The projection from the motor cortex to the inferior olive in the cat. An experimental study using axonal transport techniques.

The cortico-olivary projection has been investigated in the cat with the methods of retrograde transport of horseradish peroxidase and wheat-germ agglutinin conjugated with horseradish peroxidase as well as autoradiographic techniques using tritium-labelled amino acids. The projection arises from cells in cortical layer V and terminates mainly ipsilaterally and less densely contralaterally. The strongest termination site is in the caudal medial accessory olive adjacent to subnucleus beta. Projections to that area originate in the medial portions of areas 4 and 6 rostral to the cruciate sulcus. Regions of the motor cortex related to axial back and neck, proximal forelimb and face musculature plus the frontal eye field are represented in largely overlapping areas of the caudal medial accessory olive. A second zone of termination is present in the rostral olive at the junction of the ventral lamella of the principal olive and the medial border of the dorsal accessory olive. Projections to that area arise from a central portion of area 4 at the border between the anterior sigmoid gyrus and the presylvian gyrus. This area contains portions of the representation of the muscle groups controlling the neck and proximal forelimb (shoulder and elbow) only. The frontal eye field, which in the cat influences both extraocular and neck musculature, is also an important direct source of input to this portion of the inferior olive. Contralateral terminations are distributed symmetrically. Combining this information with the olivocerebellar distribution, cerebellar cortical areas corresponding to this direct cortical input are defined. Taking into account that the cortico-olivary fibers appear to arise only from those portions of the motor cortex involved in the control of axial and proximal forelimb muscles, it is suggested that the cortico-olivo-cerebellar projections play a preponderant role in the cerebellar control of posture.