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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOm' II 



leling these effects upon the gastrointestinal tract of 

 excitation of cortical efTector zones are observations 

 concerning facilitatory and inhibitory influences ex- 

 erted upon arterial pressure (20, 215), peripheral 

 vasomotor tone (74), respiration (20, 136), pupillary 

 reactions (251), galvanic skin reflexes (276, 277) and, 

 indeed, upon all phases of autonomic balance. 



Paralleling the results of stimulation in the reticular 

 formation, the application of a stimulus to a single 

 cortical effector locus elicits responses in many differ- 

 ent effector systems. Bailey & Sweet (20), for example, 

 found that a stimulus applied to tlie orbital surface 

 of cats and monkeys inhibited respiration, caused a 

 rise in arterial pressure and inhibited tonus in the 

 gastric musculature; comparable accounts have dealt 

 with excitation of other cortical loci (11, 215). The 

 polyphasic responses elicited by cortical stimulation, 

 however, often appeared logically interrelated and 

 patterned into a recognizable behavioral unit {244). 

 Segundo et al. (244), stimulating the several cortical 

 effector loci in unanesthetized chronically-prepared 

 animals, obser\ed patterned responses appropriate to 

 naturally-occurring behavioral situations. Threshold 

 voltages elicited arousal, arrest of motion and atten- 

 tive alertness; supraliminal excitation to the same loci 

 induced roughing of the fur, increase in respiration, 

 facilitation of movement — in substance, behavior 

 normally attributed to fear and flight. 



These responses to cortical stimulation are expressed 

 by way of the neural connections each effector locus 

 is known to have with visceral 'centers' in the reticular 

 system. Normally-developed autonomic controls in 

 higher animals and in man doubtless require cortical 

 transport through utilization of these connecting 

 pathwa\s. 



The intimate relationship between functional com- 

 ponents of \isceral and somatic effector systems is 

 effectively demonstrated in truncation experiments. 

 As a result of the classical experiments of Sherrington, 

 effects upon muscle tone and reflex activity of pro- 

 gressively lowering the brain-stem transection are 

 well known. Transection at the collicular level causes 

 the animals to exhibit mild extensor rigidity. At the 

 same time, respiration is little effected, onl)- the 

 beginnings of periodic breathing; being displayed 

 (120). In addition, \asomotor control is largely 

 retained, while temperature control through panting 

 (23) is impaired and through shivering (iio) lost. 

 Midpontine transection renders the animal far more 

 rigid and causes it to display (particularly when 

 vagotomized) highly developed periodic or apneustic 

 breathing (265). At this point, control of vasomotor 



tone and support of arterial pressure are impaired, 

 and mechanisms for control of temperature regulation 

 are lost. As the transection is lowered to midmedullary 

 levels, spasticity is lost, respiration becomes 'eupneic' 

 or ataxic, and maximum loss of suprasegmental influ- 

 ence over vasomotor tone and arterial pressure is 

 achieved. 



INFLUENCE OF RETICUL.AR FORM.'VTION 

 UPON SENS.ATION 



A recent development of the first magnitude has 

 concerned the influence of the reticular formation in 

 modifying sensory inputs to the central nervous 

 system. Ramon y Cajal (222) long ago remarked 

 upon the existence of centrifugal fibers which ap- 

 peared to terminate in relay nuclei of sensory path- 

 ways. Now, evidence exists which indicates that 

 efferent fibers of this type exert control over most, if 

 not all, afferent conduction systems from receptor 

 to cortex. This control is the subject of Clhapter XXXI 

 by Livingston in this Handbook. 



The first evidence that central systems, particularly 

 the reticular formation, influence sensory input was 

 submitted by Granit & Kaada (102) for propriocep- 

 tion. Leksell (149) had demonstrated that muscle 

 spindles were under the control of anterior horn cells 

 of small size called gamma efferents. Granit & Kaada 

 (102) and others (71, 100, loi) were able to show 

 that reticular stimulation resulted in modification of 

 gamma efferent discharge and hence of spindle 

 activity. 



Subsequently, similar controls were discovered in 

 other sensory systems. Loewenstein (166) showed 

 that the sensitivity of tactile organs in frog skin could 

 be modified by excitation of sympathetic nerves to 

 the test region and that these receptors could even 

 be made to fire spontaneously. King et al. (142, 143) 

 recorded a potential in the trigeminal nerve which 

 followed the primary response elicited by peripheral 

 stimulation and which they were able to show coin-sed 

 peripherally from its central nervous system origin. 

 It was suggested that these efferent potentials were 

 capable of modifying peripheral skin receptors and 

 might relate to such clinical disorders as trigeminal 

 neuralgia. Granit (99) observed both facilitation and 

 inhibition of retinal ganglion cells following excita- 

 tion of the midbrain tegmentum. Galambos (88) 

 found that auditory clicks recorded in the cochlear 

 nerve were inhibited by stimulation of the olivo- 

 cochlear bundle. Kerr & Hagbarth (137) were able 

 to inhibit olfactory bulb potentials by excitation of 



