THE EXTRAPYRAMIDAL MOTOR SYSTEM 



909 



neurological symptoms as a release of mechanisms 

 from the inhibitory action of higher centers. This 

 principle of release also can be used for disturbances 

 of coordination at one level or e\'en within one struc- 

 ture. Denny-Brown (40) has used it to explain invol- 

 untary movements caused by disorders of integration 

 between the rolandic and extrarolandic cortical areas. 



Independently of Hess' concepts, Denny-Brown has 

 extended this principle to lower levels. He considers 

 athetosis and dystonia as "conflicting extremes in 

 the pattern of control of the basic organization of the 

 tegmental mechanisms by rolandic and extrarolandic 

 cortex," a "disequilibrium due to loss of one mem- 

 ber of a balanced pair rather than as a release of a 

 function by loss of an inhibitory suppression of this 

 function." Such a disturbance may provide an ex- 

 planation of many of the strange symptoms of extra- 

 pyramidal di.seases. 



Parkinsonism provides a good example of the 

 fallacies arising from attempts to explain signs exclu- 

 sively in terms of the Jacksonian distinction between 

 positive signs, arising by release of inhibition, and 

 negative signs caused by loss of function of the affected 

 region. In parkinsonism the loss of associated move- 

 ments in walking attributed to destruction of the 

 substantia nigra is certainly a negative sign. However, 

 these movements return if the pallidum is also 

 destroyed. Thus these movements cannot be inter- 

 preted as being mediated solely i)y the substantia 

 nigra. 



It is not yet clear whether in epilepsie giratoire some 

 of the rotatory movements and adversive convulsions 

 are mediated by the mechanisms of the brain stem 

 and whether they are elicited b)- subcortical release 

 or by active stimulation from a cortical epileptic focus, 

 such as the frontal or occipital areas for head and eye 

 deviation. 



Extraji\ramidal System and Spinal Reflex Activity. 



As clinical observations had shown that extra- 

 pyramidal diseases of the basal ganglia have less 

 influence on reflex activity than do pyramidal lesions, 

 one might not expect much information from animal 

 experiments in this field. Therefore, only a very few- 

 such inxestigations have been made. 



Lloyd's (165) studies on reticulospinal pathways 

 included an important electrophysiological analysis 

 demonstrating the connection of the bulbar extra- 

 pyramidal structures with the motoneurons and 

 interneurons of the spinal cord, resulting in synchro- 

 nization of internuncial activitv. Tiie functional 



organization of this system is such that secondarv 

 vestibular fibers through the dorsal longitudinal 

 bundle and vestibulospinal tract, together with 

 reticulospinal fibers, constitute the main extrapvra- 

 midal input which is correlated with corticospinal 

 impulses at all levels. This system contains not only 

 short fiber relays but also long fiber connections of 

 the reticulospinal and propriospinal tracts with 

 rapid conduction rates. Facilitation of two-neuron 

 reflexes after electrical stimulation of the descending 

 bulbospinal tracts was described. 



Peacock & Hodes' (206) experiments on the modi- 

 fication of the cortical stimulation effects by stimula- 

 tion of the basal ganglia, described in the section on 

 the striatum, included the finding that monosynaptic 

 spinal reflexes were inhibited by caudate stimulation. 

 In contrast, unpublished experiments of Segundo 

 and co-workers on cats have shown that ventral root 

 discharges in the L7 segment evoked by dorsal root 

 stimulation are usually augmented but occasionally 

 reduced by repetitive stimulation of higher extra- 

 pyramidal centers (the caudatum and putamen as 

 well as the pallidum and claustrum). Diminution of 

 reflex response was obtained only from the striatum 

 (caudatum and putamen). Since dorsal root stimula- 

 tion elicits many reflex mechanisms synchronously 

 which are never acti\ated at the same time under 

 physiological conditions, such studies do not allow 

 differentiation of reflex activity except as to mono- 

 synaptic and polysynaptic reflexes. Therefore, the 

 functional interpretation of their results is very 

 limited. It may be concluded, however, that the 

 striatum appears to exert a regulatory effect on motor 

 reflex mechanisms which includes both excitatory and 

 inhibitory components. 



The recent discovery of the spinal gamma fiber 

 system for regulation of muscle spindle activity and 

 its dependence on a brain-stem mechanism has 

 aroused much interest since this systein now appears 

 to be involved in production of certain extrapyramidal 

 motor disorders. 



The basic physiological findings concerning this 

 system are described in Chapter XLI in this Handbook 

 by Eldred on posture and locomotion. Much of the 

 evidence that the activity of this system is altered in 

 extrapyramidal diseases has already been presented in 

 this chapter and need only be summarized here. 

 First, the rigidity of parkinsonism depends in con- 

 siderable degree on abnormal gamma system par- 

 ticipation in reflex control of inuscle length and 

 tension. Second, in observations on reflex reinforce- 

 ment in man, Hassler (88) and others showed that 



