An ion channel is a protein that forms a pore across a biological membrane, allowing particular ions to pass between compartments. Channels in the cell membrane connect the interior of a cell with its surroundings; others operate in intracellular membranes. Their activity underlies electrical signaling, muscle contraction, and many secretory processes. Unlike pumps, channels conduct ions passively: net movement follows the combined influence of concentration differences and electrical forces rather than driving ions against that gradient. (ncbi.nlm.nih.gov)
Structure and ion selectivity
Ion channels contain membrane-spanning regions surrounding an aqueous pathway. Their architecture varies: a functional channel may consist of several protein subunits or multiple repeated domains within one large protein. Many potassium channels contain four pore-forming subunits, whereas voltage-gated sodium and calcium channels have four homologous domains in a principal subunit. Additional associated proteins can modify channel behavior. (pubmed.ncbi.nlm.nih.gov)
The pore is not simply a hole of a particular diameter. Its narrowest region, often called the selectivity filter, creates a chemical environment that favors some ions over others. Selectivity depends on pore geometry, the arrangement of amino acid groups, electrical interactions, and the energetic cost of changing an ion’s surrounding water shell. In potassium channels, oxygen atoms in the filter coordinate potassium ions and replace interactions normally supplied by water. This helps explain why these channels discriminate against sodium despite sodium’s smaller ionic size. (nobelprize.org)
Selectivity varies substantially among families. Some channels strongly favor potassium, sodium, calcium, or chloride; others conduct several positively charged ions. An open channel can transmit millions of ions per second, making channels especially effective for rapid changes in membrane electrical properties. (ncbi.nlm.nih.gov)
Gating and classification
Gating is the transition between conducting and nonconducting conformations. Channels fluctuate between states, and a stimulus usually changes the probability of opening rather than forcing every channel into the same state. Some channels also enter an inactivated state, in which conduction stops even though the activating stimulus remains present. (ncbi.nlm.nih.gov)
Channels are commonly classified by their principal gating mechanism:
- Voltage-gated channels respond to changes in membrane potential. Charged regions within the protein act as voltage sensors and couple their movement to pore opening.
- Ligand-gated channels respond to binding of a ligand. At many chemical synapses, a neurotransmitter binds directly to a channel receptor and changes its conductance.
- Mechanosensitive channels respond to mechanical forces acting on the membrane or associated structures. PIEZO channels are examples involved in touch and body-position sensing.
- Temperature-sensitive channels respond to thermal conditions. Members of the transient receptor potential family include channels associated with heat or cold detection. (ncbi.nlm.nih.gov)
These categories overlap. A channel may respond to several inputs, such as voltage and intracellular calcium. “Leak” channels provide background conductance, but the term does not necessarily imply that their activity is entirely unregulated. (ncbi.nlm.nih.gov)
Electrochemical driving forces
Ion movement through a channel follows an electrochemical gradient, which combines a concentration-dependent component with the effect of voltage on electric charge. Consequently, an ion can move against its concentration gradient if the electrical force is sufficiently strong in the opposite direction. Channels provide a route for passive diffusion; pumps and coupled transporters establish or maintain the gradients that make this movement possible. (ncbi.nlm.nih.gov)
For a membrane permeable to one ionic species, the Nernst equation gives the voltage at which its chemical and electrical driving forces balance. At this equilibrium potential, individual ions may still cross in both directions, but their net flux is zero. Opening a selective channel therefore tends to move membrane voltage toward the equilibrium potential of the ions it conducts. The resulting current depends on both conductance and driving force, not merely on whether the channel is open. (ncbi.nlm.nih.gov)
Physiological functions
In a neuron, coordinated sodium and potassium channel activity generates and propagates an action potential. Sodium entry can produce rapid depolarization, while sodium-channel inactivation and increased potassium conductance help restore a more negative voltage. At a synapse, calcium entry into the presynaptic terminal triggers neurotransmitter release; transmitter-gated channels then alter the receiving cell’s electrical properties. (ncbi.nlm.nih.gov)
Channels also connect electrical activity with muscle contraction and control the release of signaling substances. Calcium channels in intracellular membranes release stored calcium, allowing changes in cytoplasmic calcium concentration to regulate cellular responses. These functions demonstrate that ion channels are not restricted to nerve signaling. (pdb101.rcsb.org)
Sensory channels convert physical or chemical stimuli into electrical signals. TRPV1 responds to capsaicin and noxious heat, TRPM8 participates in cold sensing, and PIEZO2 contributes to touch and proprioception. Different gating mechanisms thus provide molecular links between environmental stimuli and nervous-system activity. (nobelprize.org)
Experimental study and medical relevance
Patch-clamp recording uses a fine glass pipette sealed against a membrane to measure electrical currents, including currents through individual channels. Related voltage-clamp methods control membrane voltage while recording current, helping separate voltage-dependent gating from ionic driving forces. Structural methods, including X-ray crystallography and cryo-electron microscopy, reveal pores, selectivity filters, and conformational changes. (ncbi.nlm.nih.gov)
A mutation can impair ion-channel function by altering gating, conduction, protein folding, or delivery to the membrane. Disorders arising from channel dysfunction are termed channelopathies. A well-established example is cystic fibrosis, in which defects in the CFTR chloride channel disrupt epithelial ion transport. Certain CFTR variants also destabilize the protein, reducing the amount of functional channel available at the cell surface. (ncbi.nlm.nih.gov)