Download Neurobiology 360: Electrical and Chemical Synapses 1a) What is

October 22, 2014
Neurobiology 360: Electrical and Chemical Synapses
1a) What is the difference between a non-rectifying electrical synapse and a rectifying electrical
synapse?
A non-rectifying electrical synapse allows information to flow between two cells in either
direction (presynaptic cell  postsynaptic cell and postsynaptic cell  presynaptic cell).
A rectifying electrical synapse allows information to flow in only one direction; positive current
will flow in one direction while negative current will flow in the other direction.
1b) You are conducting a voltage clamp experiment to determine the properties of a synapse
within the central nervous system. You conduct the experiment as follows: 1) You depolarize the
presynaptic cell and record the voltage in both the pre- and the postsynaptic cell. 2) You
hyperpolarize the presynaptic cell and record from the pre- and postsynaptic cell. 3) You
depolarize the postsynaptic cell and record from the pre- and postsynaptic cell. 4) You
hyperpolarize the postsynaptic cell and record from the pre- and postsynaptic cell. Analyze each
piece of data shown below and determine what kind of synapse this is. How did you draw your
conclusion?
This is a rectifying electrical synapse. When you depolarize the presynaptic cell, we can see that
there is a response in both the pre and post synaptic cell. When we stimulate the postsynaptic
cell, however, we see that there is only a response in the postsynaptic cell and there is no
response in the presynaptic cell. A similar trend can be seen in the hyperpolarizing data. This
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means there must be a gate allowing information to flow in one direction while preventing it
from flowing in the other.
2) Compare and contrast electrical synaptic transmission with chemical synaptic transmission.
Electrical synapses in general connect two cells together via the cytoplasm (i.e. they are directly
connected and can pass information from one cell to the next). They are fast, reliable, usually
bidirectional, have no synaptic delay, and are usually depolarizing.
Chemical synapses pass information from one cell to another through a small gap (synaptic
cleft). Electrical signals must first be converted into a chemical signal in the presynaptic cell,
diffuse across the synaptic cleft, and convert back to an electrical signal in the postsynaptic cell.
They are not as fast, not as reliable, usually unidirectional, they have a 1ms synaptic delay, and
they can be both depolarizing and hyperpolarizing.
3) What is the difference between an EPSP and an IPSP? What determines whether a post
synaptic potential will be either excitatory or inhibitory?
An EPSP (excitatory postsynaptic potential) is a small depolarizing change in membrane
potential that brings the overall cell potential closer to the threshold for generating an action
potential at the axon hillock. An IPSP (inhibitory postsynaptic potential) is a small
hyperpolarizing change in membrane potential that moves the overall cell potential further from
the threshold for generating an action potential at the axon hillock. The type of ion entering the
cell usually determines whether it will be excitatory or inhibitory. An EPSP is typically
generated by a ligand-gated channel in the postsynaptic membrane that is selective for sodium or
sodium + potassium. An IPSP is typically generated by a ligand-gated channel in the
postsynaptic membrane that is selective for chloride.
4) In the following experiment, you stimulate a vertebrate motor neuron and record the voltage
from two locations in the muscle as depicted in the diagram below.
You obtain the following data:
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Why do you see a small early potential (a “hump”) in V1 but not V2?
The small potential is produced by the neurotransmitter binding to a post synaptic receptor which
causes an increase in gNa/gK. If this increase in gNa/gK causes a depolarizing potential large
enough to produce an action potential, the action potential will travel down passively from V1 to
V2. Since the depolarizing PSP is a passive response, it will decrement over time and distance
and will not be seen in V2.