Certaincells, commonly calledexcitable cells,are unique because of their ability to generate electrical signals. Althoughseveral types of excitable cells exist — including neurons, muscle cells, andtouch receptor cells — all of them useion channel receptors to convert chemicalor mechanical messages into electrical signals.

Ion channel receptors are usually multimeric proteins located in the plasma membrane. Each of these proteins arranges itself so that it forms a passageway or pore extending from one side of the membrane to the other. These passageways, orion channels, have the ability to open and close in response to chemical or mechanical signals. When an ion channel is open, ions move into or out of the cell in single-file fashion. Individual ion channels are specific to particular ions, meaning that they usually allow only a single type of ion to pass through them. Both the amino acids that line a channel and the physical width of the channel determine which ions are able to wiggle through from the cell exterior to its interior, and vice versa. The opening of an ion channel is a fleeting event. Within a few milliseconds of opening, most ion channels close and enter a resting state, where they are unresponsive to signals for a short period of time (Figure 1).

Figure 1: An example of ion channel receptor activation

An acetylcholine receptor (green) forms a gated ion channel in the plasma membrane. This receptor is a membrane protein with an aqueous pore, meaning it allows soluble materials to travel across the plasma membrane when open. When no external signal is present, the pore is closed (center). When acetylcholine molecules (blue) bind to the receptor, this triggers a conformational change that opens the aqueous pore and allows ions (red) to flow into the cell.

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Figure 2: Comparing the activation of an ion channel receptor with that of a G-protein-coupled receptor

Activation of both a G-protein-coupled receptor (a) and an ion channel receptor (b) cause a conformational change in the receptor protein. G protein activation can lead to multiple intracellular events through a variety of intracellular proteins, and this signaling can take seconds to minutes. When a G protein activates transcription, this can take up to 20 minutes. In contrast, ion channel receptors open pores in the cell membrane, causing the formation of electrical current. This receptor activation therefore causes a much faster response within the cell, on the order of milliseconds.

© 2008Nature Publishing Group Moreau, C. J.et al. Coupling ion channels to receptors for biomolecule sensing.Nature Nanotechnology3, 620-625 (2008) doi:10.1038/nnano.2008.242. All rights reserved.

Figure Detail

Electrical signals travel much more rapidly than chemical signals, which depend on the process of molecular diffusion. As a consequence, excitable cells respond to signals much more rapidly than cells that rely solely on chemical signals (Figure 2). In fact, an electrical signal can traverse the entire length of a human nerve cell — a distance of as much as one meter — within only milliseconds.

Neurons,muscle cells, and touch receptor cells are all excitable cells — which means theyall have the capacity to transmit electrical signals. Each of these cells alsohas ion channel receptors clustered on a particular part of its surface. Forexample, the receptors that respond to chemical signals are generallylocated atsynapses— or points ofnear contact between adjacent cells.

Ofthe various types of excitable cells that respond to chemical signals, neuronsare perhaps the most familiar. When electrical signals reach the end ofneurons, they trigger the release of chemical messengers calledneurotransmitters. Each neurotransmitter then diffuses from its point ofrelease on one side of the synapse to the cell on the other side of thesynapse. If the neurotransmitter binds to an ion channel receptor on the targetcell, the related ion channel opens, and an electrical signal propagates itselfalong the length of the target cell.

Neuronshave ion channel receptors specific to many kinds of neurotransmitters. Some ofthese neurotransmitters act in an excitatory capacity, bringing their targetcells ever closer to signal propagation. Other neurotransmitters exert aninhibitory effect, counteracting any excitatory input and lessening the chancethat the target cell will fire.

Skeletalmuscle cells also rely on chemical signals in order to generate electricalsignals. These cells have synapses that are packed with receptors foracetylcholine, which is the primaryneurotransmitter released by motor neurons. When acetylcholine binds to thereceptors on a skeletal muscle cell, ion channels in that cell open, and thislaunches a sequence of events that results in contraction of the cell.

Incontrast to neurons and skeletal muscle cells, some excitable cells have ionchannels that open in response to mechanical stimuli rather than chemicalsignals. These include the hair cells of the mammalian inner ear and the touchreceptor cells of both human finger pads and Venus fly traps. Cells thatrespond to touch have their ion channel receptors clustered at the position wherecontact usually occurs.