Hormones

Classifications and Mechanisms

According to their chemical structure, hormones can be classified into four groups:

• 1.
  Amino acid–derived. These hormones (e.g., thyroxine) are modified amino acids; thus, they are water soluble.
• 2.
  Polypeptides and proteins. These hormones (e.g., insulin) consist of shorter respective longer chains of amino acids; in most cases they are water soluble.
• 3.
  Steroids. Steroid hormones (e.g., cortisol) are synthesized from cholesterol; thus, they are liposoluble.
• 4.
  Eicosanoids. These hormones (e.g., prostaglandines) are made of fatty acids (arachidonic acid); thus, they are liposoluble as well.

Because of the chemical structure of the cell membrane (phospholipid bilayer), water-soluble hormones cannot pass through this barrier; thus, they have to bind to a receptor protein at the outside of the plasma membrane. Activated through a specific hormone, the receptor protein stimulates the production of another signal molecule at the inside of the cell, a so-called second messenger (e.g., cAMP). The second messenger, in turn, activates enzymes, which have a catalytic effect on metabolic processes in the cell.

In contrast, liposoluble hormones are capable of diffusing through the cell membrane directly and of initiating metabolic changes via modulation of DNA processes.

Table 1 provides an overview of some of the most important hormones of the human body.

Stress Response

The stress response is a good example of how the regulation of the hormone system functions, as the hierarchy of the hormone release as well as the feedback mechanisms of different types of hormones are very similar. In addition, the so-called stress hormones play an important role in many studies about the effects of music on the human organism.

Today, the stress response is understood as a complex interplay of nervous, endocrine, and immune systems. The following focuses on the organization and function of hormones that are involved in the adaptations an organism has to make in order to cope with stressful events or stressors.

Stressors can be classified according to the pathway of perception (mental/sensoric versus physical) or according to their origin (endogenous versus exogenous). Anxiety or hunger are examples of stress factors that are mental and endogenous; mental/sensoric and exogenous stressors are, for example, noise or a natural disaster, such as an earthquake. An injury or infection could be classified as a physical and exogenous stressor, and autoimmune diseases as physical and endogenous ones.

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