Spatial and Temporal Summation
Brief description and instructions (DRAFT):
This illustration is aimed to assist you in understanding the concept of neural summation. It is rare that any single neuron will actually cause another neuron to fire. Usually two or more are required, and then are are inhibitory connections (IPSPs) to consider.
In this illustration, one neuron has three neurons inputting into it. The two drawn in green have excitatory connections (that is, they generate EPSPs). The red neuron has an inhibitory connection (it generates IPSPs). You can control the strength, duration and when each post-synaptic potential (PSP) starts during the animation. For the excitatory connections, the voltage on the graph will move from the resting potential towards zero the number of mV indicated on the slider. The inhibitory connections work in the opposite way. For each neuron you can cause up to two action potentials to occur, the second has to occur at least 1/10th of the graph period later. Thus, by having action potentials on more than one of the input neurons you can explore spatial summation. By having more than one action potential on the same neuron, you can explore temporal summation. Remember that post-synaptic potentials can last quite long compared to an actionpotential.
When you press the Animate button, sequence you have set up will start. Shortly before the time you indicate for the PSP to start, the neuron, if it generates a PSP of a nonzero magnitude will fire an action potential down its axon. When it reaches the gray neuron, the PSP will be generated and drawn on the graph of the voltage of the axon of the gray output neuron. If the voltage at any point reaches threshold, the neuron will fire, if not, when the PSP will run its course.
Use neurons 1 and 3 (the excitatory connections) and see if you can cause the neuron to fire? Do these PSPs have to arrive at the same time? What if you increase the duration of the PSPs? What is the effect of the inhibitory connection?
Click here to open the applet. It will open a new window that will fill your screen.