ORGANIZATION OF STRETCH REFLEX INTO TWO TYPES OF DIRECT SPINAL ARCS

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Abstract
Cats (22) were decere-brated and certain aspects of the spinal organization of stretch reflex pathways were studied. A muscle strip about 2mm in diameter and situated in quadriceps femoris was stretched 2-3 mm. Responses to stretch of the individual strip and patellar reflexes were simultaneously recorded by a modified version of a method which was previously reported. Stretch reflex activity was altered by electronic stimulation of either the ipsilateral or contralateral sciatic nerves. Data from each preparation were arranged in order of increasing activity, using the magnitude of strip tension as the criterion for stretch reflex activity. In all preparations strip responses appeared by themselves at lowest levels of activity. After strip tensions were increased considerably, patellar responses began to appear. As strip tensions continued to rise, patellar responses showed an over-all increase in frequency of appearance and in amplitude. Special experiments are described, including electrical stimulation of muscle bands and recording of reflex tension in neighboring strips, which demonstrate conclusively that no patellar responses occurred at low activity. Furthermore, the data show that when strip and patellar reflexes occurred together, tensions were disproportionate and often varied independently. Consequently, the patellar responses must have been separate reflex responses distinct from the strip reflexes. It is concluded, therefore, that there are 2 types of direct spinal pathways involved in mediation of the stretch reflex; low threshold paths which activate only the strips which were stretched, and high threshold paths which innervate extra-strip musculature. The desirability of such an arrangement in producing either discrete adjustments of individual strips, or strong over-all contractions of the entire muscle is discussed.

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