HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



plore the physiological processes by which the reticular 

 formation contributes concurrently to the mediation 

 of these diverse attributes of central nervous system 

 function. 



ANATOMICAL CONSIDERATIONS 



According to Allen (8), the "fonnatio reticularis' is 

 embryologically that mass of cells in the brain stem 

 and spinal cord which is not utilized in the formation 

 or motor root or sensory relay nuclei. Phylogenetically 

 it is very old, and in organisms of primitive phyla it 

 represents the bulk of the entire central nervous 

 system. In higher vertebrates, however, in which the 

 process of encephalization is highly developed, only a 

 relatively small amount of reticular formation remains 

 in the spinal cord but that portion which occupies 

 the central brain stem from medulla to thalamus as- 

 sumes a mass of considerable proportions. Presumably, 

 the reticular formation expands in higher orders as a 

 result of the development of cerebral and cerebellar 

 hemispheres with which it is closely related func- 

 tionally (169). 



Allen observes further that in its de\elopment, the 

 reticular formation surrounds or partially surrounds 

 the sensory nuclei of the thalamus and that such 

 structures as the nucleus ruber, substantia nigra and 

 other differentiated hypothalamic and midbrain 

 nuclei should be considered probably as specialized 

 derivatives of it. Clearly, then, considered develop- 

 mentally, the reticular formation is closely related 

 caudally to collections of neurons in the gray sub- 

 stance of the spinal cord, presumaiily internuncial 

 (169), and cephalically to subcortical nuclei in the 

 forebrain such as the sub- and hypothalamic portions 

 of the thalamus and perhaps even the septal region (i ). 



The reticular formation proper begins, according 

 to Ramon y Cajal (222), in the bulb a little above 

 the decussation of the pyramids. It is centrally located, 

 being surrounded everywhere by a shell of neural 

 tissue consisting of long fiber tracts and nuclei of 

 specific conduction systems. Throughout this cen- 

 trally located area, collections of cells alternate with 

 regions which appear grossly as nuclei; Olszewski 

 (207) in an extensive study has descriised g8 such 

 masses in the reticular formation. A detailed discussion 

 of the cytoarchitectural structure of this region, how- 

 ever, would be pointless here as these collections of 

 cells do not appear in general to represent functional 

 units. Rather, as will be discussed later, the physio- 

 logical characteristics exhibited bv the reticular 



formation appear largely to be independent of visible 

 structural relationships. 



Retuulnpetal Ciinneclions 



An important source of neurons connecting with 

 the reticular formation is known to be the spinal cord. 

 These connections or collateral branches leave the 

 main axon trunks of the medial lemniscus, and spino- 

 thalamic or spinocerebellar tracts as they course 

 through the brain stem. Ramon y Cajal describes 

 such collaterals in great detail and Allen confirms 

 their presence. Additionally, it may be that there are 

 cells in the spinal cord which send axons directly to 

 the reticular formation, as suggested by Probst (219) 

 and confirmed recently by Collins & O'Leary (56) 

 by Brodal (40) and by Nauta (202). It is likely such 

 direct fibers travel in or near long funiculi in 

 ventrolateral and dorsal segments of the spinal cord. 

 Whether by collateral or direct connection, these 

 spinoreticular pathways have been repeatedly de- 

 scribed anatomically and arc known to be quite 

 prevalent (22, 40, 193, 194). 



Spinoreticular axons appear to enter the reticular 

 formation throughout the longitudinal extent of the 

 jjrain stem (8, 221), although there may be areas 

 which are particularly heavily populated with these 

 reticulopetal fibers. Morin (193) suggests that they 

 are particularly dense in the medullary region, but 

 it is clear that the reticular formation receives con- 

 nections from the spinal cord throughout its length. 



The reticular formation receives afferent fibers 

 from other structures in the i)rain stem as well as from 

 the long tract systems. Ramon y Cajal described 

 anatomical connections with the principle sensory 

 nuclei, with interstitial motor cells of tlie bulb and 

 with the quadrigeminal Jjodies. Recently, also, on the 

 basis of Golgi studies, Scheibel (23B) has confirmed 

 these observations. In addition, he describes the ex- 

 tremely wide area which the dendrites of single 

 reticular neurons are capable of covering. This ana- 

 tomical structure of reticular neurons suggests that 

 they receive synaptic contact from laterally located 

 pathways and nuclei through these remarkable den- 

 drite extensions as well as by central con\ergence of 

 collateral axons from projection systems. 



Corticifugal fibers, another source of reticulopetal 

 axons, are known to originate in the frontal convexity 

 (134, 152, 181, 187, 230), sensorimotor cortex (177, 

 222), particularly in the motor region {230), and 

 cingulate gyrus (230, 280). Axons destined for the 



