Lecture 6- NMJ Flashcards
what is the NMJ?
- a specialist synapse between an alpha motor neuron and a muscle fiber (muscle cell)
what does the ‘motor unit’ consist of?
- alpha motor neuron and it’s axon
- all the skeletal muscle fibers that it innervates
NB - one alpha motor neuron innervates many muscle fibers
What is the function of the NMJ?
- it allows a motor neuron to** transmit action potentials **(AP’s) to a muscle fiber. which causes contraction
what are the main features of a NMJ?
ie structure
- alpha motor neuron - axon terminal
- terminal bouton
- synaptic vesicles (with NT’s)
- motor end plate
What **neurotransmitter **is involved in the NMJ synapse?
- Ach - acetylcholine
what 2 molecules is Ach synthesised from?
- Acetyl CoA
- choline
Describe Ach synthesis and storage.
Synthesis
* Ach is synthesised from choline and acetyl co A
* the enzyme ChAT converts these 2 molecules to acetylcholine
Storage
* Ach is stored in synaptic vesciles in the axon terminal of the motor neuron
When Ach is released from the synaptic vesicles, what happens immediately?
- it is immediately broken down by ACHE
- Choline is actively transported back to the nerve terminal, where it forms the substrate for ChAT
What process best describes how the neurotransmitter is released at the NMJ?
- exocytosis
- form of transport in which a cell transports molecules out of the cell
Describe the **sequence of events at the NMJ **that lead to an action potential in a muscle fiber
- an action potential reaches the axon terminal, causing depolarisation
- depolarisation causes the opening of voltage gated Ca2+ channels
- this allows an influx of Ca2+
- Ca2+ ions bind to** synaptotagmins** on the synaptic vesicles
- this causes the vesicles to** fuse with the membrane** and** Ach is released** from the motor neuron into the synaptic cleft
- Ach binds to nicotinic receptors on the sarcolemma
- as the receptors are ionotropic receptors (ligand gated ion channels), they allow Na2+ and K+ entry into muscle fiber - EPP
- the EPP** exceeds threshold** and all voltage gated Na+ channels open
- AP propagates through muscle fiber
what does** MEPP** mean?
- minature end plate potential
- the smallest amount of stimulation that one neuron can send to another neuron
- blips in the resting membrane potential
what is the MEPP **amplitude **called?
quantal size
What is the safety factor of the MNJ defined as?
- refers to the ability of the NMJ transmission to remain effective under various physiological conditions and stresses
how many synaptic vesicles roughly are released form the presynaptic neuron after the influx of Ca2+? & what does this release cause?
roughly about 50 synaptic vesicles
* this release causes a production of an EPP which is at least 5 times as large a one required to trigger an AP of a muscle fibre
