Classification of cranial nerves

Summary

• Classification of cranial nerves
  • Summary of functions

Classifying cranial nerves

• Consider 3 main differences within the nerves
  • Effect – Sensory vs motor vs mixed (aka afferent vs efferent)
  • Type of organ function – General vs special 
  • Target area – Somatic vs visceral

• Overall, there are 7 types of nerves
  • 4 sensory
  • 3 motor
  • Sensory/afferent – 4 types
    • General somatic afferent -General sensation from skin and mucous membranes e.g. touch, pressure, temperature.
    • Special somatic afferent – e.g. vision, hearing, balance.
    • General visceral afferent – parasympathetic stuff – e.g. pharynx, lung, heart etc.
    • Special  visceral afferent – taste
  • Motor/efferent – 3 types
    1. General somatic efferent – To voluntary muscle
    2. Special visceral efferent – To pharyngeal arch structures
    3. General visceral efferent – To involuntary muscle/ glands
    • NB there is no special somatic efferent

Summary of function

• There are broadly 5 types of cranial nerves as described in the table below
  

  Cranial nerve	Functions
  I, II and VIII	Purely sensory
  III, IV and VI	Control eye movement and pupillary constriction
  XI and XII	Purely motor output
  V, VII, IX and X	Mixed motor and sensory functions
  III, VII, IX , X	Also have parasympathetic function

• The functional component of each cranial nerve can also be looked at in more detail in the table below
  • Note that some nerves have a whole host of functions, particularly Facial, Glossopharyngeal, and Vagus.

    

• ---

  For medical students, it is very important to have a broad idea of what each nerve is responsible for. Combining this with an understanding of the different types of fibres (e.g. the difference between GSA vs SVA), you can figure out the types of fibres in a given nerve.

  • E.g. for olfactory, you can appreciate is is involved in the sensation (afferent) of smell (special) which is felt by endogenous structures (therefore visceral). Therefore, it is special, visceral, afferent (SVA)