Membrane and Action Potential Quiz

Membrane and Action Potential

• Membrane potential defines the difference in charge across the plasma membrane
• It's measured in millivolts (mV)
• The difference arises from the unequal distribution of ions (cations & anions) inside and outside the cell.
• Opposite charges attract, while similar charges repel
• Work is needed to separate opposite charges to create potential energy.
• The magnitude of the membrane potential depends on the number of charges separated.

Learning Objectives

• Understand the generation of resting membrane potential.
• Explain the equilibrium potentials of important ions.
• Define local responses and firing thresholds.
• Describe action potential formation, characteristics, and factors impacting it.
• Explain the conduction of impulses along an axon.
• Define a compound action potential.

Membrane Potential

• Separation of opposite charges across the plasma membrane
• Represents the difference in the relative number of cations (+) and anions (-) in the intracellular fluid (ICF) and extracellular fluid (ECF).
• Basic principle: opposite charges attract, and similar charges repel.
• Work is required to separate opposite charges which generates potential energy.
• Measured in millivolts (mV).
• Only charges along the inner and outer surfaces of the membrane contribute to the potential.
• Movement of positive charges to one side creates an excess of positive charge on one side and negative charge on the other side, generating an electrical gradient (membrane potential).
• Magnitude depends on the number of charges separated.

Resting Membrane Potential (RMP)

• Constant membrane potential in non-excitable and excitable cells at rest.
• Caused by the unequal distribution of ions across the plasma membrane and their selective permeability.
• Primary ions involved: Na+, K+, and anionic proteins (A⁻).
• Na+ concentration is higher in the extracellular fluid (ECF), K+ is higher in the intracellular fluid (ICF), and A⁻ is mostly inside the cell.
• The membrane is more permeable to K+ than Na+.
• Mechanisms:
  • Na⁺-K⁺ pump actively transports 3 Na⁺ out and 2 K⁺ in, creating and maintaining concentration gradients.
  • Passive ion movement: K⁺ leaks out of the cell more readily than Na⁺ enters.
  • Net effect: inside the cell becomes slightly more negative than the outside.
• RMP typically ranges between -70 mV to -90 mV (depending on the cell type).

Equilibrium Potential

• Membrane potential where the electrical gradient exactly counterbalances the concentration gradient for an ion, resulting in no net ion movement.
• Factors affecting:
  • Ion concentration gradient (drives ion movement)
  • Electrical gradient (opposes ion movement)
• Calculated using the Nernst equation.

Equilibrium Potential for K+

• Concentration gradient: high K⁺ inside the cell, low K⁺ outside. K⁺ moves outward.
• Electrical gradient: K⁺ efflux leaves negative charge inside the cell, and inside becomes increasingly negative. An opposing electrical gradient develops, pulling K⁺ back into the cell.
• Equilibrium point: no net movement of K⁺ occurs when concentration and electrical gradients are balanced.
• EK+ = -90 mV (inside negative relative to outside).

Equilibrium Potential for Na+

• Concentration gradient: high Na⁺ outside the cell, low Na⁺ inside. Na⁺ moves inward.
• Electrical gradient: Na⁺ influx creates positive charge inside the cell. The outside becomes more negative due to unbalanced negative ions (mostly Cl⁻).
• Equilibrium point: no net movement of Na⁺ occurs when concentration and electrical gradients are balanced.
• ENa+ = +60 mV (inside positive relative to outside).

Generation of RMP

• The Na⁺–K⁺ pump actively transports Na⁺ out of and K⁺ into the cell, maintaining high extracellular Na⁺ and intracellular K⁺ concentrations.
• The resting membrane is far more permeable to K⁺ than Na⁺.
• K⁺ diffuses out of the cell down its concentration gradient, creating a negative intracellular potential.
• The small inward movement of Na⁺ opposes the outward movement of K⁺, creating resting potential around -70 mV.

Nerst Equation

• Calculates the equilibrium potential for a specific ion across a membrane based on ion concentration gradient and charge.
• Determines the membrane voltage needed to counteract ion diffusion.
• Shows the relationship between electrical and concentration gradients.
• Formula: Ex = -61 x log ([Xin]/[Xout]).

Graded Potentials

• Local changes in membrane potential, varying in magnitude or strength.
• Triggered by gated ion channels opening in a specific region of the membrane (often in response to a stimulus).
• Usually caused by Na⁺ entry, leading to depolarization.
• Magnitude diminishes as it spreads from the origin.

Action Potentials (APs)

• Brief rapid large changes in membrane potential (~100 mV)
• Temporarily reverses the membrane potential, making the inside more positive than the outside.
• Propagated non-decrementally, maintaining full strength.
• Triggered when threshold potential (-50 to -55 mV) is reached.
• Duration is consistent for a given excitable cell (~1 ms).
• Phases: resting, depolarization, repolarization, and hyperpolarization.
• Generation: -Resting potential: all voltage-gated channels (Na+ and K+) are closed. -Threshold: Na+ activation gate opens and PNa+ increases. -depolarization: Na+ enters the cell, causing explosive depolarization to +30 mV -peak: Na+ inactivation gate closes, and PNa+ falls, Na+ stops moving in. At the same time, K+ activation gate opens and PK+ rises. -repolarization: K+ leaves the cell, causing repolarization to resting potential. -hyperpolarization: K+ channels remain open after the potential reaches resting level.

Propagation of Action Potentials

• Propagated from the axon hillock to the axon terminals as non-decremental signals.
• Local current flow between active and adjacent inactive areas depolarizes inactive areas to threshold, initiating APs sequentially.

Factors affecting conduction velocity of APs

• Myelination
• Fiber diameter
• Temperature

Contiguous Conduction (unmyelinated axons)

• APs propagated by sequential depolarization of each patch of membrane along the axon.
• Na⁺ influx depolarizes the adjacent region by triggering a new AP.
• Slower conduction speed due to continuous regeneration of APs in each segment.
• Amplitude remains consistent regardless of distance.

Saltatory Conduction (myelinated axons)

• Myelin, a lipid insulator, prevents ion flow across myelinated regions to conserve current flow. APs are generated only at the nodes of Ranvier, causing current to “jump” from one node to the next.
• Faster propagation compared to contiguous conduction.
• Metabolically more efficient.

Multiple Sclerosis

• Chronic autoimmune disease of the CNS (brain and spinal cord)
• Characterized by immune-mediated damage to the myelin sheath (demyelination).

All-or-none Law

• APs follow the all-or-none law: an excitable membrane either completely responds to a threshold stimulus with a maximal AP or does not respond at all.
• The magnitude of an AP is independent of the triggering event's strength.

How Stimulus Intensity is Coded

• Increasing stimulus intensity leads to more APs being fired and an increase in the frequency of firing, not in the size of the AP.