Synaptic Transmission Flashcards
What are the two main types of synaptic transmissions?
Direct Transmission can occur at either electrical synapses or at chemical synapses with inotropic receptors. Indirect transmission occurs at chemical synapses with metabotropic receptors.
Direct Transmission Electric Synapses
Looking at Direct Transmission, are electrical synapses more or less common in the vertebrate brain?
Direct Transmission Electric Synapses
Electrical synapses are much less common in the vertebrate brain than are chemical synapses.
Direct Transmission Electric Synapses
Anatomically, Electrical synapses ______________ seen by electron microscopy. (add image)
Lack of synaptic vesicles
Direct Transmission Electric Synapses
How do electrical synapses function?
The presynaptic and postsynaptic cells are connected to each by the gap junction.
Direct Transmission Electric Synapses
What is the connection of electrical synapse mediated by?
Connexons
Direct Transmission Electric Synapses
How many subunits are in the connexons? Also, what is the function?
There are 6 subunits, which are called connexins, the surround the pore of the gap junction. Connexons in the presynaptic membrane line up with the connexons in the postsynaptic membrane –> forming the pore between the two cells. This allow the ions to flow across both membranes.
Direct Transmission Electric Synapses
True or False: Electrical synapses have a synaptic delay.
False; Due to the direct connection between cells, the electrical transmission is very fast.
Direct Transmission Electric Synapses
True or False: Chemical transmission generally has a synaptic delay of a few milliseconds.
True; due primarily to the amount of time it takes for vesicles to fuse and release NTs.
Direct Transmission Electric Synapses
In the chick ciliary ganglion cell experiment, when the postsynaptic cells are hyperpolarized, what does the electrical synapse do to the cell membrane during depolarization?
The electrical synapse no longer drives the cell to an action potential, instead if create a subthreshold. The chemical synapse will occur at the subthreshold.
Direct Transmission Electric Synapses
Again looking at the chick ciliary ganglion cell experiment, if the postsynaptic cell is further hyperpolarized, will an action potential occur?
No, neither the chemical or electrical synapse drives the cell to the threshold. However, a fast, short PSP occurs –> electrical synapse then a delayed, longer-lasting chemical postsynaptic potential occurs. This experiment illustrates that the electrical synapse do not have a synaptic delay.
Direct Transmission Electric Synapses
Choose the correct terminology. Electrical synapses are (more/less reliable) than chemical synapses.
More reliable
Direct Transmission Electric Synapses
Why do chemical synapses fail on being reliable?
Due to synaptic depression or due to the presence of neurotoxins.
Direct Transmission Electric Synapses
Where can you find electrical synapses? In what pathways?
Neuronal pathways underlying escape reflexes
direct synaptic chemical synapses
What is the best example to study direct synaptic excitation?
Neuromuscular junctions
direct synaptic chemical synapses
When the muscle membrane potential was clamped at -40mv, what did the nerve produced?
An inward current of Na ions due to the Na driving force. There is a small efflux of K+ ions but for right now, not that noticeable.
direct synaptic chemical synapses
What happened to the nerve stimulation when the membrane potential was more negative?
The endplate current increases in amplitude.
direct synaptic chemical synapses
When the membrane was depolarized what happen to the endplate current?
The endplate current decreased in amplitude, and even reversed direction?
direct synaptic chemical synapses
What is a reversal potential?
In general, the currents reversed from inward to outward after there is no net flow. An example, When the voltage clamp at -75mv, the nACh will let a large influx of Na+ ions, the inward current, and small reflux of K+ ions, the outward current. When the voltage clamp was at 10mv there is a large influx of K+ ions, there is small reflux of Na+ ions.
direct synaptic chemical synapses
Look at this IV curve, we can see that it is linear and crosses on the x-axis at 0mv. What does that indicate? What type of channels is at the end motor plate? What type of ions flows through these channels?
The linear IV curve indicates that the channels in the endplate are not voltage-gated channels. The x-axis interception showcase is not exclusive to Na+, K+, Ca2+, or Cl-.
direct synaptic chemical synapses
How do you calculate the reversal potential?
the GHK equation
direct synaptic chemical synapses
How do you determine which ions flow through the nAChR?
(a) Radioactive isotopes uptake in the postsynaptic membrane. (b) Changing the concertation of ions in the bath to see if it will change the reversal potential.
direct synaptic chemical synapses
The endplate channels are _______________ and allow ___, _____, and _____ to flow through.
Selective for cations Na+ Ca2+ K+
direct synaptic chemical synapses
True or False: The Ca2+ conductance is very large and is important for the inward and outward currents.
False
direct synaptic chemical synapses
Which direction will Na+ flow through the channel?
Inward
direct synaptic chemical synapses
Which direction will K+ flow?
Outward
direct synaptic chemical synapses
The conductance of the nAChR to Na+ is slightly (greater/less) than its conductance to K+.
Greater
direct synaptic chemical synapses
What would happen to the PSP in the muscle cell if the permeability to Na+ equaled the permeability to K+?
Smaller EPSP
direct synaptic chemical synapses
What factors influence the size of a PSP in chemical synapses?
(a) the reversal potential of the channel which will generally not change given a receptor at a given synapse is constant
(b) the number of channels opened can be increased by increasing the amount of transmitter released
(c) resting conductance through leak channels leak conductance counteracts the effect of the NT receptor channel opening
direct synaptic chemical synapses
What effect will this closing of leak channels have on the EPSP?
Increasing the magnitude of EPSP.
Chemical synapses-direct synaptic inhibition
Let’s look at Chemical synapses-direct synaptic inhibition Direct synaptic excitation is opening channels whose reversal potential is (positive/negative) to the threshold. Direct synaptic inhibition is opening channels whose reversal potential is (positive/negative) to threshold.
Positive; Negative
Chemical synapses-direct synaptic inhibition
For direct synaptic inhibition, what receptor channels generally open?
Chloride channels or less frequently potassium.
Chemical synapses-direct synaptic inhibition
What is an example of an inhibitory transmitter receptor permeable to Cl-?
GABA a receptor
Presynaptic inhibition
If GABA receptors are found on a cell that does not regulate its chloride concentration, will there be any current when these channels are open?
No current, because ECl equals Vm.
Presynaptic inhibition
Will the effects of GABA a receptor still be inhibitory?
Yes
Presynaptic inhibition
What is presynaptic inhibition?
When a synapse onto a presynaptic terminal causes a decrease in the amount of transmitter released. In this case, there is no direct effect of inhibition on the PS cell.
Presynaptic inhibition
Looking at Figure 9.16 on pg.173 In graph A, how did an EPSP occur?
Stimulating the excitatory neuron
Presynaptic inhibition
In graph B, how did it produced an IPSP? Why is the IPSP close to 0 mV?
Stimulating the inhibitory neuron. The IPSP is close to zero amplitude because the reversal potential is very close to the membrane potential.
Presynaptic inhibition
In Graph C, why the EPSP occur when the excitatory and inhibitory neuron is stimulated?
The excitatory neurons are stimulated first and then the inhibitory neuron is stimulated.
Presynaptic inhibition
In Graph D, why is the EPSP nearly abolished?
The inhibitory neuron fire first, this is because the inhibitory synapse onto the terminal to open the Cl- channels allow negative ions to enter the terminal of the excitatory neuron. Eliminating the effect of the inward Na+ current produced by the action potential, therefore few or no V.G. Ca2+ open, so little or no transmitter is released.
Presynaptic inhibition
____________also exists, excitatory transmitter released onto the presynaptic terminal depolarizes the terminal increasing the amount of transmitter released for each action potential.
Presynaptic excitation
Indirect Synaptic transmissions
What are indirect synaptic transmissions?
They are chemical synapses with metabotropic receptors.
Indirect Synaptic transmissions
What are metabotropic receptors?
They are not ion the channel as ionotropic. Instead they active G proteins which activate second messengers pathways which eventually act to open or close ion channels.
Indirect Synaptic transmissions
How many transmembrane regions does a G protein-coupled metabotropic receptor have?
Seven with an extracellular amino terminus and an intracellular carboxy-terminus.
Indirect Synaptic transmissions
What happens when an NT binds to a G-protein coupled metabotropic receptor?
Indirect Synaptic transmissions
GDP is exchanged for GTP on the alpha subunit of the G protein. This associates the alpha subunits from the beta and gamma complex activating both halves and allowing them to interact.
Indirect Synaptic transmissions
Activated G protein can directly affect _________. An example is the mAChR in the heart that activates ____ channels –> slowing the heart rate.
Also can work as a ______________. An example is a beta-adrenergic receptor on the heart, increasing the heart rate. Activated G protein activates __________ converting ATP to cAMP. Incearsing the probability of _____ channel opening.
It can directly affect ion channels. An example is the mAChR in the heart, that activates the K+ channels, and slowing the heart rate.
It also can work as a second messenger cascade. An example is a beta-adrenergic receptor on the heart, increasing the heart rate. Activated G protein activates adenyl cyclase converting ATP to cyclic AMP. Increasing the probability of CA2+ channels opening.
It can activate other signaling pathways.
Indirect Synaptic transmissions
What are the effects of metabolic receptor in learning and memory?
The effects are slower but longer lasting than the effects of inotropic receptors. The second messengers can activate transcription factors, which can alter gene expression, causing in some cases life-long changes in synapses.
