The endocrine system plays a vital role in maintaining cell integrity. It consists of a series of ductless glands that secrete chemical messengers (hormones) into the bloodstream to act on distal locations. Although these hormones vary in structure and function, together their effects maintain a stable environment that can adapt to stressors. These effects include managing the production, storage and utilization of energy, development and growth, and maintenance of intravascular volume status.
Hormones can be divided into four groups based on their chemical structure: amino acids, polypeptides, steroids, and eicosanoids. Amino acids are structurally modified from their base amino acid structures, allowing participation in signaling. Polypeptide hormones consist of chains of amino acids that are further modified by adding carbohydrates. Steroidal hormones are cholesterol-derived lipids that generally cross the cell membrane to enact their effects. Eicosanoids are also plasma membrane phospholipid-based messengers. These hormones are secreted to the surrounding interstitial spaces or travel into the bloodstream to distal sites.
There are two main signaling mechanisms based on the hormone’s solubility characteristics:
Lipid-soluble hormones cross the cell membrane and bind to cytoplasmic proteins. The hormone–protein complex promotes the transcription of a target DNA segment, stimulating the production of enzymes to enact the hormone’s effect.
Water-soluble proteins bind membrane receptors on the target cells. The receptor–hormone complex triggers the production of a secondary messenger in the cytoplasm. This second messenger cascade has a variety of effects, but ultimately results in the upregulation of target enzymes as well.
Hormone regulation occurs via neural regulation or feedback mechanisms. Direct neural regulation can be seen in catecholamine release, where preganglionic sympathetic nerve fibers synapse directly on the adrenal medulla to stimulate catecholamine release.
When the body senses changes to its equilibrium, the endocrine system acts to restore it. These changes can be an aberrant glucose level, an abnormal temperature, or a sudden physical stressor. Hormone production is upregulated when its effects are needed. When the body senses that the intended physiologic effect exceeds what is necessary, an inhibitory signal is sent to halt production of the messengers via a negative feedback loop. Positive feedback loops, however, are rare. One notable exception is that of oxytocin during labor. Once a threshold level of oxytocin is reached, oxytocin production is further increased, until labor occurs.
HYPOTHALAMUS AND PITUITARY GLAND
The hypothalamus and pituitary gland are distinct from the other members of the endocrine system. They act as the control center of the endocrine system with a wide arsenal of hormones to enact their effects. This tiered system of control offers two main benefits. First, it allows an amplification of the initial signal to generate a more significant end effect. Second, it provides multiple targets for feedback loops, offering layers of control.
The hypothalamus is located near ...