Behavior and Neurology of Lizards 
N. Greenberg and P. D. MacLean, eds. 
NIMH, 1978. 
Somatosensory Pathways in Lizards: The Identification of the 
Medial Lemniscus and Related Structures 
Sven O. E. Ebbesson* 
Depts. of Neurosurgery and Anatomy 
University of Virginia 
and 
Dept, of Neurosciences 
University of California, San Diego 
SUMMARY. The sensory pathways that relay information from the spinal cord to the brain 
are remarkably similar in all vertebrates, including lizards. In the various taxonomic groups, the 
differences in ascending spinal projections relate primarily to the relative quantity of fibers in 
a given pathway and the degree of differentiation of the various, related nuclear groups. It is 
clear, for example, that the degree of limb development and the consequent increase in number 
and differentiation of receptors are closely correlated with the size and differentiation of the 
dorsal column system (Ebbesson, 1969). In addition to a review of the general organization of 
ascending spinal pathways in lizards, this chapter provides the first description of the projec- 
tions of the dorsal column nuclei in two lizard species. 
INTRODUCTION 
One aspect of comparative neurology today 
concerns itself with the analysis of struc- 
tural variation of neuronal systems in a 
broad spectrum of vertebrates, with the hope 
that such comparisons will provide insights 
into the evolution of systems and the signifi- 
cance of structural variation in the nervous 
system. It is significant that less than a dozen 
investigators in the world are currently 
working on reptile brain anatomy, and many 
of us dilute our efforts by studying other 
vertebrates as well. If one then considers 
that no one was interested in reptilian neuro- 
anatomy 15 years ago, one realizes that much 
has been achieved in a short period of time. 
Nevertheless, only a few neuronal systems 
have been described, even superficially, and 
most of the lizard brain remains as a black 
box. This chapter concerns itself with one 
of the still obscure systems, namely the sen- 
sory pathways that relay information from 
the spinal cord to the brain. These pathways 
*Present address: Department of Anatomy, Medical 
Sciences Campus, University of Puerto Rico, 
GPO Box 5067, San Juan, Puerto Rico 00936 
are often referred to as somatosensory, but 
they carry information from a variety of 
receptors ranging from visceral receptors 
to Golgi tendon organs. It should be pointed 
out from the beginning that, in reptiles, 
nothing is known about which receptors are 
related to which ascending spinal pathways. 
However, since such pathways in lizards are 
remarkably similar to mammalian pathways, 
it is likely that many carry information 
similar to that described in mammals. 
The use of new experimental anatomical 
techniques in the last few years has radically 
altered our view of brain organization in 
poikilothermic vertebrates. The somatosen- 
sory systems in reptiles are a case in point. 
Contrary to earlier beliefs (Ariens Kappers 
et al., 1936), it has now been shown that 
fibers of spinal origin reach as far as the 
thalamus in sharks (Ebbesson, 1972), frogs, 
and reptiles (Ebbesson, 1966, 1967, 1969; 
Ebbesson and Goodman, 1976; Pederson, 
1974). Along their course through the brain- 
stem, spinal fibers issue branches to basi- 
cally the same cell aggregates that have 
been described in mammals (Mehler, 1957, 
1969; Mehler et al., 1960; Schroeder and 
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