SummaryPeripheral tissue damage or nerve injury often leads to pathological pain processes, such as spontaneous pain, hyperalgesia and allodynia, that persist for years or decades after all possible tissue healing has occurred. Although peripheral neural mechanisms, such as nociceptor sensitization and neuroma formation, contribute to these pathological pain processes, recent evidence indicates that changes in central neural function may also playa significant role. In this review, we examine the clinical and experimental evidence which points to a contribution of central neural plasticity to the development of pathological pain. We also assess the physiological, biochemical, cellular and molecular mechanisms that underlie plasticity induced in the central nervous system (eNS) in response to noxious peripheral stimulation. Finally, we examine theories which have been proposed to explain how injury or noxious stimulation lead to alterations in eNS function which influence subsequent pain experience.

In a study of 52 individuals belonging to 35 species or subspecies of passerine birds it was shown that the volume of the hippocampal complex relative to brain and body size is significantly larger in species that store food than in species that do not. Retrieval of stored food relies on an accurate and long-lasting spatial memory, and hippocampal damage disrupts memory for storage sites. The results suggest, therefore, that food-storing species of passerines have an enlarged hippocampal complex as a specialization associated with the use of a specialized memory capacity. Other lifehistory variables were examined and found not to be correlated with hippocampal volume.Some species of birds store large numbers offood items, each in a separate place, and use an accurate, long-lasting spatial memory to retrieve their stores (1-5). We show here that the hippocampal complex (dorsomedial forebrain) (6) of foodstoring passerines is larger relative to body and brain size than that of nonstorers. Thus, across a range of species, a relationship has been found between the structure of a specific brain area outside sensory and motor areas and a specific behavior. METHODSWe measured the volume of the hippocampal complex and striatum of52 individuals belonging to 35 species or subspecies distributed among 9 passerine families [taxonomy in this paper follows that of Sibley et al. (7) based on DNADNA hybridization]. We defined the hippocampal complex as including the closely interconnected hippocampal and parahippocampal areas (6). The evidence from both embryological and connectivity studies (8-10) suggests that these two structures as a whole are homologous to the mammalian hippocampal complex, although the homology ofthe different subdivisions is not known. The behavioral consequences of damage to the avian hippocampal complex show that it is broadly functionally equivalent to the mammalian hippocampus in playing an important role in certain memory tasks, including those involving spatial memory (11)(12)(13)(14)(15)(16)(17)(18). The avian hippocampal complex is a paired structure located adjacent to the midline of the dorsal telencephalon (19). It extends from the caudal limit of the striatum along approximately two-thirds of the caudal-rostral extent of the striatum. In coronal section it is bounded medially by the midline and ventrally by the lateral horns of the ventricle and by the septum (Fig. 1). The region defined as the hippocampus by Karten and Hodos (19) is a V-shaped structure of densely packed cells lying ventrally and medially (Fig. 1). In the parahippocampal area large and small neurons are sparsely and nonuniformly distributed. The lateral boundary of the parahippocampal area is characterized by a change in the size distribution of neurons. Medial to the boundary the distribution is bimodal with peaks at cell areas of about 20 pIm2 and 130-150 pum2, while lateral to the boundary the distribution is unimodal with a peak at about 20-30 ILm2 (Fig. 2): the boundary is often clearer in food-storers than i...

Three families of North American passerines – chickadees, nuthatches and jays – store food. Previous research has shown that memory for the spatial locations of caches is the principal mechanism of cache recovery. It has also been previously shown that the hippocampal complex (hippocampus and area parahippocampalis) plays an important role in memory for cache sites. The present study determined the volume of the hippocampal complex and the telencephalon in 3 food-storing families and in 10 non-food-storing families and subfamilies of passerines. The hippocampal complex is larger in food-storing birds than in non-food-storing birds. This difference is greater than expected from allometric relations among the hippocampal complex, telencephalon and body weight. Food-storing families are not more closely related to each other than they are to non-food-storing families and subfamilies, and the greater size of the hippocampal complex in food-storing birds is therefore the result of evolutionary convergence. Natural selection has led to a larger hippocampal complex in birds that rely on memory to recover spatially dispersed food caches.

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