Ligand-gated Na+ channel

Many epithelia (e.g. kidney tubules, colon, lungs, trachea and sweat ducts) have ligand-gated Na" channels. The channels are regulated by hormones. Stretch-gated Na" channels have been foimd in muscles in mammals. 

Synaptic Transmission. Figure 1 Synaptic transmission. The presynaptic terminal contains voltage-dependent Na Superscript and Ca2+ channels, vesicles with a vesicular neurotransmitter transporter VNT, a plasmalemmal neurotransmitter transporter PNT, and a presynaptic G protein-coupled receptor GPCR with its G protein and its effector E the inset also shows the vesicular H+ pump. The postsynaptic cell contains two ligand-gated ion channels LGIC, one for Na+ and K+ and one for Cl-, a postsynaptic GPRC, and a PNT. In this synapse, released transmitter is inactivated by uptake into cells. 

HT3 Depolarisation Ligand-gated Na+/K+ channel 2-methyl 5-HT Ondansetron Postsynaptic and... 

Figure 2.8 A ligand-gated ion channel receptor. ACh = acetylcholine. The binding of ACh to the a subunits opens the ion channel, allowing Na ions to flow through the channel into the cell.

An example of a ligand-gated ion channel (B) is the nicotinic cholinocep-tor of the motor endplate. The receptor complex consists of five subunits, each of which contains four transmembrane domains. Simultaneous binding of two acetylcholine (ACh) molecules to the two a-subunits results in opening of the ion channel, with entry of Na+ (and exit of some 1<+), membrane depolarization, and triggering of an action potential (p. 

Ionotropic receptors (bottom left) are ligand-gated ion channels. When they open as a result of the transmitter s influence, ions flow in due to the membrane potential (see p. 126). If the inflowing ions are cations (Na"", C, Ca ""), depolarization of the membrane occurs and an action potential is triggered on the surface of the postsynaptic cell. This is the way in which stimulatory transmitters work (e.g., acetylcholine and glutamate). By contrast, if anions flow in (mainly Cl ), the result is hyperpolarization of the postsynaptic membrane, which makes the production of a postsynaptic action potential more dif cult. The action of inhibitory transmitters such as glycine and GABA is based on this effect. 

Ionotropic receptors are ligand-gated ion channels (left half of the table). The receptors for stimulatory transmitters (indicated in the table by a ) mediate the inflow of cations (mainly Na""). When these open after binding of the transmitter, local depolarization of the postsynaptic membrane occurs. By contrast, inhibitory neurotransmitters (GABA and glycine) allow cr to flow in. This increases the membrane s negative resting potential and hinders the action of stimulatory transmitters hyperpolarization, 0). 

Ion channels (center). These receptors contain ligand-gated ion channels. Binding of the signaling substance opens the channels for ions such as Na, Ca, and Cl . This mechanism is mainly used by neurotransmitters such as acetylcholine (nicotinic receptor see p.224) and GABA (A receptor see p.354). 

Acetylcholine (ACh) is an example of an endogenous neurotransmitter that binds to more than one receptor type, the nicotinic acetylcholine receptor (nAChR) which preferentially binds nicotine and the muscarinic receptor which binds muscarine, a mushroom alkaloid. The latter is a G protein-coupled receptor while the nACh receptor is an excitatory ligand-gated ion channel that transports Na-i- ions. Nicotinic cholinergic receptors are found in the CNS, autonomic ganglia, and at the neuromuscular junction of skeletal muscles. They are a possible target for anaesthetics. 

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