Central and proprioceptive influences on the activity of levator costae motoneurones in the cat.

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Abstract
The role of central respiratory drive, muscle spindles and tendon organs in producing respiratory movements was studied in the cat by recording from motoneurons supplying a set of small axial muscles inserted between each rib and the vertebra immediately rostral, the levatores costae. The levator costae muscles are active during normal inspiration but activity is progressively stronger in the muscles located in more caudal thoracic segments. Intracellular recordings from levator costae motoneurons show a characteristic central respiratory drive potential (c.r.d.p.), comprising phases of depolarization during inspiration alternating with expiratory phased hyperpolarization due to post-synaptic inhibition. Loading or unloading the levator costae muscles increases and decreases, respectively, their normal inspiratory activity. Electrophysiological and histological analysis of levator costae afferents reveals that each muscle contains 3 to 5 spindles and 2 to 3 tendon organs. By dissecting the levator costae nerve, afferent discharges from muscle spindle primary and secondary endings and those from tendon organs were recorded in continuity and used to trigger an averager for analyzing the synaptic potentials they evoke in levator costae motoneurons. Monosynaptic excitation and oligosynaptic inhibition originate from spindle (primary and secondary endings) and from tendon organ afferents, respectively. A monosynaptic excitation has been evoked from a tendon organ. The action of the levator costae muscles on the respiratory movements of the ribs in the different thoracic spaces was analyzed in relation to their anatomical properties and to the activity of the other respiratory muscles. The levator costae muscles, because of their relatively small size and well-defined population of proprioceptors, appear to provide a favorable preparation for studying the integrative action of the motoneuron.