Types of signals

Extracellular

Signal transduction usually involves the binding of extracellular signaling molecules to receptors that face outwards from the membrane and trigger events inside. This takes place via a change in the shape or conformation of the receptor which occurs when the signal molecule "docks" or binds. Receptors typically respond only to the specific molecule or ligand for which they have affinity, and molecules that are even only slightly different tend to have no effect or else to act as inhibitors.

Most extracellular chemical signals are hydrophilic and are unable to penetrate the lipid membrane that surrounds cells. A common kind of extracellular signal is nutrient. In complex organisms this includes the ligands responsible for sensations of smell and taste. Steroids represent an example of extracellular signals that can cross the membrane to permeate cells, which they are able to do because they are lipophilic.

Intracellular

Often, but not always, the intracellular events triggered by the external signal are considered distinct from the event of "transduction" itself, which in the strictest sense refers only to the step that converts the extracellular signal to an intracellular one.

Intracellular signalling molecules in eukaryotic cells include heterotrimeric G protein, small GTPases, cyclic nucleotides, such as cyclic AMP (cAMP) and cyclic GMP (cGMP), calcium ion, phophoinositide derivatives, such as Phosphatidylinositol-triphosphate (PIP3), Diacylglycerol (DAG) and Inositol-triphosphate (IP3), and various protein kinases and phosphatases. Some of these are also called second messengers.

Intercellular

Intercellular communication is accomplished by extracellular signalling and takes place in multicellular organisms. Within endocrinology, which is the study of intercellular signalling in animals, intercellular signalling is subdivided into the following types:
- Endocrine signals are produced by endocrine cells and travel through the blood to reach all parts of the body.
- Paracrine signals target only cells in the vicinity of the emitting cell. Neurotransmitters represent an example.
- Autocrine signals affect only cells that are of the same cell type as the emitting cell. An example for autocrine signals is found in immune cells.
- Juxtacrine signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent.

Hormones

Most of the molecules that enable signalling between the cells or tissues within an individual animal or plant are known as "hormones." Hormone-initiated signal transduction takes the following steps:
1. Biosynthesis of a hormone.
2. Storage and secretion of the hormone.
3. Transport of the hormone to the target cell.
4. Recognition of the hormone by the hormone receptor protein, leading to a conformational change.
5. Relay and amplification of the signal that leads to defined biochemical reactions within the target cell. The reactions of the target cells can, in turn, cause a signal to the hormone-producing cell that leads to the down-regulation of hormone production.
6. Removal of the hormone.

Hormones and other signaling molecules may exit the sending cell by exocytosis or other means of membrane transport. The sending cell is typically of a specialized type. Its recipients may be of one type or several, as in the case of insulin, which triggers diverse and systemic effects.

Hormone signaling is elaborate and hard to dissect. A cell can have several different receptors that recognize the same hormone, but activate different signal transduction pathways; or different hormones and their receptors can invoke the same biochemical pathway. Different tissue types can answer differently to the same hormone stimulus. There are two classes of hormone receptors, "membrane-associated receptors" and intracellular or "cytoplasmic" receptors.

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