FIGURE 1 . This photomicrograph illustrates the pattern of hindpaw formalin- 
evoked Fos immunoreactivity in neurons of the L4-L5 spinal cord 
of a rat that received an intraperitoneal (IP) injection of saline. 
There is dense labeling in the superficial layers (laminae I and II) 
and in the neck of the dorsal horn (V). There are no Fos - 
immunoreactive neurons in the motoneuron (MN) pool. The 
densest labeling is in the medial part of the superficial dorsal 
horn; the lateral part (asterisk) does not receive primary afferent 
input from the plantar hindpaw. SG=substantia gelatinosa. 
SOURCE: Presley et al. 1 990. Copyright 1 990 by the Society for 
Neuroscience (New York). 
(Men6trey et al. 1989). Since the pattern of visceral primary afferent 
termination in the cord is different from that of somatic afferents (Cervero 
1985; Sugiura et al. 1989), it was expected that the pattern of Fos expression 
following visceral stimulation with formalin would differ. For this set of 
experiments, rats were anesthetized and injected (IP) with acetic acid in the 
right lower quadrant of the abdomen, which provides a model of visceral 
stimulation (T aber et al. 1969). Several important differences were found 
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