Skeletal Muscle and Exercise: Comprehensive Study Notes
Skeletal Muscle and Exercise Overview
The human body contains over 400 skeletal muscles, accounting for 40-50\% of total body weight.
Muscle Actions:
Flexors: Decrease joint angle.
Extensors: Increase joint angle.
Types of Muscle Tissue:
Smooth muscle: Involuntary, found in hollow organs.
Cardiac muscle: Involuntary, found in the heart.
Skeletal muscle: Voluntary, attached to the skeleton.
Functions of Skeletal Muscle:
Force production for locomotion and breathing.
Force production for postural support.
Heat production during cold stress.
Gross Skeletal Muscle Anatomy
Epimysium: Surrounds the entire muscle.
Perimysium: Surrounds bundles of muscle fibers, forming fascicles.
Endomysium: Surrounds individual muscle fibers.
Sarcolemma: The muscle cell membrane.
Other structures include: Bone, Tendon, Fascia, Muscle, Fascicle, Axon of motor neuron, Blood vessel, Muscle fibers, Sarcoplasmic reticulum, Nucleus, Myofibrils, and Filaments.
Muscle Cell Anatomy (Microanatomy)
Myofibrils: Contain contractile proteins.
Actin: Thin filaments.
Myosin: Thick filaments.
Sarcomere: The fundamental contractile unit of a muscle fiber, including the Z line, M line, H zone, A band, and I band.
Sarcoplasmic Reticulum (SR): Storage sites for calcium (Ca^{2+}).
Transverse Tubules (T-tubules): Extensions from the sarcolemma that penetrate deep into the muscle fiber, connecting to the SR.
Triad of the Reticulum: Composed of a Transverse tubule flanked by two Terminal cisternae of the sarcoplasmic reticulum.
Bands (Striations): Appear under a microscope due to the arrangement of actin and myosin.
A-bands (Dark stripes): Contain both actin and myosin filaments.
I-bands (Light stripes): Contain only actin filaments.
H-zone: The middle of the A-band, containing only myosin filaments.
M-line: The exact middle of the H-zone.
Z-lines: Common boundary structures that define the sarcomere.
Muscle Contraction: Neuromuscular Junction
Neuromuscular Junction: The junction between a motor neuron and a muscle fiber.
Motor Unit: A motor neuron and all the muscle fibers it innervates.
Motor End Plate: A pocket formed around the motor neuron by the sarcolemma.
Neuromuscular Cleft: A short gap between the motor neuron and the muscle fiber.
Excitation of the Muscle Cell (Steps):
An Action Potential (AP) travels down the motor neuron.
The AP arrives at the axon terminal of the motor neuron.
Acetylcholine (ACh) is released from the motor neuron into the neuromuscular cleft.
ACh crosses the synapse and binds to ACh receptors on the muscle cell membrane (sarcolemma).
ACh binding causes an End-Plate Potential (EPP), which depolarizes the muscle fiber membrane and initiates an AP in the muscle fiber.
Muscle Contraction: Cross-Bridge Cycling (Sliding Filament Theory)
Cross-Bridge Cycling in the Muscle Cell (Contraction Steps):
The Action Potential travels down the T-tubules into the muscle fiber.
The AP triggers the release of Ca^{2+} from the sarcoplasmic reticulum (SR).
Ca^{2+} binds to the troponin protein, which is located on the actin filament.
This binding causes tropomyosin to shift, uncovering the myosin binding sites on the actin filament.
A myosin head, already energized with ATP hydrolysis products (ADP + P_i), attaches to an open binding site on actin, forming a cross-bridge.
The myosin head shifts, undergoing a "power stroke," which pulls the actin filament past the myosin filament, causing the myosin to move.
If ATP is present, it binds to the myosin head, causing the myosin head to detach from the actin binding site.
The myosin head returns to its resting (cocked) position, ready for another cycle of binding and pulling, as long as Ca^{2+} and ATP are available.
Sliding Filament Theory: Describes how muscle shortens. The actin filaments slide past the myosin filaments, resulting in a decrease in the length of the sarcomere's H-zone and I-bands, while the A-band length remains constant. This sliding brings the Z-lines closer together, leading to overall muscle shortening.
Motor Units and Muscle Fiber Types
Motor Units:
A motor unit consists of a motor neuron and all the muscle cells (fibers) it innervates.
Each muscle fiber receives input from only one motor neuron.
Innervation Ratio: The number of muscle fibers that are part of a motor unit, ranging from approximately 10 fibers (for fine movements) to more than 1000 fibers (for gross movements).
Motor Neuron Pool (Motor Nucleus): The group of motor neurons that innervate a single muscle. Muscles requiring finer movements typically have a larger motor neuron pool.
The force generated by a given muscle depends on the firing rate of the alpha motor neuron.
Energy for Muscle Contraction:
ATP is essential for muscle contraction.
Myosin ATPase: An enzyme located on the myosin head that breaks down ATP (ATP
ightarrow ADP + P_i) during fiber contraction.Sources of ATP:
Phosphocreatine (PC)
Glycolysis
Oxidative phosphorylation
Muscle Fiber Types: Classified based on speed of contraction and metabolic pathways.
Type I Fibers (Slow-twitch or Slow-oxidative fibers):
Small motor neuron size (part of a smaller motor unit, < 300 fibers).
Slower nerve conduction velocity, contraction, and relaxation speed.
High fatigue resistance, low force/power output, high endurance.
High aerobic enzyme content, low anaerobic enzyme content.
High capillary density, myoglobin content, mitochondria size/density.
Small fiber diameter, red color.
Type IIa Fibers (Intermediate or Fast-oxidative Glycolytic fibers):
Larger motor neuron size (part of a larger motor unit, > 300 fibers).
Fast nerve conduction, contraction, and relaxation speed.
Intermediate to low fatigue resistance, intermediate force/power output, intermediate to low endurance.
Intermediate to low aerobic enzyme content, high anaerobic enzyme content.
Intermediate capillary density, low myoglobin content, intermediate mitochondria size/density.
Intermediate fiber diameter, white/red color.
Type IIx Fibers (Fast-twitch or Fast-glycolytic fibers, sometimes called Type IIb):
Larger motor neuron size (part of a larger motor unit, > 300 fibers).
Fast nerve conduction, contraction, and relaxation speed.
Low fatigue resistance, high force/power output, low endurance.
Low aerobic enzyme content, high anaerobic enzyme content.
Low capillary density, myoglobin content, mitochondria size/density.
Large fiber diameter, white color.
Effects of SR and Myosin ATPase:
Type II fibers have a more highly developed SR, leading to faster Ca^{2+} release (35 times faster than Type I).
The speed of Myosin ATPase varies: Fast ATPase leads to fast contraction cycling, while slower ATPase leads to slower contraction cycling.
Peak Power Output: Type IIx > Type IIa > Type I.
Measuring Muscle Fiber Type:
Muscle Biopsy: A small ( 10-100 g) piece of muscle is removed, frozen, sliced, and examined under a microscope.
Gel Electrophoresis: Utilizes the difference in size of myosin types between Type I and Type II fibers to separate and quantify them.
Typical Muscle Fiber Composition in Elite Athletes:
Distance Runners: 70-80\% Type I (Slow), 20-30\% Type IIx/IIa (Fast).
Track Sprinters: 25-30\% Type I (Slow), 70-75\% Type IIx/IIa (Fast).
Nonathletes: 47-53\% Type I (Slow), 47-53\% Type IIx/IIa (Fast).
Muscle Fatigue
Definition: A decrease in muscle force production and a reduced ability to perform work.
Size Principle of Recruitment: Motor units are recruited in an orderly fashion from smallest to largest:
Motor unit 1 (small fibers) -> Motor unit 2 (medium fibers) -> Motor unit 3 (large fibers).
Onset of Fatigue: Influenced by contraction time and intensity.
**Causes of Fatigue:
High-Intensity Exercise (~60 seconds): Accumulation of lactate, H^+ ions, ADP (adenosine diphosphate), P_i (inorganic phosphate), and free radicals.
Long-Duration Exercise ( 2-4 hours):
Muscle Factors: Accumulation of free radicals, electrolyte imbalance, and glycogen depletion.
Central Fatigue: Reduced motor drive to the muscle from the Central Nervous System (CNS).
Mechanisms of Fatigue during Intense Exercise ( 1-10 minutes):
Multifactorial, ranging from decreased Ca^{2+} release from the SR to accumulation of metabolites that inhibit myofilament sensitivity to Ca^{2+}.
Key Metabolites: Increases in P_i, H^+ ions, and free radicals.
H^+ ions: Bind to Ca^{2+}--binding sites on troponin, preventing Ca^{2+} binding and thus contraction.
P_i and Free Radicals: Modify the cross-bridge head and reduce the number of cross-bridges bound to actin, collectively promoting fatigue.
Mechanisms of Fatigue during Moderate Intensity Exercise (> 60 minutes):
Primarily caused by increased radical production and muscle glycogen depletion.
Accumulation of P_i and H^+ in the muscle typically do not contribute to fatigue during moderate intensity exercise.
Radical Accumulation: Modifies the cross-bridge head and reduces the number of cross-bridges bound to actin, thus reducing force production.
Glycogen Depletion: Reduces TCA cycle intermediates, which in turn decreases ATP production via oxidative phosphorylation.
Effect of Training, Inactivity, and Aging
Strength Training:
Increases in muscle fiber size (hypertrophy).
Possibly increases in muscle fiber number (hyperplasia), though evidence in humans is limited.
Endurance Training:
Increases oxidative capacity of muscle.
Induces an alteration in fiber type: a