Out of 12 pairs of the cranial nerves the initial 2 pairs originate from the forebrain and the next 10 pairs originate from the brainstem. They may be numbered 1 to 12 in the craniocaudal sequence of their connection on the brain. The cranial nerves are usually designated by Roman numerals.

Image showing cranial nerves at the base of the brain.

The 12 pairs of nerves are:

A tiny bundle of nerve fibres closely related to the olfactory nerves is named the 13th pair or ‘O’ pair of cranial nerves. Its precise function isn’t understood, but it’s believed to give a unique chemo-sensory nerve pathway of olfaction and impacts the secretion of luteinizing hormone-releasing factor from the hypothalamus. Additionally, it plays an essential part in smell-mediated sex behaviour. Every nerve is connected to the cerebrum, posterior to the olfactory stria of the olfactory tract close to anterior perforated substance and septal regions and spread to the nasal mucous membrane

Mnemonics

On Old Olympus’s Towering Top, A Fine Veteran German Viewed A House

Some Say Marry Money But My Bride Says Big Balls Matter More

On¬†Old¬†Olympus’¬†Towering¬†Top,¬†A¬†Finely¬†Vested¬†German¬†Viewed¬†A¬†Hawk

Ooh, Ooh, Ooh, to Touch and feel very good velvet. Such heaven!

Functional Parts

A cranial nerve is composed of motor fibres (motor nerve) or sensory fibres (sensory nerve) or both the motor and sensory fibres (mixed nerve).

The motor fibres of cranial nerves can be of the following 3 types:

  • Somatic efferent (SE) or general somatic efferent (GSE) fibres. They supply the striated muscles which grow from somites.
  • Special visceral efferent (SVE) fibres. They supply the muscles which grow from the mesoderm of pharyngeal arches.
  • General visceral efferent (GVE) fibres. They supply the glands, smooth muscles of viscera and vessels. They can be preganglionic parasympathetic fibres.

The sensory fibres of cranial nerves can be of the following 4 types:

  • General somatic afferent (GSA) fibres. They carry general sensations of pain, feel and temperature from skin and proprioceptive sensations of shaking and muscle and joint perception.
  • General visceral afferent (GVA) fibres. They carry general sensations of distension and ischemic pain from viscera.
  • Special visceral afferent (SVA) fibres. They carry unique sensations of flavor from tongue.
  • In addition to the aforementioned 3 types, the sensory fibres could be particular somatic afferent (SSA) which carry unique sensations of smell, hearing and equilibrium.

Key Points

  • The motor fibres of cranial nerves originate as outgrowths of axons from motor nuclei situated inside the central nervous system (CNS).
  • The sensory fibres originate as outgrowths of axons from cells situated inside the sensory ganglia (situated outside the CNS) and terminate in the sensory nuclei situated inside the CNS.
  • The motor and sensory nuclei inside the CNS are arranged in longitudinal columns named functional columns.

Summary