Muscle Physiology

• Muscle Contraction

  • Overview: Muscle contraction is the process by which muscles generate force,leading to movement or maintaining body position. It involves a complex interplay of proteins and ions, with the sliding filament mechanism at its core, regulated by calcium and fueled by ATP.

• Sliding Filament Mechanism:

  • Myosin filaments bind to actin filaments.

  • Myosin heads pull actin filaments towards the center of the sarcomere.

  • Sarcomere shortens, leading to muscle contraction.

• Calcium Role:

  • Calcium ions bind to troponin, causing a conformational change.

  • Tropomyosin shifts, exposing myosin-binding sites on actin.

  • Allows myosin heads to attach to actin and initiate the contraction cycle.

• ATP Role:

  • ATP binds to myosin heads, causing them to detach from actin.

  • ATP hydrolysis provides energy for myosin heads to recock and reattach.

  • ATP is essential for both muscle contraction and relaxation.

• Energy Sources:

  • ATP stored in muscle fibers provides initial energy.

  • Creatine phosphate donates a phosphate group to ADP to regenerate ATP.

  • Glycolysis breaks down glucose to produce ATP and pyruvate.

  • Oxidative phosphorylation in mitochondria generates ATP from glucose, fatty acids, or amino acids.

Skeletal Muscle Fibers

• Overview: Skeletal muscle fibers are the contractile cells responsible for movement. They are categorized into slow (red) and fast (white) fibers, differing in their metabolic properties, myoglobin content, and mitochondrial density, which influence their capacity for sustained or rapid, powerful contractions.

• Slow (Red) Fibers:

  • High myoglobin content for oxygen storage.

  • Abundant mitochondria for aerobic metabolism.

  • Adapted for sustained, endurance activities.

• Fast (White) Fibers:

  • Lower myoglobin content.

  • Fewer mitochondria.

  • Rely on glycolytic metabolism for quick, powerful contractions.

• Myoglobin:

  • Oxygen-binding protein.

  • Higher concentration in slow fibers.

  • Facilitates oxygen delivery to mitochondria.

• Mitochondria:

  • Site of aerobic respiration.

  • More numerous in slow fibers.

  • Enables efficient ATP production from oxygen and fuels.

• Aerobic Metabolism:

  • Primary energy source for slow fibers.

  • Uses oxygen to generate ATP from glucose, fats, and proteins.

  • Supports prolonged muscle activity.

• Glycolytic Metabolism:

  • Primary energy source for fast fibers.

  • Breaks down glucose to produce ATP quickly.

  • Produces lactic acid as a byproduct, leading to fatigue.

NMJ Alterations

• Overview: NMJ Alterations refer to disruptions or impairments in the normal function of the neuromuscular junction (NMJ). These alterations can result from various factors, including autoimmune diseases, toxins, and drugs, leading to impaired muscle contraction and potential paralysis.

• Myasthenia Gravis:

  • Autoimmune disorder.

  • Antibodies against acetylcholine receptors (AChRs).

  • Reduces the number of available AChRs at the NMJ.

  • Leads to muscle weakness and fatigue.

• Anticholinesterases:

  • Drugs that inhibit acetylcholinesterase (AChE).

  • Increase the amount of acetylcholine (ACh) in the synaptic cleft.

  • Prolong the duration of ACh action at the NMJ.

  • Used to treat Myasthenia Gravis by improving muscle strength.

• Neostigmine:

  • A type of anticholinesterase.

  • Reversibly inhibits AChE.

  • Increases ACh levels at the NMJ.

  • Improves neuromuscular transmission in Myasthenia Gravis.

• Botulism:

  • Caused by Botulinum toxin produced by Clostridium botulinum.

  • Blocks the release of acetylcholine (ACh) at the NMJ.

  • Results in flaccid paralysis.

• Curare:

  • A neuromuscular-blocking agent.

  • Competitively binds to AChRs, preventing ACh binding.

  • Causes muscle relaxation and paralysis.

• Synaptic Transmission:

  • Process of signal transmission at the NMJ.

  • Involves the release of ACh from the presynaptic terminal.

  • ACh binds to AChRs on the postsynaptic muscle fiber.

  • Results in muscle fiber depolarization and contraction.

Skeletal Muscle Organisation

• Overview: Skeletal muscle is a complex tissue responsible for movement, posture, and heat production. Its organization involves a hierarchy of structures from the macroscopic muscle itself down to the microscopic arrangement of proteins within muscle cells, enabling muscle contraction and force generation.

• Sarcolemma:

  • The cell membrane of a muscle fiber.

  • It surrounds the muscle fiber and regulates the passage of materials in and out of the cell.

• Myofibrils:

  • Long, cylindrical structures that run parallel within the muscle fiber.

  • They are composed of repeating units called sarcomeres, which are the basic contractile units of the muscle.

• T-Tubules:

  • Invaginations of the sarcolemma that extend into the muscle fiber.

  • They transmit action potentials from the sarcolemma to the sarcoplasmic reticulum, facilitating uniform muscle contraction.

• Sarcoplasmic Reticulum:

  • A network of tubules surrounding each myofibril.

  • Stores and releases calcium ions (Ca2+), which are essential for muscle contraction.

• Sarcomere:

  • The basic contractile unit of a muscle fiber.

  • It is delineated by Z-lines and contains the actin and myosin filaments.

• Actin:

  • A thin filament protein.

  • Forms the backbone of the thin filaments in the sarcomere.

• Myosin:

  • A thick filament protein.

  • Has heads that bind to actin, forming cross-bridges and generating force during muscle contraction.

• Troponin:

  • A protein complex bound to actin.

  • Regulates muscle contraction by controlling the interaction between actin and myosin.

• Tropomyosin:

  • A protein that covers the myosin-binding sites on actin when the muscle is at rest.

  • Prevents muscle contraction until calcium ions are present.

• Titin:

  • A giant protein that spans half of each sarcomere, from the Z-line to the M-line.

  • Stabilizes the position of the myosin filaments and provides elasticity to the muscle.

NMJ Physiology

• Overview: NMJ Physiology focuses on the function of the neuromuscular junction, the synapse between a motor neuron and a muscle fiber. This involves understanding how signals from the nervous system trigger muscle contraction, including the roles of neurotransmitters and receptors.

• Somatic Nervous System:

  • Controls voluntary movements via skeletal muscles.

  • Motor neurons directly innervate muscle fibers.

  • Single neuron pathway from CNS to skeletal muscle.

• Autonomic Nervous System:

  • Regulates involuntary functions (e.g., heart rate, digestion).

  • Innervates smooth muscle, cardiac muscle, and glands.

  • Two-neuron pathway (preganglionic and postganglionic neurons).

• Neuromuscular Junction Events:

  • Action Potential Arrival: Motor neuron action potential reaches the axon terminal.

  • Calcium Influx: Voltage-gated calcium channels open, causing Ca2+ influx.

  • Acetylcholine Release: Ca2+ triggers the release of acetylcholine (ACh) into the synaptic cleft.

  • ACh Binding: ACh binds to nicotinic receptors on the muscle fiber's motor endplate.

  • Depolarization: Influx of Na+ through ACh receptors causes depolarization (endplate potential).

  • Muscle Action Potential: If the endplate potential exceeds threshold, it initiates an action potential in the muscle fiber.

  • Muscle Contraction: The muscle action potential leads to muscle contraction.

  • ACh Degradation: Acetylcholinesterase (AChE) in the synaptic cleft rapidly degrades ACh, terminating the signal.

• Muscle Fiber Types:

  • Type I (Slow Oxidative):

    • High endurance, fatigue-resistant.

    • Relies on oxidative metabolism.

  • Type IIa (Fast Oxidative-Glycolytic):

    • Moderate endurance and strength.

    • Uses both oxidative and glycolytic metabolism.

  • Type IIx (Fast Glycolytic):

    • Powerful, but fatigues quickly.

    • Primarily uses glycolytic metabolism.