Membrane Potentials Review

Lecture 4: Membrane Potentials Review

• Sodium (Na+) and potassium (K+) play a dominant role in resting membrane potential (RMP).
• Intracellular proteins and organic phosphates, which cannot cross the cell membrane, also contribute to RMP.
• Understanding equilibrium potentials, permeability, and ion pumps is crucial to comprehending RMP generation and its negative value.

How and Why Ions Move: Gradients

• The difference in ion concentration (gradient) across the membrane drives ion movement.
• This gradient is maintained by primary or secondary active transport, creating a force for ion movement across the membrane.
• Ion channels (leak channels) allow ions to pass through the membrane, moving down their concentration gradient (high to low). For example, K+ moves from inside to outside the cell.

How and Why Ions Move: Charge

• Positive and negative ions tend to pair up in solution (opposites attract).
• Cation movement from inside to outside a cell leaves behind a negative anion, making the inside more negative and the outside more positive.
• This electrostatic gradient builds up over time.
• The negative charge inside the cell exerts a force opposing the outward movement of positively charged ions (like K+) down their concentration gradient.
• Equilibrium potential is reached when the electrostatic force equals the concentration gradient, stopping ion movement. This is calculated by the Nernst equation.

Equilibrium Potential

• The Nernst equation calculates equilibrium potential (Vm):
  • Vm = RT/zF * ln([ion outside the cell]/[ion inside the cell])
  • R = gas constant (8.314472 J. K⁻¹)
  • T = temperature (Kelvin)
  • F = Faraday's constant (9.65 x 10⁴ C mol⁻¹)
  • Z = ion charge (1 for monovalent ions like K+, 2 for divalent ions like Ca²⁺)
• Simplifying the equation at normal body temperature (37°C): RT/F ≈ 61.5
• Simplifying the equation at room temperature (18°C): RT/F ≈ 58

RMP in Different Cells and Its Roles

• Resting membrane potential (RMP) varies among different cell types.
• A table (not included in the text provided) illustrates different functions (e.g., circadian rhythm, biological sensing, or contractility) and associated cell types (neuros, fibroblasts, vascular smooth muscle cells) that are regulated by RMP.

What kind of neurons carry signals exclusively towards the brain?

• Sensory neurons

The part of the neuron that sends electrical signals over large distances is the:

• Axon

Which neuroglia are responsible for forming the myelin sheath around axons in the peripheral nervous system?

• Schwann cells

Ionic gradients across membranes are maintained by:

• Sodium/potassium pumps

The Nernst potential for potassium is:

• the equilibrium potential specific to K+

The entry of chloride ions into resting neurons:

• Causes a hyperpolarization

Which of the following values is unnecessary when finding the equilibrium potential of an ion using the Nernst equation?

• The permeability of the ion channel

What is the primary ion responsible for the depolarization phase of an action potential?

• Sodium (Na+)

During the repolarization phase of an action potential, which ion moves out of the neuron?

• Potassium (K+)

What is the typical threshold potential that must be reached for an action potential to be initiated?

• −55 mV

Which of the following statements about action potentials is true?

• Action potentials are all-or-nothing events.

Which of the following best describes the role of voltage-gated sodium channels during the action potential?

• They open in response to depolarization and allow sodium ions to enter the cell.

What is the primary reason for the refractory period following an action potential?

• To prevent the backward propagation of the action potential.

Which of the following factors does NOT affect the conduction velocity of an action potential along an axon?

• Length of the axon

During the afterpotential or hyperpolarization phase of an action potential, which of the following occurs?

• Voltage-gated potassium channels remain open longer than necessary.