Focus: Conversion of electrical signals from motor neuron to muscle fiber.
Neuromuscular Junction (NMJ): Comprises axon terminus, synapse (gap), and motor end plate.
Key Components:
Motor Neuron
Axon
Motor End Plate: Part of the muscle plasma membrane.
Synapse: Gap between motor neuron and muscle fiber.
Action Potential Propagation
Voltage-Gated Channels:
Sodium (Na+) and Potassium (K+): Facilitate rapid depolarization and repolarization.
Calcium (Ca2+): Triggers exocytosis of neurotransmitters.
Resting Membrane Potential:
Maintained by leaky channels and the sodium-potassium pump.
Neurotransmitter Release
Acetylcholine (ACh):
Exclusive neurotransmitter in motor neurons.
Released into the synapse following calcium influx.
Ion Permeability and Membrane Potential
Chemically Gated Channels:
Located at the motor end plate.
Allow simultaneous sodium influx and potassium eflux.
Sodium influx is greater due to:
Electrochemical gradient.
Smaller size compared to potassium.
Result: Membrane potential becomes more positive (depolarization).
Graded Potentials and End Plate Potential
End Plate Potential (EPP):
Converted chemical signal (ACh) into an electrical signal (EPP) in muscle cell.
Distinguished from graded potentials seen in neurophysiology.
Typically reaches super-threshold levels, often leading to an action potential.
Key Differences
Chemically Gated Channel: Allows both sodium and potassium permeability.
EPP Characteristics:
No summation of graded potentials due to single motor neuron.
Almost always results in an action potential.
Conclusion
Events at the neuromuscular junction involve conversion of electrical signals in motor neurons into chemical signals and then back to electrical signals in muscle cells.
Next video will explore conversion of EPP into an action potential leading to muscle contraction.