Comprehensive Study Notes on Foundations of Human Physiology, Bioenergetics, and Cell Physiology

Foundations of Physiology and Anatomical Relationships

• Definition of Physiology: The scientific study of how living organisms function, perform work, and execute specific biological activities to keep the organism alive and integrated.

• Definition of Anatomy: The study of body structure, physical organization, and physical appearance of organs and tissues.

• Key Comparison Between Anatomy and Physiology:

  • Anatomy focuses on structural form: "What does a body part look like?"

  • Physiology focuses on functional mechanics: "How does a body part perform its job, and how do different parts work together?"

• Complementarity of Structure and Function:

  • The architectural structure of a biological component directly determines and constrains the specific functions it can perform.

  • Example (Teeth): Incisors possess sharp, narrow edges designed specifically for cutting food, whereas molars have broad, flattened surfaces optimized for grinding and crushing food.

  • Example (Heart): The heart is structurally built with muscular walls, distinct receiving and pumping chambers, and one-way valves that collectively permit blood to be received and pumped unidirectionally through the vascular system.

• Approaches to Explaining Body Functions:

  • Teleological Approach: Explains a biological function in terms of its operational purpose or meeting a overarching bodily need (answers why a function is required). Example: Breathing occurs to obtain oxygen for the body.

  • Mechanistic Approach: Explains biological processes through step-by-step cause-and-effect relationships (answers how a function actually occurs). Example: During inhalation, the diaphragm contracts, expanding the chest cavity, which decreases intra-thoracic pressure relative to atmospheric pressure, causing air to move down the pressure gradient into the lungs.

  • Physiological studies focus heavily on the mechanistic approach to unravel biological systems.

Levels of Structural Organization in the Human Body

• The human organism is organized in a fine-grained hierarchical sequence where each level builds upon the preceding structural framework:

  • Biological Hierarchy Sequence: Chemical Level $\rightarrow$ Cellular Level $\rightarrow$ Tissue Level $\rightarrow$ Organ Level $\rightarrow$ Organ System Level $\rightarrow$ Organism Level.

• Chemical Level:

  • Atoms represent the smallest fundamental building blocks of matter.

  • Four major chemical elements account for approximately $99\%$ of total human body mass:

  • Oxygen ($\text{O}$): Vital for cellular respiration and macromolecular frameworks.

  • Carbon ($\text{C}$): The structural backbone of all organic biological compounds.

  • Hydrogen ($\text{H}$): Critical constituent of water, biological molecules, and electrochemical gradient generation.

  • Nitrogen ($\text{N}$): Essential building block for nucleic acids (DNA and RNA), amino acids, and structural/functional proteins.

  • Trace elements constitute less than $1\%$ of body mass, including Calcium ($\text{Ca}$), Phosphorus ($\text{P}$), and Potassium ($\text{K}$).

  • Molecules: Atoms combine through chemical bonds to create biological macromolecules, including proteins, carbohydrates, lipids (fats), and nucleic acids.

  • Alphabet Analogy: Individual atoms act like alphabet letters; when combined in specific combinations, they create complex, meaningful molecular structures.

• Cellular Level:

  • The cell is the fundamental unit of both structure and function in living organisms; it is the smallest structural unit capable of carrying out life-sustaining processes.

  • Plasma Membrane: A protective outer lipid barrier that encloses internal cellular contents (cytosol and eukaryotic organelles such as mitochondria, Golgi bodies, ribosomes, and the nucleus) and separates the cell from the surrounding environment.

  • Fundamental Cellular Functions:

  • Obtaining nutrients and oxygen ($\text{O}_2$) from surrounding extracellular fluids.

  • Performing chemical reactions to generate biological energy.

  • Eliminating carbon dioxide ($\text{CO}_2$) and cellular waste products.

  • Synthesizing structural components, enzymes, and functional molecules.

  • Controlling the exchange of materials across the cell membrane.

  • Sensing and responding to internal and external environmental changes.

  • Controlling cellular reproduction, maintenance, and repair.

  • Failure of Cellular Waste Elimination: If a cell loses the ability to expel waste products, toxins accumulate within the cytosol, rendering the internal environment unsuitable for normal metabolic activity and leading to cellular dysfunction or cell death.

  • Cell Specialization (Differentiation): Multicellular organisms consist of specialized cells that modify or elaborate basic cellular processes to perform specific tasks (e.g., muscle cells specialize in mechanical contraction; nerve cells specialize in generating and transmitting electrical impulses).

Primary Tissue Types and Structural Modifications

• Definition of Tissue: An organization of similarly structured cells that work cooperatively to execute specialized functional tasks.

• Four Primary Tissue Types:

  1. Muscle Tissue: Specialized for cellular contraction and mechanical force generation.

  • Skeletal Muscle: Striated, voluntary muscle connected to bones; drives intentional physical movement (e.g., walking, lifting objects).

  • Cardiac Muscle: Striated, involuntary muscle found exclusively in the heart wall; contracts rhythmically to pump blood through the circulatory system.

  • Smooth Muscle: Non-striated, involuntary muscle lining hollow internal organs and tubes (e.g., digestive tract, blood vessels); contracts to propel contents through internal pathways.

  1. Nerve Tissue: Specialized for initiating, conducting, and transmitting rapid electrical signals (action potentials).

  • Located in the brain, spinal cord, and peripheral nerves.

  • Functions as a rapid body-wide communication network. Example: Touching a painful or hot surface immediately triggers sensory signals sent via nerve pathways to coordinate a rapid protective reflex response.

  1. Connective Tissue: Specialized for connecting, supporting, anchoring, and insulating various body structures.

  • Characterized by relatively few cells dispersed throughout an abundant extracellular matrix.

  • Tendons: Connect skeletal muscles to bones.

  • Bones: Provide structural framework, mechanical leverage, and protection for internal organs.

  • Blood: Fluid connective tissue that connects distant body regions by transporting gases, nutrients, wastes, and chemical signals.

  1. Epithelial Tissue: Specialized for forming protective barriers, lining cavities, and managing material exchange.

  • Epithelial Sheets: Tightly packed layers of cells forming protective boundaries, such as the outer skin layer (epidermis) or internal linings of visceral cavities.

  • Lumen: The internal fluid-filled cavity or space within a hollow organ or tube (e.g., the intestinal tract lumen through which ingested food passes).

  • Secretory Glands: Epithelial structures specialized for synthesizing and releasing specific substances.

    • Exocrine Glands: Possess specialized ducts; release secretions through ducts onto external body surfaces or into internal organ lumens (e.g., sweat glands, digestive glands).

    • Endocrine Glands: Ductless glands; release chemical secretions (hormones) internally directly into the surrounding extracellular fluid and blood vascular system.

Organ Systems and Integrated Physiological Function

• Organ Level: Consists of two or more primary tissue types combined structurally to perform integrated, complex functions that individual tissues cannot execute alone.

  • Stomach Example: The inner surface is lined with epithelial tissue (for protection and gastric juice secretion); its wall contains smooth muscle tissue (for mechanical churning); it is innervated by nerve tissue (for regulating contraction and gland secretion); and all components are bound together by connective tissue.

• Organ System Level: A collection of organs that work synergistically to accomplish major physiological operations.

• The 11 Major Organ Systems:

  1. Circulatory System (Heart, blood vessels, blood): Transports oxygen, nutrients, carbon dioxide, wastes, electrolytes, and hormones throughout the body.

  2. Digestive System (Mouth, esophagus, stomach, intestines, accessory glands): Breaks down ingested food into absorbable nutrients and eliminates solid waste.

  3. Respiratory System (Nose, pharynx, larynx, trachea, lungs): Absorbs oxygen from air, eliminates carbon dioxide, and assists in blood $pH$ regulation.

  4. Urinary System (Kidneys, ureters, urinary bladder, urethra): Filters blood to eliminate metabolic wastes, excess water, salts, and acids; regulates ECF volume and electrolyte balance.

  5. Skeletal System (Bones, cartilage, joints): Provides structural support, protects soft tissues, stores minerals, and works with muscles to enable movement.

  6. Muscular System (Skeletal muscles, tendons): Generates physical movement, maintains posture, and produces heat to assist in body temperature homeostasis.

  7. Integumentary System (Skin, hair, nails, sweat glands): Serves as an outer protective barrier against injury and pathogens, and regulates thermal loss.

  8. Immune System (White blood cells, lymph nodes, spleen, thymus): Defends against foreign pathogens and disease-causing agents, and facilitates tissue repair.

  9. Nervous System (Brain, spinal cord, nerves, sense organs): Provides rapid electrical communication and control; senses external and internal changes and coordinates responses.

  10. Endocrine System (Hormone-secreting glands such as pituitary, thyroid, adrenal, pancreas): Controls metabolic and physiological processes through long-term chemical regulation via hormones transported in blood.

  11. Reproductive System (Gonads, reproductive tracts, essential glands): Responsible for gamete production and species continuation (not essential for individual homeostatic survival, but required for species preservation).

• Interconnectedness of Body Systems:

  • No organ system functions in isolation; systems continuously communicate and cooperate.

  • Exercise Integration Example: Muscle activity requires more oxygen and generates extra carbon dioxide and thermal heat. The respiratory system increases oxygen intake and carbon dioxide expulsion; the circulatory system accelerates blood transport to deliver oxygen to active muscles; and the integumentary system dissipates excess heat via sweat production.

  • Blood Pressure Integration Example: Maintaining stable systemic blood pressure relies on coordinated real-time interactions among the circulatory, urinary, nervous, and endocrine systems.

Principles of Homeostasis and Body Fluid Compartments

• Definition of Homeostasis: The dynamic state of relative stability in the internal environment maintained by coordinated physiological processes, despite continuous fluctuations in external environmental conditions.

• Dynamic Physiological Limits: Homeostasis does not imply a rigid, unchanging numeric point; rather, it maintains variables within specific functional physiological ranges that support normal cell survival.

  • Thermal Range Example: Core body temperature operates most efficiently within a physiological range of $36^\circ \text{C}$ to $37^\circ \text{C}$, fluctuating mildly throughout the day while remaining balanced.

• Body Fluid Compartments:

  • Intracellular Fluid (ICF): The fluid contained inside cells.

  • Extracellular Fluid (ECF): The fluid located outside cells. ECF represents the immediate internal environment that directly surrounds and bathes cells.

  • Plasma: The fluid portion of the blood contained inside blood vessels.

  • Interstitial Fluid: The fluid portion surrounding and directly contacting individual tissue cells.

• Key Homeostatically Maintained Variables:

  • Concentrations of nutrient molecules (e.g., glucose, fatty acids, amino acids).

  • Concentrations of $\text{O}_2$ and $\text{CO}_2$.

  • Concentrations of metabolic waste products.

  • $pH$ (acidity/alkalinity balance) of the internal fluid environment.

  • Concentrations of water, salt, and electrolytes (e.g., $\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{Cl}^-$).

  • Volume and pressure of extracellular fluid and blood plasma.

  • Internal core body temperature.

Homeostatic Control Systems and Feedback Mechanisms

• Essential Control Tasks:

  • Detection: Identifying deviations of regulated variables from desired physiological baseline values.

  • Integration: Processing sensory information with other relevant physiological data.

  • Adjustment: Directing effector mechanisms to restore the variable back toward its set point.

• Spatial Classifications of Homeostatic Controls:

  • Intrinsic Controls (Local Controls): Built directly into or inherent to an organ; allows local tissue self-regulation in response to local environment changes.

  • Extrinsic Controls: Control mechanisms initiated outside an organ, coordinated primarily by the nervous system or endocrine system to regulate multiple organs toward a common goal.

• Regulatory Mechanisms:

  1. Negative Feedback Loop:

  • The primary biological feedback mechanism maintaining homeostasis.

  • Operates when a system detects a change in a variable and generates a response that opposes or reverses the initial change, returning the variable toward its set point.

  • Three Components: Sensor (detects deviation) $\rightarrow$ Control Center (compares against set point) $\rightarrow$ Effector (produces compensatory response).

  • Hyperthermia Example: Core temperature increases above set point $\rightarrow$ thermal sensors detect deviation $\rightarrow$ control center activates sweat glands $\rightarrow$ sweat evaporation cools the body, opposing the temperature rise.

  • Hypothermia Response: If body temperature drops below set point, negative feedback mechanisms trigger shivering and blood vessel constriction to raise temperature, opposing the decrease.

  1. Positive Feedback Loop:

  • Less common in homeostatic regulation.

  • Amplifies or reinforces an initial change in the same direction, accelerating a process until a specific biological event is completed.

  • Parturition (Childbirth) Example: Uterine contractions force the fetus against the cervix, stretching sensory receptors $\rightarrow$ signals cause pituitary oxytocin release $\rightarrow$ oxytocin intensifies uterine contractions $\rightarrow$ further cervical stretching triggers greater oxytocin release in an escalating positive cycle until the baby is delivered.

  1. Feedforward Mechanism:

  • Operates by anticipating an impending change before the regulated variable actually drifts from its set point.

  • Behavioral Analogy: Putting on a heavy coat before stepping outdoors into cold weather.

  • Physiological Example: The presence, smell, or sight of food stimulates early insulin release from the pancreas while food is still in the digestive lumen, preparing cells to absorb glucose before blood glucose levels actually rise.

• Pathophysiology: The study of abnormal body functioning associated with disease or injury when normal homeostatic feedback systems fail or are disrupted. Severe homeostatic failure leads to functional illness or death.

Bioenergetics and Cellular ATP Production

• Subdivisions of the Cell:

  • Plasma Membrane: Encloses the cell and regulates substance exchange.

  • Nucleus: Houses genetic material (DNA).

  • Cytoplasm: Contains cytosol (gel-like fluid) and organelles (mitochondria, ribosomes, Golgi apparatus).

• Cellular Metabolism:

  • Catabolism: Chemical breakdown of complex organic molecules into simpler compounds, releasing stored chemical energy (exergonic).

  • Anabolism: Chemical synthesis of complex macromolecules from simpler precursor molecules, requiring energy input (endergonic).

• Adenosine Triphosphate (ATP) — Cellular Energy Currency:

  • Structure: Composed of adenosine linked to three phosphate groups ($\text{Adenosine}-\text{P}\sim\text{P}\sim\text{P}$).

  • Hydrolysis Reaction:     $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

  • Hydrolysis splits ATP into Adenosine Diphosphate (ADP) and inorganic phosphate ($\text{P}_i$), releasing usable free energy to drive cellular work (e.g., protein synthesis, membrane active transport, mechanical muscle contraction).

  • The reaction is reversible; ADP can be re-phosphorylated back into ATP.

• Metabolic Pathways for Cellular ATP Regeneration:

  1. Substrate-Level Phosphorylation / Creatine Phosphate System:

  • Direct transfer of a high-energy phosphate group from creatine phosphate to ADP, catalyzed by creatine kinase:        $\text{Creatine Phosphate} + \text{ADP} \xrightarrow{\text{Creatine Kinase}} \text{Creatine} + \text{ATP}$

  • Provides rapid, short-term ATP regeneration in the cytosol at the onset of intense muscle contraction.

  1. Glycolysis:

  • A 10-step enzymatic sequence occurring in the cell cytosol.

  • Converts 1 molecule of glucose ($6\text{-carbon}$) into 2 molecules of pyruvate ($3\text{-carbon}$).

  • Anaerobic process (functions independently of oxygen).

  • Requires an initial energy investment of $2\,\text{ATP}$; generates $4\,\text{ATP}$ total, yielding a net gain of $2\,\text{ATP}$ and $2\,\text{NADH}$.

  1. Pyruvate Processing (Decarboxylation):

  • Under aerobic conditions, pyruvate enters the mitochondrial matrix.

  • Pyruvate dehydrogenase complex removes a carbon as $\text{CO}_2$, reduces $\text{NAD}^+$ to $\text{NADH}$, and joins the remaining $2\text{-carbon}$ fragment to Coenzyme A, forming Acetyl-CoA.

  1. Tricarboxylic Acid (TCA) / Citric Acid / Krebs Cycle:

  • Takes place within the mitochondrial matrix.

  • Acetyl-CoA enters the cycle and undergoes a series of reactions to produce high-energy electron carriers: $\text{NADH}$ and $\text{FADH}_2$, along with $\text{CO}_2$ and a small amount of ATP/GTP.

  1. Electron Transport Chain (ETC) and Chemiosmosis (Oxidative Phosphorylation):

  • Located on the inner mitochondrial membrane.

  • $\text{NADH}$ and $\text{FADH}_2$ transfer high-energy electrons through carrier protein complexes (Complexes I–IV, containing cytochromes).

  • Electron transport releases energy in controlled steps, pumping hydrogen ions ($\text{H}^+$) from the matrix into the intermembrane space, building an electrochemical $\text{H}^+$ gradient.

  • Chemiosmosis: $\text{H}^+$ ions flow down their concentration gradient back into the matrix through ATP Synthase, driving the phosphorylation of ADP into ATP.

  • Molecular Oxygen ($\text{O}_2$) acts as the final terminal electron acceptor, binding electrons and $\text{H}^+$ to form water ($\text{H}_2\text{O}$).

• Comparative Yields:

  • Aerobic Respiration Yield: Produces a maximal theoretical yield of $32$ to $38\,\text{ATP}$ per single molecule of glucose.

  • Anaerobic Respiration / Lactic Acid Fermentation: In the absence of $\text{O}_2$, pyruvate is converted into lactic acid in the cytosol to recycle $\text{NAD}^+$, yielding only $2\,\text{ATP}$ per glucose molecule.

Plasma Membrane Structure and the Fluid Mosaic Model

• Phospholipid Bilayer Structure:

  • Composed of amphipathic phospholipid molecules.

  • Hydrophilic (water-attracting) polar phosphate heads face outward toward the aqueous extracellular fluid (ECF) and intracellular fluid (ICF).

  • Hydrophobic (water-repelling) non-polar fatty acid tails project inward toward each other, creating a central hydrophobic core.

  • The hydrophobic core acts as a barrier, preventing charged or large polar molecules from freely crossing.

• Fluid Mosaic Model:

  • Describes the membrane as a dynamic structure where lipids and proteins move laterally within the bilayer.

  • Cholesterol molecules interspersed within the hydrophobic core modulate membrane fluidity and mechanical stability across temperature changes.

  • "Mosaic": Represents the dynamic collection of membrane proteins, glycoproteins, and glycolipids embedded in or attached to the lipid bilayer.

• Outer Membrane Identity Markers:

  • Glycoproteins (carbohydrates bound to proteins) and Glycolipids (carbohydrates bound to lipids) extend into the ECF.

  • Function as cell identity markers for tissue recognition and intercellular interaction (abnormal cell markers are characteristic of cancer cells).

• Functional Classes of Membrane Proteins:

  • Ion Channels: Water-filled transmembrane pores allowing specific small ions to pass down gradients.

  • Carrier Proteins (Transporters): Bind specific target molecules, changing conformation to transport them across.

  • Receptors: Bind specific extracellular chemical messengers (ligands) to initiate intracellular signal transduction.

  • Membrane Enzymes: Catalyze chemical reactions at inner or outer membrane surfaces.

  • Cell Adhesion Molecules (CAMs): Anchor cells to adjacent cells or extracellular structures.

Cell Adhesion and the Extracellular Matrix

• Extracellular Matrix (ECM):

  • An extracellular gel-like complex carbohydrate mass intermixed with fibrous protein threads, providing structural support to cells in tissues.

  • Collagen: Forms tough fibrous cables providing high tensile strength (resists mechanical stretching forces).

  • Elastin: Provides rubber-like elasticity, allowing tissues to stretch and recoil (e.g., lungs, blood vessel walls).

  • Fibronectin: Promotes cell adhesion and anchors cells in position within tissues.

Cellular Junctions and Intercellular Communication

• Specialized Intercellular Junctions:

  1. Desmosomes (Adhering Junctions):

  • Anchor adjacent cells tightly in tissues subjected to mechanical stretching or stress (e.g., cardiac muscle, skin, uterus). "Desmosomes hold."

  1. Tight Junctions (Impermeable Junctions):

  • Firmly fuse adjacent cell membranes near epithelial surfaces, sealing off extracellular space to prevent unmanaged passage of materials between cells. "Tight junctions seal."

  1. Gap Junctions (Communicating Junctions):

  • Consist of aligned protein tunnels (connexons) bridging neighboring cells, allowing direct passage of small ions and water-soluble molecules for rapid electrical and chemical coupling (abundant in cardiac and smooth muscle). "Gap junctions communicate."

• Methods of Intercellular Communication:

  • Direct Communication: Occurs via gap junctions or direct cell-to-cell surface marker interaction.

  • Indirect Chemical Communication:

  • Paracrines: Local chemical messengers acting on adjacent target cells.

  • Neurotransmitters: Short-range chemical messengers released by neurons across synaptic clefts.

  • Hormones: Long-range chemical messengers secreted by endocrine glands into blood to reach distant target tissues.

Passive Transport Mechanisms

• Factors Determining Membrane Permeability:

  • High lipid solubility and small particle size favor rapid movement across the hydrophobic lipid bilayer.

• Unassisted Passive Transport (Does not expend ATP):

  1. Simple Diffusion:

  • Net thermal movement of molecules down a concentration gradient from high to low concentration until equilibrium is reached.

  • Favors small, non-polar, lipid-soluble molecules (e.g., $\text{O}_2$, $\text{CO}_2$, steroid hormones).

  1. Osmosis:

  • Net passive diffusion of water molecules across a selectively permeable membrane toward a region of higher non-penetrating solute concentration.

  • Aquaporins: Channel proteins that facilitate rapid osmotic water transport.

  • Hydrostatic Pressure: Fluid pressure exerted against a membrane surface, opposing osmotic water movement until dynamic equilibrium occurs.

  • Solution Tonicity Effects on Cell Volume:

    • Isotonic Solution: Equal non-penetrating solute concentration as ICF; cell volume remains unchanged.

    • Hypotonic Solution: Lower non-penetrating solute concentration than ICF; water enters cell, causing swelling and potential lysis.

    • Hypertonic Solution: Higher non-penetrating solute concentration than ICF; water leaves cell, causing cell shrinkage (crenation).

  1. Facilitated Diffusion:

  • Passive movement of polar molecules down a concentration gradient using a membrane carrier protein (no ATP spent). Example: Entry of glucose into cells via GLUT carrier proteins.

• Properties of Carrier-Mediated Transport:

  • Specificity: Carriers accommodate only specific, structurally matching molecules.

  • Saturation ($T_m$ / Transport Maximum): Transport rate reaches a maximum limit when all available carriers are occupied.

  • Competition: Structurally related molecules compete for binding sites on the same carrier protein.

Active and Vesicular Transport Mechanisms

• Active Transport Mechanisms (Requires cellular energy expenditure):

  1. Primary Active Transport:

  • Uses ATP directly to pump solutes uphill against a concentration gradient.

  • Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ ATPase): Hydrolyzes ATP directly to pump $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell per cycle, maintaining essential resting electrochemical gradients.

  1. Secondary Active Transport:

  • Uses "secondhand" potential energy stored in an ion concentration gradient (created by primary active transport) rather than hydrolyzing ATP directly.

  • Cotransport Types:

    • Symport: Moves two substances in the same direction across the membrane (e.g., $\text{Na}^+$-glucose cotransport).

    • Antiport: Moves two substances in opposite directions across the membrane (e.g., $\text{Na}^+/\text{H}^+$ exchange).

    • Uniport: Moves a single substance across the membrane (typically passive or facilitated).

• Vesicular Transport Mechanisms (Active energy-requiring transport for macromolecules):

  1. Endocytosis: Plasma membrane invaginates and pinches off to internalize external materials into intracellular vesicles.

  • Pinocytosis ("Cell drinking"): Non-selective fluid uptake of extracellular fluid droplets and solutes.

  • Phagocytosis ("Cell eating"): Selective uptake of large multimolecular particles (e.g., bacteria, cellular debris) using pseudopods to form a phagosome.

  • Receptor-Mediated Endocytosis: Highly selective uptake initiated when specific extracellular ligands bind surface receptors, triggering vesicle formation.

  1. Exocytosis: Secretory vesicles fuse with the plasma membrane to discharge internal contents into the ECF. Restores membrane surface area and inserts functional proteins/receptors.

Signal Transduction Pathways

• Definition of Signal Transduction: The process by which an extracellular chemical messenger (ligand) binding to a surface receptor is converted into specific intracellular biological responses.

• Sequential Steps of Signal Transduction:

  1. An extracellular chemical messenger arrives at the target cell surface.

  2. The messenger binds specifically to its complementary membrane receptor.

  3. Receptor binding triggers a structural change that initiates intracellular events:

  • Opening or closing of specific membrane ion channels.

  • Activation of intracellular second-messenger pathways (e.g., cyclic AMP, $\text{Ca}^{2+}$), relaying signals to target proteins inside the cell.

  1. The target cell executes a coordinated functional response (e.g., enzyme activation, altered secretion, gene expression).