Face Sensation

• Nerve fibres giving sensation to the face are carried in cranial nerve V - the trigeminal nerve.
  • The trigeminal nerve provides sensory innervation to:
    • The skin of the face
    • The ears
    • The nasal mucosa
    • The cornea
  • There are three divisions of the trigeminal nerve
    • The mandibular branch
    • The maxillary branch
    • The opthalmic branch
• Facial sensation is tested by observation for an avoidance response or reflex movement following stimulation.
  • All the divisions of the trigeminal nerve should be tested. This is achieved by:
    • Stimulation within the nostrils
    • Stimulation within the ears
    • Stimulation between the mandibles
    • Testing the corneal reflex.
      1. Pressure is exerted on the corneal surface through the eyelid.
      2. The opthalmic branch of the trigeminal nerve provides sensory (afferent) input.
      3. Efferent signals are sent via the abducens nerve (CN VI)
      4. The retractor oculi muscle retracts the eye

Masticatory muscles

• The mandibular branch of the trigeminal nerve carries motor fibres.
  • It is the only division of the trigeminal nerve that does so.
• The muscles of mastication are innervated by these fibres.
  • The masseter and temporalis muscles are particulaly influenced by the mandibular brach of the trigeminal nerve.
    • These muscles of mastication are also the easiest to assess.
    • These muscles should be closely examined for signs of atrophy.
      • Atrophy of the temporalis and masseter muscle indicates damage to the mandibular branch of CN V.

Facial Expression

• The muscles of facial expression are innervated by branches of cranial nerve VII - the facial nerve.
• The facial nerve also carries the parasympathetic supply to the lacrimal glands.
  • Horses with facial nerve dysfunction are therefore prone to corneal ulceration, due to :
    • An inability to blink
    • Poor or absent tear production

Facial Nerve Dysfunction

• Facial nerve dysfunction is common in the horse.
• It is readily identified by one or more of the following:
  • A lip droop
  • Muzzle deviation
    • Deviation is towards the normal side.
  • An ear droop
• Early or mild dysfunction may be reflected by:
  • Slight changes to nostril size
  • Nostril flare
  • Reduced ear movements in response to audible stimuli
• The branch of CN VII supplying the muzzle and nostrils crosses the vertical mandibular ramus and the surface of the masseter muscle.
  • A lesion to that side of the face, for example a kick, may cause signs confined to the nose.
  • Alternatively, a more central lesion will give both ear and nostril signs.

The Palpebral Reflex

• The palpebral reflex examines the function of both CN V (afferent) and CN VII (efferent).
• The reflex is elicited by lightly touching the eyelids and watching for reflex closure.