Discharge Pattern and Projection of Pinna Primary Afferents to the Spinal Cord Involved in the Scratch Reflex of the Cat : A Study by Neurophysiological and Histological Methods

The present study was undertaken to clarify the afferent system of the cat scratch reflex. First, the discharge properties of mechanoreceptive units from the pinna skin were studied neurophysiologi cally using microelectrodes in C1 spinalized cats. Then the projection of the pinna afferents to the spinal cord was traced anterogradely by the WGA-HRP (conjugates of horseradish peroxidase with wheat germ agglutinin) method histologically. 1) Tactile or electrical (rectangular pulses of 1 ms duration, 50 Hz) stimuli were applied to the skin surface covering the bursella, scapha, or dorsal basal part of the pinna receptive fields (RFs). After sectioning C2 and C3 dorsal roots, electromyographic recordings from ipsilateral hindlimb muscles did not exhibit any scratching rhythm. In immobilized cats, the rhythmic discharge of fictive scratch reflex was recorded in n. gastrocnemius ipsilaterally and causative pinna afferent impulses were simultaneously recorded using tungsten microelectrodes inserted into the C2 dorsal root ganglion (DRG) or dorsal root. Fifty mechanoreceptive units were recorded in total; 19 rapidly adapting (RA), 4 slowly adapting (SA) touch units and 27 hair follicle units. Fictive scratch reflex was elicited only when RA touch units responded to the pinna tactile stimuli. The latencies of scratching rhythm from the unit afferent impulses ranged from 1.5 to 6.0 (mean : 3.6) seconds. Rhythmic discharge (mean 4.2 Hz) was maintained during the nonrhythmic afferent impulses (mean : 32 impulses/second). Fictive scratching rhythm was also elicited when RA touch units were activated by electrical stimuli to the pinna RFs. Nerve conduction velocities of RA touch units ranged from 18.0 to 49.5 (mean 35.6) m/s. 2) WGA-HRP solution (5-10%, 1.5-2.5 μ1) was injected into the C2 DRG. After a 24-48 hours survival period, frozen sections (60 μm thickness) of the cervical cord from C1 to C4 segments were cut transversely or longitudinally. Labeled dorsal root fibers were traced rostrally to the middle part of C1 segment and caudally to the lower end of C3 segment. The HRP-labeled granules were observed in the ipsilateral Rexed's laminae I-III and partly in IV. These results suggest that pinna afferents involved in the scratch reflex are thin myelinated axons from RA touch receptors and terminate in the ipsilateral spinal dorsal horn of Rexed's laminae I-III from Cl to C3 segments