L3: Action Potential

"Describe osmosis in the context of water movement across a membrane."

"Osmosis is the net movement of water across a permeable membrane due to differences in solute concentration, causing water to move from an area of lower solute concentration to an area of higher solute concentration."

"Explain the concept of osmotic pressure."

"Osmotic pressure is the pressure required to prevent the movement of water across a semipermeable membrane, effectively stopping the flow of water from one compartment to another due to solute concentration differences."

"Define an ideal membrane in terms of osmosis."

"An ideal membrane is one that is only permeable to water, allowing it to flow down its concentration gradient without any interference from solutes."

"How does a nonideal membrane affect osmotic pressure?"

"In a nonideal membrane, osmotic pressure depends on the membrane's ability to distinguish between solute and solvent. If the membrane is permeable to both, intercompartmental mixing occurs, resulting in an osmotic pressure of zero."

"What is the reflection coefficient in the context of osmosis?"

"The reflection coefficient (σ) quantifies a membrane's ability to reflect solute S, with values ranging from 0 (no reflection) to 1 (complete reflection)."

"Explain the relationship between osmolarity and the number of particles in a solution."

"Osmolarity describes the concentration of osmotically active particles in a solution, typically measured in osmoles (Osm) or milliosmoles (mOsm) per liter, indicating how many particles contribute to osmotic pressure."

"How is osmolarity calculated from molarity?"

"Osmolarity is calculated by multiplying molarity by the number of particles that a solute dissociates into in solution."

"Define tonicity and its significance in cellular behavior."

"Tonicity refers to how cells behave in solutions of differing osmotic strength, affecting whether cells swell, shrink, or remain unchanged when placed in those solutions."

"What characterizes an isotonic solution?"

"An isotonic solution has an equal concentration of ions compared to the concentration of ions inside the cell, resulting in no net movement of water into or out of the cell."

"Define hypertonic solution."

"A hypertonic solution has a higher concentration of ions in the solution than in the cell."

"Describe hypotonic solution."

"A hypotonic solution has a lower concentration of ions in the solution than in the cell."

"Explain tonicity in relation to plasma membranes."

"Tonicity is the measure of the water gradient that exists between two solutions separated by a plasma membrane, highly dependent on solute concentration and permeability."

"What is the normal plasma osmolarity range in physiology?"

"The normal plasma osmolarity range in physiology is 275-300 mOsm."

"How does intracellular osmolarity relate to plasma osmolarity?"

"Intracellular osmolarity must be the same as plasma osmolarity for the cell to be in osmotic equilibrium."

"Describe the movement of water in non-plasma solutions."

"In non-plasma solutions, water will move into or out of cells depending on the relative osmotic strength of the solution compared to the internal osmolarity and the permeability of the solute."

"What happens to a cell in a hypotonic solution?"

"In a hypotonic solution, where the osmotic strength is lesser and the solute is not permeable, the cell will swell."

"Explain the effect of a hypertonic solution on a cell."

"In a hypertonic solution, where the osmotic strength is greater and the solute is not permeable, the cell will shrink."

"What occurs in an isotonic solution?"

"In an isotonic solution, where the osmotic strength is the same and the solute is not permeable, there is no net water movement."

"How does osmolarity relate to tonicity?"

"The osmolarity of a solution is not an accurate predictor of its tonicity."

"Define hyposmotic, isosmotic, and hyperosmotic in relation to tonicity."

"Hyposmotic refers to a solution with lower osmotic strength, isosmotic refers to equal osmotic strength, and hyperosmotic refers to a solution with higher osmotic strength."

"What is the effect of glycerol and urea on cell membranes?"

"Glycerol and urea are very permeable across cell membranes; a cell in a 500 mOsm glycerol solution will swell and lyse despite the solution being hyperosmotic."

"Identify the only solutes that define tonicity."

"Only non-permeable solutes define tonicity."

"Explain how small ions and non-electrolytes respond to chemical gradients."

"Small ions respond to chemical gradients similarly to non-electrolytes like glucose, but charged ions also consider electrical potential in their movement."

"What governs the diffusion of charged species?"

"Both electrical and chemical forces govern the diffusion of charged species."

"What is the law of electroneutrality?"

"The law of electroneutrality must be maintained for a bulk solution, ensuring that the total charge is balanced."

"Describe the movement of ions when a membrane becomes permeable to K and Ac."

"When the membrane becomes permeable to K and Ac, both ions will move from side 1 to side 2."

"Describe the role of the concentration gradient between two compartments in generating a diffusion potential."

"The concentration gradient between compartment 1 and 2 serves as the driving force for ion movement, leading to the formation of a diffusion potential as ions move to reach equilibrium."

"Explain the concept of a diffusing dipole in the context of ion movement."

"A diffusing dipole is created when ions with different radii, such as K+ and Ac-, move, resulting in a pair of electric charges separated by a small distance, which contributes to the generation of a diffusion potential."

"Define the Nernst Equation and its significance in ion equilibrium."

"The Nernst Equation calculates the membrane voltage at which the chemical and electrical driving forces for an ion are equal and opposite, indicating equilibrium. It is expressed as E ion = log(2.3 * RT / zF) * (C1s / C2s)."

"How is the Nernst equilibrium potential for an ion determined?"

"The Nernst equilibrium potential is determined using the Nernst Equation, which computes the electrical force that balances the concentration force, resulting in no net movement of the ion."

"Explain what happens when a membrane is only permeable to one ion."

"When a membrane is only permeable to one ion, an equilibrium potential is reached where the chemical and electrical driving forces are equal and opposite, stabilizing the ion's concentration across the membrane."

"Describe the process of measuring membrane potential using a microelectrode."

"Inserting a microelectrode into a cell allows for the measurement of the electrical potential difference between the inside and outside of the cell, which is referred to as the membrane potential."

"What is the significance of membrane potential in cells?"

"Membrane potential is crucial for cell function; all living cells have a membrane potential, while cells without it are considered dead."

"Explain how a leak channel for K+ affects the membrane potential of a cell."

"When a leak channel for K+ is inserted into the membrane, K+ ions move out of the cell down their concentration gradient, causing the inside of the cell to become more negative and the outside to become more positive."

"Define the initial state of a cell regarding membrane potential before any ion movement occurs."

"Initially, a cell has no membrane potential, with the extracellular fluid (ECF) and intracellular fluid (ICF) being electrically neutral, containing Na and Cl ions in the ECF and K and large ions in the ICF."

"How does the movement of K+ ions influence the charge distribution inside and outside the cell?"

"As K+ ions leave the cell through a leak channel, the inside of the cell becomes more negative due to the loss of positive charge, while the outside becomes more positive, creating a charge imbalance."

"Describe the concept of electrochemical equilibrium in relation to potassium ions (K+)."

"Electrochemical equilibrium occurs when the concentration gradient pushing K+ out of the cell is exactly balanced by the electrical gradient pulling K+ back into the cell, resulting in no net movement of K+."

"Explain how a membrane potential maintains electroneutrality despite potential differences."

"Membrane potential does not violate electroneutrality because the charge balance of the bulk solution is maintained, as potential differences arise from the separation of only a few charges near the membrane."

"Define equilibrium potential and its significance for ion movement."

"Equilibrium potential is the voltage that exactly opposes the driving force from the concentration gradient for an ion, resulting in no net electrochemical driving force for diffusion when reached."

"How does exceeding the equilibrium potential affect ion flow?"

"If an external current exceeds the equilibrium potential, it drives the flow of the ion against its concentration gradient, moving from a region of low concentration to a region of high concentration."

"What is the relationship between concentration gradients and electrical potentials for monovalent ions?"

"A tenfold concentration gradient of a monovalent ion corresponds to an electrical potential of 60mV, which serves as the driving force for ion movement."

"Calculate the Nernst equilibrium potential for sodium (Na+) given a concentration ratio of 100 outside to 10 inside the cell."

"E Na = 60 log (100/10) = +60mV."

"Calculate the Nernst equilibrium potential for chloride (Cl-) with a concentration ratio of 100 outside to 10 inside the cell, considering its valence."

"E Cl = -60 log (100/10) = -60mV, taking into account the valence of the ion."

"Explain the term 'reversal potentials' in the context of equilibrium potentials."

"Equilibrium potentials are sometimes referred to as reversal potentials because they indicate the voltage at which the driving force for ion movement reverses direction."

"Describe the threshold potential in nerve action potentials."

"The threshold potential is the membrane potential at which the depolarizing effects of the inward Na+ current exceed the hyperpolarizing effects of the outward K+ current."

"Explain the concept of threshold intensity in relation to action potentials."

"Threshold intensity is the minimal intensity of stimulating current that, acting for a given duration, will just produce an action potential. It varies with duration; weak stimuli require a longer duration while strong stimuli require a shorter duration."

"How does a stimulus affect the intracellular potential during an action potential?"

"A stimulus raises the intracellular potential to a threshold level, leading to the instantaneous opening of voltage-gated Na+ channels."

"What happens to the cell membrane during the depolarizing phase of an action potential?"

"During the depolarizing phase, the membrane becomes permeable to Na+, resulting in a rapid influx of Na+ due to both electrical and chemical gradients, causing the cell membrane potential to become more positive."

"Define the rapid upstroke in the context of nerve action potentials."

"The rapid upstroke, or depolarizing phase, is characterized by an increase in Na+ conductance of the cell membrane due to the activation of voltage-gated Na+ channels."

"Explain the all-or-none response in nerve action potentials."

"The all-or-none response refers to the phenomenon where the cell potential either fully reaches a certain level or does not change at all, moving toward E Na due to the chemical and electrical driving force."

"How much does the cell depolarize during an action potential?"

"The cell depolarizes by approximately 100mV, moving from around -70 or -80mV to +20 or +30mV."

"What is the role of voltage-gated Na+ channels in action potentials?"

"Voltage-gated Na+ channels open instantaneously when the threshold is reached, allowing Na+ to flow into the cell and causing depolarization."

"Describe the sequence of events that occur in the voltage-gated Na+ channel at resting membrane potential."

"At resting membrane potential, the activation gate closes the channels."

"Explain what happens when a depolarizing stimulus arrives at the voltage-gated Na+ channel."

"When a depolarizing stimulus arrives, the activation gate opens, allowing Na+ to enter the cell."

"How does the rising phase of the action potential demonstrate positive feedback?"

"The rising phase shows positive feedback as depolarization causes Na+ channel activation gates to open rapidly, leading to more Na+ entering the cell and further depolarization."

"What occurs after the activation gate of the Na+ channel opens?"

"After the activation gate opens, Na+ enters the cell until the inactivation gate closes, stopping Na+ entry."

"Define the role of the inactivation gate in the voltage-gated Na+ channel."

"The inactivation gate closes to stop Na+ entry into the cell after the activation gate has opened."

"Explain the process of repolarization in relation to the voltage-gated Na+ channel."

"During repolarization, K+ leaves the cell, and both gates of the Na+ channel reset to their original positions."

"Describe the events that lead to hyperpolarization after the action potential."

"Hyperpolarization occurs as the slower Na+ channel inactivation gate closes, K+ channels open, and K+ exits the cell."

"How does the outward K+ gradient contribute to cell repolarization?"

"The outward K+ gradient facilitates K+ flux as voltage-dependent K+ channels open, leading to cell repolarization."

"What is the final outcome of the sequence of events in the voltage-gated Na+ channel?"

"The final outcome is that the cell repolarizes after the action potential, returning to its resting membrane potential."

"Describe the refractory period in neuronal action potentials."

"The refractory period is the time during which a neuron is incapable of eliciting a normal action potential. It consists of two phases: the absolute refractory period, where no action potential can be generated regardless of stimulus strength due to the inactivation of Na channels, and the relative refractory period, where a stronger than normal stimulus is required to elicit an action potential due to higher K+ conductance."

"Explain the difference between absolute and relative refractory periods."

"The absolute refractory period occurs when no action potential can be elicited, regardless of stimulus strength, due to closed Na channel inactivation gates. The relative refractory period follows, where a stronger stimulus is needed to trigger an action potential because the membrane potential is closer to E_k and higher K+ conductance is present."

"How does the propagation of an action potential occur along an axon?"

"Propagation occurs through electrotonic conduction, where a graded potential above threshold reaches the trigger zone, opening voltage-gated Na+ channels. Na+ enters the axon, causing positive charge to flow into adjacent sections, depolarizing them. The refractory period prevents backward conduction, and the loss of K+ repolarizes the membrane."

"Define saltatory conduction and its significance in neuronal function."

"Saltatory conduction is the process by which action potentials jump from one Node of Ranvier to another along myelinated axons, significantly increasing the speed of conduction and reducing current loss."

"Describe the role of myelin in the conduction of action potentials."

"Myelin, composed of 70% lipid and 30% protein, insulates axons, increasing resistance and decreasing capacitance. This allows current to flow more efficiently down the interior of the nerve, enhancing the speed of action potential conduction."

"Explain how the structure of myelin affects neuronal signaling."

"The concentric wrapping of myelin around axons, formed by Schwann cells in the PNS or oligodendrocytes in the CNS, increases resistance and decreases capacitance, which reduces current loss and allows for faster action potential propagation."

"How does the Node of Ranvier contribute to action potential propagation?"

"The Node of Ranvier is a break in the myelin sheath that allows action potentials to be generated, facilitating saltatory conduction by enabling the action potential to jump between nodes, thus increasing conduction speed."

"What is the analogy used to describe the conduction of an action potential down an axon?"

"The conduction of an action potential down an axon is likened to energy passed along a series of falling dominos, where each domino represents a different phase of the action potential in various sections of the axon."

"Describe the impact of myelin on current loss in neurons."

"Myelin reduces current loss by increasing the resistance of the membrane and decreasing its capacitance, allowing for more efficient conduction of electrical signals along the axon."

"Describe the effect of myelination on current decay in neurons."

"Myelination reduces current decay in neurons, allowing for faster and more efficient signal transmission."

"Explain the difference in conduction velocity between myelinated and unmyelinated neurons."

"Myelinated neurons have a significantly higher conduction velocity compared to unmyelinated neurons due to the presence of myelin sheaths that facilitate rapid signal propagation."

"Define the conduction velocity of A α myelinated fibers."

"A α myelinated fibers have a conduction velocity of 42 m/s and are primarily found in motor neurons."

"How does the conduction velocity of A β fibers compare to A δ fibers?"

"A β fibers have a conduction velocity of 25 m/s, while A δ fibers have a lower conduction velocity of 17 m/s."

"List the conduction velocity and location of B myelinated fibers."

"B myelinated fibers have a conduction velocity of 4.2 m/s and are located in autonomic nervous system (ANS) neurons."

"What is the conduction velocity of unmyelinated C fibers?"

"Unmyelinated C fibers have a conduction velocity ranging from 0.3 to 0.4 m/s and are associated with sensory and post-ganglionic sympathetic neurons."

"Do myelinated fibers conduct signals faster than unmyelinated fibers?"

"Yes, myelinated fibers conduct signals significantly faster than unmyelinated fibers due to the insulating properties of myelin."

"Explain the role of axon diameter in conduction velocity."

"Larger axon diameters generally lead to higher conduction velocities, as they reduce internal resistance to the flow of electrical signals."

"Describe the relationship between axon diameter and current conduction speed."

"The larger the axon diameter, the lower the internal resistance (R), resulting in faster current conduction down the axon."

"Explain how membrane resistance affects current flow in neurons."

"A greater membrane resistance leads to lower current leakage, which allows for faster current conduction down the axon."

"Define demyelination and its association with Multiple Sclerosis."

"Demyelination refers to the loss of the myelin sheath around nerve fibers, which is a characteristic of Multiple Sclerosis, leading to impaired nerve conduction."