Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is (1) clearl y present in many extant anamniote species, this basal ganglia is cell spar se and receives only a relatively modest tegmental dopaminergic input and l ittle if any cortical input. The major basal ganglia influence on motor fun ctions in anamniotes appears to be exerted via output circuits to the tectu m. In contrast, in modern mammals, birds, and reptiles (i.e., modem amniote s), the striatal and pallidal parts of the basal ganglia are very neuron-ri ch, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical inpu t. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, o utput circuits to the cortex, however, appear to be considerably more devel oped in mammals than in birds and reptiles. The basal ganglia, thus, appear s to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movem ents, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian linea ge appears, however, to have diverged somewhat from the sauropsid lineage w ith respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement. (C) 1998 Elsevier Science B.V. All rights reserved.