INPUT PATTERNS AND PATHWAYS FROM THE 6 SEMICIRCULAR CANALS TO MOTONEURONS OF NECK MUSCLES .2. THE LONGISSIMUS AND SEMISPINALIS MUSCLE GROUPS

To reveal patterns of input from the six semicircular canals to motone urons of various neck muscles and their relationship to the mechanical actions of individual neck muscles, patterns of input to neck motoneu rons of the longissimus and the semispinalis muscle groups were invest igated in the upper cervical spinal cord of anesthetized cats. Intrace llular potentials were recorded from motoneurons of the longissimus mu scle group (obliquus capitis superior muscle. OCS; splenius muscle, SP L; longissimus muscle, LONG) and the semispinalis muscle group (bivent er cervicis muscle, BIV; complexus muscle, COMP), and effects of separ ate electrical stimulation of the six ampullary nerves on them were an alyzed in each preparation. Neck motoneurons usually received converge nt inputs from all of the six ampullary nerves, and motoneurons that s upplied a particular muscle had a homogeneous pattern of input from th e six ampullary nerves, Two different patterns of input were identifie d for motoneurons of these two muscle groups; one pattern for motoneur ons of the longissimus muscle group and the other pattern for motoneur ons of the semispinalis muscle group. Motoneurons of the OCS, the SPL, and the LONG muscles received excitation from the three contralateral ampullary nerves and inhibition from the three ipsilateral ampullary nerves. BIV and COMP motoneurons received excitation from the bilatera l anterior canal nerves (ACNs) and the contralateral lateral canal ner ve (LCN) and inhibition from the bilateral posterior canal nerves (PCN s) and the ipsilateral LCN. Latencies of postsynaptic potentials (PSPs ) evoked by stimulation of each of the six ampullary nerves indicated that the earliest component of excitatory PSPs (EPSPs) and inhibitory PSPs (IPSPs) was disynaptic in these motoneurons. However, trisynaptic IPSPs were evoked by stimulation of the contralateral PCN in a consid erable number of BTV and COMP motoneurons. In OCS, SPL, and LONG moton eurons, all of the excitation from the contralateral and all of the in hibition from the ipsilateral ampullary nerves were mediated through t he ipsilateral medial longitudinal fascicle (MLF). In BTV and COMP mot oneurons, disynaptic excitation from the contralateral ACN and LCN and disynaptic inhibition from the ipsilateral LCN and bilateral PCNs wer e mediated through the ipsilateral MLF, whereas disynaptic excitation from the ipsilateral ACN was mediated through the ipsilateral lateral vestibulospinal tract. The patterns of semicircular canal input to nec k motoneurons of these two muscle groups are related closely to the me chanical actions of individual neck muscles and the optimal stimulus t o the semicircular canals such that the connections will tend to stabi lize head position in response to head perturbations.