Lec 5 & 6 - Synaptic Transmission & NMJ Flashcards
(42 cards)
What is the time between arrival of an action potential into the presynaptic terminal and a evoked change in the postsynaptic membrane?
Synaptic delay
What is a connexon?
Specialized proteins that form channels for ion passage that have no synaptic delay and has a bidirectional transmission.
The ___________ terminal converts the electrical signal (action potential) into a chemical signal and releases a neurotransmitter.
Presynaptic
The __________ membrane converts the chemical signal back into an electrical signal.
Postsynaptic
What are some factors of electrical synapses?
Ex: heart cells
- Electrical contact
- Minimum synaptic delay
- Bidirectional
- Allows for synchronous activation
What are some factors of chemical synapses?
Ex: Neuromuscular junction
- Chemical
- 0.5-1.0 msec delay
- Unidirectional
What is a jeuromuscular junction?
Synapse between a motor neuron and a skeletal muscle fiber. Its presynapting ending is filled with vesicles containing Ach. The postsynaptic membrane has junctional folds that contain ACh receptors and acetylcholinesterase (AChE) that hydrolyzes ACh and stops its action.
What is Lambert Eaton syndrome?
Disruption of Ca2+ channel function by autoantibodies causes muscle weakness.
What are the steps in the presynaptic NMJ events?
- Action potential travels down axon to presynaptic membrane
- Membrane depolarizes and opens voltage dependent Ca2+ channels, allowing Ca2+ to rush in
- Ca2+ influx
- Ca2+ influx promotes binding of synaptic vesicles to inside membrane of presynaptic terminal
- ACh is released via exocytosis
- ACh diffuses across synaptic cleft and combines with ACh receptors on post junctional membrane
What are the steps in postsynaptic NMJ events?
- ACh binds with the nicotinic receptor and causes opening of ligand-gated cation channels (Na/K) in the postsynaptic membrane
- Sodium ions rush in to the cell and causes graded depolarization - producing endplate potential (EPP)
- Muscle membrane surrounding the endplate depolarizes and produces an action potential which propagates away from the endplate in both directions
Acetylcholine is broken down by acetylcholinesterase into what products?
Acetate and Choline (which is taken up by presynaptic terminal for resynthesis of Ach)
How is acetylcholine made in the presynaptic cell?
Choline + Acetyl Co-A
What toxin blocks the action of ACh by binding to the ACh receptor?
Curare (plant alkaloid) - This causes a decrease in EPP. This mimics the autoimmune neuromuscular disease Myasthenia gravis
Acetylcholine is a natural agonist for what two cholinergic receptors?
Nicotinic and Muscarinic
What does Neostigmine do?
Inactivates acetylcholinesterase which causes and increase in EPP
What occurs when someone has myasthenia gravis?
Destruction of ACh receptors which causes a decrease in EPP and can lead to lack of AP being formed. This is due to specific antibodies made by the immune system of the patient.
What are some cardinal features of myasthenia gravis?
Fluctuating fatigue of voluntary muscles, which worsen after exertion and improve with rest. Initially it involves weakness of the extraocular muscles. Ptosis (eyelid drooping) is very common, especially during long periods of driving or reading.
Ptosis and failure to close eyelids indicates paralysis of the orbicularis oculi muscle innervated by CN ___.
CN VII (facial)
T/F - Weakness of the extraocular muscles may resemble paralysis of CN III, IV, and VI.
TRUE
What is used to treat myasthenia gravis and curare?
- AChE blockers
- Stigmines (neostigmine, pyridostigmine, etc.) They prevent breakdown of ACh and improve neuromuscular transmission.
T/F - Myasthenia gravis affects pupillary function.
FALSE - MG does NOT affect pupillary function but CN III palsy does.
Why doesn’t MG affect the muscles of the heart and intestine?
f
Someone with severe fatigability, weakness during mastication (can’t keep jaw closed after chewing), nasally speech (weakness of soft palate), and slurred speech without difficulty in language fluency may have……
myasthenia gravis
__________________________ can cause abnormalities in eye position or movement, visual problems due to optic neuritis (inflammation of the myelin sheath of the optic nerve).
Multiple sclerosis
