Absence of linear correlation between fluctuations in area of simultaneous recorded monosynaptic responses and Hoffmann's reflexes in the rat

In this study we analyze the possible relationship between fluctuations in area of monosynaptic reflex responses (MSR) and Hoffmann's reflex (H reflexes) in the plantar closed loop pathway of the anesthetized rat. These reflexes were evoked by low-frequency stimuli applied to the sciatic nerve or lateral plantar nerve and then concurrently recorded on the distal tibial nerve or lateral plantar nerve, respectively as well as the lateral plantar muscles in the foot of the anesthetized rat. From trial to trial, H reflexes showed higher variability in area than MSR, whether the latter was recorded in the distal tibial nerve (n = 8 experiments) or in the lateral plantar nerve (n = 5 experiments). No linear correlation was found between changes in area of concurrently evoked MSR and H reflexes (r(MSR-H,n=8) = 0.11 ± 0.03 and r(MSR-H,n=5) = 0.08 ± 0.09, respectively). These findings suggest that trial-to-trial fluctuations in area of H reflexes may involve interaction of several sources of variation, among others to MSR variability (due to pre-, and post-synaptic factors influencing the excitability of spinal motoneurons) in combination with those related to peripheral mechanisms, such as trial to trial activation of a different number of muscle fibers, either by the probabilistic transmitter release from neuromuscular junctions, by activation of motor units of variable size or to fluctuations in excitability of muscle fibers.