Brain mass and cranial nerve size in shrews and moles

Consciousness makes sense in the light of evolution

I believe consciousness is a property of advanced nervous systems, and as such a product of evolution. Thus, to understand consciousness we need to describe the trajectory leading to its evolution and the selective advantages conferred. A deeper understanding of the neurology would be a significant contribution, but other advanced functions, such as hearing and vision, are explained with a comparable lack of detailed knowledge of the brain processes responsible. In this paper, I try to add details and credence to a previously suggested, evolution-based model of consciousness. According to this model, the feature started to evolve in early amniotes (reptiles, birds, and mammals) some 320 million years ago. The reason was the introduction of feelings as a strategy for making behavioral decisions.

The inferior alveolar nerve and its relationship to the mandibular canal

Previous work on the mandibular canal, mental foramen, and mandibular foramen has focused on humans and some other non-primate mammals (with small sample sizes), but little work has investigated the mandibular canal and inferior alveolar nerve (IAN) across primates. However, it is important to understand the relationship between the IAN and mandibular canal due to the IAN's close relationship to the teeth and mastication, and thus dietary adaptations. While it is assumed that most bony canals within the skull grow around and form to pre-existing nervous structures, this relationship has never been validated for the IAN and mandibular canal. MicroCT scans of 273 individuals (131 females, 134 males, and 8 unknown sex) from 68 primate species and three mammalian outgroups, and diceCT scans of 66 individuals (35 females, 23 males, and 8 unknown sex) from 33 primate species and the same mammalian outgroups were used to create 3D models of the IAN and mandibular canal from which cross-sectional areas were taken at various points on the structures. Using qualitative descriptions, phylogenetic generalized least squares analysis, and phylogenetic ANOVAs, we were able to establish three main conclusions: (1) the mandibular canal is most often not a defined canal within the mandible of primates, (2) when the canal can be identified, the IAN does not comprise most of the space within, and (3) there are significant relationships between the IAN and the corresponding canals, with most showing isometry and the mental foramen/nerve showing negative allometry.

Mammalian Skull Dimensions and the Golden Ratio (Φ)

The Golden Ratio (Phi, or Φ = 1.618…) is a potentially unifying quantity of structure and function in nature, as best observed in phyllotactic patterns in plants. For centuries, Φ has been identified in human anatomy, and in recent decades, Φ has been identified in human physiology as well. The anatomy and evolution of the human skull have been the focus of intense study. Evolving over millenia, the human skull embodies an elegant harmonization of structure and function. The authors explored the dimensions of the neurocranium by focusing on the midline calvarial perimeter between the nasion and inion (nasioiniac arc) and its partition by bregma into 2 sub-arcs. The authors studied 100 human skulls and 70 skulls of 6 other mammalian species and calculated 2 ratios: 1) the nasioiniac arc divided by the parieto-occipital arc (between bregma and inion), and 2) the parieto-occipital arc divided by the frontal arc (between nasion and bregma). The authors report that in humans these 2 ratios coincide (1.64 ± 0.04 and 1.57 ± 0.10) and approximate Φ. In the other 6 mammalian species, these 2 ratios were not only different, but also unique to each species. The difference between the ratios showed a trend toward convergence on Φ correlating with species complexity. The partition of the nasioiniac arc by bregma into 2 unequal arcs is a situation analogous to that of the geometrical division of a line into Φ. The authors hypothesize that the Golden Ratio (Φ) principle, documented in other biological systems, may be present in the architecture and evolution of the human skull.

All in the Family - Touch Versus Olfaction in Moles

Here I review, compare, and contrast the neurobiology and behavior of the common, eastern mole (Scalopus aquaticus) and the star-nosed mole (Condylura cristata). These two species are part of the same family (Talpidae) and have similar body size and general morphology. But they differ in sensory specializations, complexity of neocortical organization, and behavior. The star-nosed mole has an elaborate mechanosensory organ-the star-consisting of 22 epidermal appendages (rays) covered with 25,000 touch domes called Eimer's organs. This densely innervated structure is represented in the neocortex in three different somatosensory maps, each visible in flattened neocortical sections as a series of 11 modules representing the 11 rays from the contralateral body. The 11th ray is greatly magnified in primary somatosensory cortex (S1). Behavioral studies show the star is moved in a saccadic manner and the 11th ray is a high-resolution tactile fovea, allowing star-nosed moles to forage on small prey with unprecedented speed and efficiency. In contrast, common mole noses lack Eimer's organs, their neocortex contains only two cortical maps of the nose, and they cannot localize small prey. Yet common moles have exceptional olfactory abilities, sniffing in stereo to rapidly localize discrete odor sources originating from larger prey. In addition, common moles are shown to track odorant trails laid down by moving prey. These results highlight the surprising abilities of species once thought to be simple, and the usefulness of diverse species in revealing general principles of brain organization and behavior. Anat Rec, 2019. © 2019 American Association for Anatomy.

Synaptic organization and division of labor in the exceptionally polymorphic ant Pheidole rhea

Social insect polyphenisms provide models to examine the neural basis of division of labor and anatomy of the invertebrate social brain. Worker size-related behavior is hypothesized to enhance task performance, raising questions concerning the integration of morphology, behavior, and cellular neuroarchitecture, and how variation in sensory inputs and cognitive demands of behaviorally differentiated workers is reflected in higher-order processing ability. We used the highly polymorphic ant Pheidole rhea, which has three distinct worker size classes - minors, soldiers, and supersoldiers - to examine variation in synaptic circuitry across worker size and social role. We hypothesized that the density and size of synaptic complexes (microglomeruli, MG) would be positively associated with behavioral repertoire and the relative size of the mushroom bodies (MB). Supersoldiers had significantly larger and less dense MG in the lip (olfactory region) of the MB calyx (MBC), and larger MG in the collar (visual region) compared to minors. Soldiers were intermediate in synaptic phenotype: they did not differ significantly in MG density from minors and supersoldiers, had MG of similar size to minors in the lip, and did not differ from these two worker groups in MG size in the collar. Results suggest a complex relationship between MG density, size, behavior, and worker body size involving a conserved and plastic neurobiological development plan, although workers show strong variation in size and social role.

A Comparative Morphometric Analysis of Three Cranial Nerves in Two Phocids: The Hooded Seal (Cystophora cristata) and the Harbor Seal (Phoca vitulina)

While our knowledge about the senses of pinnipeds has increased over the last decades almost nothing is known about the organization of the neuroanatomical pathways. In a first approach to this field of research, we assessed the total number of myelinated axons of three cranial nerves (CNs) in the harbor (Phoca vitulina, Pv) and hooded seal (Cystophora cristata, Cc). Axons were counted in semithin sections of the nerves embedded in Epon and stained with toluidine blue. In both species, the highest axon number was found within the optic nerve (Pv 187,000 ± 8,000 axons, Cc 481,600 ± 1,300 axons). Generally, considering absolute axon numbers, far more axons were counted within the optic and trigmenial nerve (Pv 136,700 ± 2,500 axons, Cc 179,300 ± 6,900 axons) in hooded in comparison to harbor seals. The axon counts of the vestibulocochlear nerve are nearly identical for both species (Pv 87,100 ± 8,100 axons, Cc 86,600 ± 2,700 axons). However, when comparing cell density, the cell density is almost equal for all nerves for both species except for the optic nerve in which cell density was particularly higher than in the other nerves and higher in hooded in comparison to harbor seals. We here present the first comparative analysis of three CNs in two phocid seals. While the CNs of these closely related species share some general characteristics, pronounced differences in axon numbers/densities are apparent. These differences seem to reflect differences in e.g. size, habitat, and/or functional significance of the innervated sensory systems.