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Internal carotid artery
The internal carotid artery (Latin: arteria carotis interna) is one of two terminal branches of the common carotid artery. Each person has two internal carotid arteries that supply arterial blood to the structures found within the skull, such as the cerebral hemispheres, eyes, nose, nasal cavity and paranasal sinuses. Moreover, the internal carotid arteries with their branches contribute to the anterior circulation system of the brain and, together with the posterior circulation system, form the circle of Willis.
Origin of internal carotid artery
The internal carotid artery originates from the site where the common carotid artery bifurcates in its two terminal branches, the other one being the external carotid artery. The bifurcation site is called the carotid bifurcation. It is found within the carotid triangle of the neck at the level of the superior border of the thyroid cartilage or approximately at the third to fifth cervical vertebral (C3 - C5) level.
Course and parts of internal carotid artery
Several classification systems divide the internal carotid artery into various parts or segments based on its course and the areas through which it travels. Nowadays, two main classification systems are used. They are the Gibo classification and the Bouthillier classification.
The Gibo classification is considered the most commonly used classification. It was proposed by Gibo in 1981. This classification divides the internal carotid artery into four segments based on their relation to the anatomical structures that are crossed by the artery.
According to the Gibo classification, the internal carotid artery has the following four parts or segments:
- Cervical or extracranial part (C1) - segment that goes through the neck;
- Petrous part (C2) - portion that travels through the petrous part of the temporal bone;
- Cavernous part (C3) - part that passes through the cavernous sinus;
- Intracranial or supraclinoid part (C4) - segment that goes within the cranial cavity after the internal carotid artery pierces the dura mater.
The cervical part (C1) of the internal carotid artery extends from its origin to the external orifice of the carotid canal. It goes through the neck, ascending posteromedially to the external carotid artery and anterior to the transverse processes of the first three cervical vertebrae (C3 - C1). This segment travels within the carotid sheath together with the internal jugular vein and the vagus nerve (CN X).
The cervical segment of the internal carotid artery usually does not give any branches. It enters the skull through the carotid canal of the petrous part of the temporal bone, anterior to the jugular foramen.
The petrous part (C2) extends from the external orifice of the carotid canal to the petrolingual ligament or to the site where the internal carotid artery enters the cavernous sinus. It is divided into a short vertical portion that curves anterior and medially and continues as a horizontal portion.
The petrous part then crosses the cartilage occluding the foramen lacerum (it does not go through it) and enters the cavernous sinus. Initially, the petrous part of the artery is located anterior to the cochlea and the tympanic cavity. Only a thin, bony plate separates the artery from the tympanic cavity and the trigeminal ganglion.
The internal carotid artery is encircled and surrounded by small veins and branches of the superior cervical ganglion of the sympathetic trunk forming the carotid plexus. The petrous part gives off several small branches, including the caroticotympanic artery and Vidian artery:
- The caroticotympanic artery goes through the tympanic cavity. It enters the tympanic cavity via the foramen within the carotid canal. Here it anastomoses with the anterior tympanic branch of the maxillary artery and the stylomastoid artery of the posterior auricular artery.
- The Vidian artery is inconsistent, and it may sometimes be absent. It is also called the artery of the pterygoid canal. It travels through the pterygoid canal together with the Vidian nerve and anastomoses with the branch of the maxillary artery.
The cavernous part (C3) corresponds to the segment that travels from where the internal carotid artery enters the cavernous sinus to where it exits the sinus (close to the anterior clinoid process). It goes through the cavernous sinus.
At first, the internal carotid artery ascends towards the posterior clinoid process, and then it goes in the anterior direction along the side of the body of the sphenoid bone and curves upwards on the medial aspect of the anterior clinoid process. The cavernous segment ends at the proximal dural ring that incompletely encircles the internal carotid artery. It then pierces the dura mater and exits through the roof of the sinus. The curve of the cavernous part is known as the carotid siphon.
Several cranial nerves run through the cavernous sinus lateral to the internal carotid artery. They include the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic and maxillary divisions of the trigeminal nerve (CN V1 and CN V2) and abducens nerve (CN VI). The artery is also accompanied by an internal carotid plexus originating from the superior cervical ganglion.
The cavernous part of the internal carotid artery gives off several branches that overall supply the trigeminal ganglion, walls of the cavernous sinus and nerves going through it. They include the following branches:
- The meningohypophyseal trunk is a small vessel that usually originates from the proximal portion of the C3 segment. It may arise as a single trunk or collection of arteries. It is also known as the posterior trunk. Overall, it supplies the pituitary gland, clivus, oculomotor (CN III), trochlear (CN IV), trigeminal (CN V) and abducens (CN VI) nerves, tentorium cerebelli and adjacent dura mater.
- The inferolateral trunk or the artery of the inferior cavernous sinus arises laterally from the midportion of the C3 segment and goes in the inferior direction. It travels between the trochlear nerve (CN IV) and the ophthalmic division of the mandibular nerve (CN V1) and gives off several branches. Overall, the inferolateral trunk supplies the oculomotor (CN III), trochlear (CN IV), trigeminal (CN V) and abducens (CN VI) cranial nerves.
- Small capsular arteries that supply the wall of the cavernous sinus.
The final portion of the internal carotid artery is the intracranial or supraclinoid part (C4). After the internal carotid artery leaves the cavernous sinus and pierces the dura mater, it travels further between the optic nerve (CN II) medially and the oculomotor nerve (CN III) laterally. The artery travels to the anterior perforated substance at the medial end of the lateral sulcus and bifurcates into two terminal branches - the anterior cerebral artery and the middle cerebral artery.
Along its course, the intracranial part of the artery gives off several branches, including the ophthalmic, posterior communicating and anterior choroidal arteries. The intracranial segment can also be subdivided into three distinct subsegments in relation to the origin of these branches:
- Ophthalmic segment (C4-a or Opth) - from the origin of the ophthalmic artery to the origin of the posterior communicating artery;
- Communicating segment (C4-b or Co) - from the origin of the posterior communicating artery to the origin of the anterior choroidal artery;
- Choroidal segment (C4-c or Ch) - from the origin of the anterior choroidal artery to the bifurcation of the internal carotid artery.
The ophthalmic artery is the first major branch of the internal carotid artery arising right after the internal carotid artery pierces through the dura mater and exits the cavernous sinus. It originates medially to the anterior clinoid process of the sphenoid bone and has a relatively short intracranial course.
The ophthalmic artery pierces the dura mater, enters the optic canal via the optic foramen, and passes into the orbit together with the optic nerve (CN II). Within the orbit, the ophthalmic artery goes along its medial wall. Together with its branches, it supplies the orbit and its contents, such as the eyeball, extraocular muscles and peri-ophthalmic tissue.
The posterior communicating artery originates from the posterior aspect of the internal carotid artery. It runs posteriorly above the oculomotor nerve (CN III) and anastomoses with the terminal branch of the basilar artery - the posterior cerebral artery.
The posterior communicating artery connects the anterior and posterior circulation systems of the brain and participates in the formation of the circle of Willis. The artery gives off several small branches that pierce the posterior perforated substance. They supply the internal capsule, third ventricle, thalamus, hypothalamus, subthalamus, optic chiasm and mammillary bodies.
The anterior choroidal artery arises from the posterolateral aspect of the intracranial part of the internal carotid artery close to its termination site. The anterior choroidal artery supplies an extensive cerebral area, including structures of the limbic system, midbrain and diencephalon.
Bouthillier and his colleagues proposed another classification of internal carotid artery segments in 1996. They modified the Gibo classification by dividing the existing segments into several subsegments. Therefore, the Bouthillier classification is more detailed.
The cervical segments are identical in both classifications. The petrous segment of the Gibo classification is subdivided into petrous and lacerum segments in the Bouthillier classification. It was observed that the entire petrous part of the internal carotid artery is not always located within the petrous portion of the temporal bone but is also found in the dural ligament. The cavernous segment is similar in both classifications. And finally, the clinoid, ophthalmic and communicating segments of the Bouthillier classification correspond to the intracranial segment of the Gibo classification.
According to Bouthillier et al, the internal carotid artery is subdivided into seven segments:
- C1 - cervical segment;
- C2 - petrous (horizontal) segment;
- C3 - lacerum segment;
- C4 - cavernous segment;
- C5 - clinoid segment;
- C6 - ophthalmic (supraclinoid) segment;
- C7 - communicating (terminal) segment.
The cervical segment (C1) travels through the neck and carotid triangle. It extends from the bifurcation of the common carotid artery to the external orifice of the carotid canal. The cervical segment does not have any branches.
The petrous (horizontal) segment (C2) starts from the external orifice of the carotid canal. It extends to the posterior edge of the foramen lacerum, which marks the end of the carotid canal. The branches of this part are the Vidian and caroticotympanic arteries.
The lacerum segment (C3) starts at the posterior edge of the foramen lacerum and extends to the entrance to the cavernous sinus or the superior margin of the petrolingual ligament. This segment does not have any branches.
The cavernous segment (C4) is the portion of the artery from the entrance of the cavernous sinus to the site where the internal carotid artery leaves the cavernous sinus. It happens at the level of the proximal dural ring (anterior clinoid process). This part gives off the meningohypophyseal trunk, inferolateral trunk and capsular arteries.
The clinoid segment (C5) extends between the proximal dural ring (anterior clinoid process) and the distal dural ring (cavernous sinus roof). This segment usually does not have any branches.
The ophthalmic (supraclinoid) segment (C6) starts at the distal dural ring and ends proximal to the origin site of the posterior communicating artery. It gives off the ophthalmic artery and superior hypophyseal artery.
The communicating (terminal) segment (C7) extends from the origin of the posterior communicating artery to the bifurcation of the internal carotid artery. It passes between the optic (CN II) and oculomotor (CN III) nerves. The communicating segment gives off the posterior communicating artery and anterior choroidal artery, as well as its two terminal branches - the anterior cerebral artery and the middle cerebral artery.
Terminal branches of internal carotid artery
Each internal carotid artery ends by terminating into two branches - the anterior cerebral artery and the middle cerebral artery.
Anterior cerebral artery
The anterior cerebral artery is the smallest of both terminal branches of the internal carotid artery. It arises at the medial end of the lateral sulcus of the brain, passing anterior and medial to the optic nerve (CN II). It travels in the anteromedial direction towards the longitudinal cerebral fissure. Near the fissure, it anastomoses with the opposite anterior cerebral artery via the anterior communicating artery. Therefore, the anterior cerebral artery participates in the formation of the circle of Willis.
Both anterior cerebral arteries pass further curving around the genu of the corpus callosum. After the turn, the artery continues to go in the posterior direction towards the parieto-occipital sulcus along the upper aspect of the body of the corpus callosum. At the splenium of the corpus callosum, the branches of the anterior cerebral arteries anastomose with the branches of the posterior cerebral artery.
The anterior cerebral artery gives off various superficial and deep branches along its course. Overall, all branches can be subdivided into two major groups - cortical or terminal and central branches.
The cortical (terminal) branches supply the motor and somatosensory cortex that represents the lower limb. These branches include the following:
- Orbital branches - headed to the orbital surface of the frontal lobe; they supply the olfactory cortex, medial orbital gyrus and gyrus rectus (straight gyrus); they include the orbitofrontal and frontopolar arteries;
- Frontal branches - branches that ramify into the frontal lobe; they supply the corpus callosum, cingulate gyrus, medial frontal gyrus, paracentral lobule, superior frontal gyrus and middle frontal gyrus;
- Parietal branches - provide arterial blood to the precuneus and nearby lateral surface.
The central branches originate from the proximal part of the anterior cerebral artery. They enter the anterior perforated substance to supply the lamina terminalis, rostrum of the corpus callosum, anterior aspect of the putamen, septum pellucidum, head of the caudate nucleus and internal capsule.
Middle cerebral artery
The middle cerebral artery is the largest terminal branch of the internal carotid artery. It goes between the frontal and temporal lobes and travels laterally through the lateral sulcus towards the insular cortex. When it reaches the insula, it passes over its posterosuperior surface. The middle cerebral artery terminates by dividing into superior and inferior trunks on the lateral surface of the brain at the insula.
The superior trunk supplies the lateral aspect of the inferior frontal gyrus, including the Broca's area. The inferior trunk is responsible for the arterial blood supply of the superior temporal gyrus, including the Wernicke's area. Like the branches of the anterior cerebral artery, the branches of the middle cerebral artery also can be subdivided into cortical and central branches.
The cortical branches supply the primary motor and somatosensory cortical areas that represent the face, upper limbs and trunk. Therefore, they perfuse a large area of the lateral surface, including the frontal, parietal and temporal lobes. However, these vessels do not supply the auditory cortex and insula.
The central branches of the middle cerebral artery are relatively small. They arise within the lateral sulcus and include several perforating vessels called the lateral lenticulostriate arteries. They provide arterial blood supply to the deep subcortical structures of the brain, such as the basal ganglia and internal capsule.
- Bonasia, S., Bouthillier, A., & Robert, T. (2020). Segmental classification of the internal carotid artery: An overview. Contemporary Neurosurgery, 42(18), 6–6. https://doi.org/10.1097/01.cne.0000734832.23603.51
- Bouthillier, A., van Loveren, H. R., & Keller, J. T. (1996). Segments of the internal carotid artery: A new classification. Neurosurgery, 38(3), 425–433. https://doi.org/10.1097/00006123-199603000-00001
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