Nervous System 3 - Synaptic Transmission Flashcards
What are the sequence of events involved in transmission at a typical chemical synapse?
- An action potential invades the presynaptic terminal
- Depolarization of the terminal causes opening of voltage gated Ca2+ channels
- Ca2+ enters through channels
- Ca2+ causes vesicles to fuse with the presynaptic membrane
- Transmitter is released into synaptic cleft via exocytosis
- Transmitter binds to receptor molecules in the postsynaptic membrane
- Transmitter binds to receptor molecules in the postsynaptic membrane
- Opening or closing of postsynaptic channels
- Postsynaptic current causes excitatory or inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell
- Removal of neurotransmitter by glial uptake or enzymatic degradation
- Retrieval of vesicula membrane from plasma membrane
What happens when an Action Potential invades the Presynaptic terminal?
It causes openings of voltage gated Ca2+ channels
What does the opening of Voltage gated Ca2+ channels allow for?
The movement of Calcium inside of the cell
What does the influx of Calcium trigger?
It triggers vesicles containing transmitters to fuse with the presynaptic membrane
What happens after vesicles fuse with the presynaptic membran?
The transmitter is released in the cleft via exocytosis
What happens after transmitters are released from the vesicles?
The transmitters bind to receptor molecules in the postsynaptic membrane
What happens after transmitters bind to to receptor molecules of the postsynaptic membrane?
It causes opening or closing of the postsynaptic channels which cause excitatory or inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell
How are neurotransmitter removed?
By glial uptake or enzymatic degradation
What is the main excitatory transmitter?
Glutamate
What does Glutamate cause?
Sodium channels to open allowing sodium to enter the cell
What is the main inhibitory neurotransmitter?
GABA
What does GABA cause?
Cl- channels allowing Cl- to enter the cell causing hyperpolarization
What happens to the EPSP when Calcium channels are blocked in the presynaptic neuron?
No EPSP occurs
What does Calcium Chelator do?
It binds to calcium inside a nerve terminal preventing calcium from triggering transmitter release and therefore blocking postsynaptic potentials
What is an Endplate?
The synapse between nerve and muscle (neuromuscular junction)
What is an End-Plate potential?
An EPSP at the neuromuscular junction
What does EPSP stand for?
Excitatory postsynaptic potential
How does distance affect EPSPs?
EPSP decay in size over long distances
What does MEPP stand for?
Miniature endplate potential
What does a MEPP result from?
The release of a very small amount of neurotransmitter contained in a single vesicle
What causes a single vesicle to be released?
Sometimes due to an action potential and sometimes released spontaneously
How do we know that transmitters are released in quantal packets?
Because each packet results in the same amount of depolarization which is a MEPP
How much depolarization will the smallest Quantum produce?
0.4mV of depolarization
What is a Synaptic delay?
The time between onset of presynaptic spike and onset of EPSP
What is the neurotransmitter at the neuromuscular junction?
Acetylcholine (ACh)
What does ACh cause?
It opens channels that conduct both Na+ and K+ ions
What does AChE break down?
ACh for recycling
What happens once ACh binds to receptors?
Receptors at the NMJ open to allow sodium to enter and potassium to leave
What is an example of Transmitter and Voltage gated channels working together?
In the NMJ ACh opens channels that allow sodium to enter and K+ to leave and then voltage gated Na+ channels allow sodium to enter both working to bring the postsynaptic cell to threshold
What occurs in Myasthenia gravis?
The loss of voltage gated Ca2+ channels in presynaptic terminals of motor neurons which results in reduced neurotransmitter release at NMJ and muscle weakness
What occurs in Botulism?
Food poisoning from clostridium bacteria which impairs transmitter release at NMJ causing paralysis of muscle
What is the Reversal Potential?
The membrane potential at which there is zero net current through a transmitter gated channel because ions are flowing in and out
Which channel conducts sodium and potassium?
ACh receptor channels
What does the Reversal Potential depend on?
It depends on the relative conductances of the synaptic channel to ions and also on the equilibrium potentials for these ions
What is the rule surrounding membrane potentials and reversal potentials?
Vm will always move towards the reversal potential of the synaptic channel when its conductance increases (when the channel opens)
What is the difference between Na and K transmission in between ACtion Potential and EPSPs at NMJs?
In Action potentials have separate Na and K channels
In EPSPs acetylcholine channels conduct both Na and K
What is the Erev in EPSP?
0 mV
What are EPSPs at NMJs triggered by?
Acetylcholine
What are action potentials initiated by?
Depolarization
What are channels controlled by in EPSPs?
Transmitters; they are not sensitive to voltage
What are EPSPs blocked by?
Curare
