Neuromuscular Junction Flashcards
What is a neuromuscular junction?
- Between a motor neuron and a muscle cell
- Where an axon terminal meets the muscle fiber
- Tuned for reliable neurotransmission
- Efferent
- Highly specialised (large conc of vesicles ready to fuse)
What must happen in order for a skeletal muscle to contract?
An action potential is sent from the brain to an upper motor neuron which activates a lower motor neuron. The action potential then travels along axons along axon terminals to muscle fibers
What is the presynaptic membrane of the NMJ?
Membrane of the axon terminal
What is the postsynaptic membrane of the NMJ?
Membrane of the skeletal muscle fiber (Motor end plate)
What is the synaptic cleft of the NMJ?
Gap between the presynaptic membrane and the motor end plate
What happens when an action potential reaches the axon terminal? (NMJ)
Stimulates voltage gated calcium channels to open, allowing calcium ions from the ECF to flow into the axon terminal which can then bind to vesicles containing acetyl choline allowing them to fuse with the presynaptic membrane
What happens when vesicles containing acetyl choline fuse with the presynaptic membrane (NMJ)?
Acetylcholine is released into the synaptic cleft where is rapidly diffuses to the motor end plate where 2 acetyl choline molecules will bind to a ligand gated ion channel (nicotinic actylcholine receptor (nAChRs)) allowing them to open
What happens once the nAChRs are open?
Sodium ions enter into the skeletal muscle fiber, increasing the overall charge (DEPOLARISATION) on the post synaptic membrane (EP).
What happens once threshold potential is met? (NMJ)
Once threshold potential is met voltage gated sodium channels open and an action potential is generated and the muscle fiber contracts
What happens once the signal from the lower motor neuron stops?
Voltage gated calcium channels on the presynaptic membrane close, no influx of calcium, vesicles stop fusing and just remain in the cytoplasm of the axon terminal.
What happens to the remaining acetyl choline in the synaptic cleft?
Acetyl choline left in the synaptic cleft is degraded by acetylcholinesterase into choline and acetate. The choline is taken back into the axon terminal where it is reused by the enzyme acetylcholine transferase to make new acetylcholine
What is the arrangement of NMJs and sketal muscle fibers?
- Each skeletal muscle fiber has only one NMJ so each muscle fiber is controlled by one lower motor neurone. The axon of a lower motor neuron splits into many branches and each branch can innervate multiple adjacent muscle fibers (single motor unit)
What is a single motor unit?
A lower motor neuron and all the muscle fibers it innervates
What does the size of a motor unit depend on?
Precision of muscle control. More precise = more smaller units
