The olivo-cerebellar system: a key to understanding the functional significance of intrinsic oscillatory brain properties

Abstract

The reflexological view of brain function (Sherrington, 1906) has played a crucial role in defining both the nature of connectivity and the role of the synaptic interactions among neuronal circuits. One implicit assumption of this view, however, has been that CNS function is fundamentally driven by sensory input. This view was questioned as early as the beginning of the last century when a possible role for intrinsic activity in CNS function was proposed by Thomas Graham Brow (Brown, 1911, 1914). However, little progress was made in addressing intrinsic neuronal properties in vertebrates until the discovery of calcium conductances in vertebrate central neurons leading dendritic electroresponsiveness (Llinás and Hess, 1976; Llinás and Sugimori, 1980a,b) and subthreshold neuronal oscillation in mammalian inferior olive (IO) neurons (Llinás and Yarom, 1981a,b). This happened in parallel with a similar set of findings concerning invertebrate neuronal system (Marder and Bucher, 2001). The generalization into a more global view of intrinsic rhythmicity, at forebrain level, occurred initially with the demonstration that the thalamus has similar oscillatory properties (Llinás and Jahnsen, 1982) and the ionic properties responsible for some oscillatory activity were, in fact, similar to those in the IO (Jahnsen and Llinás, 1984; Llinás, 1988). Thus, lending support to the view that not only motricity, but cognitive properties, are organized as coherent oscillatory states (Pare et al., 1992; Singer, 1993; Hardcastle, 1997; Llinás et al., 1998; Varela et al., 2001).

Keywords: IO neurons; PO neuron oscillation; electrophysiology; intrinsic oscillatory; olivo-cerebellar.

Figure 1

Olivocerebellar circuit. (A) Cerebellar cortex. Inferior olive axons (IO, orange) activate Purkinje cells (black) through the climbing fibers and send collaterals to cerebellar nuclear cells (green) that feed back to IO and projection cerebellar nuclear cells (purple) (After Ramon y Cajal, 1911). (B) Intracellular recordings of an-all or-none complex spike elicited by climbing fiber stimulation and a simple spike elicited by mossy fiber activation. (C) Inferior olivary neurons (Ramon y Cajal, 1911). Note the spherical dendritic trees. (D) Electronmicrograph showing gap junction between spines of IO dendrites within IO glomerulus. Modified from Llinás et al. (1974). (E) Diagram of IO glomerulus. The center shows spines from IO dendrites (IODs) coupled by gap junctions. (a) coupling current path between IO dendrites, (b) current flow shunted when gabaergic synapses are active at the gap junction. (Modified from Llinás, 1974).

Figure 2

IO Electrophysiology. (A) In vitro intracellular recordings from IO neuron showing high threshold spike (a) activated by an outward pulse from a depolarized potential with respect to rest (broken line) the same outward pulse delivered from the rest potential (broken line) did not elicit a spike (b). (B) Same pulse as in (A) delivered from a hyperpolarized membrane potential level generated a low voltage activate spike (Modified from Llinás and Yarom, 1981a,b). (C) Subthreshold membrane oscillation recorded intracellularly from an IO neuron and associated Lissajeux image demonstrating oscillatory stability. (Modified from Llinás and Yarom, 1986). rp, resting potential.

Figure 3

Differences between principal olive (PO) and Dorsal cap of Kooy (DCK) neurons. (A) Drawing lf brain slice showing climbing fiber projection (red) of DCK neurons to the contralateral cerebellar flocculus, and afferent GABAergic input (green) to the DCK from the bilateral prepositus hypoglossi nuclei. (B–D) Effect of membrane potential on DCK and PO firing (in same slice). (B) Recordings from DCK neuron showing increased firing frequency with membrane potential. Each action potential is followed by a large, long-lasting afterhyperpolarization. (Action potentials were truncated and recorded with a high-potassium intracellular solution) (C) Patch recordings from a PO neuron showing subthreshold oscillations at the same membrane potentials as in (A) Amplitude increased, but frequency was unchanged. (D) Representative currents in DCK and PO neurons (in response to 100-ms depolarizing square pulses recorded by using a high-potassium electrode solution). Inward currents were followed by a small outward current in PO neurons while the same depolarizing square pulses activated a strong outward current in DCK neurons. (E) I–V curve for cells in panel (D). (F) DCK neurons had a single current peak near a membrane potential of −20 mV that was blocked by ω-Agatoxin-TK (a specific P/Q calcium channel blocker). (G) In the PO cells, two inward components, peaking near −20 and −10 mV were seen. The second component was reduced by application of ω-Agatoxin-TK and further reduced by application of ω-Conotoxin-GVIA (a specific N-type calcium channel blocker). Thus, the inward calcium currents of DCK neurons were mediated only by P/Q-type channels, while both P/Q-type and N-type channels are present in PO neurons. Fl, flocc; Pfl, paraflocculus; PHN, prepositus hypoglossi nuclei. (Modified from Urbano et al., 2006).

Figure 4

Electrophysiological properties of IO in wild-type and mutant mice. (A,B) Hyperpolarizing current injection elicited a low threshold spike from IO cell in slice from wild-type mouse (A), but not from mutant mouse (B) at resting potentials of −54 and −61 mV. Subthreshold rebound mediated by Ih was present in the mutant mouse. (C) Plot showing modulation of subthreshold sinusoidal oscillation (SSTO) amplitude by membrane potential in wild-type (black) but not in mutant (blue) mice. (D) Frequency of SSTO was lower in mutant than in wild-type mice but neither was modulated by membrane potential. (E,F) Superposition of six traces showing SSTO recorded from single IO neuron in wild-type (E) or mutant (F) mouse. Extracellular stimulation lead to phase reset of SSTO in IO cell in slice from the wild-type mouse. Such stimulation had a minor, if any, effect in the mutant mouse (F). (Modified from Choi et al., 2010).

Figure 5

The olivocerebellar loop circuit. (A) Diagram of olivocerebellar circuit. Action potentials in IO neurons (red) are generated at the crest of the subthreshold oscillations; example of subthreshold oscillations is shown in Figure 2C. These elicit complex spikes in Purkinje cells (green) and activate cerebellar nuclear cells (purple and yellow). Purkinje cell output is inhibitory to cerebellar nuclear cells where the IPSPs trigger rebound firing in cerebellar nuclear cells. Arrows indicate direction of action potential conduction. (B,C) Synaptic potentials and firing of cerebellar nuclear cells. White matter stimulation (WM stim) at increasing stimulus strength elicits graded EPSP-IPSP sequences. The first sequence (1) is due to direct stimulation of mossy fiber collaterals (EPSP) and Purkinje cell axons (IPSP). The second sequence is due to activation of the climbing fiber system (2) the Purkinje cell IPSP was strong enough to activate the rebound response (3 and spikes). (C) Average of 10 responses showing the timing of the EPSP-IPSP sequences. (Modified from Llinás and Muhlethaler, 1988).

Figure 6

IO spontaneous and stimulus-evoked oscillations. (A) Intracellular recording of spontaneous oscillations at 2 Hz interrupted by an extracellular stimulus. After extracellular stimulation the oscillations disappeared for 750 ms (boxed area) and then resumed. (B) Left. Superimposition of six individual intracellular traces (each a different color) of stimulus-evoked oscillations recorded from the same cell. Right. Power spectra. The frequency of stimulation-evoked oscillation was the same (2.0 Hz). Oscillations are clear after the stimulus-induced reset but can be barely detected before the stimulation. (C) Superposition of average of six traces of stimulus-evoked oscillations (red) and spontaneous oscillations (black). The stimulus-evoked and spontaneous oscillations have the same frequency. Calibration, 1 mV; (A) 1 s; (B) 415 ms; (C) 500 ms. (Modified from Leznik et al., 2002).

Figure 7

Optical and intracellular recordings in brainstem slice. (A) Recordings of IO subthreshold oscillations are quite similar using optical (red) and using intracellular voltage (black) methods. (Recording site indicated by asterisk in first image panel in B). (B) Spatial profiles of optically recorded oscillations at five times during the oscillation shown in panel (A). (Modified from Leznik and Llinás, 2005).

Figure 8

Olivocerebellar conduction times. (A) Drawing of cerebellum showing conduction times for evoked complex spikes (parasagittal plane 4 mm from midline) in an in vivo preparation. (B) Plot of mean conduction time (±sem) as a function of olivocerebellar fiber length (0.5 mm bins) showing that conduction time was independent of distance between IO and individual Purkinje cell. (C) Plot showing close to linear relationship between mean conduction velocity (±sem) and fiber length. (Modified from Sugihara et al., 1993).