1292 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



diffusely scattered throughout the anterior and lateral 

 funiculi of the spinal cord (205) and, while the most 

 prominent inhibition was observed ipsilaterally, 

 bilateral effects were easily demonstrated. 



The brain-stem area from which inhibition could 

 be induced was originally thought to be confined to 

 the medioventral reticular formation at the bulbar 

 level (205). Subsequent observations by Hodes et al. 

 (119), however, described effective loci throughout 

 the cephalic neuraxis to and including the septal 

 nuclei. Austin (15) likewise was able to elicit inhibi- 

 tion by stimulation of points in the region of the pos- 

 terior commissure, center median nucleus of the 

 thalamus, stria terminatis and red nucleus. The in- 

 hibitory effects first described were general and non- 

 reciprocal (171, 172, 261), but Bach (16) was able 

 to record reciprocal responses. Later, Sprague (258, 

 260) found that reciprocal effects were induced by 

 stimulation of some points within the reticular forma- 

 tion, and nonreciprocal responses from others. 

 Gernandt & Thulin (97) confirmed these effects of 

 reticular stimulation by demonstrating both reciprocal 

 and nonreciprocal inhibition of ventral root reflex 

 responses. The two-neuron stretch reflex was found 

 to be particularly susceptible to the inhibitory in- 

 fluences of the reticular formation while, concurrently, 

 the polysynaptic reflex was commonly, although not 

 uniformly, found to be facilitated (15, 97, 144)- 



The existence of interneurons in the spinal cord 

 which have exclusively inhibitory properties was 

 originally proposed by Renshaw (225) and more 

 recently expanded by Eccles d al. (70). It is convenient 

 to think that specialized cells of this kind may relate 

 closely to the mediation of suprasegmental inhibitory 

 influences, but as yet no definitive evidence confirming 

 this possibility is available. Support for the concept, 

 however, is supplied by Lettvin who found that fibers 

 from an inhibitory reticular area end in the lateral 

 portion of the internuncial pool (150) and that direct 

 stimulation of this spinal region with microelectrodes 

 suppressed motor neuron activity. 



Facilitation 



In contrast to the inhibitory effects elicited by 

 stimulation of the medullary reticular formation, 

 Magoun & Rhines found that facilitation of spinal 

 motor activity could lie initiated by comparable 

 stimulation of loci higher in the central brain stem. 

 These effects were induced by excitation of the bulbar 

 reticular formation, midbrain and pontile tegmentimi, 

 periaqueductal grey substance, sub- and hypothala- 



mus, and from the mid-line, intralaminar nuclei and 

 nucleus ventralis anterior of the thalamus. Peacock 

 & Hodes (210) subsequently observed that facilitatory 

 influences could be elicited from still more rostrally 

 located sites, for example, from the septum. 



As with inhibition, this effect (169, i 72) was elicited 

 by low threshold stimulation and could be evoked by 

 a wide variety of stimulus frequencies. Its rapidly con- 

 ducting spinal path (93, 169) was bilateral and diffuse, 

 located in the ventral and lateral funiculi. In general, 

 facilitatory fibers were found to occupy more dorsal 

 and inhibitory fibers and more \entral locations in 

 the spinal cord, and each could be sectioned differ- 

 entially (205). 



Both the two-neuron stretch reflex and the multi- 

 synaptic flexor response are subject to facilitatory in- 

 fluences from these suprasegmental areas, although 

 effects upon the myotatic response predominate in 

 flexor and extensor muscles alike (15, 144). Facilita- 

 tion of this response was accompanied either by in- 

 hibition or enhancement of the polysynaptic reflex 

 (15, 144), although inhibition predominated (144). 

 Both reciprocal {99, 260) and non-reciprocal fioi, 

 266) effects have been observed. 



The effects of brain-stem stimulation upon reflex 

 activity in the spinal cord were foimd to be rapidly 

 conducted (93), although, as with p\Tamidal excita- 

 tion (44), recruiting build-up appeared to play an 

 important role in the elicited response. Lloyd (163) 

 showed tiiat augmentation of the monosynaptic reflex 

 did not occur immediately upon arrival at the spinal 

 level of facilitating influences. The effect required 

 much longer to become maximal and, as was found 

 when studying inhibition, prolonged (15) or recurrent 

 (144) facilitation was evidenced for as long as 10 sec. 

 (15) following cessation of the brain stimulus. A re- 

 bound reversal effect also has been noted at the ter- 

 mination of a facilitatory stimulus applied to the 

 reticular formation (97, 258). 



In addition to augmentation of reflexly- or cor- 

 tically-induced movements, facilitatory influences 

 arising from stimulation of the brain stem can be 

 made suflficientlv great to evoke postural changes. 

 Sprague & Chambers (260) reported that threshold 

 bulbar stimulation near the mid-line initiated ipsi- 

 lateral flexion and contralateral extension, while 

 excitation of more laterally located points elicited the 

 opposite effect. .Such changes are related doubtless 

 to the contrasting effect upon reflex activity elicited 

 by medial and lateral loci in the RAS (97). Reticular 

 formation excitation, therefore, particularly with 



