Dynamical Systems in Neuroscience:

318 Simple Modelsthey are able to generate high-frequency trains of spikes reaching 800 Hz, like corticalFS interneurons. The simple model in Fig. 8.33 reproduces all these features. Its phaseportrait and bifurcation diagram is similar to the one in Fig. 8.15, but its dynamicshas a much faster time scale.8.4 Other interesting casesThe neocortical and thalamic neurons span an impressive range of dynamic behavior.Many neuronal types found in other brain regions have dynamics quite similar to someof the types discussed above, while many do not.8.4.1 Hippocampal CA1 pyramidal neuronsHippocampal pyramidal neurons and interneurons are similar to those of the neocortex,and hence could be simulated using the simple model presented in Sect. 8.2. Let uselaborate using the pyramidal neurons of CA1 region of hippocampus as an example.Jensen et al. (1994) suggested to classify all CA1 pyramidal neurons according totheir propensity to fire bursts of spikes, often called complex spikes. The majority (morethan 80%) of CA1 pyramidal neurons are non-bursting cells, whereas the remainingexhibit some form of bursts, which are defined in this context as sets of three or moreclosely spaced spikes. There are five different classes:• (NB) Non-bursting cells generate accommodating trains of tonic spikes in responseto depolarizing pulses of dc-current and a single spike in response to abrief superthreshold pulse of current, as in Fig. 8.34A.• (HTB) High-threshold bursters fire bursts only in response to strong long pulsesof current, but fire single spikes in response to weak or brief pulses of current, asin Fig. 8.34B.• (LTB I) Grade I low-threshold bursters fire bursts in response to long pulses, butsingle spikes in response to brief pulses of current, as in Fig. 8.34C.• (LTB II) Grade II low-threshold bursters fire stereotypical bursts also in responseto brief pulses, as in Fig. 8.34D.• (LTB III) Finally, grade III low-threshold bursters fire rhythmic bursts spontaneously,which are depicted in Fig. 8.34E using two time scales.NB neurons are equivalent to neocortical pyramidal neurons of RS type, whereas HTBand LTB I neurons are equivalent to neocortical pyramidal neurons of IB type. Theauthor is not aware of any systematic studies of the ability of IB neurons to fire stereotypicalbursts in response to brief pulses, as in Fig. 8.34Db, or to have intrinsic rhythmicactivity, as in Fig. 8.34E. Therefore, it is not clear whether there are any analogues ofLTB grade II and III neurons in the neocortex.