Direct fastigiospinal fibers in the cat

Projections o f the fastigial nucleus to the vestibular nuclear complex 6,8, the pontomedul lary reticular format ion 6,9 and the ventral thalamic complex1,3, 6 are well known. In 1956, Thomas et al. 6 reported that fastigial fibers also project to the upper cervical spinal cord, a finding which has since been overlooked. Following injection o f horseradish peroxidase (HRP) into the upper cervical spinal cord we have found numerous labeled neurons within the fastigial nucleus, indicating the existence o f a significant direct fastigiospinal pathway. To obtain further information on this pathway, we utilized retrograde axonal t ransport o f H R P to investigate the following points: (1) Is the projection f rom the fastigial nucleus to the spinal cord bilateral or unilateral? (2) Where in the fastigial nucleus are fastigiospinal cells located? (3) To what level of the spinal cord do they primarily project? Our results, described in this paper, are confirmed and extended by the physiological study which foUows 1°. Experiments were performed on 8 cats anesthetized with Nembuta l (40 mg/kg, i.p.). In 4 cats 1-6 injections of 0.2-1.0 #1 of a 50 ~ solution of H R P (Sigma, Type VI) were made into the grey matter on one side of the C8 segment using a microsyringe fitted with a glass micropipette having a tip diameter of 20-100 #m. In 2 cats similar injections were made into the grey matter at the L5 and L7 segments. In 2 other cats 0.2 /~1 injections were made into the grey and white matter on one side o f the C~ and C5 segments with a horizontal and vertical spacing of 0.3 mm to facilitate uptake of H R P by damaged axons 4. Such injections gave rise to labeling o f cells th roughout the red