Autonomic Nervous System

The nervous control of the human body, independently regulating vital functions such as respiration, metabolism, and digestion, is largely not subject to our will.There is a close functional connection between the autonomic and somatic nervous systems. The center of the autonomic nervous system mainly lies in the diencephalon and the medulla oblongata. In the autonomic nervous system, two antagonistic regulatory divisions can be distinguished: the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system causes an increase in activity in the organs it supplies, activating the organs to maximum performance (eg, the heart and circulation) by the formation of epinephrine (adrenaline) and norepinephrine (noradrenaline). The sympathetic nervous system is therefore also known as the adrenergic system.

The parasympathetic nervous system diminishes latent vitality but increases all processes that are important to recovery and restoration of the body's reserves; one of its functions is to stimulate bowel activity and the storage of food.

Hormonal Control

In addition to the nervous system, a second regulatory system keeps the body's activities harmoniously in balance: hormones (Fig 7-56).

Hormones are formed in the body and intervene in the enzyme balance of the cells in a regulatory way.They control growth, metabolism, and reproduction. Hormones are chemical substances that are extremely effective even in very small quantities, being able to exert a stimulating or inhibiting effect in the appropriate organs or cells. They appear to be ubiquitous, at least in more highly developed mammals, because the active substances in these animals are also effective in humans. This offers various possibilities of dealing with disturbed hormonal control: transplantation of the appropriate hormonal organs; direct administration of animal hormones to counteract inadequate function (hypoactivity, hypofunction); or use of antagonistic substances to restrict the hormonal effect in the case of excessive function (hyperactivity, hyperfunction).

The regulation of the body's functions by hormones is relatively slow and widely diversified; they are able to act on several bodily functions at once. The hormones and other active substances reach the cells and organs with the bloodstream; the stimulating effect produced by hormones is thus dependent on the speed of blood flow and always lasts several seconds.

The stimulating effect of hormonal control can be equated with the stimulation by nerves. This is because chemical substances (often the very same hormones) are released at the synapses and motor end plates of the nerves. The subsequent conduction of nerve impulses is simply a special type of hormonal control. The hormones and the active substances of this regulatory system are produced in the endocrine glands (Fig 7-57), which secrete their hormones into the bloodstream.

These endocrine glands include the following:

• The pineal gland (epiphysis) sits in the diencephalon; its active substance is melatonin, and it regulates sexual maturity. Disturbances inhibit sexual maturation or lead to premature development.
• The pituitary gland (hypophysis) regulates hormonal control. Disturbances lead to dwarfism, obesity, and genital underdevelopment.
• The thyroid gland (glandula thyroidea) regulates the metabolic processes by increasing the uptake of oxygen in the cells. Disturbances lead to distinctive metabolic disorders, cretinism, and Graves disease.
• The thymus gland lies in front of the pericardium and is a lymphatic organ. Its active substance has a growth-regulating effect; it inhibits the development of the sexual organs and monitors the immune system. Disturbances lead to an increased tendency to infection; hyperactivity of the thymus prevents sexual maturation.
• The adrenal glands (glandulae suprarenalis) lie on the kidneys and produce epinephrine (adrenaline) and norepinephrine (noradrenaline) to regulate the metabolism. Disturbances lead to excessive pigmentation of the skin (Addison disease or bronzed skin), a fall in blood sugar, muscle weakness, and sluggish thinking.
• The islets of Langerhans in the pancreas produce active substances to regulate the sugar content of the blood. Disturbances lead to diabetes, imbalance in blood sugar levels, and even diabetic coma.
• The reproductive glands are the male testes and the female ovaries.