Chapter 6: Signaling and Homeostasis (LO 6.1-6.5)

Kiara Case Study

• Kiara’s body is described as composed of cells that respond to intercellular signals via intracellular signaling cascades.

• To operate effectively, Kiara requires both:

  • Global control (e.g., regulation of blood pressure)

  • Local control (e.g., contraction of specific muscles and directing blood flow)

• This illustrates the need for coordinated signaling across tissues and organs to maintain overall function.

Chapter 6 Learning Objectives (Overview)

• 6.1. Create a framework to describe and compare how signaling occurs between cells. Include five examples of types of signaling.

• 6.2. Create a framework to describe and compare how signaling occurs within cells. Include general principles and specific examples.

• 6.5. Create and describe a system model of how homeostasis is maintained. Include general principles and two or three examples. Describe why homeostasis is important.

Vocabulary and Intercellular Signaling (LO 6.1)

• Intercellular signaling: communication between cells across space

• Gap junctional signaling: direct channel between adjacent cells via gap junction proteins

• Contact-dependent signaling: signaling requiring direct cell–cell contact via membrane proteins

• Receptor: protein that binds a signaling molecule (ligand)

• Ligand: small molecule that binds to a receptor

• Diffusion: passive spreading of molecules through space

• Autocrine signaling: cell releases a signal that acts on itself

• Paracrine signaling: signal diffuses to neighboring cells

• Endocrine signaling: long-distance signaling via bloodstream

• Hormone: signaling molecule used in endocrine signaling

• Neuronal signaling: long-distance signaling via neurons and synapses

• Neurotransmitter: signaling molecule released by neurons at synapses

• Neurohormone: signaling molecule released by neurons into the bloodstream

Intercellular Signaling Types (LO 6.1)

• Gap Junctional Signaling

  • Gap junctions are protein channels that connect neighboring cells

  • Ions and small molecules can pass directly through

  • Enables direct communication (e.g., coordinating cardiac contractile cells)

  • Also used between some neurons

• Contact-Dependent Signaling

  • Direct communication via membrane-associated proteins (e.g., integrin receptors)

  • Coordinates cells in tissues

  • Requires cell–cell junction proteins in addition to integrins

• Ligands and Receptors

  • Ligands bind to receptors, which are proteins, usually larger than the ligand

  • Note: sometimes cells themselves are referred to as receptors; specify by saying protein receptor when needed

• Autocrine and Paracrine Signaling

  • Diffusion of signaling molecules

  • Autocrine: signal released and received by the same cell

  • Paracrine: cytokines diffuses to nearby cells

  • Diffusion is inefficient for long-distance signaling (addressed further in 5.3)

• Endocrine Signaling

  • Long-distance signaling affecting the whole body via hormones

  • Transported by blood; hormones diffuse short distances to target cells from blood

  • Secreted by endocrine tissues; reach all body cells but act at low concentrations

  • Response depends on target cell receptors and state of responding cells

• Neuronal Signaling

  • Long-distance signaling (anywhere in the body) targeting specific cells or tissues

  • Neurotransmitters cross synapses to target cells (neurons, muscles, some endocrine glands)

  • Neurohormones released by neurons into the blood can affect the whole body

Frameworks for Signaling (6.1) and Within-Cell Signaling (6.2)

• Creating a framework to describe intercellular signaling (6.1)

  • Pick terms that belong to the concept of intercellular signaling (e.g., the six types above)

  • Organize into categories of signaling first, then add mechanisms and details later

  • Example exercise: organize terms as a map (e.g., grouping by distance, mechanism, or target)

• Creating a framework for intracellular signaling (6.2)

  • General principles: receptors sense signals, transduction cascades propagate signals inside the cell, and responses are produced via second messengers and effectors

  • Receptor types: cytosolic receptors (lipophilic ligands) and membrane receptors (lipophobic ligands)

  • Membrane-receptor subtypes include: receptor channels, G-protein coupled receptors (GPCRs), receptor enzymes, integrin receptors

  • Common second messengers: $ext{Ca}^{2+}, ext{cAMP}$

  • Signaling cascades involve transducers and amplifier enzymes that alter intracellular signaling molecules and, ultimately, target proteins

  • Outcomes can be diverse (enzymatic activity changes, gene expression, cytoskeletal rearrangements)

  • Key tip: organize terms from general to specific; use broad categories to organize details, then add specific examples

Intracellular Signaling: Types of Receptors (LO 6.2)

• Cytosolic receptors

  • Bind ligands that cross membranes (lipophilic)

  • Example: steroid hormones binding to cytosolic receptors; often affect gene expression (slower actions)

• Membrane receptors

  • Bind ligands that cannot cross membranes (hydrophilic or large)

  • Often act by altering enzyme activity (relatively fast)

Membrane Receptors (Subtypes)

• Receptor channels

• G-protein coupled receptors (GPCRs)

• Receptor enzymes

• Integrin receptors

Signaling Pathways (LO 6.2)

• A signaling molecule (ligand) binds to a receptor (membrane or intracellular)

• Receptor activation triggers a signaling cascade inside the cell

• Intracellular signaling molecules and second messengers propagate, amplify, and transduce the signal

• Cascades lead to changes in state/concentration of cellular components, resulting in a response

• Framework utility: helps organize physiology from molecular to organ level

Second Messengers and Transduction (LO 6.2)

• Second messengers: small molecules whose concentrations rise in response to receptor activation

• Common second messengers: $ext{Ca}^{2+}, ext{cAMP}$

• Transducer proteins and amplifier enzymes modulate these messengers

• Protein kinases phosphorylate target proteins to alter activity

• Example schematic (textual):

  • Signal molecule binds membrane receptor → receptor activates intracellular signaling proteins → amplifier enzymes raise second messengers (e.g., Ca^{2+}, cAMP) → kinases phosphorylate target proteins → cellular response

• Notes on signaling pathways

  • Pathways can converge (different signals produce the same response) or diverge (one signal triggers multiple responses)

  • Review additional 6.2 examples on your own; this is foundational to prerequisites in physiology

Practice and Review (Selected Quiz-Style Items)

• Long-distance signaling: select all that apply

  • a. endocrine signaling

  • b. neuronal signaling

  • c. paracrine signaling

  • d. autocrine signaling

  • Answer: a and b (long-distance); paracrine and autocrine are localized

• Signaling types that require a ligand binding to a receptor: select all that apply

  • a. Endocrine signaling

  • b. Gap junctional signaling

  • c. Neuronal signaling

  • d. Paracrine signaling

  • e. Autocrine signaling

  • Answer: a, c, d, e (endocrine, neuronal, paracrine, autocrine all involve receptor-ligand interactions; gap junctional signaling does not rely on a ligand-receptor interaction in the same way)

• Correct matches for signal transduction proteins (choose all correct):

  • a. G-protein-coupled receptor – ligand binds intracellularly, ions and small molecules flow through the receptor into the cell → False

  • b. Receptor enzyme – ligand binds extracellularly, receptor changes shape, which activates an enzyme → True

  • c. Ion channel receptor – ligand binds to receptor, opens the channel and allows ions to flow through the channel → True

  • d. Integrin receptors – bind to cytoskeleton, no ligand binding → False

Human Homeostasis and System Models (LO 6.5)

• Homeostasis: the ability to maintain a relatively stable internal environment

• Key parameters typically maintained (examples): blood pressure, blood glucose, body temperature, pH, etc. (students may write three examples)

• System model components (general): input signals, integrating center, effectors, outputs, and feedback loops

• Feedback types

  • Negative feedback: parameter is returned toward the set point (homeostatic) – stabilizes the system

  • Positive feedback: stimulation drives more stimulation in the same direction (not homeostatic in isolation)

  • Feed forward: anticipatory changes to prepare the body for a change

• Cannon’s Postulates of Homeostasis
  1) The nervous system can regulate the internal environment
  2) Many systems have tonic activity (constant, mid-range activity) above threshold and below saturation
  3) Many systems have antagonistic control (e.g., insulin lowers blood glucose; glucagon raises it)
  4) Chemical signals can have different effects on different tissues

• Neuronal and Tonic Control (example: heart rate)

  • Tonic rate of action potentials can increase or decrease to modulate response

  • System model of control of blood vessel diameter (example exercise prompt): consider how altering neuronal tone changes vascular resistance

• Neuronal and Antagonistic Control

  • Antagonistic control: balance between inputs determines the final state

  • Sympathetic (adrenergic) activation increases heart rate; Parasympathetic activation decreases heart rate

• Epinephrine (adrenaline) and adrenergic receptors

  • Epinephrine can bind alpha adrenergic receptors (in intestinal vessels) or beta adrenergic receptors (in skeletal muscle vessels) and elicit opposing effects via different second messenger pathways (notably cAMP)

  • Alpha receptor activation tends to increase certain second messenger levels and cause vasoconstriction; Beta receptor activation can cause vasodilation in skeletal muscle

  • This illustrates how a single ligand can have tissue-specific, opposing effects depending on receptor subtype

• System Models: practical guidance

  • System models focus on mechanisms; components are labeled with verbs showing interactions

  • You should create one large model or several smaller models for different objectives (6.5)

  • Strong verbs (e.g., activates, inhibits, binds, transport) help describe relationships in the model

• Heart Rate Homeostasis – Example System Model

  • Components: Nervous System (Sympathetic and Parasympathetic inputs), Integrating Center, Heart Rate, Tonic Activity, Threshold, Saturation, and target responses

  • Relationships described with verbs showing how each part influences heart rate to maintain blood pressure

  • Example labels: Sympathetic activation increases heart rate; Parasympathetic activation decreases heart rate; Integrating center modulates net output; tonic activity maintains baseline

• Practice: Build and refine your own system models for homeostasis (e.g., tonic neuronal control of heart rate)

• Assessment items (from slides): select terms applicable to the heart-rate example (e.g., sympathetic, parasympathetic, integrating center, tonic activity, antagonistic control, heart rate, etc.)

Notes on Creating Frameworks and Studying (LO 6.1, LO 6.2, LO 6.5)

• Why frameworks and models help:

  • Organizes data to improve understanding and communication

  • Allows adding details and examples as you learn more

  • Helps with chunking information for memory and exam recall

  • Enables connections between ideas and real-world relevance

• Step-by-step approach to framework construction (as illustrated in the slides):

  • Step 1: Select terms relevant to the objective (e.g., 6.1 intercellular signaling types)

  • Step 2: Organize terms into categories (distance-based, mechanism-based, etc.)

  • Step 3: Add organizing principles (patterns, groups, and relationships)

  • Step 4: Expand with examples, details, and notes

• Study tips included in the slides:

  • Write by hand or with digital tools to reinforce learning

  • Record a brief outline or podcast to reinforce understanding

  • Continuously add information as you encounter more details

  • Expect to spend roughly 2–2.5 hours after each lecture on the material

Additional Review Topics (from quiz-style prompts in the slides)

• Which signaling types are long-distance? Endocrine and Neuronal signaling

• Which signaling types require ligand binding to a receptor? Endocrine, Neuronal, Paracrine, Autocrine

• Matching signal transduction proteins to their functions (correct matches):

  • G-protein coupled receptor – ligand binds extracellularly (not intracellularly); this option is false

  • Receptor enzyme – ligand binds extracellularly, receptor changes shape, activates an enzyme → true

  • Ion channel receptor – ligand binds to receptor, opens channel, ions flow through → true

  • Integrin receptors – bind to cytoskeleton with ligand binding involved; this option as stated is false

End-of-Chapter Reflections and Final Thoughts

• There is no single right system model; multiple valid models can exist depending on scope

• Sharing models in discussions can broaden understanding and generate new ideas

• The course emphasizes understanding over rote memorization; focus on conceptual frameworks and their application

What about Kiara? (Reinforcement)

• Kiara’s body exemplifies how cells respond to intercellular signals with intracellular cascades

• Effective operation requires both global and local control mechanisms

• The combination of intercellular signaling and intracellular cascades underpins the ability to regulate systemic functions such as blood pressure and localized actions like muscle contraction and directing blood flow