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Muscular System Histology and Physiology

Structure of Muscle Tissue

• Key Components: The muscular system is composed of arteries, veins, nerves, and layers of connective tissue, including:

  • Epimysium: A fascial layer that surrounds the entire muscle.

  • Perimysium: Surrounds groups of muscle fibers known as fasciculi.

  • Endomysium: Envelops individual muscle fibers.

• Additional Microstructure:

  • Axon of Motor Neuron: Transmits signals from the nervous system to muscle fibers.

  • Neuromuscular Junction: The interface where a motor neuron meets a muscle fiber.

  • Sarcolemma: The cell membrane that surrounds a muscle fiber.

Physiology of Skeletal Muscle

• Action Potentials: Nervous systems control muscle contractions through action potentials. Key physiological concepts include:

  • Resting Membrane Potential: The charge difference across the membrane when a muscle is not stimulated. The inside of the cell is more negatively charged due to large protein molecules.

  • The higher concentration of K+ ions inside the cell is counteracted by negatively charged proteins, while Na+ ions dominate the outside environment. This electrochemical gradient is maintained by the Na/K pump, crucial for action potential generation.

Ion Channels and Action Potentials

Types of Ion Channels

• Ligand-Gated Channels: Open in response to specific chemicals (e.g., neurotransmitters like acetylcholine).

  • Ach binds to the receptor on the muscle cell, triggering Na+ channels to open and sodium ions to influx into the cell.

• Voltage-Gated Channels: Open in response to changes in membrane voltage. Each type of ion channel is specific to a particular ion, facilitating rapid ion movement across the cell membrane.

Action Potential Phases

• Depolarization: Initiated when the inside of the membrane becomes less negative, and depolarization occurs once the threshold level is reached.

• Repolarization: The membrane reestablishes its resting potential, sometimes dropping below the original voltage due to the combined effects of Na and K movement, until the resting potential is restored by the Na/K pump.

• All-or-None Principle: Indicates that a muscle fiber will either fully contract or not at all in response to an action potential.

• Propagation: Action potentials spread along the muscle membrane, generating successive potentials as they move.

Neuromuscular Junction and Muscle Contraction

Structure of the Neuromuscular Junction

• Components:

  • Presynaptic Terminal: Contains synaptic vesicles filled with neurotransmitters.

  • Synaptic Cleft: The gap between the neuron and the muscle fiber.

  • Postsynaptic Membrane: Also known as the motor end-plate where neurotransmitters bind and initiate muscular contraction.

Function of Neuromuscular Junction

• When acetylcholine (ACh) is released from the presynaptic vesicles, it diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, resulting in muscle fiber action potentials.

• Acetylcholinesterase is present to quickly degrade ACh, terminating its action and preventing excessive stimulation of the muscle fiber.

Excitation-Contraction Coupling

• This process describes how an action potential leads to the actual contraction of muscle fibers, involving:

  • Sarcoplasmic Reticulum: The organelle that regulates calcium ion release necessary for muscle contraction.

  • T Tubules and Terminal Cisternae: Facilitate rapid conduction of action potentials and calcium release.

• Cross-Bridge Cycle: Calcium binds to troponin, causing tropomyosin to shift, which exposes binding sites on actin for myosin, enabling contraction.

Muscle Twitch Phases

• The muscle twitch consists of three main phases:

  • Lag Phase: Delay between stimulus and contraction.

  • Contraction Phase: Actual muscle contract due to cross-bridge cycling.

  • Relaxation Phase: Calcium ions are pumped back into the sarcoplasmic reticulum to halt contraction.

Types of Muscle Contractions

• Isometric Contraction: Muscle length does not change with tension increase (e.g., postural muscles).

• Isotonic Contraction: Muscle changes length while maintaining tension (e.g., lifting objects).

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

Muscle Fiber Types

• Slow-Twitch (Type I): Often found in postural muscles, these fibers are fatigue-resistant and better suited for endurance activities.

• Fast-Twitch (Type II): These fibers respond quickly and are more suited for rapid, powerful actions but fatigue quickly.

• Muscle adaptations include hypertrophy (increase in muscle size) and atrophy (decrease in muscle size) based on training and disuse, respectively.

Smooth Muscle vs. Cardiac Muscle

• Smooth Muscle: Non-striated, spindle-shaped fibers that operate involuntarily. Contractions are regulated by autonomic nervous system and hormones.

• Cardiac Muscle: Striated with intercalated disks and gap junctions, ensuring synchronized contractions.

Effects of Aging on Skeletal Muscle

• Decreased muscle mass, prolonged contraction times, reduced stamina, and slowed recovery times are common in aging populations.