Skeletal Muscle Structure And Function

USMLE Guide: Skeletal Muscle Structure and Function

Introduction

The United States Medical Licensing Examination (USMLE) is a three-step examination for medical licensure in the United States. This guide aims to provide a concise overview and key points about skeletal muscle structure and function, which is an important topic frequently tested in the USMLE.

I. Overview of Skeletal Muscle

A. Definition

Skeletal muscles are voluntary muscles responsible for movement and maintaining posture. They are attached to bones by tendons and allow for locomotion, facial expressions, and other bodily movements.

B. Structure

1. Muscle Fiber: The basic unit of skeletal muscle, consisting of multiple myofibrils.
2. Myofibrils: Long cylindrical structures within muscle fibers containing contractile elements.
3. Sarcomere: The functional unit of myofibrils, responsible for muscle contraction.

C. Types of Skeletal Muscle Fibers

1. Type I (Slow Twitch): High endurance, low force production, rich in mitochondria, and myoglobin.
2. Type IIa (Fast Oxidative): Moderate endurance, moderate force production, moderate mitochondria, and myoglobin.
3. Type IIb (Fast Glycolytic): Low endurance, high force production, low mitochondria, and myoglobin.

II. Muscle Contraction

A. Sliding Filament Theory

1. Actin and Myosin: Actin (thin filament) and myosin (thick filament) slide past each other during muscle contraction.
2. Cross-Bridges: Myosin heads form cross-bridges with actin, pulling the thin filaments toward the center of the sarcomere.
3. ATP and Calcium: ATP provides energy for muscle contraction, while calcium ions (Ca²⁺) initiate the contraction process.

B. Neuromuscular Junction

1. Motor Neuron: A nerve cell that transmits signals from the brain or spinal cord to muscle fibers.
2. Acetylcholine (ACh): Neurotransmitter released by motor neurons at the neuromuscular junction.
3. Motor End Plate: Specialized part of muscle fiber where ACh receptors are located.
4. Action Potential: Electrical signal that travels along the motor neuron and initiates muscle contraction.

C. Excitation-Contraction Coupling

1. T-Tubules: Invaginations of the sarcolemma that carry the action potential deep into the muscle fiber.
2. Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum storing calcium ions needed for muscle contraction.
3. Calcium Release: Action potential triggers the release of calcium ions from the SR.
4. Cross-Bridge Formation: Calcium binds to troponin, leading to the exposure of myosin binding sites on actin.

III. Muscle Relaxation

A. Role of ATP

1. ATP Hydrolysis: ATP is required for myosin to detach from actin and allow muscle relaxation.
2. Calcium Pump: ATP powers the active transport of calcium back into the SR, reducing cytosolic calcium concentration.

B. Role of Motor Neuron

1. Cessation of Action Potential: When the motor neuron stops releasing ACh, muscle contraction ceases.
2. Acetylcholinesterase (AChE): Enzyme that breaks down ACh, preventing continued muscle stimulation.

IV. Clinical Significance

A. Muscle Disorders

1. Muscular Dystrophy: Group of genetic disorders characterized by progressive muscle weakness and degeneration.
2. Myasthenia Gravis: Autoimmune disorder causing muscle weakness due to impaired neuromuscular transmission.
3. Rhabdomyolysis: Rapid breakdown of skeletal muscle resulting in the release of myoglobin into the bloodstream, potentially leading to kidney damage.

B. Pharmacological Interventions

1. Neuromuscular Blocking Agents: Medications that inhibit neuromuscular transmission, used for anesthesia or in critical care settings.
2. Muscle Relaxants: Medications used to induce muscle relaxation, often used in surgical procedures.

Conclusion

Understanding the structure and function of skeletal muscles is essential for medical professionals. This USMLE guide provides a brief but comprehensive overview of skeletal muscle, including muscle contraction, relaxation, clinical disorders, and pharmacological interventions. Remember to study additional resources and practice questions to solidify your knowledge for the USMLE examination.