Muscular System Overview

Muscular System Overview

• Myology is the study of muscles.
• Myo = muscle, logy = study of, sarco = flesh.
• Three types of muscle tissue: skeletal, cardiac, and smooth.

Types of Muscle Tissue

• Skeletal Muscle:
  • Located attached to bones.
  • Long, cylindrical cells.
  • Multiple nuclei.
  • Striated (striped appearance).
  • Voluntary control.
  • Function: body movement.
• Smooth Muscle:
  • Found in walls of hollow organs and blood vessels.
  • Spindle-shaped cells.
  • Single nucleus.
  • Non-striated.
  • Involuntary control.
  • Function: moving substances through the body.
• Cardiac Muscle:
  • Found in the heart.
  • Branching, cylindrical cells.
  • Single nucleus.
  • Striated.
  • Involuntary control.
  • Function: pumping blood.

Properties of Muscle Tissue

• Excitability: Responds to stimuli with electrical signals.
• Contractility: Generates force and movement.
• Extensibility: Stretches without damage.
• Elasticity: Returns to original shape after stretching.

Functions of Muscular Tissue

• Producing body movements.
• Stabilizing body positions.
• Storing and moving substances within the body.
• Thermogenesis (producing heat).

Organization of Muscle Tissue

• Muscles are comprised of muscle fibers, nerves, blood vessels, and connective tissue (Epimysium, Perimysium, and Endomysium).
• Epimysium: Outer layer surrounding the entire muscle.
• Perimysium: Surrounds fascicles (bundles of muscle fibers).
• Endomysium: Penetrates each fascicle, wrapping around individual muscle fibers, primarily made of fine reticular fibers.

Organization of Muscle Tissue (Muscle Fibers)

• Muscle Fiber (muscle cell): Includes nuclei, capillary, sarcolemma (plasma membrane), sarcoplasm (cytoplasm), transverse tubules, mitochondria, sarcoplasmic reticulum (stores/releases Ca2+ for contraction), and myofibrils (contractile organelles).
• Myofibrils: Contain actin and myosin filaments (myofilaments).

Myofilaments (Actin & Myosin)

• Myosin: Main component of thick filaments, tail points toward M line, head has actin and ATP binding sites.
• Actin: Main component of thin filaments, binding sites for myosin heads.
• Tropomyosin and troponin: regulatory proteins that control muscle contraction.

Myofilaments (Actin & Myosin) - Sarcomere

• Sarcomere: Functional unit of muscle contraction.
• Z-discs: Separate one sarcomere from the next.
• A band: Entire length of myosin filaments.
• I band: Contains actin filaments only.
• H zone: Region in center of A band containing only myosin filaments.
• M line: Middle of sarcomere, responsible for holding thick filaments together.

Actin and Myosin Structure

• Myosin has a head and a tail. Myosin heads form cross-bridges.
• Actin filaments have active sites for myosin heads to bind.
• Regulatory proteins (troponin and tropomyosin) control the interaction between actin and myosin.

Skeletal Muscle Contraction Physiology

• Somatic motor neurons: Stimulate skeletal muscle fibers.
• Muscle fiber contraction: Action potentials travel along the sarcolemma and T-tubules. Initiation occurs at Neuromuscular Junction (NMJ).

Neuromuscular Junction (NMJ)

• Axon Terminal/Synaptic End Bulbs: End of motor neuron, containing synaptic vesicles (ACh).
• Synaptic cleft: Gap between neuron and muscle cell, signals transmitted chemically.
• Motor end plate: Portion of muscle fiber sarcolemma facing synaptic knob; contains ACh receptors (ligand-gated ion channels).

Neuromuscular Junction (NMJ) Physiology

• Nerve impulse and Ca2+ influx: Nerve impulse triggers calcium influx into the neuron, causing ACh release.
• Release of Acetylcholine (ACh): Ca2+ influx prompts release of ACh into the synaptic cleft.
• Diffusion and binding of ACh: ACh diffuses across the cleft and binds to ACh receptors.
• Opening of ion channels: ACh binding opens ion channels, allowing Na+ to enter the muscle fiber.
• Production of muscle action potential: Na+ influx causes a change in membrane charge, triggering a muscle action potential.
• Termination of ACh activity: Acetylcholinesterase (AChE) breaks down ACh, ending the contraction signal.

Sliding Filament Theory

• Muscle contraction results from the sliding of actin and myosin filaments past each other.
• Myosin heads bind to actin, pull actin filaments toward the center of the sarcomere (power stroke).

Summary of Skeletal Muscle Contraction

• Action potential travels along axon.
• Ca2+ enters axon terminal.
• ACh released, diffuses to motor end plate.
• ACh binds to receptors, causes Na+ channels to open.
• Action potential travels along muscle fiber membrane, triggering Ca2+ release from sarcoplasmic reticulum.
• Ca2+ exposes myosin-binding sites on actin.
• Myosin heads bind to actin, cycle repeats until contraction is complete or Ca2+ decreases.

Muscle Relaxation

• ACh breakdown stops signal.
• Ca2+ pumped back into sarcoplasmic reticulum.
• Tropomyosin and troponin block myosin-binding sites.
• Myosin heads detach from actin.
• Muscle relaxes.

Production of ATP for Muscle Contraction

• Creatine Phosphate: Quickly replenishes ATP during short, intense exertion.
• Anaerobic Glycolysis: Produces ATP without oxygen but yields less ATP and produces lactic acid.
• Aerobic Respiration: Most efficient way to produce ATP but requires oxygen, slow process.

Muscle Fatigue

• Inability to maintain force of contraction after prolonged activity.
• Reduced Ca2+ release, depletion of creatine phosphate, insufficient oxygen, and other factors contribute.

Types of Muscle Contraction

• Isotonic: Muscle changes length during contraction (movement).
  • Concentric: Muscle shortens.
  • Eccentric: Muscle lengthens.
• Isometric: Muscle does not change length during contraction (no movement but tension is maintained).

Effects of Exercise on Muscles

• Aerobic: Increases muscle endurance, strength, flexibility, and neuromuscular coordination.
• Resistance/Isometric: Strengthens muscles and improves ability to contract against resistance.