CHAPTER X X X \" I 



Spinal mechanisms involved in somatic activities 



DAVID P. C . L L O \' D \ The Rockefeller hutilute. New York City 



CHAPTER CONTENTS 



Introduction 



Constitution of Afferent Paths 



Peripheral Origins of Afferent Fibers 



Muscle Afferent Fibers 



Cutaneous Afferent Fibers 

 Constitution of Motor Paths 

 Spinal Liaison Between Afferent and Motor Pathways 



Distribution and Properties of Afferent Collaterals in the 

 Spinal Cord 



Circumscribed and Diffuse Mechanisms of Ramon y Cajal 



Divergence and Convergence 



Local Sign in Reflex Action 



After-Discharge 



Multiple and Closed Internimcial Chains 



Temporal Characteristics of Action Through Internuncial 

 Chains 

 Reflex Action of Muscular Origin 



Monosynaptic Myotatic Reflex 



Monosynaptic Reflex Relations Between Synergists 



Monosynaptic Reflex Relations Between Antagonists 



Concept of Myotatic Unit 



Lengthening Reaction or Inverse Myotatic Reflex 



Stretch Flexor Reflex 



Group III Flexor Reflexes 

 Reflex Action of Cutaneous Origin 



Low and High Threshold Flexor Reflexes 



Special Effects From Specific Regions 



Crossed Reflexes of Cutaneous Origin 



Reflex Effects of Unmyelinated Afferent Fibers 

 Spinal Organization for Reflex Control of Mid-line Structures 



INTRODUCTION 



ACTION MEDIATED BY THE SPINAL CORD differs de- 

 pending upon what structures craniad to it retain 

 functional continuity with it, and also to a degree 

 depending upon the length of time it may have been 

 isolated from these structures. Thus the decerebrate 

 preparation displays prominently, indeed in exag- 



gerated form, certain reactions that can be elicited 

 only with difficulty, in fractional form, or not at all 

 in the spinal preparation. But let the spinal prepara- 

 tion progress from the 'acute' to the 'chronic' status 

 and certain reactions, previously submerged, are 

 elicitable (76). Clearly the machinery was there yet 

 inoperative in the acute phase. Still other reactions 

 are more readily obtained in the acute spinal state 

 than in the decerebrate, indicating that continuity 

 with the brain stem brings to the spinal cord not 

 only supportive influence but, perhaps equally, 

 supressive influence. Submissive as the spinal cord 

 is to higher centers it is nonetheless an organ in its 

 own right; it contains the essential mechanisms for 

 the reflex actions it mediates. Realization of this 

 fact has increased immeasurably since the time when 

 reflex action could be judged largely only by its ex- 

 ternal expression in muscle contraction. Today with 

 the aid of delicate tests for excitability change in 

 any motor nucleus of choice, one finds revealed 

 the potentiality for action as well as action itself — ex- 

 pression, in suisliminal form, of action that would 

 take place were conditions appropriate. 



Once the spinal cord has been isolated from supra- 

 spinal influence, specifically that of the brain, the 

 power to initiate movement is lost to the animal 

 and such movements as it executes are relatively 

 stereotyped, dependent upon presentation of change 

 in the external environment, and are in character 

 related to the nature of that change. Mindful that 

 the entities called reflexes normally are probably 

 inextricably blended in the performance of the 

 animal and so in a sense are artificial entities, one 

 can remove each from its functional context for 

 analysis and classify it. What then are some of these 

 reflex entities? If one attempts, in the decerebrate 



929 



