Differential modulation of primary afferent depolarization of segmental and ascending intraspinal collaterals of single muscle afferents in the cat spinal cord

Abstract

We examined primary afferent depolarization (PAD) in the anesthetized cat elicited in 109 pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pair ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. Tests for refractoriness were made to assess whether the responses produced by intraspinal stimulation in the L3 and L6 segments were due to activation of collaterals of the same afferent fiber. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. In most fibers, stimulation of the ipsilateral posterior biceps and semitendinosus (PBSt) nerve with trains of pulses maximal for group I afferents had a qualitatively similar effect but produced a larger PAD in the L6 than in the L3 collaterals. Stimulation of cutaneous nerves (sural and superficial peroneus) with single pulses and of the posterior articular nerve, the ipsilateral reticular formation, nucleus raphe magnus and contralateral motor cortex with trains of pulses often had qualitatively different effects. They could produce PAD and/or facilitate the PBSt-induced PAD in one collateral, and produce PAH and/or inhibit the PAD in the other collateral. These patterns could be changed in a differential manner by sensory or supraspinal conditioning stimulation. In summary, the present investigation suggests that the segmental and ascending collaterals of individual afferents are not fixed routes for information transmission, but parts of dynamic systems in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs and funneled to specific targets according to the motor tasks to be performed.

Mechanosensory Gating of Proprioceptor Input to Modulatory Projection Neurons

“…For example, there might have been a suppression of GPR spike propagation through the STG, so that the GPR spikes could not reach the CoGs. Focal regulation of sensory spike propagation within central ganglia is well documented in the molluscan nervous system (Evans et al, 2003;Frost et al, 2003) and also appears to occur in the cat spinal cord (Lomeli et al, 1998;Rudomin et al, 2004). However, our results indicate that the VCN gating of GPR actions in the CoGs Figure 7.…”

Motor Cortical Modulation of Cutaneous Reflex Responses in the Hindlimb of the Intact Cat

“…The difference in the temporal profile of the responses argues against the spatial facilitation being a simple function of nonlinear motoneuronal properties (Brownstone et al 1994) as this might be expected to affect convergent inputs to a given muscle in a similar manner; clearly, this was not always the case. Last, there is also the possibility that the modulation of the cutaneous reflexes might be influenced via cortical modulation of presynaptic afferent depolarization (PAD) (Andersen et al 1964;Fetz 1968;Lundberg 1964;Rudomin et al 2004). Such modulation may be particularly important in explaining the depression of the responses observed with small C-T intervals.…”

Altered activation patterns by triceps surae stretch reflex pathways in acute and chronic spinal cord injury

“…After spinalization, collaterals from the same muscle afferent can be differentially regulated by other segmental inputs (Rudomin et al 2004a). Changes in presynaptic regulation of TS muscle afferents could explain why the same muscle stretch fails to activate some muscles after spinalization that were strongly activated in the intact state (e.g., Sol and LG) while recruiting muscles that were inactive before spinalization (e.g., St and Srt).…”