Effect of reversible dorsal cold block on the persistence of inhibition generated by spinal reflexes

Abstract

The effects of bilateral focal cooling of dorsolateral thoracic spinal cord on segmental reflex pathways to the triceps surae muscles were assessed in decerebrate cats from the reflex forces produced by single shocks or trains of electrical stimuli applied to the ipsilateral caudal cutaneous sural and the contralateral tibial nerves. The validity of the dorsal cold block technique as a substitute for acute surgical dorsal hemisection was established by showing that focal cooling reliably reproduced the stretch-induced "clasp knife" inhibition of triceps surae reflexive force seen following dorsal hemisection. Under control (warm) conditions, the inhibitory components of electrically evoked ipsilateral sural and contralateral tibial reflexes faded rapidly during sustained trains, with a resultant production of large-amplitude reflex force as measured from either the entire triceps surae or from the medial gastrocnemius muscle alone. Dorsal cold block greatly reduced the amplitude of reflexive force evoked by sustained electrical stimulation of either nerve. Indeed, the cold block completely reversed the sign of train-evoked reflexes to a net inhibition of reflex force output in one-half of the sural and one-half of the contralateral tibial stimulation experiments. Peak transient forces evoked by single shocks to the sural or contralateral tibial nerves were also sometimes reduced, but this result was more variable than for prolonged nerve stimulation. The persistence of activity in segmental inhibitory pathways during dorsal cold block, as indicated by instances of reflex sign reversal, suggests that descending bulbospinal pathways traversing the dorsolateral funiculi may be responsible for "fading" of segmental inhibitory reflex components in decerebrate cats with intact spinal cords during sustained afferent input. The possibility that the enhanced magnitude and duration of segmental inhibition during cold block will increase the likelihood of disruption of the size principle for motoneuron recruitment is also discussed.

Effects of Reversible Spinalization on Individual Spinal Neurons

“…It was suggested that PLRs in intact animals contribute substantially to postural reactions (Musienko et al, , 2010Deliagina et al, 2012). We have found that the spinal cord contains neuronal networks capable of generating PLRs in response to postural perturbations (Musienko et al, 2010).…”

“…During the experiment, the rectal temperature and mean blood pressure of the animal were continuously monitored and were kept at 37-38°C and at Ͼ90 mmHg, respectively. Recordings were started not Ͻ1 h after cessation of anesthesia, when the functional state of the preparation could be characterized by rather stable extensor tonus and pronounced PLRs (see below), which are important attributes of postural activity (Musienko et al, , 2010.…”

“…The experimental design is shown in Figure 1A. The method of induction of PLRs was similar to that described previously (Musienko et al, 2010;Hsu et al, 2012). In short, the hindlimbs of the rabbit were positioned on the horizontal platform, with limb configuration and the interfeet distance similar to that observed in freely standing rabbit.…”

“…In a recent study on decerebrate rabbits (Musienko et al, 2010), we characterized postural limb reflexes (PLRs). It was suggested that PLRs in intact animals contribute substantially to postural reactions (Musienko et al, , 2010Deliagina et al, 2012).…”

“…Changes in transmission of afferent signals to motoneurons were also shown (Bennet et al, 2004). However, there have been very few studies devoted to the effect of SCI on the activity of individual interneurons mediating different spinal reflexes, and these studies were focused on the effect of partial SCI upon spinal interneurons responding to load receptors (Miller et al, 1995;Chen at al., 2001). The lack of data about changes in the activity of spinal interneurons caused by SCI is mainly due to technical problems: difficulties in recording activity of the same interneurons before and after SCI.…”