Neural system, hormonal system and cell messengers

Stimuli from the environment are registered by the senses and transmitted to the brain. Sensory perceptions can be of a different nature. The eyes (light or the sight of other animals of the same species), the nose (sex odours), and the tactile senses register information from the environment, and the optic nerve, olfactory nerve and the sensory nerves transmit the message to the brain. The brain is able to translate the information and, if necessary, react by sending a nervous impulse through the nerve fibres to a target organ.

The hormonal system is a regulatory system that sends information by means of chemical messengers. This is a system that is regulated by feedback loops and impulses from the nervous system and several organs.

A hormone can be defined as a chemical substance, produced in a gland or tissue in the body, which evokes a specific reaction in hormone sensitive tissue. The action of the hormonal system can be subdivided according to the way the hormones reach the target cells. Acting in the autocrine way, the producing cell is also the target cell. Acting paracrine, the hormones influence the neighboring cells or organs. Endocrine hormones are transported by the blood, and acting exocrine the hormones are excreted (in the digestive tract, urinary tract, at the skin, etc.) before reaching the target organs. So far, most information is available about the endocrine functions of the hormonal system. During the past decade, the paracrine and autocrine functions have received more attention from researchers, but many aspects are still unknown.

After reaching a target cell the hormone has to provoke a reaction. For this, target cells have hormone-specific receptors. A hormone receptor is a unique molecular structure in or on the cell with a high and specific affinity for a particular hormone. After binding to the receptor the message can be passed on. This message will lead to a cell-specific response, which generally involves activation or inactivation of enzymes in the target cells.

So the hormone receptors have two important functions:
- Recognition of the specific hormone by the target cell;
- translation of the hormonal signal into a cell-specific response.

The effect of an endocrine hormone release can vary with circumstances. The number and type of receptors of a target cell are not fixed. The formation and degradation of receptors is a dynamic process. The function of one hormone in a cell can be the induction or degradation of receptors for another messenger. Furthermore, receptors can be blocked by an excess of hormones. Extra stimulation by a normally highly effective dose of hormones will then cause no further effect. Many pathological conditions in reproduction are caused by derailments at the receptor level. Most receptors need a second messenger to transmit the message. One of the best known second messengers is cyclic AMP.

After binding to the receptor, the hormone activates the adenylate-cyclase-system situated in the cell membrane. ATP is the converted into cyclic AMP. cAMP, the second messenger, in its turn, activates an inactive cAMP-protein-kinase-A that splits up into an active catalytic unit and a regulatory unit. The active catalytic unit of the protein-kinase will stimulate the phosphorylation of a protein synthesis, growth or hormone secretion.

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