Seeley's Anatomy & Physiology Chapters 9 & 10 Practice Flashcards

Chapter 9 Overview and Muscular Properties

• Functions of Skeletal Muscle

  • Movement: Production of bodily motion.

  • Posture: Maintenance of body position.

  • Joint Stability: Reinforcing and stabilizing joints.

  • Heat Production: Generation of heat through metabolic activity and contraction.

• Functional Properties of Muscle Tissue

  • Excitability: The ability to receive and respond to stimuli.

  • Conductivity: The ability to propagate electrical signals along the membrane.

  • Contractility: The ability to shorten forcibly when stimulated.

  • Extensibility: The ability to be stretched or extended.

  • Elasticity: The ability of a muscle fiber to recoil and resume its resting length after stretching.

• Connective Tissue Layers

  • Epimysium: An overcoat of dense irregular connective tissue that surrounds the entire muscle.

  • Perimysium: Fibrous connective tissue that surrounds groups of muscle fibers called fascicles.

  • Endomysium: A fine sheath of connective tissue surrounding each individual muscle fiber.

• Muscle Attachments and Fascia

  • Tendons: Cord-like structures that attach muscle to the periosteum of the bone.

  • Aponeuroses: Broad, flat, sheet-like tendons that attach muscles to other muscles or bones.

  • Fascia: Connective tissue that surrounds individual muscles and groups of muscles into functional compartments.

Muscle Organization and Structural Hierarchy

• Levels of Organization

  • Muscle: The complete organ.

  • Fascicles: Bundles of muscle fibers.

  • Muscle Fibers: Individual muscle cells.

  • Myofibrils: Rod-like contractile elements that occupy most of the muscle cell volume.

  • Sarcomeres: The functional contractile unit of the muscle fiber, located between two Z discs.

  • Myofilaments: The even smaller macromolecular structures consisting of actin (thin filaments) and myosin (thick filaments).

The Neuromuscular Junction (NMJ)

• Step-by-Step Mechanism of Signal Transmission

  • 1. A motor neuron action potential reaches the axon terminal of the neuromuscular junction.

  • 2. Voltage-gated calcium channels open in the axonal membrane.

  • 3. Calcium ($Ca^{2+}$) enters the axon terminal.

  • 4. Acetylcholine ($ACh$) is released from the axon terminal into the synaptic cleft via exocytosis.

  • 5. $ACh$ diffuses across the cleft and binds to nicotinic receptors located on the motor end plate of the sarcolemma.

  • 6. Sodium ($Na^+$) enters the muscle fiber, causing a depolarization of the sarcolemma.

  • 7. Acetylcholinesterase ($AChE$) rapidly breaks down $ACh$ in the synaptic cleft to terminate the stimulation and prevent continued contraction.

Excitation-Contraction Coupling

• Process Steps

  • 1. An action potential is propagated and travels across the entire length of the sarcolemma.

  • 2. The action potential travels deep into the muscle fiber via the T-tubules.

  • 3. DHP (dihydropyridine) receptors in the T-tubules are activated, which in turn activate ryanodine receptors on the sarcoplasmic reticulum.

  • 4. The sarcoplasmic reticulum (SR) releases stored calcium ($Ca^{2+}$) into the sarcoplasm.

  • 5. Calcium ($Ca^{2+}$) ions bind to troponin.

  • 6. The binding causes a conformational change in troponin, which moves tropomyosin away from the active sites on actin, thereby exposing the actin-myosin binding sites.

  • 7. Myosin heads bind to the exposed actin sites, forming cross-bridges.

  • 8. A power stroke occurs as the myosin head pivots, pulling the actin filament toward the center of the sarcomere.

  • 9. ATP binds to the myosin head, causing the myosin to release its grip on the actin.

  10. ATP hydrolysis (breakdown into ADP and $P_i$) provides the energy to "re-cock" the myosin head back into its high-energy position.

  • The cycle repeats continually as long as calcium is present and ATP is available. The process ends when calcium is actively pumped back into the SR.

The Sliding Filament Model of Contraction

• Sarcomere Changes During Contraction

  • The I band shortens.

  • The H zone shortens and may disappear.

  • The Z discs move closer together.

  • The A band stays the same length.

  • Filament Behavior: It is critical to note that the individual myofilaments (actin and myosin) do not shorten themselves; instead, they slide past one another to increase the degree of overlap.

Energy Requirements and ATP Sources

• Roles of ATP

  • ATP is required to detach the myosin head from the actin filament.

  • ATP provides energy to re-cock the myosin head to prepare for the next power stroke.

  • ATP powers the calcium pumps that return $Ca^{2+}$ to the sarcoplasmic reticulum.

• Sources of ATP Production

  • Creatine Phosphate: The fastest method of ATP production; provides immediate energy for short bursts.

  • Anaerobic Glycolysis: Provides short-term energy but results in the production of lactic acid.

  • Aerobic Respiration: Provides long-term energy; highly efficient but requires a steady supply of oxygen.

Types of Muscle Contraction

• Isometric Contraction

  • Tension increases within the muscle, but the muscle does not shorten or lengthen. There is no change in the joint angle.

• Isotonic Contractions

  • Isotonic Concentric: The muscle shortens while producing force (e.g., the upward phase of a bicep curl).

  • Isotonic Eccentric: The muscle lengthens while producing force (e.g., lowering a weight in a controlled manner).

Motor Units and Muscle Fatigue

• Motor Unit Dynamics

  • Definition: A motor unit consists of a single motor neuron and all the individual muscle fibers it innervates.

  • Recruitment: The process of increasing the number of active motor units to increase the total force of contraction.

• Muscle Fatigue

  • Fatigue is the physiological inability of a muscle to contract despite continued stimulation.

  • Causes: Primarily results from the buildup of metabolites, ionic imbalances ($K^+$, $Na^+$), and depletion of energy stores.

  • Clarification: Fatigue is rarely caused by a total exhaustion of ATP.

Muscle Fiber Types

• Type I Fibers (Slow Oxidative)

  • Characteristics: Slow contraction speed, high aerobic capacity.

  • Function: Fatigue resistant; used for endurance activities like maintaining posture.

• Type IIa Fibers (Fast Oxidative-Glycolytic)

  • Characteristics: Moderate contraction speed; uses both aerobic and anaerobic metabolism.

  • Function: Intermediate resistance to fatigue.

• Type IIx Fibers (Fast Glycolytic)

  • Characteristics: Fastest contraction speed, primarily anaerobic.

  • Function: Strongest force production but fatigues the fastest.

Clinical Connections and Pathophysiology

• Myasthenia Gravis

  • A condition characterized by a reduction in the number of functional acetylcholine ($ACh$) receptors at the neuromuscular junction, leading to muscle weakness.

• Botulinum Toxin

  • A toxin that blocks the release of $ACh$ from the axon terminal, preventing muscle contraction (paralysis).

• Curare

  • An alkaloid that acts by blocking $ACh$ receptors on the motor end plate, preventing the muscle from responding to nerve impulses.

• Rigor Mortis

  • Occurs after death because ATP is no longer being produced. Without ATP, the myosin heads cannot detach from actin, resulting in permanent cross-bridges and rigid muscles.

Exam Strategy and Review Tips

• Key Processes to Master

  • Memorize every specific step of excitation-contraction coupling in the correct chronological order.

  • Be able to identify and label the components of the neuromuscular junction (NMJ) and the sarcomere.

• Concepts to Differentiate

  • Understand the electrical difference between depolarization (becoming less negative) and repolarization.

  • Distinguish the mechanical differences between concentric (shortening) and eccentric (lengthening) isotonic contractions.

  • Recall exactly which zones and bands change size in the sarcomere during the sliding filament process vs. which remain constant.