The cranial nerve innervation is highly disproportionate, as far as humans are concerned. I am not sure of the advantage of being innervated by cranial nerve versus being innervated by a normal spinal nerve branches, if any.
If there is any advantage to be innervated by cranial nerves, why is there such a largely disproportionate distribution of these cranial nerves?
Only the Vagus nerve (
X) innervates parts other than the head of the animal. While there are 3 motor nerves just for the 6 eyeball muscles and the ciliary muscles (
III,IV,VI), there is only one mixed nerve (
X) innervating heart, gastro-intestinal tract and the respiratory apparatus and all smooth muscles therein. While there are three nerves for gustatory receptors, (
VII,IX,X), one for retina (
II), cochlea and vestibule (
VIII) and the olfactory receptors (
I) each, the entire plethora of intero-receptors in thoracic and abdominal viscera, aortic arch, external auditory meatus, tympanic membrane etc are carried by a single nerve (
X). I am not sure if the disproportion is limited to the disproportionate area of innervation or even to the density of innervation in the target tissue.
Below are a few possible explanations that I came across while surfing the net, all without any strong arguments.
- Is it because of the fineness of the senses in the head, i.e. the skin receptors in facial skin being higher than in the general body, providing a finer resolution of tactile reception? The special senses (vision, hearing, smelling and tasting) are also very fine in their resolution and might require more concentrated innervation and direct interpretation by the brain. But then the fingers and feet also have a high density of skin receptors, and the tactile sense can also be very fine (owing to several different kind of receptors) requiring dense innervation? Certain body movements (especially of hand) are very fine and detailed, requiring a very dense and co-ordinated motor neuron framework.
- Is it because the cranial nerves, owing to some developmental restraint or functional trade-off, can only innervate areas close to its place of origin effectively? This might explain why 11 pairs of nerves innervate only the head region and only 1 pair innervates the remaining body. I have no idea about the validity and the reason of this claim.
- Could it just be a random evolutionary outcome, there being absolutely no benefit of cranial innervation leading to random drift in the innervation patterns?