93° 



HANDBOOK OF PHVSIOLOOV 



NEUROPHYSIOLOGY II 



animal, to force the rigidly extended limb into a 

 flexor posture, the limb resists the applied force by 

 active muscular contraction. This in essence is the 

 stretch or myotatic reflex (52). Upon increasing the 

 force to a degree that might be supposed potentially 

 harmful, the limb may suddenly give way and as- 

 sume a new posture more in flexion. This happening 

 is called the lengthening reaction (93), or inverse 

 myotatic reflex (50). As the limb gives way, the 

 limb opposite may increase its extensor posture, a 

 reaction discovered by Philippson (79) and named 

 after him — Philippson's reflex. If some part of a 

 limb comes into contact with a source of hurt, that 

 limb rapidly is removed from the source by a reflex 

 of flexion, general in character, involving the several 

 joints of the limb. Associated with this ipsilateral 

 flexor reflex, and set in action by the same sorts of 

 stimulation, is the crossed extensor reflex (92). A 

 flexor stretch reflex is known and in operation plays 

 a role in walking movements. There are others, the 

 scratch reflex, hand-foot reactions and so on; but 

 it is sufficient when concern is with principles to 

 concentrate attention upon a few, the spinal mecha- 

 nisms for which are reasonably well understood. 



The reflex arc consists of afferent fibers that bring 

 to the spinal cord information as to the environment, 

 internal as well as external, and that reach the spinal 

 cord by way of the dorsal roots; of motor fibers, 

 emergent through the ventral roots, that in action 

 provoke muscular contraction and movement; and 

 of a mediate system of interneurons arranged in 

 patterns, largely unknown, but certainly of varying 

 complexity. The mediate system is engaged by all 

 but one of the known spinal reflex reactions, that 

 unique exception bring the stretch reflex (54, 55). 



Entering into the spinal mechanism from the 

 periphery are afferent fibers the action of which is 

 destined to result in the relay of information to the 

 higher centers. To what extent these are the same 

 fibers as those which feed the local reflex mechanism 

 is not reliably known. There are in turn descending 

 fibers entering into the spinal mechanism from higher 

 centers serving all manner of supraspinal influence 

 over the final product of neural activity, motion. 

 Thus the spinal cord is not only an organ in its own 

 right but is also a major highway of liaison between 

 the brain and the outer world. 



CONSTITUTION OF AFFERENT PATHS 



Dorsal roots contain myelinated fibers of all 

 diameters from the largest, approximately 22 ix, to 



10 12 14 16 18 20 /J. 



FIG. 1. Distribution with respect to diameter of the afferent 

 fibers in a "demotored' muscle nerve (heavy line) and in a 

 cutaneous nerve ifainl line-shaded area). The pile of fibers 

 centered at 1 7 M, unique to muscle nerves, is termed Group I. 

 Group II includes the pile of fibers centered about the 8 m peak, 

 and Group III those about the 3 m peak. [From Lloyd (60).] 



the smallest, approximately i /i. There are in addi- 

 tion unmyelinated fibers in profusion. A distinction 

 of great functional import is seen if one considers 

 not the entire gamut of afferent fibers gathered to- 

 gether in a dorsal root, but rather such aggregations 

 of afferent fibers as appear severally in nerves to 

 muscles and nerves to skin. 



Figure i illustrates the distribution with respect to 

 diameter of myelinated afferent fibers in a typical 

 'demotored' muscle nerve (63) and a typical skin 

 nerve (29). There are three peaks of numerical 

 preponderance; one, located at approximately 17 

 /i, consists of a group of fibers unique to the muscle 

 nerves. The other peaks, at approximately 8 m and 3 

 H are present in both muscle and skin nerves. In 

 order of decreasing magnitude the fibers gathered 

 about these three peaks are designated, respectively. 

 Group I, Group II and Group III (54, 63). Among 

 skin afferent fibers those designated Group II corre- 

 spond essentially to the alpha and beta fibers and 

 Group III to the delta fibers of earlier description 

 (24). The present terminology, yielding in priority 

 with respect to skin nerves, is the more convenient 

 when reflex action is the subject under review. 



Afferent fillers from joints play an important role 

 in somatic activities, although they have not been 

 studied as intensively as have the other sorts. They 

 are distributed with respect to diameter in much the 

 same way as are skin afferent fibers. 



