Lecture Notes: Neuromuscular Junction and Skeletal Muscle Contraction

Key Learning Goals

• Examine the structure of a neuromuscular junction.
• Understand the sequence of events at the neuromuscular junction following a stimulus.

Essential Concepts

• Anatomy of a Skeletal Muscle Cell
• Resting Membrane Potential: The polarized state of the axon terminal and sarcolemma.
• Action Potential
• Depolarization

Anatomy Overview

• Skeletal Muscle Cells: Electrically insulated from each other by endomysium.
• Motor Neurons: Stimulate each muscle cell; each neuron can stimulate several cells.
• Axon: Elongated nerve cell process carrying impulses.

Neuromuscular Junction Structure

• Axon Terminal: Swollen distal end of the axon containing acetylcholine in synaptic vesicles.
• Synaptic Cleft: Space between the axon terminal and the motor end plate of the muscle cell.
• Motor End Plate: Folded region of the sarcolemma.
• T-Tubules: Invaginations of the sarcolemma penetrating into the cell.
• Terminal Cisternae: Specialized regions of the sarcoplasmic reticulum storing calcium ions.
• Sarcomere: Contractile unit of the muscle cell from one Z-line to the next.

Action at the Neuromuscular Junction

1. Action Potential Arrival
   • Triggers voltage-regulated calcium channels to open.
   • Calcium ions enter axon terminal.
2. Release of Acetylcholine
   • Calcium causes synaptic vesicles to fuse with the membrane.
   • Acetylcholine released into synaptic cleft via exocytosis.
3. Depolarization of Motor End Plate
   • Acetylcholine binds to receptor sites on ion channels.
   • Channels open allowing sodium influx and potassium efflux.
   • Local depolarization occurs.
4. Propagation of Action Potential
   • Acetylcholine diffuses away and is broken down by acetylcholinesterase.
   • Depolarization initiates action potential along sarcolemma and T-tubules.
5. Release of Calcium Ions
   • Action potential causes calcium release from terminal cisternae.
   • Calcium triggers muscle cell contraction.

Summary

• Each skeletal muscle cell is stimulated by a motor neuron.
• Neuromuscular junction: Site where motor neuron's axon meets muscle cell membrane.
• Action potential leads to acetylcholine release, depolarization, and further action potential along sarcolemma, resulting in muscle contraction.

Interactive Learning

• Engage with interactive elements to explore each process step-by-step.
• Quiz: Test your understanding with self-assessment quiz.