CHAPTER 14. THE CENTRAL AUTONOMIC SYSTEM

OBJECTIVE:
    BE ABLE TO IDENTIFY IN BRAINSTEM AND SPINAL CORD SECTIONS THE AUTONOMIC EFFERENT NUCLEI AND THE AUTONOMIC AFFERENT PARTS OF THE SOLITARY TRACT AND NUCLEUS.

AUTONOMIC EFFERENT SYSTEM

Parasympathetic Nuclei

Edinger-Westphal Nucleus

Identify the oculomotor nucleus (Sl. 31). The preganglionic parasympathetic neurons that form the Edinger-Westphal nucleus are located dorsomedially. Other such visceromotor neurons are located near the midline at more rostral levels also (Sl. 33).

The axons of the Edinger-Westphal nucleus travel in the oculomotor nerve and synapse on cells in the ciliary ganglion which then give rise to postganglionic fibers that reach the eye via the short ciliary nerves. These postganglionic fibers terminate in the sphincter muscle of the iris and the ciliary muscle.

Superior Salivatory Nucleus

This nucleus consists of parasympathetic neurons scattered in the pontine reticular formation near the facial nucleus (Sls. 21, 22).

Its preganglionic fibers emerge from the brainstem in the nervus intermedius and travel in the facial nerve and via its major petrosal branch to the pterygopalatine ganglion and via the chorda tympani and lingual nerve to the submandibular ganglion. Postganglionic fibers from the pterygopalatine ganglion innervate the lacrimal and nasal glands, while those from the submandibular ganglion terminate in the submandibular and sublingual glands.

Inferior Salivatory Nucleus

This nucleus consists of parasympathetic neurons scattered in the reticular formation of the caudal pons and rostral medulla. Rostrally, these neurons intermingle with those of the superior salivatory nucleus and, caudally, they intermingle with neurons of the nucleus ambiguus. Most of the neurons are located near the rostral part of the nucleus ambiguus (Sls. 12, 13, 14, 15).

The preganglionic fibers from the inferior salivatory nucleus emerge in the rootlets of the glossopharyngeal nerve and, after traveling in the tympanic plexus and lesser petrosal nerve, they synapse in the otic ganglion, from whence postganglionic fibers reach the parotid gland via the auriculotemporal nerve.

Dorsal Motor Vagal Nucleus

Identify in the floor of the 4th ventricle the vagal trigone which is formed by the dorsal motor nucleus of the vagus nerve (Sls. 12, 13, 14, 15, 16). Other preganglionic parasympathetic fibers of the vagus nerve arise from neurons scattered in the reticular formation near the nucleus ambiguus at mid-medullary levels.

The preganglionic components of the vagus nerve synapse in terminal ganglia distributed throughout the thoracic and abdominal cavities.

Sacral Parasympathetic Nucleus

Identify the sacral parasympathetic nucleus in the lateral part of the intermediate zone of spinal cord segments S.2, 3, 4 (Sl. 1).

After emerging in the spinal nerves of these segments the preganglionic fibers form the pelvic splanchnic nerves which enter the various pelvic plexuses especially the inferior hypogastric. After synapsing on neurons in the pelvic plexuses (vesical, prostatic, cavernous, etc.) and in the myenteric and submucous plexuses of the colon and rectum, the parasympathetic impulses pass via postganglionic fibers to the urinary bladder, erectile tissues, and the smooth muscles and glands in the colon and rectum.

Sympathetic Nuclei

All cell bodies of preganglionic sympathetic neurons are in spinal cord segments C.8 to L.2 or 3. Identify the conspicuous lateral horn or intermediolateral nucleus (Sl. 5). Additional preganglionic sympathetic neurons are located in the more medial parts of lamina VII (intermediomedial nucleus), in the area bridging these lateral and medial nuclei (intercalated nucleus), and in the lateral funiculus near the lateral horn.

The axons of these neurons are the preganglionic sympathetic fibers emerging in spinal nerves T.1 to L.2 or 3. After entering the sympathetic trunk via the white communicating rami, these preganglionic fibers terminate on sympathetic trunk ganglion cells at all levels and on autonomic plexus ganglion cells chiefly along the abdominal aorta at the origin of its celiac, superior mesenteric, and inferior mesenteric branches. Postganglionic fibers from the sympathetic trunk ganglia are distributed via perivascular, spinal, and visceral nerves to the head, limbs, trunk, and the thoracic viscera, while those from the abdominal autonomic plexus ganglia pass via perivascular nerves to the abdominal and pelvic viscera.

AUTONOMIC AFFERENT SYSTEM

Receptors

Free nerve endings, networks of fine fibrils, Pacinian-like corpuscles and the other various forms of receptive corpuscles are located in the viscera of the body, especially in the heart and blood vessels, lungs and respiratory tract, gastrointestinal tract, urinary bladder, etc.

Cranial Paths

Autonomic afferent impulses from certain regions of the head (oral cavity, pharynx, etc.), travel centrally and enter the brainstem via the glossopharyngeal nerve. Such impulses from the thoracic and abdominal viscera enter the brainstem via the vagus nerve.

The glossopharyngeal nerve carries visceral afferent impulses arising mainly from the mucous membranes of the posterior part of the tongue, the tonsil and pharynx, and from the carotid sinus and body. Impulses from these play a major role in reflexes associated with deglutition, gagging, circulation, and respiration. The cell bodies are in the petrosal ganglion and central connections are made through the solitary tract and its nucleus.

The vagus nerve distributes visceral afferent fibers to all parts of the digestive tube from the pharynx to as far distal as the splenic flexure, to the heart and the walls of the great vessels and aortic bodies, and to the walls of the bronchial tree and interalveolar tissue of the lungs. The vagal visceral afferent fibers initiate gastrointestinal, cardiovascular, and respiratory reflexes. Their cell bodies are in the inferior (nodose) ganglion and the central connections are also made through the solitary tract and its nucleus.

Central Connections in the Brainstem

Autonomic afferent fibers from the cranial nerves enter the brainstem and pass into the solitary tract (Sls. 12, 13, 14, 15, 17, 18). Closely related to this tract is the solitary nucleus, in which the terminations of the cranial autonomic afferent fibers occur.

The cranial autonomic afferents primarily initiate viscero-visceral reflexes and viscerosomatic reflexes. The course of the secondary fibers from the solitary nucleus to conscious levels is via the reticular formation. Those axons from the solitary nucleus that mediate the various reflexes terminate in the reticular formation where the circulatory, respiratory, vomiting, etc., centers are located.

Spinal Paths

Most of the fibers conveying visceral afferent impulses to the spinal cord course in the sympathetic nerves. Those from the heart wall, coronary vessels, and lungs travel via the cardiac and pulmonary nerves to the sympathetic trunk, while those from the abdominal viscera pass to the sympathetic trunk via the splanchnic nerves. The visceral afferent fibers in the sympathetic trunk enter the spinal nerves via the white communicating rami of the thoracic and upper lumbar nerves. Hence, their cell bodies are located primarily in the dorsal root ganglia of the thoracic and upper lumbar nerves.

Afferents from the pelvic viscera travel centrally via the hypogastric plexuses and splanchnic nerves and via the pelvic parasympathetic nerves. The latter have their cell bodies located in the dorsal root ganglia of S.2,3,4.

The phrenic nerve also contains visceral afferents from the pericardium, diaphragm, hepatic ligaments and capsule, pancreas, and suprarenal glands. The afferent fibers from peripheral blood vessels travel centrally in all spinal nerves.

Central Connections of Spinal Fibers

The conduction pathways for general visceral afferent impulses within the spinal cord are uncertain. The fibers enter the cord via the lateral division of the dorsal root and terminate on cells located in the medial part of the base of the dorsal horn and intermediate zone of the thoracic, upper lumbar, and middle sacral cord segments. The ascending pathways are chiefly within the anterolateral quadrants of the spinal cord and reticular formation of the brainstem.