MR

CH4 - TISSUES PT3 - Muscle and Nervous Tissue, Membranes, and Tissue Repair

Skeletal Muscle

  • Function: muscle tissue contracts and can stretch; critical for movement and force generation.
  • Types of muscle tissue in the body: 3 types: skeletal, cardiac, smooth.
  • Skeletal muscle characteristics:
    • Very long, cylindrically shaped cells that run parallel to each other; cells can be extremely long.
    • Each long cell is also referred to as a muscle fiber.
    • Multinucleated with nuclei located at the sides (peripheral nuclei).
    • Striated pattern visible under the microscope (alternating light and dark bands).
    • Voluntary control: contraction can be consciously initiated.
    • Connects to bones via tendons; contraction enables movement at joints.
    • Requires the ability to stretch to allow opposing muscles to extend; movement involves coordination of opposing muscle groups.
    • Regeneration: skeletal muscle cells do not divide to replace damaged cells; satellite cells exist but provide very limited regeneration.
  • Important notes to avoid confusion:
    • "Muscle fiber" = a muscle cell, not a protein fiber.
    • Skeletal muscle cells are long, cylindrical, multinucleated, peripheral nuclei, and striated.

Cardiac Muscle

  • Characteristics:
    • Also striated, but cells are branched and interconnected.
    • Typically has one nucleus per cell (could have more).
    • Intercalated discs: thicker cell membranes at junctions between cells.
    • Two key components:
      • Gap junctions: allow ion and electrical signal coupling between cells.
      • Desmosomes: provide strong adhesion to prevent cells from pulling apart during contraction.
  • Functional significance:
    • Branched network enables coordinated contraction of the heart as a pump.
    • Cells must beat as a unit to efficiently propel blood.
  • Innervation and control:
    • Cardiac muscle is involuntary (not under conscious control).
  • Tissue formation:
    • Cardiac muscle forms a tissue by grouping cells; a single cell is not a tissue on its own.

Smooth Muscle

  • Characteristics:
    • Long, tapered (banana-shaped) cells with a central nucleus.
    • Not striated.
    • Involuntary control.
  • Locations:
    • Found in internal organs (stomach, intestines) and walls of blood vessels.
  • Regeneration:
    • Smooth muscle has some regenerative capacity and can divide to replace damaged tissue.
  • Contrast with other muscles:
    • Unlike skeletal (voluntary, striated) and cardiac (striated, branched), smooth muscle is non-striated and more uniform in appearance.

Regeneration and Repair of Muscle Tissues

  • Regeneration capacities:
    • Smooth muscle: some regenerative ability.
    • Skeletal muscle: individual cells do not divide; satellite cells exist but regeneration is limited.
    • Cardiac muscle: essentially no capacity for regeneration.
  • Practical implication (e.g., heart attack):
    • Damaged cardiac muscle cannot be regenerated; repair occurs via stromal cells forming a collagen scar, which does not contract.
  • Summary:
    • Muscle tissues differ in regeneration potential; smooth muscle can regenerate to some extent; skeletal and especially cardiac muscle have limited or no regenerative capacity.

Nervous Tissue

  • Main cell types:
    • Neurons: large cell bodies with long extensions; capable of conducting electrical impulses; essential for communication in the nervous system.
    • Neuroglia (glial cells): support neurons, maintain a healthy environment (e.g., controlling what passes from blood into neural tissue); more numerous than neurons and capable of regeneration.
  • Neuron details:
    • Neurons have long extensions (axons and dendrites) that can be difficult to observe fully under a light microscope because they extend away from the plane of the slide.
    • Neurons do not undergo mitosis to regenerate once damaged.
  • Neuroglia details:
    • Provide structural and metabolic support; aid in maintenance of the neuron environment.
    • Can regenerate; important in disease contexts (e.g., brain cancers like gliomas arising from glial cells).
  • Clinical note:
    • The limited regenerative capacity of neurons makes nervous tissue particularly vulnerable to irreversible damage.

Membranes

  • General concept: membranes are usually epithelial membranes consisting of epithelial tissue (free surface) + connective tissue below it, connected by the basement membrane. The connective tissue component is typically areolar tissue; the combination is often referred to as a membrane.
  • Mucous membranes (mucosa):
    • Epithelium + areolar tissue (lamina propria) underneath.
    • Goblet cells present in mucous membranes produce mucus.
    • Epithelial types can vary (e.g., simple columnar or pseudostratified epithelium), but goblet cells are a hallmark.
    • Lamina propria: the areolar tissue component of mucous membranes.
    • Common locations: respiratory tract (ciliated pseudostratified), digestive tract (simple columnar), reproductive tract, urinary tract.
    • Practical implication: physicians may refer to the mucous membrane lining of organs (e.g., stomach) when discussing endoscopy findings.
  • Serous membranes (serosa):
    • Epithelium is always simple squamous; underlying areolar tissue.
    • Function: secrete watery fluid (serous fluid) to reduce friction between moving organs.
    • Examples: peritoneum (abdominal cavity), pleura (lungs), and pericardium (around the heart).
    • Visceral vs. parietal layers: visceral covers organs; parietal lines body cavities.
  • Cutaneous membrane (skin):
    • Not an epithelial membrane in the same sense; consists of keratinized stratified squamous epithelium (epidermis) on top of areolar tissue with underlying connective tissue layers.
    • Topmost cells are dead and keratinized, providing abrasion resistance and waterproofing.
  • Synovial membranes: an exception to the epithelial membrane rule
    • Location: surrounding joints, within the joint capsule.
    • Composition: primarily areolar tissue; no true epithelium; may appear as scattered epithelial cells in areolar tissue.
    • Function: secrete synovial fluid rich in hyaluronic acid; reduces friction between articulating surfaces.
    • Key components: areolar tissue and secreted hyaluronic acid (a thick, egg-white-like fluid) in the joint space.
  • Practical takeaway:
    • Membranes are usually epithelial membranes (epithelium + areolar tissue), except synovial membranes which lack a true epithelial layer.

Tissue Repair: Overview and Key Concepts

  • Which tissues repair well and why:
    • Epithelial tissue: repairs well due to high mitotic activity.
    • Connective tissue: generally repairs well (e.g., bone healing is robust).
    • Cartilage: poor repair due to lack of direct blood supply; relies on perichondrium for nutrients.
    • Muscle tissue: repair capacity varies; smooth muscle can regenerate somewhat; skeletal and cardiac muscle have limited or no regeneration.
    • Nervous tissue: neurons do not regenerate; glial cells can regenerate to some extent; overall repair is limited.
  • Case study example: cardiac tissue after heart attack
    • Death of cardiac muscle cells cannot be replaced by regeneration; stromal (connective tissue) cells form a collagen scar.
    • The scar is strong but non-contractile, reducing the functional capacity of the heart.
  • Two major cellular players in repair:
    • Stromal cells (e.g., fibroblasts): contribute to extracellular matrix and collagen deposition (fibrosis/scar formation).
    • Parenchymal cells: the functional cells of an organ; if they can divide and replace damaged cells (regeneration), better functional recovery occurs (e.g., liver).
  • Liver as an example of regeneration:
    • Parenchymal cells of the liver can undergo mitosis and regenerate damaged tissue, leading to true restoration of function.
  • General three-step repair process (inflammation-centric view): 1) Inflammation: damaged cells release intracellular contents; mast cells degranulate and release histamine; results in vasodilation and recruitment of white blood cells; redness and swelling observed. 2) Formation of granulation tissue: fibroblasts and new blood vessels invade the damaged area to reestablish blood supply and groundwork for repair. 3) Outcome dependent on cell type involved:
    • Fibrosis (scar formation): collagen deposition by stromal cells; functional tissue replaced by scar tissue.
    • Regeneration: parenchymal cells divide and restore original tissue function (where possible).
  • Factors influencing tissue repair:
    • Adequate blood flow to the tissue and nutrient supply are essential.
    • Vitamins and nutrition support collagen formation and new cell production.
    • Age affects repair efficiency; older individuals often have reduced repair capacity due to diminished blood flow and other vascular changes.
    • Diseases like uncontrolled diabetes can impair blood vessels and hinder repair processes.
  • Summary implications for medical practice:
    • Knowing tissue type and repair capacity helps in predicting outcomes after injury or disease.
    • Therapeutic strategies may aim to improve blood supply, nutrition, or modulate stromal vs parenchymal involvement to optimize healing.

Connections to Foundations and Real-World Relevance

  • Basements membranes: foundational connection between epithelium and connective tissue; critical for nutrient diffusion from connective tissue to the epithelium.
  • Lamina propria: a specific name for areolar tissue within mucous membranes; important for clinical descriptions of mucosal surfaces (e.g., gastroscopy findings).
  • Intercalated discs in cardiac tissue illustrate how structure supports synchronized function of the heart as a pump.
  • Synovial fluid mechanics: hyaluronic-acid-rich fluid provides lubrication that minimizes wear and tear on joints.
  • Regeneration vs fibrosis: understanding the balance between stromal and parenchymal repair informs treatment strategies after myocardial infarction or liver injury.

Key Takeaways (quick recap)

  • Four tissue types: epithelial, connective, muscle, nervous. Muscles (skeletal, cardiac, smooth) differ in structure, control (voluntary vs involuntary), and regeneration capacity.
  • Nervous tissue relies on neurons for signaling and glial cells for support; neurons have limited regenerative capacity.
  • Membranes are typically epithelial membranes consisting of epithelium + areolar connective tissue; synovial membranes are a notable exception.
  • Tissue repair proceeds through inflammation, granulation tissue formation, and final outcome via fibrosis or regeneration; capacity for regeneration varies by tissue type and organismal factors such as age and nutrition.