Physiologic and morphologic properties of motoneurons and spindle afferents innervating the temporal muscle in the cat

Little is known about physiology and morphology of motoneurons and spindle afferents innervating the temporalis and on synaptic connections made betwe en the two. The present study was aimed at investigating the above issues a t the light microscopic level by using the intracellular recording and hors eradish peroxidase or biotinamide labeling techniques and by the use of suc cinylcholine (SCh) for the classification of spindle afferents in the cat. Temporalis motoneurons had dendritic trees that ranged from a spherical for m to an egg-shaped form. The shape deformation was more prominent for the d endritic trees made by motoneurons located closer to the nuclear border. No axon collaterals of the motoneurons were detected. On the basis of the val ues for the dynamic index after SCh infusion, temporalis spindle afferents were classified into two populations: presumptive groups Ia and II. The spi ndle afferents terminated mainly in the supratrigeminal nucleus (Vsup), reg ion h, and the dorsolateral subdivision (Vmo.dl) of the trigeminal motor nu cleus (Vmo). The proportion of group Ia afferent terminals was lower in the Vsup than that of group II afferents. In the Vmo.dl, the proportion of gro up Ia afferent terminals was nearly even throughout the nucleus, but that o f group II afferent terminals increased in the more outlying regions. The p roportion of terminal distribution in the central region of Vmo.dl was high er for group Ia than group II. The frequency of contacts (presumptive synap ses) made by a single spindle afferent on a motoneuron was higher for group Ia than group II. The present study provided evidence that the central org anization of spindle afferent neurons is different between groups Ia and II . J. Comp. Neurol. 406:29-50, 1999. (C) 1999 Wiley-Liss, Inc.