1 68b 



II WIlllllIlK OF I'lIM' i % 



M.I-ROPHYSIOLOGY III 



by a reduction in their size. Al the same time, the 

 innervation ratio of the motor units which compose 

 the muscle tends to increase (a single motoneuron 

 controlling a smaller number of muscle fibers). The 



delicacy of control is thereby increased. 



The activation of motor units therefore must con- 

 form to the spatiotemporal patterning which bears 

 an effective relationship to the functional character- 

 istics of the peripheral organ. This patterning depends 

 simultaneously upon the heterogeneity of the thresh- 

 old of excitability of central elements and upon the 

 modulating actions of central elements which con- 

 tribute to the creation of the "central excitatory 

 state' (107). 



The concept of the motoneuron as a 'final common 

 path' for actions of various origins (Sherrington) 

 finds support today in our knowledge of the ele- 

 mentary mechanisms of synaptic transmission. These 

 permit us to understand the subtle integrations 

 which take place in the somatodendritic membranes. 

 A line modulation of the state of reactivity of the 

 motoneurons depends upon competition of the 

 facilitating and inhibitory influences exerted in the 

 regions of synaptic influence. The study of the reac- 

 livilv of motoneurons in reflex activity has revealed 

 the graded mode of expression of certain predeter- 

 mined configurations of influences. The mecha- 

 nisms mediating tonic postural regulations, for 

 instance, are capable, as we know, even in the 

 absence of higher control, of executing harmonious 

 chords of postural orchestration upon the spinal 

 keyboard without confusion, but lacking in origi- 

 nality. 



Thus, the spinal keyboard does not receive the 

 commands from higher levels as a docile instrument 

 reads to transmit, blindly and faithfully, carefully 

 prepared orders to the muscles. It appears as a fine 

 machinery sensitive to certain types of influences, 



influenced by messages of diverse origins which, 

 directly or indirectly by way of the internuncial 

 systems, converge upon it. It already constitutes in 



itself an 'integrative' structure in the sense (h.it 



Sherrington gave to this term 1107), This structure 

 interprets the Orders thai it receives and transmits 

 them only as a function of an ever changing stale of 



receptivity. 



/ / ; ■<■■ 1 Motoneuron Keyboard 



I nder the inspiration oi Sherrington, data relative 

 to the spinal keyboard emerged progressively and 

 clearly from the research; by contrast, the studies 



relative to the organization of the cortical motor 

 area have encountered many difficulties. The in- 

 terpretation of these results, even until recently, has 

 been the subject of numerous and sometimes pas- 

 sionate controversies 1128). 



After the first observations by Jackson (116), the 

 experimental studies of Fritsch lV Hitzi" (37) and of 

 Ferrier (33) made it clear that in the region anterior 

 to the central fissure there exists some mode of 

 'representation' of movements evocable by electrical 

 stimulation of the cortical tissue. Lev ton & Sherring- 

 ton (72), by means of punctate electrical stimulation 

 of the motor cortex with just supraliminal intensities 

 in the monkey, obtained a regional 'representation' 

 of the various parts of the body. 



The existence of such a somatotopic organization 

 of the so-called motor area has been confirmed by all 

 investigators. However, the diversity and the in- 

 stability of the results obtained with direct electrical 

 stimulation of the cortex lead to a rather confusing 

 polemic; this has been discussed by Bucv (20) and 

 by Walshe (128). 



Certain authors following Foerster (35), and more 

 recently Woolsey (135) and Hines (52), advocate a 

 precise somatotopy which extends not only to the 

 muscular groups of a same region of the body, but to 

 the muscles themselves. This concept embodies the 

 so-called 'mosaic' interpretation of cortical somato- 

 topy. As in the spinal keyboard, each region contains 

 the groups of pyramidal neurons the stimulation of 

 which evokes the response of a given muscle. 



For other authors, inspired by the ideas of Jackson, 

 the cortical representations are functional. Move- 

 ments and not muscles are represented in the ascend- 

 ing frontal convolution. Jackson picturesquely 

 described this by saying that the motor cortex "thinks 

 in movements, not in muscles." According to this 

 concept, a muscle, or a fraction of a muscle, must 

 be represented as many times as there are types of 

 movements employing that muscle, hence the idea 

 of a multiplicity of 'representations' of the muscles 



1 1 seems today that in the li<_;lit of the most recent 

 studies (-■-', 97) we are drawing near to a satisfactory 

 interpretation of the contradictions of past experi- 

 ments using electrical stimulation of the cortical 



areas. We are now able i" appreciate the number 

 ami the importance of the difficulties encountered 

 in (he study of cortical motor function. These in- 

 clude: a' the extreme difficulty of systematization 

 and interpretation of pathological data; b) the 

 morphological complexity of the structure exposed to 

 the artificial influence <>i direct elecirii-.il stimula- 



