Chapter 45

• Hormones are responsible for sexual dimorphism
• Hormones are secreted molecules that circulate throughout the body and elicit a response in target cells (endocrine system)
• The nervous system is a network of specialized cells (neurons) that transmit signals along dedicated pathways
• Communication between animal cells through secreted signals can be classified by two criteria: the type of secreting cell and the route taken by the signal in reaching its target
• Endocrine signaling maintains homeostasis, mediates responses to stimuli, and regulates growth and development
• Paracrine and autocrine signaling play roles in processes such as blood pressure regulation, nervous system function, and reproduction
• An example of a local regulator are prostaglandins
• Neurotransmitters: sensation, memory, cognition, and movement
• Neurohormones: regulate endocrine signaling
• An example of a neurohormone is the antidiuretic hormone, which helps with kidney function and water balance
• Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoskeleton, enzyme activation, or a change in gene expression
• The hormone epinephrine (or adrenaline) regulates many organs in response to stressful situations
• A steroid hormone diffuses through the membrane and binds to its cytosolic receptor, then the hormone-receptor complex forms that moves into the nucleus
• In female birds and frogs, estradiol, a form of estrogen, binds to a cytoplasmic receptor in liver cells
• The hormone-receptor complex activates transcription of genes needed to produce egg yolk
• Endocrine cells are often grouped in ductless organs called endocrine glands, such as the thyroid and parathyroid glands and testes or ovaries
• In contrast, exocrine glands, such as salivary glands, have ducts to carry secreted substances onto body surfaces or into body cavities
• Hormones are assembled into regulatory pathways
• In a simple endocrine pathway, endocrine cells respond to a stimulus by secreting a hormone
• In a simple neuroendocrine pathway, the stimulus is received by a sensory neuron, which stimulates a neurosecretory cell
• A negative feedback loop inhibits a response by reducing the initial stimulus, thus preventing excessive pathway activity (example: secretin)
• Positive feedback reinforces a stimulus to produce an even greater response (example: oxytocin)
• In a wide range of animals, endocrine organs in the brain integrate function of the endocrine system with that of the nervous system
• The endocrine pathway that controls the molting of larva originates in the larval brain, where neurosecretory cells produce PTTH
• PTTH directs the release of ecdysteroid
• Bursts of ecdysteroid trigger each successive molt as well as metamorphosis
• The hypothalamus coordinates endocrine signaling
• It receives information from nerves throughout the body and initiates appropriate neuroendocrine signals
• The posterior pituitary stores and secretes hormones that are made in the hypothalamus
• The anterior pituitary makes and releases hormones under regulation of the hypothalamus
• Neurosecretory cells of the hypothalamus synthesize the two posterior pituitary hormones: antidiuretic hormone (ADH) and oxytocin
• Antidiuretic hormone (ADH) regulates physiology and behavior
• Oxytocin regulates milk secretion by the mammary glands
• The anterior pituitary controls diverse processes, such as metabolism, osmoregulation, and reproduction
• For example, prolactin-releasing hormone from the hypothalamus stimulates the anterior pituitary to secrete prolactin (PRL), which has a role in milk production
• In humans and other mammals, thyroid hormone regulates bioenergetics, in addition to other functions
• If thyroid hormone level drops in the blood, the hypothalamus secretes thyrotropin-releasing hormone (TRH), causing the anterior pituitary to secrete thyroid-stimulating hormone (TSH)
• TSH – thyroid stimulating hormone
• TRH – thy rotropinreleasing hormone
• Endocrine signaling regulates homeostasis, development, and behavior