Muscle Physiology: Contraction, Fatigue, and Fiber Types

Posted on Apr 8, 2025 in Human Biology

Muscle Fatigue

Muscle Fatigue: Muscle Damage (high tension), Substrate Depletion (CrP depletion for highly-intense exercise), Metabolite build up

Muscle Fiber Types and Metabolism

Twitch Velocity: Speed of contraction

  • Fast Fiber: Contracts quickly
  • Slow Fiber: Contracts Slowly

Metabolic: How ATP is replenished

  • Oxidative: Lots of mitochondria & capillaries
  • Glycolytic: Anaerobic, uses stored glycogen
  • Slow-oxidative fibers: Smaller & slow contract velocity, fatigue resistant
  • Fast-oxidative glycolytic: Medium, very fatiguable
  • Fast glycolytic: Fastest and largest, fast with fatigue resistance

Peripheral vs. Central Fatigue

Peripheral fatigue: Muscle is activated but there’s less tension (due to build up of metabolites, muscle damage, or substrate depletion)

Central fatigue: Doesn’t fully activate muscle anymore due to limitation of excitation of MN

Muscle Tension

Whole Muscle tension generated controlled by: Tension developed by each fiber & number of active fiber

Motor Units

Ventral horn of spinal cord: Where motor units are found/pool

Motor Unit Recruitment: A larger excitatory input to motor neuron pool recruits more motor neurons, recruits based on size. Slow oxidative first/smallest – fast oxidative – fast glycolytic

Motor Control Hierarchy

  1. Highest level: Initiates it (sensorimotor cortex; memory, emotions, and motivations)
  2. Middle Level: Plans it (Thalamus, cerebellum, brainstem)
  3. Local Level: Activates (receptors, spinal cord interneurons) Motor program works it down to local level

Smooth Muscle Contraction

Increase of Ca2+ -> Ca2+ binds to calmodulin -> activates myosin light-chain kinase (removes p from ATP) -> myosin light chain extends towards thin filament -> thick & thin interact (tension) -> myosin light-chain phosphates dephosphorylates light chain (muscle relaxes)

Smooth Muscle Characteristics

Smooth Muscle: Single nucleus, spindle-shaped, smaller than skeletal, lack striations, involuntary, responses to hormones, signals received can be excitatory or inhibitory

Smooth muscle contracts by sliding filament mechanism with thick and thin filament

Role of Cytosolic Ca2+

Role of Cytosolic Ca2+: Ca2+ comes from the ECF & plasmic reticulum. Moved in and out of the cell via calcium ATPase pump. Ca2+ is actively pumped into SR and extracellular space. Can be modulated for graded tension production.

Stimuli: Spontaneous electrical activity, ex pacemaker. NT released by ANS or other (hormones, acidity, oxygen)

Muscle Contraction Summaries

Smooth Muscle Summary: Increase cytosolic Ca2+ -> Ca2+ binds to calmodulin in cytosol -> Ca2+calmodulin complex binds to myosin light-chain kinase -> myosin light-chain kinase uses ATP to phosphorylate myosin cross-bridges -> phosphorylated cross-bridges bind to actin filaments -> cross-bridge cycle produces tension and shorting

Skeletal Muscle Summary: Increase cytosolic Ca2+ -> Ca2+ binds to troponin on the thin filaments -> conformational change in troponin moves tropomyosin out of blocking position -> myosin cross-bridges bind to actin -> cross-bridge cycle produces tension and shortening

Total Tension: Total number of myosin heads attached at once

An action potential generated at the neuromuscular Junction propagates deep inside of the muscle fibers via the t tubules

Within skeletal muscle, ca2+ triggers contraction by binding to troponin and moving tropomyosin

A small excretory command to the motor neuron pool will recruit only slow oxidative motor units

Which of the following statements is true regarding smooth muscle? Smooth muscle cells can be activated by neural inputs, stretch, hormones, or pacemakers

And in resting skeletal muscle, cross Bridge cycling does not occur because tropomyosin blocks Actin / myosin interaction

Which of the following is true with regard to the power stroke of the Cross Bridge cycle? It occurs when the myosin head binds to the actin and releases ATP and Pi

Compared to a slow fiber type, a fast fiber shortens more rapidly because It splits ATP more rapidly

A motor unit refers to all of the muscle fibers innervated by the same motor neuron

Isometric, active tension is greatest at intermediate lengths: all myosin heads having thin filament to which to attach

A somatic motor neuron can only excite skeletal muscle, it cannot inhibit skeletal muscle

In order for smooth muscle to contract, which of the following must occur? Phosphate groups must be enzymatically added to the myosin light chains

If the firing frequency of a skeletal muscle increases, tension will increase

The Detachment of the myosin head from the thin filament is caused by Binding of ATP to the myosin head

The energy system with the lowest power output but the greatest capacity is oxidative phosphorylation

Smooth muscle is involuntary and is typically found surrounding hollow tubes and organs

True about maximal eccentric contraction vs maximal concentric contraction: The maximal Eccentric contraction will generate more tension than the maximum concentric contraction / The maximum eccentric contraction will be working against a greater load than the maximum concentric contraction

Striated muscle includes skeletal muscle and cardiac muscle

Striated muscle is defined by the presence of Titan