THE GENERAL PRINCIPLES OF MOTOR INTEGRATION 



/OD 



Stretch Reflex 



Part of the fundamental proof of the stretch reflex 

 (63) was the demonstration that the afferent roots 

 pertaining to each muscle were not only necessary 

 for a stretch reflex within that muscle but for the 

 background of postural reactions in general. It is 

 possible, as Ranson & Hinsey (80) showed, to induce 

 some extension of the deafTerented forelimbs of the 

 cat by putting the decerebrated animal in the posture 

 for maximal tonic labyrinthine and neck reflex but 

 onh after the device of section of the cord below the 

 cervical enlargement. Deaflferentation raises the 

 threshold for all other reflexes, phasic as well as 

 postural, and abolishes after-discharge. 



The receptor function of the muscle spindle will be 

 discussed in a later section. Here we may note only 

 the regulation of the spindle response by the in- 

 tensity of small-fiber motor innervation (56, 58) and 

 the uniquely monosynaptic nature of the stretch re- 

 flex response (65). The receptor has most excitatory 

 efTect in the deep short-fibered postural segments of 

 the same muscle that is stretched. Thus stretch of the 

 gastrocnemius excites chiefly soleus motor neurons; of 

 the vastus, the crureus. Stretch of the quadriceps 

 excites the soleus to some smaller extent but not vice 

 versa (20, 32). The patterns of stretch effects from 

 adductors and abductors, manifestly important in 

 postural stability, have not yet been worked out. 



The adjuvant effect of the small-fibered motor 

 innervation allows delicate facilitation of this system 

 to energize powerful stretch reflexes, and it is possible 

 that most motor and certainly postural function is 

 self-energized. Little is yet known of the reflex sources 

 by which the small-fibered motor system is thus 

 facilitated, although cutaneous tactile stimulation of 

 the foot can thus proxoke gamma-fiber facilitation 

 (56), and the pontine brain-stem mechanism can 

 influence it powerfully (33, 43). 



Two local factors tend to limit the self-energizing 

 effect of the stretch reflex, the slackening of the 

 muscle spindles when the muscle contracts and the 

 high threshold inhibitory effect (autogenetic inhibi- 

 tion) of tension on the tendon organs {42). Each 

 natural stretch reflex response is therefore an equi- 

 librium in which a balance of proprioceptive excita- 

 tion counters autogenetic inhibition. This equilibrium 

 gives plasticity to the response (20) so that an in- 

 crease in length of the inuscle is followed first by 

 an increase in its contraction, then some lessening 

 of contraction immediately the new length is reached 

 (lengthening reaction). Conversely, any sudden 

 shortening of the muscle is associated with a cessa- 



tion of discharge in many units followed bv recovery 

 of some (shortening reaction). 



Coordination of Spinal Reflexes 



Even the most simple spinal reflex shows a com- 

 plexity of organization directed to purposive in- 

 tegration of the various discharging motor units. 

 Nociceptive stimulation of the outer border of the 

 foot induces flexion with some adduction, while 

 stimulation of the inner border yields flexion with 

 some abduction. The corresponding fractionation of 

 the groups of neurons representing each of the corre- 

 sponding flexor and adductor muscles has been 

 estimated quantitatively (16). The adaptation of 

 response to locus of stimulus is attributed to the 

 inborn pattern of synaptic arrangement of the cen- 

 tral terminals of the afferents from each locus. In 

 more elaborate pattern the spinal scratch reflex in- 

 volves appropriate flexion of the hind limb with ad- 

 duction, or adduction together with curvature of the 

 spine sufficient to bring the scratching paw within a 

 few millimeters of any stimulated spot in the wide 

 .sensory field. In addition this reflex secures a tonic, 

 continued postural contraction in some muscles as 

 well as a rhythmic flexion-extension of those that 

 determine the scratching movement. Likewise re- 

 flex urination and defecation when fully developed 

 are associated with appropriate posturing of the hind 

 limbs and tail and even the terminal scratch of both 

 feet. It is therefore certain that the spinal motor 

 mechanism not only presents a predetermined re- 

 ciprocal relationship of antagonists but also all the 

 requisite combination of prime movers, synergists 

 and fixation muscles appropriate for the performance 

 of spinal reflex behavior. If stimuli with conflicting 

 eff'ects are deli\ered concurrently, the spinal mecha- 

 nism either resohes the response in favor of one at the 

 expense of the other or exhibits a rhythmic alterna- 

 tion of response. 



Sherrington's studies of the scratch reflex (89, 93) 

 included the demonstration by degeneration experi- 

 ments that the reflex pathway from the cer\ical and 

 thoracic roots to the lumbosacral seginents traversed 

 the lateral tracts lying close to the gray matter in the 

 spinal cord (propriospinal tracts). For the integrated 

 spinal response of alternate stepping of the limbs the 

 presence of the grey matter of segments lying imme- 

 diately rostral to the cell columns of the chief flexors 

 and extensors in the lumbar and cervical enlarge- 

 ment appears necessary. In these regions the studies 

 of Ramon y C^ajal (79) indicated that the interneurons 

 that go\ern such coordinated spinal responses are in 



