The sympathetic preganglionic neurons are located in the intermediate gray matter along the cervical, thoracic and lumbar spinal cord segments. However, as they only establish contact with the postganglionic neurons between T1 and L5 spinal segments. The preganglionic neurons that do not make contact degenerate, so they can only be found between T1 and L5.
The sympathetic myelinated axons leave the spinal cord forming part of the ventral roots of the spinal nerves. When they reach the spinal nerve, the sympathetic fibers form the white communicating branch that reaches the sympathetic trunk. This trunk is a chain of ganglia located ventrolaterally to the vertebral column that extends from the base of the skull to the first caudal vertebra. 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 that fuses with the first four or five thoracic ganglia to form the stellate or cervicothoracic ganglion.
Not all the preganglionic neurons synapse in the ganglia of the sympathetic trunk. The ones that do not synapse they will do so in other sympathetic ganglia. Some of the postganglionic unmyelinated fibers may return, from the associated ganglion of the sympathetic trunk, to the spinal nerve, forming the gray communicating branch (parallel to the white communicating one) so they can reach capillaries, sweat glands and erector pili muscles via spinal nerve branches.
Some sympathetic postganglionic cells from the stellate ganglion reach the C6, C7, C8, and T1 spinal nerves of the brachial plexus to sympathetically innervate the forelimb. Other unmyelinated sympathetic postganglionic fibers branch off from the stellate ganglion and form the vertebral nerve. This nerve forms a plexus around the vertebral artery and vein. In their course, some fibers peal off the vertebral nerve to join the cervical spinal nerves. The vertebral nerve reaches up to C3/C4 spinal nerves. The sympathetic postganglionic innervation for C1 to C3 spinal nerves comes from 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 middle cervical ganglion, the last preganglonic sympathetic fibers incorporate to the vagal nerve (they give rise to the vagosympathetic trunk) in order to reach the cranial cervical ganglion.
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. The sympathetic fibers join the big blood vessels for innervation but not to the smaller capillaries and venules. As this innervation is segmental, the internal carotid plexus is dense for the proximal part of the artery and for the origins of its branches5. There is also a mismatch between the muscular-nerve development, and the vascular development. The muscle differentiation and innervation precedes the growth of vessels so the alternative route for sympathetic fibers is to join to the cranial nerves as they do in the limbs and trunk.
The carotico-tympanic branch leaves the internal carotid plexus and reaches the tympanic plexus, through the petrosal bone, to sympathetically innervate the middle ear mucosa, auditory tube, the mastoid air cells, and the oval and round windows. The deep petrosal nerve also branches off from the internal carotid plexus and joins the major petrosal nerve inside the petrosal portion of the temporal bone and, together, they form the nerve of the pterigoid canal (or Vidian nerve) that reaches the pterygopalatine ganglion. The sympathetic fibers bypass the ganglion without synapsing and they incorporate to the branches of the maxillary nerve. They innervate the lacrimal gland and palate. Other sympathetic fibers from the deep petrosal nerve enter the cranial cavity, at the level of the trigeminal ganglion, and join the III, IV, V and VI cranial nerves.
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.
The sympathetic fibers that innervate the smooth muscle of the periorbita and the dilator muscle of the pupil join the ophthalmic nerve and become the long ciliary nerves, and to the oculomotor nerve to form part of the short ciliary nerves. The sympathetic axons of the maxillary nerve that innervate the lacrimal gland and nasal glands are vasoconstrictor and reduce the secretion. The external carotid plexus innervates the thyroid and parathyroid glands that are vasoconstrictors in the thyroid gland and may be secretor in the parathyroid gland. It is suggested that the sympathetic activity inhibits secretion of the salivary glands through a vasoconstrictor activity so the viscous saliva formed by adrenergic activity may be due to a reduction of blood flow.
Other postganglionic fibers from the cranial cervical ganglion incorporate to the IX, X, XI and XII cranial nerves. It has been pointed out the sympathetic fibers also join the auricular branch of the vagus nerve.
One group of postgangljonic 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 so the extracranial internal carotid artery (that branches off from the occipital artery) enters the tympano-occipital fissure and gets occluded. All the postganglionic sympathetic fibers must pass through the tympanic cavity as part of the tympanic plexus. The sympathetic fibers enter the cranial cavity through an small fissure between the articulation of the petrous temporal bone and the sphenoid bone to join the ophthalmic nerve at the level of the trigeminal ganglion.
In all species, sympathetic denervation of the eye leads to a parasympathetic predominance. This situation is known as Horner’s 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.
Having reach this point it is important to notice 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 must 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 muscle. The dilator muscle does not participate actively. Differences between species must be due to the number of β adrenergic receptors in the iris.
The sympathetic nerve fibers to the abdominal region come from the thoracic and abdominal sympathetic trunk. A great nerve detaches from the sympathetic trunk between the T6 and the T10 ganglia, is the greater splanchnic nerve. It enters the abdominal cavity lateral to or trough the crus of the diaphragm to reach the celiacomesenteric plexus (located at the origin of the celiac and cranial mesenteric arteries). It innervates the aorta and the adrenal gland. 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 (lumbar splanchnic nerves) reach the ganglia located at the origin of the celiac, cranial mesenteric, renal, gonadal and caudal mesenteric arteries. 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 splachnic nerve that stimulate the release of the epinephrine and norepinephrine into the bloodstream. Thus, the sympathetic stimulation of the orgnans 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 to form the right and left hypogastric nerves. Most of these are postganglionic neurons but a few are preganglionic ones. They reach the pelvic viscera via the pelvic plexus. This retroperitoneal plexus is located on the lateral surface of the rectum and also receives parasympathetic fibers from the pelvic nerves. Some sympathetic postganglionic neurons synapse on parasympathetic postganglionic neurons in the pelvic plexus resulting in an inhibition of the parasympathetic activity, preventing the contraction of the smooth muscle of the bladder.
The tail and the pelvic limb receive sympathetic fibers through communicating rami of the sacral and caudal portions of the sympathetic trunk that join the lumbosacral plexus.
 Also called sympathetic chain or paravertebral ganglia.
 This is a ganglion corresponding to spinal segments C7 and C8 that become incorporated to the stellate ganglion.
 This is a ganglion corresponding to spinal segments C4 to C6.
 This is located ventrally to the tympanic bulla and the proximal parts of the IX, X, XI and XII cranial nerves.
 MAKLAD, A., QUINN, T. , and FRITZSCH, B. Intracranial Distribution of the Sympathetic System in Mice: DiI Tracing and Immunocytochemical Labeling. The Anatomical Record 263:99–111 (2001).
 Page 30 of "The cardiorespiratory system" by King, A. S. Blackwell Science. 1999.
 Matthews, B. and Robinson, P.P., The course of postganglionic sympathetic fibers distributed with the facial nerve in the cat. Brain Res. 1986 10;382 (1): 5560.
 From the Greek syndromos, "running together".
 From the Greek myo, "meiosis, diminution".
 From the Greek en, "inside"; opthalmos", eye".
 Autonomic Nervous System: Ophthalmic Control J. Pintor, in Encyclopedia of Neuroscience, 2009. The Neural Control of the Iris.
 From the Greek splaghon, "viscera".
 Page 280 of "Basic neuroscience. Anatomy and physiology" by Guyton, A.C., Saunders company. second ed. 1991.