The number of second messengers is small: AMPc, cGMP, diacil glycerol, inositol phosphates, calmodulin. Paracrine mediator substances is AMPc (glucagon, catecholamines, parathyroid hormone, TSH, gonadotrophins, ACTH). The hormone action is internal mediated by transducers system of external messages in intracellular messages. Transducer systems are cytoplasmic membrane receptors, nuclear.
At cellular level, signal transduction includes the movement of signals from outside the
cell to inside. This transport of signal can be of simple which include the ion channel
receptors to the most complex level that are associated with the coupling of ligandreceptor
interactions which follows many intracellular happenings.
Extracellular signaling by means of secreted molecules can be of three types in case of
animals. This classification depends upon the distance between the cells over which a
signal can act.
1) Endocrine signaling:
In this type of signaling, hormones are the signaling molecules. Inside the cells of
endocrinal organs like pancreas and liver, these hormones are synthesized. The
hormones are transported to the target cell through the blood stream, and are capable of
acting on a target cell which is far from their site of synthesis.
2) Paracrine signaling:
In contrast to the endocrine signaling, paracrine signaling is effective only in the target
cells which are nearer to the signaling cell. Neurotransmitters are the best example for
such types of signaling molecules in which the signal is transmitted from one nerve cell
to the adjacent nerve cell in the nervous tissue.
3) Autocrine signaling:
Autocrine signaling is another specific type in which the cell gives response to the
signaling molecules produced on its own. Most of the growth factors come under this