - GPe)
• Substantia Nigra (SN pars compacta - SNc; SN pars reticulata - SNr)
• Subthalamic Nucleus (STN)
• Nucleus accumbens (ventral striatum), olfactory tubercle (limbic basal ganglia)

• Striatum receives from: All areas of cerebral cortex (glutamate, excitatory)
• Striatum also receives from: SNc (dopamine via nigrostriatal pathway)
• Subthalamic nucleus receives: glutamate from cortex (hyperdirect pathway)

• GPi + SNr -> Thalamus (VL, VA, CM nuclei) -> motor cortex (frontal)
• GPi -> brainstem (superior colliculus, pedunculopontine nucleus)
• SNr -> superior colliculus -> eye movements

• D1 (direct): excites -> net facilitation of movement
• D2 (indirect): inhibits -> net facilitation of movement
• Both effects: dopamine promotes movement
• Loss of dopamine -> Parkinson's disease (hypokinesia)

Part B: Parkinson's Disease - Physiological Basis, Signs, Symptoms and Treatment (3 marks)

• Loss of dopaminergic neurons in Substantia Nigra pars compacta (SNc)
• Reduces dopamine in nigrostriatal pathway
• Direct pathway underactive (less movement facilitation)
• Indirect pathway overactive (excessive STN -> GPi activation -> thalamus over-inhibited -> reduced cortical activation)
• Net result: Hypokinesia, bradykinesia, difficulty initiating movement

• Tremor: Resting tremor (4-6 Hz, "pill-rolling") - disappears during voluntary movement (opposite of cerebellar intention tremor)
• Rigidity: Lead-pipe rigidity (uniform resistance throughout ROM); with tremor = "cogwheel rigidity"
• Akinesia/Bradykinesia: Slowness/absence of movement; festinant gait, hypomimia (mask face)
• Postural instability: Loss of postural reflexes; falls; retropulsion

• Micrographia, hypophonia (soft voice), drooling
• Shuffling gait, reduced arm swing
• Flexed posture (stooped)
• Cognitive impairment (late)
• Depression, autonomic dysfunction

• Levodopa + Carbidopa: L-DOPA crosses BBB -> converted to dopamine; carbidopa prevents peripheral conversion
• Dopamine agonists (pramipexole, ropinirole): Act on D2/D3 receptors
• MAO-B inhibitors (selegiline, rasagiline): Reduce dopamine breakdown
• COMT inhibitors (entacapone): Prolong levodopa effect
• Deep Brain Stimulation (DBS): Electrode in STN or GPi -> reduces overactivity
• Anticholinergics (benztropine): For tremor (restore ACh-dopamine balance in striatum)

---

40. ROLE OF HYPOTHALAMUS IN CONTROL OF HUNGER AND FEEDING (4 marks)

• Stimulation -> hyperphagia
• Lesion -> aphagia, starvation
• Contains: orexin/hypocretin neurons (also arousal), melanin-concentrating hormone (MCH) neurons
• Both orexin and MCH stimulate food intake

• Stimulation -> stops eating
• Lesion -> hyperphagia, obesity (VMH syndrome in rats)
• Contains: glucose-responsive neurons (activated by high glucose)

• NPY/AgRP neurons (orexigenic):
  • NPY (Neuropeptide Y): most potent appetite stimulant
  • AgRP (Agouti-related peptide): MC4R antagonist -> blocks anorexigenic MC4R signaling
  • Activated by: ghrelin, low leptin, low insulin, hypoglycemia
  • Project to: PVN, LH -> increase food intake, decrease energy expenditure
• POMC/CART neurons (anorexigenic):
  • POMC cleaved to alpha-MSH -> acts on MC3R and MC4R -> suppresses appetite
  • CART (Cocaine and Amphetamine Regulated Transcript): anorexigenic
  • Activated by: leptin, insulin, high glucose
  • Project to: PVN, LH -> decrease food intake, increase energy expenditure

• Receives NPY/AgRP and POMC/CART projections
• Integrates signals -> controls ANS and endocrine responses to feeding

• Lipostatic theory: Leptin from adipose tissue signals fat stores -> when fat high, leptin high -> inhibits NPY/AgRP, activates POMC -> reduces food intake until fat stores normalized
• Leptin resistance: Seen in obesity -> leptin high but ineffective

---

41. CHEMORECEPTORS IN BRAIN (3 marks)

• Primarily on the ventral surface of the medulla oblongata (ventral chemosensitive area / retrotrapezoid nucleus)
• Also: Pre-Botzinger complex, Raphe nuclei, Nucleus tractus solitarius (NTS)

• Respond to CO2 and H+ (NOT directly to PO2)
• Blood-brain barrier is relatively impermeable to H+ and HCO3-, but CO2 freely crosses
• CO2 + H2O -> H2CO3 -> H+ + HCO3- (via carbonic anhydrase in CSF)
• Increased PCO2 -> increased CSF H+ -> stimulates chemoreceptors

• Increased PCO2 (hypercapnia) -> stimulates central chemoreceptors -> increased ventilation (hyperventilation)
• CO2 is the MOST IMPORTANT stimulus for breathing in normal physiological state
• pH 7.35 corresponds to threshold; PCO2 rise of 1 mmHg -> VE increases ~3 L/min

• Carotid bodies (at carotid bifurcation) + Aortic bodies
• Respond to: low PO2 (< 60 mmHg, most important), high PCO2, low pH, low blood flow
• Glomus (Type I) cells -> release ACh/dopamine -> activate afferent nerve
• Carotid body afferent: CN IX (Hering's nerve) -> NTS
• Aortic body afferent: CN X -> NTS
• Primary peripheral response to hypoxia; only ~30% of CO2/pH response

• Chronic hypercapnia (COPD): CO2 response blunted; patient relies on hypoxic drive (peripheral chemoreceptors). High-flow O2 can remove hypoxic drive -> respiratory depression ("CO2 narcosis risk")
• Mountain sickness: hyperventilation (low PO2 -> peripheral chemoreceptors -> hyperventilation -> respiratory alkalosis -> lowers CSF H+ -> blunts central response initially)

---

42. PHYSIOLOGY OF LANGUAGE AND ITS DISORDERS (4 marks)

• Motor speech area - responsible for production of speech (programming of speech muscles)
• Connects to: Primary motor cortex (face area), Wernicke's area (via arcuate fasciculus)
• Broca's aphasia: Non-fluent, effortful speech; comprehension intact; patient knows what to say but can't produce it; frustrated; telegraphic speech ("book... want... book")

• Sensory/Receptive speech area - responsible for understanding/comprehension of language
• Wernicke's aphasia: Fluent but meaningless speech (jargon aphasia); poor comprehension; paraphasias (wrong words); patient unaware of errors; may sound like "word salad"

• Integrates visual input with language (reading, writing)
• Lesion: Alexia (inability to read), Agraphia (inability to write)

• White matter tract connecting Wernicke's to Broca's area
• Lesion -> Conduction Aphasia: Fluent speech, good comprehension, but cannot repeat words (disconnection)

• Heschl's gyrus - receives auditory input, feeds into Wernicke's area

---

43. GOLGI TENDON ORGAN AND MUSCLE SPINDLE (2 marks)

---

44. ALPHA BLOCK (3 marks)

• Alpha (8-13 Hz): Relaxed, eyes closed, awake; most prominent over occipital cortex
• Beta (13-30 Hz): Active thinking, eyes open, alert
• Theta (4-8 Hz): Drowsiness, children
• Delta (0.5-4 Hz): Deep sleep, severe pathology if in awake adult

• Alpha waves represent synchronized, idle oscillations of thalamocortical circuits
• Thalamic pacemaker cells fire in rhythmic bursts -> synchronized cortical activity -> alpha waves on EEG
• When subject opens eyes or performs mental task:
  • Reticular activating system (RAS) is activated
  • Increased cholinergic + noradrenergic input to thalamus
  • Thalamus switches to tonic firing mode (desynchronized)
  • Cortical neurons now process information independently -> low amplitude, high frequency (beta waves) replace alpha
  • This is "alpha block" or "EEG arousal"

• Indicator of cortical arousal and activation
• Used in EEG to test reactivity of visual cortex
• Alpha persists (not blocked) in:
  • Blind individuals (don't block to eye opening)
  • Coma (unresponsive to stimuli)
  • Certain drugs (anesthetics can paradoxically cause alpha waves)
• Alpha blocking by mental arithmetic (no visual input) - shows it's cognitive activation, not just visual

---

45. CLASSIFICATION OF NERVE FIBERS (3 marks)

Erlanger and Gasser Classification (by conduction velocity and myelination):

Lloyd Classification (for muscle afferents):

• Myelinated fibers: faster, larger diameter, saltatory conduction
• C fibers: slowest, most numerous (70% of all peripheral nerve fibers)
• Local anesthetic sensitivity: Small fibers (C, Aδ) blocked first; motor (Aα) last
• Pressure: Large fibers (touch, motor) blocked first; pain (C) last

---

46. ENDOGENOUS PAIN RELIEVING SYSTEM (3 marks)

• Located in midbrain around cerebral aqueduct
• Rich in mu-opioid receptors + endogenous opioid peptides
• Activated by: opioid drugs, stress, electrical stimulation
• PAG activation -> profound analgesia
• Sends output to: Nucleus Raphe Magnus (NRM) in medulla

• Serotonergic neurons
• Descend via dorsolateral funiculus of spinal cord
• Synapse on enkephalinergic interneurons + directly on dorsal horn neurons (Laminae I, II, V)
• Release: Serotonin (5-HT) -> inhibit pain transmission

• Major noradrenergic nucleus (pons)
• Descending noradrenergic fibers -> dorsal horn
• Release: Norepinephrine (NE) -> alpha-2 adrenergic receptors -> inhibit pain

• Enkephalinergic interneurons (SG cells):
  • Release enkephalins (met-enkephalin, leu-enkephalin)
  • Act presynaptically on C fiber terminals -> block Ca2+ entry -> reduce Substance P/Glutamate release
  • Act postsynaptically -> open K+ channels -> hyperpolarize 2nd order neuron

• Large A-beta fibers (touch) activate SG interneurons (inhibitory) -> close gate to C fiber input
• Explains: rubbing an injury relieves pain (TENS, massage)
• "Gate" = inhibitory interneuron in substantia gelatinosa (Lamina II)

• Opioid drugs (morphine) mimic endogenous system at mu receptors
• TENS (Transcutaneous Electrical Nerve Stimulation): gate control
• Acupuncture: activates opioid system (naloxone partially blocks it)
• Antidepressants (amitriptyline, duloxetine): enhance descending NE/5-HT inhibition

---

47. MOTOR CORTEX (3 marks)

• Location: Precentral gyrus (anterior to central sulcus)
• Histology: Agranular cortex (no layer IV); contains Betz cells (giant pyramidal neurons) in layer V - largest neurons in CNS
• Somatotopic map (Motor Homunculus):
  • Medial: leg/foot (paracentral lobule)
  • Middle: arm/hand (large area - fine motor)
  • Lateral: face/tongue (large area)
  • Size proportional to motor precision, NOT body size

• Bilateral planning of complex movements
• Active before movement begins (internally generated movements)
• Readiness potential (Bereitschaftspotential) originates here

• Externally cued movements
• Postural set (proximal muscles)
• Mirror neuron system

• Output: Corticospinal tract (pyramidal tract) -> brainstem motor nuclei + spinal cord alpha motor neurons
  • 30% from area 4; 30% from area 6; 40% from parietal cortex (5, 1, 2, 3)
  • 90% cross in pyramidal decussation (medulla) -> contralateral limbs
• Input: Thalamus (VL from cerebellum; VA from basal ganglia), parietal cortex (proprioception, somatosensory), SMA, PMC

• Contralateral spastic paralysis
• Increased deep tendon reflexes
• Positive Babinski sign
• Spasticity (velocity-dependent resistance)
• Loss of fine finger movements (early sign)
• Spares trunk/proximal muscles (bilateral control)

---

48. HORRIPILATION (2 marks)

• Arrector pili: Smooth muscles attached obliquely to hair follicles
• Innervated by sympathetic adrenergic (noradrenergic) fibers (post-ganglionic, unmyelinated C fibers)
• Stimulus: Cold, fear, emotional arousal
• Cold -> hypothalamic thermoregulatory center -> sympathetic activation -> norepinephrine released at arrector pili -> smooth muscle contracts -> hair stands erect

• In fur-bearing animals: Hair erection traps air layer -> increases insulation -> reduces heat loss (thermoregulation)
• In defense (animals): Piloerection makes animal appear larger (threat display)
• In humans: Vestigial response (insufficient hair for significant insulation)
• Emotional: Fear, listening to music, emotional arousal -> piloerection (psychogenic)

• Sweating, tachycardia, pupillary dilation, bronchodilation
• Associated with the fight-or-flight response

• Flare response (triple response) involves axon reflex of sensory fibers
• But horripilation is mediated by sympathetic efferent axon reflex (antidromic transmission in sympathetic fibers)

---

49. STRUCTURE AND FUNCTION OF MUSCLE SPINDLE IN STRETCH REFLEX (4 marks)

• Intrafusal fibers: Nuclear bag (2 types) + Nuclear chain (3-5)
• Ia afferents (primary, annulospiral) - both bag and chain - velocity + length
• II afferents (secondary, flower-spray) - mainly chain - static length
• Gamma motor neurons - dynamic (bag1) and static (bag2, chain)
• Capsule, lymph space, connective tissue sheath

1. External force stretches muscle (e.g., tap on patellar tendon)
2. Muscle stretches -> intrafusal fibers stretch -> annulospiral endings distort
3. Ia afferents discharge (action potentials) - proportional to rate and amount of stretch
4. Ia afferents enter spinal cord via dorsal root -> synapse monosynaptically on alpha motor neurons (SAME muscle = homonymous) in anterior horn
5. Glutamate released -> EPSP in alpha MN -> alpha MN fires -> extrafusal fibers contract -> muscle shortens
6. Simultaneously: Ia afferents activate Ia inhibitory interneuron (glycinergic) -> inhibit alpha MN of antagonist muscle (reciprocal inhibition)

• Gamma motor neurons maintain tension in intrafusal fibers
• Alpha-gamma co-activation: ensures spindle remains sensitive throughout contraction
• Without gamma: spindle unloaded when muscle shortens -> Ia goes silent -> reflex termination
• With gamma: spindle maintains sensitivity -> continuous modulation of stretch reflex

• Phasic (dynamic): Response to sudden stretch (tendon tap) - depends mainly on Ia dynamic response
• Tonic: Sustained low-level contraction in response to sustained stretch - depends on Ia static + II fibers

• Diminished/absent knee jerk: Lower motor neuron lesion (Ia afferents, alpha MN, or muscle)
• Exaggerated reflexes: Upper motor neuron lesion (loss of descending inhibition)
• Spasticity: Increased stretch reflex gain; tonic stretch reflex exaggerated (velocity-dependent)

---

50. MIGRATING MOTOR COMPLEX (MMC) (3 marks)

• Starts in distal stomach or duodenum
• Migrates aborally at ~5 cm/min
• Reaches ileum in ~90 minutes
• New complex begins in stomach before previous one reaches ileum

• Motilin (polypeptide from Mo cells in small intestine): triggers Phase III - peak levels coincide with Phase III
• Myenteric plexus (Auerbach's): coordinates propagation
• Vagus nerve: modulates but not essential
• Somatostatin: prolongs Phase I
• Feeding: immediately abolishes MMC -> replaced by irregular mixing contractions (fed pattern)

1. Housekeeping: Clears residual food, secretions, desquamated cells, bacteria from small intestine ("intestinal housekeeper")
2. Prevents bacterial overgrowth of small intestine (sweeps bacteria toward colon)
3. Clears bile and pancreatic secretions
4. Prevents constipation during fasting

• MMC absent in: Diabetic gastroparesis, scleroderma
• Bacterial overgrowth: Absent MMC -> stasis -> bacterial overgrowth -> malabsorption
• Erythromycin: Motilin receptor agonist -> prokinetic (stimulates Phase III)

---

51. ELECTROENCEPHALOGRAM (EEG) (4 marks)

• Primarily: Summated dendritic postsynaptic potentials (not action potentials) of pyramidal neurons in cortex
• Synchronization by thalamocortical loops (thalamus acts as pacemaker)
• Recorded in µV range (10-100 µV)

• International 10-20 system (electrodes spaced 10% or 20% of skull dimensions)
• Fp (frontopolar), F (frontal), C (central), P (parietal), O (occipital), T (temporal)
• Reference electrode on ear lobe (mastoid)

• Stage 1: Alpha -> Theta
• Stage 2: Sleep spindles (12-14 Hz) + K-complexes
• Stage 3: Delta (>20%)
• REM: Low amplitude, mixed (like awake) - desynchronized

1. Epilepsy - diagnosis, classification, localization of seizure focus
2. Sleep disorders - polysomnography (EEG + EMG + EOG)
3. Coma evaluation - prognosis
4. Brain death - electrocerebral silence (supportive)
5. Encephalitis - viral (HSV: temporal lobe PLEDs)
6. Intraoperative monitoring - depth of anesthesia, cerebral ischemia detection

---

52. INHIBITION OF PAIN (4 marks)

1. Peripheral Level:

• Opioids (peripheral): Opioid receptors upregulated on nociceptors during inflammation; morphine injected into inflamed joints effective
• Cooling: Reduces nociceptor sensitivity and slows conduction velocity
• Anti-inflammatory drugs (NSAIDs): Inhibit COX enzymes -> reduce PGE2/PGI2 -> reduce sensitization of nociceptors

2. Spinal Cord (Dorsal Horn) Level:

• Large diameter A-beta fibers (tactile) activate inhibitory interneurons (SG cells, Lamina II) -> inhibit pain transmission cells (T cells)
• C fiber activity: "opens gate" (inhibits SG cells) -> more pain
• A-beta activity: "closes gate" (activates SG cells) -> less pain
• Clinical: TENS, rubbing/massage, vibration

• Presynaptic: mu receptors on C fiber terminals -> reduce Ca2+ influx -> less Substance P + Glutamate release
• Postsynaptic: mu receptors on 2nd order neurons -> open K+ channels -> hyperpolarization -> less firing

• Released from postsynaptic neurons -> retrograde -> CB1 receptors on C fiber terminals -> reduce NT release (depolarization-induced suppression of inhibition)

3. Brainstem Level (Descending Inhibition):

• Stimulus: stress, opioids, fear, PAG electrical stimulation
• PAG -> NRM (serotonin) + Nucleus reticularis (NE) -> dorsolateral funiculus -> dorsal horn
• Release: 5-HT, NE, enkephalins in dorsal horn -> inhibit pain transmission

• Mu-opioid receptors abundant in PAG
• Beta-endorphin from hypothalamus activates PAG
• OFF cells (pain inhibitory) and ON cells (pain facilitatory) in RVM

4. Thalamic and Cortical Level:

• Anterior cingulate cortex (ACC): modulates affective component; cognitive control of pain
• Frontal cortex -> PAG: cognitive reappraisal, attention, placebo effects
• Placebo analgesia: mediated by endogenous opioids (blocked by naloxone)

5. Pharmacological Inhibition:

• Opioids (morphine, codeine, fentanyl): mu receptor agonists -> all levels
• NSAIDs (aspirin, ibuprofen): peripheral + central COX inhibition
• Tramadol: Weak mu agonist + NE/5-HT reuptake inhibition
• Tricyclic antidepressants (amitriptyline): block NE + 5-HT reuptake -> enhance descending inhibition
• Gabapentinoids (pregabalin, gabapentin): block Ca2+ channel alpha-2-delta subunit -> reduce NT release from nociceptors
• NMDA antagonists (ketamine): block central sensitization (wind-up)
• Capsaicin: Depletes Substance P from C fiber terminals; TRPV1 desensitization

---

53. SUMMATION AND OCCLUSION (3 marks)

• Two or more afferent fibers from DIFFERENT presynaptic neurons fire simultaneously
• Each produces a subthreshold EPSP on the same postsynaptic neuron
• Combined EPSP = sum of individual EPSPs
• If combined EPSP reaches threshold -> action potential
• Example: Two fingers touched simultaneously produce stronger sensation than one

• A SINGLE presynaptic neuron fires repetitively in rapid succession
• Each impulse produces an EPSP before previous one has decayed
• EPSPs summate in time -> reach threshold
• Requires fast firing rate (EPSPs must overlap before decaying in ~15 ms)

• A subthreshold input that brings neuron closer to threshold without firing it = facilitation
• A later suprathreshold input then more easily fires the neuron

• Each input pathway activates: (a) neurons fired suprathreshold + (b) subliminal fringe neurons (subthreshold EPSP only)
• Overlapping neurons in the "discharged zone" common to both inputs are activated by EITHER stimulus alone
• When both stimulated together: overlapping neurons NOT activated twice (already at threshold) -> combined response less than sum

---