Untitled Quiz

Muscle Tissue Overview

• Chemical energy is converted to mechanical energy, generating force, performing work, and producing movement.
• Myology is the study of muscle.
• Prefixes "myo," "mys," and "sarco" all relate to muscle.

Functions of Muscle Tissue

• Muscle tissue enables body movement.
• It maintains posture.
• It protects and supports various body parts.
• It stores and moves materials within the body.
• It generates heat.

Special Properties of Muscle Tissue

• Excitability
• Conductivity (action potentials)
• Contractility
• Extensibility
• Elasticity

Types of Muscle Tissue

Skeletal Muscle

• Attaches to bone, skin, or fascia.
• Striated appearance.
• Voluntary control of contraction and relaxation.

Cardiac Muscle

• Striated.
• Involuntary.
• Autorhythmic (pacemaker).

Smooth Muscle

• Non-striated.
• Involuntary.
• Found in arrector pili and walls of hollow organs.

Gross Anatomy of Skeletal Muscle

• A muscle is an organ.
• Fascia is connective tissue surrounding a muscle.
• Connective tissue sheaths (epimysium, perimysium, endomysium) surround the muscle, bundles of fibers, and individual fibers, respectively.
• Tendons connect muscle to bone.

Skeletal Muscle Nerve and Blood Supply

• Skeletal muscle is supplied by nerves, arteries, and veins.
• Somatic motor neurons form the neuromuscular junction.

Fusion of Myoblasts into Muscle Fibers

• Mature muscle is derived from more than 100 myoblasts.
• Mature muscle cells cannot divide further.
• Satellite cells retain the ability to regenerate new cells.

Muscle Fiber or Myofibers

• Sarcolemma is the muscle fiber membrane.
• Sarcoplasm is filled with myofibrils and myoglobin.

Sarcomere

• The sarcomere is the smallest contractile unit of a muscle.
• Located between two successive Z discs.
• Composed of overlapping myofilaments (actin and myosin).

Myofibrils & Myofilaments

• Myofibrils are separated by the sarcoplasmic reticulum (SR).
• Myofilaments are the contractile proteins within muscle tissue.

Sarcoplasmic Reticulum (SR)

• System of tubular sacs (smooth ER) enveloping myofibrils.
• Regulates calcium (Ca2+) levels.
• Stores Ca2+ in relaxed muscle.
• Releases Ca2+ during contraction via voltage-gated channels.

Transverse Tubules (T-tubules)

• Invaginations of the sarcolemma into the cell.
• Filled with extracellular fluid.
• Carry action potentials into the cell interior.
• Mitochondria are located near muscle proteins that use ATP.

Filaments & Sarcomere

• Striations are produced by the arrangement of thick and thin filaments within a sarcomere (the I-band and A-band).
• Overlap region: thick and thin filaments overlap.
• Supporting proteins: M line, titin, Z disc anchor thick and thin filaments in place.

Regions of a Sarcomere of a Myofibril

• Regions include the Z disc, M line, H zone, I band, and A band.

Proteins of Muscle

• Myofibrils contain three types of proteins:
  • Contractile: Myosin and actin
  • Regulatory: Troponin and tropomyosin
  • Structural: Titin, myomesin, nebulin, and dystrophin.

Thick Filaments

• Composed of myosin.
• Myosin heads are responsible for binding to actin.

Thin Filaments

• Composed of actin, tropomyosin, and troponin
• Myosin-binding site is typically covered by tropomyosin in relaxed muscles, preventing interaction with myosin.
• Thin filaments are held in place by Z lines.

Other Structural Proteins

• Titin anchors thick filaments to both the M line and Z disc.
• M line (myomesin) connects adjacent thick filaments and titin.
• Nebulin aligns thin filaments.
• Dystrophin links thin filaments to the sarcolemma and transmits tension to tendons.

Sliding Filament Mechanism of Contraction

• Myosin cross-bridges pull on thin filaments, causing them to slide inward.
• Z discs move closer together, shortening sarcomeres.
• Shortening of sarcomeres causes the entire muscle fiber to shorten.

Muscle Gross Anatomy Summary

• Parts of a muscle (tendon, epimysium, perimysium, endomysium, fascicle, myofibril, myofilament, organelle, and muscle fiber).

Muscle Scale

• Parts of a muscle (endomysium, perimysium, epimysium, fascicle, skeletal muscle—organ, tendon, bone, actin myofilament, myosin myofilament, sarcomere, nuclei, capillary, mitochondrion, muscle myofibrils).

Nerve Stimulus of Skeletal Muscle

• Motor neurons in the somatic nervous system innervate muscle cells.
• Motor neurons branch and form neuromuscular junctions with single muscle fibers.
• The neuromuscular junction is formed by axonal endings, the motor end plate of a muscle, and the synaptic cleft.

Synaptic Region

• Synaptic end bulbs contain synaptic vesicles (acetylcholine).
• The motor end plate has acetylcholine receptors.

Excitation of Skeletal Muscle Fiber

• Nerve signals trigger the release of acetylcholine (ACh) from synaptic vesicles.
• ACh binds to receptors on the sarcolemma.
• This leads to an action potential in the sarcolemma.
• The action potential travels down the T-tubules.
• This triggers the release of calcium (Ca2+) from the sarcoplasmic reticulum (SR).

Excitation-Contraction Coupling

• Depolarization of muscle fibers.
• Propagation of action potentials along the sarcolemma and T-tubules.
• Release of Ca2+ from the SR.

Overview of Skeletal Muscle Contraction

• Excitation of the muscle fiber (neuromuscular junction).
• Propagation of action potential along the sarcolemma and T-tubules.
• Release of Ca2+ from the sarcoplasmic reticulum.
• Cross-bridge cycling, sliding of thin filaments, and muscle shortening.

Relaxation-Acetylcholinesterase

• Enzyme acetylcholinesterase removes acetylcholine (ACh).
• Muscle action potentials cease.
• Sodium ions are pumped out of the cell.
• Cross-bridge cycling ceases as Ca2+ is transported back into the sarcoplasmic reticulum;
• tropomyosin recovers the binding site on actin.

Pharmacology of the NMJ

• Botulinum toxin prevents acetylcholine (ACh) release.
• Curare blocks ACh receptors.
• Neostigmine is a reversible acetylcholinesterase inhibitor.

Muscle Metabolism

• Muscle uses ATP during activity.
• Sources of ATP production include immediate, short-term, and long-term.

Immediate Supply of ATP: Phosphagen System

• ATP is produced using ATPase, myokinase, and creatine kinase.

Short-Term Supply of ATP: Anaerobic Cellular Respiration

• Pyruvic acid is converted to lactic acid.
• Lactic acid can return to the liver and be converted back to pyruvic acid when oxygen is available.

Long-Term Supply of ATP: Aerobic Cellular Respiration

• ATP production in mitochondria from pyruvic acid, fatty acids, and amino acids.

Creatine Phosphate

• Excess ATP produced in resting muscle is utilized to create creatine phosphate.
• Quick breakdown of creatine phosphate releases energy for muscle contraction.
• Creatine supplementation decreases innate production.

Fermentation: Anaerobic Cellular Respiration

• If oxygen is not present, pyruvic acid is converted into lactic acid.
• Lactic acid returns to the liver when oxygen is available and converted to pyruvic acid.

Anaerobic vs Aerobic Cellular Respiration

• Comparison of anaerobic and aerobic respiration for energy production.

Aerobic Cellular Respiration

• ATP production in mitochondria from pyruvic acid, fatty acids, and amino acids.

Muscle Fatigue

• Central fatigue (protective mechanism).
• Factors that contribute to muscle fatigue: Insufficient oxygen or glycogen, build-up of lactic acid and phosphates, insufficient release of acetylcholine (ACh) from motor neurons, and sodium and potassium imbalances in the cell.

Oxygen Consumption After Exercise

• Muscle tissue uses two sources of oxygen: diffusion from blood and release from myoglobin inside muscle fibers.
• Aerobic systems require oxygen to produce ATP needed for prolonged activity; elevated oxygen use is called oxygen debt.
• Factors include hemoglobin, myoglobin, glycogen stores, and ATP/creatine phosphate stores.
• Lactic acid is converted back to pyruvic acid.
• Body temperature increases.

Skeletal Muscle Fiber Types

• Three fiber types (slow oxidative, fast oxidative, fast glycolytic) are present, differing in their ATP production capacity, and function. Knowledge of each type's characteristics is crucial for understanding whole-fiber properties.

The Motor Unit

• Motor unit = somatic motor neuron and skeletal muscle fibers it stimulates.
• Total strength of contraction depends on the number of motor units activated.

Motor Unit Recruitment

• Motor units are recruited asynchronously to produce smooth muscular contractions.
• Precise movements require smaller motor units.
• Larger motor units are recruited when greater tension is needed.

Muscle Tone

• Involuntary contraction of a small number of motor units keeps muscles firm; essential for posture and maintaining blood pressure.