FUNCTIONAL-PROPERTIES OF SPINAL INTERNEURONS ACTIVATED BY MUSCULAR FREE NERVE-ENDINGS AND THEIR POTENTIAL CONTRIBUTIONS TO THE CLASP-KNIFE REFLEX

1. The goal of this study was to characterize the functional propertie s of spinal interneurons that are excited by muscular free nerve endin gs and to assess their contributions to the clasp-knife reflex. 2. The patterns of activity of 82 spinal interneurons that were excited by s queezing the Achilles tendon or manipulation of the muscle surfaces, p referential stimuli for muscular free nerve endings, were extracellula rly recorded in lamina V-VII of the L5-S1 spinal cord in decerebrated and spinalized cats. 3. Interneurons were uniformly excited by increas es in muscular length and force. Responses to muscle stretch exhibited gradual decay during maintained stretch, afterdischarge after stretch release, and adaptation to repeated stretch. Responses to isometric c ontraction induced by electrical stimulation of motor axons was also p rolonged after contraction, but did not decay during maintained contra ction. For similar increases in force, stretch evoked greater excitati on than contraction, indicating that both stretch and contraction cont ributed to interneuronal activity. Overall, the time course and magnit ude of the interneuronal responses to stretch and contraction parallel ed the time course and magnitude of the clasp-knife reflex. 4. Interne urons were powerfully excited by muscular free nerve endings, which me diate the clasp-knife reflex, and by cutaneous receptors. Only occasio nally were they excited by primary spindle or Golgi tendon organ affer ents, which suggests that activation of muscular free nerve endings me diated the interneuronal responses to stretch and contraction. 5. Simu ltaneous recordings of interneuronal activity and the clasp-knife refl ex revealed a broad correlation between interneuronal activity and cla sp-knife inhibition. 6. Because the patterns of activity of free nerve ending-responsive interneurons during stretch and contraction were si milar to the clasp-knife reflex, were closely correlated with clasp-kn ife inhibition during simultaneous interneuronal and reflex recordings , and were powerfully excited by muscular free nerve endings, it is li kely that the interneurons described above contributed to the clasp-kn ife reflex. 7. In contrast, a small number (n = 16) of interneurons we re recorded that were only weakly excited by muscular free nerve endin gs but strongly excited by group I afferents, exhibited less spontaneo us and evoked activity, and had significantly different responses to s tretch and contraction. These interneurons are less likely to have con tributed to the clasp-knife reflex.