Neuro-muscular junction Flashcards
What is the neuromuscular junction?
Motor nerve terminal at the motor endplate
Ultimate synapse
What is the function of the Schwann cells?
Overlay the terminals which prevents leakage of current
Amplitude doesn’t decrease
What happens at rest?
Random blips in the resting potential are present (MEPPs)
What happens when a nerve is stimulated?
Membrane potential depolarizes - EPP triggers and action potential when it reaches the threshold
What neurotransmitter is present in each synaptic vesicle?
ACh
How is ACh synthesized?
In the nerve terminal cytoplasm from choline and Acetyl CoA via the enzyme ChAT
How is ACh hydrolysed?
ACh is hydrolysed after it is released by acetylcholinesterase in synaptic basal lamina
What is the effect of 4-AP?
Blocks K+ chnanels - depolarization prolongs action potential
Allows more Ca2+ to enter which increases number of vesicles used
What happens when Ca2+ is lowered?
Decreases number of vesicles available to fuse
Decreases number of transmitters released due to Ca2+ voltage gated channels at terminal
Subthreshold potential generated
What happens to endocytosed vesicles?
Coated with clathrin -> incorporated into cisternae of the Golgi apparatus -> new vesicles bud off from this
What recycles synaptic vesciles?
HRP
What causes transmitter release?
Entry of Ca2+ through presynaptic voltage-gated calcium channels
What are MEPPs caused by?
Release of one vesicle
What is found at each receptor?
Pentamer of subunits enclosing a transmembrane pore that is permeable to Na+ and K+ ions
What initiates an inward current?
2 ACh molecules binding to alpha subunits of one ACh receptor
How is the Endplate potential produced?
Release of ACh in response to a nerve impulse produces a large Endplate Current (EC) in muscle fiber which causes it to depolarize
What does the EPP have to be larger than to activate sodium channels?
20mV
How is synaptic transmission depressed?
Reducing extracellular Ca2+ ion concentration or increasing extracellular Mg2+
How can an EPP be measured?
Measurements of the fluctuation of EPPs and sizes of MEPPS allow the ‘quantal content’
How can a measure of the mean quantal content (m) be produced?
Dividing the mean EPP amplitude by the mean MEPP amplitude
How many quantal units are required to reach the threshold for an action potential?
12
What is the safety factor?
Excess of release over what is required to fire an action potential, very high at NMJ
What happens with repetitive stimulation?
Amplitude and quantal content of EPPs is reduced
What happens if action potentials aren’t present?
Synaptic potentials would decay exponentially with distance from their origin, currents would leak out of the cell
