Bi 360 Week 4 Discussion Questions: 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 which is equivalent to negative current flowing in the opposite 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, there is a response in both the pre and post synaptic cell. When the postsynaptic cell is depolarized, however, there is a depolarization in the postsynaptic cell but no response in the presynaptic cell.

A similar trend can be seen in the hyperpolarizing data but in the opposite direction. This means there must be a voltage dependent gate allowing positive current 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 connect two cells together via cytoplasmic linkages (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 across a small gap (synaptic cleft). Electrical signals of the action potential must first be converted into a chemical signal (i.e., the neurotransmitter) in the presynaptic cell, diffuse across the synaptic cleft, and convert back to an electrical signal (post synaptic potential) in the postsynaptic cell. Chemical synapses 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. Both EPSPs and IPSPs are the result of the release of transmitter from a pre-synaptic cell. 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 or potassium.

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:

Why do you see a small initial depolarizing potential (a “hump”) in V1 but not V2?

The small potential is the EPSP (also known as the End Plate Potential) produced by the neurotransmitter binding to a post synaptic receptor. This particular EPSP/EPP is unusual because it is the result of an increase in both gNa and gK. This increase in gNa and gK causes a depolarizing EPP/EPSP large enough to produce a regenerative action potential which travels actively along the muscle membrane from V1 to V2. Since the depolarizing PSP is a passive response, it will decrement over distance and will not be seen in V2.