The READ Part of the Signaling Machinery

The cell membrane serves as the interface between the outside and inside of the cell. Embedded in the cell membrane are a number of specialized proteins, known as ion channels, that enable inorganic ions such as sodium (Na+), potassium (K+), calcium (Ca2+), or chloride (Cl) to cross the cell membrane. Inorganic ions carry a positive or negative electric charge. As the ion channels transport the inorganic ions from outside to inside the cell, the accumulation of a specific type of inorganic ions inside the cell will change the membrane potential to reflect the charge of that ionic species: for example, an increase in chloride (Cl ) ions will result in a more negative membrane potential. This change in membrane potential will result in a physiological response from the cell. In another example, if all the sodium channels on the cell membrane open up all at once and allow sodium (Na+) ions to rush into the cell, the membrane potential will be driven toward a positive charge. In neurons, such an event will result in a nerve impulse or action potential propagating to other nerve cells.

Ion channels are characterized by two common properties: they are ion selective and they are gated. The first implies that sodium channels will only conduct sodium ions and no other ionic species across the cell membrane. The second means that, similar to a gate, they open or close briefly in response to a specific stimulus.

At the present time, ion channels can be classified into three general groups: voltage-gated channels, mechanically gated channels, and ligand-gated channels.

Voltage-gated channels open or close with a voltage change in the membrane potential. Thus, they transduce an electrical signal into a physiological response.

Mechanically gated channels are associated with mechanoreceptors, that is, receptors that transduce a mechanical stimulus into a physiological response. Hair cells in the cochlea of the inner ear are examples of mechanoreceptors (see Fig. 2.9). Mechanically gated channels thus open under the influence of a mechanical force to allow flow of inorganic ions.

Ligand-gated channels are further subdivided into nucleotide-gated, ion-gated, or transmitter-gated channels. These channels will open when their specific ligand (nucleotide, ion, or transmitter) binds to them. Although they convert a chemical signal (the ligand) into an electrical response (change in membrane potential due to ionic influx), they tend to be insensitive themselves to changes in the membrane potential. The size of the physiological response elicited by the ligand-gated channels depends largely on the amount and availability of the ligand. In this chapter, the emphasis will be placed on transmitter-gated channels since they are common in nerve cells and they are the targets for psychoactive drugs used in the treatment of neurological conditions.

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