Lec 2: Membrane Potentials and Action Potentials

These flashcards are designed to help students understand and memorize key concepts related to membrane potentials, action potentials, and the associated physiological mechanisms.

Resting Membrane Potential

The electrical potential difference across the membrane of a resting nerve cell.

Ion Species

Different types of ions (such as Na+, K+, Cl-, Ca2+) that contribute to membrane potentials.

Nernst Potential

The membrane potential that opposes the net movement of a specific ion across the membrane.

Goldman-Hodgkin-Katz Equation

An equation that calculates the membrane potential considering the permeability and concentrations of multiple ions.

Sodium-Potassium Pump

A membrane-bound enzyme that pumps Na+ out of the cell and K+ into the cell, maintaining resting potential.

Positive Feedback

A mechanism that amplifies a response or process, such as the opening of additional sodium channels during depolarization.

All-or-None Characteristic

Refers to action potentials occurring at full strength once the threshold is reached or not occurring at all.

Myelin Sheath

An insulating layer around a nerve fiber, providing support and increasing conduction velocity.

Voltage-Gated Ion Channels

Channels that open or close in response to changes in membrane potential, allowing ions to pass.

Equilibrium Potential

The membrane potential at which the net flow of a particular ion across the membrane is zero.

Electromotive Force (EMF)

The force that drives ions across the membrane, calculated using the Nernst equation.

Concentration Gradient

The difference in concentration of ions across a membrane, driving diffusion.

Leak Channels

Channels that are always open, allowing specific ions to diffuse across the membrane.

Membrane Permeability

The ability of the membrane to allow ions to pass through, affecting the membrane potential.

Threshold Potential

The critical level to which the membrane potential must be depolarized to initiate an action potential.

Repolarization

The return of the membrane potential to its resting value after depolarization.

Hyperpolarization

An increase in membrane potential (making it more negative) following repolarization.

Depolarization

A decrease in membrane potential (making it less negative) that is part of the action potential.

Intensity of Stimulus

The strength of a stimulus that affects the frequency of action potentials generated.

Refractory Period

A period following an action potential during which the nerve fiber cannot fire again.

Sodium Concentration Inside and Outside of Cell

14 mEq/L inside, 142 mEq/L outside.

Potassium Concentration Inside and Outside of Cell

140 mEq/L inside, 4 mEq/L outside.

Capacitance of Membrane

The ability of the cell membrane to store charge, significantly decreased by myelin.

Node of Ranvier

Gaps in the myelin sheath where ion channels are concentrated.

Saltatory Conduction

The jumping of action potentials from node to node, speeding up nerve transmission.

Diffusion Potential

The potential difference generated by the diffusion of ions across a membrane.

Calcium Concentration

Influences membrane potential, typically around +130 mV in neurons.

Chloride Concentration

Involved in membrane dynamics, typically around -70 mV in neurons.

Skeletal Muscle Action Potentials

Triggered similarly as in nerve fibers, involving rapid membrane potential changes.

Electrolytes

Ionic solutes that conduct electricity and are crucial for neuronal function.

Action Potential

Rapid changes in membrane potential that propagate along the axon.

Inactivation Gate

A gate on sodium channels that closes shortly after the channel opens, stopping sodium influx.

Activation Gate

A gate on sodium channels that opens in response to depolarization, allowing sodium to enter.

Diffusion of Ions

Movement of ions from an area of higher concentration to lower concentration across the membrane.

Nernst Equation for Na+

EMF = -61 log (Ci/Co) for sodium ions to determine equilibrium.

Nernst Equation for K+

EMF = -61 log (Ci/Co) for potassium ions to determine equilibrium.

K+ Leak Channels

Channels that permit potassium ions to leak out of the cell, helping establish resting potential.

Action Potential Threshold for Nerve Fibers

Typically around -65 mV, where action potentials initiate.

Energizing of Na+-K+ Pump

Requires ATP for active transport to maintain ion gradients.

Ion Concentration Gradients

Essential for generating resting and action potentials.

Metabolic Activity in Nerves

Increases as sodium ions accumulate inside the cell during action potentials.

Membrane Potentials Dependence

Relies on the permeability of the membrane to different ions.

Resistance in Axon

Resistance to the flow of ions within the axon affecting conduction speed.

Signal Transmission in Muscle Cells

Similar to that in neurons, involving action potentials.

Influence of Myelin on Membrane Capacitance

Significantly reduces capacitance, aiding rapid signal propagation.

Role of Schwann Cells

Glial cells that myelinate peripheral nerve fibers.

Role of Oligodendrocytes

Glial cells that myelinate central nervous system nerve fibers.

Factors Influencing Membrane Potential Changes

Include ion concentration gradients and membrane permeability.

Myelination Effect on Efficiency

Conserves energy and speeds up action potential transmission.

Active Transport Mechanism

Involves protein pumps moving ions against concentration gradients.

Absolute Refractory Period

The interval during which a second action potential cannot be initiated, regardless of the stimulus strength, due to the inactivation of voltage-gated Na^+ channels.

Relative Refractory Period

The period following the absolute refractory period when an action potential can be triggered, but only by a stimulus of significantly greater intensity than the normal threshold.

Continuous Conduction

The step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane, occurring in unmyelinated axons.

Voltage-Gated K^+ Channels

Channels that open more slowly than sodium channels in response to depolarization, allowing potassium efflux to drive the repolarization phase.

Length Constant (\lambda)

A measure of how far a graded potential will spread along an axon before its amplitude decays to approximately 37\% of the original value; affected by membrane and axial resistance.

Time Constant (\tau)

The time it takes for the membrane potential to reach 63\% of its final value, determined by the product of membrane resistance and capacitance (\tau = rm cm).

After-hyperpolarization (AHP)

A temporary period following an action potential where the membrane potential is more negative than the resting potential because K^+ conductance remains elevated.

Electrochemical Driving Force

The difference between the actual membrane potential and the equilibrium potential for a specific ion (Vm - E{ion}), determining the direction and magnitude of ion flow.

Graded Potentials

Local changes in membrane potential that vary in amplitude depending on the strength of the stimulus and decay over distance.

Accommodation

A phenomenon where the threshold for an action potential increases when a cell is subjected to a slowly rising or prolonged subthreshold stimulus.

Hypocalcemia and Nerve Excitability

A condition where low extracellular Ca^{2+} levels lower the threshold for Na^+ channel activation, making the nerve fiber highly excitable.

Ohm's Law in Membrane Physiology

Relates the ionic current (I) to conductance (g) and driving force: I{ion} = g{ion}(Vm - E{ion}).