Gated Ion Channels:- Neurons Have Receptor Channels That Respond to Different Neurotransmitters
Animal cells, especially those of the nervous system, contain a variety of ion channels gated by ligands, voltage, or both. The neurotransmitters 5-hydroxytryptamine (serotonin), glutamate, and glycine can all act through receptor channels that are structurally related to the acetylcholine receptor. Serotonin and glutamate trigger the opening of cation (K+, Na+, Ca2+) channels, whereas glycine opens Cl-specific channels. Cation and anion channels are distinguished by subtle differences in the amino acid residues that line the hydrophilic channel. Cation channels have negatively charged Glu and Asp side chains at crucial positions. When a few of these acidic residues are experimentally replaced with basic residues, the cation channel is converted to an anion channel. Depending on which ion passes through a channel, the ligand (neurotransmitter) for that channel either de polarizes or hyperpolarizes the target cell. A single neuron normally receives input from several (or many) other neurons, each releasing its own characteristic neurotransmitter with its characteristic depolarizing or hyperpolarizing effect. The target cell’s Vm therefore reflects the integrated input (Fig. 12–1d) from multiple neurons. The cell responds with an action potential only if the integrated input adds up to a net depolarization of sufficient size. The receptor channels for acetylcholine, glycine, glutamate, and -aminobutyric acid (GABA) are gated by extracellular ligands. Intracellular second messengers—such as cAMP, cGMP (3,5-cyclic GMP, a close analog of cAMP), IP3 (inositol 1,4,5-trisphosphate), Ca2+, and ATP—regulate ion channels of another class, which, as we shall see in Section 12.7, participate in the sensory transductions of vision, olfaction, and gustation.
