Chemical signals are produced and released by one cell or group of cells. The released chemical signals control or influence target cells. Target cells are cells where the chemical signals act.

Chemical signals are classified in different ways. They are classified as below:

1. The classical hormones
2. Gastrointestinal or GI hormones
3. Autacoids or local hormones
4. Paracrine hormones
5. Autocrine hormones
6. Neurotransmitters

A second set of chemical agents are those which are included in such categories like

1. Second messengers
2. Cytokines or interleukins
3. Different factors like growth factors

Another way of classification or chemical signals is by their chemical nature. They are as follows.

1. Protein or peptide based chemical signals
2. Derivative of amino acids based chemical signal
3. Steroids
4. Eicosanoids

Classical Hormones

Till 1960s by the term hormone one understood only the classical hormones. Their man feature is they are produced by an endocrine or ductless gland. They are then released by these glands, carried by blood stream and reach the target organs. In the target organs this hormones produce their effects.

Example of the classical hormone is hormones produced by anterior-posterior pituitary, thyroid, parathyroid, pancreas, adrenal cortex and gonads.

Gastrointestinal or GI Hormones

Gastrointestinal hormones or GI hormones are secreted from stomach, pancreas, small intestines and gastrointestinal tract parts. This type of hormones helps in digestion of foods. These hormones are secreted from the epithelial cells of the gastrointestinal tract. They stimulate the secretion of gastrointestinal juices form different glands. For example the hormone gastrin induces the secretion of gastric juice. These hormones are secreted directly in the blood. Thus they circulate the whole body through the connective tissues. Modern results have also shown that gastrointestinal hormones also have roles in nervous system development.

Local Hormones

Local hormones are hormones which are secreted by localized cells and they act on the nearby cells. They induce their effects on the localized cells that are why they are called as localized cells. They have a very short life expectancy for a hormone and they are not carried by blood stream.

They are released during physical work or exercises. They mainly control smooth and vascular muscle dilation and concentration. They are of paracrine type hormone. Some of the examples of local hormones are eicosanoids, prostaglandins, angiotensin, neurotensin, nitric oxide, kinins, histamine, serotonin etc.

Paracrines

In some cases chemical signal producing cell produces the chemical signal which diffuses into the target cells which are situated very close to the producer cells. This type of hormone is called as paracrine hormones.

Examples of this type of paracrine hormones are histamine and insulin etc..

Autocrines

In this type of hormone the producer cell produces the chemical signal and the chemical signal influences the producer cell itself. Autocrine effects may be very important for the growth sustenance of cancer cells. In some hostile environment normal cells die or cannot grow, but cancer cells can. This is because cancer cells produce some growth factors, which by autocrine effects can cause protein synthesis of the cancer cells itself.

Neurotransmitters

Nerve fibers terminate on their target cells. The target cell can be a cardiac or smooth muscle cell or another nerve fiber. On stimulation the stimulus when reaching the nerve terminal, releases a chemical substance called neurotransmitters. The neurotransmitters then reach a target cell. The examples of neurotransmitters are noradrenalin, adrenaline, serotonin etc.

Steroid Hormones

Steroid hormones are chemical derivatives of cholesterol and behave in our body as cholesterol. Steroid hormones in our body are the steroids of the suprarenal cortex and the gonadal steroid hormones such as female sex hormones or female steroid hormones or the male sex steroid hormones.

The common characteristics of steroid hormones are:

1. They can be synthesized in our body but CPP ring cannot be further broken down in our body.
2. They are synthesized and immediately utilized that is they are not stored. They are synthesized and stores in the endocrine gland itself.
3. Receptors of steroid hormones are situated either in the cytosol or on the nucleus of the target cells.

Eicosanoids

Eicosanoids are derivatives of the polyunsaturated fatty acid or arachidonic acid. The major two classes of eicosanoids are prostaglandins and leucotrienes.

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