Resting potentials and action potentials

Abstract

Synergy between the body's various organs and tissues requires a high degree of coordination and rapid communication between cells across long distances. Communication between cells, or cell signaling, occurs by way of electrochemical signals transmitted by charged ions, the distribution of which along the cell membrane is subject to a sensitive balance. This is referred to as electrogenic transport, as opposed to electroneutral transport, which involves the transport of uncharged particles.

The distribution of charged ions along the cell membrane results in a disparity between the electrical potential on the inside of the cell and the space that surrounds it. This is referred to as membrane potential. In its quiescent state, the voltage difference across the cell membrane requires energy to uphold. This is referred to as a resting potential. The transport of charged particles through the cell membrane leads to a change in potential along the membrane and forms the basis for the transmission of information. This is referred to as an action potential.

Distribution of charge inside and outside the cell

Differences in the concentration of cations and anions inside and outside the cell lead to distinct membrane potentials (see also section on “Electrolytes” in laboratory medicine). Besides electrolytes, negatively charged particles (especially those within the cell) also affect charge distribution. The following table lists the concentrations of the most important intracellular and extracellular cations based on an idealized model of a neuron.

Cation Extracellular concentration Intracellular concentration
Na+
  • Approx. 140 mmol/L
  • Approx. 10 mmol/L
K+
  • Approx. 5 mmol/L
  • Approx. 150 mmol/L
Ca2+
  • Approx. 2 mmol/L
  • Approx. 10-5 mmol/L
H+
  • pH = 7,4
  • pH = 7,0
Cl-
  • Approx. 105 mmol/L
  • Approx. 7 mmol/L
Protein anions
  • Approx. 5 mmol/L
  • Approx. 155 mmol/L

Chemical fundamentals of conduction

Ion solutions conduct electricity. The speed of ion transport is determined by the strength of the electric field. Electric field strength is determined by ionic strength, which is a measure of the concentration of ions and their charge in a solution.

  • Calculating membrane potentials: A concentration gradient of ions along a membrane leads to a difference in charges between the inside and the outside of the cell. This results in a buildup of voltage (potential). The potential is calculated as follows:
    • Nernst equation: determines the electric equilibrium potential of a cell membrane with respect to a specific type of ion
      • Formula: Eion = 60/z log [ion]out/[ion]in
        • Eion = equilibrium potential, z = absolute value of ionic charge, [ion]out = ion concentration outside cell, [ion]in = ion concentration inside cell
  • Ion flux: rate of ion flow. Depends on the ability of ions to cross the membrane and the potential difference on both sides of the membrane, which acts as a driving force.
    • Formula: ion flux J = membrane conductivity Λ × potential difference ΔU

Physical fundamentals of conduction

Basic principles

  • Depolarization of the membrane: The influx of cations results in an increased positive charge on the inside of a cell
  • Electrotonic conduction
    • Depolarization of one section of the membrane automatically leads to the depolarization of neighboring sections (passive).
    • This effect depends on membrane resistance and the shape of the electrical conductor (axon diameter). Describing electrotonic conduction requires consideration of these two aspects.

Physical laws governing nervous conduction along the cell membrane

The laws are based on cable theory.

  • Axial resistance (ri)
    • Definition: resistance of cytosol to the forward movement of charged particles
    • Formula: ri = ρ × l / A
      • ri = resistance, ρ = specific resistance, l = length, A = surface of section
      • Measure: Ω (ohm)
    • Effect on conduction: the lower the axial resistance, the better and faster the conduction
    • Influence of nerve fiber thickness: the thicker a nerve fiber, the lower the axial resistance (Hagen-Poiseuille equation)
  • Membrane resistance (rM)
    • Definition: resistance of the cell membrane to the passage of charge carriers
    • Effect on conduction: The higher the membrane resistance, the better the conduction (as fewer charge carriers are lost).
    • Influence of nerve fiber thickness: The thicker a nerve fiber, the lower the membrane resistance.
  • Membrane capacitance (CM)
    • Definition: Membranes of nerve fibers function as a sort of capacitor and can take up a certain amount of electric charge that is not passed on.
    • Effect on conduction: the higher the membrane capacitance, the worse the conduction
    • Influence of nerve fiber thickness: the thicker a nerve fiber, the greater the membrane capacitance (as this also entails an increase in the surface area of nerve fibers, on which voltage can build up)

Membrane resistance and capacitance are inversely related (the higher the resistance, the lower the capacitance)!

Nerve conduction velocity is higher in thick nerve fibers than in thin ones, because the reduction of axial resistance has a greater effect on conduction than increased membrane capacitance!References:[1]

Resting potential

Resting potential (RP) is the membrane potential of an excitable cell (e.g., a neuron or muscle cell) at rest. It can be described as the default state of the cell and corresponds more or less to the sum of all diffusion potentials (i.e., potentials in dynamic equilibrium) of extracellular and intracellular ions. The resting potential depends on the type of cell and can range from approx. -70 mV to -90 mV.

  • Brownian motion: the random motion of particles that makes diffusion possible
  • Semipermeable membrane: The availability of ion canals with selective transport allows K+ ions (in some cases also Cl- ions) to pass easily through the cell membrane at a resting state and makes it more difficult for Na+ ions to pass through.
  • Na+/K+-ATPase and ion channels: maintain resting potential
    • K+ concentration: intracellular > extracellular
    • Na+ concentration: extracellular > intracellular

The RP corresponds more or less to the K+ equilibrium potential, which varies according to the ion channels present in the cell: neurons -70 mV, myocytes (skeletal and cardiac) ≈ -90 mV, glial cells ≈ -90 mV.

Action potential

An action potential (AP) is a sequence of changes in the voltage-gated ion channels in a neuronal cell membrane that generate a nerve impulse.

Phases

Phase Description Cellular Processes Potential
Resting potential
  • Default excitable state
  • Dynamic equilibrium of K+ and Na+
    • High extracellular concentration of Na+
    • High intracellular concentration of K+

-70 mV to -90mV

(inside -/ outside +)

Threshold potential
  • The critical electrical potential that must be reached by a stimulus to generate an AP
  • The all-or-none law
-70 mV to -50 mV
Depolarization
  • Conversion of the impulse to a transmittable electrochemical potential
  • The steeply rising portion of the potential energy curve (upstroke) from the threshold (approx. -70 to -50 mV) to the peak (approx. +30 mV)
  • Primarily driven by sodium cations (Na+)
    • Massive influx of Na+ ions through voltage-dependent Na+ channels
      • Membrane grows closer to the equilibrium potential for Na+(+60 mV)
    • Tetrodotoxin, a potentially-deadly neurotoxin present in pufferfish, blocks voltage-gated sodium channels, which inhibits action potential propagation → muscle weakness, paralysis, hypotension, and paresthesias
  • K+ channels almost entirely closed
> -50 mV
Peak
  • Voltage reaches a maximum
  • Point at which it is most like that the Na+ channels are open and the membrane potential is positive
  • At the apex of the curve, many Na+ channels become inactive again
  • K+ channels almost entirely closed

+30 mV

(inside +/ outside ‑)

Repolarization
  • Return to default state
  • Primarily driven by potassium ions (K+)
    • Voltage-dependent K+ channels open
    • Beginning of K+ efflux
  • Deactivation of voltage-dependent Na+ channels
    • Decreased Na+ influx
+30 mV to -90 mV
Afterhyperpolarization
  • After repolarization, some K+ channels remain open
    • Caused by increased activity of the K+ channels, which is due to the influx of Ca2+ during polarization; Ca2+ is only slowly removed
    • K+ efflux continues and the equilibrium potential of K+ is approached.
  • Na+ channels go from being closed and non-activatable to closed and activatable
∼ -90 mV (inside -/ outside +)
Refractory period
  • Time after depolarization in which the cell cannot be excited at all or only weakly
    • Regulation of the firing rate
    • Prevents impulses from simply returning

-70 mV

(inside -/ outside +)

Uncontrolled ion flow that is solely due to depletion of the concentration gradient is known as a leak current.

Duration of action potentials

The duration of an action potential is dependent on cell type.

Cell type Duration of AP
Neurons ∼1 ms
Skeletal muscle cells ∼10 ms
Cardiac muscle cells ∼300 ms

Control of action potentials

The membrane potential reached during depolarization does not reflect the stimulus intensity. Electrochemical impulse conduction is subject to the all-or-none principle: depolarization of the membrane is either triggered by an action potential or not. The stimulus intensity is only evident in the frequency of consecutive action potentials that follow.

Conduction of action potentials

The occurrence of an action potential through the opening of the ion channels and the resulting depolarization are local events. In order for an action potential to function as an impulse for communication, it must be directed, i.e., passed on in one direction. Neural backpropagation is usually prevented by the refractory period of the responsible Na+ channels. Stimulus conduction either ensues rapidly (along the myelinated nerve fibers via saltatory conduction) or slowly along the non-myelinated nerve fibers via continuous conduction.

The transfer speed along a nerve is above all dependent on the electrical insulation provided by myelin sheaths: The thicker the myelin sheath, the faster the conduction of the stimulus!

Electrochemical communication