The soma of the sympathetic preganglionic neurons is located in the lateral intermediate nucleus of the intermediate gray matter, along the spinal cord segments from T1 to L3-L5.
The sympathetic myelinic axons leave the spinal cord as part of the ventral roots of the spinal nerves. After leaving through the intervertebral foramen, the sympathetic fibers leave the spinal nerve, giving rise to the white communicating branch that reaches the sympathetic trunk. Some of the postganglionic (unmyelinated) fibers return from the sympathetic trunk to the spinal nerve forming the gray communicating ramus (it runs together with the white communicating branch). In this way, the sympathetic innervation can reach the capillaries, the sweat glands and the erector muscles of the hair through the branches of the spinal nerves.
The sympathetic trunk is formed by a chain of ganglia, located ventrolaterally to the vertebrae, which extends from the base of the skull to the first caudal vertebra. During development, the neurons of the intermediate gray matter are located in the cervical, thoracic, lumbar and sacral segments. However, since only the postganglionic neurons between the T1 and L5 spinal segments establish contact with the sympathetic trunk ganglia. Non-contact preganglionic neurons degenerate. The first three cervical ganglia fuse to form the cranial cervical ganglion. The fourth, fifth and sixth ganglia form the middle cervical ganglion. The seventh and eighth ganglia form the caudal cervical ganglion, which fuses with the first four thoracic ganglia to form the stellate or cervicothoracic ganglion. The spinal nerves caudal to L5 are only connected to the sympathetic trunk through the communicating gray branches.
Some sympathetic postganglionic fibers of the stellate ganglion give rise to gray communicating branches that reach the ventral branches of the C6, C7, C8 and T1 spinal nerves that form the brachial plexus. They form the sympathetic innervation to the forelimb. Other sympathetic amielynic postganglionic fibers from the stellate ganglion form the vertebral nerve that runs with the vertebral artery. In its course, some fibers detach from the vertebral nerve to join the cervical spinal nerves. The vertebral nerve reaches as far as the C3-C4 spinal nerves. The postganglionic sympathetic innervation for spinal nerves C1 to C3 comes from the postganglionic fibers of the cranial cervical ganglion that join the vertebral artery at C1 level.
Preganglionic fibers from the stellate ganglion form two branches that pass around the subclavian artery, forming the ansa subclavia, to unite at the middle cervical ganglion. The organs of the thoracic cavity receive sympathetic innervation from the stellate ganglion and the middle cervical ganglion. The fibers follow the arteries to the heart and bronchi. They are distributed to the atria and ventricles, and increase the heart rate and the force of ventricular contraction. In the lungs, they inhibit the bronchial smooth muscle in order to dilate the airway passage. From the middle cervical ganglion, the last preganglionic sympathetic fibers ascend through the vagus nerve to the cranial cervical ganglion, giving rise to the vagosympathetic trunk.
How postganglionic sympathetic fibers from the cranial cervical ganglion reach cranial structures remains confusing. Two pathways have been postulated: forming a plexus around the internal and external carotid arteries (named internal carotid plexus and external carotid plexus respectively) or joining to the cranial nerves. During development, sympathetic innervation of arteries is facilitated by secretion of axon guidance nerve growth factor by arterial smooth muscle. However, in the skin, in the absence of cutaneous innervation, neither blood vessels nor arterial differentiation occurs. However, in the skin, in the absence of skin innervation, blood vessels and arterial differentiation do not occur.
We know that in the trunk and in the limbs, the postganglionic fibers are attached to the somatic spinal nerves through the gray communicating branches. These nerves are releasing sympathetic branches to innervate, smoothly, the smooth muscle of the arteries and veins. In the head, all cranial nerves have been found to have sympathetic fibers in humans. Therefore, the autonomic innervation of the head must follow the same pattern as the spinal nerves.
The postganglionic fibers of the cranial cervical ganglion join the internal carotid artery to enter the tympano-occipital fissure into the carotid canal. In its interior, the carotico-tympanic branch detaches and reaches to the tympanic plexus (through the petrous bone) to sympathetically innervate the mucosa of the middle ear, the auditory tube, the mastoid cells, and the oval and round windows. The deep petrosal nerve (sympathetic fibers from the internal carotid plexus) joins the major petrosal nerve (parasympathetic fibers from the facial nerve) to form the nerve of the pterygoid canal (or vidian nerve) that reaches the pterygopalatine ganglion. Sympathetic fibers bypass the ganglion without synapsing, and join the branches of the maxillary nerve. They innervate the lacrimal gland, and the palate and nasal mucosa. Other sympathetic fibers of the deep petrosal nerve enter the cranial cavity, at the level of the trigeminal ganglion, and join the cranial nerves III, IV, V and VI.
Sympathetic fibers from the tympanic plexus also enter the cranial cavity at the level of the trigeminal ganglion to join the III, IV, V and VI cranial nerves. Others incorporate to the minor petrosal nerve, bypassing the otic ganglion, and join to the mandibular nerve to reach the parotid gland and the zygomatic gland.
The sympathetic fibers that supply the smooth muscle of the periorbita and the dilator muscle of the pupil join the ophthalmic nerve to form the long ciliary nerves, and the oculomotor nerve to form part of the short ciliary nerves (these latter fibers do not synapse in the ciliary ganglion). The sympathetic axons of the maxillary nerve that innervate the lacrimal gland and the nasal glands are vasoconstrictors and reduce secretion.
The sympathetic fibers that supply the thyroid and parathyroid glands are vasoconstrictive in the thyroid gland and may be secretory in the parathyroid gland. It is suggested that sympathetic activity inhibits salivary gland secretion through vasoconstrictor activity, so that viscous saliva formed by adrenergic activity may be due to reduced blood flow.
In the eye, sympathetic fibers (which form the long ciliary nerves) and parasympathetic fibers (which form the short ciliary nerves) enter the eye along with the long posterior ciliary artery and the short posterior ciliary arteries respectively, in the vicinity of the optic nerve. Horner syndrome (miosis, smaller palpebral fissure, protrusion of the semilunar fold of the conjunctiva or third eyelid, mild enophthalmos and mild congestion of the bulbar conjunctiva) is not induced by injuring the ophthalmic artery, common carotid, external carotid, or internal carotid artery. Therefore, instead of forming a plexus that runs along the blood vessels, the sympathetic innervation of the eye comes from fibers of the cranial cervical ganglion that have joined the cranial nerves to enter the eye along with the arteries.
Other postganglionic fibers from the cranial cervical ganglion incorporate to the IX, X, XI and XII cranial nerves. One group of postganglionic sympathetic fibers leave the cranial cervical ganglion and join the auricular branch of the vagus nerve at the jugular foramen. This branch runs in the petrous part of the temporal bone to join the facial nerve in the facial canal.
In the cat, there is no carotid canal, but there is a small extracranial internal carotid artery (branch of the occipital artery) that enters through the tympano-occipital fissure and dissapears. Its function is to transport the postganglionic sympathetic fibers, which form the carotypotympanic branch, so that they enter the tympanic cavity and form the tympanic plexus. Sympathetic fibers from this plexus enter the cranial cavity through a small fissure between the joint of the petrous temporal bone and the sphenoid bone to join the ophthalmic nerve at the level of the trigeminal ganglion and innervate the eye.
In all species, sympathetic denervation of the eye leads to Horner syndrome. However, in large animals signs could be easily overlooked as they are not readily apparent. It is important to remember that the internal carotid artery is present in dog and horse, and regresses and obliterates in cat, pig and ruminants. However, all species with otitis may suffer from Horner’s syndrome. This leads us to the idea that sympathetic fibers must enter through the tympanic cavity and not with the internal carotid artery. Then the differences of Horner’s syndrome between species may be due to the number of adrenergic receptors in the eye. Noradrenaline acts on β adrenoceptors relaxing the sphincter muscle of the iris and on α adrenoceptors activating the dilator muscle. Mydriasis is mainly due to the relaxation of the sphincter pupillary muscle since the dilator muscle dilator pupillary muscle does not participate actively. Differences between species may be due to the number of α and β adrenergic receptors in the iris and the anatomy of the pupil. Sympathetic denervation results in an increase in the number of α receptors activating the dilator muscle of the pupil.
The sympathetic nerve fibers of the abdominal region come from the thoracic and abdominal sympathetic trunk. The major splanchnic nerve leaves the sympathetic trunk between the T6 and T10 ganglia. It enters the abdominal cavity, trough the crus of the diaphragm, to reach the celiacomesenteric plexus (located at the origin of the celiac artery and cranial mesenteric artery). During its course, some fibers may leave the greater splanchnic nerve forming small branches called lesser splanchnic nerves. Other fibers from the lumbar sympathetic trunk form the lumbar splanchnic nerves that reach the ganglia located at the origin of the cranial mesenteric artery, renal arteries, gonadal arteries and caudal mesenteric artery. The postganglionic sympathetic fibers follow the course of the arteries to reach their destination. The adrenal gland is innervated by preganglionic fibers of the great splanchnic nerve that stimulate the release of the epinephrine and norepinephrine into the bloodstream. Adrenaline (or epinephrine) is synthesized and stored in the adrenal medulla and released into the systemic circulation. Norepinephrine (or norepinephrine) is synthesized and stored not only in the adrenal medulla, but also in the sympathetic nerves. Therefore the sympathetic stimulation of the organs takes place in two ways: directly, through sympathetic fibers, and indirectly through the hormones released by the adrenal medulla.
The sympathetic nerve fibers that innervate the organs of the pelvic cavity arise from the caudal mesenteric ganglion and form the right and left hypogastric nerves. Most of these fibers correspond to postganglionic neurons but a few are preganglionic ones. They reach the pelvic viscera through the retroperitoneal pelvic plexus, located on the lateral surface of the rectum. It is made up of sympathetic fibers from the hypogastric nerves and parasympathetic fibers from the pelvic nerves. Some sympathetic postganglionic neurons synapse on parasympathetic ganglia of the pelvic plexus resulting in an inhibition of parasympathetic activity, preventing the contraction of the smooth muscle of the bladder.
The tail and pelvic limb receive sympathetic fibers through the gray communicating branches of the lumbosacral and caudal portions of the sympathetic trunk.
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 From the Greek myo, "meiosis, diminution".
 From the Greek en, "inside"; opthalmos", eye".
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