General Information about the Neuro-humoral Regulation
If some of the terms used below are unclear to you, you can look them up in one of several online glossaries.
All vital processes that happen in our organism are controlled by the brain, which "receives information and sends commands" to the body via the nervous and the endocrine systems.
The nervous system consists of neurons, cells that conduct electrical current. Their electrical signals immediately affect the metabolism of other cells. This system triggers immediate changes in the organism but it cannot influence processes such as the synthesis of new proteins, something which the endocrine system does.
The autonomic(vegetative) nervous system is controls processes inside the organism such as breathing, heart beating, digestion - processes happening without our will. The autonomic nervous system controls the sympathetic and parasympathetic nervous systems. The sympathetic system activates processes such as rise of heart rate, rise of perspiration rate, muscle tension, etc, whereas the parasympathetic system does just the opposite - it is responsible for relaxation - lower heart rate, lower perspiration rate, relaxed muscle tissue. The centres of the sympathetic system are found everywhere inside the backbone, while the centres of the parasympathetic are situated at its ends. Both systems control vital processes and no matter which one is more active in a certain moment, the other one, although suppressed, cannot be "switched off".
The endocrine system controls the biochemical balance in the organism, growth and reproduction. This control is realized by the release of protein substances called hormones, which selectively enter cells and change either their metabolism or induce the synthesis of new proteins. It is evolutionary an older one and it its effects on the organism appear slower than that of the nervous system.
Both systems are controlled by a part of the brain called hypothalamus and their work is so precisely coordinated that we speak of neuro-humoral regulation (humoral = endocrine).
More about the endocrine system
Hormones are produced in organs called glands. A gland, situated in the brain and as big as a bean, called pituitary gland controls all the other glands in the organism. The pituitary is not a typical gland - it is built up by both gland and nervous tissue.
Hormones can be produced by non-gland tissues(the brain is one example), too and the release of such hormones is not controlled by the pituitary. A gland in the brain called pineal also secretes hormones independent from the pituitary.
Before the pituitary secrete any hormone it must receive signals from the hypothalamus. The hypothalamus regulates the pituitary by releasing neurohormones through the vessels of the pituitary stalk. Then the pituitary secretes hormones, which stimulate other glands to produce hormones.
Those glands, which secrete hormones directly into the blood are called glands with internal secretion and those which secrete hormones directly into "target" organs - glands with external secretion. The latter have their own vessels to reach the "target" organ.
Not every hormone can enter every cell. For certain cells there are certain hormones and one and the same hormone has different effects on different cells.
There are three types of hormones - polypeptides, aminoacids and steroids. Polypeptides and aminoacids change the metabolism of cells and steroids interfere with the genetics of cells - that is the synthesis of new proteins.
Hormones enter cells by forming a chemical compound with substances found either in the cell membrane or in the cell cytoplasm called receptors. A hormone will form chemical compound only with certain receptors - hormones act as a key and receptors - as a keyhole. One and the same key will not open all keyholes. Released into the blood, hormones "try at" every cell on their way and though slow it may seem, a hormone will find the target cell much quicker than we can imagine.
The structure of receptors is protein, thus the compound formed is very unstable - it lasts for a limited time only. In a cell there are either membrane only or cytoplasm only receptors. Steroids' receptors are the DNA in cell nuclei.
In the case of membrane receptors the receptor-hormone compound turns around at 180 degrees, so that the hormone "is moved" into the cytoplasm - inside the cell. Of course to say only that the receptor-hormone compound turns around at 180 degrees is too simplistic and to a great extent incorrect. The whole process of "turning" is a very complex one and consists of many stages.
Receptors contain in their structure enzymes which are activated, once the hormone-receptor compound is formed. These enzymes activate on their turn other enzymes, which ultimately change the metabolism of the cell.
Steroids form a compound together with DNA and after complex biochemical processes induce the synthesis of new proteins. Their effect is slowest related to that of polypeptide and aminoacid hormones.
Feedback to the brain
After a cell has "received" a number of hormones each of its receptors, having already formed a compound with those hormones becomes unable to form compound with other hormones. Thus the number of "free" receptors decreases and logically the number of "free" hormones in blood increases. Blood circulation brings hormones literally everywhere and soon the brain detects the increase and sends signals to the hypothalamus, which produces the neurohormones statines. Statines suppress the production of hormones in the pituitary.
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