Species differences in mechanosensory projections from the mouth to the ventrobasal thalamus

Abstract

To determine whether the largely ipsilateral, inverted representation of mouth parts in the ventrobasal thalamus of sheep was unique to that species or an expansion of a general mammalian pattern, the corresponding thalamic projections were mapped electrophysiologically in a selected series of mammals (oppossums, agoutis, squirrel monkeys, cats, raccoons, and sheep) representing major branches of evolution among therian mammals. In mapping, tungsten microelectrodes were used to record multi-unit discharges in the thalamus in response to mechanical stimulation of oral surfaces. The pattern of projections seen in sheep is not a general mammalian pattern; there is extensive variability among mammals in the laterality and internal orgainzation of the projections from the mouth. In spite of the great variability, the results suggest an hypothesis concerning phylogenetic trends: descendants of palaeoryctoid insectivores (cats, raccoons, and sheep in our sample) have extensive ipsilateral projections from the mouth, in other therian mammals (opossums, agoutis, and squirrel monkeys in our sample) the ipsilateral component is small or absent.

Organization of the Facial Nucleus in the Rat

“…Provis [1977] noted that the dorsoventral relationships of the facial muscles were maintained in their motoneuron re presentation and that the anteroposterior axis of the face was represented lateromedially on the nucleus, and her results were in general confirmed by the antidromic field potential study of Martin and Lodge [1977], However, fine details of topogra phy are not maintained in the facial nu cleus of the rat in the same way as they are in mammalian sensory relay nuclei such as the ventrobasal thalamus [Bombardieri et al, 1975]; for example, some of the moton eurons supplying the anterior auricular muscle are scattered amongst those sup plying the orbicularis oculi and the fronta lis muscles. This blurring of topographic detail may be due in part to the spread of HRP from injections sites.…”

Brain Traits through Phylogeny: Evolution of Neural Characters

“…Subdued ipsilaterality occurs apparently convergently in rabbits and primates. As we have not ed earlier [2] the advantages of ipsilaterality are obscure, but no less so than those of contralaterality.…”

Phylogeny Through Brain Traits

“…The other branch, Simpson's cohort Ferungulata [19] including the paleoryctid line [12,13] retains the primitive trait of olfactory tract fibers passing under AOF, while exhibiting the derived trait of predominantly ipsilateral projections of mouth parts to somatic sensory thalamus and neocortex [3], Note that the degree of 'dot tedness' implies the degree of differentiation toward a derived state. The major al teration from McKenna's recent ordering [14] is the divergence of Ferungulata at ing to us, since division into similar groups was earlier proposed on the basis of another brain characteristic, namely a preponderance of ipsilateral or contralateral protections to the forebrain from receptive fields of the face and mouth [3]: the ipsilateral condition is evidently the derived one, since monotremes, marsupials, and many placentals are characterized by contralateral projections. In this case, however, the placental orders showing that derived state (ipsilateral projections) are those which show the primitive (under) route of dLOT, while those showing the primitive state of projections (contralateral) possess the derived dLOT condition (through).…”

“…Cohorts Unguiculata and Glires of Simpson [19] in cluding the leptitid line of Lillegraven [12] and McKenna [13] make up one branch which shows one primitive brain trait that is shared Prototheria and Metatheria the predominance of contralateral projections of mouth parts to sensory thala mus and neocortex [3]; this branch of Eutheria also shows one derived trait: the course of dorsal lateral olfactory tract fibers through the accessory olfactory forma tion. The other branch, Simpson's cohort Ferungulata [19] including the paleoryctid line [12,13] retains the primitive trait of olfactory tract fibers passing under AOF, while exhibiting the derived trait of predominantly ipsilateral projections of mouth parts to somatic sensory thalamus and neocortex [3], Note that the degree of 'dot tedness' implies the degree of differentiation toward a derived state.…”

“…Cohorts Unguiculata and Glires of Simpson [19] in cluding the leptitid line of Lillegraven [12] and McKenna [13] make up one branch which shows one primitive brain trait that is shared Prototheria and Metatheria the predominance of contralateral projections of mouth parts to sensory thala mus and neocortex [3]; this branch of Eutheria also shows one derived trait: the course of dorsal lateral olfactory tract fibers through the accessory olfactory forma tion. The other branch, Simpson's cohort Ferungulata [19] including the paleoryctid line [12,13] retains the primitive trait of olfactory tract fibers passing under AOF, while exhibiting the derived trait of predominantly ipsilateral projections of mouth parts to somatic sensory thalamus and neocortex [3], Note that the degree of 'dot tedness' implies the degree of differentiation toward a derived state. The major al teration from McKenna's recent ordering [14] is the divergence of Ferungulata at ing to us, since division into similar groups was earlier proposed on the basis of another brain characteristic, namely a preponderance of ipsilateral or contralateral protections to the forebrain from receptive fields of the face and mouth [3]: the ipsilateral condition is evidently the derived one, since monotremes, marsupials, and many placentals are characterized by contralateral projections.…”