Parallels Between The Mammalian And Avian Telencephalon

An extensive number of structural, functional, embryological, and genetic studies support the hypothesis that the telencephalon is composed of three parts: the pallium, striatum, and pallidum regions (e.g., Puelles et al., 2000; Smith-Fernendez et al., 1998; Striedter, 1999; Swanson, 2000). The pallium can be further separated into four subdivisions. The dorsal pallium is the most conspicuous part of the mammalian telencepalon. The greater part of the mammalian telencephalon is dorsal pallium, similar to an external mantle encasing the brain that can be segregated into different areas depending on lamination patterns. Broadly speaking, these areas are functionally differentiated. However, the avian brain has significantly less dorsal pallium that makes up the telencephalon. The majority of the avian telen-cephalon is derived from the ventral and lateral pallium. In mammals, the ventral and lateral pallium each contribute parts of the amygdala and give rise to the claustrum and pyriform cortex. A fourth subdivision exists in the avian and mammalian telecephalon — the medial pallium. In the mammal, the medial pallium develops into the hippocampal formation.

The striatum is separated into two parts. The dorsal striatum consists of the caudate and putamen, which together are often referred to as the striatum in the literature, though this obfuscates the so-called ventral striatum. There is a diffuse projection from the whole of the striatum to the midbrain dopaminergic neurons (MDNs) in mammals (Gerfen, 1985; Parent and Cicchetti, 1998; Swanson, 2000), and this is paralleled in the avian brain (Brauth et al., 1978; Karten and Dubbeldam, 1973; Kitt and Brauth, 1981; Reiner, 2002). The dorsal striatum also sends axons to the dorsal pallidum, or the globus pallidus, while the ventral striatum sends projections to the ventral pallidum. In some cases, the striatal efferents to the pallidum are collaterals that branch from those striatal efferents that project to the MDNs.

The striatum receives excitatory input from diffuse topographical projections that originate throughout the entirety of the pallium (McGeorge and Faull, 1989; Webster, 1961). In fact, the principal cell type in the striatum, the spiny cell, has been estimated to have between 10,000 and 30,000 dendritic spines, all of which are hypothesized to be contacted by different isocortical or thalamic neurons in mammals (e.g., Groves et al., 1995; Wilson, 1995). The avian striatum is densely innervated by pallium afferents as well (Brauth et al., 1978; Reiner, 2002; Veenman et al., 1995). The cellular constituents of the avian striatum are electrophysiological^

and morphologically comparable to those found in the mammalian striatum. Furthermore, there is a large degree of convergence on avian spiny striatal cells, much like the mammalian striatum (Reiner, 2002; Reiner et al., 1998).

In both mammals and songbirds, the striatum serves as a nexus for the functions that different pallial areas subserve (e.g., vision, audition, motor production, and so forth). Therefore, the striatum is considered an ideal candidate for a locus of sensory and motor integration. In primates, there is evidence to suggest that reciprocal striato-nigral-striatal connections allow an interface among the different areas of the striatum and, as a result, functionally distinct areas of the pallium (Haber et al., 2000). In the mammalian (Swanson, 2000) and avian (Reiner, 1998, 2002) brain, there are diffuse reciprocal projections from the MDNs back to the entire striatum. The evidence suggests that reciprocal connections between the striatum and MDNs encourage a distributed interplay between these two systems (Joel and Weiner, 2000). One consequence of this interaction may be sensorimotor learning.

Another form of organization is superimposed on this system within the mammal. The cortical efferent to the striatum is followed by a striatal projection to the pallidum that represents another degree of convergence because the number of spiny striatal cells in relation to its primary target cells (both the globus pallidus and MDNs) reflects a 30:1 ratio in the rat and an 80:1 ratio in the monkey (e.g., Oorschot, 1996). The globus pallidus sends collaterals to the thalamus. The thalamus, in turn, sends an excitatory projection back to the same primary cortical area to form topographical, segregated loops (see Alexander et al., 1986; Matell et al., this volume). The loop is closed in the sense that the projections throughout the loop remain in register (Hoover and Strick, 1999; Kelley and Strick, 1999). This structural arrangement lends itself well to information-processing models based on coincidence detection mechanisms (e.g., Beiser et al., 1997). The presence of cortico-striatal modules gives further support to the emerging belief that the mammalian striatum contributes to cognition in addition to motor processes (e.g., Brown et al., 1997; Malapani and Rakitin, this volume; Middleton and Strick, 2000; White, 1998).

0 0

Post a comment