Lecture Notes on Neuromuscular Junction and Muscle Contraction

Overview

• Focus on skeletal muscle contraction
• Understanding how a cell knows when to contract
• Key concept: Neuromuscular junction (NMJ)

Neuromuscular Junction

• NMJ is the communication point between a motor neuron and a skeletal muscle cell
• Each muscle cell communicates with only one neuron, but a neuron can connect with multiple muscle cells

Structure of Neuromuscular Junction

• Motor neuron axon from the spinal cord to the muscle cell
• Synaptic components:
  • Presynaptic area: Contains synaptic vesicles with neurotransmitter (acetylcholine)
  • Synaptic cleft: Space between the neuron and muscle cell
  • Postsynaptic area: Muscle cell membrane with acetylcholine receptors

Function of Neuromuscular Junction

• Acetylcholine is released from presynaptic terminal into the synaptic cleft
• Binds to receptors on the postsynaptic membrane
• Initiates an action potential by allowing sodium influx

Action Potential and Muscle Contraction

• Action potential propagates along the muscle cell membrane and into T-tubules
• Voltage change triggers release of calcium from sarcoplasmic reticulum
• Calcium binds to troponin on actin, facilitating muscle contraction

Mechanism of Muscle Contraction

• Calcium binding causes tropomyosin to shift, exposing active sites on actin
• Myosin binds to actin, using ATP to move actin filaments, resulting in contraction
• Calcium is quickly pumped back into sarcoplasmic reticulum; contraction ceases

Twitch

• Response of a muscle cell to a single action potential
• Characterized by a quick contraction and relaxation
• Time from stimulus to contraction peak is approximately 60 milliseconds

Important Concepts

• Dihydropyridine (DHP) Receptor: Voltage-sensitive protein in T-tubules that triggers calcium release
• Ryanodine Receptor: Works with DHP receptor to regulate calcium flow from sarcoplasmic reticulum

Summary

• Neuromuscular junction facilitates muscle contraction through acetylcholine signaling
• Action potentials lead to calcium-mediated contraction of muscle fibers
• Understanding delays in muscle response will be discussed in future lectures