Specific relationship between excitatory inputs and climbing fiber receptive fields in deep cerebellar nuclear neurons

Abstract

Many mossy fiber pathways to the neurons of the deep cerebellar nucleus (DCN) originate from the spinal motor circuitry. For cutaneously activated spinal neurons, the receptive field is a tag indicating the specific motor function the spinal neuron has. Similarly, the climbing fiber receptive field of the DCN neuron reflects the specific motor output function of the DCN neuron. To explore the relationship between the motor information the DCN neuron receives and the output it issues, we made patch clamp recordings of DCN cell responses to tactile skin stimulation in the forelimb region of the anterior interposed nucleus in vivo. The excitatory responses were organized according to a general principle, in which the DCN cell responses became stronger the closer the skin site was located to its climbing fiber receptive field. The findings represent a novel functional principle of cerebellar connectivity, with crucial importance for our understanding of the function of the cerebellum in movement coordination.

Figure 1. Responses of DCN cells to manual skin stimulation.

(A) Location of a recorded, stained DCN neuron in the anterior interposed nucleus. The outlines of the nucleus and the electrode track are indicated with white dotted lines. (B) Sample IC recording illustrating an excitatory response. The neuron was slightly hyperpolarized from resting membrane potential. (C) Strain gauge signal indicating the time course of the force applied during the skin stimulation. (D) Averaged intracellular recording after removing spikes in software from raw data (N = 50 stimulations). (E) Spike responses for the same cell as in (B)–(D) but recorded without hyperpolarization. The bin width of the peristimulus histogram (obtained using N = 50 stimulations) is 10 ms.

Figure 2. For comparison, responses evoked by a similar stimulation as in Fig. 1 but applied within the CF receptive field of the afferent PCs to the DCN cell (PC-CFRF, see text).

Similar display as in Fig. 1.

Figure 3. Responses evoked from various skin sites.

(A) Spike response evoked by electrical skin stimulation of the skin site indicated by an ‘X’ in (C). Bin width of the peristimulus histograms in A and B, 5 ms. (B) Spike response evoked by electrical skin stimulation within PC-CFRF (asterisk marks the location in C). (C) Histograms of net spike responses evoked from various skin sites. Bin width, 10 ms. iH, ipsilateral hindlimb, coFL, contralateral forelimb (both with a distal location).

Figure 4. Summary of net spike responses evoked from various skin sites.

PFRF, parallel fiber receptive fields of afferent PCs (see text); CFRF, climbing fiber receptive fields of afferent PCs. Kat = category; cFL = contralateral forelimb; Tru. = trunk; iHL = ipsilateral hindlimb.

Figure 5. Net spike responses in DCN cells and LRN cells evoked by manual skin stimulation.

(A) Net spike response of the same cell as illustrated in Fig. 1. (B) Net spike response shown as an average peristimulus histogram of all LRN cells recorded (N = 29). The motivation for pooling the LRN cells in (B) is that DCN cells would be expected to receive multiple LRN MF synaptic inputs. Bin width in both (A) and (B) is 10 ms. (C) Reconstructed electrode track for a stimulation electrode placed in LRN, shown in a histological section and a 3D reconstruction of the brainstem, respectively.

Figure 6. Responses evoked in DCN cells by microstimulaton in LRN.

(A) Superimposed raw EPSPs recorded from one DCN cell during hyperpolarization to prevent spiking. LRN stimulation, 50 uA. (B) Histogram of spike responses evoked in the same cell but without hyperpolarization. N = 50 stimulations. Bin width 1 ms.

Figure 7. The information conveyed to DCN neurons.

The AIP is composed of multiple functional cell groups, each of which forms the output of a cerebellar corticonuclear microcomplex (ucplx). The main mossy fiber input to the DCN cells is derived from spinal motor circuits, which are conveyed to the cerebellum either directly in a spinocerebellar tract (SCT), or via a synaptic relay in the LRN, in a spino-reticulo cerebellar tract (SRCT). These tracts are in turn driven by spinal interneurons, which target specific combinations of muscles or synergies. Pentagons in the ventral horn are alpha-motorneurons encircled by dashed lines to indicate separate spinal motor nuclei, and each spinal interneuron targets a limited number of motor nuclei , as indicated by example. Hence, the information carried in SCTs/SRCTs informs the DCN cells of the final composition of the motor command, i.e. which muscle synergies that are currently driven. Also the input that drives the CFs is derived from the spinal motor circuitry (indicated by dashed line from the spinal interneurons), via relays at least in the cuneate nucleus and the inferior olive (IO). The spinal motor circuitry is driven by motor commands from the neocortex and the brainstem. The output of the DCN cell represents the cerebellar correction of ongoing motor commands and targets the neocortex and the brainstem. Since the CF receptive field identifies the microcomplex the DCN cell is located in and consequently the muscle synergy it is connected with, comparing the SCT/SRCT input with the location of the CF receptive field is equal to comparing the motor information it receives with the motor information it issues. Accordingly, our findings suggest that the input and the output of the DCN cell are functionally matched. AIP, Anterior interposed nucleus; PC, Purkinje cell; CF, climbing fiber; ucplx, microcomplex; MF, mossy fiber; IO, inferior olive; LRN, lateral reticular nucleus; SRCT, spinoreticulocerebellar tract; SCT, spinocerebellar tract.