The cerebellum of a rat can be subdivided into more than 20 compartments, as shown in a schematic view. The unfolded rat cerebellum reveals its division into various lobules, each indicated by Roman numerals. The right half of the cerebellum displays the zebrin compartment numbers, while the left hemisphere presents the names of the cerebellar lobules. The inferior olivary nucleus, found in the brain, is distinct due to its unique characteristics. The majority of its neurons are projection cells, meaning that they do not interact through local chemical synaptic connections. Instead, these olivary neurons are electrically coupled to each other via gap junctions, resulting in synchronized activity that is transmitted to the cerebellum. The inferior olivary nucleus contains the highest density of neuronal gap junctions in the central nervous system (CNS). Afferent fibers to the inferior olivary nucleus can be divided into two main classes: excitatory fibers, originating from various regions in the CNS, and inhibitory GABAergic fibers, which come from the cerebellar nuclei and a few brainstem nuclei. These afferent fibers modulate the firing rates of olivary neurons, but due to the membrane properties of these neurons, this modulation is limited to a few hertz. Olivary neurons possess the potential to be intrinsic oscillators, capable of generating synchronized activity across the cerebellar cortex. The patterns of synchronous activity delivered to the cerebellum can be altered by afferent activity, which also affects the expression of the oscillatory potential of olivary neurons. The functional significance of these patterns is still debated, but one hypothesis suggests that they provide a gating signal for synchronizing motor commands to various muscle combinations.
C, Schematicviewofanunfoldedratcerebellum,showingitssubdivisionintomorethanI 1+2+3+3−3b−2b−*2a−5a−4−d−d−6−1−a−c−5−5−e2−e1−f−Par 4−5a−4a−4b−3−3−2−1−*3b+4+a+a+a+2+*2b+d+d+c+5+5+4+5a+6+4b+6+3+3+2+2+1+1+7+e2+*f+4+4+e1+b+2−1−b−III II IV V VII VIII IXa CP Cr IIb Cr IIa Cr Ic Cr Ib Cr Ia Sim b Sim a pf DPFL VPFL FL 5 mm 1 mm Rostral Left Right Caudal IXb IXc Xa Xb Vld Vla Vlb Vlc C •Fig. 9.20, cont’d 20compartments,accordingtostainingformolecularmarkers:inthiscase,zebrinII(aldolaseC).Letters and numbers on the right half ofthecerebellumindicatethezebrincompartmentnumber.Roman numerals down the center indicatecerebellarlobules.Names on left hemisphere indicatenamesofcerebellarlobules.CP,copulapyramis;Cr,crus;DPFL,dorsalparaflocculus;FL,flocculus;Par,paramedian;pf,primaryfissure;Sim,simplex;VPFL,ventralparaflocculus.(B, ModifiedfromVoogdJ.InLlinásRR(ed).Neurobiology of Cerebellar Evolution and Development. Chicago:AmericanMedicalAssociation;1969.C, CourtesyofDr.IzumiSugihara.)
The inferior olivary nucleus is a distinctive brain region for several reasons. As already noted, its neurons are virtually all projection cells, and so there is little local chemical synaptic interaction between the cells. Instead, olivary neurons are electrically coupled to each other by gap junctions. In fact, the inferior olivary nucleus has the highest density of neuronal gap junctions in the CNS. This allows olivary neurons to have synchronized activity that gets transmitted to the cerebellum. Afferent fibers to the inferior olivary nucleus may be divided into two main classes: those transmitting excitatory input, which arises from many regions throughout the CNS, and those transmitting inhibitory GABAergic input from the cerebellar nuclei and a few brainstem nuclei. Although these afferent fibers can modulate the firing rates of olivary neurons (as is typical in most brain regions), the membrane properties of olivary neurons limit this modulation to a range of a few hertz and endows these neurons with the potential to be intrinsic oscillators. Instead of just modulating firing rates, olivary afferent activity also acts to modify the effectiveness of the electrical coupling between olivary neurons and thus changes the patterns of synchronous activity delivered to the cerebellum. Afferent activity may also modulate expression of the oscillatory potential of olivary neurons. Thus the inferior olivary nucleus appears to be organized to generate patterns of synchronous activity across the cerebellar cortex. The functional significance of these patterns remains controversial. One hypothesis is that they provide a gating signal for synchronizing motor commands to various muscle combinations.
CHAPTER 9 Organization of Motor Function •Fig. 9.21 Three-Dimensional View of the Cerebellar Cortex, Showing Some of the Cerebellar Neurons. Thecutfaceattheleftisalongthelongaxisofthefolium;thecutfaceattherightisatrightanglestothelongaxis.BC,basketcell;BCA,basketcellaxon;CF,climbing(olivocerebellar)fiber;CN,cerebellarnuclearcell;GC,Golgicell;Glm,glomerulus;GrC,granulecell;MF,mossyfiber;PC,Purkinje cell;PF,parallelfiber;SC,stellatecell.