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 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 ).
- 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.
|Ionic composition of the extracellular and intracellular fluid|
|Ions||Extracellular concentration||Intracellular concentration|
|Na+|| || |
|K+|| || |
|Ca2+|| || |
|H+|| || |
|Cl-|| || |
|Protein anions|| || |
Basics 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.
- Membrane potential: a concentration gradient determined by the presence of positively and negatively charged ions across the plasma membrane (i.e., inside and outside of the cell) that results in a buildup of voltage (potential)
- The 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: the rate of ion flow across the plasma membrane
- Dependent 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: J = Λ × ΔU
- J = ion flux
- Λ = membrane conductivity
- ΔU = potential difference
Physical fundamentals of conduction
- Depolarization of the membrane: The influx of cations results in an increased positive charge on the inside of a cell.
- Electrotonic conduction
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 ( ).
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).
Length constant (λ)
- Definition: A constant numerical value that quantifies the length that an electrical signal can travel along an axon before decaying.
- Effect on conduction: The higher the length constant, the further an electrical signal can propagate before decaying.
- Influence of nerve fiber: The presence of myelin significantly increases the length constant.
Membrane resistance and capacitance are inversely related (the higher the resistance, the lower the capacitance).
Large myelinated fibers have longer length constants and a faster conduction velocity than thin unmyelinated fibers.
Demyelinating diseases such as Guillain-Barré syndrome and multiple sclerosis lead to increased membrane capacitance, decreased membrane resistance, decreased conduction velocity, and a decreased length constant.
- Resting potential (RP): the membrane potential of an excitable cell (e.g., a neuron or muscle cell) at rest.
- 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.
- Ion distribution
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.
|Phases of action potential|
|Resting potential|| || || |
|Threshold potential|| |
|Depolarization|| || |
|Peak|| || || |
|Repolarization|| || || |
|Afterhyperpolarization|| || |
|Refractory period|| || |
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.
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.
- Stimulus intensity in relation to voltage
- Stimulus intensity in relation to current strength
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.
Continuous conduction: slow conduction of an impulse along a non-myelinated nerve fiber
- Triggering of an action potential: at an arbitrary area of the membrane where the threshold potential is exceeded
- Transmission of an action potential: Depolarization of the membrane ensures the opening of voltage-dependent Na+ channels in adjacent areas (depolarization migrates along the cell membrane).
Saltatory conduction: fast conduction of an impulse along the axons of myelinated nerve fibers
- Triggering of an action potential: only at non-myelinated areas of the membrane (nodes of Ranvier) where the threshold potential is exceeded
- Transmission of an action potential: Depolarization of a node of Ranvier leads to the opening of voltage-dependent Na+ channels at the next node of Ranvier (depolarization skips along the cell membrane).
- Two factors promote depolarization of the adjacent node of Ranvier:
- Shifts in the adjacent charge carriers lead to changes in the potential field.
- The insulation of certain areas of the membrane (internodes) prevents the passage of charges and in this way enables a continuous ion current to the next node of Ranvier.
- Two factors promote depolarization of the adjacent node of Ranvier: