Receptors
The plasma membranes of cells contain specialized proteins called receptors. Receptors allow cells to receive chemical signals that direct cellular function and activity. These chemical signals, also called signaling molecules, include hormones, neurotransmitters, and local mediators. Signaling molecules have the ability to bind to receptors and trigger a series of biochemical events in a cell's interior.
There are hundreds of different signaling molecules, each of which will only bind to a certain receptor. Neurotransmitters are the chemicals used by the nervous system to transmit nerve impulses to and from the brain. Hormones are released into the bloodstream by specialized tissues called glands. They can influence cellular events throughout the body. On a more local level, local mediators are chemicals released by many cells types to influence other cells that are nearby.
The relationship between signaling molecules and their receptors is very specific. Each receptor on a cell's surface is made to accept only one signaling molecule. However, one cell may have many different receptors on its surface. As a result, the function and activity of that cell can be influenced by several different signaling molecules.
Although there are many different receptors, they can be divided into three main categories based on their underlying molecular machinery. The three classes of receptors are ion channel receptors, receptors linked to G protein, and catalytic receptors. In general, all three types feature receptors that are embedded in a cell's plasma membrane. Each of these receptors are in contact with the cell's interior as well as its external environment.
Ion channel receptors are typically found in connection with the nervous system and provide sites for neurotransmitter binding. The action of a common neurotransmitter, such as acetylcholine, illustrates how ion channel receptors work. Acetylcholine receptors are found on neurons (nerve cells) and on muscle cells. When acetylcholine binds to its receptor, it causes the cell membrane to depolarize. Depolarization means that the membrane allows charged particles, or ions, to flow into or out of the cell. This ion flow triggers a cellular response, such as continuation of a nerve impulse to another neuron or the contraction of a muscle cell.
G protein receptors are the most diverse category of receptors. These receptors are found in cells throughout the body and are used by neurotransmitters, hormones, and local mediators. The key feature of G protein receptors is that once the receptor binds a signaling molecule, it triggers the formation of a second messenger. Common second messengers include cyclic adenosine monophosphate (cAMP), inositol triphosphate, and diacylglycerol. In effect, these second messengers convey the signaling molecule's message to the rest of the cell. Second messengers may activate or deactivate enzymes within a cell or may help to control the cellular responses to other signaling molecules. The eventual response to the signaling molecule depends on the cell type and the function it is expected to perform.
The third category of receptor is the catalytic receptor. These receptors function as enzymes. When a catalytic receptor is occupied by a signaling molecule, a chemical reaction is triggered within the cell. A common reaction is the addition of a phosphate group to another enzyme that is inside the cell. The addition of a phosphate group--phosphorylation--is one method in which an enzyme can be switched from its inactive form to its active form. An inactive enzyme is nonfunctional; an active enzyme is able to trigger specific chemical reactions. The reactions unleashed within a cell correspond with its response to the signaling molecule.
Once a receptor has been activated there are several ways in which the cellular response comes to an end. The simplest method relies on the signaling molecule detaching from the receptor. Once the receptor is no longer occupied, the response mechanism is essentially switched off. Another method revolves around the receptor becoming less sensitive to the signaling molecule. In this case, once the cellular response has been achieved, further signals have little or no effect. More drastic desensitization can be accomplished by reducing the number of receptors in the plasma membrane. Receptor down-regulation is accomplished by drawing receptors that have bound signaling molecules into the cell's interior. Once inside, the receptors are snipped from the plasma membrane and destroyed.
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