Inhibition of Spinal Reflexes by Paramedian Reticular Nucleus

Abstract

The inhibitory actions of the paramedian reticular nucleus (PRN), and its neighbouring structures i.e., midline raphe nuclei (MRN) and dorsal medullary depressor area (DMD) on the knee jerk (KnJ) and crossed extension movement (CEM) induced by central sciatic stimulation and on the L5 ventral root response (EVRR) evoked by central tibial stimulation, were studied in cats under urethane (400 mg/kg) and alpha-chloralose (40 mg/ kg) anesthesia alone, IP or further paralyzed with atracurium besylate (0.5 mg/kg/30 min), IV. Electrical stimulation of the above areas with rectangular pulses (80 Hz, 1.0 msec, 100–200 μA) decreased systemic arterial blood pressure (SAP) in an average value of: 36±3 mmHg for PRN; 19±2 mmHg for MRN; and 23±3 mmHg for DMD. The KnJ and CEM were almost completely suppressed by simultaneous PRN stimulation. The EVRR, including mono- and polysynaptic spinal reflexes with transmission velocity from 10 to 60 m/sec or above, were also suppressed. MRN stimulation only inhibited the KnJ, CEM and polysynaptic spinal reflexes with transmission velocities between 25 and 60 m/sec, but facilitated spinal reflexes with conduction velocities below 10 m/ sec. On the other hand, DMD stimulation resulted in small suppression of KnJ, CEM and inhibition of polysynaptic spinal reflexes with conduction velocities between 25 and 60 m/sec. Even though MRN and DMD partially inhibited polysynaptic spinal reflexes, the magnitude of such inhibition was much smaller than that produced by PRN (−20% and −22% vs. −48%). The above-mentioned PRN effects on SAP and EVRR persisted in chronic animals decerebellated 9–12 days before. In addition, microinjection (200 nl) of sodium glutamate (1 M), DL-homocysteic acid (0.05 M) or kainic acid (0.047 M) to PRN also decreased KnJ, CEM and EVRR. The present results suggest that the PRN, containing both cell bodies and fibers projected from elsewhere, in addition to its inhibitory effects on the autonomie nervous system, also depresses the somatic nervous system, including mono- and polysynaptic motoneuronal pools. This inhibitory action of the PRN is exerted independently of the efferent and afferent connections of the cerebellum.