Dynamical Systems in Neuroscience:

Simple Models 307purpose is to collect and low-pass filter the synaptic input.Now, we explore the active properties of dendrites and their dependence on the location,timing, and strength of synaptic input. First, let us stimulate two synapses thatinnervate two sister dendritic compartments, e.g., compartments 6 and 7 in Fig. 8.22dthat could interact via their mother compartment 5. Each synaptic input evokes astrong EPSP of 12 mV, but due to their separation, the EPSPs do not add up andno dendritic spike is fired. The resulting somatic EPSP is only 0.15 mV due to thepassive attenuation. In Fig. 8.22e we provide exactly the same synaptic input, butinto the same compartment, i.e., compartment 6. The EPSPs add up and result in adendritic spike, which propagates into the mother compartment 5 and then into thesister compartment 7 (which was not stimulated), but it fails to propagate along theapical dendrite into the soma. Nevertheless, the somatic compartment exhibits anEPSP of 1.5 mV, hardly seen in the figure. Thus, the location of synaptic stimulation,with all other conditions being equal, made a difference. In Fig. 8.22f we combine thesynaptic stimulation to compartment 6 with injection of a weak current, I all , to allcompartments of the neuron. This current represents a tonic background excitation tothe neuron that is always present in vivo. It depolarizes the membrane potential by 2.5mV and facilitates the propagation of the dendritic spike along the apical dendrite allthe way into the soma. The same effect could be achieved by an appropriately timedexcitatory synaptic input arriving to an intermediate compartment, e.g., compartment3 or 2. Not surprisingly, an appropriately timed inhibitory input to an intermediatecompartment on the apical dendrite could stop the forward-propagating dendritic spikein Fig. 8.22f.In Fig. 8.22g and h we illustrate the opposite phenomenon — back-propagatingspikes from soma to dendrites. A superthreshold stimulation of the somatic compartmentevokes a burst of three spikes, which fails to propagate along the apical dendritesalone, but can propagate if combined with a tonic depolarization of the dendritic tree.We see that dendritic trees can do more than just averaging and low-pass filtering ofdistributed synaptic inputs. Separate parts of the tree can perform independent localsignal processing and even fire dendritic spikes. Depending on the synaptic inputsto other parts of the tree, the spikes can be localized or they can forward-propagateinto the soma, causing the cell to fire. Spikes at the soma can backpropagate into thedendrites, triggering spike-time-dependent processes, such as synaptic plasticity.8.2.4 Chattering (CH) neuronsChattering neurons, also known as fast rhythmic bursting (FRB) neurons, generatehigh-frequency repetitive bursts in response to injected depolarizing currents. Themagnitude of the dc-current determines the interburst period, which could be as longas 100 ms or as short as 15 ms, and the number of spikes within each burst, typically2 to 5, as we illustrate in Fig. 8.23 using in vivo recordings of pyramidal neuron of catvisual cortex.An RS model neuron as shown in Fig. 8.12 can be easily transformed into a CH