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.

  1. 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.
  2. 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.
  3. Could it just be a random evolutionary outcome, there being absolutely no benefit of cranial innervation leading to random drift in the innervation patterns?
  • $\begingroup$ To me the third argument appears unconvincing. Talking about the first, it seems to be the most appropriate one as our special senses do put in a lot of efforts to collect every minute details of our surroundings. Also the final processing of the stimuli by these senses needs to be really very fast and accurate, whereas for the fine movements of our limbs the time duration in which a stimuli is to be analysed and responded to is comparatively more (as for quick responses the reflex arc comes into action). $\endgroup$ – Shefali Jan 5 '14 at 11:15
  • $\begingroup$ So probably there was no need for our limbs and other organs to be innervated by the cranial nerves when the spinal nerves were just enough and did their job in appropriate time interval.! $\endgroup$ – Shefali Jan 5 '14 at 11:20
  • $\begingroup$ I wonder to what extent 1) contributes to social communication (specifically, through a wider array of facial expressions). $\endgroup$ – Keegan Keplinger Jan 5 '14 at 19:40

We can somewhat rule out reason 1 because fingertips are also highly sensitive to sensory stimulus. Moreover innervation is not solely about sensory inputs but also motor functions, visceral control etc. All the nerves required for the motor function and a significant portion of visceral control nerves connect to spinal cord.

Therefore in my opinion reason 2 is the most appropriate which automatically rules out reason 3. Spinal cord is an advancement of neural organization in the chordates. Spinal cord can be viewed as a central cable in which all power lines merge (they still maintain their individuality [the spinal tracts]; they just get clustered to form an organized bundle). Evolution of spinal reflexes must have been a secondary event.

The case of vagus nerve and cranioaccessory nerve suggests that this is an ancient pathway to control the heart. Heart with a closed circulatory system is found in molluscs as well. Apart from the functional significance of the heart (which might to some extent justify direct cranial innervation), it is decently proximal to the brain as well.


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