From the Guidelines
Special Visceral Efferent (SVE) nerves are termed "visceral" rather than "somatic" due to their embryological origin and evolutionary history, despite controlling voluntary muscles. These nerves innervate muscles derived from the pharyngeal (branchial) arches, which develop from different embryological tissue than typical skeletal muscles, as noted in the most recent study 1. While traditional somatic efferent nerves innervate muscles derived from somites (body segments), SVE nerves innervate muscles that evolved from structures originally associated with the pharyngeal region and gill arches in our evolutionary ancestors. This includes muscles of facial expression, mastication, and those in the pharynx and larynx. The "special" designation indicates their unique developmental pathway and specific function, while "visceral" reflects their phylogenetic relationship to autonomic functions despite their current role in voluntary movement, as discussed in 1 and further updated in 1. This terminology preserves important distinctions in neural development and evolutionary history, even though functionally these nerves control striated muscles that perform voluntary actions. Key points to consider include:
- The embryological origin of SVE nerves from the pharyngeal arches
- Their role in innervating muscles derived from these arches
- The distinction between somatic and visceral efferent nerves in terms of their developmental pathways and functions
- The preservation of historical and evolutionary context in the terminology used to describe these nerves, as highlighted in the latest guidelines 1.
From the Research
Special Visceral Efferent (SVE) Nerves
- The term "visceral" in SVE nerves refers to their origin from the visceral (pharyngeal) arches, rather than their function in controlling voluntary muscle movements 2.
- SVE nerves are involved in the innervation of muscles derived from the branchial arches, which are responsible for functions such as swallowing, speaking, and facial expressions 3.
- The classification of SVE nerves as "visceral" is based on their embryological origin, rather than their physiological function 2.
Comparison with Somatic Nerves
- Somatic nerves, on the other hand, are involved in the innervation of muscles derived from the somites, which are responsible for voluntary movements such as walking, running, and writing 2.
- The main difference between SVE nerves and somatic nerves is their origin and the type of muscles they innervate, rather than their function in controlling voluntary movements 2.
Functional Columns of the Brainstem
- The brainstem is classically divided into functional columns, including special and general subdivisions for somatic and visceral components 2.
- The term "special visceral motor" is applied to branchiomotor nuclei, while "special visceral sensory" refers to nuclei devoted to incoming gustatory and olfactory senses 2.
- However, the use of the term "special visceral motor" has been questioned, as branchiomotor neurons function more like general somatic than general visceral motoneurons 2.