Hormones and the Endocrine System
Overview: The Body’s Long-Distance Regulators
An animal hormone is a chemical signal that is secreted into the circulatory system that communicates regulatory messages within the body.
A hormone may reach all parts of the body, but only specific target cells respond to specific hormones.
A given hormone traveling in the bloodstream elicits specific responses from its target cells, while other cell types ignore that particular hormone.
Concept 45.1 The endocrine system and the nervous system act individually and together in regulating an animal’s physiology
Animals have two systems of internal communication and regulation, the nervous system and the endocrine system.
Collectively, all of an animal’s hormone-secreting cells constitute its endocrine system. Hormones coordinate slow but long-acting responses to stimuli such as stress, dehydration, and low blood glucose levels.
Hormones also regulate long-term developmental processes such as growth and development of primary and secondary sexual characteristics. Hormone-secreting organs called endocrine glands secrete hormones directly into the extracellular fluid, where they diffuse into the blood.
The nervous and endocrine systems overlap to some extent.
Certain specialized nerve cells known as neurosecretory cells release hormones into the blood.
The hormones produced by these cells are sometimes called neurohormones. Chemicals such as epinephrine serve as both hormones of the endocrine system and neurotransmitters in the nervous system.
The nervous system plays a role in certain sustained responses—controlling day/night cycles and reproductive cycles in many animals, for example—often by increasing or decreasing secretions from endocrine glands.
The fundamental concepts of biological control systems are important in regulation by hormones.
A receptor, or sensor, detects a stimulus and sends information to a control center. After comparing the incoming information to a set point, the control center sends out a signal that directs an effector to respond.
Lecture Outline for Campbell/Reece Biology, 7 Edition, © Pearson Education, Inc.
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In endocrine and neuroendocrine pathways, this outgoing signal, called an efferent system, is a hormone or neurohormone, which acts on particular effector tissues and elicits specific physiological or developmental changes.
The three types of simple hormonal pathways (simple endocrine pathway, simple neurohormone pathway, and simple neuroendocrine pathway) include these basic functional components.
A common feature of control pathways is a feedback loop connecting the response to the initial stimulus.
In negative feedback, the effector response reduces the initial stimulus, and eventually the response ceases.
This prevents overreaction by the system.
Negative feedback regulates many endocrine and nervous mechanisms. Positive feedback reinforces the stimulus and leads to an even greater response. The neurohormone pathway that regulates the release of milk by a nursing mother is an example of positive feedback.
Suckling stimulates sensory nerve cells in the nipples, which send nervous signals that reach the hypothalamus, the control center.
The hypothalamus triggers the release of the neurohormone oxytocin from the posterior pituitary gland.
Oxytocin causes the mammary glands to secrete milk.
The release of milk in turn leads to more suckling and stimulation of the pathway, until the baby is satisfied.
Concept 45.2 Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses Hormones convey information via the bloodstream to target cells throughout the body. Other chemical signals—local regulators—transmit information to target cells near the secreting cells.
Pheromones carry messages to different individuals of a species. Three major classes of molecules...