Reticulospinal inhibition of transmission in reflex pathways

Abstract

SUMMARY1. The effect of electrical stimulation of the brain stem on reflex transmission has been investigated in decerebrate cats after partial transection of the spinal cord.2. Brain stem stimuli that do not evoke inhibitory post-synaptic potentials (IPSPs) in motoneurones or primary afferent depolarization may still effectively depress the excitatory and inhibitory synaptic actions evoked from the flexor reflex afferents (FRA) and from lb afferents. There is no effect on post-synaptic potentials from Ia afferents or on Renshaw IPSPs. The depression is not associated with any measurable change in conductance over the motoneuronal membrane.3. There is also inhibition from the brain stem of transmission from the FRA (but not from Ia and Ib afferents) to primary afferqnt terminals and to ascending spinal pathways. 4. It is concluded that this inhibition from the brain stem is exerted at an interneuronal level in spinal reflex paths.5. The inhibitory action is evoked from the region ofMagoun's inhibitory centres in the brain stem and is mediated by axons with a conduction velocity of at least 20 m/sec. The axons are distributed in the dorsal part of the lateral funicle.6. The pathway mediating the inhibition from the brain stem is named the dorsal reticulospinal system. Its possible role in maintaining the decerebrate control of reflexes is discussed and related to the problem of a selective control of some paths from a primary afferent system.

Medullary reticulospinal tract mediating the generalized motor inhibition in cats: Parallel inhibitory mechanisms acting on motoneurons and on interneuronal transmission in reflex pathways

“…It follows that the early and late IPSPs can be mediated by common inhibitory interneurons (Fig. 5D) 17 Neuroanatomical studies illustrated that fibers arising from the MRMc and the caudal raphe nucleus mostly descend in the LF, 42,46 while those from the NRGc mainly descend in the VLF. 43,46 These findings support our data indicating that the fibers mediating the middle EPSPs descend more dorsally than those mediating the early and the late IPSPs do.…”

“…17 The dorsal reticulospinal tract suppressed transmission of Ib inhibitory pathways and FRA pathways. 17 The inhibition of FRA pathways was caused by the inhibition of first order interneurons.…”

“…17 The dorsal reticulospinal tract suppressed transmission of Ib inhibitory pathways and FRA pathways. 17 The inhibition of FRA pathways was caused by the inhibition of first order interneurons. 18 In contrast, the ventral reticulospinal tract described in this study possibly inhibited FRA pathways at the level of the last order interneurons and excited a group of Ib inhibitory interneurons.…”

Stretch Hyperreflexia of Triceps Surae Muscles in the Conscious Cat after Dorsolateral Spinal Lesions

“…Also, ratings of resting extensor tone and resistance of the hindlimb to flexion revealed an ipsilateral hypertonia and were identified as the best combination of subjective tests for the DLF lesion effect. Thus, the functional effects of the T13-L3 spinal lesions seem to result from interruption of reticulospinal pathways that course through the dorsolateral funiculus (Nathan and Smith, 1955;Aggelopoulos et al, 1966;Engberg et al, 1968;Peterson et al, 1975;Jeneskog and Johansson, 1977;Kuypers, 1981).…”

“…The descending reticulospinal pathways in the DLF have been considered to be inhibitory for both muscular and cutaneous reflexes (Holmqvist and Lundberg, 1959;Sandkuhler et al., 1987; Pubols et al, 1991), although disinhibitory mechanisms have also been shown for group Ib and slower afferents (Engberg et al, 1968;Grillner, 1970;Iles et al, 1989). After interruption of DLF axons, descending modulatory influences from pathways in the ventrolateral funiculus are expected to predominate, and these have been characterized as both facilitatory and inhibitory (Jankowska et al, 1968;Kuypers, 1981;White et al, 1991;Brown, 1994;Liu et al, 1995).…”