The superior cervical ganglion (SCG) is part of the

autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...

(ANS); more specifically, it is part of the

sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...

, a division of the ANS most commonly associated with the

fight or flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...

. The ANS is composed of pathways that lead to and from

ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...

, groups of nerve cells. A ganglion allows a large amount of divergence in a neuronal pathway and also enables a more localized circuitry for control of the innervated targets. The SCG is the only ganglion in the sympathetic nervous system that innervates the head and neck. It is the largest and most rostral (superior) of the three

cervical ganglia The cervical ganglia are paravertebral ganglia of the sympathetic nervous system. Preganglionic nerves from the thoracic spinal cord enter into the cervical ganglions and synapse with its postganglionic fibers or nerves. The cervical ganglion ha ...

. The SCG innervates many organs, glands and parts of the carotid system in the head.

Structure

Location

The SCG is located opposite the second and third

cervical vertebrae In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...

. It lies deep to the sheath of the

internal carotid artery The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these b ...

and

internal jugular vein The internal jugular vein is a paired jugular vein that collects blood from the brain and the superficial parts of the face and neck. This vein runs in the carotid sheath with the common carotid artery and vagus nerve. It begins in the posteri ...

, and anterior to the

Longus capitis muscle The longus capitis muscle (Latin for ''long muscle of the head'', alternatively rectus capitis anticus major), is broad and thick above, narrow below, and arises by four tendinous slips, from the anterior tubercles of the transverse processes of t ...

. The SCG contains

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

s that supply sympathetic innervation to a number of target organs within the head. The SCG also contributes to the

cervical plexus The cervical plexus is a plexus of the anterior rami of the first four cervical spinal nerves which arise from C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medi ...

. The cervical plexus is formed from a unification of the anterior divisions of the upper four cervical nerves. Each receives a

gray ramus communicans Each spinal nerve receives a branch called a gray ramus communicans (plural rami communicantes) from the adjacent paravertebral ganglion of the sympathetic trunk. The gray rami communicantes contain postganglionic nerve fibers of the sympathetic ne ...

from the superior cervical ganglion of the sympathetic trunk.

Morphology and physiology and its neurons

The superior cervical ganglion is reddish-gray color, and usually shaped like a spindle with tapering ends. Sometimes the SCG is broad and flattened, and occasionally constricted at intervals. It formed by the coalescence of four

, corresponding to the upper four

cervical nerves A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...

, C1-C4. The bodies of these preganglionic sympathetic neurons are specifically located in the lateral horn of the spinal cord. These preganglionic neurons then enter the SCG and synapse with the postganglionic neurons that leave the rostral end of the SCG and innervate target organs of the head. There are a number of neuron types in the SCG ranging from low threshold to high threshold neurons. The neurons with a low threshold have faster

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

firing rate, while the high threshold neurons have a slow firing rate. Another distinction between SCG neuron types is made via

immunostaining In biochemistry, immunostaining is any use of an antibody-based method to detect a specific protein in a sample. The term "immunostaining" was originally used to refer to the immunohistochemical staining of tissue sections, as first described by A ...

. Immunostaining allows the classification of SCG neurons as either positive or negative for

neuropeptide Y Neuropeptide Y (NPY) is a 36 amino-acid neuropeptide that is involved in various physiological and homeostatic processes in both the central and peripheral nervous systems. NPY has been identified as the most abundant peptide present in the ma ...

(NPY), which is found in a subgroup of high-threshold neurons. Low threshold, NPY-negative neurons are secretomotor neurons, innervating salivary glands. High threshold, NPY-negative neurons are vasomotor neurons, innervating blood vessels. High threshold, NPY-positive neurons are vasoconstrictor neurons, which innervate the iris and pineal gland.

Innervation

The SCG receives input from the

ciliospinal center The ciliospinal center (in Latin: ''centrum ciliospinale'') is a structure which receives input from the pretectum, and has output to the superior cervical ganglion. It is located in the intermediolateral cell columns (IMLCC) of the spinal cord ...

. The ciliospinal center is located between the C8 and T1 regions of the spinal cord within the intermediolateral column. The preganglionic fibers that innervate the SCG are the thoracic spinal nerves, which extend from the T1-T8 region of the ciliospinal center. These nerves enter the SCG through the cervical sympathetic nerve. A mature preganglionic axon can innervate anywhere from 50-200 SCG cells. Postganglionic fibers then leave the SCG via the internal carotid nerve and the external carotid nerve. This pathway of SCG innervation is shown through stimulation of the cervical sympathetic nerve, which invokes action potentials in both the external and internal carotid nerves. These postganglionic fibers shift from multiple axon innervation of their targets to less profound multiple axon innervation or single axon innervation as the SCG neurons mature during postnatal development.

Function

Sympathetic nervous system

The SCG provides sympathetic innervation to structures within the head, including the

pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep, sleep patterns in both circadian rhythm, circ ...

, the blood vessels in the cranial muscles and the brain, the

choroid plexus The choroid plexus, or plica choroidea, is a plexus of cells that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central nerv ...

, the eyes, the lacrimal glands, the

carotid body The carotid body is a small cluster of chemoreceptor cells, and supporting sustentacular cells. The carotid body is located in the adventitia, in the bifurcation (fork) of the common carotid artery, which runs along both sides of the neck. The ca ...

, the

salivary glands The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands (parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary gla ...

, and the

thyroid gland The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connected by a thin band of Connective tissue, tissue cal ...

Pineal gland

The postganglionic axons of the SCG innervate the pineal gland and are involved in

Circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...

. This connection regulates production of the hormone melatonin, which regulates sleep and wake cycles, however the influence of SCG neuron innervation of the pineal gland is not fully understood.

Carotid body

The postganglionic axons of the SCG innervate the

and form the

internal carotid plexus The internal carotid plexus is situated on the lateral side of the internal carotid artery, and in the plexus there occasionally exists a small gangliform swelling, the ''carotid ganglion'', on the under surface of the artery. Postganglionic symp ...

. The internal carotid plexus carries the postganglionic axons of the SCG to the eye,

lacrimal gland The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each or ...

, mucous membranes of the mouth, nose, and

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

, and numerous blood-vessels in the head.

The eye

The postganglionic axons of the Superior cervical ganglion innervate the eye and lacrimal gland and cause vasoconstriction of the iris and sclera, pupillary dilation, widening of the

palpebral fissure The palpebral fissure is the elliptic space between the medial and lateral canthi of the two open eyelids. In simple terms, it is the opening between the eyelids. In adult humans, this measures about 10 mm vertically and 30 mm horizontally. Var ...

, and the reduced production of tears. These responses are important during

Fight-or-flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...

of the ANS. Dilation of the pupils allows for an increased clarity in vision, and inhibition of the lacrimal gland stops tear production allowing for unimpaired vision and redirection of energy elsewhere.

Blood vessels of the skin

The postganglionic axons of the SCG innervate blood vessels in the skin and cause the vessels to constrict. Constriction of the blood vessels causes a decrease in blood flow to the skin leading to paling of the skin and retention of body heat. This plays into the fight-or-flight response, decreasing blood flow to facial skin and redirecting the blood to more important areas like the blood vessels of muscles.

Vestibular system

The SCG is connected with vestibular structures, including the neuroepithelium of the semicircular canals and otolith organs, providing a conceivable substrate for modulation of vestibulo-sympathetic reflexes.

Clinical significance

Horner's syndrome

Horner's syndrome Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion o ...

is a disorder resulting from damage to the sympathetic autonomic nervous pathway in the head. Damage to the SCG, part of this system, often results in Horner's syndrome. Damage to the T1-T3 regions of the spinal cord is responsible for drooping of the eyelids ( ptosis), constriction of the pupil (

miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...

), and sinking of the eyeball (apparent

Enophthalmos Enophthalmos is a posterior displacement of the eyeball within the orbit. It is due to either enlargement of the bony orbit and/or reduction of the orbital content, this in relation to each other. It should not be confused with its opposite, ex ...

; not truly sunken, just appears so because of the drooping eyelid). Lesion or significant damage to the SCG results in a third order neuron disorder (see Horner's Syndrome: Pathophysiology).

Familial dysautonomia

Familial dysautonomia Familial dysautonomia (FD), also known as Riley-Day Syndrome, is a rare, progressive, recessive genetic disorder of the autonomic nervous system that affects the development and survival of sensory, sympathetic and some parasympathetic neurons ...

is a genetic disorder characterized by abnormalities of sensory and sympathetic neurons. The SCG is significantly affected by this loss of neurons and may be responsible for some of the resulting symptoms. In post-mortem studies the SCG is, on average, one-third of normal size and has only 12 percent of the normal number of neurons. Defects in the genetic coding for NGF, which result in less functional, abnormally structured NGF, may be the molecular cause of familial dysautonomia. NGF is necessary for survival of some neurons so loss of NGF function could be the cause for neuronal death in the SCG.

History

Reinnervation

In the late 19th century, John Langley discovered that the superior cervical ganglion is topographically organized. When certain areas of the superior cervical ganglion were stimulated, a reflex occurred in specified regions of the head. His findings showed that preganglionic neurons innervate specific postganglionic neurons. In his further studies of the superior cervical ganglion, Langley discovered that the superior cervical ganglion is regenerative. Langley severed the SCG above the T1 portion, causing a loss of reflexes. When left to their own accord, the fibers reinnervated the SCG and the initial autonomic reflexes were recovered, though there was limited recovery of pineal gland function. When Langley severed the connections between the SCG and the T1–T5 region of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

and replaced the SCG with a different one, the SCG was still innervated the same portion of the spinal cord as before. When he replaced the SCG with a T5 ganglion, the ganglion tended to be innervated by the posterior portion of the spinal cord (T4–T8). The replacement of the original SCG with either a different one or a T5 ganglion supported Langley's theory of topographic specificity of the SCG.

Research

Ganglia of the peripheral autonomic nervous system are commonly used to study synaptic connections. These ganglia are studied as synaptic connections show many similarities to the central nervous system (CNS) and are also relatively accessible. They are easier to study than the CNS since they have the ability to regrow, which neurons in the CNS do not have. The SCG is frequently used in these studies being one of the larger ganglia. Today, neuroscientists are studying topics on the SCG such as survival and neurite outgrowth of SCG neurons, neuroendocrine aspects of the SCG, and structure and pathways of the SCG. These studies are usually performed on rats, guinea-pigs, and rabbits.

Historical contributions

* E. Rubin studied the development of the SCG in fetal rats. Research on the development of nerves in the SCG has implications for the general development of the nervous system. *The effects of age on dendritic arborisation of sympathetic neurons has been studied in the SCG of rats. Findings have shown that there is significant dendritic growth in the SCG of young rats but none in aged rats. In aged rats, it was found, that there was a reduction in the number of dendrites. * SCG cells were used to study

nerve growth factor Nerve growth factor (NGF) is a neurotrophic factor and neuropeptide primarily involved in the regulation of growth, maintenance, proliferation, and survival of certain target neurons. It is perhaps the prototypical growth factor, in that it was on ...

(NGF) and its ability to direct growth of neurons. Results showed that NGF did have this directing, or tropic, effect on neurons, guiding the direction of their growth.

Additional images

File:Gray838.png, The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. File:Superior cervical ganglion 1.jpg, Superior cervical ganglion File:Gray840.png, Sympathetic connections of the ciliary and superior cervical ganglia. File:Gray1032.png, The position and relation of the esophagus in the cervical region and in the posterior mediastinum. Seen from behind. File:Sympathetic Trunk and SCG innervation.jpg, The Sympathetic Trunk and SCG innervation of target organs in the head.

References

External links

* {{Authority control Sympathetic ganglia