Synaptic Transmission

Question 1

Describe the process of Synaptic Transmission

Answer 1

In the axon terminal, we have these vesicles which are spheres that stores or hold neurotransmitters
Exterior is made up of a phospholipid bilayer
Neurotransmitters are waiting for excitatory input that will cause them to move towards the end of the axon terminal and spill their contents into the synaptic cleft
If an axon potential has propagated down the axon, the spreading depolarization will reach the axon terminal and open the voltage gated calcium channels (extracellularly abundant)
Massive influx of calcium into the voltage gated calcium channels into the axon terminal
In response to the influx of calcium, causes the vesicles to approach the end of the axon terminal and bind to the phospholipid bilayer releasing their contents into the synaptic cleft (exocytosis)
Now there is a standard concentration gradient
Neurotransmitter will diffuse down its concentration gradient towards the dendrites of the next neuron in the chain (ligand gated sodium channels postsynaptic terminal)
This allows sodium to flow into the cell, bringing its positive charge with it, and depolarizing the next neuron in the chain (excitatory input)
Makes it more likely the next neuron will fire an action potential

Question 2

Ionotropic Receptors

Answer 2

Synonymous with ligand gated ion channels

Results in very rapid changes in the membrane potential of the receiving neuron

Question 3

Metabotropic Receptors

Answer 3

Metabotropic receptors are membrane proteins, but are not ion channels

When a ligand binds to a metabotropic receptor, it activates a G-protein on the intracellular side of the membrane

Once activated, G-protein can switch enzymes in the neuron “on” or “off”

These changes to cell can impact how the neuron responds to neurotransmitter release in the future (Change the overall excitability of a neuron)

*Not as rapid of a change, but longer lasting change in a neuron
Metabolic changes
- Overarching changes within the neuron (signal cascade)

Question 4

What are the three things that leads to the end of synaptic transmission?

Answer 4

1. Diffusion Neurotransmitters move down their concentration gradients and diffuse away from areas of high concentration

This will cause a lower concentration of neurotransmitters in between the postsynaptic terminal and the presynaptic terminal and there won’t be enough activity to continue synaptic transmission

2. Enzymatic breakdown
   After a neurotransmitter has been released in the synaptic cleft, surrounding glia release enzymes that break down the neurotransmitter more quickly than diffusion
3. Reuptake
   Occurs within the sending neuron itself in which it takes back neurotransmitter that has been released to the synaptic cleft through a process similar to the sodium potassium pump

Question 5

What causes a positive/negative postsynaptic potential

Answer 5

Either inhibitory (IPSP) or excitatory (EPSP) depending on the charge of the ions entering/exiting

Sodium would be excitatory

Potassium efflux would be inhibitory

Chloride would also be inhibitory

Question 6

What is neural integration

Answer 6

How a neuron brings together information from multiple sources at the same time

Question 7

What does the strength of an EPSP depend on?

Answer 7

how many vesicles were released by the presynaptic neuron

At many CNS synapses, an action potential causes the release of only a single vesicle

In this case, the EPSP caused by this presynaptic neuron is tiny

In order for the postsynaptic neuron to fire an action potential, then, it must receive excitatory input from many presynaptic neurons (spatial summation) OR from multiple APs arriving in quick succession (temporal summation)

Question 8

What is spatial summation

Answer 8

The inputs of multiple neurons synapsing on a postsynaptic neuron in close physical proximity sum together

*receiving multiple inputs across space

If the sum of the inputs brings the axon hillock to threshold = action potential
If not = no action potential

Question 9

What is temporal summation?

Answer 9

Multiple EPSPs generated at a single synapse in rapid succession (within 1-15 ms of one another) sum together

If the sum of these rapid inputs brings the axon hillock to threshold = action potential
If not = not action potential