The internal carotid plexus is a network of sympathetic nerve fibers that surrounds the internal carotid artery as it ascends through the neck and into the skull. It is part of the autonomic nervous system and consists of postganglionic fibers derived from the superior cervical ganglion of the sympathetic trunk. These fibers distribute along the internal carotid artery and travel to various structures in the head, particularly influencing the eyes, blood vessels, and glands.
Location
The internal carotid plexus is located around the internal carotid artery, beginning at the point where the artery enters the base of the skull through the carotid canal in the temporal bone. It continues alongside the artery as it courses through the cavernous sinus and into the cranial cavity, supplying sympathetic innervation to structures within the head. The plexus extends branches to areas including the orbit, lacrimal gland, and meninges.
Structure and Anatomy
The internal carotid plexus is an important component of the sympathetic nervous system that surrounds the internal carotid artery and supplies autonomic fibers to various structures within the head. Below is a detailed description of its anatomy.
Formation and Composition
The internal carotid plexus is composed entirely of sympathetic fibers. These fibers are postganglionic and originate from the superior cervical ganglion, the largest of the cervical sympathetic ganglia. After synapsing in the superior cervical ganglion, the fibers form a network that wraps around the internal carotid artery as it ascends through the neck and enters the skull.
Pathway through the Carotid Canal
As the internal carotid artery passes through the carotid canal, the internal carotid plexus travels with it. This canal is part of the petrous portion of the temporal bone and serves as the entryway for the artery into the cranial cavity. The plexus remains in close proximity to the artery, ensuring that it can distribute its fibers to various branches of the artery and surrounding structures within the skull.
Cavernous Sinus Relations
Once the internal carotid artery exits the carotid canal, it enters the cavernous sinus, a venous sinus located on either side of the body of the sphenoid bone. The internal carotid plexus continues to surround the artery as it courses through the sinus. Within the cavernous sinus, the plexus is located close to important neurovascular structures, including the oculomotor nerve (CN III), trochlear nerve (CN IV), and the ophthalmic and maxillary divisions of the trigeminal nerve (CN V1 and V2).
Branches of the Internal Carotid Plexus
The internal carotid plexus gives off several important branches that supply sympathetic innervation to different regions of the head and face:
- Deep Petrosal Nerve: One of the most significant branches of the internal carotid plexus is the deep petrosal nerve, which combines with the greater petrosal nerve (a parasympathetic nerve) to form the nerve of the pterygoid canal (Vidian nerve). This nerve supplies sympathetic fibers to the lacrimal gland, nasal mucosa, and other areas.
- Cavernous Plexus: Branches from the internal carotid plexus contribute to the cavernous plexus, which innervates the blood vessels within the cavernous sinus and provides fibers to the structures surrounding it, including cranial nerves.
- Caroticotympanic Nerves: These small branches pass through the caroticotympanic canaliculi to innervate structures in the middle ear.
Distribution to Cranial Structures
The fibers of the internal carotid plexus extend to various key structures in the head:
- Ocular Structures: Sympathetic fibers from the plexus reach the dilator pupillae muscle of the eye through the ophthalmic artery, which is a branch of the internal carotid artery. These fibers help regulate the size of the pupil.
- Lacrimal Gland: Through its connection with the deep petrosal nerve and the nerve of the pterygoid canal, the internal carotid plexus influences the lacrimal gland, modulating tear production.
- Meninges: The plexus also sends sympathetic fibers to the meninges, the protective coverings of the brain, via branches of the internal carotid artery, contributing to vasomotor control of the meningeal blood vessels.
- Nasal Mucosa: Via the deep petrosal nerve and the nerve of the pterygoid canal, sympathetic fibers reach the nasal mucosa, playing a role in regulating blood flow to this area and contributing to the control of nasal secretions.
Connections with Other Plexuses
The internal carotid plexus is closely linked with other autonomic plexuses and nerves in the head:
- Cavernous Plexus: As the internal carotid plexus passes through the cavernous sinus, it contributes fibers to the cavernous plexus, which provides autonomic innervation to the structures located within the cavernous sinus, including the oculomotor, trochlear, and trigeminal nerves.
- Nerve of the Pterygoid Canal: The internal carotid plexus contributes sympathetic fibers to the nerve of the pterygoid canal through the deep petrosal nerve. This nerve innervates the nasal mucosa, palate, and lacrimal gland.
- Superior Cervical Ganglion: The internal carotid plexus is directly connected to the superior cervical ganglion, which is the origin of its postganglionic sympathetic fibers. These fibers are distributed along the internal carotid artery and extend to various cranial structures.
Histological Structure
Microscopically, the internal carotid plexus consists of postganglionic sympathetic fibers, which are primarily unmyelinated, and are supported by connective tissue. These fibers are closely associated with the walls of the internal carotid artery, allowing them to be distributed efficiently to their target organs. The plexus is surrounded by a loose connective tissue matrix that contains small blood vessels and provides a protective environment for the nerve fibers.
Function
The internal carotid plexus plays an essential role in autonomic regulation, particularly through its sympathetic fibers, which influence various structures in the head. It controls several functions related to vasoconstriction, pupil dilation, glandular secretion, and blood flow regulation. Below is a detailed description of the functions of the internal carotid plexus.
Sympathetic Control
The internal carotid plexus is made up entirely of sympathetic fibers, which are involved in regulating smooth muscle tone, vasoconstriction, and glandular activity in the head. These fibers are postganglionic, originating from the superior cervical ganglion.
Regulation of Pupillary Dilation (Mydriasis)
One of the primary functions of the internal carotid plexus is to control pupil dilation through its innervation of the dilator pupillae muscle. The plexus supplies sympathetic fibers to this muscle via branches of the ophthalmic artery, which is a branch of the internal carotid artery. When stimulated, these sympathetic fibers cause the pupil to dilate, a process known as mydriasis. This is particularly important during low-light conditions or as part of the body’s “fight or flight” response, where increased light entry into the eye allows for better vision.
Vasoconstriction of Blood Vessels in the Head
The internal carotid plexus regulates vasoconstriction of the blood vessels in the head, particularly those associated with the meninges, nasal mucosa, and skin. Sympathetic stimulation causes these blood vessels to constrict, reducing blood flow to certain areas. This is important for controlling blood distribution and maintaining blood pressure in the cranial region. For example, vasoconstriction in the nasal mucosa reduces blood flow to the area, which can limit mucus production and decrease congestion.
Regulation of Blood Flow to the Brain
Although the internal carotid plexus does not directly influence the large cerebral arteries, it plays a role in regulating blood flow to the smaller vessels of the meninges and other cranial structures. By controlling vasoconstriction in these areas, the internal carotid plexus helps maintain cerebral perfusion under various physiological conditions, such as changes in posture or blood pressure.
Control of Nasal and Lacrimal Gland Secretions
The internal carotid plexus contributes to the control of glandular secretions in the head, particularly in the lacrimal glands (which produce tears) and the nasal mucosa (which produces mucus). Sympathetic fibers from the internal carotid plexus combine with the greater petrosal nerve (a parasympathetic nerve) to form the nerve of the pterygoid canal. This nerve innervates the nasal mucosa and lacrimal glands, and sympathetic stimulation reduces secretion by these glands. For example:
- Lacrimal Glands: Sympathetic stimulation reduces tear production, typically during stress or dry conditions.
- Nasal Mucosa: Sympathetic stimulation reduces mucus production, which can help open the nasal passages during stress or heightened sympathetic activity.
Innervation of Cranial Structures
The internal carotid plexus extends fibers to several key structures in the head, ensuring that these structures are regulated according to sympathetic activity.
Innervation of the Eye and Orbit
The internal carotid plexus supplies sympathetic fibers to structures in the orbit, including the ciliary body, dilator pupillae muscle, and blood vessels of the eye. These fibers are responsible for:
- Pupil Dilation: As mentioned earlier, the plexus causes dilation of the pupil through innervation of the dilator pupillae muscle, increasing light entry into the eye.
- Vasoconstriction of Ocular Blood Vessels: The sympathetic fibers also regulate blood flow in the vessels supplying the eye, ensuring proper perfusion during sympathetic stimulation.
Innervation of the Lacrimal Gland
Sympathetic fibers from the internal carotid plexus contribute to the innervation of the lacrimal gland. These fibers reduce the production of tears in response to sympathetic stimulation. This is part of the body’s general sympathetic response to stress, where tear production is minimized to conserve fluids and focus resources on more immediate survival functions.
Innervation of the Nasal Mucosa
The internal carotid plexus provides sympathetic fibers to the nasal mucosa, helping to regulate blood flow and secretion within the nasal cavity. The deep petrosal nerve, a branch of the internal carotid plexus, joins with the greater petrosal nerve to form the nerve of the pterygoid canal, which supplies the nasal mucosa. Sympathetic stimulation leads to vasoconstriction of the blood vessels in the nasal mucosa, reducing mucus secretion and helping to keep the nasal passages clear during sympathetic activation.
Innervation of the Meninges
The internal carotid plexus sends fibers to the meninges, particularly the dura mater, which are the protective coverings of the brain. These sympathetic fibers help regulate vasoconstriction of the meningeal blood vessels, ensuring that blood flow to the brain’s protective coverings is appropriately controlled.
Role in Reflexes and Responses
The internal carotid plexus participates in several autonomic reflexes that help maintain homeostasis in response to changes in the internal and external environment.
Pupillary Light Reflex
While the internal carotid plexus does not control the pupillary constriction (which is regulated by parasympathetic fibers), it plays a key role in the pupillary dilation aspect of the pupillary light reflex. In low-light conditions or during sympathetic stimulation (e.g., stress or excitement), the sympathetic fibers from the internal carotid plexus cause the pupil to dilate, allowing more light to enter the eye.
Vasomotor Reflexes
The internal carotid plexus is involved in vasomotor reflexes that adjust blood flow in response to changes in posture or blood pressure. For example, during sudden changes in posture, such as standing up quickly, the sympathetic fibers in the internal carotid plexus can constrict the blood vessels in the head, helping to prevent pooling of blood and maintaining adequate perfusion to the brain.
Stress Response (Fight or Flight)
During stress, the internal carotid plexus is highly active, contributing to the body’s fight or flight response. This includes:
- Pupil Dilation to enhance visual acuity.
- Vasoconstriction of non-essential blood vessels in the head and neck to redirect blood to vital organs like the heart and muscles.
- Reduction in glandular secretion, minimizing the production of tears and mucus to conserve energy and fluid.
Connections with Other Nerves and Plexuses
The internal carotid plexus forms connections with other autonomic nerves and plexuses, facilitating comprehensive control over the head and neck regions.
Connection with the Deep Petrosal Nerve
The deep petrosal nerve, which arises from the internal carotid plexus, joins with the greater petrosal nerve (a parasympathetic branch of the facial nerve) to form the nerve of the pterygoid canal. This nerve carries both sympathetic and parasympathetic fibers and innervates the nasal mucosa, lacrimal gland, and palate, ensuring coordinated autonomic control of these regions.
Connection with the Cavernous Plexus
The internal carotid plexus contributes fibers to the cavernous plexus, which surrounds the cavernous sinus. This plexus is important for the innervation of structures such as the oculomotor nerve (CN III), trochlear nerve (CN IV), and the ophthalmic branch of the trigeminal nerve (CN V1). Through these connections, the internal carotid plexus influences the sympathetic innervation of the eye and orbit.
Clinical Significance
The internal carotid plexus is vital in controlling various autonomic functions in the head, particularly through its sympathetic fibers. Disruption or damage to this plexus can result in several clinical conditions:
Horner’s Syndrome
Damage to the sympathetic fibers in the internal carotid plexus can result in Horner’s syndrome, characterized by ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (loss of sweating) on the affected side of the face. This occurs when the sympathetic innervation to the eye and face is compromised, often due to trauma, tumors, or vascular issues affecting the plexus.
Carotid Artery Dissection
A dissection of the internal carotid artery can damage the internal carotid plexus, leading to symptoms such as headache, facial pain, Horner’s syndrome, and sometimes transient ischemic attacks (TIAs) due to altered blood flow and nerve damage.
Cluster Headaches
The internal carotid plexus is sometimes implicated in cluster headaches, a severe form of headache characterized by intense pain around the eye. Sympathetic dysfunction in the plexus may contribute to the autonomic symptoms associated with these headaches, such as eye redness, tearing, and nasal congestion.
Post-Surgical Complications
Surgical procedures involving the internal carotid artery or adjacent structures may inadvertently affect the internal carotid plexus, leading to sympathetic dysfunctions, including vision changes or facial autonomic symptoms.
