BIO325 Laboratory Guide #16 (20204

 

SYNAPSES III:

COMPUTER SIMULATIONS OF POSTSYNAPTIC POTENTIALS

 
The writeup for this lab
falls under category
C

 

 

We will be simulating postsynaptic potentials (PSPs) for chemical synapses using two Neurons in Action simulations. The first simulation will allow you to examine the relative time-courses of conductance changes, currents, and voltage excursions constituting excitatory postsynaptic potentials (EPSPs). It will also allow you to determine PSP reversal potentials and examine shunting inhibitory postsynaptic potentials (IPSPs). Finally, it will allow you to explore the response differences between passive and active membranes. The second tutorial in this lab will allow you to explore the way postsynaptic potentials interact or integrate in what are usually termed temporal summation. This simulation highlights the almost universally ignored facts that temporal summation interactions are at best decidedly non-linear, and at worst aggressively counter-intuitive.  

 

I. NEUROMUSCULAR JUNCTION Tutorial

 

Launch NIA2. Select The Neuromuscular Junction tutorial. In the course of the first few exercises, refamiliarize yourself with the window controls, especially:

 

            a) opening and manipulating parameter windows

            b) opening, resizing, and positioning display windows

            c) rescaling display window plots

            d) capturing and printing windows

 

Work through this entire tutorial.

 

Be sure to answer all questions within the tutorial text as you go.  Note – the stimulus and H-H values for the final section are not, in fact, sub-threshold, as claimed in the text.  Don’t spend too much time trying to reconcile this.


Data Sheet Item #1:
Produce a printout which demonstrates the reversal potential for a cholinergic (ACh) excitatory post-synaptic potential (EPP), as described at the bottom of page 59 of the NIA2 manual. Show on your graph how you measured or estimated this reversal potential value.


     

 

II. POSTSYNAPTIC INHIBITION Tutorial

 

Quit the previous tutorial.  Select the Postsynaptic Inhibition tutorial.

 

Work through this entire tutorial.

 

Be sure to answer all questions within the tutorial text as you go. 



Data Sheet Item #2:
Produce a well-labeled printout which demonstrates a “shunting” inhibitory post-synaptic potential, as described at the top of page 65 of the NIA manual).


     Note:  The final two sections "Excite a cell by disinhibiting it" and "Are there changes in excitability following an IPSP?" describe the phenomenon which we have seen earlier and talked about in class as as an "anode break" action potential.  Pay particular attention to this section and make sure that you thoroughly understand it.


Data Sheet Item #3:
Produce a well-labeled printout which demonstrates an action potential  produced by the offset of a prolonged period of hyperpolarization, as described in the last two sections of this tutorial (pp. 65 and 66).


    

 

III. INTERACTIONS OF SYNAPTIC POTENTIALS Tutorial

 

Quit the previous tutorial. Select the Interactions of Synaptic Potentials tutorial.

 

Work through this entire tutorial, paying particular attention to the following sections.  Be sure to answer all questions within the tutorial text as you go.

 

      Summation of EPSPs in a passive postsynaptic membrane


Data Sheet Item #4:
Produce a well-labeled printout of a set of traces which clearly demonstrates that EPSP do NOT sum  linearly.


 

Temporal summation of EPSPs in a postsynaptic membrane with Voltage-gated Na and K channels: Time is of the essence!


Data Sheet Item #5:
Produce a well-labeled printout of a set of traces which clearly demonstrates the peculiar time-dependence of EPSP temporal summation effects in active membrane shown in this section


          

Combining two subthreshold EPSPs

 

What are the effects of an inhibitory synaptic input (IPSP) on membrane             excitability?


Data Sheet Item #6:
Produce a well-labeled printout of a set of traces which clearly demonstrates the peculiar time-dependence of IPSP - EPSP temporal interactions in active membrane shown in this section


         

      IV. A REALLY ANNOYING THOUGHT PROBLEM

 

Consider the following situation (actually fairly common in invertebrates), where Cell A is presynaptic to Cell B via an excitatory chemical synapse.  Cells B and C are directly coupled via an electrical synapse.

 

         

 

Use what you have learned in the past two simulation labs about both electrical and chemical synapses to answer the following questions.  This would be a really good set to discuss with your classmates prior to the next exam.

 

Q1:      Explain how a quietly-resting Cell C might exert a "shunting" inhibitory effect on PSPs in Cell B via this electrical synapse. 

 

Q2:      How would the extent of this inhibitory effect depend on the relative sizes of cells B and C? 

 

Q3:      How would the extent of this inhibitory effect depend on the size (conductance) of the electrical synapse? 

 

You should recognize that these considerations also work for situations where B and C are not distinct cells, but rather are simply two different compartments in the same cell, separated by a relatively high-resistance pathway.  A common example in vertebrate neurons is that of the dendritic “spine” or “gemmule”.  These are small, roughly spherical protuberances on dendrites.  Each spine is separated from the dendritic shaft by a constricted “neck” region, which can have a relatively high internal resistance.  This produces a partial electrical isolation of the dendritic spine.  We will be discussing this in class.

 

Q4:      What if Cell/Compartment B had active membrane (could fire action potentials)? This is actually the case in many dendritic spines.  How might the internal resistance between B and C influence the ability of B to reach threshold and fire an action potential?  How might this, in turn, affect B’s influence on C?

 

You should also realize what a great potential test question this situation provides.    H’mmm.

 

 

V.  PREPARATION OF THE LAB DATA SHEET

 


Your data sheet should include all SIX of the items described in the boxes above.

Make sure that the axes of all of the graphs and print-outs are labeled and calibrated. You should certainly discuss your results and the answers to the questions with your partners and others in the lab. However, please work independently when you prepare your data sheet.

 

The writeup for this lab
 falls under category
 C