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.