BIOL5_Ch12_MuscPhys

Muscular Physiology Overview

• Focuses on the structure and function of muscle tissue.

• Includes various muscle types (skeletal, cardiac, smooth) and their contraction mechanisms.

Chapter Outline

1. Review: Structure of Skeletal Muscles

2. Motor Unit

3. Events at the Neuromuscular Junction

4. Excitation-Contraction Coupling

5. Sliding Filament Mechanism

6. Twitch, Summation, and Tetanus

7. Smooth and Cardiac Muscle Contraction

Skeletal Muscles

• Attached to bone on each end by tendons.

• Insertion: more movable attachment.

• Origin: less movable attachment.

• Contraction of muscles causes tension on tendons, moving bones at a joint.

• Flexors decrease the angle of joints; Extensors increase it.

Skeletal Muscle Structure

• Muscle fibers: muscle cells, multinucleated, striated.

• Sarcolemma: plasma membrane of muscle fibers.

• Distinctive striations visible due to the arrangement of myofibrils.

Structure of Muscle Fiber

• Each muscle fiber consists of myofibrils (1μm in diameter).

• Myofibrils: filled with myofilaments (thick and thin filaments).

• Thick filaments: made of myosin.

• Thin filaments: made of actin.

• Sarcolemma and sarcoplasm also present in muscle fibers.

Myofibrils

• Organelles containing protein fibers essential for contraction.

• Composed of sarcomeres arranged end to end.

Structure of Sarcomeres

• A band: dark area; contains thick filaments (myosin).

• H band: area in the center of the A band where actin and myosin don't overlap.

• I band: light area; contains thin filaments (actin).

• Z line/disc: center of the I band, where actin attaches.

Motor Unit

• Each motor neuron branches to innervate multiple muscle fibers.

• A motor unit consists of one motor neuron and all fibers it innervates.

• Activation leads to all muscle fibers in the motor unit contracting sequentially.

• Innervation Ratio: varying from 1:100 to 1:2000, indicating fine control.

• Recruitment: process of activating more motor units for smooth muscle operation.

Events at the Neuromuscular Junction

1. Ca2+ influx: opens voltage-gated channels.

2. Exocytosis of ACh into the synaptic cleft.

3. ACh binds to receptors, causing muscle depolarization and action potential.

4. ACh is broken down, leading to muscle relaxation.

Proteins in Muscle Contraction

• Contractile Proteins: Myosin and Actin.

• Regulatory Proteins: Troponin and Tropomyosin.

• Structural Proteins: Titin, Nebulin, Alpha-actin, Myomesin, Dystrophin.

Muscular Dystrophy

• Genetic disorders causing skeletal muscle degeneration and replacement with scar tissue.

• Duchenne Muscular Dystrophy: most common form, X-linked, leads to muscle necrosis.

Excitation-Contraction Coupling

• Step 1: Sarcolemma conducts action potentials from ACh release.

• Step 2: Depolarization leads to T-tubule action potential.

• Step 3: VG Ca2+ channels in sarcoplasmic reticulum (SR) cause Ca2+ release.

• Contraction initiated by tropomyosin movement exposing myosin binding sites.

• Relaxation: Ca2+ pumped back into SR, blocking myosin sites.

Sliding Filament Theory

• Muscle contractions involve the sliding of thin filaments over thick filaments, causing sarcomere shortening.

• A bands move closer; I bands shorten during contraction.

Muscle Metabolism

• ATP is crucial for contraction and relaxation; limited to a few seconds without replenishment.

• Methods of ATP production include:

  1. Creatine phosphate (15 seconds)

  2. Anaerobic glycolysis (2 minutes)

  3. Cellular respiration (>2 minutes).

• Muscle Fatigue: results from inadequate Ca2+, nutrient depletion, lactic acid accumulation.

Muscle Fiber Types

• Slow Oxidative (SO): endurance; high mitochondria.

• Fast Glycolytic (FG): power; fatigue easily.

• Fast Oxidative-Glycolytic (FOG): intermediate properties.

• Recruitment order: SO → FOG → FG.

Cardiac and Smooth Muscle

• Cardiac Muscle: striated, involuntary, branched, intercalated discs.

• Smooth Muscle: no striations, more actin than myosin, contracts as a single unit (visceral) or individually (multi-unit).

• Contraction regulated through Ca2+ and influenced by hormones and local factors.