Muscle Physiology

Muscle Membranes

Muscle tissue is organized into bundles, each surrounded by specific connective tissue membranes:

• Epimysium: Surrounds the entire muscle organ.

• Perimysium: Encloses muscle fascicles (bundles of muscle fibers).

• Endomysium: Wraps individual muscle fibers.

Attachment Points of Muscle

Muscles attach to bones at specific sites:

• Origin: The fixed attachment, usually proximal.

• Insertion: The movable attachment, usually distal.

Muscle Cell Structure and Physiology

Muscle cells (fibers) have specialized structures for contraction:

• Muscle fiber: The muscle cell itself.

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle cell.

• Myofibrils: Rod-like units within muscle fibers, composed of repeating sarcomeres.

Key components of myofibrils:

• Thick filament: Made of myosin.

• Thin filament: Made of actin, with regulatory proteins troponin and tropomyosin.

• Sarcomere: The functional contractile unit of muscle.

Sarcolemma and Sarcoplasmic reticulum (SR) are essential for excitation-contraction coupling:

• SR: Stores and releases Ca2+ ions upon stimulation.

• T tubules: Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

Sliding Filament Model of Muscle Contraction

Muscle contraction occurs when myosin heads bind to actin, pulling the thin filaments toward the center of the sarcomere. This process is powered by ATP and regulated by Ca2+ release from the SR.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber:

• Motor neuron: Releases acetylcholine (ACh) into the synaptic cleft.

• ACh receptor: Located on the muscle fiber membrane, binds ACh to initiate contraction.

• Acetylcholine esterase (AChE): Enzyme that breaks down ACh, terminating the signal.

Myasthenia gravis is an autoimmune disease affecting the NMJ, leading to muscle weakness.

Action Potential and Muscle Contraction

Action potentials trigger muscle contraction by opening ion channels:

• End-plate potential: Local depolarization at the NMJ.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ exits the cell.

Isometric vs. Isotonic Contraction

Muscle contractions can be classified as:

• Isometric contraction: Muscle tension increases, but length does not change.

• Isotonic contraction: Muscle changes length (shortens or lengthens) while tension remains constant.

Motor Unit

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Motor units are recruited from small to large to increase muscle tension.

Summation and Recruitment

Muscle force is increased by:

• Summation: Increased frequency of stimulation.

• Recruitment: Activation of additional motor units.

See Figures 9.12 and 9.13 for reference.

Muscle Tone

Muscle tone is the continuous and passive partial contraction of muscles, important for posture and readiness to respond.

Sources of ATP in Muscle

Muscle cells require ATP for contraction, which is generated by:

• Creatine phosphate (CP): Rapidly regenerates ATP from ADP.

• Anaerobic respiration (glycolysis): Incomplete breakdown of glucose, produces 2 ATP per glucose.

• Aerobic respiration: Complete breakdown of glucose in mitochondria, produces 32 ATP per glucose.

Muscle Fatigue and Oxygen Debt

• Muscle fatigue: Decline in ability of a muscle to generate force.

• Oxygen debt: Extra oxygen required after exercise to restore metabolic conditions.

• Muscle cramp: Involuntary, sustained muscle contraction, often due to electrolyte imbalance or fatigue.

Muscle Fiber Types

Muscle fibers are classified based on their metabolic properties:

• Oxidative fibers: Use aerobic respiration, fatigue-resistant.

• Glycolytic fibers: Use anaerobic glycolysis, fatigue quickly.

Adaptation to Exercise

Exercise induces changes in muscle:

• Aerobic exercise: Increases mitochondria, myoglobin, and capillary density.

• Resistance exercise: Increases muscle size (hypertrophy), myofibrils, and glycogen stores.

Muscle Atrophy

Disuse atrophy is the decrease in muscle mass due to inactivity.

Skeletal Muscle vs. Smooth Muscle

Comparison of muscle types:

Peristalsis is the wave-like contraction in smooth muscle, important for moving contents through hollow organs.

Muscular Dystrophy

Muscular dystrophy is a group of genetic diseases causing progressive weakness and loss of muscle mass. The most severe subtype is Duchenne muscular dystrophy, caused by mutations in the dystrophin gene.

Additional info: Some details, such as the specific figures referenced and the full list of regulatory proteins in smooth muscle, were inferred from standard Anatomy & Physiology curriculum.