Endocrine Signalling Quiz

Chapter 45: Hormones and the Endocrine System

• Hormones and other signaling molecules bind to target receptors to initiate specific response pathways.
• Feedback regulation and coordination with the nervous system are common in endocrine systems.
• Endocrine glands respond to diverse stimuli in regulating homeostasis, development, and behavior.
• Animal hormones are chemical signals secreted into the circulatory system, communicating regulatory messages within the body.
• Hormones reach all parts of the body but only target cells with receptors for that hormone.

Key Concepts of Endocrine Signaling

• Hormones, secreted into extracellular fluids, reach their targets via the bloodstream.
• Endocrine signaling maintains homeostasis, mediates responses to stimuli, regulates growth, and development.
• Local regulators act over short distances by diffusion.
• Paracrine signaling involves target cells near secreting cells.
• Autocrine signaling involves the target cell also being the secreting cell.

Signaling by Pheromones

• Members of an animal species sometimes use pheromones for communication.
• Pheromones are chemicals released into the environment.
• Pheromones can mark trails leading to food, define territories, warn of predators, or attract mates. Examples include those used by the Japanese beetle.

Chemical Classes of Intercellular Signaling Factors

• Many different chemicals function in communication between cells, with three main classes in vertebrates: polypeptides (proteins and peptides), amines derived from amino acids, and steroid hormones.
• The solubility properties of these chemicals are crucial for their function.

Chemical Classes of Hormones

• Lipid-soluble hormones (steroid hormones) readily pass through cell membranes, whereas water-soluble hormones (polypeptides and amines) do not.
• Receptor location varies with hormone type.

Protein and Amide Signaling Factors

• Many familiar hormones are encoded proteins.
• Insulin, produced as a long, inactive prohormone polypeptide, is secreted to reduce blood glucose.
• Growth factors regulate the cell cycle.
• Amine hormones are signaling factors produced from amino acids.

Lipid Signaling Factors

• Most lipid hormones are related to fatty acids and steroids.
• Prostaglandins are modified fatty acids promoting fever and inflammation.
• Many hormones, like steroids, are derived from cholesterol.

Cellular Response Pathways

• Water- and lipid-soluble hormones utilize different pathways within the body.
• Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors.
• Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and bind to receptors in the cytoplasm or nucleus.

Hormones and the Endocrine System

• If a hormone receptor is mutated such that it cannot bind to the hormone, the hormone will arrive at the cell, but nothing will happen.

Pathway for Water-Soluble Hormones

• Binding of the hormone to its receptor initiates a signal transduction pathway, which may alter the cytoskeleton, activate enzymes, or change gene expression.
• Example: The hormone epinephrine (adrenaline) has multiple effects mediating the body's response to short-term stress.
• Binding of epinephrine to receptors on the plasma membrane of liver cells triggers the release of messengers activating enzymes. One response is the activation of enzymes to break down glycogen, releasing glucose into the bloodstream

Pathway for Lipid-Soluble Hormones

• Responses to lipid-soluble hormones are typically changes in gene expression.
• A steroid hormone binding to a cytosolic receptor forms a complex that moves into the nucleus.
• In the nucleus, the receptor part of the complex acts as a transcription factor regulating transcription of specific target genes.

Multiple Effects of Hormones

• The same hormone can have different effects on different target cells depending on the receptors present in these respective cells.
• For instance, epinephrine can increase blood flow to skeletal muscle but decrease blood flow to the digestive tract.

Specialized Role of a Hormone in Frog Metamorphosis

• Thyroid hormone regulates metabolism and controls metamorphosis in frogs.

Endocrine Tissues and Organs

• Endocrine cells are often grouped in ductless organs called endocrine glands.
• Examples are thyroid and parathyroid glands, testes, and ovaries, which secrete hormones directly into surrounding fluids.
• Exocrine glands have ducts carrying secreted substances to body surfaces or cavities.

Major Human Endocrine Glands

• A diagram of the major human endocrine glands is included in the provided text to identify their location.

A Simple Endocrine Pathway

• An example of a simple endocrine pathway, such as secretin signalling, is provided.
• The stimulus of low pH in the duodenum causes S cells in the duodenum to release secretin.
• This, in turn, causes pancreatic cells to release bicarbonate, raising the pH in the duodenum.

A Simple Neuroendocrine Pathway

• In a straightforward neuroendocrine pathway, the stimulus received by a sensory neuron triggers a neurosecretory cell.
• The neurosecretory cell produces a neurohormone that enters the bloodstream and travels to target cells.
• Example: sucking by an infant initiates oxytocin-related signals affecting the hypothalamus, triggering oxytocin release from the posterior pituitary to cause milk release..

Feedback Regulation

• Negative feedback loops inhibit responses by reducing the initial stimulus, preventing overactivity.
• Positive feedback loops enhance stimulation to generate a greater response.

Invertebrates

• Insect molting and development are controlled by a combination of hormones.
• Prothoracicotropic hormone (PTTH) stimulates ecdysteroid release in insects.
• Juvenile hormone promotes larval characteristics.
• Ecdysone promotes molting/development. Both depend on levels of juvenile hormone.

Hormones and Biological Rhythms

• The pineal gland in the brain secretes melatonin.
• Light/dark cycles control melatonin release.
• Melatonin's primary functions relate to biological rhythms associated with reproduction and daily activity.
• The suprachiasmatic nucleus in the hypothalamus controls melatonin release.

Thyroid Regulation

• In humans, the thyroid hormone controls numerous functions.
• A drop in thyroid hormone levels triggers the hypothalamus to release TRH.
• TRH stimulates the anterior pituitary to release TSH, which in turn stimulates thyroid hormone release. This is a hormone cascade pathway involving negative feedback.

Disorders of Thyroid Function

• Hypothyroidism (low thyroid function) can result in symptoms such as weight gain, lethargy, and cold intolerance.
• Hyperthyroidism (excessive thyroid hormone production) can cause high temperature, sweating, weight loss, irritability, and high blood pressure.
• Malnutrition can also alter thyroid function.

Growth Hormone

• Growth hormone (GH) is secreted by the anterior pituitary gland
• GH promotes growth, affecting numerous metabolic processes.
• GH stimulates growth factor production
• Excess GH leads to gigantism; deficiency causes dwarfism.

Parathyroid Hormone and Vitamin D

• Two antagonistic hormones regulate blood calcium levels: parathyroid hormone (PTH) and calcitonin.
• PTH increases blood calcium levels via calcium release from bone, stimulating calcium reabsorption by the kidneys, and influencing vitamin D activation to promote calcium uptake from the intestines.
• Calcitonin decreases blood calcium levels promoting calcium deposition into bones and excretion by the kidneys.

Adrenal Hormones

• The adrenal glands are located atop the kidneys and consist of two parts: the adrenal medulla (inner) and the adrenal cortex (outer). Each part is responsible for a different hormonal response.

Sex Hormones

• The gonads (testes and ovaries) produce most sex hormones (androgens, estrogens, progestins)
• Testes primarily generate androgens (primarily, testosterone), which promote male reproductive system development and maintenance. Testosterone also influences muscle and bone mass and can be supplemented but carries risks.

Overview

• This text details the various hormones, the glands that produce them, and their functions in regulating body processes, development, and behavior.

Test your understanding of the key concepts in endocrine signalling. This quiz covers the roles and mechanisms of hormones, local regulators, and the differences between various types of signalling. Perfect for students studying biology or endocrinology.