Flashcards in Neuromuscular junction Deck (17)
What is the role of skeletal muscle?
Mediates voluntary movement
What is a motor unit?
The neurone and the number of fibres that it innervates
What is bouton and what is its function?
Ending terminal of the multiple fine unmyelinated branches that have branched from a motor neurone and it forms a chemical synapse with the muscular membrane at the neuromuscular junction
What causes the release of acetylcholine at the boutons?
Action potentials arising in cell body are conducted via the axon to the bouton
Where does the cell body of the neurone lie?
Ventral horn of spinal cord (brain stem)
What do junctional folds do?
Hold neurotransmitter at peaks of the folds
Describe the pre-synaptic processes
1)choline is transported into the terminal by the choline transporter (symport with Na+)
2)Ach is synthesised in the cytosol from choline and acetyl coenzyme A
3)ACh is concentrated in vesicles by the vesicular ACh transporter
4)arrival of action potential at the end terminal which causes depolarization and the opening of voltage-activated Ca2+ channels allowing it's entry to the terminal
5)Ca2+ causes vesicles "docked" at active zones to fuse with the presynaptic membrane (exocytosis) -Ach diffuses into the synaptic cleft to activate post-synaptic nicotinic ACh receptors in the endplate region
Describe the post-synpatic processes
1)Nicotinic ACh receptors are assembled and surround a central cation selective pore. The pore contains a gate that is closed in the absence of ACh, but open when 2 molecules of ACh bind to the exterior of the receptor
2) The open channels is roughly equally permeable to Na+ and K+ but does not conduct anions
3)When the gate is open Na+ enters the muscle cell (influx) whilst K+ exits (efflux) simultaneously through many receptors at the end plate
What is miniature endplate potential?
The electrical response to one quantum of transmitter , due to the activation of nicotinic ACh receptors at the endplate and they summate to produce the end-plate potential
What happens when end-plate potential exceeds the threshold?
Triggers an "all or none" propagated action potential that initiates contraction
what causes an end plate potential (e.p.p) ?
The activation of the nicotinic acetylcholine receptors at the endplate and is a graded response
if the e.p.p is large enough, what happens?
Triggers the opening of voltage activated Na+ channels around the end plate causing an action potential which is an "all or none response "
What is one to one coupling?
Normally one action potential in the motor nerve triggers I one action potential in the muscle and subsequently a "twitch"or contraction of muscle
What happens when drugs or toxins reduce the amplitude of e.p.p?
It doesn't reach the threshold for the opening of voltage-activated Na+ channels , block neuromuscular transmission because no muscle action potential is generated
What is a physiological reality?
Muscle fibre with voltage-activated Na+ channels- an action potential propagates from the endplate over the length of the muscle fibre
What causes rapid termination of neuromuscular transmission?
The hydrolysis of ACh by acetylcholinesterase (AChE), an enzyme associated with the end plate membrane