The Autonomic Nervous System

The cranial nerves | Main Anatomy Index

The Autonomic Nervous System

• The autonomic nervous system (ANS) is the part of the nervous system, both afferent and efferent, that innervates various body systems that are not under voluntary control.
• These include the constriction and dilation of blood vessels, the activity of the viscera, the secretion of glands (fibres supplying glands are called secretomotor fibres), and assist the endocrine system to maintain a constant internal environment (homeostasis).

• The afferent neurons have their peripheral receptors in the wall of viscera and blood vessels, and their cell bodies in the dorsal root ganglia or cranial nerve ganglia. Their central processes end in the dorsal grey column of the spinal cord or the brainstem.
• The efferent neurons supply the smooth muscles in the wall of hollow viscera and blood vessels. They can be either excitatory or inhibitory.

Sympathetic and Parasympathetic Systems

• Functionally, the ANS is divided into sympathetic and parasympathetic systems. Read "Table 4. Functions of the Autonomic Nervous System (ANS), p. 29" from Moore's "Clinically Oriented Anatomy 3^rd Edition" for a larger list.

• The sympathetic system prepares the body for "fright, fight or flight". It increases the heart rate and ventricular contraction, dilates the blood vessels in skeletal muscles, constricts blood vessels in the skin and guts, increases blood sugar level, stimulates sweating, dilates the pupils, inhibits activities of the guts and gastric secretion.

• The parasympathetic system is more active at rest, having in general anabolic effects. For example, it slows down the heart rate, constricts the pupils, increases gastric secretion and intestinal motility.

General Layout of the ANS

The simplified classical view of the design of the ANS is as follows:

The efferent pathway is made up of 2 neurons:

1. a preganglionic neuron, (myelinated), located in the spinal cord or brainstem, synapsing with;
2. postganglionic neuron(s) (unmyelinated) in an autonomic ganglion.

• In general the sympathetic ganglia are further from the target organs, and the parasympathetic ganglia are located near or in the wall of target organs.

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The Sympathetic System

• The sympathetic outflow from the CNS is thoracolumbar, emerging from the spinal cord segments T1-L3.
• The sympathetic ganglia form a string of beads called the sympathetic trunk, lying on either side of the vertebral bodies.
• The two trunks extend from C1 to the level of the coccyx where they unite at the midline at the ganglion impar.
• The preganglionic fibres leave the spinal cord with the ventral root of the corresponding spinal nerve, and pass to the corresponding sympathetic ganglion via the white ramus communicans (white because the fibres are myelinated). In that ganglion, they may:

1. Synapse with postganglionic fibres which leave the sympathetic trunk in the grey rums communicans, rejoins the same ventral ramus, then are distributed to the target organs (smooth muscles of blood vessels, sweat glands...)
2. Travel up or down the trunk to synapse with postganglionic fibres in other ganglia (e.g. cervical, lumbar, sacral, which do not receive direct rami communcantes from the cervical, lower lumbar or sacral segments).
3. Pass through the ganglion without relay to synapse in prevertebral ganglion (e.g. coeliac ganglion, known to lay people as "solar plexus").

Sympathetic Supply to the Head and Neck

The preganglionic fibres, which supply the head and neck, arise from the spinal segments T1-T2. They enter the sympathetic trunk and travel upward to synapse in one of the three ganglia in the neck:

1. The superior cervical ganglion (located anterior to vertebrae C1, C2).
2. The middle cervical ganglion (small, often absent, lying anterior to C6).
3. The inferior cervical ganglion which is usually fused with the first thoracic ganglion to form the cervicothoracic (or stellate) ganglion (located anterior to C7 and the neck of the first rib).

Each of these ganglia gives rise to:

1. Cardiac branches.
2. Branches to blood vessels, sweat glands, hair follicles in the neck and head. The superior ganglion sends branches to spinal nerves C1-C4, the middle ganglion to spinal nerves C5-C6, and the cervicothoracic ganglion to spinal nerves C7, 8-T1.
3. Vascular branches which pass to the vertebral, common, internal and external carotid arteries. Some of these branches "hitch-hike" along the arteries and their branches to reach their targets in the head and neck. The superior ganglion sends branches along the internal and external carotid arteries to reach structures in the orbit, face, nasal and oral cavities and pharynx. The middle ganglion sends branches along the inferior thyroid artery to reach the larynx, trachea and upper oesophagus. The inferior ganglion sends branches to the subclavian and vertebral arteries.

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The Parasympathetic System

The parasympathetic system outflow is craniosacral, emerging from:

1. The oculomotor, facial, glossopharyngeal, and vagus nerves.
2. The spinal cord segments S2, 3, 4. These fibres travel in the branches of sacral nerves S2-S4 (nervi erigentes) to the pelvic viscera.

• The parasympathetic system supplies the heart, glands, and smooth muscles of the viscera not the sweat glands, blood vessels or erector pilorum muscles.

Parasympathetic Supply to the Head and Neck

• Click here for a diagram of the parasympathetic ganglia along the branches of the facial (CN VII) and glossopharyngeal (CN IX) nerves.

There are four ganglia in the head: ciliary, pteryopalataline, otic, and submandibular.

1. Oculomotor Nerve

• The parasympathetic fibres enter the orbit with the inferior division of the nerve and synapse in the ciliary ganglion, which is located just lateral to the optic nerve.
• The postganglionic fibres with the short ciliary nerves enter the eye to supply the sphincter pupillae (that constricts the pupil) and ciliary muscles.
• Parasympathetic fibres also travel along the superior branch of the oculomotor nerve to supply the smooth muscle component of levator palpebrae superioris.

2. Facial Nerve

• The parasympathetic fibres supply the lacrimal gland, the mucous glands of the nose and palate, the submandibular and sublingual salivary glands.

• The fibres which supply the lacrimal, nasal and palatine glands leave the facial nerve (at its genu inside the petrous bone as the greater petrosal nerve), passes through the pterygoid canal (at the root of the pterygoid process as the nerve of the pterygoid canal), and terminates in the pterygopalatine ganglion (located in the pterygopalatine fossa).
• The postganglionic fibres travel in the branches of the ganglion or the maxillary nerve to reach the nasal and palatine glands.
• The fibres supplying the lacrimal gland travel with the zygomatic nerve (a branch of the maxillary nerve, CN V²) to enter the orbit, then pass into the lacrimal branch of the ophthalmic nerve to reach the gland.

• Other preganglionic fibres leave the facial nerve in its chorda tympani branch. This nerve passes over the internal surface of the tympanic membrane before emerging from the skull through a small fissure, the petrotympanic fissure, in the temporal bone. It joints the lingual nerve (a branch of the mandibular nerve) to travel to, and synapse in, the submandibular ganglion, which is suspended from the lingual nerve.
• The postganglionic fibres either pass into the submandibular gland or rejoin the lingual nerve to reach the sublingual gland.

3. Glossopharyngeal Nerve

• The parasympathetic fibres supply the parotid gland.
• The preganglionic fibres travel with the tympanic branch into the tympanic plexus in the middle ear. The fibres emerging from this plexus as the lesser petrosal nerve, which synapses in the otic ganglion (located medial to the mandibular nerve, just below the foramen ovale).
• The postganglionic fibres then hitchhike the auriculotemporal nerve (a branch of the mandibular nerve, CN V²) to reach the parotid gland.

• Other preganglionic fibres of CN IX are secretomotor to the oropharynx. These fibres pass the pharynx via the pharyngeal plexus. They synapse on the postganglionic cells, which are distributed throughout the wall of the oropharynx.

4. Vagus Nerve

• The parasympathetic fibres supply the glands of the laryngopharynx, the larynx, the cardiac muscles, and the smooth muscles and glands of the thoracic and abdominal viscera.
• Preganglionic fibres travel with the branches of the vagus nerve to reach their targets in the neck, thorax, and abdomen. Then they synapse on postganglionic cells, which are located in the wall of the target organs, rather than in specific ganglion.

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