A technique for recording from intact phrenic nerve afferents.

Recent evidence has suggested that phrenic nerve afferents can influence respiratory motor drive. This paper presents a technique whereby the activity of single phrenic nerve afferents can be recorded from uncut dorsal root filaments. Cervical dorsal roots 4, 5, and 6 were exposed by dorsal laminectomy in 10 anesthetized, spontaneously breathing cats. A stimulating electrode was placed on the right whole phrenic nerve low in the neck. The animal was then placed in a spinal suspension frame. Dissection of the dorsal root filaments was performed with probes made of fine tungsten wire. Single filaments were isolated intact from the dorsal root fascicles and placed across a tungsten electrode. Fiber classification was performed by determining conduction velocity. Monopolar recordings were made from a total of 38 fibers. Tonic activity was observed in 21, respiratory-related activity was evident in 15, and two fibers were silent but could be recruited by phrenic nerve stimulation. The conduction velocities ranged from 2.2 to 103 m/s. Approximately one-half of the fibers had conduction velocities of less than 20 m/s. This technique offers a way to record the activity of diaphragm afferents while maintaining the integrity of possible reflex pathways. Application of this method should prove helpful in elucidating the possible role of the various diaphragm afferents in the control of respiratory motor drive.