Internal carotid artery

The internal carotid artery (ICA) is a major artery that supplies oxygenated blood to the brain, eyes, and parts of the face. It is one of the two terminal branches of the common carotid artery, the other being the external carotid artery. The ICA plays a crucial role in the cerebral circulation system.

Location

The internal carotid artery originates from the common carotid artery at the level of the fourth cervical vertebra, around the upper border of the thyroid cartilage. It ascends vertically in the neck, traveling without giving off any branches until it enters the skull. The ICA passes through the carotid canal in the temporal bone to enter the cranial cavity. Once inside the skull, it runs through the cavernous sinus and continues toward the base of the brain, where it forms part of the Circle of Willis, supplying critical areas of the brain and eyes.

Anatomy

The internal carotid artery (ICA) is a vital artery in the head and neck that supplies blood to the brain, eyes, and other structures within the skull. It has a complex course as it ascends from the neck, passes through the skull, and ultimately reaches the brain. Below is a detailed description of the anatomy of the internal carotid artery.

Origin

The internal carotid artery originates from the common carotid artery, which bifurcates at the level of the fourth cervical vertebra (C4) near the upper border of the thyroid cartilage. At this bifurcation, the common carotid artery divides into two branches: the internal carotid artery (which supplies blood to the brain and eyes) and the external carotid artery (which supplies blood to the face and neck).

Cervical Segment (C1)

Course in the Neck

• After its origin from the common carotid artery, the internal carotid artery ascends through the neck, traveling within the carotid sheath. This sheath is a fibrous connective tissue that also contains the internal jugular vein and the vagus nerve (cranial nerve X). The artery runs vertically without giving off any branches in the neck.
• The ICA is located deep in the neck, lying just lateral to the pharynx and the vertebral column. It is medial to the internal jugular vein and is closely related to the cranial nerves as it ascends.

Petrous Segment (C2)

Entry into the Skull

• The internal carotid artery enters the skull through the carotid canal, a bony passage located in the petrous part of the temporal bone. This portion of the artery is known as the petrous segment.
• Within the carotid canal, the artery ascends and then bends anteriorly and medially. This S-shaped bend in the artery is referred to as the carotid siphon and serves as a key anatomical landmark in neurovascular imaging and surgery.

Relations within the Petrous Part

The petrous segment of the internal carotid artery is closely related to the middle ear cavity, and it is separated from it only by a thin bony plate. This proximity to the middle ear structures makes it susceptible to injury in cases of trauma or infection involving the ear.

Cavernous Segment (C3)

Course through the Cavernous Sinus

• After exiting the petrous part of the temporal bone, the internal carotid artery enters the cavernous sinus, a venous structure located at the base of the skull on either side of the sella turcica (the depression in the sphenoid bone where the pituitary gland resides).
• The cavernous segment of the ICA runs through the sinus, forming another S-shaped curve known as the carotid siphon, which is a key feature of the artery in this region. Here, it is surrounded by important cranial nerves (III, IV, V1, V2, and VI), which control eye movements and facial sensation.

Relations with Cranial Nerves

The ICA’s close relationship with the cranial nerves in the cavernous sinus, particularly the abducens nerve (cranial nerve VI), makes it susceptible to compression in conditions such as cavernous sinus thrombosis or aneurysms of the artery.

Clinoid Segment (C4)

Exit from the Cavernous Sinus

• After passing through the cavernous sinus, the internal carotid artery enters the clinoid segment, where it passes over the anterior clinoid process of the sphenoid bone. This segment marks the transition from the cavernous part of the artery to its final intracranial course.
• The clinoid segment is short and is a transitional area where the artery becomes more closely associated with the brain’s meninges and cerebral arteries.

Supraclinoid Segment (C5)

Entry into the Cranial Cavity

After passing through the clinoid segment, the ICA enters its supraclinoid segment within the cranial cavity. This is where the artery begins to give off important branches that supply the brain and eyes.

Branches from the Supraclinoid Segment

• Ophthalmic Artery: One of the first branches of the internal carotid artery in this segment is the ophthalmic artery, which supplies blood to the eyes and orbital structures.
• Posterior Communicating Artery: The ICA also gives rise to the posterior communicating artery, which forms part of the Circle of Willis and connects the internal carotid artery with the posterior cerebral artery, providing collateral blood flow to the brain.
• Anterior Choroidal Artery: This small branch supplies parts of the optic tract, internal capsule, and cerebral peduncle.

Cerebral Segment (C6)

Termination and Contribution to the Circle of Willis

The cerebral segment of the internal carotid artery represents the terminal portion of the artery. Here, the ICA divides into two major branches:

• Anterior Cerebral Artery (ACA): This artery supplies the medial and superior parts of the frontal lobes and the superior parietal lobes.
• Middle Cerebral Artery (MCA): The MCA is the larger of the two terminal branches and supplies the lateral aspects of the frontal, temporal, and parietal lobes, including regions involved in motor control, language, and sensory perception.

Formation of the Circle of Willis

The internal carotid artery, through its branches, contributes to the Circle of Willis, a circular anastomotic system of arteries at the base of the brain that provides collateral circulation. The Circle of Willis includes contributions from the anterior cerebral, anterior communicating, posterior communicating, and posterior cerebral arteries, forming a crucial blood supply network for the brain.

Anatomical Relations

• Internal Jugular Vein and Vagus Nerve: In the neck, the internal carotid artery is located medial to the internal jugular vein and is closely related to the vagus nerve (cranial nerve X), which runs within the carotid sheath. This proximity is important for surgeries or procedures involving the carotid arteries, as damage to these structures can result in significant complications.
• Cranial Nerves: In its course through the cavernous sinus, the internal carotid artery is in close relation to several cranial nerves, including the oculomotor nerve (cranial nerve III), trochlear nerve (cranial nerve IV), ophthalmic branch of the trigeminal nerve (cranial nerve V1), maxillary branch of the trigeminal nerve (cranial nerve V2), and the abducens nerve (cranial nerve VI). This relationship is significant because aneurysms or other pathologies involving the ICA in this region can affect these nerves, leading to ocular and facial symptoms.
• Sphenoid and Temporal Bones: As the internal carotid artery ascends through the skull, it passes through the carotid canal in the temporal bone and runs close to the sphenoid bone. Its bony relationships are important in cases of skull base fractures or surgical interventions in this region.

Function

The internal carotid artery (ICA) is one of the main arteries supplying oxygenated blood to essential structures in the head, particularly the brain, eyes, and other vital areas of the skull. The ICA’s functions are primarily related to maintaining cerebral circulation and supporting the functions of the brain and vision. Below is a detailed breakdown of the functions of the internal carotid artery.

Blood Supply to the Brain

Oxygenation of Cerebral Hemispheres

• One of the primary functions of the internal carotid artery is to provide oxygenated blood to large parts of the brain. It is a key component of the cerebral circulation, supplying blood to the anterior and middle portions of the brain. Through its branches, particularly the anterior cerebral artery (ACA) and the middle cerebral artery (MCA), the ICA ensures that the brain receives the oxygen and nutrients needed for its functions.
• The middle cerebral artery (MCA) is the larger terminal branch of the ICA and supplies most of the lateral aspects of the cerebral cortex, which are involved in motor control, sensory processing, language, and higher cognitive functions. The anterior cerebral artery (ACA) supplies the medial and superior parts of the frontal and parietal lobes, supporting cognitive functions, decision-making, and voluntary movement.

Support for Cerebral Circulation through the Circle of Willis

• The ICA contributes to the Circle of Willis, an arterial ring at the base of the brain that serves as a critical structure for maintaining blood flow to the brain, even in cases of arterial blockage or narrowing. The ICA provides blood to the anterior cerebral artery and the posterior communicating artery, which form important parts of the Circle of Willis.
• The posterior communicating artery, a branch of the ICA, connects the ICA to the posterior cerebral artery (supplied by the vertebral arteries). This connection helps maintain cerebral perfusion and provides a route for collateral circulation, which is vital in the event of arterial occlusions or strokes.

Blood Supply to the Eyes and Orbit

Supply to the Eyes via the Ophthalmic Artery

• One of the key branches of the internal carotid artery is the ophthalmic artery, which supplies blood to the eyes, orbit, and lacrimal apparatus. The ophthalmic artery enters the orbit through the optic canal and gives rise to multiple branches that supply the eyeball, extraocular muscles, eyelids, and surrounding tissues.
• This arterial supply is critical for maintaining vision, as the ophthalmic artery provides blood to the retina through its branch, the central retinal artery, which supplies the inner layers of the retina, including the optic nerve head. Any compromise in the blood flow from the ophthalmic artery can lead to vision problems, including loss of sight due to ischemia or retinal damage.

Vascularization of the Orbital Structures

The ophthalmic artery’s other branches, such as the lacrimal artery, ethmoidal arteries, and supra-orbital artery, also ensure that the orbital structures, including the muscles that move the eye, the lacrimal gland (which produces tears), and the nasal sinuses, receive adequate blood supply. This supports the normal functioning of the eye and surrounding tissues, including eye movement, tear production, and nasal air filtration.

Blood Supply to the Forehead and Scalp

Contribution to the Facial and Scalp Circulation

• The internal carotid artery also indirectly contributes to the blood supply of the forehead and scalp through its ophthalmic branch. The supratrochlear and supra-orbital arteries, which arise from the ophthalmic artery, provide blood to the skin, muscles, and soft tissues of the forehead and scalp.
• These arteries support the health and function of the facial tissues by delivering oxygenated blood, ensuring the skin remains nourished and capable of healing. They also play a role in thermoregulation by helping regulate heat exchange in the scalp and facial regions.

Support for the Meninges

Blood Supply to the Dura Mater

• The internal carotid artery provides blood to parts of the dura mater, the thick outer layer of the meninges that protect the brain and spinal cord. Through branches like the anterior meningeal artery (from the ophthalmic artery), the ICA supports the vascularization of the dura mater in the anterior cranial fossa.
• This blood supply is essential for maintaining the health and integrity of the meninges, which provide structural support, protection against trauma, and a pathway for venous drainage from the brain.

Contribution to Cerebral Metabolism

Delivery of Oxygen and Nutrients

• As a major arterial source for the brain, the internal carotid artery plays a crucial role in the delivery of oxygen and glucose to the brain, which are vital for neuronal activity. The brain has a high metabolic demand, and the ICA ensures that key regions involved in cognitive functions, memory, and motor control receive the necessary nutrients to sustain these activities.
• By maintaining cerebral perfusion, the ICA supports essential brain processes, including consciousness, decision-making, voluntary movement, sensory perception, and language processing. Interruptions in the blood supply from the ICA can lead to severe consequences such as strokes, ischemia, or cognitive impairment.

Facilitation of Collateral Circulation

Collateral Flow in Case of Blockage

• One of the critical roles of the internal carotid artery is its ability to facilitate collateral circulation in the brain. The Circle of Willis serves as an important collateral pathway, and the ICA’s contribution to this structure through the anterior cerebral and posterior communicating arteries ensures that blood can be rerouted in cases of blockage or arterial narrowing.
• This function is particularly significant in stroke prevention, as collateral circulation can help preserve brain tissue by providing alternative routes for blood flow when a major artery is compromised.

Blood Supply to Deep Brain Structures

Supply to the Internal Capsule, Basal Ganglia, and Optic Pathways

• The internal carotid artery’s branch, the anterior choroidal artery, plays an important role in supplying deep brain structures such as the internal capsule, basal ganglia, optic tract, and parts of the hippocampus. These areas are critical for functions like motor control, sensory processing, and visual processing.
• The anterior choroidal artery’s supply to the internal capsule ensures that important motor and sensory pathways connecting the cerebral cortex to the spinal cord are maintained. Similarly, the blood supply to the optic tract supports vision by preserving the pathways that transmit visual information from the eyes to the brain.

Clinical Significance

The internal carotid artery (ICA) is critically significant because it supplies blood to essential areas of the brain, eyes, and parts of the face. It plays a vital role in maintaining cerebral circulation, and any compromise to its function can lead to serious medical conditions. Blockage or narrowing of the ICA, often due to atherosclerosis, can result in ischemic stroke, which can lead to neurological deficits like paralysis, speech impairments, or cognitive dysfunction.

The ICA is also a key structure in conditions such as carotid artery stenosis, where plaque buildup reduces blood flow to the brain, increasing the risk of stroke. Aneurysms of the ICA, particularly in the cavernous segment, can compress nearby cranial nerves, leading to visual disturbances, double vision, or eye movement dysfunction.

In the Circle of Willis, the ICA provides collateral blood flow to other parts of the brain, helping maintain cerebral perfusion in case of arterial blockage. Its involvement in ophthalmic artery occlusion can lead to sudden vision loss due to ischemia of the retina. Therefore, the ICA is crucial in both neurology and ophthalmology, with its dysfunction posing serious risks to brain and eye health.