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HANDBOOK OF PHYSIOLOGY ' 



NEUROPHYSIOLOGY II 



precentral syrus. The thumb complex had the lowest 

 threshold, the face complex the highest. Increasing 

 the frequency of stimulation dissolved this simple 

 pattern of representation into the classical motor map. 

 They have examined the cortical representation of 

 motor units of the first dorsal interosseous muscle of 

 the hand, with a view to testing the degree of un- 

 varying inevitability or of causal lability of trans- 

 mission along this neural pathway. They conclude 

 that neither of these events is absolute and both obtain 

 partially. Motor units in the first dorsal interosseous 

 muscle were acti\ated by single pulse stimulation of 

 different motor points. Responses were variable with 

 units fluctuating in latency and order of firing or not 

 firing at all, and there was no more than a tendency 

 to absolute or rigid relationships between any cortical 

 point and a responding spinal motoneuron. The evi- 

 dence suggested, however, that certain motoneurons 

 may be summoned into action with special ease. 



Liddell & Phillips (271) found no systematic 

 lengthening of latency of motor unit responses from 

 the center to the periphery of the cortical area, sug- 

 gesting that there need be, as a rule, no additional 

 synapses on any horizontal path running from the 

 periphery to the middle (or low threshold part) of the 

 stimulable cortical area. They consider their findings 

 consistent with the view that the neural path from 

 cortex to spinal motoneurons arises from all parts of 

 the cortical area stimulated and not only from its 

 lowest threshold part. 



Autonomic Cotutimitants of Cortual Sliniii/alion 



Brief reference may be made here to the autonomic 

 effects which follow stimulation of the motor areas of 

 the cortex, since there is evidence that they may be 

 mediated through fibers of the pyramidal tract (89, 

 246, 329, 382). If limb and kidney volumes are simul- 

 taneously recorded during stimulation of area 6 in 

 the monkey, limb volumes increase while kidney vol- 

 umes diminish. There is also a sharp rise in systolic 

 arterial pressure which is independent of limb move- 

 ments since it is seen in the curarized animal. This shift 

 in blood volume is from viscera to muscles, rather than 

 from viscera to skin (182, 210). 



Vasodilator responses follow stimulation of the 

 motor cortex of the dog in an area between the cruci- 

 ate sulcus and the sulcus considered to be homologous 

 to the fissure of Rolando ( 1 39), and subcortical \'asodi- 

 lator points seen in these experiments may indicate 

 the participation of a corticohypothalamic pathway 

 with at least some fibers passing in the internal cap- 



sule to the anterior hypothalamus where relays de- 

 scend to lower lexels. Lund (284, 285) has described 

 an analogous autonomic response in the cat where 

 v'ery active \asoconstrictor reactions can be ob- 

 tained from the frontal pole from an area corre- 

 sponding closely with that outlined by Rose & 

 VVooLsey (378) as the orbitofrontal projection area of 

 the dorsomedial nucleus of the thalamus. Vasocon- 

 striction is confined largely to the skin and is accom- 

 panied by a rise in the systolic arterial pressure. Lund 

 suggests that in \ie\v of the overlap of the vasomotor 

 and the motor centers of the cortex, \oluntary muscu- 

 lar work constitutes the normal physiological stimulus 

 for the cortical vasomotor center, and that specific 

 autonomic foci may share a representation with ap- 

 propriate somatic areas. 



Little is yet known about the anatomical pathways 

 mediating these autonomic effects, although the physi- 

 ological evidence suggests that cortical influences 

 relay in hypothalamic and mesencephalic nuclei, in 

 addition to more direct pathways through the pyrami- 

 dal tract. Eliasson et al. (139) have drawn attention 

 to the finding that cortical stimulation affects par- 

 ticularly the sympathetic \asodilator outflow, and that 

 tliis is apparently concerned solely with the integra- 

 tive control of muscle blood flow. Direct stimulation 

 of the hypothalamus mimics in many respects the 

 autonomic responses to cortical stimulation, with 

 redistribution of splanchnic blood to active muscles 

 in a fashion consistent with the role assigned to the 

 hypothalamus in response patterns of emotional 

 arousal in emergency situations. 



The results of cortical stimulation suggest that 

 these cortical vasodilator neurons are concerned pri- 

 marily with the initial adjustment of muscle blood 

 flow during exercise and occur with such speed that a 

 neural rather than a humoral mechanism is neces- 

 sarily involved. It is further postulated that these 

 sympathetic vasodilator nerves may be involved in 

 the control of muscle blood flow even if the organism 

 is not under stress. Although the sympathetic vasodi- 

 lator nerves may play a part in the initial rise of blood 

 flow in the muscles at the start of muscular exercise, 

 they are not necessarily involved in the regulation 

 of muscle blood flow during exercise. Nevertheless, 

 some difficulty attaches to the interpretation of the 

 apparent absence of tonic effects of cortical volleys 

 on the vessels of skeletal muscles since most observa- 

 tions have been made in animals under general anes- 

 thesia. This topic is discussed at length by Uvnas in 

 Chapter XLIV of this work. 



