Lecture 11: Neuromuscular Junction Flashcards
An action potential arrives at the nerve terminal and causes
- Synaptic membrane to depolarize
The depolarization of the synaptic membrane triggers
- Voltage-gated calcium channels (N-type) to open there
- Allows entry of calcium ions
- Raises their concentration within
- Leading to movement, fusion and release of the content of transmitter vesicles in the nerve terminal
The transmitter, acetylcholine (ACh), takes
- 10μs to diffuse across the 20 nm gap (synaptic cleft) from the pre-synaptic release site to the post-synaptic membrane on the myocyte (the end plate)
ACh binds to and opens
- Nicotinic ACh receptors (nAChR) on the tops of folds in the end plate membrane
- AChRs form channels called ligand-gated channels (they are triggered by binding of ACh not by voltage change)
The movement of ions depolarizes the end plate membrane, which is detected by
- Additional voltage-gated Na+ channels in the bottoms of folds of the end plate membrane
The Na+ channels open, thereby further increasing
- The amount of depolarization of the post-synaptic membrane
The current flowing at the end plate is sufficient to cause
- The spread of the depolarization (called the end plate potential [EPP]) to the surrounding membrane outside the end-plate region
- End plate membrane has a very high threshold to AP production and does not itself fire APs
Activation of voltage-gated Na+ channels in the extra-junctional membrane, which has lower threshold to AP production, leads to
- Triggering of an action potential that is propagated away from the NMJ and depolarizes the entire muscle fiber
Muscle action potential production occurs at
- The extra-junctional membrane
Synthesis of ACh via Choline-o-acetyltransferase (ChAT)
- Choline not synthesized by motor neuron while acetyl CoA is
Destruction of ACh is achieved by
- Acetylcholinesterase (AChase)
Acetylcholinesterase (AChase)
- Attached to basal lamina (fine connective tissue filling synaptic space)
- Breaks ACh into acetate choline
- Diffusion away (very little)
- Some ACh is taken back up in about 30-40 msec (very little)
Choline is actively reabsorbed within
- 5-10 msec
- Na-dependant process
Characteristics of end plate potentials (EPPs)
- Graded potential (assive, non-regenerative, electrotonic, decremental)
- Magnitude and duration normally large enough to evoke muscle action potential (AP)
- Always excitatory
Between _____ are released per action potential in nerve terminal
- 100 - 300 vesicles (several quanta)
Threshold is always reached with EPPs because
- ACh released is 3+ times greater than that needed
Nature of the ACh receptor
- About 107 - 108 AChRs per end plate
- Density 20,000/um2 (may be maximal of 50,000/um2)
- Channel is closed until an ACh molecule attaches to binding site on each alpha (a) subunits
Five subunits of ACh receptor
- Two alphas
- One each of beta, delta, epsilon
- Together these create a non-specific cation channel
Binding of ACh molecule to alpha site causes
- A conformational change, resulting in the opening of channel
- Positive ions flow through channel
- Negative ions do not due to high density of negative charges at its mouth
Denervation effects of ACh receptor
- Spread of AChR over entire sarcolemma
Denervation supersensitivity
- Replacement of e subunit with g subunit leads to increased sensitivity to ACh
- Channel opening time becomes 10X shorter
- Only 1 ACh molecule is needed to open channel
Examples of ACh mimetics
- Methacholine
- Carbachol
- Nicotine
ACh mimetics (Methacholine, Carbachol, Nicotine)
- Not destroyed by cholinesterase
- Dissipated slowly
- Leads to prolonged EPPS
- Can causing repeated APs and thus spasms
All ACh mimetics prolong
- Na channel opening times
ACh inhibitors
- Curare
- Alpha-bungarotoxin
Curare
- d-tubocurarine the most active ingredient
- Binds competitively to the AChR
- Effect lasts 30’ to 8 hrs, based on dosage
- Ineffective if introduced orally
Antidote to curare
- AChEs
- They prolong the half-life of ACh
- Allow these molecules to compete out the curare
Alpha-bungarotoxin (snake venom)
- Neurotoxin that binds ACh receptor almost irreversibly
- Other snake alpha toxins also paralyze prey by binding to ACh receptor
Anti-cholinesterses
- Stigmine drugs
- Edrophonium
- Nerve gas (diisopropyl fluorophosphate)
Stigmine drugs
- Physostigmine
- Neostigmine (a derivative)
- Inhibit the cholinesterase and therefore prolong the action of ACh
Edrophonium
- Synthetic anti-cholinesterase acts similarly
Nerve gas (diisopropyl fluorophosphate)
- Inhibits AChE for weeks
ACh release inhibitors
- Botulinus toxin
- Tetanus toxin
- Pb and Cd
Botulinus toxin
- From C. botulinum
- Reduces amount of AChreleased by disrupting SNARE docking proteins involved in neurotransmitter release
- Leads to flaccid paralysis
Tetanus toxin
- From C. tetani
- Enters a NMJ and travels retrogradely along a-motor neurons to spinal cord and enters presynaptic boutons of inhibitory neurons to disrupt SNARE docking proteins involved in neurotransmitter release
- Leads to spastic paralysis, e.g. risus sardonicus of facial muscles
Pb and Cd
- Inhibit transmitter release by diminishing Ca entry in presynaptic terminal
Reuptake inhibitors
- Hemicholinium
Hemicholinium
- Blocks choline reuptake into terminal
- Reduces amount in quanta (later stages)
Myasthenia Gravis
- Autoimmune
- Progressive disease
Transmission: sequence of events
- Depolarization of axon terminal membrane
- Opening of Cav channels in terminal membrane
- Entry of Ca into axon terminal
- Fusion of synaptic vesicles with membrane
- Release of ACh into synaptic cleft
- Diffusion of ACh across synaptic cleft
- Binding of ACh to AChR in postsynaptic membrane
- Generation of postsynaptic potential
Calcium entry into the axon terminal is
- An absolute requirement
Neurotransmitter is released in discrete amounts ot
- Quanta (packets)
- Size of the quanta may vary from system to system
All spontaneous post-synaptic potentials are _____ of these quanta
- Integer multiples
- i.e. 2x, 3x or 4x as large
- ever fractionally larger, i.e. 1.5x, 2.6682x
ACh receptor protein
- Pentameric protein
- Muscle variety permeable to all cations but only Na+ ions pass
Mimetics effects on EPPs
- Prolong synaptic activity
- Nicotine, succinylcholine, methacholine
- Longer lasting EPPs
- Greater probability of EPPs reaching extra-junctional membrane and firing of action potentials
Inhibitors effects on EPPs
- Reduce/abolish activity
- Curariform drugs (tubocurarine)
- Used in surgery to cause flaccid paralysis
Enhancers effects on EPPs
- Anti-cholinesterases
- Drugs that inhibit the breakdown of ACh
- Stigmine-derived drugs (eserine)
Pathologies of EPP
- ACh release inhibitors (botulinus, tetanus, Pb2+)
- Myasthenia gravis
- Channelopathies
EPP Channelopathies
- Lambert-Eaton syndrome (Cav antibodies)
- Slow channel syndrome (prolonged AChR opening)
