Neuromuscular Junction and Muscle Contraction Process

Overview

• Skeletal muscle contraction process starts with a signal from the brain.
• This signal travels as an action potential through neurons.

Signal Transmission

• Upper Motor Neuron: Initiates in the cerebral cortex.
• Lower Motor Neuron: Receives signal in the anterior horn of the spinal cord.
• Action potential travels down the axon to axon terminals.

Neuromuscular Junction Components

1. Presynaptic Part
   • Membrane of an axon terminal.
2. Postsynaptic Part
   • Membrane of a skeletal muscle fiber.
   • Also called the motor end plate.
3. Synaptic Cleft
   • Space between nerve terminal and motor end plate.

Mechanism of Muscle Contraction

1. Action Potential at Axon Terminal
   • Opens voltage-gated calcium channels.
   • Calcium ions enter the axon terminal.
2. Role of Synaptic Vesicles
   • Contain neurotransmitter acetylcholine.
   • Calcium influx leads to vesicle fusion with axon terminal membrane.
   • Acetylcholine is released into the synaptic cleft by exocytosis.
3. Acetylcholine Binding
   • Binds to nicotinic acetylcholine receptors on the motor end plate.
   • Opens acetylcholine-gated ion channels.
   • Sodium ions influx, carrying positive charges into the muscle fiber.
4. Membrane Potential Change
   • Postsynaptic membrane potential changes from -90 mV to -45 mV.
   • Initiates an action potential along the muscle membrane.
   • Causes muscle contraction.

Acetylcholine Removal

• Enzyme Acetylcholine Esterase
  • Rapidly hydrolyzes acetylcholine in the synaptic cleft.
• Diffusion
  • Small amounts of acetylcholine diffuse out of the synaptic cleft.

Conclusion

• Acetylcholine removal prevents sustained muscle contraction.
• Understanding this process is crucial for appreciating how muscle contractions are controlled and terminated.

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