Signal transduction is the transmission of molecular signals from a cell's exterior
to its interior. Molecular signals are transmitted between cells by the secretion
of hormones and other chemical factors, which are then picked up by different
cells. Sensory signals are also received from the environment, in the form
of light, taste, sound, smell, and touch. The ability of an organism to function
normally is dependent on all the cells of its different organs communicating
effectively with their surroundings. Once a cell picks up a hormonal or sensory
signal, it must transmit this information from the surface to the interior
parts of the cell, for example, to the nucleus. This occurs via signal transduction
pathways that are very specific, both in their activation and in their downstream
actions. Thus, the various organs in the body respond in an appropriate manner
and only to relevant signals.
All signals received by cells first interact with specialized proteins in the
cells called receptors, which are very specific to the signals they receive.
These signals can be in various forms. The most common are chemical signals,
which include all the hormones and neurotransmitters secreted within the body
as well as the sensory (external) signals of taste and smell. The internal
hormonal signals include steroid and peptide hormones, neurotransmitters,
and biogenic amines, all of which are released from specialized cells within
the various organs. The external signals of smell, which enter the nasal compartment
as gaseous chemicals, are dissolved in liquid and then picked up by specialized
receptors. Other external stimuli are first received by specialized receptors
(for example, light receptors in the eye and touch receptors in the skin),
which then convert the environmental signals into chemical ones, which are
then passed on to the brain in the form of electrical impulses.
Once a receptor has received a signal, it must transmit this information effectively into the cell. This is accomplished either by a series of biochemical changes within the cell or by modifying the membrane potential by the movement of ions into or out of the cell. Receptors that initiate biochemical changes can do so either directly via intrinsic enzymatic activities within the receptor by activating intracellular messenger molecules. Receptors may be broadly classified in four groups that differ in their mode of action and in the molecules that activate them.
The largest family of receptors are the G-protein-coupled receptors (GPCRs), which depend on guanosine triphosphate (GTP) for their function. Many neurotransmitters, hormones and small molecules bind to and activate specific Gprotein-coupled receptors.
A second family of membrane-bound receptors are the receptors for tyrosine kinases (RTKs). They function by phosphorylating themselves and recruiting downstream signaling components.
Ion channels are proteins open upon activation, thereby allowing the passage of ions across the membrane. lon channels are responsive to either ligands or to voltage changes across the membrane, depending on the type of channel. The movement of ions changes the membrane potential, which in turn changes cellular function.
Steroid receptors are located within the cell. They bind cell-permeable molecules such as steroids, thyroid hormone, and vitamin D. Once these receptors are activated by ligand, they translocate to the nucleus, where they bind specific DNA sequences to modulate gene expression.
The intracellular component of signal propagation, also known as signal transduction, is receptor-specific. A given receptor will activate only very specific sets of downstream signaling components, thereby maintaining the specificity of the inside signal inside the cell. In addition, signal transduction pathways amplify the incoming signal by a signaling cascade (molecule A activates several molecule B's, which in turn activate several molecule C's) resulting in an appropriate physiological n by the cell.
Several small molecules within the cell act as intracellular messengers. These include cAMP cyclic guanosine monophosphate (cGMP), nitric oxide (NO), and Ca2+ ions. Increased levels of Ca2+ in the cell can trigger several changes, including activation of signaling pathways, changes in cell contraction and motility, or secretion of hormones or other factors, depending on the call type. Increased levels of nitric oxide cause relaxation of smooth muscle cells and vasodilation by increasing cGMP leves within the cell. Increasing cAMP levels can modulate signaling pathways by activating the enzyme protein kinase A (PKA).
One of the most important functions of cell signaling is to control and maintain normal physiological balance within the body. Activation of different signaling pathways leads to diverse physiological responses, such as cell proliferation, death, differentiation, and metabolism. Signaling pathways in cells may can act with each other and serve as signal integrators. For example, negative and positive feedback loops in pathways can modulate signals within a pathway; positive interactions between two signaling pathways can increase duration of signals; and negative interactions between pathways can block signals.