Muscle Contraction: Tension and Motor Units

Length-Tension Relationship

• Muscle fibers generate peak tension when thick and thin filaments have optimal overlap.
• Thicker muscle fibers have more myofibrils, producing more tension than thinner fibers

Motor Unit Recruitment

• This process increases the number of active motor units.
• The size of activated motor units impacts muscle tension.
• Muscles needing precise movements use small motor units, exerting small amounts of force.
• Motor units of an entire muscle do not contract in unison.
• Asynchronous recruitment means some motor units contract while others relax.
• This pattern delays muscle fatigue, enabling sustained contractions.
• Smaller motor units activate first, followed by larger units as force needs increase.
• Recruiting or deactivating a motor unit results in minor changes in muscle tension.

Muscle Tone

• A small amount of tension develops in a muscle due to weak, involuntary motor unit contractions.
• Different motor units establish muscle tone, remaining alternately active and inactive.
• Muscle tone helps maintain blood pressure and posture.

Movement of Skeletal Muscles

• Muscle attachment sites include the origin, which is the stable portion, and the insertion, which is the moveable portion.
• Its muscle action occurs when its insertion pulls towards the origin, creating movement.

Examples of Movement

• During agonist muscle contraction, a movement occurs.
• During antagonist muscle contraction, the opposite of the movement occurs at the same joint.

Lever Systems and Leverage

• Skeletal muscles enact movement by pulling on bones acting as levers, with joints working as fulcrums.
• A lever, a rigid structure (i.e., bone), moves around a fixed joint - fulcrum.
• The effort (E) causes movement via exerted muscle tension (force).
• The load or resistance opposes movement as the weight of the body that is moved.

Mechanical Advantage and Disadvantage

• Levers can operate at a mechanical advantage or disadvantage depending on the arrangement of effort, fulcrum and load.

Muscle Contractions

• Isotonic contractions create movement, change muscle length, and maintain constant tension.
• Isometric contractions do not create movement; muscle length remains constant as tension increases.

Isotonic Contractions

• During concentric isotonic contraction, tension exceeds the load, and the muscle shortens.
• During eccentric isotonic contraction, muscle length increases while previously shortened biceps lengthens in a controlled manner.
• During eccentric contraction, the tension exerted by myosin crossbridges resists movement of a load and slows the lengthening process

Muscle Fiber Types

• Slow oxidative fibers are fatigue-resistant.
• Fast oxidative-glycolytic fibers have moderate fatigue resistance.
• Fast glycolytic fibers have low resistance to fatigue.

Muscle Fiber Types Chart

• Slow oxidative fibers have the smallest diameter, and a large amount of myoglobin
• Fast oxidative-glycolytic fibers has an intermediate diameter and a large amount of myoglobin
• Fast glycolytic fibers are the largest, with the smallest amount of myoglobin
• Slow oxidative fibers, fast oxidative-glycolytic fibers and fast glycolytic fibers have many, many, and few mitochondria respectively
• Slow oxidative fibers, fast oxidative-glycolytic fibers and fast glycolytic fibers have many, many, and few capillaries respectively
• Slow oxidative, Fast oxidative-glycolytic and Fast glycolytic fibers have red, red-pink and white (pale) colors respectively.
• Slow oxidative, Fast oxidative-glycolytic and Fast glycolytic fibers have high capacity, intermediate capacity and low capacity for generating ATP.
• ATP generation occurs in Slow oxidative fibers with aerobic respiration, Fast oxidative-glycolytic fibers with aerobic respiration and anaerobic glycolysis, and Fast glycolytic fibers with anaerobic glycolysis.
• Myosin ATPase hydrolyzes ATP slowly in slow oxidative fibers, and rapidly in fast oxidative-glycolytic and fast glycolytic fibers.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers have slow, fast and fast contraction velocities respectively.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers have high, intermediate and low fatigue resistance respectively.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers have the lowest amount, intermediate amount and highest amount of creatine kinase respectively.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers have low, intermediate and high glycogen stores respectively.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers are recruited first, second and third respectively.
• Fibers are abundant in slow oxidative fibers in posture muscles such as those in the neck, lower limb muscles in fast oxidative-glycolytic fibers, and upper limb muscles in fast glycolytic fibers.
• Slow oxidative, fast oxidative-glycolytic and fast glycolytic fibers partake in maintaining posture and aerobic activities, walking/sprinting and rapid/intense movement of short duration respectively.

Smooth Muscle

• Smooth muscle fibers possess thick and thin filaments but lack transverse tubules and have minimal sarcoplasmic reticulum.
• Contraction and relaxation happen more slowly than in striated muscle.

Smooth Muscle Contraction

• The autonomic nervous system controls smooth muscle contraction.
• Varicosities release neurotransmitters.

Two Forms of Smooth Muscle

• Single-unit smooth muscle contracts together in fibers.
  • For example, visceral muscles.
• Multi-unit smooth muscle contracts independently.
  • For example, pupillary muscles.

Smooth Muscle Autorhythmicity

• Two potential types exist in smooth muscle cells. Pacemaker potentials are spontaneous and always reach threshold. Slow wave potentials exhibit constant threshold-failing depolarizations and repolarizations.
• Contraction can occur via autorhythmicity or other stimuli.

Smooth Muscle Activity

• Regulation occurs via excitatory and inhibitory actions that promote contraction or relaxation.
• The stress-relaxation response causes initial increased tension when stretched, followed by a release after about a minute.
• ATP is produced aerobically and anaerobically.

Cardiac Muscle

• Intercalated discs are at the junction between two muscle cells. Functional syncytium ensures that all cells work together as one unit. Autorhythmicity generates electrical potentials within cardiac cells. It does not require nervous stimulation.

Cardiac muscle relationship

• Action potential and refractory period are related.

Summary of Three Muscle Types By Feature

• Skeletal, Cardiac and Smooth muscles have striated, striated and non-striated (smooth) microscopic appearance and features respectively
• Skeletal and Cardiac muscles vs Smooth muscles are voluntary and involuntary respectively
• Appendicular muscles such as arms and legs, the heart and walls of hollow viscera are made of Skeletal, Cardiac and Smooth muscles
• Skeletal, Cardiac and Smooth muscles are are very large 10–100 μm, large 10–20 μm, and small 3–8 μm respectively
• Skeletal, Cardiac and Smooth muscles have abundant, some and small amounts of sarcoplasmic reticulum, respectively
• Skeletal and Cardiac muscles have transverse tubules, and smooth muscles do not
• Only cardiac and smooth muscles exhibit autorhythmicity

Summary of Three Muscle Types By Feature

• Skeletal, Cardiac and Smooth muscle uses sarcoplasmic reticulum, has sarcoplasmic reticulum with extracellular fluid, and features calmodulin for contraction Cardiac muscle uses troponin/tropomyosin.
• The different muscle types has fast, moderate and slow contraction speeds

Muscle Regeneration

• Hypertrophy Enlargement of existing cells Occurs in skeletal, cardiac, and smooth muscle Can help repair damaged tissue
• Hyperplasia Increase in the number of fibers Occurs via cell division Can occur in limited types of smooth muscle