Table of Contents
What “Excitation” Means in Biology
In biology, excitation is a rapid, short-lived change in the state of a cell triggered by a stimulus. It is especially important in nerve and muscle cells, but the basic principles are physical and chemical:
- Cells maintain different ion concentrations inside and outside.
- This leads to an electrical voltage across the cell membrane (the membrane potential).
- If a stimulus is strong enough, this voltage briefly changes in a characteristic way.
- This change can travel along a cell (e.g., along a nerve fiber) and be passed to another cell.
This chapter covers the general physical and chemical basis of excitation. How plants, animals, sense organs, or whole nervous systems use these principles is treated in other chapters.
The Cell Membrane as an Excitable Boundary
Selective permeability
Every cell is surrounded by a membrane made of a lipid bilayer with embedded proteins. For excitation, two properties are crucial:
- The membrane is impermeable to ions (charged particles like Na\^+, K\^+, Cl\^−) on its own.
- Special membrane proteins control ion movement:
- Ion channels – pores that open for specific ions.
- Transporters and pumps – proteins that move ions, often using energy.
Because of this, ion concentrations on both sides of the membrane can be kept very different.
Main ions involved
In most animal cells (values vary by cell type and species):
- Inside the cell (intracellular):
- High concentration of K\^+ (potassium)
- Many negatively charged proteins and organic anions (A\^−)
- Relatively low Na\^+ (sodium) and Cl\^−
- Outside the cell (extracellular):
- High Na\^+ and Cl\^−
- Lower K\^+
This asymmetry is the basis for voltage differences across the membrane.
Electric Potential and Resting Membrane Potential
Charge separation and voltage
Because different ions cannot freely cross the membrane, charges are unequally distributed:
- The inside of the cell is slightly negative relative to the outside.
- This charge difference creates an electrical voltage called the membrane potential:
$$V_\text{m} = V_\text{inside} - V_\text{outside}$$
When the cell is not excited, this is the resting membrane potential.
Typical values:
- Nerve and muscle cells: about $-60$ to $-80\ \text{mV}$.
- Many other cells: often somewhat less negative, but still below $0\ \text{mV}$.
Forces acting on ions
Two forces act on each ion type:
- Concentration gradient – drives diffusion from high to low concentration.
- Electrical gradient – opposite charges attract, like charges repel.
Together, they form the electrochemical gradient. At rest, some ions are close to a balance of these forces, others are kept away from their preferred balance by pumps.
The Nernst potential (equilibrium potential)
For an ion that can pass through the membrane and reach equilibrium between chemical and electrical driving forces, the resulting voltage is described by the Nernst equation:
$$
E_\text{ion} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_\text{outside}}{[\text{ion}]_\text{inside}} \right)
$$
Where:
- $E_\text{ion}$ – equilibrium potential for that ion
- $R$ – gas constant
- $T$ – absolute temperature
- $z$ – ion charge number (e.g. $+1$ for K\^+, Na\^+)
- $F$ – Faraday constant
For K\^+ at body temperature, with typical concentration ratios, $E_\text{K}$ is around $-90\ \text{mV}$; for Na\^+, $E_\text{Na}$ is around $+60\ \text{mV}$.
The resting membrane potential lies between these values, closer to $E_\text{K}$ because the resting membrane is more permeable to K\^+ than to Na\^+.
(How these equations are used in detail and how multiple ions contribute is often described with the Goldman equation, which is treated elsewhere if needed.)
The sodium–potassium pump
To maintain the ion gradients over time, cells use the Na\^+/K\^+-ATPase (“sodium–potassium pump”):
- It uses energy from ATP.
- For each cycle it typically:
- Pumps 3 Na\^+ ions out of the cell.
- Pumps 2 K\^+ ions into the cell.
This has two effects:
- It maintains the concentration differences of Na\^+ and K\^+.
- It is slightly electrogenic (moves more positive charges out than in), contributing a small amount to the negative resting potential.
From Resting State to Excitation
Depolarization and hyperpolarization
Changes in membrane potential can be described as:
- Depolarization: the membrane potential becomes less negative (moves toward or above 0 mV).
- Hyperpolarization: the membrane potential becomes more negative than the resting potential.
- Repolarization: a return to the resting potential after a change.
In general:
- Depolarization often moves the cell toward excitation.
- Hyperpolarization typically makes excitation less likely (inhibitory).
Stimuli: physical and chemical
Cells can be stimulated in different ways:
- Physical stimuli:
- Electrical current (from an electrode, or another cell).
- Mechanical deformation (stretching, pressure).
- Temperature changes.
- Light (in photoreceptor cells).
- Chemical stimuli:
- Binding of signaling molecules (neurotransmitters, hormones).
- Changes in extracellular ion concentrations.
The common feature: they ultimately change the state of ion channels in the membrane.
Ion channels as molecular switches
Ion channels can be opened or closed under specific conditions:
- Voltage-gated channels – open or close when the membrane potential changes.
- Ligand-gated channels – open when a specific molecule (ligand) binds.
- Mechanically gated channels – open in response to mechanical force.
- Leak channels – are usually open, setting the resting permeability.
An opened channel allows its specific ion (e.g. K\^+, Na\^+, Ca\^{2+}, Cl\^−) to diffuse according to its electrochemical gradient, briefly changing the membrane potential.
Graded Potentials and Threshold
Excitation begins with local voltage changes:
- A stimulus opens a limited number of ion channels.
- The resulting current (ion flow) changes the membrane potential locally.
- The size of this change depends on stimulus strength and distance from the stimulus site.
These are called graded (local) potentials:
- They are continuous in size: weak stimulus → small change; strong stimulus → larger change.
- They decrease with distance as they spread along the membrane.
- They can add up in time and space:
- Several stimuli close in time can sum.
- Simultaneous stimuli at different sites can combine their effects.
If depolarization reaches a certain critical value, the threshold potential, a qualitatively different process can begin: the action potential. The detailed mechanism of action potentials and their conduction is treated in a later chapter; here the idea is:
- Subthreshold depolarization: no action potential occurs.
- Suprathreshold depolarization: an action potential is triggered.
This introduces the concept of an “all-or-none” event built on top of graded local changes.
Excitability and Refractory Periods
Excitable vs. non-excitable cells
Not all cells respond to stimuli in the same way:
- Excitable cells (e.g., nerve cells, muscle cells, some plant cells) can:
- Generate rapid, large changes in membrane potential (action potentials).
- Conduct these changes over longer distances.
- Non-excitable cells (e.g., many epithelial cells) still have membrane potentials and channels, but do not normally generate propagating action potentials.
The difference lies mainly in:
- The type and density of ion channels.
- How they respond to voltage changes.
Refractory periods (concept)
After an intense excitation (such as an action potential), excitable cells temporarily become less excitable:
- Absolute refractory period:
- No new action potential can be triggered.
- Relative refractory period:
- A stronger-than-normal stimulus is required.
This behavior arises from the conformational states of voltage-gated channels (open, closed, inactivated). It limits the frequency of excitation and is essential for directional propagation along nerve fibers; mechanistic details are explained in the chapter on conduction of excitation.
Ionic Basis of Inhibition
Not every change in membrane potential promotes excitation. Certain ion flows stabilize or deepen the resting potential:
- Opening of K\^+ channels typically leads to K\^+ leaving the cell:
- Inside becomes more negative → hyperpolarization.
- Opening of Cl\^− channels allows Cl\^− to enter (in many cells):
- Also makes the inside more negative → hyperpolarization.
Such changes make it harder for the cell to reach threshold. In networks of cells, this is the basis of inhibitory signals. The way one cell uses neurotransmitters to produce inhibition in another is addressed in later chapters on transmission between excitable cells.
Chemical Energy and Excitation
Maintaining the conditions for excitability is energetically costly:
- Ion pumps (especially Na\^+/K\^+-ATPase) consume ATP.
- Gradients of Na\^+, K\^+, Ca\^{2+} and others represent stored energy.
- When ion channels open during excitation, the cell temporarily allows this stored energy to be released as ionic currents and voltage changes.
Thus, excitation is an example of how biological systems convert chemical energy into electrical and mechanical processes at the cellular level, linking metabolism to information processing in nerves and muscles.
Overview: From Basic Physics to Biological Function
To summarize the biological basics of excitation:
- The cell membrane with its specific ion channels and pumps creates stable resting conditions (resting potential).
- Stimuli act by changing the state of these channels.
- Resulting ion flows create graded potentials, which can sum and, if strong enough, reach threshold.
- Above threshold, excitable cells generate all-or-none electrical events that can propagate and serve as signals.
- Refractory behavior and hyperpolarizing currents ensure that excitation is temporally and spatially controlled.
- Energy from ATP is continuously used to keep the system ready for the next excitation.
How these principles are adapted in different organisms, how excitation travels along cells, and how it is passed between cells are the subjects of the following chapters in this section.