Hormones and the Endocrine System: Chapter 45 Study Notes

Hormones and the Endocrine System

Overview of the Endocrine and Nervous Systems

The endocrine system and nervous system are two major regulatory systems in animals, responsible for communication and coordination of body functions. The endocrine system uses chemical signaling via hormones, while the nervous system uses electrical and chemical signals via neurons.

• Endocrine system: Composed of ductless glands that secrete hormones into the bloodstream to regulate processes such as growth, metabolism, and reproduction.

• Nervous system: Consists of specialized cells called neurons that transmit signals rapidly along dedicated pathways.

• These systems often overlap in function, integrating responses to internal and external stimuli.

Key Terms and Concepts

• Hormone: A secreted molecule that circulates through the body and stimulates specific target cells that possess receptors for that hormone.

• Endocrine gland: Ductless organ that secretes hormones directly into the bloodstream (e.g., thyroid, parathyroid, testes, ovaries).

• Exocrine gland: Gland with ducts that secrete substances onto body surfaces or into body cavities (e.g., salivary glands).

Intercellular Information Flow

Types of Cell Signaling

Animal cells communicate through secreted signals, classified by the type of secreting cell and the route taken by the signal:

• Endocrine signaling: Hormones travel through the bloodstream to distant target cells.

• Paracrine signaling: Local regulators act on nearby cells by diffusion.

• Autocrine signaling: Local regulators act on the same cell that secreted them.

• Synaptic signaling: Neurons release neurotransmitters at synapses to communicate with target cells.

• Neuroendocrine signaling: Neurosecretory cells release neurohormones into the bloodstream.

• Pheromonal signaling: Chemicals released into the environment to communicate with other individuals of the same species.

Functions of Endocrine Signaling

• Maintains homeostasis

• Mediates responses to stimuli

• Regulates growth and development

• Triggers changes underlying sexual maturity and reproduction

Local Regulators and Examples

Paracrine and Autocrine Signaling

Local regulators are molecules that act over short distances, reaching target cells by diffusion. They play roles in blood pressure regulation, nervous system function, and reproduction.

• Paracrine signaling: Target cells are near the secreting cells.

• Autocrine signaling: Target cell is also the secreting cell.

• Prostaglandins: Local regulators involved in immune responses and blood clotting.

Local Regulator Example: Nitric Oxide (NO)

• Nitric oxide (NO): A gaseous local regulator released by blood vessel walls when oxygen levels fall.

• NO diffuses into smooth muscle cells, activating enzymes that relax the cells, causing vasodilation and increased blood flow.

Synaptic, Neuroendocrine, and Pheromonal Signaling

Synaptic and Neuroendocrine Signaling

• Neurotransmitters: Molecules released at synapses, binding to receptors on target cells.

• Neurohormones: Secreted by neurosecretory cells, diffuse into the bloodstream to reach target cells.

Signaling by Pheromones

• Pheromones: Chemicals released into the environment for communication among members of the same species.

• Functions include marking trails, defining territories, warning of predators, and attracting mates.

Chemical Classes of Hormones

Major Classes

Hormones are classified into three major chemical classes:

Class	Example	Solubility
Polypeptides	Insulin	Water-soluble
Steroids	Cortisol	Lipid-soluble
Amines	Epinephrine, Thyroxine	Water-soluble (most), Lipid-soluble (some)

Cellular Hormone Response Pathways

Water-Soluble Hormones

• Secreted by exocytosis

• Travel freely in the bloodstream

• Bind to cell-surface receptors

Lipid-Soluble Hormones

• Diffuse across cell membranes

• Travel in the bloodstream bound to transport proteins

• Diffuse through the membrane of target cells

• Bind to receptors in the cytoplasm or nucleus of target cells

Response Pathways

• Water-soluble hormone responses may include:

  • Activation of enzymes

  • Change in uptake or secretion of molecules

  • Rearrangement of the cytoskeleton

  • Initiation of changes in gene transcription

• Lipid-soluble hormone responses typically involve changes in gene expression.

Example: Epinephrine (Water-Soluble)

• Binds to G protein-coupled receptors on target cell membranes

• Triggers a cascade involving synthesis of cyclic AMP (cAMP): $\text{ATP} \rightarrow \text{cAMP}$

• Activates enzymes responsible for glycogen breakdown into glucose

Example: Estradiol (Lipid-Soluble)

• Binds to cytoplasmic receptor in liver cells (e.g., in female birds and frogs)

• Estradiol-receptor complex activates transcription of the vitellogenin gene, needed for egg yolk production

Other Lipid-Soluble Hormones

• Thyroxine, vitamin D, and other non-steroid lipid-soluble hormones have nuclear receptors

• Hormone-receptor complexes bind to DNA and stimulate transcription of specific genes

Multiple Responses to a Single Hormone

The same hormone can elicit different effects on target cells depending on the type of receptor and signal transduction pathway present.

• Epinephrine: Triggers various responses in different tissues, forming the basis of the "fight-or-flight" response.

Endocrine Tissues and Organs

Major Endocrine Glands

• Thyroid gland

• Parathyroid glands

• Pituitary gland

• Adrenal glands

• Testes (in males)

• Ovaries (in females)

• Pineal gland

Exocrine Glands

• Have ducts to carry secretions to body surfaces or cavities (e.g., salivary glands)

Hormone Pathways and Feedback Regulation

Simple Endocrine Pathways

• Endocrine cells respond directly to a stimulus by secreting a hormone

• Hormone travels to target cells, interacts with receptors, and triggers a physiological response

Example: Secretin Pathway

• Acidic contents in the duodenum stimulate endocrine cells to secrete secretin

• Secretin causes pancreas to release bicarbonate, raising pH in the duodenum

Simple Neuroendocrine Pathways

• Stimulus is received by a sensory neuron, which stimulates a neurosecretory cell

• Neurosecretory cell secretes a neurohormone into the bloodstream

Example: Oxytocin Pathway

• Suckling by an infant stimulates nerve signals in the mother

• Hypothalamus triggers release of oxytocin from the posterior pituitary

• Oxytocin causes mammary glands to secrete milk

Feedback Regulation

• Negative feedback: Response reduces the initial stimulus (e.g., increased pH from secretin shuts off further secretin release)

• Positive feedback: Response reinforces the stimulus (e.g., oxytocin release during suckling increases milk secretion and further oxytocin release)

Coordination of Endocrine and Nervous Systems

Integration in Animals

• Endocrine organs in the brain integrate endocrine and nervous system functions

• In vertebrates, the hypothalamus coordinates endocrine signaling and communicates with the pituitary gland

• The posterior pituitary stores and secretes hormones made in the hypothalamus

• The anterior pituitary makes and releases hormones under hypothalamic regulation

Pituitary Hormones

Posterior Pituitary Hormones

• Antidiuretic hormone (ADH): Regulates physiology and behavior, including water balance

• Oxytocin: Regulates milk secretion by mammary glands

Anterior Pituitary Hormones

• Controls metabolism, osmoregulation, and reproduction

• Release is regulated by hypothalamic hormones

• Prolactin (PRL): Stimulates milk production

Hormone Cascade Pathways

• Sets of hormones from the hypothalamus, anterior pituitary, and target endocrine gland form a hormone cascade

• Anterior pituitary hormones in these pathways are called tropic hormones

Anterior Pituitary Hormone	Target	Effect
FSH and LH	Testes or ovaries	Tropic
TSH	Thyroid	Tropic
ACTH	Adrenal cortex	Tropic
Prolactin	Mammary glands	Nontropic
MSH	Melanocytes	Nontropic
GH	Liver, bones, other tissues	Tropic and nontropic

Additional info:

• Hormone regulation is essential for maintaining homeostasis, development, and behavior.

• Hormone cascades allow for amplification and fine-tuning of physiological responses.