The Somatosensory System In Prosimian Primates

Prosimian primates are varied in appearance and lifestyle (Wolfheim, 1983; Fleagle, 1999). They include lemurs, lorises, and galagos. Tarsiers have also been classified with prosimians, but form a distinctly separate line of primate evolution, one of a specialized, visual, nocturnal predator. Of the prosimians, the somatosensory system has been significantly studied only in galagos. There have been limited studies of S1 organization in other prosimians (Krishnamurti et al., 1976; Carlson and Fitzpatrick, 1982; Fitzpatrick et al., 1982), and thus we know that this S1 has the typical mammalian organization. There have been no experimental studies on the soma-tosensory system of tarsiers, now considered an endangered primate. Thus, we are uncertain about the generality of conclusions about galagos to other tarsiers and pros-imians, but all prosimians appear to have only a moderate level of neocortical expansion. From this, we would not expect large, complex, cortical somatosensory systems.

The somatosensory system of galagos seems very similar to the basic, generalized somatosensory system of other mammals, but not nearly as expanded or specialized as in other primates. Yet, the dorsal column relay nuclei in the brain stem seem somatotopically organized, much as in simian primates (Coq et al., 2000), and distinct simian-like ventroposterior (VP), ventroposterior inferior (VPI), and ventro-posterior superior (VPS) nuclei are apparent in the thalamus (Kaas, 1982; Wu et al.,

FIGURE 1.3 Somatosensory cortex of prosimian galagos. Five areas have been defined including S1 (area 3b), a rostral bordering area (R) or area 3a, a caudal bordering area (C), which may correspond to area 1, a secondary somatosensory area, S2, and the parietal ventral area, PV. Part of the temporal lobe has been cut away to show S2 and PV, which are located on the upper bank of the lateral sulcus. The primary (V1) and secondary (V2) visual areas are shown as reference. Connection patterns of S2 and PV suggest that several additional somatosensory areas exist in parietal cortex of galagos.

FIGURE 1.3 Somatosensory cortex of prosimian galagos. Five areas have been defined including S1 (area 3b), a rostral bordering area (R) or area 3a, a caudal bordering area (C), which may correspond to area 1, a secondary somatosensory area, S2, and the parietal ventral area, PV. Part of the temporal lobe has been cut away to show S2 and PV, which are located on the upper bank of the lateral sulcus. The primary (V1) and secondary (V2) visual areas are shown as reference. Connection patterns of S2 and PV suggest that several additional somatosensory areas exist in parietal cortex of galagos.

1996). Somatosensory cortex includes a primary area, S1, rostral and caudal bordering areas, R and C, and S2 and PV (Figure 1.3; Sur et al., 1980; Wu et al., 1995, 1996, 1997). In galagos, S1 occupies a strip of koniocortex that is similar to area 3b of monkeys, but not as well differentiated. The representation of the body surface in S1 resembles that of area 3b of monkeys, and thalamic inputs are from SA and RA neurons of the ventroposterior nucleus. All these observations indicate that this single S1 representation is area 3b, and not a configuration of fields or area l as proposed by some investigators. The rostral area, R, is almost certainly homologous with area 3a of other primates. The rostral area of galagos appears to be activated by muscle spindles, and it does have a motor component as does area 3a of monkeys. In addition, the rostral area receives thalamic input from the ventroposterior superior nucleus (VPS) just dorsal to VP. The caudal area is in the same relative position as area l, and it has some of the architectonic features of area l. In addition, it receives projections from area 3b, as does area 1. Nevertheless, the caudal area is not as highly responsive to cutaneous stimuli as area 1 in most monkeys. In anesthetized galagos, neurons are either unresponsive to cutaneous stimuli or require intense stimulation, much as the caudal area does in some non-primates (e.g., Slutsky et al.,

2000). Because this more intense stimulation would activate both cutaneous and deep receptors, muscle spindles may contribute to the activation of the caudal area, but this is uncertain.

Areas S2 and PV form additional representations of the body surface, as they do in other mammals. As in non-primate mammals, S2 receives major thalamic inputs from VP (Wu et al., 1996) and S2 remains responsive to cutaneous stimulation after S1 lesions (Garraghty et al., 1991). Thus, S2 of galagos is activated in parallel with S1 by the VP neurons. However, S1 projects to both S2 and PV, and S2 and PV both project to primary motor cortex conforming to the generalized mammalian pattern. Injections of neuronal tracers in S2 also label regions rostroventral to PV and caudal to the caudal area 'C,' suggesting the locations of additional somatosen-sory areas. Posterior parietal cortex is limited in extent compared to simian primates, and its organization is largely unknown. Overall, somatosensory cortex in prosimians seems only moderately changed from the basic nonprimate pattern.

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