Muscle Lecture 5: Muscle Contraction and Relaxation in Human A&P

Introduction

• Delivered by Professor Bob Long Jr.
• Focus on how action potentials from neurons lead to muscle contraction.

Review of Muscle Structure

• Muscle Composition: Muscle is composed of bundles called fascicles.
• Muscle Tissue Layers:
  • Epimysium: Outer covering of the muscle.
  • Perimysium: Surrounds each fascicle.
  • Endomysium: Surrounds individual muscle fibers (myofibers).
• Myofibrils: Tubular structures within muscle cells.
• Sarcomeres: Repeating subunits within myofibrils.

Molecular Structure

• Actin and Myosin:
  • Myosin heads can bind to active sites on actin.
  • Active sites on actin are covered by tropomyosin, held in place by troponin.

Cellular Organelles

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum in muscle cells, stores calcium ions.
• T-Tubules: Invaginations of the sarcolemma that transmit action potentials deep into the muscle cell.

Muscle Contraction Mechanism

1. Neuronal Action Potential:
   • Begins in the cerebral cortex, travels down neurons in the spinal cord to a motor neuron.
2. Synaptic Transmission:
   • Synaptic Knobs: End of the neuron's axons contain synaptic vesicles with neurotransmitter acetylcholine (ACh).
   • Synaptic Cleft: Gap between neuron and muscle cell.
   • ACh binds to receptors on the muscle cell membrane, opening sodium channels.
3. Depolarization:
   • Sodium influx leads to depolarization, generating an action potential in the muscle cell.
   • Action potential travels down T-tubules.
4. Calcium Release:
   • Action potential triggers calcium release from the SR into the cytosol.
   • Calcium binds to troponin, which moves tropomyosin, exposing actin sites for myosin binding.
5. Cross-Bridge Cycling:
   • Myosin heads bind to actin, pulling actin filaments, causing muscle contraction.

Muscle Relaxation

1. Action Potential Ends:
   • Neuronal signals stop, closing voltage-gated calcium channels.
2. Calcium Reuptake:
   • Calcium is pumped back into the SR by active transport.
3. Repositioning of Tropomyosin:
   • Without calcium, troponin reverts, moving tropomyosin to block myosin binding sites on actin.
4. Acetylcholine Breakdown:
   • ACh in the synaptic cleft is broken down by acetylcholinesterase, stopping signal transmission.

Key Enzymes and Proteins

• Acetylcholine: Neurotransmitter that initiates muscle contraction.
• Acetylcholinesterase: Enzyme that breaks down acetylcholine, aiding in muscle relaxation.
• Calcium Ion Channels: Both voltage-gated and chemically gated, crucial for signaling and contraction.

Summary

• Mechanisms of contraction involve complex steps from neural signals to muscle fiber responses.
• Relaxation reverses contraction processes by removing stimuli and calcium.

Study Tips

• Watch associated animations and videos for visual understanding.
• Utilize provided worksheets and notes for practice and review.