The pattern of complex spike activity
results from a combination of factors. First, after entering the
cerebellum single climbing fibers branch and connect with up to ten
Purkinje cells within a sagittally oriented microzone, inducing
synchrony in rostro-caudal direction (Armstrong
et al. 1973a; Armstrong et al. 1973c; Armstrong et al. 1973b;
Sugihara et al. 2001);
second, inferior olivary neurons are densely electrotonically coupled
by dendrodendritic gap junctions, further enhancing the potential for
synchrony (Llinas
et al. 1974; Sotelo et al. 1974; De Zeeuw et al. 1995; Lang 2002;
Marshall et al. 2007);
and third, due to a unique set of conductances neurons of the
inferior olive show sub-threshold oscillations (STO's) in their
membrane potential with frequencies ranging from 1 to 3 Hz and/ or 3
to 10Hz (Llinas
& Yarom 1981; Yarom Y. & Llinás 1981; Llinas & Yarom
1986; Khosrovani et al. 2007),
on the beat of which their action potentials may ride (Llinas
et al. 1974; Bloedel & Ebner 1984; Mathy et al. 2009;
Bazzigaluppi et al. 2012; Gruijl et al. 2012).
To what extent the tendency to oscillate and to fire in synchrony
interact is still under debate (Devor
& Yarom 2002a; Devor & Yarom 2002b; Leznik & Llinás
2005; Placantonakis et al. 2006; Welsh 2002).
For example, pharmacological blockage of gap junction
coupling does not abolish olivary oscillations (Leznik
& Llinás 2005; Lang 2002),
but enhancing the oscillatory rhythm with harmaline can enhance the
level of synchrony (Llinas
& Sasaki 1989; Lang et al. 1996; Jacobson et al. 2009).
Intra-olivary coupling delays have not
been studied in great detail and it is not completely clear at the
systems electrophysiological level to what extent gap junction
coupling in the olivocerebellar system can influence the oscillatory
properties and related ensemble pattern formation. So far,
cross-correlation studies of complex spike activity have largely
focused on the zero-time bin peaks in the anesthetized or non-moving
preparation, trying to tackle the mechanisms underlying synchrony
(Zeeuw
et al. 1996; Lang et al. 1996; Wylie et al. 1995)
or restrict themselves to non-moving periods of awake recordings
(Lang
et al. 1999). It it also
known that different phases of movement can recruit different
sub-sets of Purkinje cells, by that creating dynamic patterns of
complex spike synchrony (Welsh
et al. 1995).