Input patterns and pathways from the six semicircular canals to motoneurons of neck muscles. I. The multifidus muscle group.

1. The pattern of connections between the six semicircular canals and neck motoneurons of the multifidus muscle group was investigated by recording intracellular potentials from motoneurons in the upper cervical cord of anesthetized cats. 2. Synaptic potentials were recorded in motoneurons of the rectus capitis posterior (RCP) muscle at C1, the obliquus capitis inferior (OCI) muscle at C1 and C2, and the cervical multifidus muscle (Multi) at C4 in response to electrical stimulation of individual ampullary nerves of the six semicircular canals. Excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs, respectively) were evoked by separate stimulation of individual ampullary nerves in all of the neck motoneurons. Virtually all of the neck motoneurons received convergent inputs from the six ampullary nerves. 3. Motoneurons that supplied a single muscle had a homogeneous pattern of input from the six semicircular canals. There were two patterns of input from the six semicircular canals to motoneurons of the multifidus muscle group. RCP and Multi motoneurons were excited by stimulation of the bilateral anterior canal nerves (ACNs) and the contralateral lateral canal nerve (LCN) and inhibited by stimulation of the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. This input pattern is similar to that previously observed in other dorsal extensor muscles, whereas the other input pattern observed in OCI motoneurons is entirely new. OCI motoneurons at C1 and C2 were excited by stimulation of the ipsilateral ACN, PCN, and the contralateral LCN and inhibited by stimulation of the contralateral ACN, PCN, and the ipsilateral LCN. 4. Most postsynaptic potentials (PSPs) were disynaptic, but there were trisynaptic inhibitory connections between the contralateral ACN and PCN and OCI motoneurons, and between the contralateral PCN and RCP motoneurons. 5. The pathways for mediating these inputs from different semicircular canals to neck motoneurons were determined by making lesions in the lower medulla. Transection of the ipsilateral medial longitudinal fascicle (MLF) abolished the following potentials: all disynaptic PSPs in RCP motoneurons except the disynaptic EPSPs from the ipsilateral ACN, and in OCI motoneurons, disynaptic PSPs from the bilateral LCNs, and disynaptic IPSPs from the contralateral PCN. Complete bilateral section of the MLF did not affect the disynaptic EPSPs from the ipsilateral ACN in RCP motoneurons, the disynaptic EPSPs from the ipsilateral ACN and PCN in OCI motoneurons, nor the trisynaptic IPSPs from the contralateral ACN and PCN in COI motoneurons and from the contralateral PCN in RCP motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)