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NEUROPHYSIOLOGY - COMPLETE EXAM NOTES

5 Bridge Topics + 8 Modules | Based on Guyton & Hall, Ganong, Kandel

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BRIDGE 1: Resting Membrane Potential & Action Potential

Resting Membrane Potential (RMP)

• K+ is the dominant ion determining RMP. K+ concentration inside (150 mEq/L) >> outside (5 mEq/L). K+ leaks outward through K+ leak channels, leaving negative charges inside.
• Na+ is kept mostly outside (145 mEq/L outside, 12 mEq/L inside) by: (a) few resting Na+ channels, and (b) active transport via the Na+/K+ ATPase pump (pumps 3 Na+ out for every 2 K+ in - electrogenic, contributes ~-3 mV directly).
• Cl- and large organic anions are also contributors but K+ dominates.
• The Goldman equation accounts for all permeable ions; the Nernst equation calculates equilibrium potential for a single ion (EK = -90 mV; ENa = +60 mV).

• Maintains concentration gradients by active transport (uses ATP).
• 3 Na+ out / 2 K+ in per cycle.
• Inhibited by ouabain/cardiac glycosides.

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Action Potential (AP)

• Absolute Refractory Period (ARP): During rapid depolarization and early repolarization. Na+ channels are inactivated - no AP possible regardless of stimulus strength. Corresponds to inactivation gate closure.
• Relative Refractory Period (RRP): Late repolarization. A stronger-than-normal stimulus can fire an AP. K+ channels still open, membrane slightly hyperpolarized.

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BRIDGE 2: Neurotransmitters & Their Receptors

Key Neurotransmitters

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BRIDGE 3: Muscle Spindle, Stretch Reflex & Gamma Loop

Muscle Spindle Structure

• Intrafusal fibers (inside spindle capsule, in parallel with extrafusal fibers):
  • Nuclear bag fibers (dynamic): Detect rate of stretch (Ia afferents primarily). Subtype: bag1 (dynamic), bag2 (static).
  • Nuclear chain fibers (static): Detect static length (Ia + II afferents).
• Extrafusal fibers: Regular force-generating muscle fibers, innervated by alpha (α) motor neurons.
• Afferents:
  • Group Ia (annulospiral): Wrap around bag AND chain fibers. Fast-conducting (70-120 m/s). Respond to both rate of stretch AND static stretch.
  • Group II (flower spray): On chain fibers only. Respond to static length.
• Gamma (γ) motor neurons: Innervate the polar (contractile) ends of intrafusal fibers. Maintain spindle sensitivity when the whole muscle shortens.

Stretch (Myotatic) Reflex

Gamma Loop

• Reticulospinal tract (main controller)
• Vestibulospinal, rubrospinal tracts
• UMNL → hyperexcitability of gamma loop → hypertonia, hyperreflexia.

Golgi Tendon Organ (GTO) & Clasp-Knife Reflex

• Located at muscle-tendon junction. In series with extrafusal fibers.
• Ib afferents → inhibitory interneuron → inhibit α motor neuron (autogenic inhibition / inverse myotatic reflex).
• Activated by strong muscle tension (both passive stretch AND active contraction, but predominantly active).
• Clasp-knife phenomenon in UMNL: Initial high resistance to passive stretch (hyperactive stretch reflex), then sudden collapse (GTO Ib inhibition "kicks in" at sufficient tension). This is the "clasp-knife" quality of spasticity.

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BRIDGE 4: Somatotopic Organization & Brain Blood Supply

Motor Cortex (Primary - Area 4, Precentral Gyrus)

• Medial aspect (near longitudinal fissure): Lower limb (important! - ACA territory)
• Middle of precentral gyrus: Upper limb, trunk
• Lateral (near Sylvian fissure): Face, lips, tongue (MCA territory)

• Premotor cortex (Area 6): Planning, sequencing complex movements. Contains the frontal eye fields (Area 8) - controls voluntary conjugate gaze.
• Supplementary motor area (SMA, medial Area 6): Initiation of voluntary movement; programming complex bilateral movements.
• Broca's area (Area 44/45, inferior frontal gyrus, dominant hemisphere): Motor speech - coordinates muscles for articulation. MCA territory.

Cortical Areas for Speech & Language

• Broca's area (Area 44/45): Dominant (left) inferior frontal gyrus. Production/motor programming of speech.
• Wernicke's area (Area 22): Dominant superior temporal gyrus. Comprehension of speech.
• Arcuate fasciculus: White matter tract connecting Wernicke's to Broca's.

Cerebral Blood Supply

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BRIDGE 5: CSF Dynamics & Intracranial Pressure (Monro-Kellie Doctrine)

CSF Production & Circulation

• Production: Choroid plexus of lateral ventricles (mainly) + 3rd and 4th ventricles. Rate: ~500 mL/day; total volume ~150 mL (circulating).
• Flow: Lateral ventricles → Foramen of Monro → 3rd ventricle → Aqueduct of Sylvius → 4th ventricle → Foramina of Luschka (lateral) & Magendie (midline) → Subarachnoid space → Upward over cerebral cortex.
• Absorption: Arachnoid granulations (villi) into superior sagittal sinus. Pressure-dependent passive process.

• Pressure: 70-180 mmH2O (lateral decubitus)
• Protein: 15-45 mg/dL
• Glucose: 60-80% of blood glucose
• Cells: 0-5 lymphocytes/mm3
• Clear, colorless ("water-clear")

Monro-Kellie Doctrine

1. CSF displaced into spinal subarachnoid space
2. Venous blood squeezed out
3. Arterial compression (last resort, dangerous)

High-Altitude Cerebral Edema (Relevance to Q36)

• Mechanism: Rapid ascent → hypobaric hypoxia → hypoxic pulmonary vasoconstriction AND cerebral vasodilation.
• Hypoxia triggers release of VEGF, NO, free radicals → increased blood-brain barrier permeability → vasogenic edema (fluid leaks into extracellular space).
• Also hypoxia → impairs Na+/K+ ATPase → cytotoxic edema (cells swell).
• Rising cerebral blood volume + edema → increased ICP → headache, ataxia, confusion.
• Slow ascent allows acclimatization: increased RBC production (EPO-driven), 2,3-DPG increases (right-shifts O2-Hb curve), renal bicarbonate excretion compensates respiratory alkalosis.
• Treatment: Descent, O2 supplementation, dexamethasone (reduces vasogenic edema), acetazolamide (prophylaxis - carbonic anhydrase inhibitor, causes metabolic acidosis, stimulates ventilation).

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MODULE A: The Basic Unit - Signal Generation & Transmission

Q20. Saltatory Conduction

Myelin Sheath

• Formed by Schwann cells (PNS) and oligodendrocytes (CNS).
• Wraps around the axon in concentric layers of lipid-rich membrane, acting as electrical insulation.
• Interrupted at regular intervals (~1 mm) by the nodes of Ranvier - where the axon membrane is exposed and densely packed with voltage-gated Na+ channels.
• Internodal segments have almost no Na+ channels.

Mechanism of Saltatory Conduction

1. Depolarization at one node generates an AP.
2. Current flows through the low-resistance axoplasm along the internodal segment to the next node.
3. The myelin prevents current from leaking out in between nodes (high-resistance insulation).
4. This current depolarizes the membrane at the next node to threshold.
5. AP fires at the new node. The previous node becomes refractory (prevents backward propagation).

Advantages Over Continuous Conduction

• Fiber diameter (larger = faster)
• Degree of myelination
• Temperature (higher temp = faster, within physiological range)

• Slowed or blocked AP conduction (conduction block).
• Internodal Na+ channels insufficient for conduction.
• Clinically: weakness, sensory loss, optic neuritis, cerebellar signs - depending on lesion site.
• Symptoms worsen with heat (Uhthoff's phenomenon) - elevated temp further reduces Na+ channel conductance.

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Q16. Synaptic Transmission

Types of Synapses

Steps of Chemical Synaptic Transmission

1. AP arrives at axon terminal (bouton).
2. Depolarization opens voltage-gated Ca2+ channels (N-type and P/Q-type) in the terminal membrane.
3. Ca2+ influx → Ca2+ binds to proteins (synaptotagmin acts as Ca2+ sensor) on synaptic vesicles.
4. Vesicle docking and fusion with presynaptic membrane (SNARE complex: synaptobrevin/VAMP on vesicle; syntaxin + SNAP-25 on membrane).
5. Exocytosis → NT released into 20-50 nm synaptic cleft.
6. NT diffuses across cleft and binds to postsynaptic receptors.
7. Postsynaptic response: Ion channel opening (ionotropic) or second messenger cascade (metabotropic).
8. Termination: (a) Enzymatic degradation (ACh by AChE), (b) Reuptake into presynaptic terminal (dopamine, serotonin, NE, glutamate), (c) Diffusion away from cleft.

Ionotropic vs. Metabotropic Receptors

Excitatory vs. Inhibitory Postsynaptic Potentials

• EPSP: NT opens Na+ or Ca2+ channels → membrane depolarizes toward threshold. Brought about by glutamate (AMPA), ACh (nicotinic). EPSPs summate spatially and temporally.
• IPSP: NT opens Cl- channels (GABA-A, glycine) → membrane hyperpolarizes (moves away from threshold) OR opens K+ channels (GABA-B). Makes firing less likely.

Spatial & Temporal Summation

• Spatial summation: Multiple synapses from different neurons active simultaneously → EPSPs add up.
• Temporal summation: Same synapse fires rapidly → EPSPs overlap and add up before decaying.
• If summed depolarization reaches threshold at the axon hillock, an AP fires.

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Q21. Synaptic Delay

1. Ca2+ channel opening time (~0.1 ms)
2. Ca2+ diffusion to release sites (~0.1 ms)
3. Vesicle docking, fusion, exocytosis (~0.1-0.2 ms)
4. NT diffusion across cleft (~0.05 ms)
5. Receptor binding and channel opening (~0.05 ms)

• It allows the direction of signal flow to be determined (signals travel ONLY pre → post at chemical synapses, i.e., one-way valve).
• It is the basis for measuring the number of synapses in a reflex arc - each 0.5 ms of extra delay beyond simple conduction time represents one synapse.
• In complex integrative pathways with many synapses, the cumulative delay becomes significant.
• The monosynaptic stretch reflex has the shortest reflex time (only 1 synapse → ~25-30 ms total for the knee jerk).
• Polysynaptic reflexes are slower due to additional synaptic delays at each interneuron.

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Q27. Synaptic Plasticity

Short-Term Plasticity

• Synaptic facilitation: Successive APs in rapid succession → residual Ca2+ in terminal → enhanced NT release. Lasts ms to seconds.
• Synaptic depression: High-frequency stimulation → depletion of readily-releasable vesicle pool → reduced NT release (fatigue).
• Post-tetanic potentiation (PTP): Following a burst (tetanus) of high-frequency stimulation, synaptic strength remains enhanced for minutes. Due to elevated presynaptic Ca2+ and activated kinases.

Long-Term Potentiation (LTP) - The Basis of Memory

1. At rest, the NMDA receptor is blocked by Mg2+ (voltage-dependent block at -70 mV).
2. During weak stimulation: Glutamate binds NMDA-R, but Mg2+ block prevents Ca2+ entry. AMPA-R opens → small EPSP.
3. During strong/repetitive stimulation (associativity condition): AMPA-R opens, membrane depolarizes sufficiently → Mg2+ block relieved → both glutamate binding AND membrane depolarization required (co-incidence detection) → NMDA-R opens → Ca2+ floods into postsynaptic cell.
4. Ca2+ activates CaMKII (Ca2+/calmodulin-dependent protein kinase II) → phosphorylates existing AMPA receptors (increasing conductance) AND triggers insertion of new AMPA receptors into the synapse → more Na+ enters per glutamate pulse → stronger EPSP = LTP.

• Requires protein synthesis and gene expression.
• CREB (cAMP response element binding protein) is phosphorylated → transcription of new synaptic proteins.
• Structural changes: growth of new dendritic spines, synaptogenesis.

Long-Term Depression (LTD)

• Low-frequency stimulation → small, sustained Ca2+ rise → activates phosphatases → AMPA receptor internalization → weakened synapse.
• Thought to underlie forgetting and fine-tuning of circuits.

Hebbian Plasticity

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Q25. Renshaw Cell Inhibition

Circuit

Alpha motor neuron → axon collateral → Renshaw cell
                                              ↓ (glycine - inhibitory)
                          Same AND neighboring alpha motor neurons

1. An α motor neuron fires and sends an axon collateral to synapse on a Renshaw cell (via ACh at nicotinic receptors).
2. The Renshaw cell fires and releases glycine back onto the α motor neuron (and neighboring α motor neurons).
3. This hyperpolarizes the α motor neuron, reducing its firing rate.

Functional Significance

• Recurrent inhibition: Self-limiting feedback - prevents runaway excitation of motor neurons (motor neuron fatigue prevention).
• Surround inhibition: Renshaw cells inhibit neighboring motor neurons → focuses and sharpens motor output by suppressing "noisy" neurons (analogous to lateral inhibition in sensory systems).
• Stabilizes motor output: Smooths out motor neuron discharge, contributing to controlled, graded muscle contraction.
• Disinhibition: Renshaw cells also inhibit Ia inhibitory interneurons → reducing reciprocal inhibition. This can allow simultaneous co-contraction of agonist-antagonist pairs.

• Tetanus toxin (tetanospasmin) blocks glycine AND GABA release from inhibitory interneurons (including Renshaw cells and Ia inhibitory interneurons) → loss of inhibitory control → tetanic spasm (spastic paralysis, opisthotonus).
• Strychnine blocks glycine receptors → similar clinical picture (convulsions, hyperreflexia).

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MODULE B: Sensory Input - The Pain Pathway

Q6 & Q19. Pain - Definition, Pathway & Referred Pain

Definition of Pain (IASP 2020)

• Acute pain: Protective, short-duration. Associated with tissue injury/inflammation.
• Chronic pain: > 3 months. Involves central sensitization.
• Nociceptive pain: Activation of nociceptors by thermal, mechanical, or chemical stimuli.
• Neuropathic pain: Damage/dysfunction of the somatosensory system itself (burning, allodynia, hyperalgesia).
• Fast pain (sharp, pricking): Mediated by Aδ (A-delta) fibers (myelinated, 5-30 m/s). Superficial, well-localized.
• Slow pain (burning, aching, throbbing): Mediated by C fibers (unmyelinated, 0.5-2 m/s). Deep, poorly localized, more distressing.

Pain Pathway Diagram

NOCICEPTORS (free nerve endings in skin/muscle/viscera)
     |
     | (Aδ + C fibers - in peripheral nerve)
     |
DORSAL ROOT GANGLION (1st order neuron - cell body here)
     |
     ↓
DORSAL HORN of Spinal Cord
   - Substantia gelatinosa (Rexed lamina II) - modulation
   - Lamina I (marginal zone) - main relay for Aδ
   - Lamina V - wide dynamic range neurons (Aδ + C + mechanoreceptors)
     |
     | (CROSSES MIDLINE within 1-2 spinal segments in anterior white commissure)
     |
     ↓
ANTEROLATERAL QUADRANT → SPINOTHALAMIC TRACT
   - Lateral spinothalamic: pain + temperature
   - Anterior spinothalamic: crude touch + pressure
     |
     ↓
THALAMUS (2nd order neuron synapse)
   - VPL (Ventral Posterolateral) for body
   - VPM (Ventral Posteromedial) for face (from trigeminal tract)
   - Intralaminar nuclei (for affective/arousal component)
     |
     ↓
SOMATOSENSORY CORTEX (1st + 2nd, Postcentral gyrus = Brodmann areas 3,1,2)
   - Localization and intensity discrimination of pain (sensory-discriminative component)
PLUS
ANTERIOR CINGULATE CORTEX + INSULAR CORTEX
   - Emotional/affective component (the "suffering" aspect of pain)

• Substantia gelatinosa (SG) cells gate the transmission of pain signals.
• Large-fiber (Aβ) activity (touch, vibration) activates SG cells → closes the gate → inhibits pain transmission neurons in lamina V.
• C fiber activity inhibits SG cells → opens the gate → pain transmission increases.
• Descending control from the brain can also modulate the gate.
• Clinical application: Transcutaneous electrical nerve stimulation (TENS) - activates Aβ fibers → closes gate → pain relief.

Descending Pain Modulation

1. Periaqueductal gray (PAG) in midbrain - receives input from cortex, hypothalamus, limbic system. Activated by opioids, stress.
2. PAG activates Nucleus Raphe Magnus (NRM) in medulla (serotonergic).
3. NRM sends fibers down dorsolateral funiculus to dorsal horn.
4. In dorsal horn: NE (from locus coeruleus) and 5-HT activate inhibitory interneurons releasing enkephalins and dynorphins → presynaptically inhibit C fiber terminals (block NT release) AND postsynaptically hyperpolarize pain transmission neurons.
5. Net effect: Pain signals damped at dorsal horn level.

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Referred Pain

1. Convergence-Projection Theory (most accepted):
   • Visceral afferents and somatic afferents from different locations converge on the same spinal cord neurons (wide dynamic range neurons in lamina V).
   • The brain has learned to associate activation of these neurons with somatic inputs (from early development).
   • When visceral input activates these neurons, the brain misinterprets (projects) the source to the usual somatic territory.
   • Explains why cardiac pain → left arm (T1-T4 dermatome).
2. Facilitation theory: Visceral pain lowers the threshold of adjacent neurons → spontaneous somatic neuron firing → perceived as pain in that somatic area.
3. Axon branching (convergence peripherally): Some visceral afferents have collateral branches to somatic areas (contributes to local hyperalgesia and allodynia at the referred site).

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Q34. Placebo as a Pain Reliever

Physiological Mechanisms

1. Endogenous opioid release:
   • The strongest evidence. Placebo analgesia is partially reversed by naloxone (opioid antagonist).
   • Expectation of pain relief activates PAG → NRM → dorsal horn opioid release.
   • PET imaging shows increased activity in PAG, anterior cingulate, and prefrontal cortex during placebo analgesia.
   • Activation of μ-opioid receptors in prefrontal cortex, ACC, and PAG correlates with magnitude of placebo response.
2. Endocannabinoid system:
   • CB1 receptor activation contributes (partly naloxone-resistant placebo effect blocked by CB1 antagonist rimonabant).
3. Dopaminergic reward circuits:
   • Placebo activates mesolimbic dopamine pathways (nucleus accumbens) → positive expectation reduces the affective/unpleasant component of pain.
4. Conditioned responses:
   • Classical conditioning: If a patient has previously received effective analgesics, the same setting/context can trigger analgesic responses even with an inert agent.
5. Psychological factors:
   • Expectation, patient-provider relationship, verbal suggestion, context all modulate descending analgesic pathways via prefrontal-limbic → PAG axis.

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MODULE C: Autonomic & Integrative Control

Q8, Q12, Q26. Functions of the Hypothalamus

Anatomical Overview

• Located below the thalamus, forming the floor and lateral walls of the 3rd ventricle.
• Major nuclei: Supraoptic (SON), Paraventricular (PVN), Arcuate, Ventromedial (VMH), Dorsomedial (DMH), Lateral hypothalamic area (LHA), Anterior, Posterior, Preoptic, Suprachiasmatic (SCN), Mammillary bodies.

Functions of the Hypothalamus (Enumerated)

Thermoregulation (Q26)

• Peripheral thermoreceptors (warm and cold receptors in skin) → signal via spinothalamic tracts to hypothalamus.
• Central thermoreceptors in the preoptic area of the anterior hypothalamus directly sense blood temperature.

• Preoptic/Anterior hypothalamus (POAH): Responds to heat → activates heat-loss mechanisms. Warm-sensitive neurons here fire more as temperature rises.
• Posterior hypothalamus: Integrates peripheral cold signals → activates heat-conservation and heat-generation mechanisms.

1. Cutaneous vasodilation (↑blood flow to skin, sympathetic inhibition of vasoconstrictor tone).
2. Sweating (sympathetic cholinergic fibers to eccrine glands).
3. Behavioral: Moving to cooler environment, less clothing.
4. Panting (in animals).

1. Cutaneous vasoconstriction (sympathetic α1-mediated, shunts blood to core).
2. Piloerection (sympathetic α1 → arrector pili → traps air layer - vestigial in humans).
3. Shivering thermogenesis (posterior hypothalamus → motor tracts → rhythmic involuntary muscle contractions - most effective).
4. Non-shivering thermogenesis: Brown adipose tissue (BAT) - sympathetic β3 adrenoceptors → activate UCP1 (uncoupling protein) → mitochondrial heat without ATP synthesis. Important in neonates and cold-adapted adults.
5. Hormonal: TSH/T3/T4 ↑ (chronic cold adaptation), catecholamines ↑ metabolic rate.
6. Behavioral: Adding clothing, seeking warmth.

• Pyrogens (bacterial LPS, IL-1β, IL-6, TNF-α) → stimulate hypothalamic cells to produce PGE2 (via COX-2).
• PGE2 raises the set point of the hypothalamic thermostat.
• Body then thermoregulates to the NEW higher set point (hence chills + vasoconstriction + shivering at onset, even though body is getting hotter).
• Antipyretics (aspirin, paracetamol) inhibit COX → block PGE2 → lower set point back to normal → sweating + vasodilation (defervescence).

Food Intake Regulation (Q12) & Hypothalamic Obesity (Q35)

• VMH (Ventromedial Hypothalamus) = Satiety center: Stimulation → stops eating. Lesion → hyperphagia and obesity (VMH syndrome).
• LHA (Lateral Hypothalamic Area) = Feeding/hunger center: Stimulation → eating. Lesion → aphagia and weight loss.

• NPY/AgRP neurons (orexigenic - stimulate feeding): Activated by fasting/ghrelin; inhibited by leptin/insulin. Release NPY (↑eating) and AgRP (MC4R antagonist - blocks satiety signals).
• POMC/CART neurons (anorexigenic - suppress feeding): Activated by leptin/insulin. Release α-MSH (agonist at MC4R → reduces food intake via PVN signaling).

• Cause: Damage to VMH or hypothalamic structures (craniopharyngioma, surgery, radiation, trauma, inflammation, Prader-Willi syndrome).
• Mechanism: Disruption of leptin/insulin signaling → loss of satiety signals; often accompanied by hyperphagia, increased vagal tone (↑insulin secretion), and low sympathetic tone (↓energy expenditure).
• Features: Severe intractable obesity; hyperphagia; metabolic syndrome; cognitive impairment; growth hormone deficiency; panhypopituitarism (if pituitary also damaged).
• Distinct from common obesity: Leptin levels often normal or high (but signal not received due to disrupted pathways).

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Q11. Sympathetic Transmission & Q32. Effect of Sympathetic Stimulation on CVS

Sympathetic Transmission

• Located in the intermediolateral cell column (lateral horn) of T1-L2 spinal cord segments.
• Short preganglionic fibers → synapse in sympathetic chain ganglia OR prevertebral ganglia.
• NT: ACh → Nicotinic (N₂) receptors on postganglionic neurons.

• Located in paravertebral (sympathetic chain) or prevertebral ganglia (celiac, superior/inferior mesenteric).
• Long postganglionic fibers → target organs.
• NT: Norepinephrine (NE) → adrenergic receptors (α1, α2, β1, β2, β3) on target organs.
• Exception: Sweat glands and adrenal medulla - postganglionic sweat gland fibers release ACh (cholinergic sympathetic). Adrenal medulla is a modified ganglion: preganglionic ACh → chromaffin cells release adrenaline (80%) and NE (20%) directly into blood.

• Reuptake (NET = norepinephrine transporter) - primary route (~75-80%).
• MAO (in mitochondria) + COMT (extrasynaptic) → metabolize to VMA (vanillylmandelic acid).

Effect of Sympathetic Stimulation on CVS (Q32)

• Adrenaline: strong β1 AND β2 effects → ↑HR + ↑contractility + bronchodilation + vasodilation in muscle. Lower doses → fall in TPR (β2 >> α1).
• NE: strong α1 + β1, weak β2 → vasoconstriction + ↑HR + ↑BP. Reflex bradycardia can occur due to baroreceptor activation.

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MODULE D: Higher Cognitive Functions

Q7 & Q9. Sleep Cycle & NREM Sleep

Sleep Architecture Overview

• NREM (Non-Rapid Eye Movement) - 75-80% of total sleep
• REM (Rapid Eye Movement) - 20-25% of total sleep

NREM Sleep Stages (Q9)

REM Sleep (Paradoxical Sleep)

Neurochemical Regulation of Sleep-Wake

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Q17. Physiological Basis of Memory

Types of Memory

                    MEMORY
                   /        \
       DECLARATIVE             NON-DECLARATIVE
      (Explicit)               (Implicit)
     /          \              /    |    \
  Episodic    Semantic    Procedural  Priming  Conditioning
 (events)   (facts)     (skills)    
 Hippocampus  Hippocampus  Basal ganglia  Neocortex  Amygdala/
 + temporal   + neocortex  + Cerebellum             Cerebellum

Key Brain Structures

• Hippocampus (medial temporal lobe): Encoding of new declarative memories; spatial memory (place cells, grid cells). H.M. (Henry Molaison) - bilateral hippocampectomy → profound anterograde amnesia with preserved procedural memory and remote memories.
• Amygdala: Emotional memories, fear conditioning. Enhances hippocampal encoding (stress hormones → amygdala → enhances hippocampal LTP). Explains why emotional events are remembered better.
• Prefrontal Cortex: Working memory, strategic retrieval, source memory.
• Cerebellum: Eyeblink conditioning, adaptation of motor programs.
• Basal Ganglia (striatum): Habit formation, reward-based learning (dopaminergic).

Synaptic Mechanisms (Links to Q27)

• LTP at Schaffer collateral - CA1 synapses in hippocampus (NMDA-receptor dependent) is the primary candidate mechanism for memory encoding.
• Memory consolidation: Two stages:
  1. Synaptic consolidation (hours): Protein synthesis, AMPA receptor insertion.
  2. Systems consolidation (weeks-years): Sleep-dependent reactivation of hippocampal-neocortical circuits (hippocampus "replays" experiences to neocortex during slow-wave sleep) → gradual transfer of memories to neocortex for long-term storage.

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Q31. Physiological Significance of Emotion

Neural Substrates - The Limbic System

• Amygdala: Fear, anxiety, threat detection, emotional memory. Receives multimodal sensory input. Projects to hypothalamus (autonomic response), brainstem (fight/flight), cortex (attention, appraisal).
• Prefrontal cortex (PFC): Emotional regulation, suppression of amygdala (top-down control). Damage → disinhibition, personality change (Phineas Gage).
• Anterior cingulate cortex (ACC): Pain affect, empathy, conflict monitoring.
• Insular cortex: Interoception (awareness of internal body state), disgust, pain affect.
• Nucleus accumbens: Reward, pleasure, motivation (dopaminergic).
• Hypothalamus: ANS expression of emotion (heart rate, blood pressure, sweating).

Physiological Significance of Emotion

1. Survival function: Fear activates amygdala → hypothalamus → sympathoadrenal response (fight/flight) → prepares organism for threat. Disgust → avoidance of toxins.
2. Social bonding: Oxytocin (hypothalamus → posterior pituitary) released during bonding, trust, mother-infant attachment.
3. Learning and memory: Emotional salience enhances memory encoding via amygdala-hippocampal interaction. Negative/positive emotional content is remembered better than neutral.
4. Motivation: Reward system (VTA dopamine → nucleus accumbens) drives goal-directed behavior.
5. Communication: Facial expressions, tone of voice (prosody) - essential for social species.
6. Decision making: Somatic marker hypothesis (Damasio) - emotional "gut feelings" (somatic markers stored as body-state representations) guide rational decision making. VMF PFC patients → poor decisions despite intact logic (Elliot, Iowa gambling task).
7. Pain modulation: Emotional state modulates pain via descending systems (fear → hyperalgesia via amygdala; positive emotion → hypoalgesia via PAG).
8. Immune function: Chronic negative emotions (stress, depression) → sustained HPA axis activation → cortisol → immunosuppression, increased disease susceptibility.

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Q23 & Q33. Motor Speech Centre & Motor Aphasia

Language Areas of the Brain

Motor Aphasia (Broca's Aphasia) - Q33

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MODULE E: The Final Common Pathway

Q15. Pyramidal & Extrapyramidal Tracts - Corticospinal Tract

Classification of Descending Motor Tracts

DESCENDING MOTOR TRACTS
├── PYRAMIDAL (Corticospinal + Corticobulbar)
│   └── Direct pathway: Cortex → Spinal cord (no synapse en route)
└── EXTRAPYRAMIDAL
    ├── Rubrospinal (red nucleus → spinal cord)
    ├── Reticulospinal (RF → spinal cord) - MOST important extrapyramidal
    │   ├── Medullary reticulospinal (lateral) - inhibitory
    │   └── Pontine reticulospinal (medial) - facilitatory
    ├── Vestibulospinal (vestibular nucleus → spinal cord)
    │   ├── Lateral vestibulospinal - ipsilateral, extensors
    │   └── Medial vestibulospinal - bilateral, neck/upper trunk
    └── Tectospinal (superior colliculus → cervical cord) - head turning

Corticospinal Tract (CST)

• 40% from Primary motor cortex (Area 4)
• 30% from Premotor + SMA (Area 6)
• 30% from Somatosensory cortex (Areas 3, 1, 2)

Motor cortex (precentral gyrus + premotor) - Layer V Betz cells (largest)
        ↓
Internal capsule - Posterior limb (anterior 2/3)
        ↓
Basis pedunculi (cerebral peduncles, midbrain)
        ↓
Pons - dispersed between pontine nuclei
        ↓
Medullary pyramids (re-assembled)
        ↓
DECUSSATION (85-90%) at pyramidal decussation (junction of medulla/spinal cord)
   → Lateral corticospinal tract (LCST) - crossed, in lateral funiculus
        ↓
Synapse in ventral horn (Rexed laminae V-VII, IX)
   → Direct monosynaptic on alpha motor neurons (for fine hand movement)
   → Via interneurons (for most movements)
        ↓
Alpha motor neurons → skeletal muscle (LOWER MOTOR NEURON)

Uncrossed (10-15%) → Anterior corticospinal tract (ACST) - midline
   → Crosses at segmental level for axial muscles bilaterally

• Sacral fibers most lateral (outermost)
• Lumbar next
• Thoracic inside
• Cervical most medial

• Cervical (C8-T1): Direct monosynaptic connection to alpha motor neurons of intrinsic hand muscles - critical for fine finger movements. Only primates (and humans especially) have this. Loss → loss of finger individuation.
• Other levels: mostly via interneurons.

• Voluntary skilled movements, especially distal extremity (fine finger movements).
• Controls speed, force, and fractionation of movement.
• Corticobulbar fibers control cranial nerve motor nuclei (CN V, VII, IX, X, XI, XII) - most with bilateral cortical representation, except lower face and tongue which have mainly contralateral representation.

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Q2. Upper vs. Lower Motor Neuron Lesions

Key Comparison (The Classic "Must-Know" Table)

• Loss of corticospinal tract → loss of descending inhibition on gamma motor neurons → gamma motor neuron hyperactivity → increased intrafusal tension → hyperexcitable stretch reflex arc → resistance to passive stretch (spasticity).
• Also: loss of inhibitory reticulospinal fibers from medullary RF.
• The inhibitory reticulospinal fibers travel in the lateral corticospinal tract area → any lesion here disrupts inhibition → spasticity.

• The alpha motor neuron IS the final common pathway (Sherrington). Damage to it = no effector regardless of intact upper centers. Reflex arc broken.

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Q3. Tendon Reflexes

Common Tendon Reflexes and Their Segmental Levels

• 0: Absent
• 1+: Hypoactive/diminished
• 2+: Normal
• 3+: Hyperactive
• 4+: Clonus

• Jendrassik maneuver: Patient clenches teeth/hands → facilitates the reflex (via reticular formation activation). Used to elicit reflexes in tense patients.
• UMNL → hyperreflexia; LMNL → hyporeflexia.
• Cerebellar lesion → pendular reflexes (oscillate).
• Hypothyroidism → delayed relaxation phase (pseudomyotonia).
• Anxiety, hyperthyroidism → hyperreflexia.

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Q4. Clasp-Knife Rigidity in UMNL (PB)

1. Initial resistance (the "clasp"): UMNL → loss of descending inhibition → gamma motor neuron hyperactivity → increased spindle sensitivity → hyperexcitable monosynaptic stretch reflex → strong contraction when stretched.
2. Sudden give (the "knife giving way"): As tension in the stretched muscle builds sufficiently, Golgi Tendon Organs (GTOs) are activated. Ib afferents → inhibitory interneurons → autogenic inhibition (inverse myotatic reflex) of the same alpha motor neurons → sudden loss of contraction → limb collapses.
3. Velocity-dependence: Clasp-knife is velocity-dependent (more resistance at faster stretch) - reflects the dynamic (rate-sensitive) component of the stretch reflex via Ia fibers.

• Lead-pipe: uniform resistance throughout range, velocity-independent (rigidity = extrapyramidal).
• Cogwheel: Lead-pipe rigidity + superimposed tremor = ratchet-like feel.
• UMNL: Clasp-knife spasticity (velocity-dependent, sudden give, affects antigravity muscles preferentially).

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Q14 & Q24. Babinski's Sign

Physiological Mechanism of Positive Babinski Sign

1. Normal (negative) plantar response: In a neurologically intact adult, plantar stimulation activates local spinal reflex circuits, and the dominant corticospinal input inhibits the extensor response - resulting in toe flexion.
2. UMNL disrupts corticospinal inhibition: The corticospinal tract (especially the lateral CST) normally suppresses a primitive flexion withdrawal reflex arc involving the extensor hallucis longus and interossei.
3. Disinhibition of the flexion withdrawal reflex: Without corticospinal inhibition, plantar stimulation activates a polysynaptic flexion withdrawal-type response: dorsiflexion of the big toe (EHL contracts) and fanning of other toes. This is considered a release sign - unmasking of a primitive reflex.
4. Alternative theory (Landau): Babinski sign represents a fragment of the flexor reflex (defensive withdrawal): the full reflex would be dorsiflexion of ankle + knee + hip (triple flexion withdrawal). EHL contraction (big toe dorsiflexion) is part of this.

• Babinski: Stroke lateral sole foot.
• Chaddock: Stroke around lateral malleolus.
• Gordon: Squeeze calf.
• Oppenheim: Stroke tibial crest downwards.
• Schaeffer: Squeeze Achilles tendon. All elicit the same extensor response in UMNL.

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MODULE F: The Coordinator - Cerebellum

Q10, Q22, Q28. Cerebellum - Connections, Functions & Disorders

Anatomical Overview

• Anterior lobe (spinocerebellum, paleocerebellum): Spinal input, controls limb tone and posture.
• Posterior lobe (cerebrocerebellum, neocerebellum): Largest. Receives cortical input, plans skilled movements.
• Flocculonodular lobe (vestibulocerebellum, archicerebellum): Vestibular input, controls balance and eye movements.

• Vermis (medial): Axial and proximal limb control.
• Paravermal (intermediate) zone: Distal limb coordination.
• Lateral hemispheres (dentate zone): Planning, timing, and execution of skilled voluntary movements.

Connections (Inputs)

• Climbing fibers: From inferior olivary nucleus → enter via ICP → synapse directly on Purkinje cell dendrites (powerful 1:1 synaptic contact, generates complex spikes). Carries error signals.
• Mossy fibers: From all other sources (spinal cord, pontine nuclei, vestibular nuclei) → synapse on granule cells → parallel fibers → Purkinje cells (simple spikes via thousands of weak contacts).

Cerebellar Cortex Circuit (Q1 - The Ultimate LQ)

Mossy fiber → Granule cell → Parallel fiber → Purkinje cell dendrites (excitation)
                                      ↓ also
                               Basket/Stellate cells → inhibit Purkinje cells (surround inhibition)

Climbing fiber (inferior olive) → directly excites Purkinje cell (complex spike, error signal)

Purkinje cell (GABA) → INHIBITS deep cerebellar nucleus (DCN) tonic firing
                              ↓
DCN tonically excites upper motor neurons (thalamus → cortex; brainstem motor centers)

When cerebellar input arrives:
1. DCN initially excited by mossy fiber collaterals
2. Then Purkinje cell GABA inhibits DCN (delayed inhibition)
3. Net effect: timed burst-pause output from DCN → precise timing of movements

• Pathway: Left cerebellar hemisphere → SCP → crosses in midbrain → right VL thalamus → right motor cortex → descends in corticospinal tract → crosses again at pyramidal decussation → controls LEFT side movements.
• Two crossings = ipsilateral coordination.

Cerebellar Functions

1. Motor coordination: Ensures smooth, accurate movements (coordination of agonist-antagonist timing).
2. Timing: Precise timing of muscle contractions (millisecond accuracy).
3. Error correction (comparator function): Compares intended movement (efference copy from cortex) with actual movement (proprioceptive feedback) → computes error → corrects via loops through thalamus and motor cortex.
4. Balance and posture: Via flocculonodular lobe and fastigial nucleus → vestibulospinal and reticulospinal tracts.
5. Ocular motor control: Smooth pursuit, vestibulo-ocular reflex (VOR) correction, gaze-holding.
6. Motor learning: Adaptation of movements (e.g., adapting to prism glasses). Climbing fiber error signals + parallel fiber activity → LTD of parallel fiber - Purkinje cell synapses → recalibration.

Clinical Features of Cerebellar Lesions

• Dysdiadochokinesia - inability to perform rapid alternating movements (e.g., pronation-supination)
• Ataxia - wide-based, staggering gait (cerebellar ataxia); titubation (head/trunk tremor)
• Nystagmus - jerky eye movements; gaze-evoked nystagmus (fast phase toward side of lesion); past-pointing to the side of lesion
• Intention tremor - tremor that increases on approaching target (finger-nose-finger test); absent at rest
• Sluurred speech (dysarthria) - scanning/staccato speech; explosive or slurred
• Hypotonia - decreased muscle tone (ipsilateral); pendular reflexes

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MODULE G: The Modulator - Basal Ganglia

Q5, Q29. Basal Ganglia - Functions & Disorders

Anatomy

• Striatum = Caudate nucleus + Putamen + Nucleus accumbens (ventral striatum).
• Globus Pallidus = GPe (external) + GPi (internal).
• Substantia nigra = SNpc (pars compacta, dopaminergic) + SNpr (pars reticulata, GABAergic output).
• Subthalamic nucleus (STN): Glutamatergic; located between internal capsule and SN.

Circuits

Motor cortex (glutamate) → Striatum (D1 receptors)
                                  ↓ GABA (inhibit)
                           GPi / SNpr (tonically active GABA)
                                  ↓ GABA inhibited (disinhibited)
                            Thalamus (VL/VA) → EXCITED
                                  ↓ Glutamate
                            Motor cortex → MOVEMENT

Motor cortex (glutamate) → Striatum (D2 receptors)
                                  ↓ GABA (inhibit)
                           GPe (tonically active GABA)
                                  ↓ GABA inhibited
                            STN → EXCITED → Glutamate
                                  ↓ Glutamate (excites)
                           GPi / SNpr → MORE active → more GABA
                                  ↓ GABA (strongly inhibit)
                            Thalamus → SUPPRESSED → less cortical drive → SUPPRESSES movement

• D1 receptors on direct pathway neurons: excitatory → activates direct pathway → MORE movement.
• D2 receptors on indirect pathway neurons: inhibitory → suppresses indirect pathway → LESS suppression of movement.
• Net dopamine effect: FACILITATES movement by exciting GO and inhibiting NO-GO.

Functions of Basal Ganglia

1. Selection and suppression of movements: Facilitates desired movements, suppresses competing unwanted movements (action selection).
2. Motor learning: Reward-based learning of motor sequences (dopaminergic teaching signals from VTA/SNpc).
3. Habit formation: Stimulus-response habits via dorsal striatum.
4. Scaling amplitude and velocity: Deficient in Parkinson's (hypometric, slow movements).
5. Motivational drive to move: Via ventral striatum (nucleus accumbens) and limbic connections.
6. Cognitive and executive functions: Caudate nucleus → prefrontal cortex loops.

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Q18. Parkinson's Disease - Physiological Basis

Pathophysiology

• Mitochondrial dysfunction (Complex I inhibition - MPTP model).
• Oxidative stress (dopamine metabolism generates reactive species).
• Proteasomal dysfunction - failure to clear abnormal proteins.
• Possible prion-like propagation of α-synuclein (Braak staging - starts in peripheral/medullary centers, spreads rostrally).

NORMAL: SNpc dopamine → D1 (excites direct) + D2 (inhibits indirect)
                     → balanced Go/No-Go → smooth movement

PARKINSON'S (loss of dopamine):
  Direct pathway UNDERACTIVE (less D1 stimulation) → GPi NOT inhibited → thalamus tonically SUPPRESSED
  Indirect pathway OVERACTIVE (less D2 inhibition) → GPe inhibited → STN disinhibited → GPi more active → thalamus MORE suppressed
  
  Net: GPi/SNpr hyperactivity → thalamus strongly inhibited → motor cortex reduced drive → HYPOKINESIA

Clinical Features (4 Cardinal Signs - TRAP)

• Tremor (resting tremor) - "pill-rolling," 4-6 Hz, suppressed by voluntary movement. Due to disrupted BG-thalamo-cortical oscillations (see Q37 below).
• Rigidity - lead-pipe (uniform, velocity-independent) throughout ROM; cogwheel = rigidity + tremor. Due to increased GPi inhibition → disrupted descending control of spinal motor neurons.
• Akinesia/bradykinesia - slowness, small amplitude of movements (hypometria). Due to inability to release sufficient motor cortex activation (suppressed thalamus).
• Postural instability (late) - impaired righting reflexes, falls. Loss of postural control circuits.

Q37. Resting Tremors in Basal Ganglia Dysfunction (PB)

1. Loss of dopaminergic modulation → imbalance in direct and indirect pathways → GPi hyperactivity.
2. Pathological oscillations in the basal ganglia-thalamo-cortical circuit:
   • GPi over-inhibits thalamic neurons (VL nucleus).
   • The thalamic neurons, when over-inhibited, enter burst-firing mode (low-threshold calcium T-type channels → periodic bursting at 4-6 Hz).
   • These thalamic bursts drive rhythmic oscillations in motor cortex → rhythmic motor neuron discharge → tremor.
   • The STN is also thought to generate oscillatory activity (abnormal beta oscillations 13-30 Hz in Parkinson's motor circuits; resting tremor is often 4-6 Hz oscillation distinct from beta).
3. Cerebellar-thalamic circuits may also contribute in some patients (transcranial magnetic studies show cerebellar involvement in some Parkinson's tremor).

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Q30. Huntington's Disease (SQ)

• Selective loss of GABAergic medium spiny neurons (MSNs) in the striatum (caudate and putamen).
• Indirect pathway neurons (D2 receptors, projecting to GPe) die FIRST → less GABAergic inhibition of GPe → GPe more active → more STN inhibition → less GPi activity → LESS thalamic inhibition → MORE movement = chorea.
• Later: Direct pathway also affected → more rigidity (late stages resemble Parkinson's - Westphal variant in young-onset).
• Cortical neurons also degenerate → dementia, psychiatric symptoms.

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MODULE H: Injury & Special Pathology

Q13. Degenerative & Regenerative Changes After Peripheral Nerve Injury

Classification of Nerve Injuries (Sunderland/Seddon)

Degenerative Changes (Distal to injury - WALLERIAN DEGENERATION)

• Chromatolysis: Dissolution of Nissl bodies (rough ER) → cell body swells, nucleus shifts to periphery. Represents upregulation of metabolic machinery for axon regeneration (shift from neurotransmission to growth mode).
• May progress to cell death if injury close to cell body or if regeneration fails.

• Denervation atrophy: Progressive loss of muscle bulk (Type 2 >> Type 1 fibers initially).
• Upregulation of extrajunctional AChRs (ACh supersensitivity throughout the entire muscle fiber membrane, not just at NMJ).
• Fibrillations (spontaneous single muscle fiber contractions) - seen on EMG, not clinically visible.
• Fasciculations (motor unit firing) - seen clinically when anterior horn cells are affected (LMN).

Regenerative Changes

• Within hours of injury, the proximal axon stump begins sprouting multiple growth cones.
• Growth cones guided by: Bands of Büngner (physical channels), neurotrophic factors (NGF, BDNF), CAMs (cell adhesion molecules - L1, N-CAM), laminin/fibronectin in basal lamina.

• Degree of injury (Sunderland grade)
• Age (younger = better regeneration)
• Proximity of injury to target (longer distance = more time, more misdirection risk, more target atrophy)
• Gap between ends (surgical repair needed if >5 mm)
• Type of nerve (pure motor/sensory = better; mixed = misdirection of motor into sensory tubes)

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Q36. Cerebral Edema During Slow Ascent at High Altitude

Pathophysiology

• Hypoxia → ↑HIF-1α → ↑VEGF → disruption of tight junctions of blood-brain barrier → protein-rich fluid leaks into extracellular space of brain.
• Also: hypoxic vasodilatation of cerebral vessels → ↑cerebral blood flow + ↑capillary hydrostatic pressure → more leakage.

• Severe hypoxia → failure of Na+/K+ ATPase → Na+ and water accumulate intracellularly → cell swelling.

Why Slow Ascent is Protective (Acclimatization)

1. Hypoxic ventilatory response (HVR): Peripheral chemoreceptors (carotid body) sense ↓PO2 → stimulate ventilation → ↓PaCO2 → respiratory alkalosis. Respiratory alkalosis → inhibits further ventilatory drive (partial brake).
2. Renal compensation: Kidneys excrete HCO3- (takes 2-3 days) → corrects respiratory alkalosis → removes the "brake" on ventilation → sustained hyperventilation → better oxygenation.
3. Hematopoietic adaptation: ↓PO2 → EPO secretion from peritubular cells → ↑reticulocytes → ↑RBC mass → ↑oxygen-carrying capacity (days-weeks).
4. 2,3-DPG increase: RBCs increase 2,3-DPG synthesis → right-shifts Hb-O2 dissociation curve → O2 released more readily to tissues.
5. Cerebral autoregulation: With time, cerebral vessels adapt; reduced tendency for vasogenic edema.
6. Acetazolamide prophylaxis: Inhibits carbonic anhydrase → metabolic acidosis → mimics renal compensation → stimulates ventilation earlier.

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What You Got

# NEUROPHYSIOLOGY - COMPLETE EXAM NOTES

5 Bridge Topics + 8 Modules | Based on Guyton & Hall, Ganong, Kandel

• BRIDGE 1 - Resting Membrane Potential & Action Potential
• BRIDGE 2 - Neurotransmitters & Their Receptors
• BRIDGE 3 - Muscle Spindle, Stretch Reflex & Gamma Loop
• BRIDGE 4 - Somatotopic Organization & Brain Blood Supply
• BRIDGE 5 - CSF Dynamics & Intracranial Pressure (Monro-Kellie)
• MODULE A - Saltatory Conduction, Synaptic Transmission, Synaptic Delay, Synaptic Plasticity, Renshaw Cell
• MODULE B - Pain Definition, Pain Pathway Diagram, Referred Pain, Placebo
• MODULE C - Hypothalamus (all functions), Thermoregulation, Food Intake / Leptin/Ghrelin, Hypothalamic Obesity, Sympathetic Transmission, CVS Effects
• MODULE D - Sleep Stages & EEG, Memory Types & LTP, Emotion & Limbic System, Motor Aphasia vs Wernicke's Aphasia
• MODULE E - Corticospinal Tract (origin/course/termination), UMNL vs LMNL table, Tendon Reflexes, Clasp-Knife, Babinski Sign
• MODULE F - Cerebellum (connections, 5 cortical cell types, circuit, DANISH features, localization)
• MODULE G - Basal Ganglia circuits (Direct/Indirect), Parkinson's, Resting Tremor, Huntington's
• MODULE H - Peripheral Nerve Injury (Sunderland grading, Wallerian degeneration, Bands of Büngner), Cerebral Edema at Altitude