Evidence for restricted central convergence of cutaneous afferents on an excitatory reflex pathway to medial gastrocnemius motoneurons

Abstract

1. We previously reported that excitatory postsynaptic potentials (EPSPs) produced by low-threshold electrical stimulation of the caudal cutaneous sural nerve (CCS) occur preferentially and with the shortest central latencies in the medial gastrocnemius (MG) portion of the triceps surae motor nuclei. The present study employs the spatial facilitation technique to assess interneuronal convergence on the short-latency excitatory pathway from CCS to MG by several other ipsilateral hindlimb afferents [the lateral cutaneous sural (LCS), caudal cutaneous femoral (CCF), saphenous (SAPH), superficial peroneal (SP), posterior tibial (TIB), and posterior articular (Joint) nerves]. 2. Spatial facilitation of CCF EPSPs in MG motoneurons was demonstrated with conditioning stimulation of the LCS, CCF, SAPH, SP, and TIB nerves, but was most readily and consistently observed with CCF conditioning. Facilitation of CCS and CCF EPSPs was obtained in individual MG motoneurons with a wide range of condition-test intervals. 3. CCF EPSPs in MG motoneurons produced by twice threshold (2T) afferent stimulation had a mean latency of 4.8 ms and often appeared as slowly rising, asynchronous potentials. On the other hand, 2T CCS EPSPs had a mean latency of 2.8 ms and appeared as sharper rising, less variable depolarizations. The optimum condition-test interval for facilitation of CCS and CCF EPSPs was found to be 5.2 ms on average, with CCS stimulation delayed from that of CCF. The longer latency of CCF EPSPs and the finding that the minimum condition-test interval was on the order of 3.9 ms suggests that convergence occurs late in the excitatory CCF pathway to MG motoneurons. 4. Convergence between excitatory pathways to MG from CCF and CCS afferents is discussed with regard to the original observations of Hagbarth on the location of cutaneous receptive fields and excitation of ankle extensors. In addition, evidence for the segregation of these specialized reflex pathways from those involved in general flexion reflexes is discussed.

Connexin36 identified at morphologically mixed chemical/electrical synapses on trigeminal motoneurons and at primary afferent terminals on spinal cord neurons in adult mouse and rat

“…In this respect, it is well known that in the lumbar spinal cord, individual Ia spindle afferents make multiple monosynaptic connections throughout the motor pool (Brown and Fyffe 1978), while group II spindle afferents make monosynaptic connections with smaller EPSPs and in only a portion of the motoneuron pool (Sypert et al , 1980). Because motoneuron excitation from tendon organ afferents (Eccles et al, 1957) and cutaneous afferents (see Labella and McCrea 1990) occurs at longer than monosynaptic latencies, these afferents do not form monosynaptic connections to motoneurons and therefore, do not form mixed synapses in lamina IX. Thus, the majority of mixed Cx36/vglut1 synapses at least in lamina IX are likely terminals of monosynaptic connections between Ia muscle spindle afferents and motoneurons.…”

Step Training-Dependent Plasticity in Spinal Cutaneous Pathways

“…However, a complete investigation of motoneuronal properties after chronic spinalization and step training is clearly needed to understand their contribution to the plasticity of sensory transmission. Latencies of cutaneous responses in hindlimb motoneurons are minimally trisynaptic (Lundberg et al, 1977;Baker and Chandler, 1987b;Fleshman et al, 1988;LaBella et al, 1989;LaBella and McCrea, 1990), and such linkage is appropriate for R1 responses in this study. There is an exceptional disynaptic linkage between SP and FDL motoneurons during the depolarized phase of fictive stepping in decerebrate cats (cf.…”

Sensory-evoked perturbations of locomotor activity by sparse sensory input: a computational study

“…In addition to the many different sensory modalities present, sensorimotor pathways can be functionally synergistic or opposed based upon the location of the sensory input and/or the sensory modality conveyed (e.g., cutaneous afferents from skin overlying knee extensor vs. cutaneous afferents from skin overlying knee flexors ;Hagbarth 1952). Thus functionally related sensory inputs are more likely to summate, whereas functionally unrelated sensory inputs are less likely to summate or may even negate each other (Brink et al 1983;LaBella and McCrea 1990;Perrier et al 2000;Rudomin 2009). With a sparse distribution of sensory input, there are more "channels" available for different afferent information, and thus more possibilities for summation or negation.…”