Behavior and Neurology of Lizards 
N. Greenberg and P. D. MacLean, eds. 
NIMH, 1978. 
The Organization of Descending Pathways in the Tegu Lizard 
and Some Comments on the Evolution of Motor Systems 
William L. R. Cruce 
Department of Anatomy 
College of Medicine 
Howard University 
and 
Department of Neurobiology 
Armed Forces Radiobiology Research Institute 
SUMMARY. The organization of descending fiber systems was studied in the tegu lizard, Tup- 
inambis nigropunctatus , using anatomical techniques and behaviorial observations. Fibers in 
the dorsolateral funiculus of the spinal cord which originate in the red nucleus, the metencephalic 
tegmentum and several myelencephalic nuclei have sparse bilateral projections to spinal laminae 
V, VI, and VII. Fibers in the ventromedial funiculus which originate in the interstitial nucleus, 
the vestibular nuclei, and the medial reticular nuclei of the metencephalon have dense bilateral 
projections to spinal laminae VIII and medial IX. Fibers in the ventrolateral funiculus, which 
originate in the metencephalic tegmentum and the myelincephalic reticular and raphe nuclei 
have projections into lamina VII overlapping the terminal fields of the dorsolateral and ventro- 
medial pathways. Following high spinal hemisection, descending coordination of movement 
of trunk musculature is lost, while control of limb movements is only slightly affected. Compari- 
sons are made with other species, especially mammals, and a number of common features are 
noted both in the organization of the descending pathways and in the organization of their 
target cells in the spinal cord. The data suggest that the major evolutionary changes in motor 
systems occurred at the level of synaptic organization of their terminals in the spinal cord. 
INTRODUCTION 
The spinal motoneuron is the "final com- 
mon pathway” (Sherrington, 1906) for all 
motor acts involving musculature of the 
trunk or limbs. The activity of spinal moto- 
neurons is initiated or modulated by synaptic 
input from spinal interneurons; both moto- 
neurons and interneurons are, in turn, influ- 
enced by fibers descending upon them from 
supraspinal regions. The anatomical and 
functional organization of this descending 
input will be the subject of the present com- 
munication. 
In mammals there are two major groups of 
descending motor pathways: those originat- 
ing in the brainstem (bulbospinal pathways) 
* Present Address: Dept, of Neurobiology, Northeast- 
ern Ohio Universities College of Medicine Rootstown, 
Ohio 44272 
and those originating in the forebrain, pri- 
marily from precentral motor cortex. Fibers 
originating in motor cortex may act directly 
upon interneurons and motoneurons of the 
spinal cord (corticospinal pathway) or in- 
directly via projections to the brainstem 
nuclei which give rise to the bulbospinal 
tracts (cortico-bulbospinal pathways). 
The bulbospinal pathways in mammals 
may be subdivided into two groups on the 
basis of their patterns of termination in the 
spinal cord (Kuypers, 1973). One group of 
fibers, the lateral brainstem pathway, ter- 
minates in the lateral part of the spinal gray, 
unilaterally, and acts upon interneurons and 
motoneurons controlling distal extremity 
muscles, i.e., the hand and foot. The other 
group of fibers, the ventromedial brainstem 
pathway, terminates in the ventromedial part 
105 
