LIGHT-MICROSCOPIC OBSERVATIONS OF THE CONTACTS MADE BETWEEN 2 SPINDLEAFFERENT TYPES AND ALPHA-MOTONEURONS IN THE CAT TRIGEMINAL MOTOR NUCLEUS

Previous studies indicate that cat jaw-muscle spindle afferents can be divided into two types (type I and II) on the basis of their axonal t rajectories. The present study examined the relationship between spind le afferent fibers and their target masseter cy-motoneurons in the cat by using the intracellular horseradish peroxidase (HRP) injection tec hnique, and provided several new findings on the synaptic organization generated between the two. Five type I afferent fiber-motoneuron pair s and nine type II afferent-motoneuron pairs were well stained with HR P. The following conclusions were drawn: 1) A motoneuron received cont acts from only one collateral of any given spindle afferent. 2) The nu mber of contacts made between an afferent and a motoneuron ranged from one to three. 3) The contacts made by a spindle afferent were on the same dendrite or dendrites branching from the same primary dendrite. 4 ) The vast majority of the contacts made by an afferent on a motoneuro n were distributed in the dendritic tree within 600 mu m from the soma , i.e., in the proximal three fourths of the dendritic tree. The diffe rences observed between the two afferent types were as follows. First, type II afferent terminals made contacts on more distal dendrites of the motoneurons than did type I afferent terminals. Second, the contac ts made between a type I afferent and a motoneuron were clustered toge ther, but those made between a type II afferent and a motoneuron were widely dispersed. The present results provided the general rules of sy naptic contacts between the spindle afferents and masseter a-motoneuro ns, and demonstrated that the spatial distribution of synaptic contact s on the dendritic tree was different between type I and type II affer ents. (C) 1996 Wiley-Liss, Inc.