FUNCTIONAL CLASSIFICATION OF AFFERENT PHRENIC-NERVE FIBERS AND DIAPHRAGMATIC RECEPTORS IN CATS

1. Single afferent fibres with receptive fields in the diaphragm (272 units) dissected from the right phrenic nerve were classified accordin g to the following properties: reaction to contraction of the diaphrag m, resting activity, conduction velocity, location and properties of r eceptive fields, and reaction to injection of bradykinin and lactic ac id into the internal thoracic artery. Nine additional fibres dissected from the phrenic nerve had receptive fields outside the diaphragm. Th e experiments were performed on chloralose-anaesthetized cats. 2. Nine ty-six fibres (36%) had high resting activity when unloaded by contrac tion of the diaphragm, had low-threshold receptive fields in the muscl e and were mostly group II and III fibres. They probably innervated mu scle spindles. 3. Eighty-eight fibres (32%) were vigorously activated by contraction of the diaphragm. They had low-threshold receptive fiel ds located in the musculotendinous border and central tendon. Their co nduction velocity was in the range for group II and III fibres. We inf er that they may innervate tendon organs. 4. Eighty-eight fibres (32%) were slightly affected or not affected by diaphragmatic contraction. They had low- and high-threshold receptive fields located mostly in th e muscular part of the diaphragm, and negligible resting activity. Mos t of them were group III and IV afferent fibres and were activated whe n bradykinin and lactic acid were applied to their receptive fields. P ossibly these low- and high-threshold receptors innervated diaphragmat ic ergo- and nociceptors, respectively. 5. Sensory outflow from the di aphragm was found to be somatotopically organized, so that fibres with receptive fields in the sternocostal portion were predominantly locat ed in the upper phrenic nerve root, and those with lumbar receptive fi elds were in the lower root. 6. It is concluded that the phrenic nerve contains fibres from several distinct classes of sensory receptors: m uscle spindles, tendon organs, ergoceptors and nociceptors. The sensor y diaphragmatic outflow to the spinal cord is somatotopically organize d.