Muscular_system_7.

Muscular System

Histology and Physiology

• Key Components:

  • Artery: Supplies blood to muscle tissues.

  • Vein: Drains blood away from muscle tissues.

  • Nerve: Provides nerve supply for muscle control.

  • Epimysium: Connective tissue that surrounds entire muscles (fascia).

  • Perimysium: Connective tissue that surrounds bundles of muscle fibers (fascicles).

  • Endomysium: Connective tissue that surrounds individual muscle fibers.

  • Axon of motor neuron: Transmits signals to muscle fibers.

  • Synapse / Neuromuscular Junction: Site where the neuron communicates with the muscle fiber.

  • Sarcolemma: Plasma membrane of muscle fibers.

  • Muscle Fiber: The actual muscle cell.

Physiology of Skeletal Muscle

• Nervous System Control:

  • Muscle contractions are controlled by action potentials.

  • Resting Membrane Potential:

    • Membrane voltage difference across muscle fiber membranes; polarized due to an imbalance of ions.

    • Inside the cell: more K+; negatively charged proteins trap K+.

    • Outside the cell: more Na+, positively charged due to higher Na+ concentration.

    • Na/K Pump: Maintains the ion concentrations necessary for action potentials.

Ion Channels

Types of Ion Channels

• Ligand-gated Channels:

  • Open when a specific molecule (ligand) binds (e.g., neurotransmitters like ACh).

  • Na+ channels open when ACh attaches to receptors, allowing Na+ to flow into the cell.

• Voltage-gated Channels:

  • Open and close in response to membrane voltage changes.

  • Specific to a type of ion.

Action Potentials

Phases of Action Potentials

• Depolarization:

  • Inside becomes less negative; threshold reached triggers depolarization.

• Repolarization:

  • Returns the membrane potential to resting levels; overshoot can occur below resting potential.

  • Na/K pump restores resting potential after action potentials.

• All-or-None Principle:

  • Action potential occurs fully or not at all.

• Propagation:

  • Action potentials spread along the membrane; a new action potential occurs at each successive location.

• Frequency: How often action potentials occur in a given time frame.

Neuromuscular Junction

• Components:

  • Synapse: Connection point between a neuron and muscle fiber.

  • Presynaptic Terminal: End of motor neuron with vesicles containing neurotransmitters.

  • Synaptic Cleft: Space between neuron and muscle fiber.

  • Postsynaptic Membrane: Muscle membrane that receives neurotransmitters.

Function of Neuromuscular Junction

• Neurotransmitter:

  • Released from axon terminal into the synaptic cleft (e.g., ACh).

• AChE (Acetylcholinesterase):

  • Degrades ACh in the synaptic cleft, preventing excessive stimulation of the muscle fiber.

Excitation-Contraction Coupling

• Mechanism linking muscle excitation (action potential) to contraction.

• Involves:

  • Sarcolemma

  • Transverse (T) Tubules: Extensions of the sarcolemma that penetrate into muscle fibers.

  • Sarcoplasmic Reticulum (SR): Stores calcium ions; releases Ca2+ for contraction.

  • Troponin/Ca2+: Calcium binds to troponin, triggering muscle contraction.

Muscle Contraction Details

• Cross-Bridge Formation:

  • Formation of connections between actin and myosin during contraction.

  • Calcium binding to troponin changes shape, allowing cross-bridges to form.

• Relaxation:

  • Calcium returns to the SR; binding sites on actin are blocked again.

Muscle Twitch

• Phases:

  • Lag Phase: Time from stimulus to initiation of contraction.

  • Contraction Phase: Fibers contract in response to action potentials.

  • Relaxation Phase: Muscle fibers lengthen and return to resting state.

Stimulus Strength and Muscle Contraction

• All-or-None Law:

  • Muscle fibers contract fully or not at all when threshold is reached.

• Motor Units:

  • Comprises a motor neuron and the muscle fibers it innervates.

Types of Muscle Contractions

• Isometric: Muscle tension increases without changing length.

• Isotonic: Muscle changes length while tension remains constant (concentric/eccentric).

Muscle Fatigue Types

• Psychological: Relates to emotional state.

• Muscular: Results from ATP depletion.

• Synaptic: Caused by a lack of ACh at NMJ.

Energy Sources for Muscle Contraction

• ATP: Immediate energy source. Produced from:

  • Creatine Phosphate: Store energy to synthesize ATP during rest.

  • Anaerobic Respiration: Produces ATP without oxygen; generates lactic acid.

  • Aerobic Respiration: Requires oxygen; more efficient at producing ATP, CO2, and water.

Muscle Fiber Types

• Slow-Twitch Fibers (Type I):

  • Better for endurance; more myoglobin, mitochondria, fatigue-resistant; found mainly in postural muscles.

• Fast-Twitch Fibers (Type II):

  • Better for short, intense bursts of effort; fewer mitochondria; predominates in sprinters.

Smooth Muscle Characteristics

• Not striated, spindle-shaped, involuntary control.

• Calcium-induced contraction: Calcium binds to calmodulin, activating myosin kinase for contraction.

Cardiac Muscle

• Striated, autorhythmic, unique intercalated discs for communication.

• Generates longer action potentials and has a higher calcium requirement for contraction.

Effects of Aging on Skeletal Muscle

• Reduced mass, increased response time, decreased stamina, and lower recovery rates.