Lecture 04_RV

Homeostasis and Signalling

Introduction

Professor Rob Vandenberg, School of Medical Sciences, Faculty of Medicine and HealthFocus on Homeostasis and Signalling, Feedback Loops, and Human Biology 2024

Major Concepts

Stability Maintenance in the Body:

• Feedback loops: Positive and negative loops play critical roles in maintaining homeostasis.

• Communication: Cells, tissues, organs, and the external environment communicate through various signaling mechanisms to maintain internal stability.

• Process Model: The process of homeostasis can be modeled as Stimulus → Sensor → Control → Effector, where a specific change in the environment triggers a response to restore equilibrium.

Homeostasis

Body Temperature Regulation

• Body Temperature Falls:

  • Blood vessels constrict (vasoconstriction) to conserve heat, reducing blood flow to the skin.

  • Sweat secretion is minimized to prevent heat loss.

  • Shivering occurs as involuntary muscle contractions generate heat to raise body temperature.

• Body Temperature Rises:

  • Blood vessels dilate (vasodilation), increasing blood flow to the skin, allowing heat dissipation.

  • Sweat glands secrete fluid which evaporates, further cooling the body.

• Normal Body Temperature:

  • The normal range of body temperature is typically around 36.1°C to 37.2°C (97°C to 99°F) and is regulated through negative feedback mechanisms to ensure it stays within this range.

Negative Feedback

Definition

• Negative feedback refers to the maintenance of internal consistency; it describes a feedback mechanism that counters changes from set-points, promoting stability in physiological conditions.

Limits

• Systems maintain a lower limit, an upper limit, and a set point to regulate physiological parameters effectively.

Feedback Loops

Types of Feedback Loops

• Negative Feedback:

  • Negative feedback loops are essential for processes such as regulating body temperature and blood glucose levels, returning the system to its set point after deviations.

• Positive Feedback:

  • Positive feedback loops contract changes by amplifying the original stimulus, moving the system further away from its starting state, often seen during processes such as childbirth (e.g., oxytocin during labor).

Systems for Homeostasis

Nervous System

• The nervous system provides rapid electrical communication throughout the body, allowing for immediate responses to stimuli and maintaining homeostasis by regulating physiological functions.

Endocrine System

• The endocrine system utilizes slower-acting chemical signals (hormones) to regulate homeostasis over longer periods, coordinating complex physiological processes such as metabolism, growth, and reproduction.

Endocrine System Overview

Functions and Mechanisms

• Various organs release hormones directly into the bloodstream, acting upon target cells to mediate physiological responses.

• Hormones: Chemical or organic molecules that serve as messengers, coordinating various bodily functions.

• Endocrine Definition: Internal secretion refers to the type of secretion related to glands that discharge substances directly into the blood.

• Endocrine Gland: These are ductless glands that release internal secretions into the bloodstream or lymphatic system, differing from exocrine glands that use ducts to secrete on epithelial surfaces.

Organs Involved in Endocrine Function

• Brain: Regulates hormonal functions and bodily responses.

• Pituitary gland: Often dubbed the master gland, controlling other endocrine glands.

• Pineal gland: Regulates sleep through melatonin secretion.

• Hypothalamus: Links the nervous system to the endocrine system and regulates key homeostatic functions.

• Other glands include the Thymus, Thyroid, Pancreas, Adrenal glands, Ovaries, and Testes.

Functions of Hormones

• Reproduction, Growth, and Development: Hormones like sex steroids, thyroid hormones, and prolactin orchestrate these processes.

• Maintenance of Internal Environment: Hormones such as aldosterone, parathyroid hormone, and vitamin D play critical roles in sustaining overall health and homeostasis.

• Energy Production, Utilization, and Storage: Insulin, glucagon, thyroid hormones, cortisol, and growth hormone regulate energy balance within the body.

Signalling Mechanisms

Insulin as a Signalling Molecule

• Function: Insulin binds to specific receptors on the cell membrane, triggering intracellular signaling pathways that lead to various cellular actions, including glucose uptake.

General Signalling Process

• Insulin interaction with target cells requires a high-affinity and specificity binding to receptors, which elicits a tailored response in those tissues.

Response to Signals

• Cellular responses to signaling can include: secretion, contraction, movement, cell division, survival, catabolism, or differentiation.

Second Messengers in Signalling

Characteristics and Function

• Water-soluble Hormones: Such as insulin and peptide hormones cannot cross cellular membranes freely and require transmembrane receptors to initiate signaling cascades.

• Phosphorylation Dynamics: The binding of ligands to receptors activates intracellular signaling pathways through phosphorylation, a key mechanism in regulating cellular activities.

Phosphorylation Effects

• Dramatic changes: Phosphorylation alters a protein’s charge and conformation, resulting in significant changes in its functional activity.

• Phosphorylation Cascades: Signaling cascades mediated by protein kinases allow for signal amplification and the elicitation of diverse biological responses, ensuring effective communication within cells.

Types of Signalling

• Direct Phosphorylation vs. Second Messengers: By differentiating between direct phosphorylation (cell signaling) and secondary messengers like cAMP and Ca²⁺, we can understand various signaling pathways.

• Calcium’s Role: Calcium concentration fluctuations within a cell induce profound cellular actions like neurotransmitter release in neurons, muscle contraction, and cellular metabolism regulation.

Steroid Hormones and Their Mechanisms

Steroid Hormone Action

• Oestradiol Example: A key player in reproductive physiology, oestradiol rapidly crosses cell membranes, interacts with intracellular receptors, and regulates gene transcription linked to growth and menstruation.

Hormone-Receptor Complex Mechanism

• Upon entering the target cell, the hormone binds to its specific intracellular receptor, resulting in a hormone-receptor complex that translocates to the nucleus. This complex subsequently influences the transcription of target genes, modulating physiological processes.

Pituitary Gland and Its Functions

Overview of Pituitary Gland

• Referred to as the master gland due to its regulatory control over various endocrine glands and numerous essential bodily functions.

Pituitary Hormones

• Produces a range of hormones including:

  • TSH (Thyroid-stimulating hormone): Influences thyroid function and metabolism.

  • ACTH (Adrenocorticotropic hormone): Stimulates cortisol production from the adrenal glands.

  • Growth hormone: Essential for growth, metabolism regulation.

  • Antidiuretic hormone (ADH): Regulates water retention in the kidneys.

  • Oxytocin: Facilitates childbirth and lactation.

  • Prolactin: Stimulates milk production.

  • Gonadotropic hormones (FSH, LH): Control reproductive processes.

Specific Functions of Hormones

• Oxytocin: Induces uterine contractions during labor and triggers milk ejection during breastfeeding.

• ADH (Antidiuretic Hormone): Released in response to elevated blood osmolarity or decreased blood volume/pressure, it prevents water loss through renal mechanisms, hence supporting fluid balance in the body.

Learning Objectives

• Understand and articulate the roles of positive and negative feedback loops in maintaining homeostasis.

• Describe intercellular communication mechanisms and their physiological implications.

• Identify major intracellular signs and their respective functions within different biological contexts.

• Appreciate the pituitary gland's pivotal role in hormonal regulation across various organ systems, and its critical influence on homeostasis.