What are ion channels?
Ion channels are transmembrane proteins that contain a specialized structure, called a pore, that permits particular ions to cross the neuronal membrane.
Some of the ion channels also contain other structures that are able to sense the electrical potential across the membrane. Such voltage-gated channels open or close in response to the magnitude of the membrane potential, allowing the membrane permeability to be regulated by changes in this potential.
Other types of ion channels are gated by extracellular chemical signals such as neurotransmitters, and some by intracellular signals such as second messengers.
Still others respond to mechanical stimuli, temperature changes, or a combination of such effects.
Voltage-Gated Ion Channels
Voltage-gated ion channels are selectively permeable to each of the major physiological ions—Na+, K+, Ca2+, and Cl–. Na+ channels, Ca2+ channels, K+ channels and Cl– channel have been discovered and all identified with multiple genes.
Ligand-Gated Ion Channels
Ligand-Gated Ion Channels are a type of ion channels respond to chemical signals (ligands) rather than to changes in the membrane potential.
The most important of these ligand-gated ion channels in the nervous system is the class activated by binding neurotransmitters. These channels are essential for synaptic transmission and other forms of cell-cell. Whereas the voltage-gated ion channels underlying the action potential typically allow only one type of ion to permeate, channels activated by extracellular ligands are usually less selective, allowing two or more types of ions to pass through the channel pore.
Other ligand-gated channels are sensitive to chemical signals arising within the cytoplasm of neurons, and can be selective for specific ions such as K+ or Cl–, or permeable to all physiological cations. Such channels are distinguised by ligand-binding domains on their intracellular surfaces that interact with second messengers such as Ca2+, the cyclic nucleotides cAMP and cGMP, or protons.
Stretch- and Heat-Activated Channels
Still other ion channels respond to heat or membrane deformation.
Heatactivated ion channels, such as some members of the transient receptor potential (TRP) gene family, contribute to the sensations of pain and temperature and help mediate inflammation. These channels are often specialized to detect specific temperature ranges, and some are even activated by cold.
Other ion channels respond to mechanical distortion of the plasma membrane and are the basis of stretch receptors and neuromuscular stretch reflexes. A specialized form of these channels enables hearing by allowing auditory hair cells to respond to sound waves.
• Purves D, Augustine G J, Fitzpatrick D, et al. Neuroscience, chapter 4[J]. 2004.