Overview
This lecture explains how nerve impulses (action potentials) trigger skeletal muscle contraction through the neuromuscular junction in a series of coordinated steps.
Nerve Impulses and Muscle Contraction
- Nerve impulses, or action potentials, travel from the brain or spinal cord to skeletal muscles to trigger contraction.
- Action potentials move along motor neurons to reach skeletal muscle fibers.
Neuromuscular Junction and Signal Transmission
- The neuromuscular junction is a chemical synapse where a motor neuron excites a skeletal muscle fiber.
- It consists of axon terminals (nerve endings) and the motor endplate (on the muscle fiber).
Steps of Signal Transmission at the Neuromuscular Junction
- Step 1: An action potential travels down the motor neuron to the axon terminal.
- Step 2: Voltage-gated calcium channels open, allowing calcium ions to enter the terminal.
- Step 3: Calcium influx causes synaptic vesicles to release acetylcholine (ACh) into the synaptic cleft by exocytosis.
- Step 4: Acetylcholine crosses the synaptic cleft and binds to receptors on the muscle fiber.
- Step 5: These receptors are ligand-gated cation channels, which open upon ACh binding.
- Step 6: Sodium ions enter, and potassium ions exit the muscle fiber, making the membrane potential less negative.
- Step 7: When the membrane potential reaches the threshold, an action potential is generated in the muscle fiber (sarcolemma).
Termination of Neural Transmission
- Neural transmission stops when acetylcholine is removed from the synaptic cleft.
- Acetylcholine is removed by diffusion away from the synapse or breakdown by acetylcholinesterase into acetic acid and choline.
- Choline is taken up by the neuron for reuse in making acetylcholine.
Key Terms & Definitions
- Action Potential — a rapid electrical signal traveling along neurons.
- Neuromuscular Junction — the synapse between a motor neuron and a skeletal muscle fiber.
- Acetylcholine (ACh) — neurotransmitter that activates muscle contraction.
- Acetylcholinesterase — enzyme that breaks down acetylcholine in the synaptic cleft.
- Sarcolemma — the cell membrane of a muscle fiber.
Action Items / Next Steps
- Review the seven steps of neuromuscular transmission for understanding and memorization.