Signal Transduction (Wiki)
This is the Wiki page which provides an overview of transmembrane signaling. Many of the links below are taken from this page. I've provided explicit links to these details as much for my own information as for yours.
"In biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers ..."
"Within multicellular organisms, there is a diverse number of small molecules and polypeptides that serve to coordinate a cell's individual biological activity within the context of the organism as a whole. These molecules have been functionally classified as:
It is important to note that most of these classifications do not take into account the molecular nature of each class member. For example, as a class, neurotransmitters consist of neuropeptides such as endorphins and small molecules such as serotonin and dopamine. Hormones are also a generic class of molecule able to initiate signal transduction, these include insulin (a polypeptide), testosterone (a steroid), and epinephrine (an amino acid derivative, in essence a small organic molecule).
The classification of one molecule into one class or another is not exact. For example, epinephrine and norepinephrine secreted by the central nervous system act as neurotransmitters. However, epinephrine when secreted by the adrenal medulla acts as a hormone."