Neuromuscular Junction and Action Potential Transmission

Nerve Impulses and Skeletal Muscle Contraction

• Nerve impulses (action potentials) travel from the brain/spinal cord to trigger skeletal muscle contraction.

• The action potential propagates down a motor neuron to a skeletal muscle fiber.

• The neuromuscular junction is where a motor neuron excites a skeletal muscle fiber. It's a chemical synapse.

  • Consists of contact points between the axon terminals of a motor neuron and the motor end plate of a skeletal muscle fiber.

Events at the Neuromuscular Junction (7 Steps)

1. Action Potential Arrival: An action potential travels the length of the motor neuron's axon to the axon terminal.

2. Calcium Influx: Voltage-gated calcium channels open, and Ca^{2+} ions diffuse into the axon terminal.

3. Acetylcholine Release: Calcium entry triggers synaptic vesicles to release acetylcholine (ACh) via exocytosis.

4. ACh Diffusion and Binding: Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors on the muscle fiber. These receptors contain ligand-gated cation channels.

5. Ligand-Gated Channels Open: The ligand-gated cation channels open upon ACh binding.

6. Ion Flux and Membrane Potential Change: Sodium ions (Na^+, shown in red) enter the muscle fiber, and potassium ions (K^+, shown in blue) exit.

   • The inward flux of Na^+ is greater than the outward flux of K^+, causing the membrane potential to become less negative (depolarization).

7. Action Potential Propagation: When the membrane potential reaches a threshold value, an action potential propagates along the sarcolemma (muscle cell membrane).

Termination of Neural Transmission

• Neural transmission ceases when acetylcholine is removed from the synaptic cleft through two mechanisms:

  1. Diffusion: Acetylcholine diffuses away from the synapse.

  2. Enzymatic Degradation: Acetylcholine is broken down by the enzyme acetylcholinesterase into acetic acid and choline.

• Choline is then transported back into the axon terminal for resynthesis of acetylcholine.

• Nerve impulses (action potentials) travel from the brain/spinal cord to trigger skeletal muscle contraction.

• The action potential propagates down a motor neuron to a skeletal muscle fiber, initiating a series of events that lead to muscle contraction.

• The neuromuscular junction is where a motor neuron excites a skeletal muscle fiber. It's a chemical synapse, a highly specialized site for signal transmission.

  • Consists of contact points between the axon terminals of a motor neuron and the motor end plate of a skeletal muscle fiber. The motor end plate is a specialized region of the muscle fiber's plasma membrane (sarcolemma) that contains a high density of acetylcholine receptors.