7. Synaptic Integration: Spatial and Temporal Summation
For this example I have chosen a stylized motorneuron whose structure
is similar to that of the much studied spinal motorneuron of the cat.
This model motoneuron is rather complete, with three dendrites, soma,
an axon hillock, an initial segment of axon
leading to a myelinated fiber. The channel densities in each of these elements
have been laboriously chosen to match as many experimental observations
on the spinal motorneuron of the cat. The lengths of the initial myelinated
segments had to be carefully chosen to allow antidromic impulse invasion
of the soma. The synaptic parameters have been collected into a simgle panel
and a slider is used to locate 3 synapses on three different
dendrites. Mike Hines has arrange a special version of the spatial voltage
profiles conconcatinating the voltages in the 3 dendrites in a single
graphical window and showing them in different colors.
Another graph shows the
voltage at several locations as a function of time.
The default parameter settings cause a spike to be generated: it actually
is initiated in the myelinated region of the cell and propagates back
into the initial segment of the axon and thence into the soma. This
back-propagation of the spike also causes passive depolarization
of the dendrites. This in turn reduces the driving voltage (the synaptic
equilibrium potential is taken to be at 0 mV) for subsequent
synaptic inputs. This inhibitory effect can be quite long lasting
because of the time taken for the dendrites to settle back down to
the resting potential.
To see these aspects,
Run this program which is the same as used
in section 6.2.
The importance of the time of activation of synapses at given locations can be shown by reversing the delay times
in the synaptic panel. Similarly the importance of the locations of synapses, for given time delays, can be explored by
changing the synapse locations with the sliders in the synaptic panel.