SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 



943 



FIG. ig. Conditioning of flexor 

 and extensor motoneurons by 

 Group II afferent volleys. .-1. Fa- 

 cilitation of knee-flexor monosyn- 

 aptic reflexes by afferent volleys 

 from the deep peroneal nerve. B. 

 Inhibition of fle.xor longus (a 

 physiological extensor) monosyn- 

 aptic reflexes by afferent \olleys 

 from tibialis posterior. In neither 

 instance does Group I, monosyn- 

 aptic or disynaptic, reflex condi- 

 tioning occur. [From Lloyd (59).] 



FIG. 20. Lejl. Effect of stretch-evoked afferent discharge from the gastrocnemius upon the flexor 

 hamstring monosynaptic reflex response. Upper records contain the reflex responses; lower records 

 measure the tension in the gastrocnemius. A. Control reflex response. B. Facilitated reflex response 

 during stretch which is signalled by rise in the lower record. Time, i msec. Right. Effect upon flexor 

 hamstring moncsynaptic reflex test of gastrocnemius muscle spindle afferent activity provoked by 

 tetanic stimulation of 20 isolated fusimotor fibers (small motor fibers or gamma efferents) supplying 

 intrafusal muscle fibers of the gastrocnemius. A. Control response. B. Facilitated response, the small 

 deflections indicating stimuli to fusimotor fibers. Time, 2 msec. [Unpublished records kindly provided 

 by Hunt (cf. 36).] 



volleys) but for the most part with central delay occa- 

 sioned by the presence of interneurons in the pathway. 

 An important step in establishing the secondary 

 endings of muscle spindles, and the Group II fibers 

 afferent from spindles, as the origin of a true stretch 

 flexor reflex is to show that natural stimulation, as 

 by stretch or contraction of the intrafusal muscle 

 fibers, can lead to a flexor reflex result (36). Figure 20 

 on the left shows that mild stretch of the gastrocnemius 

 facilitates a monosynaptic reflex of the fle.xor ham- 



strings, and on the right shows the same result on 

 contraction of intrafusal muscle fibers in gastroc- 

 nemius. It is, of course, essential that the conditioning 

 and test muscles be selected for known absence of 

 Group I reflex interconnection since the means em- 

 ployed for natural stimulation are adequate also for 

 activation of the Group I fibers. 



In parallel with excitation of flexors inhibition of 

 extensors takes place; in short, the total pattern of a 

 flexor reflex is thrown into action. This is true whether 



