INPUT PATTERNS AND PATHWAYS FROM THE 6 SEMICIRCULAR CANALS TO MOTONEURONS OF NECK MUSCLES .1. 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 re cording intracellular potentials from motoneurons in the upper cervica l cord of anesthetized cats. 2. Synaptic potentials were recorded in m otoneurons of the rectus capitis posterior (RCP) muscle at C-1, the ob liquus capitis inferior (OCI) muscle at C-1 and C-2, and the cervical multifidus muscle (Multi) at C-4 in response to electrical stimulation of individual ampullary nerves of the six semicircular canals. Excita tory or inhibitory postsynaptic potentials (EPSPs or IPSPs, respective ly) 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. Motoneur ons that supplied a single muscle had a homogeneous pattern of input f rom the six semicircular canals. There were two patterns of input from the six semicircular canals to motoneurons of the multifidus muscle g roup. RCP and Multi motoneurons were excited by stimulation of the bil ateral anterior canal nerves (ACNs) and the contralateral lateral cana l nerve (LCN) and inhibited by stimulation of the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. This input pattern is sim ilar to that previously observed in other dorsal extensor muscles, whe reas the other input pattern observed in OCI motoneurons is entirely n ew. OCI motoneurons at C-1 and C-2 were excited by stimulation of the ipsilateral ACN, PCN, and the contralateral LCN and inhibited by stimu lation of the contralateral ACN, PCN, and the ipsilateral LCN. 4. Most postsynaptic potentials (PSPs) were disynaptic, but there were trisyn aptic inhibitory connections between the contralateral ACN and PCN and OCI motoneurons, and between the contralateral PCN and RCP motoneuron s. 5. The pathways for mediating these inputs from different semicircu lar canals to neck motoneurons were determined by making lesions in th e lower medulla. Transection of the ipsilateral medial longitudinal fa scicle (MLF) abolished the following potentials: all disynaptic PSPs i n 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 secti on 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 contra lateral ACN and PCN in OCI motoneurons and from the contralateral PCN in RCP motoneurons. 6. These two patterns of input from the six semici rcular canals to motoneurons of neck muscles of the multifidus group a re consistent for their biomechanical properties.