The hypothalamus is the coordinating center for the nervous and endocrine responses to internal and external stimuli. The hypothalamus constantly monitors the body’s homeostasis by analyzing input from the periphery and the central nervous system (CNS) and coordinating responses through the autonomic, endocrine, and nervous systems. In effect, it is the “master gland” of the neuroendocrine system. This title was once given to the pituitary gland because of its many functions and well-protected location (see The Pituitary Gland).
The hypothalamus has various neurocenters—areas specifically sensitive to certain stimuli—that regulate a number of body functions, including body temperature, thirst, hunger, water retention, blood pressure, respiration, reproduction, and emotional reactions. Situated at the base of the forebrain the hypothalamus receives input from virtually all other areas of the brain, including the limbic system, cerebral cortex, and the special senses that are controlled by the cranial nerves—smell, sight, touch, taste, and hearing. Because of its positioning the hypothalamus is able to influence, and be influenced by, emotions and thoughts. The hypothalamus also is located in an area of the brain that is poorly protected by the blood–brain barrier, so it is able to act as a sensor to various electrolytes, chemicals, and hormones that are in circulation and do not affect other areas of the brain.
The hypothalamus maintains internal homeostasis by sensing blood chemistries and by stimulating or suppressing endocrine, autonomic, and CNS activity. In essence, it can turn the autonomic nervous system and its effects on or off. The hypothalamus also produces and secretes a number of releasing hormones or factors that stimulate the pituitary gland, which in turn stimulates or inhibits various endocrine glands throughout the body (Figure 718.1). These releasing hormones include growth hormone–releasing hormone, thyrotropin releasing hormone (TRH), gonadotropin-releasing hormone, corticotropin-releasing hormone, and prolactin-releasing hormone. The hypothalamus also produces two inhibiting factors that act as regulators to shut off the production of hormones when levels become too high: Growth hormone (GH) release–inhibiting factor (somatostatin) and prolactin (PRL)-inhibiting factor (PIF). Recent research has indicated that PIF may actually be dopamine, a neurotransmitter. Patients who are taking dopamine-blocking drugs often develop galactorrhea (inappropriate milk production) and breast enlargement, theoretically because PIF also is blocked and prolactin (PRL) levels continue to rise, stimulating breast tissue and milk production. Research is ongoing about the chemical structure of several of the releasing factors.
The hypothalamus is connected to the pituitary gland by two networks: A vascular capillary network carries the hypothalamic-releasing factors directly into the anterior pituitary, and a neurological network delivers two other hypothalamic hormones— antidiuretic hormone (ADH) and oxytocin—to the posterior pituitary to be stored. These hormones are released as needed by the body when stimulated by the hypothalamus.
1. Where are the major endocrine glands located?
A. In the head, shoulders, knees, and toes
B. In the abdomen, joints, brain, and spinal column
C. In the brain, neck, abdomen, and groin
D. In the gall bladder, appendix, tonsils, and spleen 2. Which gland is sometimes called “master gland”, though it is only about the size of a pea?
A. Pituitary
B. Adrenal
C. Pineal
D. Hypothalamus 3. The part of the brain that links the nervous system and the endocrine system is
A. Pancreas
B. Hypothalamus
C. Pituitary gland
D. Pineal gland 4. The process by which an organism’s internal environment is kept stable in spite of changes in the external environment is referred to as
A. Positive Feedback
B. Negative Feedback
C. Homeostasis
D. Targeted Cells
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