Biology 1103/1109 Muscle Physiology Week 11 Notes

Anatomy of the Neuromuscular Junction

Anatomy of the Neuromuscular Junction

  • Motor Neurons: Nerves that extend from the brain/spinal cord to muscles.

  • Synaptic Terminals: Swollen endings of the axon; close to, but do not contact the muscle cell sarcolemma.

  • Neuromuscular Junction: The interface between motor neuron and muscle cell.

  • Motor Unit: A motor neuron and all muscle cells it innervates.

  • Acetylcholine (ACh): Neurotransmitter stored in synaptic vesicles inside end bulbs.

Process of Muscle Contraction

  1. Nerve Impulse: Triggers Ca²⁺ release of ACh from synaptic vesicles into the synaptic cleft.

  2. ACh Binding: ACh diffuses across the cleft and binds to receptors on the sarcolemma.

  3. Ion Channel Opening: Binding opens ligand-gated Na⁺ channels; Na⁺ ions rush into the cell.

  4. Action Potential Generation: Change in voltage creates a muscle action potential.

  5. Sarcoplasmic Reticulum (SR): Ca²⁺ pumps actively transport Ca²⁺ into the SR; relaxation state has higher Ca²⁺ inside SR.

  6. Troponin-Tropomyosin Complex: Covers myosin-binding sites on actin; when Ca²⁺ binds, it shifts, exposing binding sites.

  7. Crossbridge Formation: Myosin heads attach to actin; contraction cycle repeats as long as Ca²⁺ and ATP are available.

  8. Shortening: Myofilaments slide over each other, pulling the H zone and I band closer together.

Physiology of Muscle Relaxation

  • Stopping ACh Effects: Enzymatic degradation or reuptake ceases ACh action.

  • Troponin-Tropomyosin Repositioning: When Ca²⁺ is removed, the complex covers myosin-binding sites again.

Muscle Tone

  • Definition: Sustained, partial contraction of skeletal muscle, maintaining posture.

Histology of Cardiac Muscle

  • Intercalated Discs: Structure connecting cardiac cells; contains gap junctions and desmosomes.

  • Gap Junctions: Allow ion passage for synchronous contraction.

  • Desmosomes: Reinforce cellular connections, ensuring integrity during contraction.

  • T Tubules: In cardiac muscle, associated with SR; involved in excitation-contraction coupling.

Mechanism of Cardiac Muscle Contraction

  1. Depolarization: Initiated by neurotransmitters on the plasma membrane.

  2. Calcium Entry: Voltage-gated channels open, allowing Ca²⁺ influx from extracellular space.

  3. SR Calcium Release: Ca²⁺ influx triggers SR release; both sources increase calcium in sarcoplasm.

  4. Cross-Bridge Formation: Ca²⁺ binds to troponin, initiating contraction through the sliding filament model.

Functional Significance of Self-Excitatory Cardiac Muscle Cells

  • Pacemaker Cells: Generate spontaneous depolarizations, initiating heartbeats, regulating heart rate via depolarization frequency.

Histology of Smooth Muscle

  • Caveolae: Invaginations in the sarcolemma, containing calcium channels.

  • Myofilament Arrangement: Myofilaments arranged helically; lacks sarcomeres.

  • Dense Bodies: Anchor points for myofilaments, transmitting contraction force.

Mechanism of Smooth Muscle Contraction and Relaxation

  1. Action Potential Transmission: Triggered by neurotransmitters, propagating through gap junctions.

  2. Calcium Influx: Opens calcium channels in the sarcolemma and SR, increasing intracellular Ca²⁺.

  3. Calmodulin Activation: Ca²⁺ binds to calmodulin, leading to myosin head phosphorylation.

  4. Myosin-Actin Interaction: Formation of cross bridges and contraction ensues.

  5. Relaxation Mechanism: Calcium is pumped out; calmodulin deactivates, reducing myosin ATPase activity.

Regulation of Smooth Muscle Contraction

  • Neurotransmitters: Different effects on smooth muscle depending on the tissue.

  • Hormonal Regulation: e.g., cholecystokinin relaxes sphincter, gastrin stimulates stomach contractions.

  • Chemical Factors: Substances like histamine and CO₂ can stimulate contraction without action potentials.

Peristalsis

  • Definition: Sequential contraction and relaxation of smooth muscle layers in various organs.

  • Muscle Layer Arrangement: Longitudinal and circular muscle layers facilitate coordinated movement through lumens (e.g., esophagus, stomach, intestines).