Muscle Contraction Mechanisms

Twitch

• A rapid single muscle contraction due to a single stimulus
• The latent period is the time between the stimulus and contraction initiation
• The reason for this delay is that the impulse is propagated from the plasma membrane (PM) to the T-tubules to the sarcoplasmic reticulum (SR)
• The refractory period is the time when the muscle cannot respond to a second stimulus because sodium ions are still entering the cell
• The contraction period is the time from the initial contraction to maximum contraction force. The relaxation period is the time when muscle fibers return to their original position.
• Muscle fiber type affects the duration of the contraction period.

Wave Summation

• This occurs when a second stimulus is applied before the complete relaxation of the first stimulus.
• Leads to increased contraction strength.
• This appears to contradict the "all or none" principle, however, it is explained by increased calcium ion availability.

Treppe / Staircase Effect

• An increase in the force of muscle contraction with successive stimulations (with relaxation)
• Responsible for the "warm up" effect.
• Due to increased calcium ion availability, just like wave summation.

Tetany

• Sustained contraction resulting from successive stimulations
• Leads to increased contraction strength

Incomplete / Unfused Tetany

• There is partial relaxation before the next contraction

Complete / Fused Tetany

• Lacks partial relaxation

Clostridium Tetani

• An anaerobic bacterium that secretes a neurotoxin
• Leads to continuous muscle stimulation
• Result = tetanus (lockjaw)
• The toxin blocks GABA (inhibitory neurotransmitter), causing continuous firing of motor nerves and spastic paralysis.

Muscle Stretch

• Contraction force is strongest when actin and myosin overlap is maximized (greatest number of cross-bridges)
• Maximum overlap and highest tension occur when the muscle is slightly stretched.

Tension is lowest when:

• Sarcomeres are excessively stretched

Smooth Muscle

• Short cells with a single nucleus
• Increased mitotic capability compared to skeletal or cardiac muscle
• Actin and myosin are randomly arranged (smooth) – sarcomeres are not present.
• Lacks T-tubules and has a small SR
• Most of the calcium ions are extracellular and found in caveolae
• Contraction is similar to skeletal muscle: requires calcium ions, ATP, and the sliding filament mechanism.
• Contraction differs from skeletal muscle: uses signal transduction / second messenger systems / hormones and neurotransmitters.
• Lacks troponin – uses calmodulin to bind calcium ions.
• Calmodulin activates myosin ATPase / kinase, which activates the thick filament and leads to contraction.
• Slow, sustained contractions that last several seconds.

Single & Multiunit Smooth Muscle

• Single unit / Unitary / Visceral Smooth Muscle

  • Most common type
  • Located in the walls of hollow organs (e.g., GI tract, blood vessels)
  • Gap junctions allow cell to cell impulse transmission and the organ contracts as a unit.
  • Uses a pacemaker potential to initiate the impulse.
  • Controlled by the autonomic nervous system and endocrine system.
  • Arrangements in the GI tract: outer longitudinal layer and inner circular layer.
  • Peristalsis: rhythmic wave-like contraction of smooth muscle to propel "food" along the GI tract.

• Multi-Unit Smooth Muscle

  • Cells are more randomly organized.
  • Location: large airways, large arteries, eye, arrector pili, etc.
  • Lacks pacemaker cells and gap junctions.
  • Contraction is mainly mediated by the autonomic nervous system, hormones, and reflexes.

Disorders & Myopathies

• Flaccidity: Reduction of muscle tone, usually neurological (e.g., spinal cord damage). Muscles do not contract properly, leading to flaccid paralysis and atrophy.
• Spasticity: Excessive muscle tone leading to rigidity (spastic paralysis). Example: late-stage Parkinson's disease.
• Myalgia: Muscle pain due to any muscle disorder. Examples: fibromyalgia (tendons, ligaments, and fascia exhibit great pain), accompanied by fatigue, depression, etc.
• Spasm: Sudden involuntary muscle twitch, cause unclear, may be mild or severe.
• Tic: Spasmodic contraction of a muscle, often affecting the eye or face, may be psychological.
• Cramp: Painful, spasmodic muscle contraction without relaxation.
• Fasciculation: Involuntary and uncoordinated muscle contraction visible under the skin (e.g., ALS)
• Fibrillation: Uncoordinated muscle contraction not visible under the skin, often used to refer to cardiac muscle, can accompany a heart attack.
• Tendinitis: Inflammation of a tendon.
• Strain: "Pulled muscle," excessive stretching or tearing of a muscle fiber, causing inflammation and pain.
• Fibromyositis: Inflammation of muscle fibers, connective tissue coverings, tendons, and joint capsules, associated with pain, stiffness, and soreness.
• Charley horse: Fibromyositis of the quadriceps femoris due to a contusion.
• Botulism: Food poisoning caused by Clostridium botulinum. The toxin prevents acetylcholine release and action, causing paralysis.
• Convulsions: Violent involuntary muscle contractions, many causes.
• Myofascial Trigger Points / Muscle Knots: Irritable area of fascia surrounding taught muscles. Idiopathic, possibly due to overload, stress, or homeostatic imbalance.
• Amyotrophic Lateral Sclerosis (ALS): Aka Lou Gehrig's Disease. Progressive loss of motor function due to destruction of motor neurons. Does not affect mental capacity. 30,000 cases in the US. Some forms are genetic, most are "sporadic" and linked to free radical damage.
• Muscular Dystrophy: A group of genetic disorders characterized by progressive muscle weakness and degeneration.
• Dystrophin: The mutant gene / protein that stabilizes the sarcolemma during contraction.
• Duchenne Muscular Dystrophy: Sex / X-linked disorder characterized by weakness and shortening of muscles. Onset by age 3, wheelchair bound by age 12.
• Myotonic Muscular Dystrophy: Autosomal, slower progression, and affects fewer muscles.
• Myasthenia Gravis: Autoimmune disease affecting women aged 30-50, characterized by weak, fatigued muscles. Cause: auto-antibodies to the acetylcholine receptor.
• Aging: Muscle may atrophy and strength may decrease up to 50% by age 80. Muscle is lost and replaced by dense connective tissue (fibrosis) = sarcopenia.

Exercise Effects on Muscle

• Endurance Exercise: E.g., cycling or running

  • Increases muscle vascularization (probably all fiber types)
  • Increase in the number of mitochondria
  • Increased levels of enzymes for the Krebs cycle and electron transport system
  • Also, increases respiratory and circulatory system capacity.

• Strength Exercise: E.g., weightlifting

  • Results in hypertrophy: increased fiber diameter. Note: not due to increased mitosis of myocytes!