Neuromuscular Junction

Overview

• Previous Video: Discussed skeletal muscle structure and sarcomere.
• Current Focus: Neuromuscular junction and transmission.

Anatomy of the Motor Neuron

• Spinal Cord Cross Section: Midbrain, pons, medulla.
• Anterior Gray Horn:
  • Contains cell bodies of somatic motor neurons.
  • Unmyelinated (gray matter).
  • Also known as the ventral gray horn.

Somatic Nervous System

• Voluntary Control: E.g., moving arm to flex the biceps.
• Supplies Skeletal Muscles: Under voluntary control.

Motor Neuron Pathway

• Alpha Motor Neuron: Supplies skeletal muscles.
• Action Potential:
  • Depolarization caused by sodium influx.
  • Sodium channels open at threshold potential.
  • Sodium influx causes inside to become electropositive.

Action Potentials and Depolarization

• Voltage-Gated Sodium Channels: Open, allowing sodium in, depolarizing membrane.
• Synaptic Bulb/Synaptic Terminal:
  • Action potentials travel here.
  • Voltage-gated calcium channels open at positive 30 mV.
  • Calcium influx triggers neurotransmitter release.

Acetylcholine Synthesis and Transport

• Vesicle Formation: In cell body, transported to synaptic bulb.
• Acetylcholine:
  • Composed of choline (nutrient) and acetate.
  • Synthesized in synaptic bulb from dietary choline and mitochondrial acetyl-CoA.
  • Enzyme: Choline acetyltransferase.

Neurotransmitter Release

• Vesicle Transporters: Concentrate acetylcholine inside vesicles.
• Proton Pumping: Uses ATP to create gradient for secondary transport.
• SNARE Proteins:
  • V-Snares: Synaptotagmin, Synaptobrevin.
  • T-Snares: SNAP25, Syntaxin.
  • Calcium facilitates SNARE interaction for vesicle fusion.

Exocytosis Process

• Calcium Role: Triggers SNARE protein intertwining.
• Vesicle Fusion: Releases acetylcholine into synaptic cleft by exocytosis.

Repolarization and Resting Phase

• Potassium Channels:
  • Open at positive 30 mV, allowing potassium efflux.
  • Hyperpolarizes cell back to negative 70 mV (resting potential).
• Calcium Channels Close: Stops acetylcholine release during refractory period.

Next Steps

• Upcoming Video: Effects of acetylcholine on muscle cells.

This lecture provides an in-depth explanation of the neuromuscular junction and its associated processes, from action potential initiation to neurotransmitter release. Understanding these mechanisms is critical in muscle physiology and neurophysiology.