Hormones are any substance in the body that carries a signal to regulate growth, differentiation and function of a variety of cells. The more signals needed, the more hormones in an organism. The big-brained human body is no different. In higher animals signal pathways of many hormonal systems originate in the brain. Because hormones are so specific in communicating between cells, we can expect yet more discovery of hormones in the future to add to the large number already found.
Their are major categories of hormones: peptide and protein, thyroid, catecholamine and steroid. The hormones are specific in targeting cognate receptors that are expressed for specific hormones (1). For example, catecholamine hormones epinephrine and norepinephrine as well as peptide hormones target specific cell surface receptors while steroid hormones target intracellular receptors.
Endocrine hormones are those synthesized in a gland and travels to reach distant target cells. Paracrine hormones are secreted by a cell and travel a short distance to reach a neighboring cell's receptors. Autocrine hormones are produced in a cell that functions as a target for the paracrine hormones.
Hormonal cascade systems may involve a number of hormones. A system may operate through a releasing hormone, anterioir pituitary tropic (or posteror pituitary) hormones and ultimate hormones (1). The posterior pituitary system branches off the right of the hypothalamus and is the system involving oxytocin and vasopressin.
A signal is generally transmitted first through the central nervous system, then innervate the hypothalamus secreting a releasing hormone and it begins the amplified cascade. Generally there is a feedback loop (ultra-short, short, or long) and a final hormone binds a cognate receptor to stop secretion or synthesis of releasing hormone.
If a hormone is not systemic, acting in an anatomically restricted site, it is said to be a local hormone. A hormone that functions locally as an autocrine and paracrine hormone is the neurotransmitter acetylcholine. Acetylcholine in neuron-neuron interaction acts as an excitatory transmitter. It is released after stimulation of a neuron, travels across the synapse and binds to specific nicotinic-acetylcholine receptor.
1. Devlin TM. Textbook of Biochemistry with Clinical Correlations. Philadelphia: Wiley-Liss, 2002, pp893-4.