First off, a hormone is a molecule that is secreted into the blood, or hemolymph for those insect dudes, and communicates regulatory messages throughout the body. Now although the hormone can be passed everywhere in the body, the target cells require receptors for that hormone, or else it really can’t do anything. The chemical signaling is done by 2 major systems in the body. One is the endocrine system. The other is the nervous system. The endocrine system use hormones to regulate practically everything in the body. The nervous system uses neurons (those “finger” and “tail” thingies) to transmit signals along pathways.

The hormones and signaling molecules can bind to target receptors, triggering specific response pathways:

Different Signaling Molecules

Hormones

The endocrine system is split into different parts scattered throughout the body. Some parts in these organs, and other parts in those. Endocrine cells that don’t stick to another organ, but are clumped together with really no other friends are called endocrine glands. These glands secrete the hormones right into the surrounding fluid. Exocrine glands secrete the stuff onto the skin surface.

Local Regulators

Local Regulators are molecules that travel only to the cell’s neighbors by simple diffusion. These regulators help do paracrine and autocrine signaling. Paracrine signaling involves a cell oozing out hormones to help out another neighboring cell. Autocrine cells act only on it.

Neurotransmitters and Neurohormones

Neurotransmitters are tiny little molecules that diffuse into a neighboring nerve cell. Neurosecretory cells are found in the brain. These cells make neurohormones, which like to travel through bloodstream rather than through the synapses between nerve cells.

Pheromones

Pheromones are just chemicals used by the same species to communicate. Though, they are also thought to be sex inducers. Spray some on yourself and you might get laid faster.

The chemical classes of Hormones

Hormones come in different shapes and sizes. How they are made classify them into the three groups, polypeptides, amines, and steroids.

Cellular Response Pathways

Water soluble hormones can move in blood and stick directly onto cell receptors. Lipid soluble hormones travel on transport proteins, then they diffuse into a cell and activate the cell from inside.

Multiple Effects of Hormones

Some hormones can attach to different cells, but then the cell responds differently, so technically one hormone can have lots of functions. It’s like they’re saving types of proteins in bad economic times.

More Local Regulator stuff

They include cytokines, those dudes form the immune system, the growth factors, which make you buff, the nitric oxide(NO), which is the thing found in Viagra, and Prostaglandins, which promote fever and inflammation.