Skeletal Muscle: Structure and Function

Skeletal Muscle Overview

• Skeletal muscle physiology, function, and adaptation are key topics

Gross Structure of Skeletal Muscle

• Skeletal muscles number around 600 in the human body
• Parallel fibers within each muscle direct force along the muscle's longitudinal axis
• Muscle fiber length ranges from millimeters (eye muscles) to 30 cm (large leg muscles)
• Epimysium connects the entire muscle to intramuscular tissue sheaths and forms tendons
• Perimysium surrounds fascicles, bundles of muscle fibers
• Endomysium separates individual muscle fibers from neighboring ones
• Sarcolemma encloses each muscle fiber's cellular contents
• Sarcoplasm houses nuclei, genes, mitochondria and specialized organelles
• Sarcoplasmic reticulum provides structural integrity to the muscle fiber

Skeletal Muscle Levels of Organization

• Tendons link muscle ends to the periosteum, the bone's covering
• Muscle force is transmitted via connective tissue to tendons, pulling on bone
• The origin of a muscle is its tendon attachment to a stable skeletal part, usually proximal
• The insertion is the point where the muscle attaches distally to a moving bone

Muscle Composition

• Muscle tissue is 75% water
• Muscle tissue is 20% protein
• Myosin, actin, and tropomyosin are the most abundant muscle proteins
• Salts and other substances constitute approximately 5% of muscle composition
• During intense exercise, the vascular bed supplies substantial blood to active tissues therefore increasing O2
• Strenuous physical activity raises intramuscular pressure, which may reduce local blood flow during contractions

Capillarization

• Trained muscles exhibit an increased capillary-to-muscle fiber ratio
• Enhanced capillary microcirculation boosts waste removal and facilitates oxygen, nutrient, and hormone delivery
• Endurance-trained athletes can have 40% more capillaries per muscle area
• Vascular stretch from increased blood flow during exercise promotes capillary development

Sarcomere Structure

• Cross-striation patterns are present in the myofibril
• I band represents a lighter area
• A band represents a darker area
• The center of the A band contains the H zone
• The Z line bisects the I band and adheres to the sarcolemma for structural integrity
• The sarcomere is a repeating unit between Z lines
• The sarcomere comprises the functional unit of a muscle fiber
• Sarcomere length affects the muscle's functional properties

Pennation Effects

• Differences in sarcomere alignment and muscle length influence force- and power-generating capacity
• Fusiform and pennate are two types of muscle arrangements
• Pennate muscles differ from fusiform muscles, they have shorter and more fibers and a limited range of motion

Contractile Filaments

• Actin filaments, myosin, troponin-tropomyosin complex, and M bridges are key components of contractile filaments

Complex Tubule System

• Mitochondria, cleft, sarcolemma, basal lamina, glycogen, T-tubule, the sarcoplasmic reticulum form the architecture of a muscle fiber

Sliding-Filament Model

• Muscle shortens or lengthens as thick and thin filaments slide past each other
• Length of the filaments do not change
• Relative size changes occur within the sarcomere's zones and bands
• A force is exerted at the Z bands

Actin, Myosin, and ATP Link

• Myosin crossbridges detach from actin when ATP binds to the actomyosin complex
• The myosin crossbridge returns to its original state, ready to bind again
• Dissociation of actomyosin occurs with Actomyosin + ATP producing Actin + Myosin-ATP
• Myosin ATPase splits ATP to provide energy for muscle action

Excitation-Contraction Coupling

• An electrical discharge in the muscle initiates chemical events that release intracellular Calcium ions, Ca2+
• This leads to muscle action
• Intracellular Ca2+ controls a muscle fiber's contractile and metabolic activity
• Actin + Myosin ATPase combine to form Actomyosin + ATPase in muscle contraction
• Binding of active sites on actin and myosin activates myosin ATPase to split ATP
• Energy then causes the myosin crossbridge to produce muscle tension
• Actomyosin ATP break down into Actomyosin + ADP + Pi + Energy

Relaxation

• Actin and myosin return to their original states
• Muscle stimulation ends, Calcium ion (Ca2+) activity ceases and troponin is released in order to inhibit actin-myosin interaction
• Calcium ion (Ca2+) pumps into the sarcoplasmic reticulum
• Deactivation prevents mechanical linking of crossbridges and inhibits Myosin ATPase activity

Muscle Activation Sequence: Steps 1-5

• Step 1: An action potential triggers acetylcholine (ACh) release, it diffuses across the synaptic cleft, and attaches to ACh receptors on the sarcolemma
• Step 2: Muscle action potential then depolarizes tubules at the sarcomere's junciton
• Step 3: T-tubule system depolarizes, causing release of calcium ions (Ca2+) from sarcoplasmic reticulum
• Step 4: Calcium ions (Ca2+) then bind troponin–tropomyosin in actin to combine with myosin
• Step 5: Myosin-ATP activates myosin ATPase splitting ATP, causing crossbridge movement

Muscle Activation Sequence: Steps 6-9

• Step 6: ATP binds to myosin to break actin-myosin bond which causes disassociation from the crossbridge.
• Step 6: Thick and thin filaments then slide past each other and the muscle shortens.
• Step 7: While calcium ion (Ca2+ ) concentration remains high, crossbridge activation continues.
• Step 8: Muscle stimulation stops and calcium ion (Ca2+) returns to sarcoplasmic reticulum
• Step 9: Calcium ion (Ca2+) removal restores inhibitory action of troponin–tropomyosin in the presence of ATP

Human Muscle Fiber Types

• Contraction time: Type I Slow, Type IIa Moderately fast, Type IIx Very fast
• Motor neuron size: Type I Small, Type IIa Medium, Type IIx Very large
• Fatigue resistance: Type I High, Type IIa Fairly high, Type IIx Low
• Activity Type: Type I Aerobic, Type IIa Long-term anaerobic , Type IIx Short-term anaerobic
• Max use duration: Type I Hours, Type IIa Less than 30 mins Type IIx, Less than 1 min