Mossy fiber neck and second-order labyrinthine projections to cat flocculus.

In five decerebrate cats and in two under N2O-O2 anesthesia, a bipolar electrode array was inserted into a granular layer in the rostral flocculus. At this location MF field potentials were evoked by stimulation of the ipsilateral vestibular nerve and of the C2 dorsal root ganglion. Stimulation through the array was used to fire brain stem neurons antidromically. The activity of these neurons was recorded extracellularly with glass microelectrodes. Stimulation of the C2 dorsal root ganglion evoked in the caudal lateral brain stem a field potential caused by the arrival of impulses in primary afferent fibers. Cells projecting to the flocculus also responded to the stimulus, usually monosynaptically. These neurons were fired by stimulation of the area of the atlantoaxial joint. They did not respond to stimulation of the contralateral C2 dorsal root ganglion, and responded only rarely to stimulation of the dorsal rami of the C2 and C3 spinal nerves. Stimulation of the vestibular nerve was ineffective. Another group of cells projecting to the flocculus was fired at short latency by stimulation of the ipsilateral vestibular nerve. These neurons were usually inhibited by stimulation of the contralateral vestibular nerve. They were not affected by stimulation of any neck afferents. The location of many recording locations was identified by means of fast green dye marks. Most neurons relaying neck activity were in group x of Brodal and Pompeiano (4), a few were in the external cuneate nucleus, and one was in the descending vestibular nucleus. Neurons relaying labyrinthine activity were in the descending vestibular nucleus; one was found in the medial nucleus, in which tracks were made only rarely. There are two parallel pathways relaying neck and labyrinthine activity to the flocculus. While they are separate at the level of the brain stem, they converge in the same rostral areas of the flocculus. The neck afferent information may be required for proper performance of the flocculus in eye-movement control.