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CNS Physiology - Complete MBBS University Exam Notes

All answers formatted as flowcharts, tables, and structured diagrams for exam writing. Sources: Guyton & Hall, Costanzo Physiology, Neuroanatomy through Clinical Cases, Adams & Victor's Neurology

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1. SPINOTHALAMIC TRACTS - Diagram + Sensations

ANTEROLATERAL (SPINOTHALAMIC) TRACT

STIMULUS (Pain, Temperature, Crude Touch, Pressure)
            │
            ▼
    RECEPTOR in skin/tissue
            │
            ▼
  1st ORDER NEURON
  Cell body in DORSAL ROOT GANGLION
  Axon enters spinal cord via DORSAL ROOT
  → Travels 1-2 segments in LISSAUER'S TRACT
            │
            ▼
  2nd ORDER NEURON
  Cell body in DORSAL HORN
  (Lamina I - Marginal zone, Lamina V - Rexed)
  Axon CROSSES in ANTERIOR COMMISSURE
  (takes 2-3 segments to cross → lesion affects
   contralateral side a FEW SEGMENTS BELOW)
            │
            ▼
  Ascends in ANTEROLATERAL WHITE MATTER
  Somatotopy: Legs → LATERAL, Arms → MEDIAL
            │
            ▼
  Through MEDULLA (lateral, between olive & ICP)
  → PONS (lateral to medial lemniscus)
  → MIDBRAIN
            │
            ▼
  3rd ORDER NEURON
  Synapses in:
  ┌─────────────────────────────────────────────┐
  │ VPL (Ventral Posterolateral nucleus of      │
  │  Thalamus) → Discriminative pain/temp       │
  │ Intralaminar nuclei (CL nucleus) → Arousal  │
  │ Mediodorsal nuclei → Emotional aspects      │
  └─────────────────────────────────────────────┘
            │
            ▼
  PRIMARY SOMATOSENSORY CORTEX (Areas 3,1,2)
  Postcentral gyrus

• Pain (sharp & burning)
• Temperature (hot & cold)
• Crude touch
• Pressure
• Tickle, itch

---

DORSAL COLUMN - MEDIAL LEMNISCAL TRACT (Posterior/Dorsal)

STIMULUS (Fine touch, Vibration, Proprioception,
          2-point discrimination, Stereognosis)
            │
            ▼
    RECEPTOR (Meissner's, Pacinian, Muscle spindle)
            │
            ▼
  1st ORDER NEURON
  Cell body in DORSAL ROOT GANGLION
  Axon enters cord → travels IPSILATERALLY
  in DORSAL (POSTERIOR) COLUMN
  ┌──────────────────────────────┐
  │ Gracile fasciculus (MEDIAL)  │ ← Leg (T7 & below)
  │ Cuneate fasciculus (LATERAL) │ ← Arm (T6 & above)
  └──────────────────────────────┘
            │
            ▼
  2nd ORDER NEURON
  Nucleus GRACILIS & Nucleus CUNEATUS
  in MEDULLA (at cervicomedullary junction)
  Axons cross as INTERNAL ARCUATE FIBERS
  → form MEDIAL LEMNISCUS
            │
            ▼
  Ascends IPSILATERAL in medulla, pons, midbrain
  (contralateral to original stimulus)
            │
            ▼
  3rd ORDER NEURON
  VPL of THALAMUS
            │
            ▼
  PRIMARY SOMATOSENSORY CORTEX (Areas 3,1,2)
  Postcentral gyrus

• Fine/discriminative touch
• Vibration sense
• Conscious proprioception (joint position)
• Two-point discrimination
• Stereognosis
• Graphesthesia

---

2. CEREBELLAR CONNECTIONS, FUNCTIONS & DISORDERS

╔══════════════════════════════════════════════════════════╗
║              CEREBELLAR CONNECTIONS                      ║
╠══════════════════════════════════════════════════════════╣
║  INPUTS (AFFERENTS)                                      ║
║  ┌────────────────────────────────────────────────────┐  ║
║  │ 1. Spinocerebellar tracts                          │  ║
║  │    • Dorsal (posterior) → ICP → Ipsilateral        │  ║
║  │    • Ventral (anterior) → SCP → Contralateral      │  ║
║  │    Carries: proprioception from muscles/joints     │  ║
║  │                                                    │  ║
║  │ 2. Corticopontocerebellar (CPC) → MCP              │  ║
║  │    Motor cortex → Pontine nuclei → MCP →           │  ║
║  │    Contralateral cerebellar hemisphere              │  ║
║  │                                                    │  ║
║  │ 3. Vestibulocerebellar → ICP → Flocculonodular     │  ║
║  │    Vestibular nuclei → Archicerebellum             │  ║
║  │                                                    │  ║
║  │ 4. Olivocerebellar → ICP → All cortex              │  ║
║  │    Inferior olivary nucleus → Climbing fibers      │  ║
║  └────────────────────────────────────────────────────┘  ║
║                                                          ║
║  OUTPUTS (EFFERENTS)                                     ║
║  ┌────────────────────────────────────────────────────┐  ║
║  │ Deep cerebellar nuclei (medial→lateral):           │  ║
║  │ Fastigial → Dentate → Emboliform → Globose         │  ║
║  │ ("Fat Daddy Eats Grass")                           │  ║
║  │                                                    │  ║
║  │ DENTATE nucleus (largest) → SCP →                  │  ║
║  │ Decussates → Red nucleus/VL thalamus →             │  ║
║  │ Motor cortex → controls contralateral limb         │  ║
║  │ NOTE: Double cross → ipsilateral control           │  ║
║  │                                                    │  ║
║  │ FASTIGIAL nucleus → ICP → Vestibular nuclei        │  ║
║  │ → Balance/posture                                  │  ║
║  └────────────────────────────────────────────────────┘  ║
╚══════════════════════════════════════════════════════════╝

CEREBELLAR DISORDERS
├── DECOMPOSITION OF MOVEMENT - movements broken into parts
├── DYSMETRIA - over/undershooting (past-pointing)
├── DYSDIADOCHOKINESIA - failure of rapid alternating movements
├── INTENTION TREMOR - tremor during movement (≠ resting tremor)
├── HYPOTONIA - pendular knee jerk
├── NYSTAGMUS - eyes oscillate (flocculonodular lesion)
├── SCANNING/STACCATO SPEECH - dysarthria
├── ATAXIC GAIT - wide-based, staggering
└── REBOUND PHENOMENON - failure to check movement

MNEMONIC: "DANISH" = Dysdiadochokinesia, Ataxia, Nystagmus,
           Intention tremor, Slurred speech, Hypotonia

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3. BASAL GANGLIA - Connections, Functions & Disorders

BASAL GANGLIA COMPONENTS:
Striatum (Caudate + Putamen) + Globus Pallidus (GPe + GPi)
+ Subthalamic Nucleus (STN) + Substantia Nigra (SNc + SNr)
+ Amygdala

╔══════════════ DIRECT PATHWAY (EXCITATORY NET) ══════════╗
║  Cortex ──(+Glu)──► Striatum ──(-GABA, D1)──►          ║
║  GPi/SNr ──(-GABA)──► Thalamus (inhibited less) ──(+)──► ║
║  Motor Cortex → ↑ MOVEMENT                              ║
╚═════════════════════════════════════════════════════════╝

╔══════════════ INDIRECT PATHWAY (INHIBITORY NET) ════════╗
║  Cortex ──(+Glu)──► Striatum ──(-GABA, D2)──►          ║
║  GPe ──(-GABA)──► STN (disinhibited) ──(+Glu)──►       ║
║  GPi/SNr ──(-GABA)──► Thalamus (MORE inhibited) ──(+)──► ║
║  Motor Cortex → ↓ MOVEMENT                              ║
╚═════════════════════════════════════════════════════════╝

DOPAMINE FROM SNc:
• D1 receptors → ACTIVATES Direct pathway → ↑ movement
• D2 receptors → INHIBITS Indirect pathway → ↑ movement
(Both dopamine effects facilitate movement)

• Planning and initiation of voluntary movement
• Scaling of movement amplitude and force
• Suppression of unwanted movements
• Cognitive and affective functions
• Procedural (habit) learning

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4. PARKINSON'S DISEASE - Case History, Physiology, Pathology

Case History (Exam Format):

60-year-old male, retired teacher, presents with 3-year history of tremor in right hand at rest, difficulty initiating walking, stooped posture, expressionless face, soft monotonous voice. Wife notes he moves slowly getting dressed.

PARKINSON'S DISEASE - FLOWCHART

PATHOLOGY:
Loss of dopaminergic neurons in SUBSTANTIA NIGRA PARS COMPACTA
+ LEWY BODIES (intraneuronal α-synuclein aggregates)
Normal: SNc dopamine → activates DIRECT pathway +
        inhibits INDIRECT pathway → NET: facilitates movement

In PD: ↓ Dopamine →
├── ↓ D1 stimulation → ↓ Direct pathway → ↓ motor facilitation
└── ↓ D2 inhibition → ↑ Indirect pathway → ↑ STN activity
                    → ↑ GPi inhibition → ↓ Thalamic output
                    → ↓ Motor cortex activation

RESULT: HYPOKINESIA (reduced movement)

SIGNS & PHYSIOLOGICAL BASIS:

SIGN                  │ PHYSIOLOGICAL BASIS
──────────────────────┼────────────────────────────────────────
RESTING TREMOR        │ Loss of dopamine → oscillatory activity
(pill-rolling, 4-6Hz) │ in STN-GPi loop; disappears on intention
                      │
RIGIDITY              │ ↑ Gamma motor neuron activity → ↑ muscle
(Cogwheel/Leadpipe)   │ spindle sensitivity → ↑ stretch reflex
                      │ in agonist AND antagonist
                      │
BRADYKINESIA/AKINESIA │ ↓ Motor cortex activation via thalamus
(slowness/absence of  │ ↓ Scaling of movement (supplementary
movement)             │ motor area dysfunction)
                      │
POSTURAL INSTABILITY  │ Loss of postural righting reflexes;
(festinant gait,      │ ↓ cerebellar-BG coordination;
propulsive)           │ flexed posture from rigidity
                      │
MASKED FACE           │ Rigidity of facial muscles
(hypomimia)           │ ↓ spontaneous facial motor programs
                      │
MICROGRAPHIA          │ ↓ Amplitude of movement programming
                      │
MONOTONE SPEECH       │ ↓ Control of vocal muscles amplitude
(hypophonia)          │
                      │
AUTONOMIC FEATURES    │ Degeneration extends to autonomic
(constipation,        │ ganglia & Lewy bodies in ANS
orthostatic hypotension)│

L-DOPA + Carbidopa (AADC inhibitor, prevents peripheral conversion)
→ crosses BBB → converted to Dopamine in CNS
+ Dopamine agonists (Bromocriptine, Pramipexole)
+ MAO-B inhibitors (Selegiline)
+ Amantadine (releases dopamine)
+ Deep Brain Stimulation (STN or GPi)

---

5. MUSCLE SPINDLE & STRETCH REFLEX

MUSCLE SPINDLE STRUCTURE:
┌─────────────────────────────────────────────┐
│         INTRAFUSAL MUSCLE FIBER             │
│  ┌─────────────────────────────────────┐    │
│  │ Nuclear BAG fiber (dynamic response)│    │
│  │ Nuclear CHAIN fiber (static response│    │
│  └─────────────────────────────────────┘    │
│                                             │
│  AFFERENTS:                                 │
│  • Ia (Annulospiral) → Bag + Chain fibers   │
│    Detects RATE of stretch (dynamic)        │
│  • II (Flower-spray) → Chain fibers mainly  │
│    Detects STATIC length                    │
│                                             │
│  EFFERENTS:                                 │
│  • γ static → Chain fibers → sets gain     │
│  • γ dynamic → Bag fibers → dynamic resp   │
└─────────────────────────────────────────────┘

Muscle STRETCHED
      │
      ▼
Ia afferent ACTIVATED (from nuclear bag fiber)
      │
      ▼
Enters DORSAL HORN of spinal cord
      │
      ├──► Monosynaptic synapse → Alpha motor neuron
      │    of SAME muscle → CONTRACTION (agonist)
      │
      └──► Via interneurons → INHIBIT alpha motor neuron
           of antagonist (RECIPROCAL INHIBITION)

      ▼
Muscle CONTRACTS (resists stretch)

Higher centers → γ motor neurons → Contract intrafusal fiber →
→ Maintain Ia sensitivity during muscle contraction
→ Adjust "gain" of stretch reflex
→ Important for posture & fine motor control

• Deep tendon reflexes (knee jerk = L2,L3,L4; ankle = S1,S2)
• UMN lesion → ↑ reflexes (gamma overactivity)
• LMN lesion → ↓ or absent reflexes

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6. PROPERTIES OF RECEPTORS

PROPERTIES OF SENSORY RECEPTORS
├── 1. ADEQUATE STIMULUS - each receptor has a specific
│        stimulus to which it is maximally sensitive
│        (e.g. photoreceptors → light)
│
├── 2. TRANSDUCTION - conversion of stimulus energy
│        into electrical (receptor) potential
│
├── 3. RECEPTOR POTENTIAL - graded, not all-or-none
│        proportional to stimulus intensity
│
├── 4. ADAPTATION
│    ├── FAST (Phasic): Meissner's, Pacinian
│    │   Respond to CHANGE, not maintained stimulus
│    └── SLOW (Tonic): Merkel's, Ruffini's
│        Respond throughout stimulus duration
│
├── 5. GENERATOR POTENTIAL → exceeds threshold →
│        Action potential in afferent nerve
│
├── 6. LAW OF SPECIFIC NERVE ENERGIES (Muller)
│        Any stimulation of a nerve → sensation
│        specific to that nerve (sight, sound, etc.)
│
├── 7. WEBER-FECHNER LAW ⭐
│    "The perceived intensity of sensation is
│    proportional to the LOGARITHM of stimulus intensity"
│        S = K × log(I/I₀)
│    S = sensation magnitude
│    I = stimulus intensity
│    I₀ = threshold intensity
│    K = constant
│    Explains why: 1 candle in dark is noticeable but
│    not 1 candle added to 1000 candles
│
└── 8. LAW OF PROJECTION / PHANTOM LIMB ⭐
     Whatever the point of stimulation of a sensory
     nerve, the sensation is PROJECTED to the
     peripheral receptor area (e.g. pain felt at
     fingertip when ulnar nerve hit at elbow)
     PHANTOM LIMB: After amputation, stimulation of
     nerve stump → sensation projected to (missing)
     limb → patient "feels" absent limb
     Basis: brain interprets impulses as coming from
     original receptor location

---

7. SYNAPSE - Classification & Properties

CLASSIFICATION OF SYNAPSES

BY STRUCTURE:
├── CHEMICAL SYNAPSE (most common in CNS)
│   ├── Axodendritic (most common)
│   ├── Axosomatic
│   ├── Axoaxonic (presynaptic inhibition)
│   └── Dendrodendritic
│
└── ELECTRICAL SYNAPSE
    └── Gap junctions, bidirectional, no delay
        (e.g. cardiac muscle, smooth muscle)

BY FUNCTION:
├── EXCITATORY → EPSP (depolarization)
│   Neurotransmitters: Glutamate, ACh, Norepinephrine
└── INHIBITORY → IPSP (hyperpolarization)
    Neurotransmitters: GABA, Glycine

BY LOCATION:
├── Peripheral (NMJ)
└── Central (CNS synapses)

---

8. PAIN PATHWAY

PAIN PATHWAY (NOCICEPTIVE)

STIMULUS (tissue damage, noxious chemical/thermal/mechanical)
      │
      ▼
FREE NERVE ENDINGS (nociceptors)
      │
      ▼
PRIMARY AFFERENTS:
├── Aδ fibers (thin myelinated) → FAST/SHARP pain
│   "First pain" (immediate, well-localized)
└── C fibers (unmyelinated) → SLOW/BURNING pain
    "Second pain" (delayed, poorly localized)
      │
      ▼
DORSAL ROOT GANGLION (1st order neuron cell body)
      │
      ▼
DORSAL HORN of spinal cord:
├── Aδ → Lamina I (marginal zone), Lamina V
└── C fibers → Lamina I, II (substantia gelatinosa)
Neurotransmitters: Substance P, Glutamate (NMDA & AMPA)
      │
      ▼
CROSSING: Anterior commissure (2-3 segments above entry)
      │
      ▼
ANTEROLATERAL PATHWAY:
├── SPINOTHALAMIC TRACT → VPL thalamus
│   → S1 cortex (location, intensity, quality)
│
├── SPINORETICULAR TRACT → Reticular formation →
│   Intralaminar thalamus → diffuse cortex
│   (arousal, motivational aspects)
│
└── SPINOMESENCEPHALIC TRACT → PAG (midbrain)
    (activates descending inhibition)
      │
      ▼
CORTICAL PROCESSING:
├── S1 cortex (postcentral gyrus) - discriminative
├── S2 cortex (superior temporal) - intensity
├── Anterior cingulate cortex - emotional/suffering
└── Insular cortex - autonomic responses

• Aδ & C fibers → Trigeminal ganglion → Descend to nucleus caudalis (in medulla, equivalent to spinal dorsal horn) → Cross → VPM thalamus → S1 cortex

---

9. PAIN SUPPRESSION MECHANISMS & GATE CONTROL

STIMULUS ACTIVATES PAG (Periaqueductal Gray)
Midbrain around aqueduct of Sylvius
      │
      ▼ (Enkephalin neurons)
RAPHE MAGNUS NUCLEUS (lower pons/upper medulla)
+ Nucleus Reticularis Paragigantocellularis (medulla)
      │
      ▼ (Serotonin fibers in dorsolateral column)
DORSAL HORN (spinal cord)
      │
Local interneurons release ENKEPHALIN
      │
      ▼
PRESYNAPTIC & POSTSYNAPTIC INHIBITION
of incoming Aδ and C fiber pain signals

• Endorphins: β-endorphin (pituitary), dynorphins
• Enkephalins: met-enkephalin, leu-enkephalin
• Act on μ, δ, κ opioid receptors
• ↓ Substance P release (presynaptic)
• ↓ cAMP → hyperpolarization (postsynaptic)

        Aβ (touch/vibration) ──────────────────┐
                                               ▼
                              + (excites)  SG CELL ────(inhibits)──►
                                          (Substantia                 │
                                           Gelatinosa)                │
                                               ▲                      │
        Aδ / C (pain) ─────────────────────────┘ (inhibits SG)       │
                                                                       │
                                                                       ▼
                                                               TRANSMISSION CELL
                                                               (T cell, Lamina V)
                                                                       │
                                                                       ▼
                                                               BRAIN (Pain perceived)
─────────────────────────────────────────────────────────────────────────────────
KEY POINTS:
• Aβ (non-pain) fibers → EXCITE SG cell → SG CLOSES gate → pain BLOCKED
• Aδ/C (pain) fibers → INHIBIT SG cell → gate OPENS → pain reaches T cell
• Descending control from brain also closes/opens gate
CLINICAL APPLICATIONS:
• TENS (Transcutaneous Electrical Stimulation) → stimulates Aβ → closes gate
• Acupuncture → stimulates large fibers → closes gate
• Rubbing an injury → activates Aβ → reduces pain

---

10. HYPOTHALAMUS - Nuclei, Connections & Functions

HYPOTHALAMIC NUCLEI (Anterior → Posterior)

ANTERIOR GROUP:
├── PREOPTIC nucleus → controls GnRH release, sexual behavior
├── ANTERIOR nucleus → heat dissipation (COOLING center)
│   Parasympathetic control → sweating, vasodilation
└── SUPRACHIASMATIC nucleus → circadian rhythm (biological clock)
    receives direct retinal input (retinohypothalamic tract)

TUBERAL (MIDDLE) GROUP:
├── VENTROMEDIAL nucleus (VMH) → SATIETY center
│   Lesion → hyperphagia, obesity
├── LATERAL HYPOTHALAMIC AREA (LHA) → FEEDING/HUNGER center
│   Lesion → anorexia, starvation
├── ARCUATE nucleus → releases GnRH, dopamine (controls pituitary)
└── DORSOMEDIAL nucleus → behavioral responses

POSTERIOR GROUP:
├── POSTERIOR nucleus → heat conservation (WARMING center)
│   Sympathetic → vasoconstriction, shivering
├── MAMMILLARY bodies → memory (part of Papez circuit)
└── SUPRAOPTIC & PARAVENTRICULAR nuclei (overlap anterior/tuberal)
    Secrete: ADH (AVP) & Oxytocin → released from posterior pituitary

AFFERENTS TO HYPOTHALAMUS:
├── From limbic system (hippocampus, amygdala) via fornix & stria terminalis
├── From brainstem (reticular formation, autonomic nuclei)
├── From thalamus (mediodorsal nucleus)
└── From retina (suprachiasmatic)

EFFERENTS FROM HYPOTHALAMUS:
├── Hypothalamohypophyseal tract → posterior pituitary (ADH, oxytocin)
├── Tuberoinfundibular tract → median eminence → portal blood → anterior pituitary
├── Descending to autonomic centers (sympathetic & parasympathetic)
└── To limbic system (fornix), thalamus, brainstem

---

11. BROWN-SÉQUARD SYNDROME

HEMISECTION OF SPINAL CORD

IPSILATERAL SIDE (same as lesion):
├── MOTOR: UMN paralysis below lesion
│   (Corticospinal tract is ipsilateral below decussation)
├── LOSS of PROPRIOCEPTION, VIBRATION, FINE TOUCH below lesion
│   (Dorsal columns travel ipsilaterally before crossing in medulla)
└── LMN signs AT LEVEL of lesion (flaccid, wasting)

CONTRALATERAL SIDE (opposite to lesion):
└── LOSS of PAIN & TEMPERATURE 2-3 segments below lesion
    (Spinothalamic tract crosses 2-3 segments after entry)

IPSILATERAL at lesion level:
└── Band of anesthesia (all modalities) at lesion level

┌────────────────────────────────────────────────┐
│              LESION (Right side, T6)           │
├────────────────────────────────────────────────┤
│ RIGHT side below T6:                           │
│  • UMN paralysis (leg)                         │
│  • Loss of vibration, proprioception, fine touch│
│  • Vasodilation, anhidrosis (sympathetic loss) │
│                                                │
│ LEFT side below T8 (2-3 segments below T6):    │
│  • Loss of pain and temperature                │
└────────────────────────────────────────────────┘

CAUSES: Spinal cord injury, MS, tumor, disc herniation

---

12. CEREBROSPINAL FLUID (CSF)

CSF PRODUCTION & CIRCULATION:

CHOROID PLEXUS (lateral ventricles mostly)
      │ ↓ 500 ml/day produced, ~150 ml at any time
      ▼
LATERAL VENTRICLES → FORAMEN OF MONRO
      ▼
3rd VENTRICLE → AQUEDUCT OF SYLVIUS (Cerebral aqueduct)
      ▼
4th VENTRICLE
      │
      ├──► Foramen of MAGENDIE (median) → Subarachnoid space
      └──► Foramina of LUSCHKA (lateral) → Subarachnoid space
      ▼
SUBARACHNOID SPACE (around brain and cord)
      ▼
ARACHNOID VILLI (granulations) → SUPERIOR SAGITTAL SINUS
(reabsorbed by bulk flow into venous blood)

• Mechanical cushion (buoyancy) - brain weighs 50g in CSF vs 1400g in air
• Transport of nutrients and waste
• Regulation of intracranial pressure
• Immunological surveillance
• Temperature regulation
• Extracellular fluid of brain

---

13. BLOOD-BRAIN BARRIER (BBB)

STRUCTURAL BASIS:
┌─────────────────────────────────────────────────┐
│  Capillary LUMEN                                │
│       │                                         │
│  TIGHT JUNCTIONS between endothelial cells      │
│  (Zona occludens - no paracellular diffusion)   │
│       │                                         │
│  CONTINUOUS BASEMENT MEMBRANE                   │
│       │                                         │
│  PERICYTES (contractile cells on outside)       │
│       │                                         │
│  ASTROCYTE foot processes (induce/maintain BBB) │
│       │                                         │
│  BRAIN PARENCHYMA                               │
└─────────────────────────────────────────────────┘

• Area postrema (vomiting trigger), median eminence, posterior pituitary, subfornical organ, OVLT
• Allow brain to "sample" blood composition

---

14. PYRAMIDAL TRACTS

CORTICOSPINAL TRACT (Lateral + Anterior)

ORIGIN:
├── Primary motor cortex (Area 4) - 30%
├── Premotor cortex (Area 6) - 30%
├── Somatosensory cortex (Areas 3,1,2,5,7) - 40%

COURSE:
Motor cortex → Corona radiata
      ↓
Posterior limb of INTERNAL CAPSULE
(anterior 2/3)
      ↓
CEREBRAL PEDUNCLE (middle 3/5 of crus cerebri)
      ↓
PONS (scattered through pontine nuclei)
      ↓
PYRAMID of MEDULLA
      ↓
PYRAMIDAL DECUSSATION (85-90% cross)
      ↓
LATERAL CORTICOSPINAL TRACT (crossed)    +    ANTERIOR CST (uncrossed)
in lateral funiculus                          in anterior funiculus
      ↓                                              ↓
Alpha motor neurons in                   Crosses at spinal cord level
VENTRAL HORN                             (axial muscles bilaterally)

TERMINATION:
└── Synapse on ventral horn alpha motor neurons
    (direct monosynaptic for fine finger movements)
    (via interneurons for most connections)
    → Lower motor neurons → skeletal muscles

SOMATOTOPY in internal capsule:
Leg fibers (most posterior) → Arm → Face (most anterior)

FUNCTIONS:
├── Voluntary skilled movements (especially fine finger)
├── Speed and agility of movement
├── Control of distal muscles
└── Fractionation of movements (independent finger movement)

---

15. PAPEZ CIRCUIT & LIMBIC SYSTEM

PAPEZ CIRCUIT (1937):

HIPPOCAMPUS (memory formation)
      │ (via FORNIX)
      ▼
MAMMILLARY BODIES (hypothalamus)
      │ (via MAMMILLOTHALAMIC TRACT)
      ▼
ANTERIOR NUCLEUS of THALAMUS
      │ (via THALAMOCORTICAL RADIATION)
      ▼
CINGULATE CORTEX (Areas 23, 24)
(emotional coloring of experience)
      │ (via CINGULUM bundle)
      ▼
PARAHIPPOCAMPAL GYRUS (ENTORHINAL CORTEX)
      │ (via PERFORANT PATH)
      ▼
Back to HIPPOCAMPUS ◄───────────────────────

CIRCUIT: HiPPO → Mammillary bodies → Anterior thalamus →
         Cingulate → Parahippocampal → HiPPO
MNEMONIC: "Hippocrates Made All Circuits Perfect"

• Emotional behavior (fear, anger, pleasure, sadness)
• Learning and memory (hippocampus)
• Olfaction (primary olfactory cortex)
• Autonomic regulation
• Sexual/reproductive behavior
• Feeding behavior
• Fight or flight response (amygdala)

---

16. UMN vs LMN LESION DIFFERENCES

FEATURE          │ UMN LESION              │ LMN LESION
─────────────────┼─────────────────────────┼──────────────────────
Location         │ Above anterior horn cell│ Anterior horn cell or
                 │ (brain, cord above AHC) │ below (nerve, muscle)
─────────────────┼─────────────────────────┼──────────────────────
Tone             │ ↑ SPASTICITY            │ ↓ FLACCIDITY
                 │ (clasp-knife rigidity)  │
─────────────────┼─────────────────────────┼──────────────────────
Reflexes (DTR)   │ ↑ HYPERREFLEXIA         │ ↓ HYPOREFLEXIA
                 │                         │ or AREFLEXIA
─────────────────┼─────────────────────────┼──────────────────────
Plantar reflex   │ EXTENSOR (Babinski +ve) │ FLEXOR (normal)
─────────────────┼─────────────────────────┼──────────────────────
Wasting/atrophy  │ DISUSE ATROPHY          │ SEVERE atrophy
                 │ (mild, late)            │ (neurogenic, early)
─────────────────┼─────────────────────────┼──────────────────────
Fasciculations   │ ABSENT                  │ PRESENT
─────────────────┼─────────────────────────┼──────────────────────
Fibrillations    │ Absent                  │ Present (on EMG)
(on EMG)         │                         │
─────────────────┼─────────────────────────┼──────────────────────
Paralysis        │ Spastic paralysis       │ Flaccid paralysis
─────────────────┼─────────────────────────┼──────────────────────
Clonus           │ PRESENT                 │ Absent
─────────────────┼─────────────────────────┼──────────────────────
Abdominal reflex │ ABSENT                  │ Present (if reflex arc
                 │                         │ intact)
─────────────────┼─────────────────────────┼──────────────────────
Voluntary control│ Lost (distal > proximal)│ Lost (in distribution
                 │                         │ of nerve/root)
─────────────────┼─────────────────────────┼──────────────────────
Examples         │ Stroke, MS, cord trauma │ Polio, GBS, disc
                 │                         │ prolapse, motor neuropathy

---

17. SYMPATHETIC vs PARASYMPATHETIC

FEATURE          │ SYMPATHETIC             │ PARASYMPATHETIC
─────────────────┼─────────────────────────┼──────────────────────
Origin           │ T1-L2 (thoracolumbar)   │ CN III,VII,IX,X + S2-S4
─────────────────┼─────────────────────────┼──────────────────────
Ganglia location │ PARAVERTEBRAL           │ Close to/within organ
                 │ (sympathetic chain)     │ (terminal ganglia)
─────────────────┼─────────────────────────┼──────────────────────
Pre/Post ratio   │ Short pre, long post    │ Long pre, SHORT post
─────────────────┼─────────────────────────┼──────────────────────
Preganglionic NT │ Acetylcholine (nicotinic)│ Acetylcholine (nicotinic)
─────────────────┼─────────────────────────┼──────────────────────
Postganglionic NT│ NOREPINEPHRINE          │ ACETYLCHOLINE (muscarinic)
                 │ (except sweat, adrenal) │
─────────────────┼─────────────────────────┼──────────────────────
Heart rate       │ ↑ (beta-1)              │ ↓ (M2)
─────────────────┼─────────────────────────┼──────────────────────
Blood vessels    │ Vasoconstriction        │ Vasodilation (limited)
─────────────────┼─────────────────────────┼──────────────────────
Bronchi          │ Bronchodilation (β2)    │ Bronchoconstriction
─────────────────┼─────────────────────────┼──────────────────────
GI motility      │ ↓                       │ ↑
─────────────────┼─────────────────────────┼──────────────────────
GI sphincters    │ Contract (↑ tone)       │ Relax (↓ tone)
─────────────────┼─────────────────────────┼──────────────────────
Pupils           │ MYDRIASIS (dilate)      │ MIOSIS (constrict)
─────────────────┼─────────────────────────┼──────────────────────
Salivary gland   │ Viscous secretion       │ Watery, profuse secretion
─────────────────┼─────────────────────────┼──────────────────────
Urinary bladder  │ Relax detrusor          │ Contract detrusor
                 │ Contract sphincter      │ Relax sphincter
─────────────────┼─────────────────────────┼──────────────────────
Sweating         │ ACh (sympathetic)       │ Not supplied
─────────────────┼─────────────────────────┼──────────────────────
Overall role     │ "FIGHT or FLIGHT"       │ "REST and DIGEST"

---

18. AUTONOMIC FUNCTION TESTS

TESTS FOR SYMPATHETIC FUNCTION:
├── Cold pressor test (immerse hand in cold water →
│   normal: BP rises by 10-20 mmHg)
├── Isometric exercise test (sustained handgrip →
│   normal: BP rises)
├── Valsalva maneuver (4 phases):
│   Phase I: BP rises (↑intrathoracic pressure → aortic compression)
│   Phase II: BP falls (↓venous return) → reflex tachycardia
│   Phase III: Brief further fall
│   Phase IV: OVERSHOOT of BP → reflex bradycardia
│   Valsalva ratio = max HR during phase II / min HR in phase IV
│   Normal ≥ 1.2; <1.0 = autonomic failure
└── Sympathetic skin response (galvanic skin response)

TESTS FOR PARASYMPATHETIC FUNCTION:
├── Heart rate variability with DEEP BREATHING
│   (respiratory sinus arrhythmia)
│   Normal: max-min HR ≥ 15 bpm
│   Diabetic neuropathy → reduced variability
├── Lying to Standing test
│   Normally: HR ↑ at 15th beat after standing
│   then ↓ at 30th beat (30:15 ratio ≥ 1.04)
└── Atropine test (IV atropine → ↑ HR if
    parasympathetic is functioning)

---

19. EEG (Electroencephalogram)

EEG WAVE TYPES:

WAVE  │ FREQUENCY │ AMPLITUDE │ CONDITION
──────┼───────────┼───────────┼────────────────────────────────
ALPHA │ 8-13 Hz   │ 50 μV     │ Relaxed, eyes closed, awake
      │           │           │ Disappears with mental activity
──────┼───────────┼───────────┼────────────────────────────────
BETA  │ 14-30 Hz  │ 20 μV     │ Alert, mental activity, eyes open
      │           │           │ Anxiety, sedative drugs (BZDs)
──────┼───────────┼───────────┼────────────────────────────────
THETA │ 4-7 Hz    │ Variable  │ Drowsiness, emotional stress
      │           │           │ Normal in children; abnormal adults
──────┼───────────┼───────────┼────────────────────────────────
DELTA │ 0.5-4 Hz  │ 100+ μV   │ Deep sleep (Stage 3/4, NREM)
      │           │           │ Pathological if awake (tumor, lesion)
──────┼───────────┼───────────┼────────────────────────────────
GAMMA │ >30 Hz    │ Low       │ Complex cognitive functions

• Epilepsy: Spike and wave complexes (3/sec in absence seizures)
• Encephalitis: Diffuse slowing
• Brain death: Isoelectric (flat) EEG
• Sleep staging
• Depth of anesthesia monitoring

---

20. REM vs NREM SLEEP

FEATURE          │ REM SLEEP               │ NREM SLEEP (Stages 1-4)
─────────────────┼─────────────────────────┼──────────────────────
EEG              │ Low amplitude,          │ High amplitude,
                 │ HIGH FREQUENCY          │ LOW FREQUENCY
                 │ (like wakefulness)      │ (theta/delta)
─────────────────┼─────────────────────────┼──────────────────────
Eye movements    │ RAPID eye movements     │ Slow or absent
─────────────────┼─────────────────────────┼──────────────────────
Muscle tone      │ ATONIA (paralyzed)      │ Reduced but present
─────────────────┼─────────────────────────┼──────────────────────
Dreams           │ Vivid, narrative dreams │ Bland, thought-like
─────────────────┼─────────────────────────┼──────────────────────
Autonomic        │ Variable (↑HR, ↑BP,     │ Regular, stable
activity         │ irregular breathing)    │
─────────────────┼─────────────────────────┼──────────────────────
Penile erection  │ PRESENT                 │ Absent
─────────────────┼─────────────────────────┼──────────────────────
Memory           │ Consolidation of        │ Consolidation of
consolidation    │ procedural/emotional    │ declarative memory
─────────────────┼─────────────────────────┼──────────────────────
Growth hormone   │ Not secreted            │ SECRETED (Stage 3/4)
─────────────────┼─────────────────────────┼──────────────────────
% of total sleep │ ~25% (90-100 min total) │ ~75%
─────────────────┼─────────────────────────┼──────────────────────
Cycles per night │ 4-5 cycles (~90 min)    │ Early night: more NREM
                 │ REM longer later        │ Later: more REM
─────────────────┼─────────────────────────┼──────────────────────
Control center   │ Reticular activating    │ Raphe nuclei (serotonin)
                 │ system, locus coeruleus │ VLPO (ventrolateral
                 │ turned OFF              │ preoptic area, GABA)
─────────────────┼─────────────────────────┼──────────────────────
Disorders        │ REM sleep behavior      │ Sleepwalking,
                 │ disorder, nightmares    │ night terrors, bedwetting

---

21. BELL-MAGENDIE LAW

BELL-MAGENDIE LAW:
"DORSAL (POSTERIOR) ROOTS are SENSORY;
 VENTRAL (ANTERIOR) ROOTS are MOTOR"

DORSAL ROOT:           VENTRAL ROOT:
├── Sensory neurons    ├── Motor neurons
├── Cell bodies in     ├── Cell bodies in
│   dorsal root ganglion│  ventral horn (AHC)
├── Afferent impulses  ├── Efferent impulses
│   (toward cord)      │   (away from cord)
└── Touch, pain, temp, └── Alpha, beta, gamma
    proprioception         motor neurons

Lesion DORSAL root → Loss of sensation ONLY
Lesion VENTRAL root → Loss of motor function ONLY
Mixed spinal nerve (after merging) → Both affected

---

22. RENSHAW CELL INHIBITION & FEEDFORWARD INHIBITION

RENSHAW CELL INHIBITION (RECURRENT/FEEDBACK INHIBITION):

Alpha motor neuron (fires) ──collateral──► RENSHAW CELL
                                              │ (interneuron, inhibitory)
                                              │ (uses GLYCINE)
                                              ▼
                                    Back to ALPHA MOTOR NEURON
                                    (and to other motor neurons)
                                    → INHIBITION (hyperpolarization)

FUNCTION:
├── Limits duration and frequency of motor neuron discharge
├── Prevents excessive or prolonged contraction
├── Stabilizes motor output (negative feedback)
├── Allows precise, graded muscle contraction
└── "Automatic gain control"

CLINICAL: Tetanus toxin blocks inhibitory interneurons →
spastic paralysis (Renshaw cell inhibition lost)
Strychnine: blocks glycine receptors → same effect


FEEDFORWARD INHIBITION (SURROUND/LATERAL INHIBITION):

Stimulus ──► Group Ia afferent ──► Alpha motor neuron (A)
                │                   (EXCITATION → contraction)
                │
                └──► via interneuron ──► Antagonist motor neuron (B)
                                         (INHIBITION)
                                         = RECIPROCAL INHIBITION

Also called: DIRECT/DISYNAPTIC INHIBITION

FUNCTION:
├── Smooth, coordinated movement (relaxes antagonist while agonist contracts)
├── Stretch reflex: Ia fiber excites agonist, inhibits antagonist
└── Used in flexor-extensor coordination

DIFFERENCE:
Renshaw = FEEDBACK (recurrent collateral, same motor neuron)
Feedforward (Reciprocal) = DIRECT from afferent via interneuron to antagonist

---

23. RADIATING & REFERRED PAIN

RADIATING PAIN:
Definition: Pain felt along the course of a nerve, radiating from
origin outward, following nerve distribution

Basis:
Irritation of nerve/nerve root → impulses travel along entire
nerve → pain perceived along distribution

Examples:
├── Sciatica: L4-L5 or L5-S1 disc prolapse → pain radiates
│   from back → buttock → posterior thigh → leg → foot
├── Femoral neuralgia (L2,L3,L4): anterior thigh
├── Trigeminal neuralgia: along V1,V2,V3 distribution
└── Carpal tunnel: median nerve pain to fingers

──────────────────────────────────────────────────────────

REFERRED PAIN:
Definition: Visceral pain felt at a body surface area remote
from the diseased organ, sharing the same dermatome

Basis (CONVERGENCE-PROJECTION THEORY):
Visceral and somatic afferents CONVERGE on the SAME
2nd order neuron in dorsal horn.
Brain "projects" the pain to the somatic area
(more familiar pain origin) → patient feels it at skin surface

Examples:
┌─────────────────┬──────────────────────────────┐
│ Organ           │ Referred location             │
├─────────────────┼──────────────────────────────┤
│ Heart (MI)      │ Left arm, jaw, shoulder (T1-T4)│
│ Appendix        │ Umbilicus (T10), then RIF     │
│ Diaphragm       │ Shoulder tip (C3,C4 - phrenic)│
│ Gallbladder     │ Right shoulder tip, right scapula│
│ Kidney          │ Groin, inner thigh (T10-L1)   │
│ Liver           │ Right shoulder               │
│ Uterus          │ Lower back, thighs           │
│ Ureter stones   │ Groin, genitalia             │
└─────────────────┴──────────────────────────────┘

---

24. TYPES OF LEARNING & MEMORY

LEARNING TYPES:
├── NON-ASSOCIATIVE:
│   ├── HABITUATION: Repeated same stimulus →
│   │   ↓ response (e.g., ignoring background noise)
│   │   Mechanism: ↓ neurotransmitter release
│   └── SENSITIZATION: Intense/noxious stimulus →
│       ↑ response to subsequent stimuli
│       Mechanism: ↑ Ca²⁺ entry → ↑ transmitter release
│
├── ASSOCIATIVE:
│   ├── CLASSICAL CONDITIONING (Pavlov):
│   │   Neutral stimulus + Unconditioned stimulus
│   │   → Neutral becomes Conditioned stimulus
│   │   → Conditioned response
│   └── OPERANT/INSTRUMENTAL CONDITIONING:
│       Behavior reinforced/punished → modified
│       Positive/negative reinforcement
│
└── HIGHER LEARNING:
    ├── Observational/Imitation
    └── Insight learning

MEMORY TYPES:
├── DECLARATIVE (Explicit) - requires hippocampus:
│   ├── EPISODIC: Personal events (what, when, where)
│   └── SEMANTIC: Facts, concepts, knowledge
│
└── NON-DECLARATIVE (Implicit):
    ├── PROCEDURAL: Skills, habits (BG, cerebellum)
    ├── PRIMING: Facilitated processing of stimuli
    └── CONDITIONING: Learned associations (amygdala)

MEMORY STAGES:
ENCODING → CONSOLIDATION → STORAGE → RETRIEVAL

SHORT-TERM MEMORY:
├── Duration: seconds to minutes
├── Capacity: 7±2 items (Miller's law)
└── Mechanism: Reverberating circuits (short-term synaptic change)

LONG-TERM MEMORY:
├── Duration: days to lifetime
├── Mechanism: LONG-TERM POTENTIATION (LTP)
│   NMDA receptor activation → Ca²⁺ influx →
│   AMPA receptor insertion → ↑ synaptic efficacy
│   → structural synaptic changes (new synapses)
└── Requires: protein synthesis, hippocampus (for consolidation)

---

25. PREFRONTAL CORTEX FUNCTIONS

PREFRONTAL CORTEX (Areas 9, 10, 11, 12, 46, 47)
├── WORKING MEMORY (temporary holding of information)
├── EXECUTIVE FUNCTIONS:
│   ├── Planning and decision making
│   ├── Impulse control
│   ├── Cognitive flexibility
│   └── Abstract reasoning
├── PERSONALITY & SOCIAL BEHAVIOR
├── Attention and concentration
├── Language (Broca's area = Area 44,45 = premotor frontal)
├── Emotional regulation (orbitofrontal cortex)
└── Moral judgment

Prefrontal lobotomy/lesion → "Frontal lobe syndrome":
Disinhibition, poor judgment, personality change,
inability to plan, perseveration

---

26. ALZHEIMER'S DISEASE

PATHOLOGY:
├── Neuronal LOSS mainly in:
│   Hippocampus → entorhinal cortex → association cortex
│
├── AMYLOID PLAQUES (Senile plaques):
│   Extracellular deposits of β-AMYLOID (Aβ)
│   from APP (Amyloid Precursor Protein) by β & γ secretase
│
├── NEUROFIBRILLARY TANGLES:
│   Intraneuronal hyperphosphorylated TAU protein
│   (normally stabilizes microtubules)
│
└── Loss of CHOLINERGIC neurons
    (nucleus basalis of Meynert → ↓ ACh)

GENETICS:
├── Familial early-onset: APP, Presenilin 1&2 mutations
└── Late-onset: ApoE4 allele (chromosome 19)

FEATURES (in order):
Episodic memory loss → Language problems →
Disorientation → Behavioral changes → Global dementia

TREATMENT:
├── AChE inhibitors: Donepezil, Rivastigmine, Galantamine
│   (↑ ACh at synapses)
└── Memantine (NMDA antagonist, for moderate-severe)

---

27. VESTIBULAR APPARATUS - Posture & Equilibrium

VESTIBULAR APPARATUS:
├── SEMICIRCULAR CANALS (3: horizontal, anterior, posterior)
│   Detect ANGULAR ACCELERATION (rotational movement)
│   Receptor: Crista ampullaris → hair cells in cupula
│
└── OTOLITH ORGANS:
    ├── UTRICLE: detects LINEAR ACCELERATION (horizontal)
    └── SACCULE: detects LINEAR ACCELERATION (vertical/gravity)
    Receptor: Macula → hair cells + otoconia (calcium carbonate)

REFLEXES FOR POSTURE/EQUILIBRIUM:
      │
      ▼
Vestibular nuclei (in medulla/pons)
      │
      ├──► VESTIBULOSPINAL TRACT
      │    → Alpha motor neurons of trunk/limb extensors
      │    → Maintains UPRIGHT POSTURE (antigravity muscles)
      │
      ├──► VESTIBULO-OCULAR REFLEX (VOR)
      │    Head turns right → eyes deviate left
      │    Keeps image stable on retina
      │    → Nystagmus (fast phase toward direction of head turn)
      │
      ├──► RIGHTING REFLEXES
      │    Restores body/head to normal position
      │
      └──► CEREBELLUM (flocculonodular lobe)
           Integration of vestibular with cerebellar control

Clinical: Vestibular dysfunction → vertigo, nystagmus,
nausea, past-pointing, Romberg positive

---

28. EXTRAPYRAMIDAL TRACTS

EXTRAPYRAMIDAL TRACTS (all motor tracts except corticospinal):

TRACT              │ ORIGIN          │ FUNCTION
───────────────────┼─────────────────┼──────────────────────────────
RUBROSPINAL        │ Red nucleus      │ Flexor tone, limb movement
                   │ (midbrain)      │ (works with CST)
───────────────────┼─────────────────┼──────────────────────────────
RETICULOSPINAL     │ Reticular        │ Medullary: facilitates flexors
(Medial + Lateral) │ formation        │ Pontine: facilitates extensors
                   │ (pons, medulla) │ Muscle tone, autonomic
───────────────────┼─────────────────┼──────────────────────────────
VESTIBULOSPINAL    │ Lateral vestib.  │ Extensor facilitation
(Lateral + Medial) │ nucleus (Deiters)│ Anti-gravity posture, balance
───────────────────┼─────────────────┼──────────────────────────────
TECTOSPINAL        │ Superior         │ Head turning in response to
                   │ colliculus       │ visual/auditory stimuli
                   │ (midbrain)      │ Orienting reflex
───────────────────┼─────────────────┼──────────────────────────────
OLIVOSPINAL        │ Inferior olive   │ Coordination, uncertain
                   │                 │ (may be vestigial)

---

29. PYRAMIDAL vs EXTRAPYRAMIDAL TRACTS

FEATURE         │ PYRAMIDAL              │ EXTRAPYRAMIDAL
────────────────┼────────────────────────┼──────────────────────────
Tracts          │ Corticospinal          │ Rubrospinal,
                │ Corticobulbar          │ Reticulospinal,
                │                        │ Vestibulospinal,
                │                        │ Tectospinal
────────────────┼────────────────────────┼──────────────────────────
Origin          │ Motor cortex (Area 4,6)│ Basal ganglia, cerebellum,
                │                        │ brainstem nuclei
────────────────┼────────────────────────┼──────────────────────────
Pathway         │ Through medullary      │ Multisynaptic, diffuse
                │ pyramids               │ (NOT through pyramids)
────────────────┼────────────────────────┼──────────────────────────
Function        │ SKILLED voluntary      │ Muscle tone, posture,
                │ movement               │ balance, crude movement,
                │ Especially distal,     │ coordination of axial &
                │ fractionated movement  │ proximal muscles
────────────────┼────────────────────────┼──────────────────────────
Lesion signs    │ UMN signs              │ Rigidity, resting tremor,
                │ Spasticity, Babinski   │ choreiform movements,
                │ Hyperreflexia          │ ataxia, ballismus
────────────────┼────────────────────────┼──────────────────────────
Consciousness   │ Voluntary (conscious)  │ Largely subconscious

---

QUICK EXAM REVISION: KEY FLOWCHARTS Summary

---