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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

CHAPTER 41 ANIMAL HORMONES 41.1 What Are Hormones and How Do They Work? - Control and regulation require information. - In multi-cellular animals, most of this information is transmitted as electric signals and as chemical signals. The electrical signals are impulses generated by the nervous system, conducted along cell processes of nerve cells to their targets on specific cells. The chemical signals are hormones, secreted by cells of the endocrine system into the extracellular fluid. - The two informational systems of the body are the nervous and endocrine systems. Hormones can act locally or at a distance - Endocrine Cells: The cells that secrete hormones. - Target Cells: The cells that have receptors for hormones. - Circulating Hormones: Hormones secreted into the extracellular fluid can diffuse into the blood, which distributes them throughout the body so they can activate target cells far from the site of release. Testosterone is a circulating hormone. - Some hormones are released in such tiny quantities, or are so rapidly inactivated by enzymes, or taken up so efficiently by local cells that they never diffuse into the blood in sufficient amounts to act on distant cells. - Paracrine Hormones: Hormones that affect only target cells near their release site. e.g., histamine, one of the mediators of inflammation. - The most local action of a hormone can have is when there are receptors on the same cell that released it. - When a hormone influences the cell that released it, it is said to have autocrine function. Such functions can provide negative feedback to control rates of secretion. - Some endocrine cells exist as single cells within a tissue. - Hormones of the digestive tract are secreted by isolated endocrine cells in the wall of the stomach and small intestines. - Endocrine Glands: Hormones that are secreted by aggregations of endocrine cells forming secretory organs. endocrine: the glands do not have ducts that lead to the outside of the body; they secrete their products directly into the extracellular fluid. A single endocrine gland may secrete several different hormones. Exocrine Glands are contrasting; they have ducts that carry their products to the surface of the skin (e.g., sweat glands) or to the surface of the body passageway that leads to the outside of the body (salivary glands). www.notesolution.comHormonal communication arose in early evolution - Plants do not have nervous systems, but they do have hormones. - The most primitive of the multi-cellular animals, the sponges, also do not have a nervous system but they do have chemical communication. - Even a protest, the social amoeba, which produces multi-cellular fruiting bodies by the aggregation of individuals, coordinates the aggregation with a chemical signal, cAMP. Hormones from the head control molting in insects - The largest groups of arthropods are the insects, and like all arthropods they have rigid exoskeletons. Their growth is episodic, punctuated with molts (shedding of the exoskeleton). Each growth stage between two molts is called an instar. - Sir Vincent Wigglesworth He conducted experiments on the blood-sucking bug Rhodnius prolixus. Rhodnius prolixus o A hardy experimental animal. o It can live a long time even after it is decapitated. He formed a hypothesis that something diffusing slowly form the head of the bug controls molting. He decapitated two Rhodnius: one shortly after its blood meal and another that had its blood meal a week earlier. o The two decapitated bodies were connected with a short piece of glass tubing that allowed body fluid transfer between them. They both molted. - Two hormones working in sequence regulate molting: 1. Prothoracicotropic hormone (PTTH) Cells in the brain produce PTTH and because of this it is called the brain hormone. It is transported to and stored in a pair of structures called the corpora cardiac attached to the brain. After appropriate stimulation (which for Rhodnius is a blood meal), it is released from these structures and it diffuses in the extracellular fluid to an endocrine gland, the prothoracic gland. It stimulates the prothoracic gland to release the hormone ecdysone. 2. Ecdysone It is a lipid-soluble steroid molecule that readily enters its target cells (mostly cells of the epidermis). In target cells, it binds to a receptor that is probably ancestral to the vertebrate testosterone receptor. The hormonereceptor complex acts as a transcription factor and induces expression of the genes for enzymes involved in digesting the old cuticle and secreting a new one.
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