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CHAPTER 45 - CHEMICAL SIGNALS IN ANIMALS Introduction Hormones are chemical signals. Specific target cells respond to specific hormones. A. An Introduction to Regulatory Systems The endocrine system consists of: Endocrine cells, which are hormone-secreting cells. Endocrine glands , which are hormone-secreting organs. 1. The endocrine system and the nervous system are structurally, chemically, and functionally related Neurosecretory cells secrete hormones into the blood. Chemicals such as epinephrine serve as both hormones of the endocrine system and signals in the nervous system. Feedback regulation is a feature of both the endocrine and nervous systems. Negative feedback regulates many endocrine and nervous mechanisms. 2. Invertebrate regulatory systems clearly illustrate endocrine and nervous system interactions Invertebrates have a wide variety of hormones that are involved in the regulation of homeostasis. An example from hydra: One hormone stimulates asexual reproduction. Another hormone prevents sexual reproduction. An example from a mollusk: The hormone that regulates egg laying also inhibits feeding and locomotion. Arthropods have extensive endocrine systems that regulate molting in insects and crustaceans. The hormone ecdysone triggers molting. Ecdysone also promotes the development of adult features. Neurosecretory cells in the brain produce brain hormone ( BH ), which regulates the secretion of ecdysone. Juvenile hormone ( JH ) promotes the retention of larval characteristics. B. Chemical Signals and Their Modes of Action 1. A variety of local regulators affect neighboring target cells Growth factors : proteins and polypeptides that stimulate cell proliferation. Example: nerve growth factor (NGF) affects certain embryonic cells, developing white blood cells, and other kinds of cells Nitric oxide (NO) Though a gas, NO is an important local regulator. When secreted by neurons, it acts as a neurotransmitter. When secreted by white blood cells, it kills bacteria and cancer cells. When secreted by endothelial cells, it dilates the walls of blood vessels. Prostaglandins (PGs) : modified fatty acids. PGs secreted by the placenta stimulate uterine contractions during childbirth.
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Other PGs play a role in inflammation and the blood flow to the lungs. 2. Most chemical signals bind to plasma-membrane proteins, initiating signal-transduction pathways .
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