Figure 1. View of an ion channel within the cell membrane. The lipid bilayer is shown surrounding the ion channel protein complex. The location of the ion pore that spans the membrane is indicated.

Figure 2. The approximate time course and magnitude of the change in membrane potential in an excitable cell during an action potential. The value of the membrane potential is shown on the y-axis and the time on the x-axis. During the action potential the value of the cell membrane potential rapidly sweeps from –70 to +40 mV and then returns to –70 mV in about 1 ms. The relative permeabilities of the cell membrane to potassium (P_K) and sodium (P_Na) are indicated at the start, peak and end of the action potential.

Figure 3. Single-channel recording using the patch clamp technique. Examples of currents recorded from single voltage-gated calcium channels in a neuronal cell membrane are shown. The positions of closed [C] and open [O] levels are shown. The probability that a channel will be open increases as the membrane potential is depolarized because the channel is voltage-gated. The amount of current that flows through the channel is the same at a given membrane potential but decreases as the membrane potential is depolarized because the driving force on the ion decreases as it approaches its equilibrium potential.

Figure 4. Views of the three major classes of ion channels; tetrameric, pentameric and hexameric. In each case, the pore of the channel is located through the centre of the protein complex. Within each subunit the protein crosses the membrane 2–6 times forming transmembrane spanning regions.

Figure 5. Inside the pore of a  K⁺-selective ion channel. This diagram is based on the reported crystal structure of a  K⁺-selective ion channel (MacKinnon et al.). The two balls located within the narrow selectivity filter region of the pore represent two dehydrated  K⁺ions. The inner and outer vestibules of the pore are filled with water (1 Å=0.1 nm).

Figure 6. Architecture of five different transmembrane spanning patterns in individual ion channel subunits and domains. The vertical cylindrical structures represent the transmembrane alpha helices (TM) that span the lipid bilayer and shorter helices that contribute to the ion pore region (P). Six-TM one-pore domain channels include voltage-gated  Na⁺,  Ca²⁺ and  K⁺ channels, IP₃ receptors, cyclic nucleotide-gated and TRP channels. The fourth TM alpha helix (S4) of the domain is the voltage sensor, it contains a series of basic amino acids shown by the positive symbols. These domains associate as tetramers (Figure 4); inwardly rectifying  K⁺ channels comprised two-TM one-pore domains. These domains associate as tetramers: KCNK channels contain four-TM two-pore domains; ionotropic glutamate receptors contain three-TM one-pore domains that associate as tetramers; and nicotinic receptors, 5-HT3 and glycine channels four-TM one-pore domains that associate as pentamers.

Figure 7. One way to look at ion channel gating. In this model, the channel twists between a closed and open conformation. The physical gate regulating ion flux in  K⁺ selective ion channels lies deep in the pore beyond the narrow selectivity filter at the cytoplasmic face.