Simultaneous Assessment of Homonymous and Heteronymous Monosynaptic Reflex Excitability in the Adult Rat

In order to successfully perform motor tasks such as locomotion, the central nervous system must coordinate contractions of antagonistic and synergistic muscles across multiple joints. This coordination is largely dependent upon the function of proprioceptive afferents (PAs), which make monosynaptic connections with homonymous motoneurons. Homonymous pathways have been well studied in both health and disease but their collateral fibers projecting to heteronymous, synergistic muscles receive relatively less attention. This is surprising given that PA collaterals have significant effects on the excitability of heteronymous motoneurons, and that their synaptic terminal density is activity dependent. It is likely that the relative lack of literature is due to the lack of a preparation which allows synergistic heteronymous pathways to be assessed in vivo. Here, we describe a method to simultaneously evoke homonymous and heteronymous (synergistic) monosynaptic reflexes (MSRs) and study their modulation by descending pathways in adult rats. Through stimulation of the medial plantar nerve, we were able to produce an H reflex in the intrinsic foot (IF) muscles of the hind paw with a latency of 10.52 ± 3.8 ms. Increasing the stimulus intensity evoked a robust signal with a monosynaptic latency (11.32 ± 0.35 ms), recorded in the ipsilateral gastrocnemius (Gs). Our subsequent analyses suggest that Gs motoneurons were activated via heteronymous afferent collaterals from the medial plantar nerve. These reflexes could be evoked bilaterally and were modulated by conditioning stimuli to the cortex (Cx) and reticular formation. Interestingly, cortical stimulation was equally efficient at modulating both ipsilateral and contralateral reflexes, indicating that cortical modulation of lumbar sensory afferents lacks the laterality demonstrated by studies of cortical muscle activation. This technique represents a novel, relatively simple way to assess heteronymous afferent pathways in normal motor control as well as in models of motor disorders where adaptive and maladaptive plasticity of PAs and descending systems affects functional outcomes.