Cell: ●◆ Lysosomes Cell membranes and transport across Cell membrane (imp) ● Cytoskeleton

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Cell Biology: Lysosomes, Cell Membrane & Transport, Cytoskeleton

1. LYSOSOMES

Structure

Lysosomes are membrane-bound vesicular organelles that bud off from the trans-Golgi network. They are typically spherical, ranging from 0.05 to 0.75 µm in diameter, and filled with ~40 different hydrolytic (acid hydrolase) enzymes - proteases, nucleases, lipases, phosphatases, glycosidases, and sulfatases. In TEM they appear electron-dense.

The lysosomal pH is ~4.8-5.0, maintained by proton pumps in the lysosomal membrane that actively pump H⁺ into the lumen.
At the neutral cytosolic pH (~7.2), leaked lysosomal enzymes become inactive - a key safety mechanism protecting the cell.
Lysosomes are most numerous in cells with high phagocytic activity (macrophages, neutrophils).

Lysosomal Enzyme Targeting (M6P Pathway)

Lysosomal enzymes are synthesized in the rough ER, then transported to the Golgi apparatus. In the cis-Golgi, a phosphotransferase adds mannose-6-phosphate (M6P) residues to the N-linked oligosaccharides of hydrolases destined for lysosomes. M6P receptors in the trans-Golgi network recognize these proteins and package them into vesicles that deliver them to lysosomes - diverting them away from the secretory pathway.

Clinical pearl - I-cell disease: Phosphotransferase deficiency means M6P tags cannot be added, so lysosomal enzymes are secreted extracellularly instead of reaching lysosomes. Cells accumulate undigested material (inclusions), affecting the skeleton and nervous system.

Functions of Lysosomes

Function	Mechanism
Heterophagy	Phagosomes/pinocytotic vesicles fuse with lysosomes → heterolysosome → digestion; indigestible material remains as a residual body
Autophagy	Defunct organelles wrapped in double-membrane autophagosome (derived from ER); fuses with lysosome for digestion and nutrient recycling
Autolysis	Severe cell damage → lysosomal membrane rupture → hydrolases digest entire cell contents
Tissue regression	Uterine involution post-partum, muscle atrophy during inactivity, mammary gland regression after lactation
Bactericidal	Lysosomes contain lysozyme (dissolves bacterial cell wall), lysoferrin (sequesters iron), and acid pH (~4.8) that inactivates bacterial metabolism
Bone resorption	Osteoclasts secrete lysosomal enzymes into a restricted extracellular space to degrade bone matrix

Residual bodies accumulate as lipofuscin granules in long-lived post-mitotic cells (neurons, cardiac muscle).

Lysosomal Storage Diseases

Caused by deficiency of a specific lysosomal enzyme → substrate accumulates in secondary lysosomes/residual bodies → cellular dysfunction.

Disease	Defective Enzyme	Substrate Stored	Tissue Affected
Tay-Sachs	Hexosaminidase A (GM₂-gangliosidase)	Gangliosides	Nervous system
Gaucher	Glucocerebrosidase	Glucocerebroside	Liver, spleen
Hurler syndrome (MPS I)	α-L-Iduronidase	Glycosaminoglycans	Skeleton, CNS
I-cell disease	Phosphotransferase (M6P formation)	Multiple substrates	Skeleton, CNS
McArdle syndrome	Muscle phosphorylase	Glycogen	Skeletal muscle

2. CELL MEMBRANE AND TRANSPORT (★ HIGH YIELD)

Cell Membrane Structure

The plasma membrane is a phospholipid bilayer with two main types of proteins:

Channel proteins - have watery pores allowing nearly free movement of water and selected ions (e.g., aquaporins, ion channels)
Carrier proteins - bind specific molecules/ions, undergo conformational change to transport them across

The lipid bilayer itself is permeable to lipid-soluble substances (O₂, CO₂, steroid hormones, ethanol) but impermeable to most water-soluble and charged substances.

Transport pathways through the cell membrane

Figure: Transport pathways - simple diffusion, facilitated diffusion, and active transport (Guyton & Hall)

A. DIFFUSION (Passive Transport - no energy required)

Simple Diffusion

Random molecular movement down a concentration gradient through lipid bilayer interspaces or protein channels.
Rate follows Fick's Law: proportional to the concentration gradient, membrane surface area, and membrane permeability.
Factors driving diffusion across a membrane:
1. Concentration gradient (high → low)
2. Electrical potential (for ions): positive charge attracts anions, repels cations
3. Pressure difference: higher pressure side has more kinetic energy → net movement toward lower pressure

Nernst Potential

At equilibrium for an ion, the electrical and concentration gradients exactly balance each other. The voltage at which this occurs:

EMF (mV) = ±61 × log (C₁/C₂) (at 37°C for monovalent ions)

Facilitated Diffusion

Uses carrier proteins - moves substances down their concentration gradient but faster than simple diffusion.
No energy required - movement is still from high to low concentration.
Example: glucose transport into cells (GLUT transporters); the rate shows saturation kinetics.

Osmosis

Net diffusion of water through a selectively permeable membrane from lower solute concentration → higher solute concentration.
The osmotic pressure that would exactly prevent net water movement is the osmotic pressure of the solution.
Red blood cell membrane allows water diffusion equivalent to ~100× its volume per second.

B. ACTIVE TRANSPORT (Energy required, moves against gradient)

Primary Active Transport

Uses ATP directly. Key example: Na⁺-K⁺ ATPase pump

Na⁺-K⁺ ATPase pump mechanism:

3 Na⁺ bind on the inside of the pump
2 K⁺ bind on the outside
ATPase activity is activated → ATP is cleaved to ADP + Pᵢ
Conformational change extrudes 3 Na⁺ out and brings 2 K⁺ in
Net result: one positive charge moved outward per cycle (electrogenic)

Functions of the Na⁺-K⁺ pump:

Maintains low intracellular Na⁺ and high intracellular K⁺
Controls cell volume - without it, osmotic entry of water would cause cells to swell and burst (intracellular anions attract cations → osmosis)
Generates the resting membrane potential (electrogenic - produces inside-negative)
In electrically active neurons: 60-70% of total energy expenditure goes to this pump

The Na⁺-K⁺ pump can run in reverse - if electrochemical gradients are experimentally large enough, it synthesizes ATP from ADP + Pi.

Secondary Active Transport (Co-transport / Counter-transport)

Uses the Na⁺ gradient created by Na⁺-K⁺ ATPase as the energy source (indirect).
Co-transport (symport): Na⁺ and another solute move in the same direction (e.g., Na⁺-glucose co-transporter in intestinal epithelium)
Counter-transport (antiport): Na⁺ and another solute move in opposite directions (e.g., Na⁺-Ca²⁺ exchanger - Ca²⁺ pumped out while Na⁺ enters)

Summary Table: Types of Transport

Type	Energy	Direction	Example
Simple diffusion	No	Down gradient	O₂, CO₂, lipid-soluble drugs
Facilitated diffusion	No	Down gradient	Glucose (GLUT), fructose
Osmosis	No	Water down osmotic gradient	Water via aquaporins
Primary active	ATP	Against gradient	Na⁺-K⁺ pump, Ca²⁺-ATPase
Secondary active (symport)	Na⁺ gradient	Against gradient	Na⁺-glucose, Na⁺-amino acid
Secondary active (antiport)	Na⁺ gradient	Against gradient	Na⁺-Ca²⁺ exchanger, Na⁺-H⁺

C. Vesicular Transport (Bulk Transport)

Endocytosis: cell membrane invaginates to engulf extracellular material
- Phagocytosis: large particles (bacteria, cell debris) - mainly in macrophages/neutrophils
- Pinocytosis: fluid droplets with dissolved substances
- Receptor-mediated endocytosis: specific molecules bind cell surface receptors → clathrin-coated pit → vesicle (e.g., LDL receptor pathway)
Exocytosis: vesicle fuses with plasma membrane to release contents extracellularly (opposite of endocytosis)

3. CYTOSKELETON

The cytoskeleton is an intracellular scaffold of structural proteins that maintains cell shape, polarity, organelle organization, and enables cell migration and division. It is composed of three major filament types, constantly undergoing assembly and disassembly.

Cytoskeletal elements: microfilaments, intermediate filaments, microtubules

Figure: The three cytoskeletal elements with their diameters and protein subunits (Ganong's)

A. Microtubules

Diameter: 25 nm (hollow tubes); walls ~5 nm thick, inner cavity 15 nm
Composition: α- and β-tubulin heterodimers polymerize into 13-protofilament hollow tubes
- γ-tubulin: associated with centrosomes (MTOCs) and initiates microtubule nucleation
Polarity: "+" end (plus end) elongates rapidly; "-" end (minus end) typically embedded in the MTOC/centrosome near the nucleus
Dynamics: temperature-sensitive; assembly at (+) end, disassembly at (-) end; GTP binding facilitates formation
Functions:
- Tracks for molecular motor proteins (kinesins, dyneins) to transport vesicles, organelles, secretory granules, mitochondria
- Form the mitotic spindle - segregates sister chromatids during cell division
- Core of primary cilia (9+0 arrangement) - regulate proliferation/differentiation; defects cause polycystic kidney disease
- Core of motile cilia (9+2 arrangement) and flagella

Kinesins (anterograde motors): move cargo toward the "+" end (away from nucleus) Dyneins (retrograde motors): move cargo toward the "-" end (toward nucleus/centrosome)

Drugs affecting microtubules:

Colchicine / Vinca alkaloids (vincristine, vinblastine): inhibit microtubule polymerization → disrupt mitotic spindle → arrest cell division
Taxol (paclitaxel): stabilizes microtubules (prevents depolymerization) → also arrests mitosis

B. Intermediate Filaments

Diameter: 8-14 nm (between micro and macro - hence "intermediate")
Composition: large, heterogeneous family of ropelike polymers
Key property: do not actively reorganize like actin/microtubules; provide tensile strength and resist mechanical stress
Tissue-specific expression (used as tumor markers):

Protein	Cell Type	Clinical Use
Vimentin	Mesenchymal cells (fibroblasts, endothelium)	Marker for sarcomas
Desmin	Muscle cells	Scaffold for actin-myosin contraction
Cytokeratins (30+ types)	Epithelial cells	Markers for carcinomas; lung vs. GI distinction
Neurofilaments	Neurons	Structural strength and rigidity of axons
GFAP (Glial fibrillary acidic protein)	Glial cells	Marker for astrocytomas
Nuclear lamins	All nucleated cells (nuclear lamina)	Define nuclear shape, gene regulation

Connect desmosomes and hemidesmosomes → link neighboring cells and ECM mechanically
Absent/abnormal IF → skin blistering (epidermolysis bullosa)

C. Microfilaments (Actin Filaments)

Diameter: 5-9 nm (thinnest)
Composition: G-actin (globular monomers) polymerize into F-actin (filamentous, double-stranded helix)
Most abundant protein in mammalian cells - up to 15% of total cellular protein; sequence highly conserved (88% identical between yeast and rabbit)
Polarity: new subunits added at "+" end, removed from "-" end (actin treadmilling)
Functions:
- Cell shape changes and migration (lamellipodia formation, focal adhesion complexes)
- Reach tips of microvilli (intestinal brush border)
- Interaction with myosin → muscle contraction (sarcomeres) and non-muscle motility
- Vesicular transport and epithelial barrier regulation
- Actin nucleating proteins (e.g., Arp2/3 complex) regulate branching/polymerization

Molecular Motors

Three superfamilies of ATPase motor proteins (100-500 kDa) transport cargo along cytoskeletal tracks:

Motor	Track	Direction	Functions
Kinesin	Microtubules	Anterograde (- to + end)	Vesicle/organelle transport away from nucleus; mitosis
Dynein (cytoplasmic)	Microtubules	Retrograde (+ to - end)	Transport toward nucleus/centrosome
Myosin II	Actin	-	Muscle contraction; cytokinesis
Myosin V	Actin	-	Vesicle transport; organelle positioning

Clinical Correlations - Cytoskeleton

Polycystic kidney disease: mutations in proteins of the primary cilia complex (microtubule-based)
Kartagener syndrome (primary ciliary dyskinesia): dynein arm defects → immotile cilia → bronchiectasis, situs inversus, male infertility
E-cadherin loss (linked to actin via adherens junctions): explains discohesive invasion in lobular breast carcinoma and some gastric cancers
Epidermolysis bullosa: defective keratin or desmosomal proteins → skin blistering under mechanical stress

Sources:

Guyton and Hall Textbook of Medical Physiology - Chapters 2 & 4
Robbins, Cotran & Kumar Pathologic Basis of Disease - Chapter 1
Ganong's Review of Medical Physiology - Chapter 2
Junqueira's Basic Histology - Text and Atlas, 17e - Chapter 2

Explain each with diagram and flowchart GIT: Salivary glands GIT secretions Enteric nervous system Hormonal and Neural control of gastric secretions Liver ● Pancreas ◆ Biliary secretions GallbladderCardiac cycle ECG Cardiac output(short) Conduction system of heart Definitions: Systole, diastole, Preload, afterload, stroke volume, Ejection fractionANS: Divisions of ANS Difference between Sympathetic and Parasympathetic nervous system ◆ Effect of Sympathetic and Parasympathetic nervous system on different organs O Synthesis and secretion of Acetylcholine and nor epinephrineNeural control mechanism: Neuron and its classification (imp) Action potential Refractory period Difference between neurotransmitters and neuromodulatorsCNS: Cerebrum Cerebellum (imp) Brainstem (imp) Lobes of brain (Short) Synapse and its types (imp) ◆ Types of sleep(short)

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🧬 COMPLETE SYSTEMS REVIEW: GIT · CARDIAC · ANS · CNS

Sources: Guyton & Hall Physiology, Costanzo Physiology, Robbins Pathology, Ganong's Review, Sleisenger & Fordtran's GI, Junqueira's Histology

PART I: GASTROINTESTINAL SYSTEM (GIT)

1. SALIVARY GLANDS

Three Major Pairs:

Gland	Location	Secretion Type	Duct
Parotid	Anterior to ear	Serous (watery, amylase-rich)	Stensen's duct
Submandibular	Floor of mouth	Mixed (serous + mucous)	Wharton's duct
Sublingual	Floor of mouth	Predominantly mucous	Multiple ducts of Rivinus

Composition of Saliva:

Volume: ~1-1.5 L/day
pH: 6.0-7.4 (slightly acidic to neutral)
Contents: Water (99%), Salivary amylase (ptyalin), Mucin, Lysozyme, IgA, Lingual lipase, Bicarbonate (buffers acid), Electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻)

FLOWCHART: SALIVA PRODUCTION & MODIFICATION
─────────────────────────────────────────────
Acinar cells (secrete primary saliva)
    │  Isotonic, similar to plasma
    ▼
Striated ducts (modify saliva)
    │  Na⁺ & Cl⁻ reabsorbed
    │  K⁺ & HCO₃⁻ secreted
    ▼
Final saliva = HYPOTONIC (low Na⁺, high K⁺)
    │
    ▼
At HIGH flow rates → less time for modification
→ saliva becomes MORE like plasma (↑Na⁺)

Neural Control of Salivation:

SALIVARY SECRETION CONTROL
───────────────────────────────────────
STIMULI: Food, smell, taste, thought,
         conditioned reflexes, nausea

         ┌─────────────────────┐
         │  Salivatory Nuclei  │
         │(Superior & Inferior)│
         │   in Brainstem      │
         └────────┬────────────┘
         ┌────────┴────────────┐
         ▼                     ▼
   PARASYMPATHETIC         SYMPATHETIC
   (dominant)              (minor role)
   CN VII → submandibular  T1-T3 → vasoconstrict
   CN IX  → parotid        → thick mucous saliva
         ▼
   ACh → muscarinic M3 receptors
         ▼
   ↑ Watery, enzyme-rich saliva
   (↑ volume, ↑ amylase)

2. GIT SECRETIONS (Overview)

REGION → SECRETION → KEY COMPONENTS
──────────────────────────────────────────────────
Stomach    → Gastric juice  → HCl, pepsinogen, intrinsic
                               factor, mucus, gastrin
Duodenum   → Brunner's glands → Alkaline mucus (neutralizes acid)
Pancreas   → Pancreatic juice → Enzymes + HCO₃⁻ (8 L/day)
Liver      → Bile            → Bile salts, bilirubin, cholesterol
Small Int. → Succus entericus → Enzymes (disaccharidases,
                                peptidases), mucus
Large Int. → Mucus           → Goblet cell secretion only

Gastric Secretions in Detail:

Cell Type	Location	Product	Function
Chief cells	Fundus/body	Pepsinogen	Activated by HCl → Pepsin (protein digestion)
Parietal cells	Fundus/body	HCl + Intrinsic factor	Kills bacteria; IF binds B12
G cells	Antrum	Gastrin	Stimulates parietal cells
D cells	Antrum/fundus	Somatostatin	Inhibits gastrin & HCl
Mucous neck cells	Neck of glands	Mucus + HCO₃⁻	Protects mucosa
ECL cells	Fundus	Histamine	Stimulates parietal cells via H2

3. ENTERIC NERVOUS SYSTEM (ENS)

The ENS is called the "second brain" - it contains ~100 million neurons and can function independently of the CNS.

LAYERS OF ENS
──────────────────────────────────────────────
  LUMEN
    │
  Mucosa ─────── Mucosal plexus (subglandular)
    │
  Submucosa ──── MEISSNER'S PLEXUS (Submucosal)
    │            • Controls secretion
    │            • Controls blood flow
    │            • Absorptive functions
  Muscularis
  externa ─────  AUERBACH'S PLEXUS (Myenteric)
    │            • Located between circular &
    │              longitudinal muscle layers
    │            • Controls MOTILITY (peristalsis)
  Serosa

ENS Neuron Types:

Sensory (afferent) neurons: detect luminal contents, stretch, chemical stimuli
Interneurons: process signals locally
Motor neurons: stimulate or inhibit smooth muscle and secretory cells

Key ENS Neurotransmitters:

Excitatory: ACh, Substance P
Inhibitory: VIP (vasoactive intestinal peptide), NO, ATP

Peristaltic Reflex:

PERISTALTIC REFLEX (Law of the Intestine)
──────────────────────────────────────────
 Bolus distends intestinal wall
         │
         ▼
 Sensory neurons activated
         │
    ┌────┴──────────────┐
    ▼                   ▼
 ORAL side           CAUDAL side
 (behind bolus)      (ahead of bolus)
    │                   │
 Excitatory           Inhibitory
 neurons              neurons
    │                   │
 Circular muscle    Circular muscle
 CONTRACTS          RELAXES
    │                   │
    └──── Bolus moves caudally ────┘

4. HORMONAL AND NEURAL CONTROL OF GASTRIC SECRETION

Three Phases of Gastric Secretion:

┌─────────────────────────────────────────────────────┐
│           THREE PHASES OF GASTRIC SECRETION         │
├─────────────────────────────────────────────────────┤
│                                                     │
│  CEPHALIC PHASE (~30% of total acid)                │
│  Trigger: Sight, smell, taste, thought of food      │
│  Pathway: Cortex → Dorsal vagal nucleus             │
│           → Vagus (CN X) → ACh                     │
│  Effects: ↑ HCl (parietal cells)                   │
│           ↑ Pepsinogen (chief cells)                │
│           ↑ Gastrin (G cells)                       │
│                                                     │
│  GASTRIC PHASE (~60% of total acid)                 │
│  Trigger: Food in stomach (stretch + peptides)      │
│  Pathway: Local ENS reflexes + Vagal reflexes       │
│           + Gastrin release from G cells            │
│  Effects: Maximal HCl secretion                     │
│           G cells → Gastrin → ECL cells             │
│           → Histamine → H2 → Parietal cells → HCl  │
│                                                     │
│  INTESTINAL PHASE (~10% of total acid)              │
│  Trigger: Chyme entering duodenum                   │
│  INITIALLY: Mild gastrin-like stimulation           │
│  THEN: INHIBITION dominates                         │
│  Inhibitory signals (Enterogastric Reflex):         │
│  • Acid (pH<2): Secretin → ↓ Gastrin               │
│  • Fat: CCK + GIP → ↓ Gastric motility & acid      │
│  • Distension: Neural reflex → ↓ acid              │
└─────────────────────────────────────────────────────┘

HCl Secretion by Parietal Cells:

PARIETAL CELL - HCl SECRETION MECHANISM
─────────────────────────────────────────
                LUMEN
                  ▲
                  │  H⁺ (via H⁺/K⁺-ATPase)
                  │  Cl⁻ (via Cl⁻ channel)
                  │
     ┌────────────┴───────────────┐
     │       PARIETAL CELL        │
     │                            │
     │  CO₂+H₂O → H₂CO₃          │
     │     ↓ (carbonic anhydrase) │
     │  H⁺ + HCO₃⁻               │
     │  H⁺ → pumped to lumen      │
     │  HCO₃⁻ → blood             │
     │  (the "alkaline tide")      │
     └────────────────────────────┘
                  ▲
         Three stimulants:
    1. Gastrin (CCK-B/gastrin receptor)
    2. Histamine (H2 receptor → ↑cAMP)
    3. ACh (M3 receptor → ↑Ca²⁺)

Key GIT Hormones Table:

Hormone	Source	Stimulus	Actions
Gastrin	G cells (antrum)	Peptides, distension, vagus	↑HCl, ↑pepsinogen, ↑motility, trophic
Secretin	S cells (duodenum)	Acid (pH<4.5) in duodenum	↑Pancreatic HCO₃⁻, ↓gastrin, ↑bile
CCK	I cells (duodenum)	Fat + protein in duodenum	↑Pancreatic enzymes, gallbladder contraction, ↓gastric emptying
GIP	K cells (duodenum)	Fat + glucose	↑Insulin release (incretin), ↓gastric acid
VIP	ENS neurons	Distension	Relax smooth muscle, ↑intestinal secretion
Motilin	M cells (small bowel)	Fasting (inter-digestive)	Initiates MMC (migrating motor complex)
Somatostatin	D cells	Acid, fat, protein	Inhibits ALL GI hormones and secretions

5. LIVER

Functions of the Liver (Organized):

LIVER FUNCTIONS - OVERVIEW
────────────────────────────────────────────────
        ┌──────────────────────────┐
        │          LIVER           │
        └──────────┬───────────────┘
    ┌───────────────┼───────────────────┐
    ▼               ▼                   ▼
METABOLIC       SYNTHETIC           EXCRETORY
────────        ─────────           ─────────
• Glucose       • Albumin           • Bile production
  homeostasis   • Clotting factors  • Bilirubin excretion
• Lipid         (I,II,V,VII,IX,X)   • Drug detoxification
  metabolism    • Globulins         • Cholesterol excretion
• Amino acid    • Complement        
  metabolism    • IGF-1             
• Ketogenesis   • Thrombopoietin    

    ▼               ▼                   ▼
STORAGE         IMMUNE              ENDOCRINE
───────         ──────              ─────────
• Glycogen      • Kupffer cells     • Angiotensinogen
• Fat-soluble     (phagocytosis)    • 25-OH Vitamin D
  vitamins      • Immune            • Thrombopoietin
  (A,D,E,K)       surveillance      • IGF-1
• Vitamin B12   
• Iron (ferritin)

Bile Production & Bilirubin Metabolism:

BILIRUBIN METABOLISM FLOWCHART
─────────────────────────────────────────────────
RBCs destroyed (Kupffer cells/spleen)
          │
          ▼
   Hemoglobin → Heme
          │
          ▼
   Heme → Biliverdin → BILIRUBIN (unconjugated)
                        (fat-soluble, indirect)
                        Bound to albumin in blood
                              │
                              ▼
                     LIVER (hepatocytes)
                     Glucuronyl transferase
                     Conjugates with glucuronic acid
                              │
                              ▼
               CONJUGATED BILIRUBIN (water-soluble, direct)
                              │
                     ┌────────┴────────┐
                     ▼                 ▼
               Excreted into       Small amount
               bile → duodenum     → urine
                     │
                     ▼
               Intestinal bacteria
               convert → Urobilinogen
                     │
               ┌─────┴──────┐
               ▼            ▼
           Stercobilin    Reabsorbed
           (feces color)  → liver/kidney
                          → urobilinogen in urine

6. PANCREAS

Exocrine vs Endocrine:

PANCREAS
    ├── EXOCRINE (99% of tissue)
    │       └── Acinar cells → Digestive enzymes
    │       └── Ductal cells → HCO₃⁻ rich fluid
    │
    └── ENDOCRINE (Islets of Langerhans, 1%)
            ├── α cells (20%) → Glucagon
            ├── β cells (70%) → Insulin
            ├── δ cells (5%)  → Somatostatin
            └── PP cells (5%) → Pancreatic polypeptide

Pancreatic Exocrine Enzymes:

Type	Enzyme	Substrate	Activated By
Proteases	Trypsinogen → Trypsin	Proteins	Enterokinase (brush border)
	Chymotrypsinogen	Proteins	Trypsin
	Proelastase	Elastin	Trypsin
Lipases	Pancreatic lipase	Triglycerides	Bile salts (colipase)
	Phospholipase A2	Phospholipids	Trypsin
Amylase	Pancreatic amylase	Starch/glycogen	Secreted active
Nucleases	DNase, RNase	DNA, RNA	Secreted active

Control of Pancreatic Secretion:

PANCREATIC SECRETION CONTROL
────────────────────────────────────────────
  PHASE           STIMULUS        EFFECT
  ─────────────────────────────────────────
  Cephalic        Vagus (ACh)     Enzymes (small ↑)
  
  Gastric         Vagus           Enzymes (moderate)
  
  Intestinal      Secretin        ↑↑ HCO₃⁻ & water
  (DOMINANT)     (acid in         (neutralize acid)
                  duodenum)
                  
                  CCK             ↑↑ Enzymes
                  (fat+protein)   (digest food)
                  
  NB: Secretin + CCK have SYNERGISTIC effect
      (together > sum of individual effects)

7. BILIARY SECRETIONS & GALLBLADDER

Bile Composition:

Bile salts (primary: cholic acid, chenodeoxycholic acid; secondary: deoxycholic, lithocholic)
Phospholipids (lecithin)
Cholesterol
Bilirubin conjugates
Water, electrolytes

Enterohepatic Circulation:

ENTEROHEPATIC CIRCULATION OF BILE SALTS
──────────────────────────────────────────────
LIVER synthesizes primary bile salts
         │  (~0.5 g/day new synthesis)
         ▼
Bile → Gallbladder (stored & concentrated 10-20x)
         │
         ▼  (CCK trigger → gallbladder contracts)
   Duodenum (bile salts emulsify fats)
         │
         ▼
   Jejunum (micelle formation + fat absorption)
         │
         ▼
   Terminal ILEUM ← active reabsorption (95%)
         │              (Na⁺-bile salt co-transporter)
         ▼
   Portal vein → Liver → re-secreted into bile
         │
    5% lost in feces → replaced by new synthesis
    
   Total bile salt pool = 2-4 g
   Recirculates 6-10x per day

Gallbladder Functions:

GALLBLADDER
    │
    ├── STORAGE: Holds 40-70 mL bile
    │
    ├── CONCENTRATION: Absorbs H₂O, Na⁺, Cl⁻
    │   → Concentrates bile 10-20 fold
    │
    ├── SECRETION: Mucus secretion
    │
    └── CONTRACTION:
        Stimulus: CCK (fat/protein in duodenum)
        + Vagal stimulation
        - Somatostatin inhibits
        Mechanism: CCK → smooth muscle contraction
                   + relaxes Sphincter of Oddi
                   → bile flows into duodenum

PART II: CARDIAC SYSTEM

8. CARDIAC CYCLE

(Based on Costanzo Physiology)

CARDIAC CYCLE - 7 PHASES (Left Heart)
══════════════════════════════════════════════════════
ECG:      [P wave]  [QRS]         [T wave]
           ──────────────────────────────────
Heart:    Atrial    Isovol.  Ejection  Isovol.  Filling
          Systole   Contrac.          Relax.
══════════════════════════════════════════════════════

PHASE A: ATRIAL SYSTOLE
• Trigger: P wave (atrial depolarization)
• Mitral valve OPEN, Aortic valve CLOSED
• Atrium contracts → final ventricular filling
• LV volume at maximum = EDV (~130 mL)
• Heart sound: S4 (not heard normally)
─────────────────────────────────────────────────────
PHASE B: ISOVOLUMETRIC VENTRICULAR CONTRACTION
• Trigger: QRS complex (ventricular depolarization)
• ALL VALVES CLOSED
• LV pressure rises rapidly
• Volume UNCHANGED (isovolumetric)
• Heart sound: S1 (mitral valve closure = "LUB")
─────────────────────────────────────────────────────
PHASE C: RAPID VENTRICULAR EJECTION
• Aortic valve OPENS (LV pressure > Aortic pressure)
• LV ejects ~70% of stroke volume rapidly
• Aortic pressure rises to peak (systolic ~120 mmHg)
• Heart sound: None
─────────────────────────────────────────────────────
PHASE D: REDUCED VENTRICULAR EJECTION
• Ejection slows; T wave occurs (repolarization)
• LV volume reaches minimum = ESV (~60 mL)
• Aortic pressure begins to fall
─────────────────────────────────────────────────────
PHASE E: ISOVOLUMETRIC VENTRICULAR RELAXATION
• Aortic valve CLOSES (LV pressure < Aortic pressure)
• ALL VALVES CLOSED
• Volume UNCHANGED
• Heart sound: S2 ("DUB") = aortic valve closure
• Dicrotic notch on aortic pressure tracing
─────────────────────────────────────────────────────
PHASE F: RAPID VENTRICULAR FILLING
• Mitral valve OPENS (LV pressure < LA pressure)
• Passive filling: 70-80% of filling occurs here
• Heart sound: S3 (may be heard in children/HF)
─────────────────────────────────────────────────────
PHASE G: REDUCED FILLING (DIASTASIS)
• Slow passive filling continues
• Cycle returns to Phase A (next P wave)
══════════════════════════════════════════════════════
Key Values:
  EDV = End-Diastolic Volume  = ~130 mL
  ESV = End-Systolic Volume   = ~60 mL
  SV  = Stroke Volume (EDV-ESV) = ~70 mL
  EF  = Ejection Fraction (SV/EDV) = 55-70%

9. ECG (Electrocardiogram)

ECG WAVEFORM & INTERVALS
────────────────────────────────────────────
         P        QRS      T
         wave     complex  wave
          ▲        ▲        ▲
   ───────┤   ─────┤  ──────┤──────────
          │   │   │  │      │
          └───┘   └──┘      

P–R interval: 0.12–0.20 s
QRS duration: < 0.12 s
QT interval:  0.35–0.45 s

WAVE / SEGMENT = EVENT
────────────────────────────────────────────
P wave         → Atrial DEPOLARIZATION
PR interval    → AV node DELAY (conduction)
QRS complex    → Ventricular DEPOLARIZATION
ST segment     → Ventricular PLATEAU phase
T wave         → Ventricular REPOLARIZATION
U wave         → Purkinje fiber repolarization
                 (prominent in hypokalemia)

NOTE: Atrial repolarization is HIDDEN within QRS

ECG Leads:

Limb leads: I, II, III (Einthoven's triangle); aVR, aVL, aVF
Chest leads: V1-V6 (precordial)
Axis: Normal = -30° to +90°; Left axis deviation (left ventricular hypertrophy, LBBB); Right axis deviation (RVH, RBBB, PE)

10. CARDIAC OUTPUT (Short)

KEY FORMULAS
─────────────────────────────────────────────
CO = HR × SV
   = Heart Rate × Stroke Volume
   = 72 beats/min × 70 mL/beat
   = ~5 L/min (at rest)

SV  = EDV - ESV = 130 - 60 = 70 mL

EF  = SV/EDV × 100
    = 70/130 × 100 = ~55%
    (Normal ≥ 55%; <40% = reduced EF = systolic HF)

CI  = CO/BSA = ~3.2 L/min/m² (Cardiac Index)

Determinants of CO:

CARDIAC OUTPUT DETERMINANTS
─────────────────────────────────────────
    ┌──────────────────────┐
    │    CARDIAC OUTPUT    │
    └─────────┬────────────┘
         ┌────┴────┐
         ▼         ▼
    HEART RATE   STROKE VOLUME
    (Chronotropy) (Inotropy)
         │         │
         │    ┌────┴────────────┐
         │    ▼        ▼        ▼
         │  PRELOAD  AFTERLOAD CONTRACTILITY
         │    │        │        │
         │  Frank-  Aortic    SNS/
         │  Starling pressure  Catecholamines
         │  law     TPR        Calcium

Definitions:

Term	Definition
Systole	Phase of ventricular contraction; from mitral closure to aortic closure
Diastole	Phase of ventricular relaxation and filling; from aortic closure to next mitral closure
Preload	Ventricular wall stress at END of diastole; clinically = EDV or LVEDP; ↑ by volume overload
Afterload	Ventricular wall stress during ejection; clinically = SVR (systemic vascular resistance) or aortic pressure; ↑ by hypertension
Stroke Volume	Volume ejected per beat = EDV - ESV (~70 mL)
Ejection Fraction (EF)	SV/EDV × 100; normal ≥55%; reflects systolic function

11. CONDUCTION SYSTEM OF THE HEART

CARDIAC CONDUCTION SYSTEM
══════════════════════════════════════════════════
SA NODE (Sinoatrial Node)
│  • Located: Right atrium, near SVC junction
│  • Intrinsic rate: 60-100 bpm (DOMINANT pacemaker)
│  • Spontaneous depolarization (funny If channels)
│
▼  Conduction via internodal pathways
   (anterior, middle, posterior tracts)
│
▼  ATRIA depolarize (P wave on ECG)
│
▼  AV NODE (Atrioventricular Node)
│  • Located: Floor of right atrium (Koch's triangle)
│  • Intrinsic rate: 40-60 bpm
│  • KEY FUNCTION: DELAYS conduction 0.1-0.2 sec
│    → Allows atria to complete contraction
│      before ventricles contract
│
▼  Bundle of His (AV bundle)
│  • Penetrates fibrous skeleton of heart
│
▼  LEFT and RIGHT Bundle Branches
│  • Run in interventricular septum
│
▼  PURKINJE FIBERS
│  • Fastest conduction velocity: 4 m/s
│  • Intrinsic rate: 20-40 bpm
│  • Spread impulse to ventricular endocardium
│
▼  Ventricular myocardium depolarizes
   (QRS complex on ECG)
   Endocardium → Epicardium direction
══════════════════════════════════════════════════
CONDUCTION VELOCITIES:
  Purkinje fibers:  4 m/s  (fastest)
  Ventricular muscle: 1 m/s
  AV node:          0.05 m/s (slowest → delay)
  Atrial muscle:    1 m/s

PART III: AUTONOMIC NERVOUS SYSTEM (ANS)

12. DIVISIONS OF ANS

AUTONOMIC NERVOUS SYSTEM
═══════════════════════════════════════════════════════
              ANS
         ┌────┴────┐
         ▼         ▼
   SYMPATHETIC  PARASYMPATHETIC    (+ Enteric NS)
   "Fight or    "Rest & Digest"
    Flight"

13. SYMPATHETIC vs PARASYMPATHETIC - COMPARISON TABLE

Feature	SYMPATHETIC	PARASYMPATHETIC
Origin	Thoracolumbar (T1-L2)	Craniosacral (CN III,VII,IX,X; S2-S4)
Pre-ganglionic fiber	Short	Long
Post-ganglionic fiber	Long	Short
Ganglia location	Paravertebral chain / prevertebral	Near/within target organ
Pre-ganglionic NT	ACh (nicotinic N₁ receptor)	ACh (nicotinic N₁ receptor)
Post-ganglionic NT	Norepinephrine (adrenergic)	ACh (muscarinic receptor)
Exception	Sweat glands, adrenal medulla, some vasodilators use ACh	-
Adrenal medulla	Direct innervation → releases Epi/NE into blood	-

14. EFFECTS ON DIFFERENT ORGANS

ORGAN EFFECTS - SYMPATHETIC vs PARASYMPATHETIC
══════════════════════════════════════════════════════════
ORGAN           SYMPATHETIC              PARASYMPATHETIC
──────────────────────────────────────────────────────────
Heart rate      ↑↑ (β1)                  ↓↓ (M2)
Contractility   ↑↑ (β1)                  ↓ (M2)
Blood vessels   Constrict (α1)           Dilate (some)
                Dilate skeletal (β2)
Bronchi         Dilate (β2)              Constrict (M3)
Pupils          Dilate (mydriasis) (α1)  Constrict (miosis)(M3)
                Ciliary relaxation →     Ciliary contraction →
                far vision               near vision
GI motility     ↓ (α2, β2)              ↑↑ (M3)
GI sphincters   Contract (α1)           Relax (M3)
Salivary glands Thick, viscous mucous    Profuse watery saliva
                secretion (α)            (M3)
Bladder wall    Relax detrusor (β2)      Contract detrusor (M3)
Urethral        Contract (α1)            Relax (M3)
sphincter
Liver           Glycogenolysis (β2, α1)  Glycogen synthesis
Pancreas        ↓ Insulin secretion (α2) ↑ Insulin secretion
Sweat glands    ↑ Sweating (M - ACh!)    None (most)
Piloerection    ↑ (α1)                  None
Ejaculation/    Ejaculation (α1)         Erection (M)
Sexual          "Point and Shoot"        "Erection"
Adrenal medulla Epi + NE release         -
                (nicotinic)
══════════════════════════════════════════════════════════
Memory: SYMPATHETIC = "4 Ds": Dilation (pupils/bronchi),
        Dry (mouth), Defecation↓, Don't digest

15. SYNTHESIS OF ACETYLCHOLINE (ACh) AND NOREPINEPHRINE (NE)

Acetylcholine Synthesis & Breakdown:

ACh SYNTHESIS & DEGRADATION
──────────────────────────────────────────────────
PRE-SYNAPTIC NEURON:

Choline (from diet/reuptake)
    +
Acetyl-CoA (from mitochondria)
    │
    ▼ Choline acetyltransferase (ChAT)
    │
ACETYLCHOLINE (stored in vesicles)
    │
    ▼ Action potential → Ca²⁺ influx
    │ → Exocytosis into synapse
    ▼
ACh binds RECEPTORS:
  • Nicotinic (N1: ganglia; NMJ) → Ion channel (fast)
  • Muscarinic (M1-M5) → GPCR (slow)

DEGRADATION (in synaptic cleft):
ACh ──(Acetylcholinesterase)──→ Choline + Acetate
              │
    Choline re-uptaken into presynaptic terminal
    (rate-limiting step for ACh synthesis)

Norepinephrine (NE) Synthesis:

NE SYNTHESIS (Catecholamine Pathway)
──────────────────────────────────────────────────
TYROSINE (dietary amino acid)
    │
    ▼ Tyrosine hydroxylase (rate-limiting enzyme)
    │ (inhibited by end-product NE → feedback)
DOPA
    │
    ▼ DOPA decarboxylase (aromatic L-amino acid
    │  decarboxylase)
DOPAMINE
    │
    ▼ Dopamine β-hydroxylase (in vesicles)
    │
NOREPINEPHRINE (stored in vesicles)
    │
    ▼ In adrenal MEDULLA only:
    │  PNMT (phenylethanolamine-N-methyltransferase)
    │  (induced by cortisol from adrenal cortex)
    ▼
EPINEPHRINE (adrenaline)

DEGRADATION:
NE → MAO (monoamine oxidase) - intraneuronal
NE → COMT (catechol-O-methyltransferase) - synaptic
→ Final product: VMA (vanillylmandelic acid)
   (measured in urine to diagnose phaeochromocytoma)

PART IV: NEURAL CONTROL MECHANISMS

16. NEURON AND ITS CLASSIFICATION

Structure of a Neuron:

NEURON STRUCTURE
──────────────────────────────────────────────
DENDRITES (receive signals)
    │
    ▼
CELL BODY / SOMA
  • Contains nucleus, Nissl bodies (RER+ribosomes)
  • Site of protein synthesis
    │
    ▼
AXON HILLOCK (trigger zone - lowest threshold)
    │
    ▼
AXON (transmits signals, may be myelinated)
    │
    ▼
AXON TERMINALS / BOUTONS
  • Contains synaptic vesicles with NT
  • Ca²⁺-dependent NT release

Classification:

By Number of Processes:

Type	Description	Example
Multipolar	1 axon + many dendrites	Motor neurons, most CNS neurons
Bipolar	1 axon + 1 dendrite	Retinal ganglion, cochlear
Unipolar (Pseudounipolar)	Single process divides into 2	Dorsal root ganglion neurons (sensory)
Anaxonic	No true axon	Some CNS interneurons

By Function:

Type	Function
Afferent (sensory)	Carry signals TO CNS from periphery
Efferent (motor)	Carry signals FROM CNS to effectors
Interneurons	Connect neurons within CNS (most numerous)

By Axon Diameter/Myelination (Erlanger-Gasser):

Fiber Type	Diameter	Myelination	Speed	Function
Aα	13-20 µm	Heavy	80-120 m/s	Motor, proprioception
Aβ	6-12 µm	Heavy	35-75 m/s	Touch, pressure
Aγ	3-6 µm	Light	15-30 m/s	Muscle spindle efferents
Aδ	1-5 µm	Light	5-30 m/s	Fast pain, cold, pressure
B	<3 µm	Light	3-15 m/s	Preganglionic autonomic
C	0.2-1 µm	None (unmyelinated)	0.5-2 m/s	Slow pain, warmth, postganglionic

17. ACTION POTENTIAL

(Based on Guyton & Hall)

ACTION POTENTIAL PHASES
══════════════════════════════════════════════════════
mV
+40 ┤         ╭──────╮
    │        /  Peak  \   Overshoot
  0 ┤───────╯          \
    │      ↑Depolariz.  ╲  Repolarization
-55 ┤   THRESHOLD        ╲
    │                     ╲    After-
-70 ┤ Resting              ╲───hyperpolarization
    │ potential              Resting potential
══════════════════════════════════════════════════════
         Time (ms) →  0        1        2

Phase-by-Phase:

Phase	mV Range	Ion Movement	Channel State
Resting	-70 mV	K⁺ leaks out (K⁺ leak channels)	Na⁺ channels closed; K⁺ channels partially open
Depolarization	-70 → +40 mV	Na⁺ rushes IN	Voltage-gated Na⁺ channels OPEN (activation gate opens)
Repolarization	+40 → -70 mV	K⁺ rushes OUT	Na⁺ channels inactivate (inactivation gate closes); K⁺ channels open
After-hyperpolarization	Below -70 mV	Excess K⁺ efflux	K⁺ channels slow to close
Return to resting	-70 mV	Na⁺-K⁺ pump restores gradients	Pump active

Threshold:

Must reach -55 mV (threshold) to trigger AP
All-or-none law: either full AP or none
AP amplitude does NOT change with stimulus strength; frequency of APs encodes signal intensity

18. REFRACTORY PERIOD

REFRACTORY PERIODS
──────────────────────────────────────────────────
ABSOLUTE REFRACTORY PERIOD (ARP)
  • During: Depolarization phase + most of repolarization
  • Na⁺ channels are INACTIVATED (inactivation gate closed)
  • NO stimulus, however strong, can trigger another AP
  • Duration: ~1-2 ms (nerve); ~250 ms (cardiac)
  
RELATIVE REFRACTORY PERIOD (RRP)
  • After ARP, during after-hyperpolarization
  • Na⁺ channels recovering; K⁺ channels still partly open
  • A STRONGER-THAN-NORMAL stimulus CAN trigger AP
  • Duration: ~5-15 ms (nerve)

CLINICAL SIGNIFICANCE:
  • ARP prevents tetanus in cardiac muscle
    (cardiac ARP = ~250 ms ≈ systole duration)
    → Heart cannot be tetanized (unlike skeletal muscle)
  • Limits maximum firing frequency of neurons
  • Ensures unidirectional AP propagation

19. NEUROTRANSMITTERS vs NEUROMODULATORS

Feature	NEUROTRANSMITTERS	NEUROMODULATORS
Action speed	Fast (ms)	Slow (seconds to hours)
Site of action	Synapse (local)	Diffuse (wide area)
Mechanism	Bind ionotropic receptors (ion channels)	Bind metabotropic receptors (GPCR)
Effect	Direct depolarization/hyperpolarization (EPSP/IPSP)	Modify neuronal excitability, modulate NT release
Duration	Brief	Prolonged
Examples	ACh, Glutamate, GABA, Glycine	Dopamine, Serotonin, NE (in CNS), Neuropeptides, Endorphins
Released by	Neurons at specific synapses	Neurons broadly; may be released from axon en passant

PART V: CNS

20. CEREBRUM

CEREBRAL HEMISPHERES
════════════════════════════════════════════════
    FRONTAL    PARIETAL    OCCIPITAL
    LOBE       LOBE        LOBE
    ┌──────┐   ┌───────┐   ┌───────┐
    │Motor │   │Sensory│   │Visual │
    │Exec. │   │Spatial│   │cortex │
    │Broca's│  │       │   │       │
    └──────┘   └───────┘   └───────┘
                  TEMPORAL LOBE
                 ┌────────────┐
                 │ Auditory   │
                 │ Wernicke's │
                 │ Memory(Hippo)│
                 └────────────┘

21. LOBES OF THE BRAIN

Lobe	Key Areas	Functions
Frontal	Primary motor cortex (precentral gyrus), Prefrontal cortex, Broca's area (L hemisphere)	Voluntary motor control, executive function, planning, personality, expressive speech
Parietal	Primary somatosensory cortex (postcentral gyrus), Association areas	Touch, pain, temperature, proprioception, spatial awareness, body image
Temporal	Primary auditory cortex (Heschl's gyrus), Wernicke's area (L), Hippocampus, Amygdala	Hearing, language comprehension, memory formation, emotion
Occipital	Primary visual cortex (V1 - striate cortex)	Vision, visual processing
Insular (insula)	Deep within lateral sulcus	Visceral sensation, taste, empathy, pain
Limbic	Cingulate gyrus, hippocampus, amygdala	Emotion, memory, motivation, olfaction

Functional Motor/Sensory Map:

Homunculus: Body parts are mapped topographically on motor and sensory cortices
Inverted representation: Leg on medial surface; hand/face on lateral surface
Proportional to innervation density: Hands and face have disproportionately large cortical representation

22. CEREBELLUM

(Marked as IMPORTANT)

CEREBELLUM STRUCTURE
════════════════════════════════════════════════
         ANTERIOR LOBE
         (Paleocerebellum)
         │ Spinocerebellum
         │ Receives: Spinal cord input
         │ Controls: Limb/trunk muscle tone
         │
         POSTERIOR LOBE
         (Neocerebellum / Pontocerebellum)
         │ Largest lobe
         │ Receives: Corticopontine fibers
         │ Controls: Fine voluntary movements,
         │           motor planning
         │
         FLOCCULONODULAR LOBE
         (Archicerebellum / Vestibulocerebellum)
         │ Oldest part
         │ Receives: Vestibular nuclei input
         │ Controls: Balance, eye movements, posture

Cerebellar Cortex Layers & Cells:

CEREBELLAR CORTEX (3 layers)
───────────────────────────────────────────────────
Outer  MOLECULAR LAYER
       • Basket cells, Stellate cells (inhibitory)
       • Parallel fibers (axons of granule cells)

Middle PURKINJE CELL LAYER
       • Purkinje cells (ONLY OUTPUT of cerebellar cortex)
       • Large, flask-shaped neurons
       • Use GABA (INHIBITORY to deep nuclei)
       • Receive input from: Climbing fibers (from inferior
         olive - 1:1 ratio) + Parallel fibers (mossy fiber
         pathway)

Inner  GRANULAR LAYER
       • Granule cells (most numerous neurons in brain!)
       • Golgi cells (inhibitory)

Cerebellar Inputs & Outputs:

CEREBELLAR CIRCUITRY
──────────────────────────────────────────────────────
INPUT:
Mossy fibers (spinal, pontine, vestibular info)
    → Granule cells → Parallel fibers → Purkinje cells

Climbing fibers (from inferior olivary nucleus)
    → Directly to Purkinje cells (error signals)
    (1 climbing fiber : 1 Purkinje cell)

OUTPUT:
Purkinje cells (GABA - inhibitory)
    → Deep cerebellar nuclei (Dentate, Interposed, Fastigial)
    → Thalamus (VL nucleus) → Motor cortex
       OR
    → Brainstem nuclei (red nucleus, reticular formation)
──────────────────────────────────────────────────────
DEEP NUCLEI (mnemonic: "Don't Eat Ice Cream Fridays"):
• Dentate    → Voluntary movement, planning
• Emboliform → Limb movement (with Globose = Interposed)
• Globose    → Limb movement
• Fastigial  → Balance and posture

Cerebellar Functions & Signs of Lesion:

Function	Signs of Cerebellar Lesion
Coordination of voluntary movement	Dysmetria (past-pointing)
Motor planning/timing	Intention tremor (tremor during movement)
Smooth pursuit of eyes	Nystagmus
Balance (archicerebellum)	Truncal ataxia (wide-based gait)
Muscle tone regulation	Hypotonia
Rapid alternating movements	Dysdiadochokinesia
Speech coordination	Dysarthria (scanning/staccato speech)

Key rule: Cerebellar lesions cause IPSILATERAL signs (cerebellar fibers cross twice → same side)

23. BRAINSTEM

(Marked as IMPORTANT)

BRAINSTEM DIVISIONS
═════════════════════════════════════════════════
         ┌─────────────────────────────┐
         │         MIDBRAIN             │
         │ (Mesencephalon)              │
         │ • Superior/inferior colliculi│
         │   (visual/auditory reflex)   │
         │ • Red nucleus               │
         │ • Substantia nigra (DA)     │
         │ • CN III (oculomotor)       │
         │ • CN IV (trochlear)         │
         └─────────────┬───────────────┘
                       │
         ┌─────────────▼───────────────┐
         │            PONS             │
         │ • Apneustic & Pneumotaxic   │
         │   centers (breathing)       │
         │ • Pontine reticular         │
         │   formation (ARAS)          │
         │ • Middle cerebellar peduncle│
         │ • CN V (trigeminal)         │
         │ • CN VI (abducens)          │
         │ • CN VII (facial)           │
         │ • CN VIII (vestibulocochlear│
         └─────────────┬───────────────┘
                       │
         ┌─────────────▼───────────────┐
         │          MEDULLA            │
         │ • Vital centers:            │
         │   - Cardiovascular (vasomotor│
         │     & cardiac)              │
         │   - Respiratory (DRG + VRG) │
         │   - Vomiting center (CTZ)   │
         │   - Coughing, swallowing,   │
         │     hiccup centers          │
         │ • Pyramidal decussation     │
         │ • CN IX, X, XI, XII         │
         │ • Inferior olivary nucleus  │
         └─────────────────────────────┘

Key Brainstem Tracts:

Tract	Location	Function
Corticospinal	Lateral funiculus	Voluntary motor (decussates at medulla)
Medial lemniscus	Medial part	Fine touch, proprioception, vibration
Spinothalamic	Anterolateral	Pain, temperature, crude touch
MLF (Medial longitudinal fasciculus)	Paramedian	Coordinates eye movements; connects CN III/IV/VI

24. SYNAPSE AND ITS TYPES

Structure of a Chemical Synapse:

CHEMICAL SYNAPSE STRUCTURE
─────────────────────────────────────────────────
PRE-SYNAPTIC TERMINAL
  • Synaptic vesicles (contain NT)
  • Active zones (docking sites)
  • Voltage-gated Ca²⁺ channels
  • Mitochondria (ATP for NT synthesis)

SYNAPTIC CLEFT (20-50 nm)
  • NT diffuses across
  • Degradative enzymes (AChE)

POST-SYNAPTIC MEMBRANE
  • Receptors (ionotropic or metabotropic)
  • Post-synaptic density (PSD)

Sequence of Synaptic Transmission:

AP arrives at presynaptic terminal
          │
          ▼
Depolarization → Voltage-gated Ca²⁺ channels OPEN
          │
          ▼
Ca²⁺ influx → Vesicle fusion with membrane
          │
          ▼
NT released by exocytosis into cleft
          │
          ▼
NT binds postsynaptic receptors
          │
     ┌────┴─────┐
     ▼           ▼
 IONOTROPIC    METABOTROPIC
 (Ligand-gated (GPCR → 2nd
  ion channel)  messenger)
 Fast (ms)      Slow (sec-min)
     │
  Na⁺ in → EPSP
  Cl⁻ in → IPSP

Types of Synapses:

By Location:

Type	Description
Axodendritic	Axon → Dendrite (most common)
Axosomatic	Axon → Cell body (inhibitory often)
Axoaxonic	Axon → Axon (presynaptic inhibition/facilitation)
Dendrodendritic	Dendrite → Dendrite (found in olfactory bulb)

By Function:

Type	Mechanism	NT Examples
Excitatory (EPSP)	Na⁺ influx → membrane depolarization	Glutamate (AMPA/NMDA), ACh
Inhibitory (IPSP)	Cl⁻ influx or K⁺ efflux → hyperpolarization	GABA (GABA-A), Glycine

By Transmission Type:

Type	Mechanism
Chemical synapse	NT diffuses across cleft (most common)
Electrical synapse (gap junction)	Direct electrical coupling; bidirectional; fast; seen in cardiac muscle, smooth muscle, some CNS areas

25. TYPES OF SLEEP

SLEEP TYPES & STAGES
══════════════════════════════════════════════════
NREM (Non-REM) Sleep - 75-80% of sleep
────────────────────────────────────────
Stage 1 (N1): Light sleep; theta waves (4-8 Hz)
              Hypnic jerks; easily awakened
              
Stage 2 (N2): Sleep spindles + K-complexes
              True sleep; 45-55% of total sleep
              
Stage 3 (N3): Delta waves (<2 Hz, >75 µV)
              SLOW WAVE SLEEP / Deep sleep
              Hardest to awaken
              GH secreted here (growth hormone)
              Restorative sleep
              Sleepwalking, night terrors occur here

REM Sleep - 20-25% of sleep
────────────────────────────────────────
• EEG: Low voltage, mixed frequency (resembles wake!)
• Dreaming occurs
• Rapid eye movements (hence REM)
• Muscle atonia (actively paralyzed by brainstem)
• ↑ Heart rate, ↑ Respiratory rate (irregular)
• Penile/clitoral tumescence
• Memory consolidation (especially procedural)
• Controlled by: Cholinergic neurons (pons)
• Inhibited by: Noradrenergic (LC) & serotonergic (Raphe)

SLEEP ARCHITECTURE (one night)
────────────────────────────────────────
N1→N2→N3→N2→REM (cycle ~90 minutes)
  ↓   ↓   ↓       ↓
First half: More deep NREM (N3)
Second half: More REM, longer REM periods

NEUROTRANSMITTERS:
  Sleep onset: ↑ Adenosine, ↑ GABA
  NREM: ↑ Serotonin (raphe), ↑ NE (locus coeruleus)
  REM: ↑ ACh (brainstem - PPT/LDT nuclei)
  Wake: ↑ Histamine (tuberomammillary nucleus)
        ↑ Orexin/Hypocretin (lateral hypothalamus)
  [Orexin deficiency = Narcolepsy]
══════════════════════════════════════════════════

QUICK SUMMARY MAPS

GIT SECRETION CONTROL
Cephalic (CN X) → Gastric (Gastrin/ENS) → Intestinal (CCK/Secretin)
     30%               60%                     10% (then inhibits)

CARDIAC CYCLE SEQUENCE
SA node → Atria contract (P) → AV delay → Bundle of His →
Purkinje → Ventricles contract (QRS) → Repolarize (T)

ANS SUMMARY
Sympathetic: Fight/Flight | NE | α+β receptors | Thoracolumbar
Parasympathetic: Rest/Digest | ACh | Muscarinic | Craniosacral

ACTION POTENTIAL
Rest(-70) → Stimulus → Threshold(-55) → Na⁺ in (+40) →
K⁺ out → Repolarization → AHP → Rest(-70)

Sources: Guyton & Hall Textbook of Medical Physiology | Costanzo Physiology 7e | Ganong's Review of Medical Physiology | Robbins, Cotran & Kumar Pathologic Basis of Disease | Sleisenger & Fordtran's GI & Liver Disease | Junqueira's Basic Histology 17eAll 25 topics across 4 major systems are now covered. Here's a quick navigation guide to what was included:

📋 What's Covered - Navigation Index

🫁 GIT (7 Topics)

Salivary Glands - 3 major pairs, saliva composition, neural control flowchart
GIT Secretions - All regions + gastric cell types table
Enteric Nervous System - Auerbach's vs Meissner's plexus, peristaltic reflex flowchart
Hormonal & Neural Control of Gastric Secretion - 3 phases flowchart, parietal cell HCl mechanism, GIT hormones table
Liver - All 5 categories of function, bilirubin metabolism flowchart
Pancreas - Exocrine enzymes, control of secretion (Secretin + CCK synergy)
Biliary Secretions & Gallbladder - Enterohepatic circulation flowchart, gallbladder functions

❤️ Cardiac (5 Topics)

Cardiac Cycle - All 7 phases (A-G) with valves, heart sounds, ECG correlation
ECG - All waves/intervals and their electrical events
Cardiac Output - Formulas + determinants diagram
Conduction System - SA→AV→Bundle→Purkinje pathway with velocities
Definitions - Systole, Diastole, Preload, Afterload, SV, EF

🧠 ANS (4 Topics)

Divisions of ANS - Sympathetic vs Parasympathetic overview
Comparison Table - 10 features compared side-by-side
Organ Effects - Complete table (heart, lungs, GI, eyes, bladder, glands)
ACh & NE Synthesis - Step-by-step biosynthetic pathways with enzymes

🧬 Neural/CNS (9 Topics)

Neuron Classification - By processes (uni/bi/multipolar), function, fiber type (Aα-C)
Action Potential - Phases, ion movements, voltage-gated channels
Refractory Period - ARP vs RRP, clinical significance (cardiac tetany prevention)
NT vs Neuromodulators - 7-feature comparison table
Cerebrum - Overview + functional map
Lobes of Brain - 6 lobes with key areas and functions
Cerebellum - 3 divisions, cortical layers (Purkinje cells!), deep nuclei, lesion signs
Brainstem - Midbrain/Pons/Medulla contents, CN nuclei, vital centers
Synapse Types - Structural types, transmission sequence flowchart, EPSP/IPSP
Types of Sleep - NREM stages + REM, sleep architecture, neurotransmitters of sleep

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