THE RETICULAR FORMATION 



1293 



supramaximal stimulation, can augment the ex- 

 citatory state of spinal neuron pools already subject 

 to tonic reflex inputs to a degree which is sufficient to 

 elicit movement (163, 258). 



Suprasegmental Influences upon Segmental 

 Motor Neuron Activity 



Like pyramidal (44, 146, 164) and reflex systems, 

 (except the myotatic reflex) the reticular formation 

 appears to exert its facilitatory and inhibitory spinal 

 influences principally through interneurons rather 

 than upon motor neurons directly (163). Sustained 

 reticular excitation recruits expanding pools of in- 

 terneurons and these in turn summate their influences 

 spatially upon motor neurons (163). Lloyd (165) sug- 

 gests that faster reticulospinal influences may set the 

 stage for impulses arriving over the more slowly-con- 

 ducting pyramidal tract, and clearly both of these 

 excitations sum together with local tonic inputs to 

 elicit appropriate motor neuron excitation. Reticular 

 influences appear to be more enduring than are those 

 conducted via the pyramidal tract. Kleyntjens et al. 

 (144) found that facilitatory effects elicited by py- 

 ramidal activation lasted only for the duration of the 

 stimulus, while reticular excitation elicited prolonged 

 facilitation which outlasted the stimulus by many 

 seconds (15, 144). 



The excitatory state of the anterior horn cell is 

 modified by changes in depolarization induced 'syn- 

 aptically' in it (69) and, presumably, reticular forma- 

 tion excitation elicits spinal facilitation or inhibition 

 by initiating such potential shifts. Local catelectro- 

 tonus induced experimentally in the spinal cord was 

 found to augment motor neuron firing (21, 36) while 

 anelectrotonus appears to inhibit segmental motor 

 activity (151, 266, 267). Comparable results were 

 seen to follow stimulation applied to the reticular 

 formation (94). 



The reticular formation is now known to exert 

 powerful controls over sensory receptor and con- 

 ductor systems. This phenomenon will be discussed 

 in more detail later but must be mentioned here in 

 connection with motor systems, as gamma efferent 

 neurons which control activity of muscle spindles are 

 subject to reticulospinal influences (71, 101, 102). 

 Through this mechanism, so far found to be predom- 

 inately inhibitory in nature, suprasegmental struc- 

 tures exert powerful controls over reflex activity by 

 modifying spindle discharge rates. 



Reticulopetal Inputs to Reticular Formation 



Whether or not the reticular formation is capable 

 of independent tonic activity, certainly afferent inputs 

 from the cerebral cortex, cerebelluin, basal ganglia, 

 vestibular nuclei and sensory transmission systems to 

 the central brain stem influence it critically in the 

 performance of its contributions to muscular control. 



CEREBRAL coRTE.x. All active cortical loci, along with 

 their multiplicity of other functions including arousal, 

 are known to exert an important measure of control 

 over voluntary and reflex somatic motor function. 

 Stimulation of some of these regions excites motor 

 movements; classically, such activity is known to arise 

 from activation of the motor and premotor cortex 

 as well as from frontal and paraoccipital eye fields. 

 Contrastingly, excitation of other cortical loci appears 

 to inhibit rather than initiate motor movement. Hines 

 (118) first reported inhibition of existing muscular 

 contraction by stimulating area 4S (between areas 4 

 and 6) and subsequently Dusser de Barenne and asso- 

 ciates (66, 68) designated four other zones as 'sup- 

 pressor' strips; these loci were situated in the frontal 

 and parietooccipital eye fields, postcentral gyrus and 

 cingulate gyrus. All of these cortical loci, stimulation 

 of which either initiates or inhibits movement, are 

 known to have intimate anatomic and physiologic 

 relationships with the reticular formation and to exert 

 a considerable measure of control over its activities. 

 A principal reason for considering these loci as 

 "suppressor' strips was that their excitation was 

 thought to express an inhibitory action upon the 

 myotatic reflex by way of the reticular formation 

 (177). In support of this concept was the observation 

 that resection of area 4S resulted in transitory spastic- 

 ity, presumably because this loss of suppressor input 

 to the reticular formation permitted unbalanced 

 facilitatory influence from the brain stem or vestibule 

 to augment myotatic activity in the spinal cord (156, 

 160). Also, Sloan & Jasper (250) showed that arrested 

 activity similar to that induced by stimulation of 

 thalamic portion of the reticular system (121) is 

 elicited in unanesthetized animals by stimulation of 

 the cingulate gyrus. Comparable cessation of move- 

 ment has been noted by Segundo et al. (244), by 

 Kaada (131) and by Clark et al. (55) following stim- 

 ulation of the active cortical loci — 'suppressor' as 

 well as movement-inducing. Clark et al. (55) correctly- 

 indicated, however, that tone was not eliminated by 

 cingulate stimulation in the dog; the animal ceased 

 to move but did not collapse, and suppression of 



