Autonomic Innervation
of Mouth and Pharynx
Autonomic (general visceral efferent) nerves
innervate glands and the smooth muscle. The smooth muscle of the mouth and
pharynx is largely found in the walls of blood vessels and erector pili muscles
in the skin. Glands receive both sympathetic and parasympathetic axons that
modulate their activity. Typically, this occurs via a two-neuron chain with the
cell body of the first neuron in the central nervous system and the cell body of
the second neuron in a peripheral ganglion.
For the palatine glands, the cell body
of the first-order parasympathetic neuron is located in the superior salivatory
nucleus of the pons, and the axon of this neuron follows the nervus intermedius
root of VII, the greater petrosal nerve of VII, and then the nerve of the
pterygoid canal (vidian nerve) to reach the pterygopalatine ganglion, where
it synapses with the cell body of the second-order neuron. The axon of this
second-order neuron follows the palatine nerves and their branches to be
distributed along the palate. For sympathetic input to the palatine glands, the
first-order neuron cell body is located in the intermediolateral cell column of
the upper thoracic segments of the spinal cord. The axon of this neuron follows
the anterior root of the related thoracic nerve, the spinal nerve, and the
anterior primary ramus to the white ramus communicans, along which it passes to
the sympathetic ganglion at that level. Thereafter the first-order axon ascends
in the sympa- thetic trunk to synapse with the second-order neuron in the superior
cervical ganglion. The axon of this second order neuron enters the
periarterial plexus of the nearby internal carotid artery and may take two
courses. One follows the plexus superiorly to the carotid canal and then leaves
the plexus as the deep petrosal nerve, which joins the greater petrosal nerve
in the foramen lacerum to form the nerve of the pterygoid canal. The
sympathetic fibers pass through the sphenopalatine ganglion without synapse and
follow the palatine nerves to their distribution. The other course follows
periarterial plexuses all the way to the distribution of the greater and lesser
palatine arteries.
For the innervation of the
submandibular and sub-lingual glands, the first-order parasympathetic neuron
reaches the facial nerve, as described above for the innervation of the
palatine glands, and then follows the chorda tympani branch to the
lingual nerve. The axon then accompanies the lingual nerve until it leaves by a
branch to the submandibular ganglion, where the pathway to the
sublingual gland synapses. Many of the fibers carrying impulses to the
submandibular gland itself pass through this ganglion to synapse in small
ganglia on the surface on the gland. Axons of the second-order neurons go
directly to the submandibular and sublingual glands. Those destined for the
latter may follow the lingual nerve on their way. The sympathetic supply to
these two glands follows the course described above for the palatine gland as far
as the periarterial plexus. From there, axons follow arteries to the
submandibular and sublingual glands.
For the innervation of the parotid
gland, the first-order parasympathetic neuron has its cell body in the inferior
salivatory nucleus of the medulla, and the axon of this neuron follows the
glossopharyngeal nerve, its tympanic branch, and then the lesser
petrosal nerve to the otic ganglion, where it synapses with the
second-order neuron cell body. The axon of this neuron follows the auriculotemporal
nerve to the parotid gland. The sympa- thetic innervation is similar to
that described above for the submandibular and sublingual glands.
For the parasympathetic supply of
small glands not discussed, one must assume that axons of second-order neurons
with cell bodies in the parasympathetic ganglia, described above, follow nerves
going to the area, or that the parasympathetic fibers in VII, IX, or X synapse
in small ganglia in the area. For the sympathetic supply, second-order neuron
cell bodies in the superior cervical ganglion can send axons by any convenient
nerve or periarterial plexus.
