Lectures 5.1 & 5.2: Resting Membrane Potential & The Action Potential

Resting Membrane Potential (Review) & The Action Potential

Lecture Objectives

• Describe the physiological basis of a cell’s resting membrane potential (RMP), including membrane permeability to ions, types of ion channels responsible for the RMP, and the electrochemical gradients for key ions.
• List in order and describe the principal events associated with an action potential.
• Define the terms depolarization, repolarization, and hyperpolarization.
• Define graded potential and describe the difference between EPSPs and IPSPs.
• Define the phrase ‘all-or-none’ in the context of the neuronal action potential.
• Compare and contrast the characteristics of graded vs. action potentials.
• Describe the basis of absolute and relative refractory periods and explain their importance.

Bioelectricity

• Resting Membrane Potential: -70mV
• Action Potential

Na+/K+ ATPase

• With the energy of 1 ATP molecule, the Na+/K+ ATPase pumps:
  • 3 Na+ ions OUT of the cell
  • 2 K+ ions IN to the cell
• The Na+/K+ ATPase maintains concentration gradients for both Na+ and K+!

Resting Membrane Potential – Ion Gradients & Permeability

• The sodium-potassium ATPase pump (Na+/K+ ATPase) maintains the ion concentration gradients for Na+ and K+.
  • For each ATP used:
    • 3 Na+ ions out
    • 2 K+ ions in
• At rest, the membrane is only permeable to K+ through potassium leak channels.

Resting Membrane Potential

• Membrane is permeable to K+.
• Movement of K+ ions is influenced by two competing forces:
  1. Concentration gradient
  2. Electrical gradient
• This results in charge separation (buildup of potential energy) across the membrane.
• This is the potential energy stored across the membrane of a living cell at rest.
  • i.e., resting membrane potential.

Equilibrium Electrical Potential Difference

• Nerst Equation: $V<em>K = \frac{RT}{zF} In \frac{[K^+]</em>{in}}{[K^+]_{out}}$
  • R = Gas constant: $8.31 \frac{J}{Kmol}$
  • T = Temperature in Kelvin
  • z = Valence of the ion
  • F = Faraday's constant ($96,500 \frac{coulombs}{mol}$)
  • [K+]in = Intracellular potassium concentration
  • [K+]out = Extracellular potassium concentration

Electrical Potential Difference

• There is an ‘electrical potential difference’ (PD) across the membrane of all living cells at rest.
  • Measured in millivolts (mV) – Typical values: -50 mV to -100 mV.
• Resting membrane potential is best described as:
  1. A characteristic of ALL cells
  2. Inside negative (positive charges will be pulled into the cell if given the opportunity)
  3. K+-dominated (the K+ concentration gradient is opposed to the K+ electrical gradient)

Review: Resting Membrane Potential

Changes in Membrane Potential

• The membrane potential is not always at rest (-70 mV).
• How can we change the membrane potential?
  • Changes in membrane permeability produce large changes in the membrane potential.
• How can we change membrane permeability?
  • Make membrane more permeable to K+? Open K+ channels
  • Make membrane more permeable to Na+? Open Na+ channels

Channel-Mediated Ion Permeability

• Regulation of channel-mediated ion permeability allows cells to generate electrical signals.
• In order to manipulate membrane potential:
  1. Maintain (stable) Na+ and K+ gradients (Na+/K+ ATPase)
  2. Vary the activity of specific ion channels

Ion Channels

• Ion channels are integral membrane proteins that can be open or closed.
• Some ion channels are routinely open (ion leak channels).
  • K+-leak channels are always open and are the basis for the inside-negative resting PD.

Regulated Open States of Ion Channels

• Some ion channels have their ‘open states’ regulated:
  • Chemically (‘ligand’)-gated channels open when a signal molecule binds to the channel protein (e.g., ACh).

Chemically-Gated ('Ligand-Gated') Ion Channels

• Ligand-gated channel closed. Chemical stimulus opens the channel.

Regulated Open States of Ion Channels (cont.)

• Mechanically-gated channels open when the membrane gets stretched.

Mechanically-Gated Ion Channels

• Mechanically-gated channel closed. Mechanical stimulus opens the channel.

Regulated Open States of Ion Channels (cont.)

• Voltage-gated channels open when the membrane potential gets less negative (‘depolarized’).

Voltage-Gated Ion Channels

• Voltage-gated K+ channel closed. Change in membrane potential opens the channel.

Neuronal Action Potential

• Neurons are ‘excitable cells’ that can change membrane potential to generate an electrical signal (Muscle cells are also excitable).
• Principal mechanism of generating an electrical signal: Opening/closing of voltage-gated Na+ channels.

Ion Channels in Neuron Membranes

• Ligand-gated and mechanically-gated ion channels are found in dendrites and cell body and open/close in response to binding of neurotransmitter or stretch.
• Voltage-gated ion channels are found in axon, axon branches, and synaptic end bulbs and open/close in response to membrane depolarization.

Neuronal Action Potential

• Membrane potential changes over time during an action potential, including:
  • Resting membrane potential
  • Stimulus leading to depolarization
  • Depolarizing phase
  • Reversal of polarization
  • Repolarizing phase
  • After-hyperpolarizing phase
  • Threshold

Generating an Action Potential: 4 Steps

1. Local depolarization
2. Depolarization to threshold  voltage-gated Na+ channels open
3. Voltage-gated Na+ channels close
4. Voltage-gated K+ channels open  hyperpolarization

Generating an Action Potential Step 1: Local Depolarization

• Graded potentials involve local changes in the cell’s membrane potential.
  • Occur in dendrites and cell body.
  • Size varies with strength of stimulus.
  • Generated by chemically-gated and mechanically-gated channels.

Graded Potentials

• Two types:
  • Inhibitory postsynaptic potential (IPSP): hyperpolarizing (more negative)
  • Excitatory postsynaptic potential (EPSP): depolarizing (less negative)
• An action potential begins with a local depolarization.

Generating an Action Potential Step 2: Depolarization to Threshold

• Graded potentials produce enough depolarization to reach threshold  opening of voltage-gated Na+ channels.
• Occurs at the axon hillock.
• Results in increased Na+ permeability…

Depolarization to Threshold: Positive Feedback Cycle

• Increased Na+ permeability leads to Na+ entering the cell, carrying positive charge.
• Inside of cell gets more positive (more depolarization).
• More Na+ channels open.

Generating an Action Potential Step 2: Depolarization to Threshold (cont.)

• Once step 2 is reached, there is no turning back.
• Action potentials are all-or-none – if they start, they will travel all the way to the end of the axon.

Generating an Action Potential Step 3: Na+ Channels Close

• Shortly after voltage-gated Na+ channels open, they spontaneously close (inactivate).

Generating an Action Potential Step 4: K+ Channels Open

• Depolarization also opens voltage-gated K+ channels.
  • These open slower than Na+ channels.
• Inactivation of voltage-gated Na+ channels AND activation of voltage-gated K+ channels  ‘repolarization’ of PD back toward resting value.

The After-Hyperpolarizing Phase

• The after-hyperpolarizing phase occurs after repolarization but before returning to resting PD.
• The cell ‘overshoots’ resting PD (gets more negative).

Summary of Voltage-Gated Ion Channel Opening and Closing

• Diagram showing the states of Na+ and K+ channels during an action potential, including:
  • Resting state
  • Depolarization
  • Repolarization

Graded Potentials vs. Action Potentials

Refractory Periods

• Refractory period = a period during which it is difficult or impossible to generate a second action potential.
• Two types:
  1. Absolute Refractory Period:
     • Immediately following inactivation of voltage-gated Na+ channels
     • Membrane cannot be re-stimulated to produce another action potential
     • Involves ‘resetting’ of voltage-gated Na+ channels
  2. Relative Refractory Period:
     • A new action potential can be produced, but doing so requires a larger than normal stimulation
     • Involves ‘resetting’ of voltage-gated K+ channels

Absolute vs. Relative Refractory Period

• Diagram illustrating the absolute and relative refractory periods in relation to an action potential.

Importance of Refractory Periods

1. Establishes maximum rate (frequency) of action potentials.
2. Influences the characteristics of action potential propagation.
   • Ensures forward propagation (axon hillock  axon terminal).

Suggested Learning Activities

• Draw a cell and illustrate the relative concentrations of Na+ and K+ inside and outside of the cell at rest.
• Add the Na/K ATPase and draw the direction of ion movement through this pump.
• Add the chemical and electrical gradients for Na and K at rest.
• How would the electrical and chemical gradients change if the cell membrane was positive inside?
• Draw an action potential, label the different phases, and list the specific channels that are responsible for each phase.
• Create a table to illustrate the difference between action potentials and graded potentials.