Exercise 7: Simulation of a motorneuron - spatial and temporal summation of synaptic activity

PLEASE PRINT OUT THESE PAGES FIRST AND THEN KEEP THE PRINTED TEXT BESIDE YOU AS A GUIDE WHEN YOU LOAD AND RUN "NEURON". THIS WILL SAVE YOU HAVING TO JUGGLE BETWEEN NEURON AND NETSCAPE WINDOWS ONCE YOU HAVE THE SIMULATION LOADED.



 

To begin working with this chapter you should have downloaded and installed Neuron, as described in Chapter 1.
 

 

Preparing for the simulation: This simulation brings together all of the concepts presented in Exercise 1 through 5. Load the simulation.
This simulation of a complete  nerve cell is offered to demonstrate some features of synaptic integration and impulse generation. For this example we have chosen a stylized motoneuron whose structure is similar to that of the much studied spinal motoneuron of the cat. Both the model and some of the language used here to describe it were developed by John Moore and Michael Hines at Duke and Yale universities. This model motoneuron is rather complete, consisting of a soma, an axon hillock, an initial segment of axon leading to a long myelinated fiber, and a dendritic tree collapsed into three dendrites as shown in this figure copied from the panel in the lower left of your screen.

 

The channel densities in each of these elements have been laboriously chosen to match as many as possible of available experimental observations on the spinal motoneuron of the cat. The motorneuron accepts three excitatory synapses (shown in blue), one on each dendrite. The location of the synapses on each dendrite is determined by a number between 0 and 1 in the top left-hand panel as shown below. When activated, the synapses depolarize the initial segments of the axon and the soma, resulting in action potentials being generated by voltage-dependent channels in the axon and soma. These action potentials travel down the axon, out of the spinal cord and excite muscle to contract.

A value of 0 means that the synapse is placed at the cell soma, a value of 1 places it at the end of the dendrite. When the simulation is loaded, the synapses are spaced at three different, increasing distances along their respective dendrites. In addition to their location, the delay before each synapse is activated can be determined. When the simulation is loaded, the synapses are activated at three different, increasing times; 1, 4 and 7 ms respectively. The conductance of the post synaptic membrane change (the strength) of the synapses can be changed.

Click on the Init & Run button on the lower right of the screen to run the simulation. In the graph of membrane potential vs. time, you should see the effect on the membrane potential, as recorded in the axon, of the activation of the three synapses (see diagram above for voltage electrode position). Neither of the three active synapses triggers an action potential.