NEUROLOGY
caudal oropharynx only on the side of the muscle
atrophy. You are puzzled and take a second and more
thorough look to confirm your findings. Why is there
foamy saliva in the back of his mouth, you wonder?
Thinking through the causes of muscle atrophy—
disuse versus neurogenic atrophy—you think that it
must be neurogenic atrophy, given how quickly the
owner reported the onset of the atrophy. But then
which nerve? Ah, but of course! You remember your
vet school neurology training and recall that cranial
nerve V, the trigeminal nerve, innervates the muscles
of mastication. All those mnemonics you memorized
to learn the cranial nerves and their functions are
paying off. But then how to account for the abnormal
saliva accumulation in Max’s oropharynx?
A quick trigeminal nerve review
The trigeminal nerve has three main branches—the
ophthalmic, maxillary and mandibular nerves. All three
branches relay sensory information from the head
and face to the brain. 1 Only the mandibular branch
is responsible for providing motor function to the
muscles of mastication—the masseter, temporalis,
lateral and medial pterygoids, rostral portion of the
digastricus, and mylohyoideus muscles—as well
as some lesser-known muscles such as the tensor
tympani muscle (involved in modulating the ossicles
in the middle ear) and the tensor veli palatani muscle
(involved in opening the pharyngeal orifice of the
auditory tube).
Clinically, unilateral dysfunction of the mandibular
branch of the trigeminal nerve is easy to recognize,
often because of the dramatic muscle atrophy of the
masseter and temporalis muscles. Pterygoid muscle
atrophy is inferred by the enophthalmos, as the
pterygoid muscles provide ventral and medial support
of the eye’s position in the orbit. The third eyelid will
elevate passively with the enophthalmia.
The salivary link
What probably was not a focus in your veterinary
curriculum was the role that the trigeminal nerve
serves in the autonomic nervous system. Branches of
the trigeminal nerve act as a conduit for the distribution
of parasympathetic innervation to target organs such
as the lacrimal and salivary glands.
Anyone who has suffered from dry eye
(keratoconjunctivitis sicca, or KCS), dry mouth
(xerostomia) or dry nose (xeromycteria) can understand
the life-altering effects such conditions can cause.
Since veterinary patients can’t self-report symptoms,
most owners don’t recognize altered autonomic
function until severe end-stage consequences are
present. To appreciate this, one only need examine a
dog with untreated KCS. The lack of tear production
leads to corneal opacification from corneal oedema,
neovascularisation, pigment deposits, squamous
metaplasia and hyperkeratinisation of the cornea,
which results in visual deficits. 2 People with
xerostomia suffer from severe oral discomfort, dental
caries, speech problems and difficulty eating and
swallowing. 3
In dogs, there are four major salivary glands—the
parotid, zygomatic, sublingual and submandibular
glands. 4 Although structural diseases of the salivary
glands (i.e. sialocoele, sialadenitis, salivary gland
neoplasms and sialadenosis) are well-known in
veterinary medicine, pure functional disturbances of
the salivary glands have yet to be well-described.
Quite detailed Neurological Anatomy. Important
points in bold for ease of reading...ed
Each of the salivary glands is innervated by the
sympathetic and parasympathetic nervous system.
As a whole, the autonomic nervous system has both
afferent (sensory) and efferent (motor) components.
Efferents are composed of a two-neuron system—
preganglionic and ganglionic neurons. Sometimes
these are referred to as first-order and second-order
neurons, respectively. The preganglionic neuron is
located in the central nervous system. Preganglionic
axons synapse with the ganglionic neurons in various
ganglia. Postganglionic axons innervate target organs
such as glands, the heart, the lungs and many other
organs.
For sympathetic innervation, preganglionic neurons
are located in the intermediate gray matter of the
thoracic and cranial lumbar spinal cord. For most of
the body, the ganglionic neurons are located in the
paired sympathetic trunk, which runs bilaterally along
the ventrolateral aspect of the vertebral column.
For structures of the head, ganglionic neurons are
located in the cranial cervical ganglia near the ventral
and caudal aspect of the skull. Long postganglionic
axons course to their target organs. Sympathetic
postganglionic axons use norepinephrine as their
neurotransmitter.
For parasympathetic innervation, preganglionic
neurons are located in the brain stem, adjacent to the
motor neurons for cranial nerves III (oculomotor),
VII (facial), XI (glossopharyngeal) and X (vagus), as
well as sacral spinal cord segments. The ganglionic
neurons are often located in ganglia close to their
intended target organs. Short parasympathetic
postganglionic axons course to their target organs
and use acetylcholine as their neurotransmitter.
For the salivary glands, cranial nerves VII (facial) and
IX (glossopharyngeal) provide for parasympathetic
innervation. 5 Specifically, preganglionic neurons
for the parasympathetic innervation of the
zygomatic and parotid glands are provided by the
parasympathetic nuclei of the glossopharyngeal
Issue 04 | SEPTEMBER 2018 | 21