Classically, hormones are described as substances that are produced and secreted from one organ and that travel via the circulation to other organs to direct physiological processes. The endocrine system is often described as a hierarchical system with instructions flowing from the central nervous system as neurotransmitters through the hypothalamus and/or the pituitary gland to peripheral organs and tissues. In actuality, the endocrine system has many points of information input, both from within the animal and from the environment, and feedback loops producing a highly integrated interactive system that maintains fine control over homeostasis and productive processes. Greater elucidation of endocrine regulation has revealed hormone action on nearby cells without transport through the circulatory system. These actions, without bloodstream transport, are classified as paracrine, affecting cells of a different type than those that produced them, and autocrine, affecting cells of the same type as those that produced them.
Naturally occurring protein hormones are peptide polymers of l-amino acids. Biologically active analogues of naturally occurring hormones containing d-amino acids have been synthesized. Amino acid polymers of less than 100 amino acids are generally considered peptides and larger polymers are considered proteins. Protein hormones are polar compounds that affect target tissues by binding to specific cell-surface receptors, initiating a cascade of intracellular signals directing specific pathways.
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