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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



Sherrington grouped these lengthening and shorten- 

 ing reactions, and the stretch reflex, together with 

 the phasic knee jerk and clonus phenomena, under 

 the term "myotatic reflexes' (170). A limh doprixed 

 of myotatic reaction ijy se\erance of its dorsal roots is 

 incapable of postural contraction, although neuro- 

 muscular power may be demonstrably adequate to 

 bear the body's weight during clicitation of non- 

 myotatic reflexes [e.g. the crossed extension reflex 

 (43)]. However, intact segmental reflex arcs are not 

 all that is required for effective standing. Myotatic 

 reflexes in a spinal animal are much less impressive 

 than in the decerebrate preparation, and even months 

 after cord transection the hindquarters of a dog col- 

 lapse during standing. Needless to say, the monkey, 

 cat or dog with combined dorsal root and spinal 

 cord section manifests complete flaccid paralysis 

 (263, 264). 



It is e\ident that supraspinal contributions are es- 

 sential to cffecti\e postural contraction. These, 

 together with the afferent and efferent aspects of 

 posture, are to be considered in the following sections. 

 Only a few generalities concerning the many contribu- 

 tions of central ner\ous structures to tonic contraction 

 can be mentioned, the reader being referred to other 

 chapters for further details. 



AFFERENTS CONCERNED IN POSTURE 



Sensory fibers, if they are to figure prominently 

 in reflex posture, should ha\e the.se qualifications: a) 

 the adequate stimulus for the sensory ending should 

 be that of gravity acting upon the body parts, /)) 

 the afferent discharge should be sustained under this 

 stimulus and <) centra' connections of the afferent 

 fiber should result in facilitation of motor pools of 

 antigravity mu.scles. Sensory modalities will be dis- 

 cussed with these features in mind. 



Afferent s from Muscle 



The principal afferents in muscle are the annulo- 

 spiral and flower-spray endings of muscle spindles, 

 and the tendon organs of Golgi [cf. reviews by Granit 

 (94) and Barker (9)]. 



MUSCLE SPINDLES. True spindle organs of muscle are 

 those intramuscular receptors having specific con- 

 tractile fibers as well as sensory wrappings; they 

 are found widely in skeletal muscles of crustaceans, 

 amphibians, reptiles, birds and mammals. Almost all 



skeletal muscles may contain them, with the excep- 

 tion of facial, internal ear and infrahyoid muscles; 

 extraocular (42) and glossal (40) muscles, in which 

 they were earlier thought to be lacking, now are 

 known to contain spindles in some animals, including 

 man. Oljviously, the function of these organs is not 

 exclusively postural. 



A spindle from cat gastrocnemius mu.scle, to take a 

 t\pical example, consists of a fusiform sheath of 

 connecti\e tissue which looseh encloses 3 to 10 small 

 muscle fibers. Each of these intrafusal fibers has two 

 tapering, striated motor poles and a central, unstri- 

 ated region containing a dozen or more nuclei in a 

 nuclear bag. Presumably, the nuclear bag is not con- 

 tractile, nor perhaps even conductile, which may 

 account for the presence of one or more motor end- 

 plates on each motor pole. These motor fibers are 

 small enough (the mode being at 5 to 6 fi) to contrib- 

 ute to the gamma wa\e of a neurogram produced 

 by stimulation of a mixed nerx-e at a distance from a 

 pickup electrode. Extrafusal fibers of the gross muscle 

 are supplied with alpha-sized motoneurons (with a 

 mode of 16 m). 



Direct evidence of electromyographic potentials 

 from intrafusal fibers in the \ery thin tenuissimus 

 muscle of the cat ( 154), and the indirect evidence of 

 accelerated afferent discharge during selective stimu- 

 lation of gamma efferent fibers (165) indicate that 

 intrafusal fibers are contractile. Their small size and 

 rarity of occurrence in any cross-section of a muscle 

 belly ([ to 10 spindles in cat medial gastrocnemius) 

 have rendered futile efforts to detect contributions to 

 muscle tension b\' intrafusal fibers (94, 154, 165, 210). 

 Apparently gamma motoneurons innervate only in- 

 trafusal fibers, for selecti\-e stimulation of these 

 efferents does not directly increase postiu-al tone (210). 

 The relation to postural tone of the sensory circuit 

 through spindles will be mentioned in a later section. 



Sensory endings of two types are wrapped about the 

 indi\idual intrafusal fibers: a primary ending en- 

 circling in annulospiral fashion the nuclear bag, and 

 a secondary ending, or flower-spray terminal, one of 

 which generally clasps proximal portions (myotube 

 regions) of each motor pole. Although easily dis- 

 tinguishable upon the basis of axonal conduction 

 rates, the responses of the two types of spindle endings 

 under \arious stimuli are so similar that positive 

 identification from ijeha\ior alone is not always pos- 

 sible, fiowever, by several indices, such as the be- 

 havior during twitches and irregularity of discharge, 

 nuclear bag endings seem to be more phasic in reac- 

 tion (59). The minimal tension of the muscle at which 



