The posterior auricular nerve arises from the facial nerve

Facial nerve

The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...

 close to the stylomastoid foramen

Stylomastoid foramen

Between the styloid and mastoid processes of the temporal bone is the stylomastoid foramenIt is the termination of the facial canal, and transmits the facial nerve and stylomastoid artery.- Clinical Relevance :...

 and runs upward in front of the mastoid process

Mastoid process

The mastoid process is a conical prominence projecting from the undersurface of the mastoid portion of the temporal bone. It is located just behind the external acoustic meatus, and lateral to the styloid process...

; here it is joined by a filament from the auricular branch

Auricular branch

Auricular branch can refer to any one of several different structures having to do with the ear or hearing:*Nerves**The auricular branch of the vagus nerve - "ramus auricularis nervi vagi"...

 of the vagus and communicates with the posterior branch of the great auricular as well as with the lesser occipital.

As it ascends between the external acoustic meatus and mastoid process it divides into auricular and occipital branches.

• The auricular branch supplies the auricularis posterior and the intrinsic muscles on the cranial surface of the auricula
  Ear
  The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
  
  .

• The occipital branch, the larger, passes backward along the superior nuchal line of the occipital bone
  Occipital bone
  The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
  
   and supplies the occipitalis.

External links

• http://www.dartmouth.edu/~humananatomy/figures/chapter_47/47-5.HTM