Overview
This lecture explains how motor neurons communicate with muscle cells at the neuromuscular junction to initiate muscle contraction, covering the key processes, structures, and signaling involved.
Structure of the Neuromuscular Junction
- The neuromuscular junction is where a motor neuron's axon terminal meets a muscle cell.
- The muscle cell's membrane has in-foldings that increase surface area for more ion channels.
- These in-foldings contain many sodium and calcium channels to facilitate signal transmission.
Signal Transmission Process
- An action potential travels down the motor neuron to the axon terminal.
- Depolarization (influx of sodium) and some calcium enter the axon terminal.
- Calcium triggers vesicles containing the neurotransmitter acetylcholine (ACh) to move to the membrane.
- Vesicles fuse with the axon terminal membrane, releasing ACh into the synaptic cleft via exocytosis.
Postsynaptic Activation and Muscle Contraction
- ACh binds to nicotinic acetylcholine receptors on the muscle cell membrane.
- This opens sodium channels, causing sodium to enter and depolarize the muscle cell membrane.
- Depolarization leads to opening of voltage-gated calcium channels, increasing intracellular calcium.
- Calcium from the sarcoplasmic reticulum is released by calcium-induced calcium release, amplifying the contraction signal.
Propagation of Muscle Contraction
- Released calcium binds to proteins that trigger muscle contraction.
- Gap junctions connect muscle cells, allowing ions to pass and synchronize contractions.
- As neighboring cells activate, the contraction response scales up to entire muscle fascicles.
Key Terms & Definitions
- Neuromuscular junction — Site where a neuron communicates directly with a muscle cell.
- Axon terminal — The end of a neuron where neurotransmitters are released.
- Synaptic cleft — The space between the neuron terminal and muscle cell membrane.
- Acetylcholine (ACh) — Neurotransmitter used at the neuromuscular junction.
- Exocytosis — Process of vesicles merging with the membrane to release contents out of the cell.
- Nicotinic receptor — A receptor on the muscle membrane that binds ACh and opens sodium channels.
- Sarcoplasmic reticulum — Organelle in muscle cells storing calcium ions.
- Calcium-induced calcium release — Mechanism where calcium entry triggers further calcium release from the sarcoplasmic reticulum.
- Gap junction — Protein channel connecting neighboring muscle cells.
- Syncytium — A functional unit where cells function together as one by sharing ions or signals.
Action Items / Next Steps
- Review the processes of exocytosis and endocytosis.
- Study the roles of sodium and calcium channels in muscle excitation.
- Read more about the sliding filament model to connect it to muscle contraction.