Peripheral inflammation alters the excitability of dorsal horn interneurons and increases flexor reflex strength (Dubner & Ruda, 1992); however, its effect on the spinal stretch reflex is not well understood. The stretch reflex is a muscle contraction in response to muscle stretch. We hypothesize that the acute inflammation caused by an injection of lipopolysaccharide (LPS) will cause an increase in spinal cord excitability. To test this hypothesis, we measured Hoffman’s (H) reflex, the electric analog of the stretch reflex in adult mice receiving an injection of LPS (.5mg/kg) or saline (200μl). Adult male and female mice (C57Bl/6) were anesthetized; then, the sciatic nerve was exposed and stimulated at current strengths from H-wave threshold (T) to 8T (20 x 0.1 ms pulses at 0.1 Hz). Recording electrodes were placed in the foot. We measured the maximum M wave amplitude (Mmax), maximum H wave amplitude (Hmax) and latencies of both waves. We compared the ratio of the maximal H wave over the maximal M wave (Hmax/Mmax), which reports the percentage of motor neurons activated by electrical stimulation of Group Ia muscle sensory neurons. Increased spinal cord excitability would be reflected in a larger Hmax/Mmax. We found that LPS-induced inflammation does not alter the Hmax/Mmax. While we found no evidence of changes in spinal cord excitability, inflammation could be altering Group Ia muscle spindle afferent responses to stretch. Future studies will test whether stretch reflex strength is altered by inflammation.



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.