Notes on Synaptic Transmission between Motor Neurons and Muscle Fibers

Motor Neuron and Muscle Fiber Interaction

• Motor Neurons: Specialized cells that transmit signals from the central nervous system to muscles.

  • Have long projections called axons.

• Synaptic Transmission Process:

  1. Action Potential Arrives at Axon Terminal:
  • The process begins when an action potential reaches the terminal end of the axon of a motor neuron.
  1. Depolarization and Channel Opening:
  • Voltage-gated Na+ channels open, leading to depolarization of the axon terminal.
  • This depolarization triggers voltage-gated Ca2+ channels to open.
  1. Calcium Ion Entry:
  • Ca2+ ions enter the cell, initiating the process of neurotransmitter release.
  • Calcium influx causes synaptic vesicles containing Acetylcholine (ACh) to fuse with the presynaptic membrane.
  1. Neurotransmitter Release:
  • ACh molecules diffuse across the synaptic cleft, the gap between the presynaptic neuron and the postsynaptic membrane of the muscle fiber.
  1. Receptor Binding and Postsynaptic Potential:
  • ACh binds to receptors on the postsynaptic membrane (motor end plate of muscle cell), opening cation channels.
  • Opening these channels causes depolarization of the postsynaptic membrane, which can lead to the generation of an action potential in the muscle fiber.
  1. Action Potential in Postsynaptic Cell:
  • The depolarization spreads and fires an action potential in the muscle cell, leading to contraction.
  1. Termination of Signal:
  • The enzyme Acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft, terminating the action of ACh.
  • The components of ACh are then reabsorbed by the presynaptic cell for recycling.

• Importance of This Process:

  • Efficient Communication: Essential for muscle contraction and voluntary movement.
  • Signal Termination: Prevents continuous stimulation of muscles, which could lead to spasms or paralysis.

• Key Terms:

  • Synaptic cleft: The space between the presynaptic and postsynaptic membranes where neurotransmission occurs.
  • ACh receptors (AChR): Specific proteins on the postsynaptic membrane that bind to acetylcholine and initiate muscle depolarization.