Superior cervical ganglion

The superior cervical ganglion is the largest of the three cervical sympathetic ganglia and is located in the neck. It is positioned at the level of the second and third cervical vertebrae (C2–C3) and lies anterior to the transverse processes of these vertebrae. The ganglion is situated posterior to the internal carotid artery and internal jugular vein, within the deep cervical fascia. It is also near the base of the skull, just below the foramen lacerum, through which it communicates with structures in the head and neck. This ganglion is part of the sympathetic trunk and supplies autonomic innervation to the head, neck, and upper thorax.

Structure and Anatomy

Location

The superior cervical ganglion is the largest of the cervical sympathetic ganglia and is located in the upper portion of the neck. It is positioned along the sympathetic trunk at the level of the second and third cervical vertebrae (C2 and C3). The ganglion is found anterior to the transverse processes of these vertebrae and is situated deep within the cervical fascia, behind the internal carotid artery and internal jugular vein.

The ganglion lies near the base of the skull, just below the foramen lacerum, which is an opening in the skull through which the ganglion communicates with various structures in the head. This high position allows the superior cervical ganglion to provide autonomic innervation to the head, neck, and upper thorax.

Structure

The superior cervical ganglion is elongated and spindle-shaped, with its size being greater than the other cervical ganglia. It can measure about 2-3 cm in length, making it the largest of the cervical sympathetic ganglia. The ganglion consists of a collection of sympathetic neuron cell bodies and is surrounded by connective tissue. These neuron cell bodies relay autonomic signals, specifically sympathetic fibers, to various structures in the head, neck, and thorax.

The superior cervical ganglion is part of the sympathetic trunk, a long chain of ganglia running parallel to the vertebral column. It serves as a key relay station for sympathetic signals originating from the thoracic spinal cord and traveling to the head and neck.

Connections with Sympathetic Chain and Nerves

The superior cervical ganglion is connected to the sympathetic trunk through both ascending and descending fibers, and it is linked to nearby nerves and plexuses:

• Sympathetic Trunk: The ganglion is connected to the sympathetic trunk, a long chain of interconnected ganglia running along both sides of the vertebral column. It receives preganglionic fibers from the thoracic spinal cord via the thoracic sympathetic ganglia, specifically from spinal segments T1 to T4.
• Preganglionic Fibers: Preganglionic sympathetic fibers enter the superior cervical ganglion through white rami communicantes, after originating from the lateral horn of the thoracic spinal cord. These fibers synapse within the ganglion.
• Postganglionic Fibers: The superior cervical ganglion sends out postganglionic fibers that innervate a variety of target tissues, including the eyes, blood vessels, sweat glands, salivary glands, and parts of the heart. These fibers travel along with blood vessels, cranial nerves, and other autonomic structures to reach their targets.

Branches of the Superior Cervical Ganglion

The superior cervical ganglion gives rise to numerous branches that supply sympathetic innervation to different structures in the head, neck, and upper thorax. The main branches include:

• Internal Carotid Nerve: One of the most significant branches of the superior cervical ganglion is the internal carotid nerve, which follows the course of the internal carotid artery. This nerve forms the internal carotid plexus, which distributes sympathetic fibers to the blood vessels and structures within the head, including the eye (dilator pupillae muscle) and the lacrimal gland.
• External Carotid Nerve: This nerve branch accompanies the external carotid artery and forms the external carotid plexus, which supplies the blood vessels and glands of the face, including the parotid salivary gland and sweat glands of the skin.
• Pharyngeal Branches: The superior cervical ganglion sends fibers to form the pharyngeal plexus, which supplies autonomic innervation to the muscles and mucosa of the pharynx, playing a role in swallowing and other autonomic functions in the throat.
• Cardiac Branches (Superior Cardiac Nerve): The superior cervical ganglion contributes to the cardiac plexus by sending fibers that regulate heart rate and force of contraction. The superior cardiac nerve descends into the thoracic cavity, where it connects with the cardiac plexus and contributes to autonomic regulation of the heart.
• Cervical Spinal Nerves: The ganglion also gives off gray rami communicantes that connect to the ventral rami of cervical spinal nerves (C1 to C4). These fibers provide sympathetic innervation to the skin of the neck and scalp, particularly to sweat glands and blood vessels, helping to control thermoregulation and blood flow.

Relationship to Other Structures

The superior cervical ganglion is located deep within the neck, and it has close anatomical relationships with several important structures:

• Internal Carotid Artery: The ganglion lies behind the internal carotid artery, and one of its primary branches, the internal carotid nerve, forms the internal carotid plexus. This relationship is critical for the ganglion’s role in controlling blood vessel tone and regulating autonomic functions in the head and face.
• Internal Jugular Vein: The superior cervical ganglion is situated posterior to the internal jugular vein, a major venous structure that drains blood from the brain and face. The proximity to both the carotid artery and the internal jugular vein makes the ganglion an important structure in the autonomic control of vascular function.
• Vagus Nerve (Cranial Nerve X): The ganglion lies near the vagus nerve, which is the principal parasympathetic nerve supplying the thoracic and abdominal organs. While the vagus nerve is responsible for parasympathetic innervation, the superior cervical ganglion provides sympathetic input, and the close proximity of these two structures reflects the balance of autonomic control in the region.
• Pharynx and Esophagus: The ganglion contributes fibers to the pharyngeal plexus, which is important for autonomic control of the pharynx and esophagus. This plexus is involved in regulating swallowing and other autonomic functions related to the throat and upper digestive tract.

Blood Supply

The blood supply to the superior cervical ganglion comes from small branches of the carotid arteries, primarily the internal carotid artery. This artery provides oxygenated blood to the ganglion and the surrounding structures. Additionally, the vertebral arteries and thyroid arteries may also contribute small branches that supply the ganglion. Venous drainage occurs via the internal jugular vein and other venous structures in the neck.

Variability

The size, shape, and exact position of the superior cervical ganglion can vary between individuals. In some cases, the ganglion may be larger or smaller than average, and the precise course of its branches may differ slightly. The ganglion may also have multiple segments or appear more diffuse in some individuals, though it generally maintains its role as the primary cervical sympathetic ganglion.

Function

The superior cervical ganglion is a critical component of the sympathetic nervous system and is responsible for relaying autonomic signals to a variety of structures in the head, neck, and upper thorax. Its functions primarily involve regulating blood vessel tone, glandular secretions, and smooth muscle activity in these areas. Below are the specific functions of the superior cervical ganglion, categorized by the organs and systems it innervates.

Sympathetic Control of the Eyes

The superior cervical ganglion plays a significant role in regulating the eyes by providing sympathetic innervation through its connection to the internal carotid plexus and nerves that travel to the eye:

• Pupillary Dilation (Mydriasis): The ganglion sends postganglionic fibers to innervate the dilator pupillae muscle in the eye. Sympathetic stimulation from the superior cervical ganglion causes the pupils to dilate (mydriasis), allowing more light to enter the eye. This function is critical in low-light conditions or during the fight-or-flight response, when the body prepares to respond to a threat by increasing visual acuity.
• Eyelid Elevation: The ganglion also influences the superior tarsal muscle (Müller’s muscle), which helps keep the upper eyelid elevated. Sympathetic stimulation keeps the eyelids in an elevated position during alertness, enhancing the ability to see and respond to the environment.

Regulation of Blood Vessels in the Head and Neck

The superior cervical ganglion provides sympathetic innervation to the blood vessels in the head and neck, regulating vasoconstriction and blood flow to various structures.

• Vasoconstriction of Blood Vessels: Sympathetic fibers from the superior cervical ganglion cause vasoconstriction of the arteries in the head and neck, including branches of the internal and external carotid arteries. This regulation of blood vessel tone helps control the distribution of blood to different tissues. During times of stress, the ganglion helps reduce blood flow to the skin and superficial structures by constricting blood vessels, preserving blood flow for vital organs such as the brain and muscles.
• Blood Flow to the Brain and Skin: The ganglion influences blood flow through its control of the cerebral arteries and cutaneous vessels. By modulating the diameter of these arteries, the ganglion helps maintain appropriate blood supply to the brain, while also controlling skin perfusion in response to temperature changes or emotional states.

Control of Sweat Glands in the Face and Scalp

The superior cervical ganglion plays a key role in regulating sweat glands in the face, scalp, and upper neck through its postganglionic sympathetic fibers.

• Activation of Sweat Glands: Sympathetic fibers from the superior cervical ganglion innervate sweat glands in the skin of the face and scalp. During physical exertion, increased temperature, or stress, these fibers stimulate the release of sweat, which helps cool the body by thermoregulation. This process is vital for maintaining normal body temperature and preventing overheating.
• Regulation of Sweating in Emotional Responses: The superior cervical ganglion also influences emotional sweating, which can occur in response to stress, anxiety, or excitement. This type of sweating often affects the face, palms, and scalp, and is triggered by the autonomic nervous system.

Innervation of Salivary and Lacrimal Glands

The superior cervical ganglion provides sympathetic fibers to the salivary glands and lacrimal glands, influencing glandular secretion.

• Reduction of Salivary Secretion: Sympathetic fibers from the superior cervical ganglion innervate the parotid, submandibular, and sublingual salivary glands, reducing salivary secretion during periods of stress or heightened sympathetic activity. This inhibition of saliva production is part of the fight-or-flight response, conserving energy for more immediate bodily functions like responding to stress.
• Lacrimal Gland Innervation: The superior cervical ganglion sends fibers to the lacrimal gland via the internal carotid plexus. Sympathetic stimulation may slightly reduce tear production, especially during stress or emotional arousal, helping to preserve bodily fluids in situations where energy and resources are diverted to survival functions.

Regulation of Heart Rate and Blood Pressure

Through its superior cardiac nerve, the superior cervical ganglion contributes to sympathetic innervation of the heart, affecting both heart rate and blood pressure.

• Heart Rate Acceleration: The superior cervical ganglion sends postganglionic fibers to the cardiac plexus, where they help increase heart rate (positive chronotropy) and enhance the force of heart contractions (positive inotropy). This action is crucial during the body’s fight-or-flight response, where increased heart output is needed to pump more blood to muscles and vital organs.
• Blood Pressure Regulation: By influencing heart rate and contractility, the ganglion helps maintain blood pressure, particularly during times of stress or physical activity. The increased cardiac output from the ganglion’s sympathetic stimulation helps ensure that sufficient blood is delivered to tissues under heightened metabolic demand.

Sympathetic Innervation of the Pharynx and Larynx

The superior cervical ganglion contributes fibers to the pharyngeal plexus and other nerves that innervate the pharynx and larynx. These fibers influence muscular tone and secretions in the throat and voice box.

• Pharyngeal Function: Sympathetic fibers help regulate the muscles of the pharynx, modulating swallowing and other autonomic functions in the throat. Although the parasympathetic system controls most glandular secretions, the sympathetic system contributes to the overall autonomic balance.
• Laryngeal Control: The ganglion sends fibers that influence the larynx, playing a role in controlling vocal fold tension and airflow regulation during speech and breathing.

Thermoregulation and Piloerection

The superior cervical ganglion plays a role in thermoregulation and the body’s response to cold through its control over blood vessels and skin structures:

• Piloerection (Goosebumps): Sympathetic fibers from the superior cervical ganglion innervate the arrector pili muscles in the skin, which are responsible for causing piloerection (the standing up of hairs on the skin), commonly referred to as goosebumps. This response occurs during cold exposure or emotional arousal and helps insulate the skin by trapping a layer of air.
• Vasoconstriction in Response to Cold: The ganglion helps regulate vasoconstriction in the skin’s blood vessels in response to cold temperatures, reducing blood flow to the skin to conserve heat and maintain core body temperature.

Modulation of Autonomic Reflexes

The superior cervical ganglion participates in several important autonomic reflexes that help maintain homeostasis:

• Baroreceptor Reflex: The ganglion contributes to the baroreceptor reflex, which helps regulate blood pressure by adjusting heart rate and vascular tone in response to changes in blood pressure. This reflex ensures that blood pressure remains stable during changes in posture or when blood volume changes.
• Thermoregulatory Reflexes: The superior cervical ganglion is involved in reflex pathways that control sweating and vasodilation or vasoconstriction in response to changes in external temperature. These reflexes are critical for maintaining a stable internal environment and responding to fluctuations in temperature.

Role in the Fight-or-Flight Response

The superior cervical ganglion plays a key role in the body’s fight-or-flight response, a physiological reaction that occurs in response to stress or danger:

• Increased Heart Rate and Blood Pressure: The ganglion accelerates heart rate and increases the force of heart contractions, ensuring that the body can respond to stress or physical demands.
• Redistribution of Blood Flow: The ganglion causes vasoconstriction in the skin and non-essential organs, redirecting blood to the brain, heart, and muscles for immediate action during stressful situations.
• Pupillary Dilation and Eyelid Elevation: The ganglion dilates the pupils and elevates the upper eyelid, increasing the amount of light entering the eyes and enhancing visual awareness during high-stress situations.

Clinical Significance

The superior cervical ganglion plays a vital role in the sympathetic regulation of functions in the head, neck, and upper thorax, making it clinically significant in various conditions. Injury or dysfunction of the ganglion can lead to Horner’s syndrome, characterized by ptosis (drooping eyelid), miosis (constricted pupil), anhidrosis (lack of sweating), and enophthalmos (sunken appearance of the eye) on the affected side. This condition results from damage to the sympathetic pathways that run through the ganglion.

The ganglion is also targeted in therapeutic procedures such as stellate ganglion blocks, which are used to treat chronic pain syndromes, complex regional pain syndrome (CRPS), or vascular conditions like Raynaud’s phenomenon. Additionally, its role in regulating blood pressure, heart rate, and sweating can be implicated in autonomic disorders that affect these functions. Its close proximity to the internal carotid artery and jugular vein makes it a key consideration during neck surgeries or vascular interventions.