MUSCLE NOTES FROM LECTURE VIDEO

Muscle Types

I. Types of Muscle Tissue

• The human body contains three muscle types:
    1. Skeletal muscle:
       - Attached to the skeleton via tendons.
       - Functions to support and enable movement of the skeleton.
       - Contractions are initiated by action potentials from motor neurons.
       - Controlled voluntarily.
    2. Smooth muscle:
       - Located in the walls of hollow organs and tubes.
       - Functions to facilitate internal movements.
       - Controlled involuntarily by the autonomic nervous system, hormones, and other signals.
    3. Cardiac muscle:
       - Found exclusively in the heart.
       - Functions to propel blood through the circulatory system.
       - Regulated involuntarily by the autonomic nervous system, hormones, and other signals.

Muscle Terminology

• Muscle Equivalent
    - Muscle cell: Muscle fiber
    - Plasma membrane: Sarcolemma
    - Cytoplasm: Sarcoplasm
    - Mitochondria: Sarcosomes
    - Sarcoplasmic reticulum (SR): Modified endoplasmic reticulum

Skeletal Muscle

9.1 Structure

• Skeletal muscles:
    1. Composed of cylindrical, multinucleated muscle fibers (cells) derived from myoblasts.
    2. Linked to bones by tendons at each end.
    3. Satellite cells:
       - Undifferentiated stem cells that proliferate and differentiate into myoblasts in response to damage.
    4. Hypertrophy:
       - Increase in size of muscle fibers, often due to injury or exercise.

• Components and Functions of a Skeletal Muscle Fiber
    - Fiber Components:
      - Sarcolemma: Cell membrane, contains T-tubules that conduct action potentials.
      - Sarcoplasm: Cytoplasm of muscle fiber, contains myofibrils, SR, and other organelles.
      - Myofibrils: Composed of actin (thin filaments) and myosin (thick filaments).
      - Sarcoplasmic Reticulum: Stores calcium ions.
      - Mitochondria/Sarcosomes: Sites for ATP production; glycogen is stored for energy.
      - Multiple Nuclei: Muscle fibers have multiple nuclei due to fusion of myoblasts.
      

Molecular Mechanisms of Skeletal Muscle Contraction

9.2

1. Contraction Definition: Activation of cross-bridges, the force-generating sites within muscle fibers.

2. Relaxation: Mechanisms that generate force are turned off, leading to lowered tension.

Neuromuscular Junction

1. A neuromuscular junction is the synapse of a motor neuron with a muscle fiber's motor end plate.

2. The motor end plate is the section of the muscle fiber membrane directly underneath the axon terminal.

3. All neuromuscular junctions are excitatory.

4. The synaptic junction contains acetylcholinesterase, an enzyme that degrades ACh, similar to ACh-mediated synapses found in the nervous system.

Motor Units

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

2. A single motor neuron can control multiple muscle fibers; however, each muscle fiber is innervated by a branch of only one motor neuron.

3. A motor unit contracts as a whole when the motor neuron generates an action potential.

4. A single muscle may comprise numerous motor units.

Excitation-Contraction Coupling

• Definition: Sequence leading from action potentials to muscle fiber contraction.

1. Thin Filaments:
      - Composed mainly of actin, with regulatory proteins troponin and tropomyosin.
      - Actin has binding sites for myosin.
      - Troponin:
        - Interacts with actin and tropomyosin.
        - Composed of three subunits:
          - I: Inhibitory.
          - T: Tropomyosin-binding.
          - C: Calcium-binding.
      - Tropomyosin: Rod-shaped, composed of two polypeptides, blocks myosin-binding sites in a relaxed state.

2. Thick Filament:
      - Primarily made of myosin molecules with cross-bridges that have binding sites for actin and an enzymatic site (myosin-ATPase).

3. Calcium's Role:
      - Relaxation occurs when calcium levels decrease.
      - Calcium binds to troponin, causing tropomyosin to shift and expose binding sites for cross-bridges.

Sliding-Filament Mechanism

1. Force generation involves the overlapping thick and thin filaments shortening the sarcomeres.

2. Filament lengths do not change; only overlap varies.

Crossbridge Cycle

• Involves cyclical formation and movement of actin and myosin, each cycle producing minimal movement.

• ATP Functions:
    1. Allosterically induces cross-bridges to detach from actin.
    2. Provides energy for cross-bridge movement.
     

Sequence of Muscle Contraction

• Order of Events:
    1. Action potential triggers Ca2+ release.
    2. Cross-bridge cycling begins.
    3. ACh is released from motor neurons and binds to receptors at motor end plate, generating an end-plate potential (EPP).
    4. Ca2+ actively transported back to sarcoplasmic reticulum.
    5. Action potential propagates along the sarcolemma and into T-tubules.
    6. Ca2+ binds to troponin, exposing myosin-binding sites.
    7. Tropomyosin blocks binding sites on actin.

Muscle Mechanics

9.3 Mechanics of Single-Fiber Contraction

1. Muscle tension: Force exerted by a contracting muscle.

2. Load: Resistance against which the muscle contracts.

3. The mechanical response of a muscle fiber is known as a twitch.

Features of Isotonic Twitch

1. Latent Period: Time from action potential to contraction onset due to excitation-contraction coupling processes.

2. Contraction Time: Time from the beginning of tension development to peak tension.

3. Relaxation Time: Time from peak tension to zero tension.

4. Types of Isotonic Contraction:
       - Concentric: Muscle shortens as tension exceeds load.
       - Eccentric: Muscle lengthens as load exceeds tension.
      

Types of Isometric Contraction

• A situation where tension is developed while muscle length does not change.

• Two scenarios:
     1. Muscle supports a load at a constant position.
     2. Load exceeds tension developed.

Effects of Load on Muscle

1. Increased load increases latent period and decreases distance of shortening, affecting velocity of contraction.

Frequency-Tension Relation

Summation

1. Defined as increased muscle tension from successive action potentials.

Tetanus

1. Tetanus: A sustained contraction from repetitive stimulation. Shows two forms:
       - Unfused Tetanus: Incomplete contraction with partial relaxation.
       - Fused Tetanus: Complete contraction without relaxation.

Length-Tension Relation

1. Passive Tension: Increases with stretch due to titin.

2. Active Tension: Greatest at optimal fiber length, or L0.

Skeletal Muscle Energy Metabolism

ATP Production

1. ATP generated via three methods:
      1. Phosphorylation of ADP by creatine phosphate.
      2. Oxidative phosphorylation of ADP in mitochondria.
      3. Glycolytic pathway in the cytosol.

2. Energy Sources:
       - Low-Intensity Exercise: Primarily muscle glycogen use at onset, transitioning to glucose and fatty acids, and finally fatty acids during prolonged activity.

Muscle Fatigue

1. Decline in muscle tension from prior contractile activity.

2. Acute Fatigue Causes:
      - Decrease in ATP concentration.
      - Increase in ADP, Pi, Mg2+, H+, oxygen free radicals.
      - Effects include:
        1. Decreased Ca2+ uptake by the sarcoplasmic reticulum.
        2. Reduced sensitivity of thin filaments to Ca2+.
        3. Inhibition of binding and movement of cross-bridges.

3. Central Command Fatigue: Inability of the cerebral cortex to send excitatory signals to motor neurons.

Types of Skeletal Muscle Fibers

Fiber Types

1. Fast-twitch fibers vs. slow-twitch fibers:
      - Fast-twitch fibers have a higher power output and fatigue rapidly.
      - Slow-twitch fibers have a slower speed with endurance.

2. Glycolytic vs. Oxidative Fibers:
      - Oxidative fibers contain myoglobin, an oxygen-binding protein.

Fiber Characteristics

Whole-Muscle Contraction

1. Muscle contractions are influenced by fiber type distribution and their maximal contraction speed and strength.

2. Control of Muscle Tension:
      - Total tension relies on the amount of tension developed by each fiber and the number of contracting fibers.

3. Motor Unit Size:
      - Smaller motor units for delicate movements; larger motor units for coarse control.
      - Recruitment: Process of increasing muscle tension by activating more motor units.

Muscle Adaptation to Exercise

1. Regular activity leads to hypertrophy of muscle fibers and changes in ATP production capacity.

2. Atrophy Types:
      - Disuse Atrophy: From lack of use (such as a limb in a cast).
      - Denervation Atrophy: From nerve damage.

Skeletal Muscle Disorders

1. Muscle Cramps: Involuntary tetanic contractions due to dehydration or electrolyte imbalances.

2. Hypocalcemic Tetany: Excessive contractions when extracellular Ca2+ drops, leading to spontaneous Na+ channel opening.

3. Muscular Dystrophy: Genetic disorders resulting from defects in stabilizing muscle membrane proteins. E.g., Duchenne muscular dystrophy causes progressive degeneration.

4. Myasthenia Gravis: Autoimmune disorder affecting ACh receptors, leading to loss of skeletal muscle activation ability.

Structure of Smooth Muscle

9.8 Smooth Muscle

1. Composed of spindle-shaped cells with single nuclei and capable of division.

2. Thick and thin filaments lack organization into myofibrils and are non-striated.

3. Contain thick myosin and thin actin filaments without troponin; filaments anchored to plasma membrane or dense bodies.

Smooth Muscle Contraction Control

1. Cross-bridge cycling regulated by calcium-dependent myosin light-chain kinase.

2. Dephosphorylation is required for muscle relaxation, mediated by myosin light-chain phosphatase.

Membrane Activation in Smooth Muscle

1. Smooth muscle responses can be graded, with inputs being excitatory or inhibitory.

2. Smooth Muscle Tone: Baseline cross-bridge activity under weak stimuli.

Sources of Cytosolic Ca2+ in Smooth Muscle

1. Release from the sarcoplasmic reticulum (SR).

2. Extracellular Ca2+ sources contribute to contraction.

Cardiac Muscle

9.10 Cardiac Muscle

• Combines features of both skeletal and smooth muscle.

1. Nodal Cells: Automatically generate action potentials (automaticity or autorhythmicity).

2. Cardiac Muscle Characteristics:
      - Striated with centralized one to two nuclei.
      - Contraction follows the sliding-filament mechanism.
      - Intercalated disks contain desmosomes and gap junctions for communication between cells.

Action Potentials in Cardiac Muscle

1. Absolute refractory period lasts about 250 milliseconds, preventing tetanic contractions.

Comparative Table of Muscle Cell Characteristics