Skeletal Muscle Physiology Quiz

Muscle Module: Responses to Disuse, Training, and Aging (Sarcopenia)

• This module examines how muscles react to various conditions, including disuse, training, and aging.
• It also investigates the underlying mechanisms of muscle fatigue.

Muscle Atrophy vs. Muscle Hypertrophy

• Atrophy: A decrease in muscle size due to a reduction in the number or size of myofibers (muscle fibers).
• Hypertrophy: An increase in muscle size, typically due to an increase in the size or number of myofibers.
• Muscle degradation and synthesis interplay in response to various factors.

Causes of Sarcopenia (Aging-Related Muscle Loss)

• Age-related (primary): Genetic factors, hormonal changes, mitochondrial dysfunction, apoptosis of muscle fibers.
• Disuse: Physical inactivity, immobilization, weightlessness.
• Illness-related (cachexia): Cancer, chronic inflammation, infections like sepsis
• Other issues: Chronic obstructive pulmonary disease (COPD), Diabetes, Renal failure, AIDS, Burn injury, Fasting, Sepsis, and Neurodegenerative Diseases.

Mechanisms for Age-Related Muscle Loss (Sarcopenia)

• Muscle atrophy is related to a combination of inactivity and aging-related processes.
• Chronic inflammation
• Reduced protein synthesis
• Increased protein breakdown
• Loss of lean body mass

Muscle Fiber Type and Responses to Disuse

• Type I (slow oxidative) fibers: Important for endurance activities.

• Type II (fast oxidative and fast glycolytic) fibers: Essential for short bursts of strength.

• Both fiber types can atrophy with disuse. Type II fibers display a greater loss with disuse.

Disuse Atrophy and Generalizations

• Disuse atrophy is typically reversible.
• Significant in load-bearing conditions, such as bed rest or spaceflight
• Weightlessness and bed rest cause significant atrophy in humans.
• Immobilization in limb suspension causes considerable atrophy in animals.

Adaptations to Training

• Muscle fiber size: Hypertrophy (increase in size), Hyperplasia (increase in number)

• Enzyme activity: Increase in oxidative and/or glycolytic enzymes

• Isoform expression: Change in the type of myosin ATPase and Ca++ ATPase

• Organelles and Structures: Increased mitochondria; more extensive extracellular scaffolding, increased capillarization, more extensive network

• Endurance-type training specifically leads to metabolic adaptations.

• Increased use of fats as fuel and reduced reliance on carbohydrates.

Removal of Calcium Ions to Stop Muscle Contraction

• Sodium-calcium exchanger and calcium pump in the sarcolemma remove calcium from the cell.
• Calcium pumps in the sarcoplasmic reticulum (SR) actively transport Ca++ out of the cytoplasm.
• Calcium-binding proteins (e.g., calreticulin, calsequestrin) bind calcium within the SR, preventing further activation

Cross-Bridge Cycle

• Breakdown of ATP provides the energy for the power stroke, which allows myosin heads to bind and then release actin, then repeat.

Muscle Fiber Type Characteristics

• Slow oxidative (type I): Slow contraction speed, fatigue-resistant, aerobic respiration

• Fast oxidative-glycolytic (type IIa): Intermediate contraction speed and fatigue resistance. Aerobic/anaerobic respiration.

• Fast glycolytic (type IIb): Fast contraction speed, easily fatigued. Anaerobic respiration.

• These differences in metabolic pathways affect activities best suited to each fiber type.

Motor Units

• Functional groups of interconnected motor neurons innervating a group of muscle fibers.
• Motor units work together to coordinate contraction.
• Motor units are of different types, and are recruited in an orderly fashion according to size.

Fiber Type Classification Schemes

• Color: Red fibers rich in myoglobin are oxidative versus white fibers.
• Contraction speed: Slow versus Intermediate speed versus fast
• Fatigability: Slow versus intermediate speed versus fast
• Metabolism: Aerobic versus anaerobic

Muscle Atrophy Associated with Aging

• Decrease in muscle fiber number and size
• Decrease in motor neurons number
• Motor unit reorganization
• Decreases in force generation
• Changes in excitation-contraction coupling

Dynamic Strength Changes with Aging

• Gradual decline in strength associated with age, typically starting around 40-50 years of age. This decline occurs more prominently in dynamic strength measures than muscle mass.

Strength Loss Considerations

• Strength loss is not uniform throughout the body: Lower body strength is more greatly affected during aging;
• Possible mechanisms include relative amount of disuse, distal axonal degradation, and differing susceptibilities to aging across muscle fiber types (Type II fibers are more susceptible).

Problems with Studying Fatigue

• Variability: Fatigue can be influenced by factors like motivation, nutrition, environment, and training status.
• Experimental protocols vary significantly
• Extrapolating in vitro results to in vivo and clinical scenarios requires cautious interpretation.
• Separating fatigue from cellular damage

Mechanisms for Skeletal Muscle Fatigue

• Central fatigue (CNS): Decrease in motor units recruited due to factors like psychological issues or biochemical changes within the brain and spinal cord.
• Peripheral fatigue: Physical changes within a muscle itself. Mechanisms include ionic shifts, metabolic insufficiency, reactive oxygen species, changes in excitation-contraction coupling, and force decline.

Other Degenerative Features of Aged Muscles

• Hyaline degeneration (glassy changes) and vacuoles at fiber ends
• Fat and connective tissues replacing muscle fiber tissue
• Necrosis (cell death) associated with infiltration of immune cells
• Increased central nucleation (multi-nuclei fibers)
• Fiber splitting
• Impaired mitochondrial function
• Accumulation of SR material

Clinical Implications of Sarcopenia

• Loss of muscle mass leads to reduced basal metabolic rate
• Contributing to metabolic disorders like type 2 diabetes and osteoporosis
• Decreased muscle strength & functional capacity associated with difficulties in activities of daily living.
• Increased risk of falls & injuries
• More difficulty maintaining physical activity.
• High risk of fractures, even minor falls.

Preventing and Managing Sarcopenia

• High-resistance training
• Exercise programs involving combined resistance and aerobic exercise
• Increased physical activity levels
• Nutritional adjustments.

Test your knowledge on the nuances of skeletal muscle physiology, exploring topics like muscle contraction determinants, fatigue mechanisms, and the effects of disuse. Understand how various physiological concepts relate to muscle functionality and performance. This quiz encompasses key elements that every student of exercise science should be familiar with.