Internal carotid artery
The internal carotid artery is a major branch of the common carotid artery, supplying several parts of the head with blood, the most important one being the brain. There are two internal carotid arteries in total, one on each side of the neck. They originate from the carotid bifurcation, travel through the carotid sheath in a superior direction along the neck, and enter the skull through the external opening of carotid canal. Each artery is divided into seven segments according to the areas through which it passes. Along its course, the internal carotid artery gives rise to many branches, ultimately dividing into its two terminal ones called the anterior and middle cerebral arteries.
The clinical importance of internal carotid arteries is evident during action movies or martial arts classes where self-defense moves are performed, for example a rear naked choke or a strike to one side of the neck. During these actions, anatomical structures like the carotid sinus (baroreceptor) and internal carotid arteries are either squeezed or directly hit, resulting in an instant knock-out.
Cervical (C1), Petrous (C2), Lacerum (C3), Cavernous (C4), Clinoid (C5), Ophthalmic (C6), Communicating segments (C7)
Mnemonic: Please Let Children Consume Our Candy
Caritocotympanic (C2), Vidian (C2), Meningeal (C4), Inferior Hypophyseal (C4), Superior Hypophyseal (C6), Ophthalmic (C6), Posterior Communicating (C7), Anterior Choroidal (C7)
Terminal Branches: Anterior Cerebral (C7), Middle Cerebral Arteries (C7)
Mnemonic: A VIP’S COMMA
This article will explain the embryology, functions, origin, course, and branches of the internal carotid artery. It will also help you learn them using mnemonics, saving you countless reviews and a lot of time in the long run.
- Segments and course
- Clinical aspects
During fetal development, vasculogenesis (formation of new vessels from haematoangioblastic stem cells) is responsible for the formation of two dorsal aortae, six pairs of primitive pharyngeal (branchial) arch arteries, and other vascular structures. The third pharyngeal arch artery fuses with the distal components of the dorsal aortae to give rise to the internal carotid artery. The embryological dorsal aortae (left and right) ultimately form the descending aorta.
Several main and terminal branches of the internal carotid artery start developing from the 4th gestational week onwards when it bifurcates into anterior and posterior components. The former will differentiate into the anterior cerebral, middle cerebral, and anterior choroidal arteries. In turn, the posterior component will form the foetal posterior cerebral artery and the posterior choroidal artery, which will later develop into their adult forms.
As you probably know, the brain is one of the most essential organs in the human body. It oversees the regulation of physiological homeostasis, executes cognitive functions, and micromanages integral body functions subconsciously. To sustain this continuous physiological demand, it requires a constant and large blood supply. To put it into perspective, the brain weighs approximately 2% of your entire body weight, but receives 15-20% of the daily cardiac output.
There are two sources responsible for feeding the brain with its much needed arterial blood called anterior and posterior circulations. The internal carotid arteries are part of the anterior circulation, which is responsible for supplying the forebrain. The two circulations of the brain anastomose and form an anatomical structure called the circle of Willis.
Why are there two circulations and so many sources of arterial blood to the brain? It all comes to the significance of this organ in our daily lives. If any of the paths or branches of either circulation becomes blocked or fails in some way, the remaining and hopefully viable circulation can still bring fresh arterial blood to the brain through a different path. It is similar to going to your favourite (or not so favourite) anatomy class. If the main way or access point is obstructed, you will need to take a detour to reach the lecture hall or dissection room.
More information about the arteries supplying the brain is provided below.
First things first, there are two internal carotid arteries in total, one on the right and one on the left of the neck. They both originate from their respective common carotid arteries from a point called the carotid bifurcation situated at a level between the third and fourth cervical vertebrae (C3-C4).
Situated close to the carotid bifurcation are two anatomical structures called carotid sinus and carotid body. The carotid sinus or bulb is a dilation acting as a baroreceptor for detecting blood pressure changes, while the carotid body acts as a chemoreceptor for acid-base disturbances.
Segments and course
Classifications and names
There are two main ways to categorize the segments of the internal carotid artery. The newest one was put forward in 1998 dividing the artery into four parts, the names of which are related to the areas or anatomical structures through which the internal carotid artery passes:
- Cervical part (neck)
- Petrous part (temporal bone)
- Cavernous part (cavernous sinus)
- Intercranial part (after piercing the dura mater)
However, many medical specialties actually use an older method described in 1996, known as the Cincinnati Classification (Bouthillier et. al., 1996). The names are also related to the areas through which the artery passes, but it is more detailed. According to this idea, there are seven segments:
- C1 – Cervical Segment
- C2 – Petrous Segment
- C3 – Lacerum Segment
- C4 – Cavernous Segment
- C5 – Clinoid Segment
- C6 – Ophthalmic (Supraclinoid) Segment
- C7 – Communicating (Terminal) Segment
You can easily remember the intracranial segments of the Cincinnati Classification using the mnemonic ‘Please Let Children Consume Our Candy’.
The two classifications are not like oil and water or completely distinct. The cervical segments correspond, while the C2 and C3 segments are referred to collectively as the petrous part. The cavernous parts also match, while C5, C6, and C7 are referred to as the intracranial components.
In this article, we’ll use the Cincinnati Classification to describe the pathway of the internal carotid artery because it is more detailed. You can easily revert to the newest classification by combining the trajectories of various segments according to the explanations in the previous paragraph.
- Cervical segment (C1): You can easily remember the contents by thinking about the mnemonic ‘I See 10 CC’s in the IV’. This part of the internal carotid artery (I.C. or ‘See’) travels superiorly through the carotid triangle of the neck enveloped in the carotid sheath, together with the common carotid artery (CC), internal jugular vein (IV), vagus nerve (CN 10), deep cervical lymph nodes, and sympathetic nerve fibers. As it travels superiorly, the artery passes anteriorly to the transverse processes of C1-C3, advancing towards the carotid canal in the temporal bone of the skull.
- Petrous segment (C2): inside the carotid canal, this segment travels superiorly before taking an anteromedial course; after which it travels along a superomedial course advancing towards the foramen lacerum.
- Lacerum segment (C3): this short segment travels over the cartilage occluding the foramen lacerum (not through it!) and ends at the petrolingual ligament. This anatomical structure is located on the wall of the cavernous sinus.
- Cavernous segment (C4): it courses superiorly along the posterior clinoid process of the sphenoid bone, making its way towards the anterior clinoid process before finally emerging through the roof of the cavernous sinus. Within the cavernous sinus, the internal carotid artery travels superomedially to CN VI (abducens nerve) and medially to CN III (oculomotor nerve), CN IV (trochlear nerve), and CN V1 and CN V2 (ophthalmic and maxillary divisions of the trigeminal nerve, respectively).
- Clinoid segment (C5): after exiting the cavernous sinus at the proximal dural ring, this segment travels a very short distance until the distal dural ring.
- Ophthalmic segment (C6): from the distal dural ring, the C6 segment travels parallel and horizontal in an inferolateral position with respect to CN II (optic nerve). The end point is the origin of the posterior communicating artery.
- Communicating segment (C7): this terminal segment bifurcates into its terminal branches before ending at the anterior perforated substance.
Like every major artery in the human body, the internal carotid has several branches that are often asked about in anatomy exams. They stem from several segments (C2, C4, C6, and C7), the only exceptions being the cervical (C1), lacerum (C3), and clinoid (C5) segments do not give rise to any branches.
- Caroticotympanic artery (C): it originates from the petrous part (C2) and travels through the tympanic cavity via the foramen within the carotid canal. It subsequently anastomoses with the anterior tympanic branch of the stylomastoid artery and the maxillary artery.
- Vidian artery (V): also called the artery of the pterygoid canal, or the pterygoid artery, originates from the petrous part (C2) of the internal carotid artery. The Vidian artery courses through the pterygoid canal together with the Vidian nerve, or nerve of the pterygoid canal. It finally anastomoses with a branch of the greater palatine artery. It is important to note that not every individual has the Vidian artery, meaning that it is inconsistent.
- Meningeal artery (M): it stems from the cavernous segment (C4) and supplies the dura mater of the anterior cranial fossa. In the end, it anastomoses with the meningeal branch of the posterior ethmoidal artery. In addition, the cavernous segment also gives off several small arteries that supply the trigeminal ganglion, the walls of two dural venous sinuses (inferior petrosal and cavernous), together with the nerves contained in their vicinity.
- Inferior hypophyseal artery (I): also coming from the cavernous segment (C4), this artery supplies the neurohypophysis, the posterior part of the pituitary gland. This artery terminates in the pituitary portal system.
- Superior hypophyseal artery (S): it stems from the ophthalmic segment (C6), supplying the infundibulum and median eminence of the hypothalamus, as well as the pars tuberalis of the anterior pituitary gland.
- Ophthalmic artery (O): also stemming from the ophthalmic segment (C6), this artery passes through the optic canal, ultimately entering the orbit and supplying its contents.
Posterior communicating artery (P): it originates from the communicating segment (C7) and anastomoses with posterior cerebral artery coming off the basilar artery, contributing to the formation of the circle of Willis.
- Anterior choroidal artery (A): also originating from the communicating segment (C7), this artery follows a complex course by crossing the optic tract to supply the crus cerebri, then recrossing it to project onto and supply the lateral geniculate body of the thalamus. It then passes through the choroidal fissure to enter the lateral ventricle and supply its choroid plexus. In addition, the anterior choroidal artery supplies the mesencephalic, diencephalic, and telencephalic anatomical derivatives.
Once the internal carotid artery has followed the previously described course and given off its eight branches, it divides into the two terminal branches described below:
- Anterior cerebral artery (A): it is a terminal branch of the internal carotid artery originating from the communicating segment (C7). It travels towards the longitudinal cerebral fissure, where it anastomoses with its contralateral counterpart via the short anterior communicating artery. As a result, they contribute to the formation of the circle of Willis. The paired arteries then travel along the medial surface of the brain through the longitudinal cerebral fissure in a posterior direction along the genu of the corpus callosum. Here, each vessel anastomoses with the ipsilateral posterior cerebral artery. The anterior cerebral artery gives off cortical and central branches. The cortical branches include the parietal, orbital, and frontal branches. The parietal branches supply the precuneus, while the orbital branches are responsible for the olfactory cortex, medial orbital gyrus, and gyrus rectus of the frontal lobe. The frontal branches provide blood to the paracentral lobule, medial frontal, and cingulate gyri, and the corpus callosum. The central branches of the anterior cerebral artery supply the rostrum of the corpus callosum, the septum pellucidum, and areas surrounding the anterior part of the putamen (head of the caudate nucleus and local area of the internal capsule).
- Middle cerebral artery (M): it is the second and largest terminal branch of the internal carotid artery, also originating from the communicating segment (C7). The middle cerebral artery travels through the lateral fissure before coursing over the insula. It subsequently divides to supply the lateral cortical surfaces along with the insula. It also has cortical and central branches, similar to the anterior cerebral artery. The cortical branches include the frontal, orbital, parietal, and temporal branches. The frontal arteries perfuse the inferior frontal, middle, and precentral gyri. The lateral orbital parts of the frontal lobe, as well as the frontal gyrus, are supplied by the orbital branches. The inferior parietal lobe, the inferior part of the superior parietal lobe, and the postcentral gyrus receive blood from the parietal branch. Several temporal arteries then go on to perfuse the lateral aspect of the temporal lobe. The central branches are relatively small and include the lenticulostriate arteries that pass through the anterior perforated substance to supply the lentiform nucleus and the posterior limb of the internal capsule.
You can easily remember the branches of the internal carotid artery by arranging the previous letters included in brackets into the mnemonic ‘A VIP’S COMMA’.
If you want to cement your knowledge about the internal carotid artery and learn more about the cerebrum in order to improve your conceptualization about the topic, take a look below:
Infarction and cerebrovascular accidents
Hypoperfusion of any organ results in a decrease in oxygen and nutrient supply to these tissues. Consequently, if the tissues remain hypoperfused for a prolonged period they will die–a process known as infarction.
In the brain, this condition manifests as a cerebrovascular accident (stroke). The anterior circulation is the site of approximately 70% of cerebral infarcts, with the middle cerebral artery being the offending artery in about 90% of these cases. Lesions of the anterior communicating artery (which supplies the medial surface of the cerebrum) accounts for as little as 2% of cases.
Patients may present with sudden manifestations of symptoms or symptoms that developed within 24 hours. The symptoms are often focal, meaning that a collection of neurological deficits affect a specific region or function of the body. Some examples include cognitive impairment (impaired speech), unilateral weakness and sensory impairment, and unilateral visual loss. There are other generalized symptoms such as headache, syncope, or in severe cases, seizures and coma.
Atherosclerotic plaques can also build up earlier in the course of the internal carotid artery and not only in its terminal branches. If such a narrowing (stenosis) is present, the artery can be surgically accessed for the plaque to be physically eliminated. This procedure is called carotid endarterectomy.
Basal skull fractures
As you’ve seen previously, the internal carotid artery enters the skull via the carotid canal. Therefore, fractures of the base of the skull can easily tear the internal carotid artery at this particular point resulting in an arteriovenous fistula inside the cavernous sinus. To put it simply, an abnormal communication is created between an artery, the internal carotid in this case, and a vein or venous system, this being the cavernous sinus.
Since the internal carotid artery contains arterial blood, it is under high pressure created by cardiac contractions. If an arteriovenous fistula is present, this arterial blood will flow into the low pressure system of the cavernous sinus and force blood retrogradely into its venous tributaries. Such tributaries include the ophthalmic veins which drain the orbit and its contents. If the pressure within them increases, the eyeball of the patient will protrude (exophthalmia) and the conjunctiva will be become engorged with blood (chemosis). In addition, an engorged cavernous sinus can put pressure on the structures passing through it, such as cranial nerves III, IV, V1, V2, and VI, ultimately producing distinct clinical signs and symptoms.