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Head and Neck Anatomy: Part III – Cranial Nerves

Course Number: 598

Nerve Nomenclature

In this course, to avoid confusion we will try to be clear about the transitions from one nerve to another. Because the signals in sensory nerves are running towards the CNS as smaller nerves merge together to form larger nerves, we will speak of them as joining. Motor nerves on the other hand start as larger nerves and divide to form smaller nerves and we will refer to that as splitting. Many nerves have both a sensory and motor component and we will refer to those as branching. This is not perhaps how all texts will use the nomenclature and it is only done here to try to alleviate confusion within this course.

In the study of individual cranial nerves those that are pure sensory nerves will be started at their origin in the sense organs and traced back to the CNS. Pure motor nerves and mixed motor and sensory nerves will be traced from the CNS to the periphery.

Another fine point that we will ignore in the quest for keeping the verbiage to a minimum is that many cranial nerves, even ones we will classify as sensory will have some sympathetic motor fibers that are travelling with the nerve. The ones classed as motor nerves carry visceral sensory information from sense organs within the muscle that sense tension and muscle length. However, these fibers are found in small numbers and generally originate elsewhere. Therefore, here we will ignore them while classifying nerves as sensory, mixed or motor. While some authors are very exacting and classify them by all the fibers that are in a nerve, most use the convention of using the types of fibers at the point of origin or, in the case of sensory nerves, the termination point.

To make the text less repetitive the terms motor and efferent will be used interchangeably as will sensory and afferent. We will also use the equivalent terms autonomic and visceral to break up the monotony when describing the parasympathetic or sympathetic nerves but will use the specific terms for the division involved. Pre-synaptic and pre-ganglionic will also be used to describe the fibers that have not yet synapsed in an autonomic ganglion as will post-synaptic and post-ganglionic.

Lastly it must be noted that all cranial nerves occur in pairs, one on the right and one on the left, as they are symmetrical often the text will refer to a nerve in the singular but that means we are just looking at one of the pair, not that there is just a single entity.