Dam80

APs were recorded extracellularly using a low-noise AC differential amplifier (DAM 80, World Precision Instruments, Sarasota, FL). The activity was monitored on-line, filtered (0.3 to 10 kHz), amplified (x10,000), digitized at 20 kHz using a 1401 interface (CED, Cambridge, UK), and stored on a PC. APs were discriminated off-line using Spike 2 software (CED). The root-mean-square (RMS) value of pre-stimulus noise was calculated and 5 times that value was set as the detection threshold for AP spikes. The time at which the stimulus first exceeded the threshold was deemed as the onset of the AP. To avoid erroneous discrimination, we only studied single-unit APs temporarily separated from other neural activities by at least 3 msec in any record. CDs were measured as the time between the onset of stimulus artifact and the onset of recorded APs. CV was computed from the CD and the distance between stimulating and recording electrodes. Axial stretch was quantified by stretch ratio λ (i.e., the deformed nerve length divided by the in vitro nerve length at the zero-stretch state). The zero-stretch nerve length was measured as the distance between the proximal and distal ligatures (Fig. 7A). Data were presented as means ± standard error (SE). Statistical analysis was performed using SigmaStat v4.0 (Systat Software, San Jose, CA). Differences were considered significant when p < 0.05.

In an additional session, we tranquilized each monkey with ketamine (5–10 mg/kg IM) and delivered trains of 12 biphasic, cathodal-first pulses, 200 μs per phase, at 333 Hz and currents up to 100 μA, using a biphasic pulse generator and stimulus isolator (BPG-1 and BSI-1, BAK Electronics, Inc., Umatilla, FL). Currents were monitored by measuring the voltage drop across a 100 Ω in-series resistor with a high-impedance amplifier (DAM80, World Precision Instruments, Sarasota, FL). As trains were generated by a micro1401 interface (Cambridge Electronic Design, Cambridge, UK) at random intervals between 1 and 4 seconds, we observed and palpated the animal’s face, upper extremity, trunk, and lower extremity for evoked twitches.

NCV from mice tails were measured in a lab-built instrument inside a grounded Faraday cage. Mice were anesthetized using a constant flow of isoflurane. The external body temperature was maintained at 37 °C with a chargeable warm heating pad (Kent Scientific Inc). The body temperature was continuously monitored with a thermal camera (Micro-Epsilon). Tail skin and stainless steel recording electrodes (~29 ga, World Precision Instruments) were sanitized with 70% isopropanol. The recording electrodes were inserted 1 cm distal to the base of the tail. The nerve compound action potential was measured using an extracellular amplifier (DAM80, World Precision Instruments). Stimulating electrodes were inserted approximately 30 mm distal to the recording electrodes. Voltage pulses (2–5 msec duration, 0.6–4 V) were delivered to the stimulating electrodes from a battery powered stimulus isolator (Model 2200, A-M Systems). Recorded data from the extracellular amplifier and from the stimulator were collected and analyzed with PowerLab 8/35 (ADInstruments Inc.) and used to calculate nerve conduction velocity of the compound action potential after completion of the experiment. Mice were returned to their home cages for recovery.