A rich variety of channels has been isolated and analyzed from a wide range of cell membranes. Invariably intrinsic proteins, they contain numerous\u00a0amino acid<\/a>\u00a0sequences that\u00a0traverse<\/a>\u00a0the membrane, clearly forming a specific hole, or pore. Certain channels open and close spontaneously. Some are gated, or opened, by the chemical action of a signaling substance such ascalcium<\/a>,\u00a0acetylcholine<\/a>, or\u00a0glycine<\/a>, whereas others are gated by changes in the electrical potential across the membrane. Channels may possess a narrow specificity, allowing passage of only\u00a0potassium<\/a>\u00a0or\u00a0sodium<\/a>, or a broad specificity, allowing passage of all positively charged ions (cations<\/a>) or of all negatively charged ions (anions<\/a>). There are channels called gap junctions that allow the passage of\u00a0<\/span>molecules<\/a>\u00a0between pairs of cells (see below<\/em>\u00a0The cell matrix and cell-to-cell communication<\/a>).<\/p>\n

The gating of channels with a capacity for\u00a0<\/span>ion<\/a>\u00a0transport is the basis of the many nerve-nerve, nerve-muscle, and nerve-gland interactions underlying neurobiological behaviour. These actions depend on the\u00a0electric potential<\/a>\u00a0of the cell membrane, which varies with the prevailing\u00a0constituents<\/a>\u00a0in the cell\u2019senvironment<\/a>. For example, if a channel that admits only potassium ions is present in a membrane separating two different potassium chloride solutions, the positively charged potassium ions tend to flow down their concentration gradient through the channel. The negatively charged chloride ions remain behind. This separation of electric charges sets up an electric potential across the membrane called the\u00a0diffusion<\/a>potential. The size of this potential depends on, among other factors, the difference in concentrations of the permeating ion across the membrane. The cell membrane in general contains the channels of widely different ion specificities, each channel contributing to the overall membrane potential according to the permeability and concentration ratio of the ion passing through it. Since the channels are often gated, the membrane\u2019s potential is determined by which channels are open; this in turn depends on the concentrations of signaling molecules and may change with time according to the membrane potential itself.<\/p>\n

Most cells have about a tenfold higher concentration of sodium ions outside than inside and a reverse concentration ratio of potassium ions. Free calcium ions can be 10,000 times more concentrated outside the cell than inside. Thus, sodium-, potassium-, and calcium-selective membrane channels, by allowing the diffusion of those ions past the cell membrane and causing fluctuations in the membrane\u2019s electric potential, frequently serve as transmitters of signals from nerve cells. Ion diffusion threatens to alter the concentration of ions necessary for the cell to function. The proper distribution of ions is restored by the action of ion pumps (see below<\/em>\u00a0Primary active transport<\/a>).<\/p>\n