Synaptic coupling of single cutaneous afferents of the median nerve with ankle muscles, and their discharge characteristics to rapid light touch displacements

• Author / Creator

  Toth, Aidan L

• Lightly touching a stable reference can greatly improve balance. Unexpected displacement of a light touch reference at the finger can lead to rapid activation of ankle muscles in standing participants and transcutaneous electrical stimulation of the median nerve generates short latency reflexes in ankle muscles. Together, this evidence suggests that cutaneous afferents in the hand are functionally coupled with the ankle muscles and might be involved in generating these interlimb balance responses, however the specific afferents serving these responses is unknown. For this thesis I wished to delineate the specific cutaneous afferents from the hand that mediate these interlimb responses. Specifically, it was hypothesized that activity of individual cutaneous afferents of the median nerve innervating the hand will modulate the electromyographic (EMG) activity of ankle muscles at short (<120 ms) latencies. It was also hypothesized that type I cutaneous afferents of the hand will show a distinct increase in firing frequency during a rapid displacement of a light touch contact across their receptive field. To investigate these hypotheses, this study was performed in two parts. In all experiments, microneurography of the median nerve at the level of the wrist was used to record discretely resolved fast and slow adapting type I and II (FAI, FAII, SAI, and SAII) afferents from 18 and 9 participants in Part one and two of the study respectively. In Part one, the discharge from discretely resolved afferents was used for generating spike-triggered average surface EMG traces of the bilateral tibialis anterior (TA) and soleus (SOL) muscles. Responses were identified when the averaged EMG trace exceeded a 99% confidence band, calculated from the background EMG activity, for a minimum of ≥ 2.5 ms. In Part two, twenty alternating touch contact displacements (1.25 cm displacement, 120 ms duration), ten away from the participant and ten towards, were delivered across the receptive fields of the identified afferent. The firing characteristics were qualitatively compared across the afferent types. Over 55 experiments, 53 single afferents were recorded and used for subsequent analysis, 42 in Part one and 11 in Part two. From Part one, 34/42 (81%) of the individual cutaneous afferents recorded produced demonstrable responses in the spike-triggered average EMG trace at latencies <120 ms. All afferent types demonstrated responses in both TA and SOL, with a bias of observed responses towards the early latency epoch (40-80 ms) in TA. In Part two, 11 individual cutaneous afferents were recorded, and our second hypothesis was partially supported as we demonstrated that type I and II afferents in the glabrous skin of the hand become active and remain more active than baseline throughout the rapid displacement of a light touch contact across its receptive field. These results suggest that cutaneous afferents in the hand consistently form interlimb connections with muscles of the lower limb at latencies implicating spinal-mediated pathways, and the same population of afferents are readily excited by a rapid displacement of a light touch contact. Therefore, cutaneous afferents in the hand can be recruited and utilized to alter motoneuron pool excitability in muscles important to balance control, at latencies relevant for balance corrective responses.

• Subjects / Keywords

  • Cutaneous afferents
  • Balance control
  • Light touch
  • Microneurography
  • Interlimb

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-xyja-6642

• License

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