Comprehensive Study Notes on Human Physiology, Homeostasis, Membrane Transport, and Neuro-Endocrine Signaling

Fundamentals of Physiology and Homeostasis

• Definition of Physiology:

  • Physiology is the study of how the body and its component parts function.

  • It focuses explicitly on the underlying mechanisms of body processes.

  • It is distinct from anatomy, which focuses on what and where body structures are located.

  • Physiology is inherently interdisciplinary, drawing upon principles from mathematics, physics, biochemistry, and organic chemistry.

• Levels of Biological Organization:

  • Atoms combine to form Molecules (studied in Chemistry).

  • Molecules form Cells, which group into Tissues (studied in Molecular and Cell Biology).

  • Tissues construct Organs, which coordinate as Organ Systems, leading to the complete Organism (studied in Physiology).

  • Organisms interact in Populations of one species, Ecosystems of different species, and the global Biosphere (studied in Ecology).

• Integrative Physiology:

  • Focuses on how multiple functional organ systems collaborate to maintain human function.

  • Major functional systems include: Integumentary, Respiratory, Digestive, Circulatory, Urinary, Nervous, Endocrine, Musculoskeletal, Reproductive, and Immune systems.

• Major Core Themes in Physiology:

  1. Structure and Function Across All Levels: Relationships between molecular/cellular anatomy and functional capacity.

  2. Energy Transfer, Storage, and Use: Metabolic pathways converting energy for biological work.

  3. Information Flow, Storage, and Use: Signal transduction and communication to achieve coordination within organisms.

  4. Homeostasis and Control Systems: Dynamics maintaining internal environment stability.

Homeostasis and Feedback Mechanisms

• Definition of Homeostasis:

  • Homeostasis is defined as the maintenance of a relatively stable internal environment (specifically the Extracellular Fluid, ECF) despite ongoing internal and external environmental fluctuations (stresses).

  • Homeostasis represents a dynamic "Steady State," NOT a state of physical or chemical equilibrium.

• Homeostatic Balance and Disease Dynamics:

  • An organism in homeostasis subjected to external or internal changes undergoes internal alterations that threaten homeostasis.

  • The body initiates dynamic compensatory responses:

    • Successful Compensation: Restores homeostatic steady state, maintaining wellness.

    • Failed Compensation: Results in homeostatic breakdown, causing illness or disease.

• Negative Feedback Mechanisms:

  • The primary mechanism for maintaining homeostatic steady state around a setpoint.

  • The response of the system counteracts or reverses the initial stimulus, effectively shutting off the response loop once steady state is restored.

• Positive Feedback Mechanisms:

  • The response of the system reinforces or amplifies the initial stimulus, driving the regulated variable progressively further from the initial setpoint.

  • Creates an unstable, escalating loop that requires an outside factor or event to terminate the process.

Biological Compartmentation and Fluid Composition

• Body Fluid Compartments:

  • Total body water is divided into two primary compartments:

    • Intracellular Fluid (ICF): Constitutes $2/3$ of total body water volume (fluid within cells).

    • Extracellular Fluid (ECF): Constitutes $1/3$ of total body water volume (fluid outside cells). ECF is subdivided into:

      • Interstitial Fluid: Fluid filling the microscopic spaces between tissue cells and capillaries.

      • Blood Plasma: The liquid matrix of the vascular system.

• Ion Concentration Differences across Compartments:

  • Intracellular Fluid (ICF):

    • Sodium ($\text{Na}^+$): Low ($\text{15}\, \text{mmol/L}$)

    • Potassium ($\text{K}^+$): High ($\text{120}\, \text{mmol/L}$)

    • Chloride ($\text{Cl}^-$): Low ($\text{10}\, \text{mmol/L}$)

    • Bicarbonate ($\text{HCO}_3^-$): Low ($\text{10}\, \text{mmol/L}$)

    • Proteins: Very High ($\text{108}\, \text{mmol/L}$)

  • Interstitial Fluid (ECF):

    • Sodium ($\text{Na}^+$): High ($\text{140}\, \text{mmol/L}$)

    • Potassium ($\text{K}^+$): Low ($\text{5}\, \text{mmol/L}$)

    • Chloride ($\text{Cl}^-$): High ($\text{103}\, \text{mmol/L}$)

    • Bicarbonate ($\text{HCO}_3^-$): High ($\text{21}\, \text{mmol/L}$)

    • Proteins: Extremely Low / Minimal

  • Blood Plasma (ECF):

    • Sodium ($\text{Na}^+$): High ($\text{140}\, \text{mmol/L}$)

    • Potassium ($\text{K}^+$): Low ($\text{5}\, \text{mmol/L}$)

    • Chloride ($\text{Cl}^-$): High ($\text{103}\, \text{mmol/L}$)

    • Bicarbonate ($\text{HCO}_3^-$): High ($\text{21}\, \text{mmol/L}$)

    • Proteins: High ($\text{76}\, \text{mmol/L}$)

• Membranes Separating Compartments:

  • Tissue Membranes: Multicellular macro-structures composed of epithelial and connective tissue layers (e.g., pericardial sac surrounding the heart).

  • Cell Membranes (Plasma Membrane): Microscopic phospholipid bilayers enclosing individual cells.

• Fluid Mosaic Model of Cell Membranes:

  • Phospholipid Bilayer: Amphipathic molecules with hydrophilic heads facing aqueous ICF/ECF compartments and hydrophobic fatty acid tails forming the interior layer.

  • Cholesterol: Inserted into the lipid bilayer to regulate membrane fluidity and stability.

  • Membrane Proteins:

    • Transmembrane (Integral) Proteins: Cross the entire lipid bilayer.

    • Peripheral Proteins: Bound loosely to inner/outer surfaces; removable without disrupting bilayer integrity.

    • Lipid-Anchored Proteins: Covalently attached to lipid molecules within the membrane.

    • Glycoproteins & Cytoskeleton-Linked Proteins: Essential for cellular identity, structural support, and signaling.

Membrane Transport Processes

• Overview of Transport Processes:

  • Homeostatic maintenance requires selective exchange across cell membranes via two core mechanisms:

    1. Passive Transport: Movement of substances that does not require added metabolic energy (uses molecular kinetic energy and potential energy stored in concentration/electrical gradients).

    2. Active Transport: Movement of substances that requires added metabolic energy (ATP hydrolysis or coupled energy) to drive molecules against their electrochemical gradient.

• Forces Driving Passive Transport:

  • Chemical Force: Operates down a concentration gradient (from higher concentration to lower concentration) driven by random thermal motion.

  • Electrical Force: Operates on charged molecules/ions driven by electrostatic attraction or repulsion due to membrane potential separation.

• Modes of Passive Transport:

  • Simple Diffusion: Movement of membrane-penetrating solutes directly through the phospholipid bilayer down their concentration gradient. Effective over extremely short distances. Operates for small, nonpolar, lipophilic molecules and gases (e.g., Urea, $\text{O}_2$, $\text{CO}_2$).

  • Facilitated Diffusion: Movement of non-membrane-penetrating solutes down their electrochemical gradient assisted by transmembrane protein channels or carriers (e.g., GLUT transporters for glucose, aquaporins for water).

Osmolarity, Osmosis, and Tonicity

• Concentration: The amount of solute in moles present in a liter of solution ($\text{moles/L}$, Molar, or $\text{M}$).

• Osmosis: Passive net movement of water across a selectively permeable membrane from an area of lower solute concentration (higher water concentration) to higher solute concentration (lower water concentration).

• Osmotically Active Particles: Solute particles that exert an osmotic draw on water molecules.

• Osmolarity:

  • Defined as the concentration of osmotically active, dissociated particles per liter of solution ($\text{Osmoles/L}$, OsMolar, or $\text{OsM}$; or $\text{mOsM}$).

  • Salts dissociate into ions in aqueous solution; each individual ion acts as an independent osmotically active particle.

• Comparing Solution Osmolarities:

  • Iso-osmotic: Two solutions containing equal concentrations of osmotically active particles.

  • Hyper-osmotic: A solution with a higher concentration of osmotically active particles relative to another.

  • Hypo-osmotic: A solution with a lower concentration of osmotically active particles relative to another.

• Toxicity:

  • Refers to the physiological effect of a solution on cell volume ("tone") at dynamic equilibrium, determined by non-penetrating solute concentrations relative to the cytoplasm.

  • Standard intracellular cytoplasm osmolarity is baseline $\text{300}\, \text{mOsM}$.

  • Isotonic Solution: Solution has an osmotic pressure equal to the cytoplasm; net water movement is zero, cell volume remains unchanged.

  • Hypotonic Solution: Solution has a lower effective non-penetrating solute concentration than the cytoplasm; net water enters the cell, causing cell swelling and potential lysis.

  • Hypertonic Solution: Solution has a higher effective non-penetrating solute concentration than the cytoplasm; net water leaves the cell, causing cell shrinkage (crenation).

• Steps for Determining Solution Tonicity:

  1. Determine concentration of all solutes in both ICF and ECF compartments.

  2. Calculate total initial osmolarity of both compartments.

  3. Identify membrane permeability of solutes; calculate net movement of penetrating solutes until equalized.

  4. Determine direction of net water movement across the membrane driven by remaining non-penetrating osmotic gradients.

  5. Classify solution tonicity based on resulting change in equilibrium cell volume.

Active and Specialized Transport

• Primary Active Transport:

  • Directly utilizes metabolic energy from ATP hydrolysis to move ions or molecules against their electrochemical gradient.

  • Prototypical Example: $\text{Na}^+/\text{K}^+\text{-ATPase}$ (Sodium-Potassium Pump).

    • Extrudes $3\, \text{Na}^+$ ions out of the cell and imports $2\, \text{K}^+$ ions into the cell per ATP molecule consumed.

    • Electrogenic pump maintaining low internal $\text{Na}^+$ and high internal $\text{K}^+$.

• Secondary Active Transport:

  • Uses potential energy stored in the concentration gradient of one molecule (established by primary active transport) to move a second molecule "uphill" against its concentration gradient.

  • Prototypical Example: SGLT (Sodium-Glucose Secondary Active Transporter).

    1. $\text{Na}^+$ in intestinal/renal lumen binds to high-affinity site on extracellular face of SGLT.

    2. $\text{Na}^+$ binding induces conformational change creating a high-affinity binding site for glucose.

    3. Glucose binds to SGLT, inducing a conformational shift that opens the transporter to the ICF.

    4. $\text{Na}^+$ is released into cytosol down its concentration gradient; $\text{Na}^+$ dissociation alters glucose site to low-affinity, releasing glucose into ICF against its concentration gradient.

• Vesicular Transport:

  • Endocytosis: Membrane invaginates to form intracellular vesicles for uptake of macromolecular cargo.

  • Receptor-Mediated Endocytosis:

    1. Ligand binds specific cell surface receptor.

    2. Receptor-ligand complex migrates to clathrin-coated pits.

    3. Endocytosis internalizes clathrin-coated vesicle.

    4. Vesicle sheds clathrin coat and fuses with endosome.

    5. Acidic endosomal environment dissociates receptors and ligands.

    6. Ligands routed to lysosomes or Golgi complex; receptors recycled via transport vesicles back to cell membrane via exocytosis.

  • Phagocytosis: Actin-mediated engulfment of bacteria or foreign particles.

  • Exocytosis: Secretory vesicles fuse with plasma membrane to release contents into ECF.

• Functional Classification of Transport Membrane Proteins:

  • Channel Proteins: Create continuous, water-filled transmembrane pores.

    • Gated Channels: Transition between open/closed states in response to signals (voltage, ligand, mechanical).

    • Open Channels (Pores / Leak Channels): Remain open continuously.

  • Carrier Proteins: Bind substrate on one side, undergo conformational change, and release substrate on opposite side (never open to both ECF and ICF simultaneously).

    • Uniport Carriers: Transport only a single substrate type (e.g., GLUT).

    • Cotransporters: Transport two or more different substrates.

      • Symport Carriers: Move substrates in the same direction across the membrane.

      • Antiport Carriers: Move substrates in opposite directions across the membrane.

• Epithelial Transport across Tissue Membranes:

  • Paracellular Transport: Substrata movement through tight junctions/intercellular spaces between adjacent cells.

  • Transcellular Transport: Substrata movement through mucosal/apical membrane, cytosol, and basolateral membrane of transporting cell.

  • Absorption: Transport from organ lumen to ECF.

  • Secretion: Transport from ECF to organ lumen.

Cellular Signaling Dynamics

• Signal Cascades and Amplification:

  • Signal Cascades: Sequentially linked enzyme activations where step A activates B, B activates C, culminating in product formation.

  • Signal Amplification: A single primary ligand binding to a membrane receptor activates an amplifier enzyme, producing hundreds to thousands of second messenger molecules.

• G Protein-Coupled Receptor (GPCR) – Adenylyl Cyclase Pathway:

  1. Primary signal molecule (ligand) binds GPCR, activating attached intracellular G protein.

  2. G protein activates Adenylyl Cyclase (amplifier enzyme).

  3. Adenylyl Cyclase converts ATP into cyclic AMP (cAMP; second messenger).

  4. cAMP binds and activates Protein Kinase A (PKA).

  5. Activated PKA phosphorylates specific target proteins, initiating cellular response.

• Receptor Pharmacology:

  • Receptor Agonist: Exogenous or endogenous ligand that binds a receptor, activating it and triggering a biological response.

  • Receptor Antagonist: Molecule that binds a receptor with affinity but lacks intrinsic activity, blocking or inhibiting agonist-induced responses.

Membrane Potential ($V_m$) and Quantitative Biophysics

• Membrane Potential ($V_m$):

  • Defined as the electrical potential difference (voltage) across the plasma membrane, expressed in millivolts ($\text{mV}$).

  • Reflects charge separation across the membrane; by convention, ECF is defined as $\text{0}\, \text{mV}$, and $V_m$ describes the inner cytoplasmic face charge relative to ECF.

  • Standard resting membrane potential ($V_{m,\text{rest}}$) in excitable cells is typical $\text{-70}\, \text{mV}$.

• Experimental Measurement of Membrane Potential:

  • Glass microelectrode (recording electrode) inserted into cytoplasm; second reference electrode placed in extracellular saline bath ($\text{0}\, \text{mV}$).

  • Voltmeter measures charge differential ($V_m = \text{-70}\, \text{mV}$).

• Terminology for Potential Shifts:

  • Depolarization: Shift in membrane potential making cytoplasm less negative (more positive) relative to $V_{m,\text{rest}}$.

  • Repolarization: Shift in membrane potential returning toward $V_{m,\text{rest}}$ following a depolarization.

  • Hyperpolarization: Shift in membrane potential making cytoplasm more negative than $V_{m,\text{rest}}$.

• Equilibrium Potential ($E_{\text{ion}}$):

  • The membrane potential ($V_m$) at which electrical electrostatic force is equal in magnitude and opposite in direction to chemical concentration force acting on a specific permeable ion.

  • At equilibrium potential, net transmembrane ion flux is zero.

• The Nernst Equation:

  • Calculates equilibrium potential ($E_{\text{ion}}$) for any single ion species at body temperature ($37^\text{o}\text{C}$):

$E_{\text{ion}} = \frac{61}{z} \text{log} \times \frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}}$

• In the Nernst equation, zz represents the electrical charge or valence of the ion (for example, +1+1 for Na+Na+ and K+K+, or −1−1 for Cl−Cl−).

• Goldman-Hodgkin-Katz (GHK) Equation:

  • Calculates resting membrane potential ($V_m$) taking into account concentrations and relative membrane permeabilities ($P$) of all permeable ions ($\text{Na}^+$, $\text{K}^+$, and $\text{Cl}^-$):

$V_m = 61 \text{log} \times \frac{P_{\text{Na}}[\text{Na}^+]_{\text{out}} + P_{\text{K}}[\text{K}^+]_{\text{out}} + P_{\text{Cl}}[\text{Cl}^-]_{\text{in}}}{P_{\text{Na}}[\text{Na}^+]_{\text{in}} + P_{\text{K}}[\text{K}^+]_{\text{in}} + P_{\text{Cl}}[\text{Cl}^-]_{\text{out}}}$

Neuronal Functional Anatomy and Ion Channels

• Neurons: Functional structural units of the nervous system specialized for electrical signal generation and propagation.

• Anatomical Components:

  • Soma (Cell Body): Metabolic hub; integrates synaptic input signals.

  • Dendrites: Branching, non-myelinated input structures that convey local incoming signals via Graded Potentials.

  • Axon: Myelinated or unmyelinated output process propagating self-regenerating Action Potentials to target terminals.

  • Axon Hillock & Trigger Zone: Conical junction between soma and initial axon segment. Contains dense concentration of voltage-gated $\text{Na}^+$ and $\text{K}^+$ channels; serves as the neuron's integrating center where action potentials are initiated if threshold ($\text{-55}\, \text{mV}$) is met.

• Categories of Membrane Ion Channels:

  • Leak Channels: Continuously open pores; resting neuronal membranes possess vastly more $\text{K}^+$ leak channels than $\text{Na}^+$ leak channels.

  • Gated Channels: Open/close selectively in response to specific triggers. Exist in three conformational states:

    1. Closed, capable of activating.

    2. Open (activated).

    3. Inactivated (closed, incapable of activating).

  • Gated Channel Types:

    • Ligand-Gated Channels: Open upon binding chemical messengers (found mainly on dendrites and soma).

    • Mechanically-Gated Channels: Open upon physical stretch or pressure deformation (found on sensory neurons).

    • Voltage-Gated Channels: Open upon changes in $V_m$ (found predominantly along axons).

• Voltage-Gated Sodium ($\text{Na}^+$) Channel Gating Dynamics:

  • Possesses two distinct gates: an Activation Gate (fast-opening) and an Inactivation Gate (slow-closing, ball-and-chain).

  • Resting State ($\text{-70}\, \text{mV}$): Activation gate closed; inactivation gate open.

  • Depolarization to Threshold ($\text{-55}\, \text{mV}$): Activation gate opens rapidly, causing explosive $\text{Na}^+$ influx.

  • Peak Depolarization ($\text{+30}\, \text{mV}$): Inactivation gate closes (slow-timed delay), halting $\text{Na}^+$ entry.

  • Repolarization: Activation gate closes and inactivation gate opens, resetting channel to resting state.

Graded Potentials vs. Action Potentials

• Graded Potentials:

  • Localized, variable-amplitude changes in $V_m$ occurring on soma and dendrites.

  • Magnitude correlates directly with stimulus intensity; decays exponentially over distance (decremental conduction).

  • Can be depolarizing (EPSP) or hyperpolarizing (IPSP).

• Action Potentials:

  • Rapid, stereotype, uniform-amplitude electrical signals propagated down axons without decrement.

  • All-or-None Event: Initiated only if graded potentials reaching trigger zone equal or exceed threshold potential ($\text{-55}\, \text{mV}$).

• Phases of an Action Potential:

  1. Resting Phase: $V_m = \text{-70}\, \text{mV}$. Voltage-gated $\text{Na}^+$ and $\text{K}^+$ channels closed.

  2. Depolarization Phase: Graded potential reaches threshold ($\text{-55}\, \text{mV}$). Fast voltage-gated $\text{Na}^+$ activation gates open -> rapid $\text{Na}^+$ influx down electrochemical gradient -> positive feedback loop accelerates membrane depolarization to peak overshoot ($\text{+30}\, \text{mV}$).

  3. Repolarization Phase: At $\text{+30}\, \text{mV}$, $\text{Na}^+$ inactivation gates close; slow voltage-gated $\text{K}^+$ channels open fully -> $\text{K}^+$ efflux rushes out of cell -> $V_m$ falls rapidly toward resting potential.

  4. Hyperpolarization Phase (Undershoot): Voltage-gated $\text{K}^+$ channels are slow to close, allowing continued $\text{K}^+$ efflux -> $V_m$ hyperpolarizes toward $E_{\text{K}}$ ($\text{-90}\, \text{mV}$).

  5. Return to Resting Potential: Voltage-gated $\text{K}^+$ channels close. Leak channels and $\text{Na}^+/\text{K}^+\text{-ATPase}$ maintain baseline resting potential ($\text{-70}\, \text{mV}$).

Refractory Periods and Propagation

• Refractory Periods:

  • Absolute Refractory Period: Time span during depolarization and early repolarization where voltage-gated $\text{Na}^+$ channels are either open or inactivated. Absolutely no second action potential can be triggered, regardless of stimulus magnitude. Ensures unidirectional propagation and limits max firing rate.

  • Relative Refractory Period: Follows absolute period during hyperpolarization undershoot. Voltage-gated $\text{Na}^+$ channels have reset to deactivated closed state, but voltage-gated $\text{K}^+$ channels remain open. A suprathreshold stimulus (larger-than-normal) CAN initiate a second action potential.

• Action Potential Propagation Mechanisms:

  • Continuous Conduction: Occurs in unmyelinated axons. Sequential depolarization of adjacent membrane segments. Relatively slow conduction velocity.

  • Saltatory Conduction: Occurs in myelinated axons. Myelin sheath (produced by Schwann cells in PNS or oligodendrocytes in CNS) insulates internodal regions. Action potentials "jump" between non-insulated gaps called Nodes of Ranvier, where voltage-gated $\text{Na}^+$ channels are highly clustered. Greatly speeds up conduction velocity and conserves metabolic energy.

Synaptic Transmission and Signal Integration

• Synapse Structure: Region of communication between presynaptic terminal and postsynaptic target cell.

• Mechanisms of Chemical Synaptic Transmission:

  1. Action potential depolarizes presynaptic axon terminal.

  2. Depolarization opens voltage-gated Calcium ($\text{Ca}^{2+}$) channels; $\text{Ca}^{2+}$ rushes into terminal down concentration gradient.

  3. $\text{Ca}^{2+}$ influx triggers exocytosis of neurotransmitter-filled synaptic vesicles.

  4. Neurotransmitter diffuses across synaptic cleft and binds specific postsynaptic membrane receptors.

  5. Receptor activation initiates postsynaptic cellular response.

• Neurotransmitter Termination Mechanisms:

  1. Reuptake: Transport back into presynaptic terminal or surrounding glial cells.

  2. Enzymatic Degradation: Inactivation by specific extracellular enzymes in synaptic cleft (e.g., acetylcholinesterase).

  3. Diffusion: Passive movement away from synaptic cleft into blood vessels.

• Postsynaptic Potentials:

  • Excitatory Postsynaptic Potential (EPSP): Graded depolarization caused by ionotropic $\text{Na}^+$ influx or closing $\text{K}^+$ channels; drives $V_m$ closer to threshold, increasing action potential probability.

  • Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization caused by ionotropic $\text{Cl}^-$ influx or $\text{K}^+$ efflux; drives $V_m away from threshold, decreasing action potential probability.\n\n\n\n* **Synaptic Summation Dynamics:**\n * **Spatial Summation:** Simultaneous graded potentials originating from different spatial presynaptic terminals converge and summate at the trigger zone.\n * **Temporal Summation:** Multiple graded potentials originating rapidly in sequence from a single presynaptic terminal summate over time at the trigger zone.\n\n\n\n\n\n# Reflex Arc Pathways and Endocrine Control\n\n* **Universal Reflex Arc Steps:**\n\n\text{Stimulus} ightarrow \text{Sensor/Receptor} ightarrow \text{Input Signal (Afferent)} ightarrow \text{Integrating Center} ightarrow \text{Output Signal (Efferent)} ightarrow \text{Target/Effector} ightarrow \text{Response} ightarrow \text{Feedback Loop}$$

• Classification of Reflex Pathways:

  1. Simple Neural Reflex: Mediated strictly by nervous system.

     • Example: Patellar tendon (knee jerk) stretch reflex. Tendon tap stretches muscle spindle receptor -> afferent sensory neuron -> spinal cord integration center -> somatic motor neuron effector -> quadriceps contraction. Simultaneously, interneuron inhibits motor neuron to antagonist hamstring (reciprocal inhibition).

  2. Neuroendocrine Reflex: Neural input triggers secretion of a neurohormone into blood.

     • Example: Oxytocin-mediated release of breast milk in response to suckling stimulus.

  3. Complex Neuroendocrine Reflex: Neural and multiple endocrine integrating centers operating in sequence.

     • Example: Growth Hormone (GH) axis. Hypothalamus releases GHRH/Somatostatin -> anterior pituitary releases GH -> liver secretes Insulin-like Growth Factors (IGFs) -> bone/tissue growth.

  4. Simple Endocrine Reflex: Endocrine tissue acts directly as sensor and integrating center.

     • Example: Beta cells of pancreas directly detect elevated blood glucose -> secrete insulin -> target tissues increase glucose uptake -> blood glucose lowers to homeostatic setpoint.

• Comparison of Neural and Endocrine Control Systems:

  • Specificity:

    • Neural Reflex: Highly specific; each neuron terminates on a single target cell or limited adjacent cells.

    • Endocrine Reflex: Systemic/Widespread; most cells are exposed to bloodborne hormones; response depends on receptor expression.

  • Nature of Signal:

    • Neural Reflex: Electrical impulses along axon, followed by chemical neurotransmitters across synapses.

    • Endocrine Reflex: Chemical signals (hormones) secreted directly into vascular system for blood distribution.

  • Speed:

    • Neural Reflex: Rapid execution (milliseconds).

    • Endocrine Reflex: Slower distribution and onset of action (seconds to hours).

  • Duration of Action:

    • Neural Reflex: Brief and short-lived.

    • Endocrine Reflex: Sustained, longer-lasting responses.

  • Coding for Stimulus Intensity:

    • Neural Reflex: All action potentials are identical in magnitude; intensity coded by frequency of action potentials.

    • Endocrine Reflex: Stimulus intensity correlated directly with concentration/amount of hormone secreted into blood.