Muscle system

Introduction

  • Chapters ten and twelve are anticipated to be the hardest in the semester.
  • Importance of visuals: students will need to take detailed notes with illustrations since several will appear on the final exam.
  • Transitioning understanding for 24-02, upcoming class, involves adapting to different teaching styles and content.

Types of Muscle Tissue

  • Three types of muscle tissue: skeletal, cardiac, and smooth.
    • Skeletal Muscle:
    • Characteristics:
      • Voluntary control
      • Striated appearance due to transverse bands
      • Multinucleated (can have 2-50 nuclei per muscle fiber)
    • Cardiac Muscle:
    • Characteristics:
      • Involuntary control
      • Striated and branched structure
      • Each cell can connect with multiple others
    • Smooth Muscle:
    • Characteristics:
      • Involuntary control
      • Non-striated, smooth appearance
      • Each cell contains a single nucleus

Functions of Skeletal Muscle

  • Production of motion (muscle contractions enable movements).
  • Maintenance of body position (e.g., head and neck orientation).
  • Support of soft tissues (muscle tissue provides support when bone is not present).
  • Guards openings (e.g., orbicularis oculi around the eyes).
  • Maintenance of body temperature through heat production during muscle contraction (e.g., erector pili muscles causing goosebumps).
  • Calcium storage (the skeletal system stores calcium; parathyroid hormone facilitates calcium release from bones).
  • Proteins and amino acids are stored in muscles to be utilized during stress (e.g., during starvation).

Excitable Membranes

  • Definition: Membranes that can change their electrical membrane potential in response to stimuli (important feature of muscle and nerve cells).
  • Resting membrane potential: usually more negative inside the cell (e.g., -70 to -90 mV for muscle tissues).
  • Ion concentration: higher positive charge outside, negative inside.
    • Excitability: - Alters membrane potential when certain ion channels are opened or closed.
  • Ion channels:
    • Sodium has a positive charge and can influence membrane potential when it enters the cell.

Nerve Control and Action Potentials

  • All muscle movement is controlled by the nervous system; muscles will not contract independently.
  • Details of the action potential process:
    • Neurons at rest are approximately -70 mV.
    • Depolarization occurs upon reaching a threshold (approximately -55 mV), leading to opening of voltage-gated channels.
    • When enough sodium enters the cell, the potential can reach +30 mV (the action potential).
    • Signal propagation occurs where the action potential opens more voltage-gated channels down the axon.

Anatomy of Skeletal Muscle

  • Muscle fiber (cell):
    • Contains contracts and is characterized by length, multinucleation, and striated features.
    • Muscle fiber is composed of numerous myofibrils.
  • Connective Tissue Components:
    • Epimysium: wraps around entire muscle.
    • Perimysium: around fascicles (bundles of muscle fibers).
    • Endomysium: around individual muscle fibers.
    • All connective tissues converge to form tendons that attach muscle to bone.

Myofibrils and Myofilaments

  • Myofibrils are bundles of myofilaments (actin and myosin).
  • Myosin (thick filaments): composed of myosin proteins, interwoven with heads projecting outwards.
  • Actin (thin filaments): made of globular proteins (G-actin) that link to form fibrous strands (F-actin).
  • Sarcoplasmic Reticulum (SR):
    • Smooth endoplasmic reticulum specialized for calcium storage essential for muscle contraction.
    • T-tubules: Extensions of the sarcolemma (muscle cell membrane) that facilitate the transmission of action potentials deep into muscle fibers.
    • Triad Structure: A T-tubule flanked by two terminal cisternae (bulges of the SR).

Sarcomere Structure

  • The sarcomere is the functional unit of muscle fibers, extending from one Z line to the next.
  • Key Zones and Bands in Sarcomeres:
    • Z-disks: boundaries of the sarcomere.
    • A band: length of thick filaments (myosin), does not change during contraction.
    • I band: region containing only actin, shrinks during contraction.
    • H zone: area between actin fibers within the A band, decreases during contraction.
    • Zone of Overlap: Increased during contraction, where actin and myosin overlap each other.

Sliding Filament Theory

  • During muscle contraction, filaments don’t shorten; they slide over each other (myosin pulls actin).
  • Calcium ions released from the SR bind to troponin to expose myosin binding sites on actin, allowing for contraction to occur.
  • This reveals that contraction is enabled through the interaction of myosin heads with actin at actin’s active sites.

Conclusion

  • Skeletal muscle contains complex systems involving neural control and biochemical processes, leading to contraction and muscle movement.
  • Understanding these systems is essential in exploring muscle physiology further, particularly in practical applications related to exercise and muscle repair or growth.