Evolutionary and developmental understanding of the spinal accessory nerve

Area of Science:

• Neuroscience
• Evolutionary Biology
• Comparative Anatomy

Background:

• The spinal accessory nerve (SAN) innervates the cucullaris muscle, a key neck muscle in jawed vertebrates.
• The SAN's developmental and evolutionary classification has been a long-standing debate.
• The cucullaris muscle's unique position between branchiomeric and somatic muscle groups adds complexity.

Purpose of the Study:

• To clarify the developmental and evolutionary origins of the spinal accessory nerve (SAN).
• To investigate the phylogenetic trajectory leading to the gnathostome-specific SAN.
• To determine the SAN's classification by examining its innervation and cytoarchitecture.

Main Methods:

• Assessed innervation and cytoarchitecture of the spinal nerve plexus in lampreys.
• Reviewed existing studies on SAN in diverse vertebrate species and their embryos.
• Reconstructed an evolutionary sequence of developmental neuronal patterning.

Main Results:

• Identified a lamprey-like spinal nerve plexus innervating the infraoptic muscle as a potential precursor to the cucullaris muscle.
• Established an evolutionary pathway linking ancestral neuronal patterning to the modern SAN.
• Provided evidence for the SAN's origin within a primitive spinal nerve plexus.

Conclusions:

• The spinal accessory nerve (SAN) likely evolved from a spinal nerve plexus found in ancestral jawless vertebrates.
• The cyclostome-type infraoptic muscle is a probable precursor to the cucullaris muscle, innervated by this ancestral plexus.
• This study clarifies the evolutionary history and developmental basis of the SAN, a defining feature of jawed vertebrates.