Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Anthropology (PhD)

Degree Level

Graduate

Department

Anthropology

Advisor/Mentor

Claire Terhune

Committee Member

J. Michael Plavcan

Second Committee Member

Peter Ungar

Keywords

3D segmentation, biology, cranial nerves, inferior alveolar nerve, iodine staining, primate diet, sensory nerves, trigeminal nerve

Abstract

The inferior alveolar nerve (IAN) enters the mandible via the mental foramen, supplies nervous sensation to the mandibular teeth as it travels through the mandibular canal, and exits the mandibular foramen to send information to the brain to maintain chewing cycles and protect the teeth from damage. Although bony canals and foramina have been shown to form around soft-tissue structures, there are some examples (e.g., the hypoglossal nerve/canal) where the nervous structures do not comprise most of the canal/foramina space. It is important to know the size of nerves because it has been established that larger nerves convey more information at faster rates. However, no previous work has established the size of the IAN in primates or if the mandibular canal and associated foramina can be used as proxies for the nervous tissues. The purpose of this dissertation is to assess the variation seen in the IAN in using both a hard-tissue dataset comprised of tooth and mandibular canal measurements and a soft-tissue dataset comprised of the cross-sectional area (CSA) and volumetric measurements of the IAN. These two datasets explore the relationship between the IAN and the roots and enamel surfaces of I1, C1, P4, and M1, the CSA and volume of the mandibular canal, and the dietary categories of primates.

Overall, the IAN is related to the bony structures of the mandible by size across primates. There were significant relationships between the tooth surface areas and the IAN throughout the mandible, with most showing either isometric or negatively allometric relationships. Additionally, the nerve CSA measurements at the mental foramen, mandibular foramen, beneath P4, beneath M1, and overall canal volumes showed significant relationships with the corresponding IAN measurements. However, while these relationships may be significant there is no evidence to support the hypothesis that the IAN fills most of the mandibular canal.

Teeth are under strong selective pressure to change shape in response to a change in environment or diet in primate species. Therefore, it was hypothesized that the nervous tissues – because of their direct relationship to the teeth by supplying somatic sensation – would be under these same selective pressures. However, there was only one significant relationship found between the shape of the premolar tooth and the nervous tissue variables, with no other teeth showing significant relationships with the shape of the tooth’s surface. These relationships were further supported when there were no significant relationships between the IAN and dietary categories – reinforcing the conclusion that there are little to no differences in IAN size across primates based on diet.

All cranial nerves in mammals are highly conserved in their shape, pathways, and functions, indicating strong selective pressures to maintain these nerves for their vital functions. These data showed that the IAN – a termination of cranial nerve V – is highly constrained across primates (and some mammal species) and is more likely related to the overall size of the mandible rather than selective pressures such as diet.

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