Koki Kawamura

The Inferior Olive. Notes on its Comparative Anatomy, Morphology, and Cytology

It is appropriate, before discussing the olivocerebellar connections, to review briefly some features of the normal olive, since these are of relevance for the interpretation of data obtained in studies of the connections of the olive.

Identification of cells of origin of tectopontine fibers in the cat superior colliculus: An experimental study with the horseradish peroxidase method

The pontine projections from the superior colliculus in the cat have been studied by means of retrograde axonal transport of horseradish peroxidase (HRP). Following injections of HRP in the dorsolateral pontine nucleus, where the tectopontine fibers terminate, a fair number of labeled cells are found throughout the rostrocaudal extent of the ipsilateral superior colliculus. Relatively few of the labeled cells are of medium size (25-40 micron in diameter), more than 80% are small (10-25 micron), but no large cells are labeled. The cell bodies giving rise to tectopontine fibers are distributed in tectal layers deeper than the optic stratum (including this), with only a few in the deeper portion of the superficial gray layer. There are only few labelled cells in the relatively large lateral portion of the intermediate and deep gray layers were the largest neurons (more than 40 micron) are located. Most of these presumably belong to the tectoreticular and the tectospinal projections. The tectal neurons, distributed in various collicular layers, are supposed to receive different kinds of information from other parts of the central nervous system, e.g. from the retina, the cerebral cortex, the brain stem reticular formation, the spinal cord etc. The dorsolateral pontine nucleus appears to have a particular function in the integration of the input from the superior colliculus with those from other sources, especially from the inferior colliculus and the auditory cerebral cortex.

Cells of origin of corticopontine and corticotectal fibers in the medial and lateral banks of the middle suprasylvian sulcus in the cat. An experimental study with the horseradish peroxidase method

Cells of origin of corticopontine and corticotectal fibers in the medial and lateral banks of the cat middle suprasylvian sulcus (MSs area) were identified by means of the retrograde axonal transport of horseradish peroxidase. Somal diameters of labeled cells seen in the MSs area were measured after injection of the enzyme in the pons and the tectum. Diameters of corticopontine and corticotectal neurons are 12-35 microm (mean and SD: 18.9 + or - 3.2 microm) and 12-55 microm (mean and SD: 27.7 + or - 10.6 microm), respectively. These two kinds of neurons are both pyramidal in shape, located in layer V and the corticotectal neurons are, in general, larger and more polymorphic than the corticopontine cells. It thus appears that the two descending fibers originate from different, and independent sets of pyramidal neurons in layer V of the cortex.

An HRP study of neural pathways to neocortical olfactory areas in monkeys

Afferent fiber projections to the two orbitofrontal olfactory areas of monkeys were studied using the horseradish peroxidase (HRP) technique. After injections of HRP into the lateroposterior (LPOF) or centroposterior (CPOF) area of the orbitofrontal cortex, some differences were found in the distribution of labeled cells between the projections to the LPOF and CPOF. These results, along with those of previous electrophysiological investigations, suggest the following conclusions: (1) the extrathalamic olfactory pathway to the LPOF identified by Tanabe et al. has relay neurons primarily in the substantia innominata and the amygdala and, secondarily, in the prorhinal cortex and the hypothalamus; (2) direct fibers to the LPOF from the amygdala and the prorhinal cortex pass through the areas ventral to the thalamus; (3) the transthalamic olfactory pathway to the CPOF identified by Yarita et al. has relay neurons concentrated primarily in the magnocellular portion of the mediodorsal nucleus of the thalamus.

The Olivocerebellar Projection

The existence of fibers passing from the inferior olive to the cerebellum was recognized by early workers in neuroanatomy (see Brodal 1940b). Partly on the basis of experimental findings, partly on the basis of cell loss found in the olive in human beings with cerebellar afflictions, some authors indicated particular termination sites for olivary fibers within the cerebellum.

Commissural afferents to the cortex surrounding the posterior part of the superior temporal sulcus in the monkey

Commissural afferents to the cortex surrounding the posterior part of the superior temporal sulcus were studied in Japanese monkeys by the horseradish peroxidase method. After injection of the enzyme, many callosal neurons were labeled contralaterally in the cortical area corresponding to the injection site (homotopical area) and in other regions (heterotopical area). Most of the callosal neurons were triangular in shape, occurring for the most part in layer III of both homotopical and heterotopical areas (about 75-90% of the total number of labeled cells). Mean diameters of the cell bodies were about 11-13 micron.

General Principles of Organization of the Olivocerebellar Projection and Some Functional Correlations

The subjects of this section will be considered in the light of the data discussed in the preceding chapters. For reasons discussed in Section 3.2 comparisons between results obtained with different methods often have to be tentative. This is the case particularly when attempts are made to correlate sites of ending of olivary afferents with sites of origin of olivary efferents.51 Nevertheless, on many points the evidence concerning the olivocerebellar projection is fairly conclusive (see Sect. 3). On other points final conclusions have so far not been possible.

Main Afferent Connections of the Inferior Olive

As seen from Sect. 3, a minor division of the olive usually gives off fibers to various cerebellar regions, and all lobes or lobules of the cerebellum receive afferents from more than one olivary subdivision.