Muscle Physiology: Contraction and Energy Use

• Muscle physiology involves the study of how muscles function, including contraction mechanisms, energy use, and adaptation to exercise
• Muscle contraction is initiated by nerve impulses, relying on complex molecular interactions
• Energy metabolism in muscles involves ATP production through different metabolic pathways
• Muscle fiber types vary and influence muscle performance

Muscle Contraction Mechanisms

• The sliding filament theory describes muscle contraction as the sliding of actin and myosin filaments past each other
• Myosin heads attach to actin filaments, forming cross-bridges that pull the actin filaments towards the center of the sarcomere
• ATP provides the energy for myosin head detachment and reattachment, enabling the sliding process
• Calcium ions regulate muscle contraction by binding to troponin, which exposes the myosin-binding sites on actin
• Action potentials trigger the release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction
• Relaxation occurs when calcium ions are pumped back into the sarcoplasmic reticulum, causing troponin to block myosin binding
• Rigor mortis is the stiffening of muscles after death, caused by the depletion of ATP, preventing myosin detachment from actin

Adaptations To Exercise

• Endurance training leads to increased mitochondrial density, improving the muscle's capacity for aerobic metabolism
• Resistance training causes muscle hypertrophy, increasing the size and strength of muscle fibers
• Capillarization increases with endurance training, enhancing oxygen delivery to muscle fibers
• Muscle adaptations are specific to the type of exercise, with different changes occurring in response to endurance versus resistance training
• Satellite cells contribute to muscle repair and growth by fusing with existing muscle fibers or forming new fibers
• Hormones such as testosterone and growth hormone play a role in muscle growth and adaptation

Energy Metabolism In Muscles

• ATP is the primary energy currency for muscle contraction
• Muscles use different metabolic pathways to regenerate ATP
  • Creatine phosphate system: Provides a quick burst of energy for short-duration, high-intensity activities
  • Anaerobic glycolysis: Breaks down glucose to produce ATP in the absence of oxygen, resulting in lactate production
  • Aerobic metabolism: Uses oxygen to produce ATP from carbohydrates, fats, and proteins in the mitochondria
• The contribution of each energy system depends on the intensity and duration of the activity
• Muscle fatigue can result from depletion of energy substrates, accumulation of metabolic byproducts, and neural factors
• Oxygen debt (EPOC) refers to the excess post-exercise oxygen consumption used to restore energy stores and clear metabolic byproducts

Neuromuscular Junction

• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber
• Acetylcholine (ACh) is the neurotransmitter released at the neuromuscular junction
• When an action potential reaches the motor neuron terminal, ACh is released into the synaptic cleft
• ACh binds to receptors on the muscle fiber membrane (sarcolemma), causing depolarization and initiating an action potential in the muscle fiber
• Acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft, terminating the signal and allowing muscle relaxation
• Diseases like myasthenia gravis disrupt the neuromuscular junction by attacking ACh receptors, leading to muscle weakness

Muscle Fiber Types

• Type I (slow-twitch) fibers are fatigue-resistant and primarily rely on aerobic metabolism
  • They are suited for endurance activities
  • They contain more mitochondria and myoglobin
• Type IIa (fast-twitch oxidative) fibers have intermediate characteristics, using both aerobic and anaerobic metabolism
  • They are faster than type I fibers but fatigue more quickly
• Type IIx (fast-twitch glycolytic) fibers are powerful but fatigue quickly, relying mainly on anaerobic glycolysis
  • They are suited for short-duration, high-intensity activities
• Muscle fiber type composition varies between individuals and can be influenced by genetics and training
• Fiber type conversion can occur with training, but the extent is limited

ATP

• ATP (adenosine triphosphate) is the primary source of energy for muscle contraction, nerve impulse propagation, and other cellular processes
• ATP consists of an adenosine molecule attached to three phosphate groups
• The energy is stored in the chemical bonds between the phosphate groups
• ATP hydrolysis (breakdown) releases energy that powers cellular activities, resulting in ADP (adenosine diphosphate) and inorganic phosphate
• Muscles store only a limited amount of ATP, so it must be continuously regenerated
• Creatine phosphate, anaerobic glycolysis, and aerobic metabolism are the primary pathways for ATP regeneration in muscles
• The rate of ATP regeneration determines the intensity and duration of muscle activity
• Different muscle fiber types have varying capacities for ATP production through different metabolic pathways, influencing their function