The systems of the body are in general under the control of the nervous system and the endocrine system. In this article I will discuss the endocrine system briefly. There are in the body many endocrine glands, the master of which is the pituitary gland. The pituitary gland controls or regulates the function of most other endocrine glands in the body.
It is located below the hypothalamus at the base of the skull. The pituitary itself is under the hormonal control of the hypothalamus which secret hormones that are regulated by a negative feedback and which are called releasing hormones. These hormones stimulate the release of 6 hormones from the anterior part of the pituitary gland in addition to two other hormones from the posterior part of the pituitary gland.
The secretion of hormones from the anterior pituitary gland is controlled in most cases by a positive feedback from hormones that are secreted in the hypothalamus and which are called releasing hormones. They are called releasing hormones because the stimulate the release of the pituitary hormones into the blood circulation.
All the hormones that are secreted by the anteior pituitary gland are regulated by a positive feedback from hormones in the hypothalamus except one hormone which is called prolactin which is regulated by a negative feedbck from the hormone dopamine. A negative feedback means that an increase in the secretion of dopamine renders a decrease in the secretion of the hormone prolactin and vice versa.
Prolactin is a hormone which stimulates the secretion of milk from the breast. Its excess in the blood renders a state of infertility. This situation can occur in cases in which dopamine antagonists are administered by psychotic patients which suffer from psychosis.
Dopamine is clinically important. Its excessive secretion in the brain can cause a state of psychosis. Schizophrenic patients usually have high level of dopamine in their blood. Deficiency of the hormone dopamine occurs in patients which suffer from parkinson disease.
Patients of parkinson disease that usually take dopamine agonists to controll their symptoms of the disease can have as side effect psychotic episodes. Dopamine is an inhibitory neurotransmitter and its deficiency as occurs in parkinson disease leads to specific symptoms such as tremors of the extremeties.
The endocrine system is tightly related to the function of the nervous system. For example, the adrenal medulla secretes the hormones epinephrine and norepinephrine in response to neural stimuli of the sympathetic nervous system. Two hormones that are secreted by the pituitary gland and the thyroid gland do not have a specific effect on target organs. Instead they mediate their effect on the whole body.
These hormones are the growth hormone which is secreted by the anterior pituitary and the thyroid hormone which is secreted by the thyroid gland. THe growth hormone functions by causing growth in all parts of the body. In addition and similarly the thyroid hormone functions by increasing the rate of many metabolic reactions in the body. Thus its effect is general and is not targeted at a specific organ.
Other hormones that are secreted by the pituitary gland target in their function a specific organ. Thus they are selective in their function. An example is the adrenocorticotropic hormone that is secreted by the anterior pituitary gland. It is specific for the stimulation of the adrenal cortex which in turn secretes the hormones aldosterone and cortisol which in turn have specific functions. Cortisol being more general in its action having several effects including raising the sugar level in the blood in addition to elevation of blood pressure.