Action Potential Propagation and Transmission 3051 Spring 2025 PDF

Action Potential Propagation and Transmission Joe Sepe, PhD Session Learning Objectives (February 7th, 2025): Students will be able to: Describe the process of action potential propagation from the axon hillock to the axon terminal in unmyelinat...

Action Potential Propagation and Transmission Joe Sepe, PhD Session Learning Objectives (February 7th, 2025): Students will be able to: Describe the process of action potential propagation from the axon hillock to the axon terminal in unmyelinated and myelinated neurons. Differentiate the conduction velocity difference between these types of neurons. Recall and draw the events of synaptic transmission in the presynaptic cell. Describe the types of synapses Electrical Chemical Recall post-synaptic potentials (PSPs) and the ions responsible for depolarization (excitatory) and hyperpolarization (inhibitory) Excitatory post-synaptic potentials (EPSP) Inhibitory post-synaptic potentials (IPSP) Action Potential Propagation The propagation of the action potential from the axon hillock to the axon terminal is typically one-way because the absolute refractory period follows along in the “wake” of the moving action potential Axon hillock has the highest density of voltage-gated Na+ channels Action Potential Propagation Unmyelinated Axons AP begins at the “trigger zone” But the propagation is slow (axon hillock) and propagates to the axon terminal AP Propagation in Myelinated Neurons Oligodendrocytes in CNS, Schwann cells in PNS Myelin creates increased membrane resistance to current flow, so ions instead flow along path of least resistance (axon interior) instead of against the high resistance membrane Saltatory conduction: action potentials “jump” from one Node of Ranvier (brief break in the myelin) to the next as they propagate along a myelinated axon MUCH faster propagation: 1 mph vs 225 mph Large diameter also increases conduction velocity, Node of Ranvier: independently of myelin High density of voltage- gated Na+ channels AP Propagation 1. One way (starts at axon hillock and travels toward axon terminal). Refractory periods prevent moving in retrograde direction after starting at axon hillock. 2. Velocity increases with axon diameter 3. Velocity increases with myelination (Nodes of Ranvier; Saltatory conduction) Myelination when you need speed, such as motor neurons (to muscle) or sharp pain Non-myelinated when no hurry, such as regulation of digestion, dull (aching) pain Relevant pathologies: multiple sclerosis, diabetes mellitus Synapses Functional association of a neuron to: another neuron effector organs (muscle or gland) Two types: Electrical Chemical Synaptic Transmission: Electrical Two (or more) excitable cells linked together by gap junctions Very rapid signal transmission. Allows for multiple cells in a tissue/organ to “behave as one” Gap junction Where in the body?: Heart cells Smooth muscle cells Some brain cells Synaptic Transmission: Chemical Slower than electrical synapse Presynaptic More complexity and neuron modification Communication from neuron to: Synapse Other neurons Muscle cells Postsynaptic Gland cells neuron And others Chemical Synaptic Transmission Presynaptic Events: Converting an electrical signal into a chemical (neurotransmitter) signal 1. AP propagates to axon terminal 2. Voltage-gated Ca2+ channels open 1. 3. Rapid influx of Ca 2+ activates vesicle exocytosis, allowing vesicles to fuse with plasma membrane 2. (calcium-induced exocytosis) 4. Neurotransmitter diffuses across synaptic cleft (15 nm) onto postsynaptic cell 3. 4. Ca2+ binds to synaptotagmin, causing SNARE complex to draw vesicle to plasma membrane, which then fuses and creates a pore Postsynaptic Events at Excitatory Synapse 1. NT binds to postsynaptic receptor 2. Ligand-gated channels open (chemical gating) 3. Cations flow through channel into cell (mainly Na+ or Ca2+) 4. Net effect is depolarization (an excitatory postsynaptic potential; EPSP: makes an AP more likely) Very small: ~0.5 mV per NT:R complex. Summation is key! Postsynaptic Events at Inhibitory Synapse 1. NT binds to postsynaptic receptor 2. Ligand-gated channels open (chemical gating) 3. EITHER K+ flows out or Cl- flows into cell 4. Net effect is hyperpolarization or inhibits further depolarization (an inhibitory postsynaptic potential; IPSP: a graded potential that makes an action potential less likely) Chemical Synaptic Transmission Postsynaptic potentials are brief because: 1. NT rapidly binds and unbinds R + NT R:NT Synapses are rife for pharmacological 2. NT reuptake into presynaptic terminal modification. (pumps) and/or 3. NT diffusion away from the synapse and/or Removal of NT from the synapse 4. Enzymatic destruction of NT Chemical Signals: Neurotransmitters There are many different types of neurotransmitters Ultimately, it is the RECEPTOR (and associated ion channel) that determines whether an IPSP or EPSP develops. However, some NTs are consistently excitatory or inhibitory. Typically Excitatory: Typically Inhibitory: Acetylcholine GABA Glutamate Glycine Which types of ion channels would these NTs open?  Chemical Synaptic Transmission: can you recall and list these events in order? Presynaptic Events: Postsynaptic Events: 1. AP propagates to axon terminal 1. NT binds to receptor 2. Voltage-gated Ca2+ channels 2. Ligand-gated channels open open 3. Ca2+ initiates vesicle exocytosis 3. Ions flow 4. Neurotransmitter diffuses across 4. Net effect is a graded synapse potential (EPSP or IPSP) Action Potentials Joe Sepe, PhD Session Learning Objectives (February 5th, 2025): Students will be able to: Differentiate between a graded potential and an action potential. Define and describe each phase of the action potential, including the flux of ions and types of channels they flow through (e.g. voltage-gated): o Threshold o Upstroke o Peak o Repolarization o After-hyperpolarization Explain how electrical signals are converted to chemical signals. Compare and contrast voltage-gated Na+ channels from voltage-gated K+ channels, and the phases they contribute to during an action potential. In-Class Activity 145 mM Na+ 127 mM Cl- 15 mM Na+ 2 mM Cl- Chemical Driving Electrical Driving At each membrane potential (Vm): Vm Force Force Net Flux In In In 1. Circle the direction in which the -110 mV Out Out Out chemical and electrical driving Zero Zero Zero forces are acting on Chloride (Cl-) In In In in the table; and -90 mV Out Out Out Zero Zero Zero 2. Predict the net flux of Cl- at each In In In membrane potential. + 60 mV Out Out Out Zero Zero Zero In In In + 90 mV Out Out Out Zero Zero Zero Ionic Basis of Action Potentials Rapid Repolarization Occur in response to graded potentials (via Depolarization ligand-gated ion channels) that summate to reach threshold At threshold (typically at -55 mV), voltage- gated Na+ channels open and an action potential is initiated (where at on the cell?) An action potential (AP) consists of 3 distinct phases. Understanding what’s happening with the changing permeability of Na+ and K+ at each phase is helpful when thinking about APs. Afterhyperpolarization Phase 1: Rapid Depolarization Rapid depolarization As threshold is reached (due to (upstroke) summation of graded potentials), voltage-gated Na+ channels open and the membrane rapidly depolarizes as sodium quickly enters the cell. Voltage-gated K+ channels remain closed. What’s the most positive the membrane potential can get under typical conditions? Why? Voltage-Gated Na+ Gating Properties Open Closed Inactive Notice the unidirectional arrows. Cannot go from Open directly to Closed. Fast to open, fast to inactivate/close Costanzo, Figure 1.14 Phase 2: Repolarization Caused by sudden decrease in Na+ permeability (voltage-gated Na+ channels inactivate) AND Sudden increase in K+ permeability (opening of voltage- gated K+ channels) Net effect: less positive (Na+) charge entering the cell, and lots of positive charge (K+) leaving the cell Voltage-Gated K+ Gating Properties Closed Open Slow to open, slow to close Guyton, Figure 4.5 Phase 3: Afterhyperpolarization Voltage-gated Na+ channels are all inactivated/closed (no sodium can enter the cell through these channels) Voltage-gated K+ are slow to close, so potassium continues to leave the cell and hyperpolarize the membrane to a potential more negative than RMP Eventually, the voltage-gated K+ channels close and Na+/K+-ATPase pumps and leak channels bring cell back to RMP What’s the most negative the membrane potential can get under typical conditions? Why?  Graded potentials depolarize Flow Chart of Channel Events the membrane enough to reach threshold Voltage-gated Na + channels open Voltage-gated Na + channels inactivate while voltage-gated K+ channels open  Testing Your Understanding Can you align the changes in ion channel permeability to the change in ion conductance to the change in membrane potential? Costanzo, Figure 1.13 Refractory Periods During and immediately after an action potential, the membrane is less excitable than it is at rest. This period of reduced excitability is called the refractory period. Two types: Absolute: It is not possible for the cell to generate a 2nd action potential, regardless of stimulus size. Voltage-gated Na+ channels are not available to open (and thus can’t have ions flow through) Relative: It is possible to generate a 2nd action potential, but only in response to a larger than normal stimulus. At least some voltage-gated Na+ channels have entered the “closed” state, and are thus available to open. Action Potentials 1. Are “all-or-none”. Once membrane is depolarized to threshold, amplitude is independent of the size of the initiating event (stimulus). 2. Are not graded by stimulus size. ECF conditions, such as a higher ECF concentration of Na +, can influence AP properties. 3. Can not summate due to refractory period. Absolute refractory period Relative refractory period 4. Do not decrease with distance from stimulus. They propagate over long distances. Membrane, Graded, and Action Potentials Joe Sepe, PhD Session Learning Objectives (February 3rd, 2025): Students will be able to: Explain how the resting membrane potential is generated. Explain how chemical and electrical components of driving force influence the movement of substances across cell membranes. Understand how the permeability of different ions contributes to the Vm. Define the Nernst Equation and use the equation to explain the flow of ions. Define graded potential and describe their key characteristics Define and describe changes in membrane potential: o Depolarization o Overshoot o Repolarization o Hyperpolarization o Resting membrane potential Equilibrium Potential (Nernst Potential) For any given concentration gradient of Nernst Equation: *Know this a single ion, the membrane potential equation that exactly opposes the concentration gradient is known as the equilibrium potential (aka Nernst Potential). At the equilibrium potential (Eion), the movement of an ion across the cell membrane due to its concentration gradient is opposed by the movement of an ion in the opposite direction due to its electrical gradient. If the cell is only permeable to one ion, then that ion’s equilibrium potential (Eion) will be the cell’s membrane potential (Vm). Ion concentrations How do we determine which direction ions will flow? Intracellular Fluid (ICF) Extracellular Fluid (ECF) Na+ 15 mM Na+ 145 mM K+ 150 mM K+ 5 mM Membrane potential (Vm) = -70 mV Inside (relative to outside): high [K+], low [Na+] In-Class Question Intracellular Fluid (ICF) Extracellular Fluid (ECF) Na+ 15 mM Na+ 145 mM K+ 150 mM K+ 5 mM Membrane potential (Vm) = -70 mV What is the equilibrium potential of K + in the above conditions? What will the net flux of K+ be in the resting cell? In, out, or no (zero) net flux? Membrane Potential When a cell is permeable to more than one ion (K+ and Na+, for example), both affect the resting membrane potential in proportion to their relative permeabilities and conductances. Vm is a weighted average of each ion’s flux, and can be calculated using the Goldman-Hodgkin-Katz Equation (you are not responsible for this equation). In real cells, relative permeability determines which ion dominates in determining RMP (K+ is king)  Signaling through Dynamic Changes in Membrane Potential Overshoot refers to the development of a charge reversal Overshoot (membrane now positive) Changes in membrane potential allow cells to receive and send Repolarization is moving information back toward the negative resting potential Depolarization occurs when ion movement reduces the charge imbalance towards zero. Hyperpolarization is the development of even more negative charge inside the cell (below resting Vm) A cell is polarized because its interior is different (more negative) than its exterior Electrical Signals Neurons communicate by generating electrical signals in the form of changes in membrane potential. Ion channels open and close to generate changes in membrane potential (deviate from resting membrane potential) Types of ions channels (for electrical signaling): Leak channels: always open Ligand-gated: chemicals Voltage-gated: voltage changes Mechanically-gated: changes in shape (e.g. baroreceptors) 2 Types of Electrical Signals 1) Graded Potentials: Small and short distance 2) Action Potentials: Large and long distance Graded Potentials are Proportional to the Size of the Stimulus Graded Potentials can be Excitatory (Depolarizations) or Inhibitory (Hyperpolarizations) In-Class Activity 145 mM Na+ 4 mM K+ 15 mM Na+ 140 mM K+ Chemical Driving Electrical Driving Vm Net Flux Force Force In In In At each membrane potential (Vm): -110 mV Out Out Out Zero Zero Zero 1. Circle the direction in which the In In In chemical and electrical driving -90 mV Out Out Out forces are acting on sodium (Na+) Zero Zero Zero in the table; and In In In 2. Predict the net flux of Na+ at each + 60 mV Out Out Out membrane potential. Zero Zero Zero In In In + 90 mV Out Out Out Zero Zero Zero Graded Potentials Decay (Decrease) with Distance from the Stimulus Site Graded Potentials can Summate Graded Potentials 1. Are proportional to the size of the stimulus 2. Decrease with distance from the stimulus site Graded potentials 3. Can be depolarizations or hyperpolarizations can summate and 4. Can summate with each other induce (trigger) an action potential 5. Are short-distance signals that rely only on local flow of ions 6. AKA sensory receptor potentials; generator potentials Membrane Potential + bonus osmolarity content Joe Sepe, PhD  KNOW that the ICF is 300 mOsm/L nonpenetrating solute. Calculate osmolarity of the ECF. Calculate the tonicity (how many nonpenetrating solutes) of the ECF If the ECF solution is hypotonic to the cell, cell volume at equilibrium will increase (swell). If the ECF solution is hypertonic to the cell, cell volume at equilibrium will decrease (shrink). If the ECF solution is isotonic to the cell, cell will at equilibrium will not change. Osmolarity Solution Osmolarity Tonicity Outcome 80 mM NaCl 150 mM NaCl 300 mM Ethanol 150 mM NaCl + 100 mM Ethanol 100 mM NaCl + 150 mM Glucose 80 mM NaCl 150 mM NaCl 300 mM Ethanol 150 mM NaCl + 100 mM Ethanol 100 mM NaCl + 150 mM Glucose  The Relationship Between Osmolarity and Tonicity OSMOLARITY TONICITY Hypoosmotic Isosmotic Hyperosmotic Hypotonic X X X Isotonic X X Hypertonic X Place an “X” if the combination is possible, leave blank if not Session Learning Objectives (1/31/2025): Students will be able to: Explain how the resting membrane potential is generated. Explain how chemical and electrical components of driving force influence the movement of substances across cell membranes. Recall the Nernst Equation and be able to use the equation to explain the flow of ions. Predict changes in membrane potential caused by alterations in ion concentration gradients or drugs that block ion channel function. Membrane Potential Separation of charge creates an electrical potential. When this separation occurs across the plasma membrane, we call it a membrane potential. Separated charges have the potential to do work. Phospholipid bilayer has a high electrical resistance and acts as a capacitor. Open ion channels in the cell membrane are a low-resistance pathway for movement of ions (current) Electrochemical Driving Force A combination of both the chemical and electrical gradients. Describes how both concentration and membrane charge affect ion movement. Compare the ion’s equilibrium potential (Eion) to the membrane potential (V m) in order to determine which way (into or out of the cell) the ion will move (equation on next slide). Ions “want” the membrane potential to be at their equilibrium potential. The flow of ions is essential for the generation of a membrane potential. No ion flow, no Vm. Generation of Resting Membrane Potential How is the RMP generated? 1. The Na+/K+ pump 3 Na+ out of the cell, 2 K+ in, losing positive charge (electrogenic) The pump sets up and maintains the ICF gradient for Na+ and K+ 2. K+ and Na+ Leak channels K+, a positive charge, is always leaving the cell through “leak” channels (these channels are always open). Na+ is always leaking in, but a much smaller amount than K+ leaving (fewer Na+ leak channels than K+) Ion concentrations How do we determine which direction ions will flow? Intracellular Fluid (ICF) Extracellular Fluid (ECF) Na+ 15 mM Na+ 145 mM K+ 150 mM K+ 5 mM Membrane potential (Vm) = -70 mV Inside (relative to outside): high [K+], low [Na+] Equilibrium Potential (Nernst Potential) For any given concentration gradient of Nernst Equation: *Know this a single ion, the membrane potential equation that exactly opposes the concentration gradient is known as the equilibrium potential (aka Nernst Potential). At the equilibrium potential (Eion), the movement of an ion across the cell membrane due to its concentration gradient is opposed by the movement of an ion in the opposite direction due to its electrical gradient. If the cell is only permeable to one ion, then that ion’s equilibrium potential (Eion) will be the cell’s membrane potential (Vm).  Have a Great Weekend! If you have any questions at all, or just want to chat, please don’t hesitate to reach out or stop by my office! Neurophysiology Joe Sepe, PhD Penetrating and Non-Penetrating Penetrating: Non-Penetrating: Urea Glucose Ethanol Na+ Cl- K+ Mg2+ Ca2+ (ions in general) Today’s Learning Objectives (January 29 th, 2025): Students will be able to: 1. Diagram the major divisions of the human nervous system and define their functions. 2. Describe the structure of the neuron and the direction of signal propagation in it. 3. Differentiate the cells of the nervous system (neurons and glial cells) and the roles they play in nervous system function. 4. Compare and contrast presynaptic and postsynaptic neurons. 5. Describe the components of the electrochemical gradient. Roles in Homeostasis 1. Control system that receives information about internal (ECF) and external environment, integrates it, and directs activities of cells throughout the body to maintain homeostasis. One of two control systems (endocrine being the other). 2. Learning, memory 3. Language 4. Intelligence Directs processes of cells and organs throughout the body to maintain homeostasis Central (CNS) and Peripheral (PNS) CNS: brain + spinal cord PNS afferent neurons (their activity “affects” what will happen next into the CNS), into the CNS. PNS efferent neurons (“effecting” change: movement, secretions, etc.), projecting out of the CNS. PNS: Afferent and Efferent divisions Cells of the Nervous System Costanzo, Figure 3.3 Cells of the Nervous System 90% of cells of the nervous system are glial cells. They support the function of neurons. 4 types of glial cells: 1. Astrocytes: help monitor and regulate ECF of CNS. Supply metabolic fuel to neurons. Important for blood-brain barrier. 2. Oligodendrocytes: synthesize myelin in the CNS. 3. Schwann cells: synthesize myelin in the PNS. 4. Microglia: proliferate following neuronal injury. Serve as scavengers to remove cell debris; have immune function. Cells of the Nervous System Neurons communicate with each other across gaps called synapses; neurotransmitter chemicals are released by presynaptic neurons at axon terminals; act on postsynaptic neurons. Neuronal Networks Converge and Diverge Convergence Divergence Multiple presynaptic One presynaptic neuron neurons communicate onto communicates with multiple one postsynaptic neuron postsynaptic neurons How do neurotransmitters become the communication link between neurons?  Communication between neurons is based on changes in the plasma membrane’s permeability to ions which in turn changes the membrane potential 1. Resting membrane potential 2. Equilibrium potential (aka Nernst potential) 3. Graded/receptor potential 4. Action potential Cells and Membrane Potentials Membranes can be electrically charged. The charge on a membrane influences the net flux of charged ions. Membrane charge can oppose or increase ion movement across the plasma membrane Electrochemical gradient describes how ions will move across the plasma membrane Membrane Potential All cells have a potential difference across their plasma membrane (measured in millivolts) Reference Separation of charge creates an electrode in the ECF electrical potential. When this separation occurs across the plasma membrane, we call it a membrane potential. Recording electrode in Expressed as the potential the ICF (difference in charge) of the inside of the cell relative to the outside At “rest”, the membrane potential is negative (inside relative to the outside). Exact value varies between cell types. Typical neuron has a resting membrane potential of -70 mV. Driving Forces 1) Chemical Driving Force This is a concentration gradient Substances move down their concentration gradient Driving Forces 2) Electrical Driving Force Electrical gradient Opposites charges attract, like charges repel Electrochemical Driving Force (Gradient) A combination of both the chemical (concentration) and electrical gradients Describes how both concentration and membrane charge affect ion movement. Membrane charge is synonymous with membrane voltage (Vm) Neurophysiology Joe Sepe, PhD  Mechanism In-Class Activity of Action Mr. Jimothy Halpert, a sale representative for a regional paper distribution company, is a competitive rec league basketball player. Recently, he was challenged to a game of basketball by the warehouse workers of the paper company. According to Mr. Halpert, in order to gain a competitive advantage, he ingested flowers from foxglove plants (Digitalis purpurea) grown by his co-worker who owns and operates a local beet farm. He believes the plant “helps his heart beat stronger.” 1. What cellular transporter does digitalis (aka Digoxin; cardiac glycoside), the active ingredient in foxglove plants, inhibit? Is this an example of a primary or secondary active transporter? Digitalis inhibits the myocardial Na+/K+ ATPase pump, which is a primary active transporter. 2. Through what transporter does the compound exert its main effect (i.e. is there an indirect target of the compound?)? Is this an example of a primary or secondary active transporter? By inhibiting the Na+/K+ ATPase pump, intracellular Na+ increases. This results in the NCX (Sodium-Calcium Exchanger) activity decreasing, and even working in reverse, causing intracellular Ca2+ levels to rise, leading to increased contractility. By inhibiting the Na+/K+ ATPase pump, intracellular Na+ increases. This results in the NCX (Sodium-Calcium Exchanger; a secondary active transporter) activity decreasing, and even working in reverse, causing intracellular Ca2+ levels to rise, leading to increased contractility. Modes of Signal Transmission: Neurohormone Receptor Axon Cell 2 Cell 1 Axon Terminal Bloodstream Penetrating and Non-Penetrating Penetrating: Non-Penetrating: Urea Glucose Ethanol Na+ Cl- K+ Mg2+ Ca2+ (ions in general) Today’s Learning Objectives (January 29 th, 2025): Students will be able to: 1. Diagram the major divisions of the human nervous system and define their functions. 2. Describe the structure of the neuron and the direction of signal propagation in it. 3. Differentiate the cells of the nervous system (neurons and glial cells) and the roles they play in nervous system function. 4. Compare and contrast presynaptic and postsynaptic neurons. 5. Describe the components of the electrochemical gradient. Roles in Homeostasis 1. Control system that receives information about internal (ECF) and external environment, integrates it, and directs activities of cells throughout the body to maintain homeostasis. One of two control systems (endocrine being the other). 2. Learning, memory 3. Language 4. Intelligence Directs processes of cells and organs throughout the body to maintain homeostasis Central (CNS) and Peripheral (PNS) CNS: brain + spinal cord PNS afferent neurons (their activity “affects” what will happen next into the CNS), into the CNS. PNS efferent neurons (“effecting” change: movement, secretions, etc.), projecting out of the CNS. PNS: Afferent and Efferent divisions Cells of the Nervous System Costanzo, Figure 3.3 Cells of the Nervous System 90% of cells of the nervous system are glial cells. They support the function of neurons. 4 types of glial cells: 1. Astrocytes: help monitor and regulate ECF of CNS. Supply metabolic fuel to neurons. Important for blood-brain barrier. 2. Oligodendrocytes: synthesize myelin in the CNS. 3. Schwann cells: synthesize myelin in the PNS. 4. Microglia: proliferate following neuronal injury. Serve as scavengers to remove cell debris; have immune function. ChimeIn: In Class Question A molecule is released from a cell and binds to an extracellular receptor on the same cell that released it, which is an example of signal transmission. A. Hydrophobic; autocrine B. Hydrophilic; autocrine C. Hydrophobic; paracrine D. Hydrophilic; paracrine Cells of the Nervous System Neurons communicate with each other across gaps called synapses; neurotransmitter chemicals are released by presynaptic neurons at axon terminals; act on postsynaptic neurons. Neuronal Networks Converge and Diverge Convergence Divergence Multiple presynaptic One presynaptic neuron neurons communicate onto communicates with multiple one postsynaptic neuron postsynaptic neurons How do neurotransmitters become the communication link between neurons?  Communication between neurons is based on changes in the plasma membrane’s permeability to ions which in turn changes the membrane potential 1. Resting membrane potential 2. Equilibrium potential (aka Nernst potential) 3. Graded/receptor potential 4. Action potential Cells and Membrane Potentials Membranes can be electrically charged. The charge on a membrane influences the net flux of charged ions. Membrane charge can oppose or increase ion movement across the plasma membrane Electrochemical gradient describes how ions will move across the plasma membrane Membrane Potential All cells have a potential difference across their plasma membrane (measured in millivolts) Reference Separation of charge creates an electrode in the ECF electrical potential. When this separation occurs across the plasma membrane, we call it a membrane potential. Recording electrode in Expressed as the potential the ICF (difference in charge) of the inside of the cell relative to the outside At “rest”, the membrane potential is negative (inside relative to the outside). Exact value varies between cell types. Typical neuron has a resting membrane potential of -70 mV. Driving Forces 1) Chemical Driving Force This is a concentration gradient Substances move down their concentration gradient Driving Forces 2) Electrical Driving Force Electrical gradient Opposites charges attract, like charges repel Electrochemical Driving Force (Gradient) A combination of both the chemical (concentration) and electrical gradients Describes how both concentration and membrane charge affect ion movement. Membrane charge is synonymous with membrane voltage (Vm) Thursday Office Hours: 9am-10am via Zoom If you have any questions at all, or just want to chat, please don’t hesitate to reach out or stop by my office! Session 3: Signal Transmission and Osmolarity Joe Sepe, PhD January 27th, 2025 Session Learning Objectives: Students will be able to: Describe the cellular location of receptors for hydrophobic and hydrophilic molecules, and downstream effects of receptor binding. List and describe the major modes of signal transmission: Autocrine Paracrine Endocrine Neurotransmitters Neurohormones Become familiar with signal transduction and the steps of second messenger signaling cascades.  Hydrophilic Signaling Molecule Typically bind to extracellular receptors (on external surface of the plasma membrane) and then: 1: Initiate second messenger cascades 2: alter ion channel conformations (open or close) They typically have a fast effect (modify existing proteins) and are metabolized/excreted quickly Example: Norepinephrine (more on this later) Guyton, Figure 75.7 Hydrophobic Signaling Molecule Typically bind to intracellular receptors and then Typically have a slow effect and are alter gene transcription and protein synthesis. metabolized/excreted slowly. Example: steroid hormones Guyton, Figure 75.6 Modes of Signal Transmission: Autocrine Receptor Cell 2 Cell 1 Bloodstream Modes of Signal Transmission: Paracrine Receptor Cell 2 Cell 1 Bloodstream Modes of Signal Transmission: Neurotransmitter Receptor Axon Synapse Cell 2 Axon Terminal Bloodstream Modes of Signal Transmission: Endocrine Receptor Cell 2 Cell 1 Bloodstream Modes of Signal Transmission: Neurohormone Receptor Axon Cell 2 Cell 1 Axon Terminal Bloodstream Signal Transduction Norepinephrine and Epinephrine are classic examples 3. Adenylyl cyclase (AC) 1. First messenger molecule activated (ligand) binds its receptor Don’t need to learn this specific pathway now, 2. G protein complex 4. Increased levels of cAMP. This is but be aware of second (activated) the second messenger! messengers and signal transduction. We will PKA revisit this in the cardiovascular unit. 5. Phosphorylates target proteins around the cell (biologic/physiologic effect) Katzung, Figure 9.2  Osmolarity Primer Water can cross most cell membranes through channels called aquaporins Always a passive process, and therefore always down its concentration gradient (this movement is called osmosis) Osmolarity: total solute concentration of a solution per unit volume (typically per liter). Not the same as molarity, as some solute particles dissociate in solution (Example: NaCl) In Physiology, we often use milliosmole (mOsm) Normal osmolarity of ECF and ICF is ~300 mOsm More on this in this Tightly regulated, as minor changes can have drastic week’s lab consequences for cells (tonicity) Guyton, Figure 4.10 Session 2: The Cell Membrane Joe Sepe, PhD January 24th, 2025 Session Learning Objectives: Students will be able to: Explain the importance of cell membranes being selectively permeable. Differentiate between active and passive transport mechanisms across cell membranes. Define the following processes and list common substances transported via each mechanism: o Simple diffusion o Facilitated diffusion o Primary active transport o Secondary active transport Broadly know the ECF and ICF values (mOsM) for Na+, K+, Cl-, Ca2+, and glucose (i.e.. Inside relative to outside of cell) Predict how gradients affect molecular movement. Last Time: Organ systems maintain homeostasis of ECF through control systems Many ECF variables/conditions are regulated. Examples: ECF Na+ (sodium) ECF K+ (potassium) pH Temperature Blood glucose These variables/conditions are under constant threat: Changes in external environment (cold, drought, etc…) Activities of the cells (metabolic waste products, etc…) How does the body maintain homeostasis (i.e. How are these variables regulated within the normal range?)? Accomplished through control systems, primarily Negative Feedback Cell Structure: A Brief Overview Good review in the textbook of the various organelles, but we won’t spend much time on it here. Nucleus Mitochondria Endo(sarco)plasmic reticulum Cytoplasm and Cytosol Hydrophobic: water fearing (aka lipophilic) Plasma Membrane Hydrophilic: water loving (aka lipophobic)  The Cell (Plasma) Membrane “Life” = the ability to locally violate the 2nd Law of Thermodynamics (increasing entropy; randomness/disorder). Cells maintain a non-random distribution of molecules across their cell membranes. A combination of phospholipids and proteins within the plasma membrane makes cells ”selectively permeable”. The selectively permeable membrane impedes the penetration of most water-soluble substances, large polar molecules (glucose), and charged particles (ions). Guyton, Figure 2.3 ECF ≠ ICF Hydrophilic, charged, and/or large molecules can NOT cross the plasma membrane (without assistance). Hydrophilic signaling molecules, such as insulin, usually bind to an extracellular receptor (and initiate second messenger cascades; fast effect). Hydrophobic (lipophilic) molecules can cross the plasma membrane. Typically bind to an intracellular receptor and alter gene transcription and protein synthesis (slow effect). (don’t memorize specific values for now, but How can hydrophilic/charged/large instead notice the trends: what’s high on the molecules cross the membrane? outside vs the inside?) Crossing the Plasma Membrane 1. Simple Diffusion 2. Facilitated Diffusion (through channel proteins) 3. Active Transport (primary and secondary) 4. Osmosis (diffusion of water) 5. Vesicular Transport Facilitated Diffusion No energy required Requires a transmembrane protein Moves a substance down its concentration/electrochemical gradient (either into or out of the cell) Costanzo, Figure 1.3  Ion Channels (Facilitated Diffusion) Charged particles, like ions, can’t cross the plasma membrane. When ion channels open (in response to a ligand binding, change in voltage, etc…) then the ion is free to cross the membrane, following its electrochemical gradient (more on this next week). Being familiar with relative ion concentrations will help you to predict ion movement (and often times pharmacological mechanism of action). Guyton, Figure 4.5 Transport Rate Across the Membrane Simple diffusion Simple diffusion through the plasma membrane does not reach a max under physiological conditions. Carrier-mediated transport Substances that require a transport carrier or channel will have a maximum transport rate (Tmax). Carrier-mediated processes saturate. This is why urine is sweet in diabetes mellitus (only so much glucose can be reabsorbed in the proximal nephron, as it requires a carrier protein). Primary Active Transport Requires energy source (usually in the form of ATP) to directly transport substances across the plasma membrane Requires transmembrane proteins (called pumps) Pumps against a concentration gradient (therefore creating/maintaining the gradient)  Primary Active Transport Example: Na+/K+-ATPase Pump 3 Na+ are pumped out of the cell while 2 K+ are pumped into the cell, both against their concentration gradients. Directly uses ATP. Guyton, Figure 4.12 Secondary Active Transport Cotransport Pumps against a concentration gradient Energy provided by another molecule’s gradient that was previously created by primary active transport Molecules going in same (uses energy, but not directly) direction across membrane Countertransport Two main varieties: 1. Cotransport (symport) 2. Countertransport (antiport) Molecules going in opposite directions across membrane  Secondary Active Transport Example: Sodium-dependent glucose cotransporters (SGLT1 and SGLT2) Na+ binds to carrier, creating a high-affinity binding site for glucose (ECF). Glucose binding changes carrier conformation so that binding sites now face ICF. Na+ is released into ICF, following its concentration gradient. Release changes glucose-binding site to low-affinity. Glucose is released against concentration gradient into ICF. Guyton, Figure 66.9 Is this an example of cotransport or countertransport? Vesicular Transport: Endocytosis and Exocytosis Endocytosis Exocytosis Molecules from the ECF enter the Intracellular vesicle fuses with the cell through vesicles formed from plasma membrane and releases its the plasma membrane contents into the ECF  Proteins in Phospholipid Bilayer Allow for Selectivity of Membrane Named after the substances they allow to pass (e.g. water channels are called aquaporins) Proteins allow for the cell to be selectively permeable. Simple diffusion: diffusion directly across the cell membrane (follows concentration gradient). Examples: hydrophobic molecules (steroid hormones), small molecules (O2) Active Transport Facilitated diffusion: molecule requires a Primary: directly uses energy to push molecules membrane protein to cross cell membrane against their concentration gradient (follows concentration/electrochemical Secondary: uses potential energy stored in the concentration gradient). Examples: ions (Na+) gradient to push other molecules against their concentration gradient. Guyton, Figure 4.2  The Cell Membrane is Dynamic In most cells, all types of membrane transport are always occurring. Example: cardiac myocyte Guyton, Figure 9.7 Physiology 3051 Joe Sepe, PhD Physiology and Homeostasis January 22nd, 2025  Welcome! Dr. Joe Sepe [email protected] Office: Jackson Hall 6-130 Office Hours: Mondays: 2-3pm, PWB 4-156 Thursdays: 9-10am, virtual (Zoom posted to Canvas) Happy to answer any questions via email, Zoom, or in person! Unit 1 Schedule: 1. Homeostasis & Cell Physiology 2. The Cell (Plasma) Membrane 3. Cells, Signal Transmission, & Osmolarity 4. Neurons & System Overview 5. Membrane Potentials 6. Graded & Action Potentials 7. AP Propagation and Transmission 8. Synaptic Transmission 9. Central and Peripheral Systems 10.Autonomic Nervous System Session Learning Objectives: Students will be able to: Define homeostasis and explain how the internal environment is regulated through negative feedback. Differentiate between negative and positive feedback mechanisms, and contrast to feedforward regulation. Differentiate between the teleological (the “why”) and the mechanistic (the “how”) approaches to physiological processes. List the two major fluid compartments in the body and the typical (relative) fluid volumes in each compartment. Describe the levels of organization for cells, tissues and organs  What is Physiology? Physiology is the study of the normal functioning of a living organism, from molecules to cells to tissues, to organs and organ systems, to whole body- separately and integrated The basis of medical practice. Though we often use pathophysiology to describe a disease, this first requires a strong foundation in normal, healthy functioning (physiology) Function and Mechanism “Why” versus “How” Function tells us about the why. Example: living on land presents a challenge to preventing dehydration, and this is why the kidneys can produce concentrated urine. This is the teleological approach to physiology. Mechanisms explain the how, typically at the cellular level. Example: how the kidneys produce concentrated urine (membrane transport, selective permeability, etc..). This is the mechanistic approach to physiology. Our goal: Study the mechanisms to help understand the function, in both health and disease The Core Concepts of Physiology “The stability of the internal environment is the condition for the free and independent life.” -Claude Bernard, 1865 Michael and McFarland, 2011.  Organism Body Plans: Homeostatic Challenge Simple Complex External and Internal Environments Cells, Tissues, Organs, and Organ Systems Levels of Organization: Cells (smallest living units): 4 major cell types in the human body: Epithelial Connective Nerve cells Muscle cells cells tissue cells Cells have unique challenges within a body (multicellular organisms) A Simplified Body Plan Outside body External and Internal Environments Inside body Separated by epithelial membrane Epithelial barrier is continuous  External Environment Source of nutrients and oxygen Repository for wastes Most cells have no direct exchange 2. with external environment Examples: 1. Surroundings external to skin 3. 2. Air in lungs 3. Food in stomach and intestines 4. Urine in bladder Internal Environment Immediate environment of most cells Includes both interstitial fluid and plasma 4. (both are extracellular fluid; ECF) Plasma is fluid around blood cells Interstitial fluid (also known as 1. tissue fluid) is fluid around all other cells Body Fluid Compartments Homeostasis Regulation of an organism’s internal environment (within limits) compatible with cell survival. Maintenance of a relatively constant internal environment. Relies on organ systems that detect and respond to deviations in physiological variables from their “set point” values.  Systemic, Diffuse Response Regulated by Two Control Systems: Nervous and/or Endocrine Systems Nervous Endocrine Speed of Action: Endocrine System Slower response: minutes, to hours, and even longer Diffuse targets: entire tissues and organ systems can be impacted Peptide Hormone Steroid Hormone Speed of Action: Nervous System Very fast response (milliseconds), often on a specific target Regardless of the speed of action, both the endocrine and nervous systems use negative feedback to regulate functions.  Terms and Components of Negative Feedback Control System in Homeostasis Commonly the brain 4. (Compare to set point) 3. 5. Overall, negative (at/towards) (Exit) feedback opposes 2. Detection 6. the initial change. 1. Regulated 7. variable deviates from normal range (begin flowchart here). 8. Neg feedback towards normal range Negative Feedback Example: Blood Glucose Lab 3: Membrane, Graded, and Action Potentials Katharine Mamizuka Spring 2025 REMINDER!! Please do not begin your lab quiz until the questions are displayed  Learning Objectives: This lab is a review of learning objectives for Classes 5 and 6, plus action potentials. Extra practice questions for outside of lab (and to be covered in lecture): 1. Explain how an increase in extracellular K+ concentration can have a dramatic effect on the Equilibrium potential for K, the resting membrane potential, and the excitability of cells. 1. Define threshold, and relative and absolute refractory period. Explain the state of ion channels and the stimulus amplitude required to generate an action potential during each. The Nernst Equation Calculates the potential at which the inward and outward flow of a particular ion is balanced (i.e., the flow into and out of the cell through one type of channel is matched in magnitude, but opposite in direction), resulting in no net movement of that ion across the membrane Does NOT calculate resting membrane potential Eion = equilibrium potential for a particular ion (mV) [in] = intracellular concentration of the ion [out] = extracellular concentration of the ion Z = the valence of the ion 61 = a constant that takes into account the gas constant, temperature and faradays electrical constant. Membrane Potential When a cell is permeable to more than one ion (K+ and Na+, for example), the net flow of all ions affect the resting membrane potential in proportion to their relative permeabilities and conductances. Vm is a weighted average of each ion’s flux, and can be calculated using the Goldman-Hodgkin-Katz Equation (you are not responsible for this equation). In real cells, relative permeability determines which ion dominates in determining RMP (K+ is king)  Thinking about the Vm Chemical Driving Force Electrical Driving Force Net Flux electrochemical gradient -110 mV In Out In Out In Out Zero Zero Zero In In In -90 mV Out Out Out At each membrane potential (Vm), 1) Zero Zero Zero In In In circle the direction in which the chemical -70 mV Out Out Out and electrical driving forces are acting Zero Zero Zero on K+; 2) predict the net flux of K+. In In In 0 mV Out Out Out Zero Zero Zero EK = -90 mV + 60 mV In Out In Out In Out Zero Zero Zero In In In + 90 mV Out Out Out Zero Zero Zero Electrical Signals Neurons communicate by generating electrical signals which are changes in membrane potential Ion channels open and close to generate changes in membrane potential (deviate from resting membrane potential) Electrical Signaling Types of ion channels (for electrical signaling): ★ Leak channels: always open ★ Ligand-gated: chemicals ★ Voltage-gated: voltage changes ★ Mechanically-gated: changes in shape (e.g. baroreceptors) Action Potentials At threshold, voltage-gated Na+ channels open and the membrane rapidly Voltage-gated Na+ begin to depolarizes inactivate while voltage-gated K+ channels open, repolarizing the membrane Graded potentials (ligand gated ion channels) can summate and bring the membrane to threshold Graded Potentials vs. Action Potentials Graded Potentials Action Potentials Amplitude varies with stimulus intensity Amplitude is all-or-none (i.e., threshold (i.e., response is graded) must be hit) Depolarizing or hyperpolarizing Initiated by depolarization Summation No summation Mediated by ligand-gated channels Mediated by voltage-gated channels External Study Resource (Optional) Ninja Nerd: Resting Membrane, Graded, Action Potentials Lab 3 Activity  Part 1: Equilibrium Potential, Resting Membrane Potential and the Electrochemical Gradient (Draw the cells on your sheet of paper) Cell 1 Tool Kit Na Na Na Na Na Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + + + + + + Leak Sodium Leak Sodium Channel Channel Electrical Electrical Gradient Gradient Vm = -70 mV Chemical Chemical Gradient Gradient Cell 2 Tool Kit K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Leak Leak Potassium Potassium Channel Channel Chemical Chemical Vm = -70 mV Gradient Gradient Cell 3 Tool Kit K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Leak Leak Leak Potassium Potassium Potassium Channel Channel Channel Leak Sodium Leak Sodium Leak Sodium Channel Channel Channel Vm = -70 mV Part 2: Action Potential Review (Draw the neuron on your sheet of paper)  Note: LG means “Ligand Gated” Neuron Tool Kit Section 1 Section 2 LG Na+ Channe l  Physiology 3051 Lab 1: Diffusion Joe Sepe, PhD Spring 2025 Learning Objectives 1. Explain how the kinetic energy of molecules influences the distribution of molecules in a gas or a liquid (be sure to include a discussion of collisions). Define net diffusion. 2. List the factors that affect the rate of net diffusion of molecules and explain how they affect diffusion. Diffusion: Why is diffusion important? The body has two ways of delivering material to every cell. For long distances, the body uses the blood For short distances, the body used diffusion. Diffusion works optimally over SHORT distances (less than ~70 𝜇m). Cells typically do not grow to be larger than 70 𝜇m because this would not allow nutrients to diffuse in and out of the cell properly. A human hair is about 80𝜇m On average every cell in the body is within 70𝜇m of a blood vessel. This is how your body exchanges O 2, CO2, glucose, ions, waste products etc… Principles of Diffusion Kinetic Energy Temperature & Molecular Weight Concentration Gradients Diffusion: Kinetic energy Kinetic energy is the energy of motion. All molecules at the same temperature have the same kinetic energy. ROTATIONAL VIBRATIONAL Diffusion relies on kinetic energy and is a passive process (does not require energy input). Diffusion: Concentration gradients o Diffusion occurs rapidly over short distances but is very slow over long distances. o Molecules move from an area of high concentration to an area of lower concentration. o Net movement of molecules occurs until the concentration of a molecule is equal within a given space. Concentration gradient Diffusion equilibrium Diffusion: Temperature and Molecular Weight o Diffusion is directly related to temperature o Increase temperature → increase rate of diffusion o Diffusion is inversely related to molecular weight and size o Increase molecular weight→ decreased rate of diffusion N: 14.1 g/mol NH3 H: 1.01 g/mol (ammonia) -------------------- MW: 17 g/mol Na: 23 g/mol NaCl Cl: 35 g/mol (table salt) -------------------- MW: 58 g/mol Learning Objectives 1. Compare the rate of diffusion in gas versus a liquid. Explain what is happening at the molecular level that accounts for this difference. Think about the density of these two states of matter and about collisions of molecules... 2. For experiment #2 Explain how a pH indicator allows us to observe the diffusion of OH- ions. 3. For experiment #2 Describe the relationship between distance and time with respect to diffusion of solute in a liquid. 4. Use the equation that you derived from your experimental data in experiment #2 to compare how far net diffusion of OH- ion will get in 10 msec versus 100 msec. (Note: msec = 1/1000 sec). (NOTE: remember to keep the units straight – we measured distance in mm and time in minutes). 5. Explain the advantages and disadvantages of diffusion as a means of moving molecules in the body and describe where and over what short of scale (ex: micrometers vs millimeters vs centimeters) it is a useful means of moving molecules in the body. Basics Knowledge Questions Net diffusion will occur in which direction? Experiment 1 compartment 1 compartment 2 25 Cº Diffusion will occur faster in which experiment? Experiment 1 Experiment 2 compartment 1 compartment 2 compartment 1 compartment 2 25 Cº 35 Cº  Molecule A weighs 25g/mol & Molecule B weighs 32g/mol. Which of the following statements is TRUE? A. Molecule B will diffuse faster than Molecule A in a gas B. Molecule B will diffuse faster than Molecule A in a liquid C. Molecule A will diffuse faster than molecule B in any medium D. Both molecules will diffuse at the same rate. Experiment 1: Diffusion in a gas Diffusion: In a gas Start Time: ________ End Time: ________ Total Time: ________ NH3 (g) HCl NH3 (g) = 17 g/mol Molecular Weight HCl (g) = 36 g/mol 25 Cº Temperature 25 Cº Same Concentration gradient Same NH3 (g) + HCl (g) → NH4Cl (s) NH3 (g) 1 2 3 HCl Based on what you just learned, where do you think the precipitate will form in the tube? a.Region 1 b.Region 2 c.Region 3 Why do you think that?  Does more collisions mean more diffusion? Experiment 1 Experiment 2 Diffusion: Gas Vs. Liquid The concentration of the red molecules is the same 25 Cº in both gas and liquid. Gas Liquid is more densely packed Temperature is the same in both the liquid and gas. 25 Cº Liquid Will the red molecules diffuse faster in a liquid or a gas? Experiment 2: Diffusion in a liquid Ionic molecules dissociate in water What you need to know about pH In this experiment, we will use a dye called Phenolphthalein as a pH indicator. Based on the color we can infer pH. pH10 Colorless Pink! Colorless Extremely Extremely acidic basic 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Higher [H+] Lower [H+] Lower [OH-] Higher [OH-]  pH~7 OH- in the dish will begin to diffuse into the tubes filled with agar. What will happen to pH in the tube? pH~7 When the tube begins to change color what does that tell you about OH- ion pH~13 diffusion?  1: Place tube in the solution 2: Take measurements at corresponding times 3: Plot the measurements on the graphs pH~7 4: Use d2 vs t graph to generate an equation that can predict diffusion times. (TAs can help if you need it) 5: Make calculations for the time it takes molecules to diffuse to the MIDDLE of agar tube.