Reticulospinal neurons with and without monosynaptic inputs from cerebellar nuclei

Abstract

An account is given of the responses of 557 medial reticular neurons with axons projecting down the spinal cord. All 30 experiments were on decerebrated unanesthetized cats paralyzed by Flaxedil. Recording from single neurons was by extracellular glass microelectrodes. Identification was first by location (confirmed by subsequent histology) in the medial reticular nucleus of medulla or pons, and second by antidromic activation from cord stimulation at C2 and L2 segmental levels. Axonal conduction velocities were calculated from the latency differential between L2 and C2 antidromic responses, and were usually in the range of 90-140 m/s; but about 25% were slower, ranging down to 30 m/s. Stimulation by electrodes in the ipsilateral and contralateral fastigial nuclei differentiated reticulospinal neurons into two classes according to whether they did or did not receive monosynaptic inputs, the respective populations of fully investigated neurons being 270 and 174. The fastigioreticular neurons were distinguished by a higher background frequency with mean values of 28 as against 15/s. There were also significant diffences in both the excitatory and inhibitory responses to afferent volleys from forelimb and hindlimb nerves. Comparison of the respective latency histograms showed that the responses of neurons with a fastigial input had an excess of latencies in the ranges that can be correlated with the latency histograms observed for fastigial responses. Thus, there is evidence for the effectiveness of the fastigial input and so for the pathway with monosynaptic linkage: Purkinje cells of cerebellar vermis yields fastigial neurons yields medial reticular neurons projecting down the spinal cord. Adequate stimulation of cutaneous receptors by pad taps and air-jet stimulation of hairy skin in a disppointingly small action when compared with fastigical responses. Explanations of this deficiency are suggested. Another discrpancy from the fastigial responses is that the medial reticular neurons have much wider receptive fields with little discrimination between ipsilateral and contralateral and between forelimb and hindlimb. Stimulation of the ipsilateral tegmental tract was tested on 183 reticulospinal neurons, 112 being with fastigial inputs. In about half there was a powerful monosynaptic excitation, which would identify such neurons as being on the pathway from mesencephalic and diencephalic centers to the spinal cord. There is a general discussion of transmission across successive synaptic relays, where specificity is sacrificed to integration.

Descending Signals From the Pontomedullary Reticular Formation Are Bilateral, Asymmetric, and Gated During Reaching Movements in the Cat

“…It is possible therefore that this signal should be seen as one providing inhibitory sculpting of the activity in the ipsilateral extensors. This would be compatible with one of the suggested functions for the cerebellum (Armstrong and Edgley 1984; Eccles et al 1967), which, via the fastigial nucleus, provides strong monosynaptic input to RSNs in the PMRF (Eccles et al 1975;Mori et al 2000). It is possible that this functional subset corresponds to the population of RSNs described by Takakusaki et al (1989Takakusaki et al ( , 2001Takakusaki et al ( , 2003 in the decerebrate cat as producing powerful inhibition of extensor muscle activity.…”

Why are voluntary head movements in cervical dystonia slow?

“…Inhibitory reticulospinal bursts neurons receive projections from the deep cerebellar nuclei [20, 21]. The first possibility, i.e., the premature excitation of the antagonistic, inhibitory burst neuron, might be attributed to an disturbed cerebellar signal [22].…”

Neuronal Basis of Crossed Actions from the Reticular Formation on Feline Hindlimb Motoneurons

“…This factor was not of major consequence for ipsilateral actions of reticulospinal fibers in view of the high conduction velocities of these fibers (90 -140 m/sec; Shapovalov, 1969;Grillner et al, 1971;Eccles et al, 1975) and because their axon collaterals terminate within fairly restricted areas (Matsuyama et al, 1997). Crossing axon collaterals of reticulospinal tract fibers (Fig.…”