Sodium and Other Ion Channels

¶ … sodium and other ion channels considered gated? Ions moving by facilitated diffusion can traverse the plasma membrane through channels created by proteins. These embedded transmembrane proteins allow the formation of a concentration gradient between the extracellular and intracellular contents. These ion channels are said to be 'gated' if they can be opened or closed. Ligand gated channels open or close in response to the binding of a small signaling molecule or ligand (Keramidas et al.). Some ion channels are gated by extra cellular ligands; some by intracellular ligands.

In both cases, the ligand is not the substance that is transported when the channel opens. The binding of neurotransmitter acetylcholine opens sodium channels in certain synapses. Voltage gated channels are found in neurons and muscle cells. They open or close in response to changes in the charge across the plasma membrane. For example, as an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane.

This allows the influx of sodium into the neuron and thus the continuation of the nerve impulse. Ion channels are highly specific filters, allowing only desired ions through the cell membrane. Voltage gated sodium channels are crucial for the propagation of action potentials in excitable membranes. They cause the cell membrane to depolarize by allowing the influx of sodium ions into the cell. However due to the size and hydrophobic nature of the channel protein, it has not been fully resolved by x-ray crystallography.

The secondary and some of the tertiary structures can only be deduced from the primary amino acid sequence. Voltage gated sodium channels consist of an ?-subunit responsible for selectivity and voltage gating (Errington, Stohr and Lees; Sands, Grottesi and Sansom). However some sodium channels also have one or two smaller subunits called ?-1 and ?-2. The protein has 4 homologous domains containing multiple potential ?-helical transmembrane.