Synapse Physiology Model

You can download the program from

http://mphywww.tamu.edu/davis/models/synapse.html.

Modeling of Synaptic Integration

Written by M.J. Davis, Texas A&M University College of Medicine;
modified by D. G. Ward, Modesto Junior College

Objective:

To model with simulation or with diagrams the major features of synapses and the processing of nervous signals, including temporal and spatial summation, excitatory and inhibitory neurotransmission and presynaptic inhibition, at the level of 85% proficiency for each student.

In order to achieve this objective, you will need to be able to:

Compare and contrast Temporal Summation and Spatial Summation.
Explain the post-synaptic potentials seen with a combination of temporal and spatial summation.
Explain the post-synaptic potentials seen with the opening of various ion channels that reflect excitatory and inhibitory neurotransmission.
Explain pre-synaptic inhibition.

Materials and Methods:

The simulation program is SYN3P-R2.EXE for Windows based computers.

To load the program, Double-click on

After it loads, the program runs once to redraw the two displays. Run the program either by clicking on the "Run" button (à) in the upper left-hand corner of the program window, or by selecting "Run" from the "Operate" menu at the top of the program page. The program can be stopped at any time by clicking on the stop button (stop-sign symbol) in the upper left-hand corner of the program window. If necessary, the model parameters can be reset to their initial conditions at any time by selecting "Reinitialize All to Default" from the "Operate" menu at the top of the screen.

For Mac, selecting "About" from the "Help" menu will pause the model and bring up a series of informational screens about the model and the experimental set-up that would be used to make actual recordings in live neurons.

Orientation and Brief Description of the Model:

Load and start the program as described above. While the program is running, but before beginning the laboratory exercise, look at the computer screen and note that there are three general areas displayed on the screen. These areas include:

A. Animated Display of Neuronal firing (top left side of screen):

On the left side of the screen is shown a pictorial diagram of the anatomical connections between two pre-synaptic neurons (cells A and B) and one post-synaptic neuron (cell C). A diagram of a recording electrode placed near the axon hillock of cell C indicates the site of membrane potential (E_m) measurements.

The default settings for the model are such that when the program is first initiated, one pre-synaptic cell (cell A) will be stimulated with one pulse at the lowest stimulation (firing) rate. Cells A and B each synapse on the cell body of C. At any time the user may pull down the "Operate" menu at the top of the program page and select "Reinitialize All to Default" to reset the program to the initial conditions.

B. Controls (lower left side of screen):

Below the animation are located various controls to alter the stimulus parameters, the anatomical arrangement between the cells, and the membrane properties of the post-synaptic cell. Each pre-synaptic stimulus will produce an action potential in the respective pre-synaptic cell, which will travel down to that cell's terminal bouton and release transmitter

The controls are organized such that there are four for each pre-synaptic cell, two for the post-synaptic cell (C), and two general controls. The "Status" control for each pre-synaptic cell refers to whether that cell is being stimulated (by an external stimulator which is not shown) to fire an action potential. "Firing Rate" determines the interpulse interval if multiple stimuli are delivered to the pre-synaptic cell (1 = lowest rate, 10 = highest rate). "Pulse Delay" controls the timing between the stimulation of the two pre-synaptic cells. "Time Constant" and "Length Constant" control these properties for the membrane of cell C. "# Pulses" determines the number of stimuli delivered to the pre-synaptic cell(s) and can be used to fire a "train" of action potentials. "Wiring" controls the anatomical arrangement between the cells, i.e. whether cell A synapses on cell C or on cell B. It is only used to demonstrate pre-synaptic inhibition.

C. E_m vs Time Graph for Cell C (right side of screen):

The display on the right shows the change in membrane potential (E_m) of cell C as a function of time. This display provides quantitative information about the impact of pre-synaptic stimulation on the post-synaptic cell C. For this reason, it is more important than the animation display, but many students are less confused if they can see that the animation is synchronized with the change in the membrane potential (E_m). The timing of the pre-synaptic stimulus pulses is marked on the membrane potential (E_m) vs time graph by gray pulses just below the membrane potential (E_m) trace.

D. Optional controls.

Increasing the ‘Display Time’ allows the drawing to be slowed down for use on faster computers. Turning the “Remember Previous Trace” switch ‘On’ preserves the previous graph, allowing the traces associated with two sets of parameters to be easily compared.

Temporal summation

This exercise demonstrates how stimulation of a single pre-synaptic cell generates EPSPs that summate over time.

A) RESET all variables and set cell A = fire. Click the "Run" button.
Note the shape of the EPSP (a rapid rise in membrane potential (E_m) with exponential decay back to the resting potential). What accounts for this shape?

B) Set "# Pulses" = 4 and run the model.
Note that a train of four action potentials (APs) travelling down cell A produces four individual EPSPs on cell C without any summation.

C) Now sequentially increase the firing rate of A from 1 to 9 and RUN the model after each increase.

Temporal summation should be observed; what is the maximum amplitude of the summed EPSP?

At the highest firing rate (= 9), how many pulses are required to reach the threshold (-60 mV) for firing an AP on cell C?

(progressively increase the “# pulses” and RUN each time to find out)

What should happen to the membrane potential (E_m) tracing if the threshold is reached?

Spatial summation

This exercise demonstrates how stimulation of multiple presynaptic inputs can generate EPSPs that sum over distance.

A) RESET all variables and set cell A = fire. Click the "Run" button.

B) Set A and B = fire. Click the “Run” button.
Compare the amplitude of the EPSP on cell C when A fires 1 pulse vs when A and B both fire one pulse in synchrony.

C) Increase the delay of cell B to ≈ 1.4 ms to more clearly observe that summation occurs,

At what value of delay of cell B (B_delay) does no summation occur?

What is the maximum EPSP amplitude that can be achieved in this model with spatial summation alone (i.e. without increasing the # of pulses)?

Combinations of Spatial and Temporal Summation

A) RESET, set A=fire, Pulses = 5, A_rate= 8. Click the "Run" button.

What is the maximum EPSP amplitude?
What type of summation is involved?

B) Now add excitatory input from cell B (B = fire, B_rate= 5).

What is the maximum EPSP amplitude now?

C) Progressively increase B_rate to determine at what minimal rate an AP fires on cell C:

D) Keep these settings and then increase the delay of cell B (B_delay) to 1.2 ms (this should change the degree of summation slightly but enough to prevent an AP from firing).

What happens?

E) For another example of the interaction between temporal and spatial summation, RESET, set A=fire, B=silent, A_rate=9, Pulses=5, delay = 1.6 then RUN.
Note that threshold is not quite reached, but if cell B fires even once (set B=fire to see this), an AP fires on cell C.

Effects of Changing Post-synaptic Conductance (ion channel type)

A) RESET, set A=fire. Measure the amplitude of the EPSP; and the default conductance setting:

This could simulate the effects of ACh as the transmitter released from cell A, because ACh typically increases the membrane conductance of the postsynaptic cell to both K⁺ and Na⁺ ions.

B) To illustrate how this can cause a depolarization (despite K efflux), compare the size of this EPSP with one resulting from a selective increase in Na conductance. Change A_cond="Na"):

Why is it different?

C) Now set A_cond="Cl" and note the magnitude of the EPSP:

Explain the difference:

D) Compare this with the IPSP amplitude when A_cond = "K+Cl":
or when A_cond="K"

If changes in Cl conductance have "no effect" on the membrane potentioal (E_m) why is the IPSP amplitude smaller when A_cond="K+Cl" than when A_cond="K" alone?

Presynaptic inhibition

A) RESET all variables, then set "Wiring" = A-B and RUN once.

B) Set A = fire, B = silent.

Now what effect does cell A firing have on the membrane potential (E_m) of cell C?

Why?

C) Now set A = silent, B = fire. You should see a "normal" EPSP on cell C.

What happens when cell A fires also (set A = fire)?

What mechanism underlies this phenomenon?

D) Keep the settings the same. What happens if the delay of cell B (B_delay) is progressively increased to 3.0?

Why?