Overview
This lecture explains the neuromuscular junction, focusing on how nerve impulses lead to skeletal muscle contraction through neurotransmitter release.
Structure and Function of the Neuromuscular Junction
- The neuromuscular junction is where a motor neuron connects to a skeletal muscle fiber to enable contraction.
- Motor neuron cell bodies are located in the anterior (ventral) gray horn of the spinal cord.
- The somatic nervous system controls skeletal muscles and is under voluntary control.
Action Potential Transmission
- The alpha motor neuron conducts action potentials from the spinal cord to muscle fibers.
- Action potentials occur when voltage-gated sodium channels open, allowing sodium influx and depolarizing the neuron's membrane.
- Depolarization is a wave of increasing positive charge that travels along the axon.
Neurotransmitter Release Mechanism
- At the axon terminal (synaptic bulb), depolarization opens voltage-gated calcium channels at around +30 mV.
- Calcium influx triggers vesicles containing acetylcholine (ACh) to move toward and fuse with the cell membrane.
- ACh is synthesized in the synaptic bulb from choline (obtained via diet) and acetate (from mitochondrial acetyl-CoA), catalyzed by choline acetyltransferase.
Vesicle Loading and Exocytosis
- Protons are actively pumped into synaptic vesicles using ATP, creating a gradient that helps import ACh via secondary active transport.
- SNARE proteins control neurotransmitter release: V-SNAREs (synaptotagmin, synaptobrevin) on vesicles and T-SNAREs (SNAP-25, syntaxin) on the membrane.
- Calcium acts as a cross-link, allowing SNAREs to intertwine, bringing the vesicle to the membrane for exocytosis and ACh release.
Membrane Repolarization
- Voltage-gated potassium channels open, allowing potassium to exit and repolarize the membrane to about -70 mV.
- Repolarization closes calcium channels, stopping further ACh release and placing the terminal in a refractory period.
Key Terms & Definitions
- Neuromuscular Junction — Connection point between a motor neuron and a muscle fiber.
- Alpha Motor Neuron — The neuron supplying skeletal muscle fibers.
- Depolarization — Reduction of membrane potential making the inside more positive.
- Acetylcholine (ACh) — Neurotransmitter enabling muscle contraction.
- SNARE Proteins — Protein family mediating vesicle fusion (synaptotagmin, synaptobrevin, SNAP-25, syntaxin).
- Exocytosis — Process of vesicle contents (ACh) released into the synaptic cleft.
Action Items / Next Steps
- Review the next lecture/video covering how acetylcholine acts on muscle cell receptors and alters membrane potential.