SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 



939 



FIG. 12. Monosynaptic reflex response obtained by a single 

 shock stimulation of, and by recording from, the tibial nerve. 

 Following break in the recording there is seen in A the tail of 

 the action potential conducted directly from stimulating to 

 recording leads and, approximately 4 msec, later, the mono- 

 synaptic reflex volley. In B the reflex volley is lost following 

 severance of appropriate dorsal roots. [From Lloyd (59).] 



FIG. 13. Proof of stretch origin of monosynaptic reflex. A. 

 Afferent response evoked by stretch of gastrocnemius and re- 

 corded from the first sacral dorsal root at a gi\en point. B. 

 Segmental monosynaptic reflex evoked by stimulation at that 

 same point on the dorsal root and recorded from the first sacral 

 \entral root at a given point. C Reflex response evoked by 

 stretch of gastrocnemius recorded from that point on the ven- 

 tral root. The sum of the latencies in A and B approximates 

 that in C, hence response C is monosynaptic. [From Lloyd (59).] 



to realize that the natural reflex operates under 

 closed circuit conditions permitting 'feedback' (32), 

 whereas the monosynaptic reflex is usually observed 

 under open loop conditions. 



Many studies have been made of tendon jerk and 

 myotatic reflex latency (cf. 52, 55) and its brevity 

 has been conceded generally. It was often thought of 

 as monosynaptic; and indeed on a basis of latency, 

 the tendon jerk was even thought to be an idiomuscu- 

 lar reaction dependent only upon muscle tone which 

 was in turn dependent upon intact reflex connections. 

 Not until exact measurements of synaptic delay (71) 

 and of conduction time were availaijle was it possible 

 to prove the exact manner of central connection. 



Monosynaptic Reflex Relations Between Synergists 



As has been noted, the field of action of a mono- 

 synaptic reflex is limited by anatomical limitation of 

 monosynaptic connection established by any given 

 afferent fiber source. As a generality, there is no 



monosynaptic excitatory connection between muscles 

 that act upon different joints. Thus quadriceps and 

 gastrocnemius, or biceps and tibialis anterior, are 

 without monosynaptic reflex interconnection (50). 

 There is, however, between two muscles or two frac- 

 tions, or heads, of a muscle that act in concert upon a 

 given joint, established monosynaptic reflex connec- 

 tion. In usual circumstances monosynaptic reflex 

 afTerent impulses from one muscle or head of such a 

 pair facilitate action by the motoneurons of the other 

 (cf. fig. 14) but do not discharge those motoneurons. 

 The difference between homonymous connections 

 (those between the afferent fibers and motoneurons of 

 a single muscle) that transmit readily and heterony- 

 mous connections (those between synergists) that do 

 not transmit readily are essentially quantitative (64, 

 68) and appear to result from differences in number 

 and aggregation of individual active synaptic contacts 

 (knobs) (38). 



Heteronymous monosynaptic reflex transmission 

 will take place to a small degree in several experi- 

 mental circumstances: during the period of increased 

 response following a high frequency stimulation called 

 posttetanic potentiation (fig. 15), during repetitive 

 stimulation at frequencies between approximately 60 

 and 1 50 or more per sec . ( i ) , if a high degree of ' back- 

 ground activity' is present as in well-developed de- 

 cerebrate rigidity (i) or in long spinal reflex action 

 (67, 68), and in the chilled preparation (64). 



The muscles bound together by excitatory mono- 

 synaptic connection in common action at a joint con- 

 stitute a synergic unit; thus the parts of the triceps 

 surae (soleus, inedial and lateral gastrocnemius) form 

 such a unit as do the biceps femoris posterior and 

 semitendinosus (fig. 14). 



Monosynaptic Reflex Relations Between Antagonists 



Monosynaptic reflex action of afferent fibers upon 

 motoneurons is not limited to excitation. There are 

 inhibitory connections (57, 58) and, as in the case of 

 excitation, the field of inhibitory monosynaptic action 

 is limited by anatomical limitation of connection. The 

 monosynaptic reflex afferent fibers from a muscle of 

 a given joint inhibit action by the motoneurons of its 

 direct antagonist at that joint but not of those acting 

 at other joints (fig. 16). Thus the tibialis anterior and 

 triceps, flexor and extensor of the ankle, respectively, 

 are linked by inhibitory interconnection as are the 

 quadriceps and biceps of the knee, whereas the mus- 

 cles of the ankle and of the knee are independent in 

 inhibitory monosynaptic action as in excitatory. 



