The posterior communicating artery is a critical artery in the brain that forms part of the Circle of Willis. It connects the anterior and posterior circulations, linking the internal carotid artery with the posterior cerebral artery.
Location
The posterior communicating artery is located at the base of the brain, running horizontally from the internal carotid artery to the posterior cerebral artery on each side. It lies within the subarachnoid space and is part of the Circle of Willis, positioned near the optic chiasm, hypothalamus, and pituitary stalk. The artery forms a bridge between the anterior circulation (supplied by the internal carotid arteries) and the posterior circulation (supplied by the vertebrobasilar system). It courses over the lateral aspect of the midbrain, where it is in close relation to the oculomotor nerve (cranial nerve III).
Structure and Anatomy
The posterior communicating artery (PComA) is a vital arterial structure that plays an essential role in linking the anterior and posterior cerebral circulations. It forms a part of the Circle of Willis and connects the internal carotid artery to the posterior cerebral artery. Below is a detailed description of its anatomy.
Origin
The posterior communicating artery arises from the internal carotid artery at its termination, just before the bifurcation of the internal carotid into the middle cerebral artery and anterior cerebral artery. This branch extends laterally and posteriorly, connecting the anterior and posterior circulations of the brain.
Course
The posterior communicating artery travels horizontally across the base of the brain, running posteriorly from its origin at the internal carotid artery towards its termination at the posterior cerebral artery (PCA). It passes over the lateral aspect of the midbrain and runs parallel to the optic tract, situated above the optic nerve, and is in proximity to several important neural structures, including the oculomotor nerve (cranial nerve III).
The posterior communicating artery courses within the subarachnoid space, a cerebrospinal fluid-filled space between the arachnoid mater and pia mater of the brain.
Relations
- Anteriorly: The posterior communicating artery is related to the internal carotid artery, from which it originates.
- Posteriorly: The artery terminates by anastomosing with the posterior cerebral artery, contributing to the posterior circulation of the brain.
- Medially: The posterior communicating artery is close to the optic chiasm and the pituitary stalk (infundibulum), key structures at the base of the brain.
- Laterally: It runs in close proximity to the oculomotor nerve (cranial nerve III), which emerges from the brainstem between the midbrain and pons and passes near the posterior communicating artery. Compression or injury to this nerve by aneurysms involving the posterior communicating artery can lead to oculomotor nerve palsy.
Branches
The posterior communicating artery gives off small perforating branches as it courses towards the posterior cerebral artery. These branches supply portions of the midbrain and hypothalamus, providing blood to important neural structures. These branches are relatively small but critical in ensuring oxygenated blood reaches deep regions of the brain.
Perforating Branches: These branches supply blood to parts of the diencephalon, including the hypothalamus, and portions of the midbrain, contributing to the perfusion of the structures surrounding the Circle of Willis.
Termination
The posterior communicating artery terminates by anastomosing with the posterior cerebral artery (PCA). This junction completes the connection between the anterior and posterior circulations of the brain, allowing for blood flow between these two systems. The posterior cerebral artery, in turn, supplies the occipital lobe, temporal lobe, and other posterior regions of the brain.
Anastomoses
The posterior communicating artery is an integral component of the Circle of Willis, an arterial ring that connects the major arteries of the brain and provides collateral circulation in case of vascular occlusion. The anastomotic connections formed by the posterior communicating artery include:
- Anteriorly: The posterior communicating artery connects with the internal carotid artery, forming a key link between the anterior and posterior circulations.
- Posteriorly: It anastomoses with the posterior cerebral artery, which arises from the basilar artery, part of the vertebrobasilar system.
This anastomotic network ensures that blood can flow between the internal carotid and vertebrobasilar systems, maintaining cerebral perfusion in the event of arterial occlusion or reduced blood flow in one of the major arteries.
Variations
There are several anatomical variations associated with the posterior communicating artery:
- Hypoplasia: In some individuals, the posterior communicating artery may be underdeveloped (hypoplastic), reducing its ability to contribute to collateral circulation.
- Fetal-Type Posterior Cerebral Artery: In some cases, the posterior cerebral artery arises predominantly from the internal carotid artery rather than the basilar artery. In this variation, the posterior communicating artery is enlarged, and the basilar artery’s contribution to the posterior cerebral artery is reduced.
These variations can influence the function of the Circle of Willis and affect the brain’s ability to maintain collateral circulation in the event of vascular blockage or narrowing.
Function
The posterior communicating artery (PComA) plays a crucial role in the cerebral circulation by connecting the anterior and posterior circulatory systems of the brain. It provides an essential pathway for collateral blood flow, ensuring that the brain remains adequately supplied with oxygenated blood, especially in the event of arterial blockages or narrowing. Below is a detailed description of its specific functions.
Connection Between Anterior and Posterior Circulations
The primary function of the posterior communicating artery is to serve as a bridge between the anterior circulation (supplied by the internal carotid artery) and the posterior circulation (supplied by the vertebrobasilar system). This connection allows blood flow between the anterior and posterior parts of the brain, ensuring balanced and continuous blood supply to critical brain regions.
- Anterior Circulation: The anterior circulation is mainly supplied by the internal carotid arteries, which give rise to the anterior cerebral artery and the middle cerebral artery, providing blood to the frontal and parietal lobes, as well as portions of the temporal lobes.
- Posterior Circulation: The posterior circulation is supplied by the vertebral arteries, which converge to form the basilar artery. The basilar artery then bifurcates into the posterior cerebral arteries (PCA), which supply the occipital lobe, temporal lobe, thalamus, and portions of the midbrain.
The posterior communicating artery connects these two circulations by linking the internal carotid artery to the posterior cerebral artery. This connection is essential for maintaining a balanced distribution of blood between the anterior and posterior regions of the brain.
Provides Collateral Circulation (Redundancy)
Another critical function of the posterior communicating artery is to provide collateral circulation, ensuring that blood can flow through alternative pathways if one of the main arteries becomes blocked or narrowed. In the event of an occlusion in the internal carotid artery or vertebrobasilar system, the posterior communicating artery allows blood to reroute through the Circle of Willis, ensuring that the brain regions normally supplied by the blocked artery still receive adequate blood flow.
- Internal Carotid Occlusion: If the internal carotid artery is obstructed, the posterior communicating artery allows blood from the posterior cerebral artery to flow forward and supply the territories of the anterior circulation.
- Vertebrobasilar Occlusion: If there is a blockage in the vertebral arteries or the basilar artery, the posterior communicating artery allows blood from the internal carotid system to flow backward through the posterior communicating artery to supply the posterior cerebral arteries.
This collateral pathway is critical for preventing ischemic damage to the brain in conditions such as stroke or transient ischemic attacks (TIAs). The presence of a functioning posterior communicating artery significantly reduces the risk of permanent brain damage when vascular occlusions occur.
Supports the Function of the Circle of Willis
The posterior communicating artery is a key component of the Circle of Willis, an arterial ring at the base of the brain that ensures continuity of blood flow between the anterior and posterior circulations. The Circle of Willis is responsible for distributing blood throughout the brain and protecting against ischemic events. The posterior communicating artery’s role in the Circle of Willis is to connect the anterior and posterior circulatory systems, allowing for the redistribution of blood when necessary.
Circle of Willis: The Circle of Willis includes the anterior cerebral arteries, posterior cerebral arteries, anterior communicating artery, and posterior communicating arteries. The posterior communicating artery is crucial in maintaining the integrity of this network, as it provides the link between the internal carotid arteries (anterior system) and the posterior cerebral arteries (posterior system).
Through its involvement in the Circle of Willis, the posterior communicating artery ensures that the brain can maintain adequate blood flow even in the event of arterial occlusion, aneurysm, or vascular injury.
Supplies Blood to Deep Brain Structures
The posterior communicating artery also contributes directly to the blood supply of critical deep brain structures, including portions of the hypothalamus, thalamus, and midbrain. This is accomplished through small perforating branches that arise from the posterior communicating artery as it courses along the base of the brain.
- Hypothalamus: The hypothalamus is a key regulatory center in the brain, responsible for controlling autonomic functions such as temperature regulation, thirst, hunger, and hormone release. The perforating branches of the posterior communicating artery help supply this region with the oxygenated blood it needs to function.
- Thalamus: The thalamus is a critical relay center for sensory information, sending sensory inputs to the appropriate cortical areas. The posterior communicating artery contributes to the blood supply of the posterior thalamus, particularly through its perforating branches.
- Midbrain: The posterior communicating artery, along with the posterior cerebral artery, supplies portions of the midbrain, which is involved in motor control, vision, and auditory processing.
By nourishing these deep brain structures, the posterior communicating artery supports essential functions such as sensory processing, autonomic regulation, and motor control.
Relation to Cranial Nerves
The posterior communicating artery is closely related to the oculomotor nerve (cranial nerve III), which emerges from the midbrain and passes near the artery. This relationship is clinically significant because aneurysms of the posterior communicating artery can compress the oculomotor nerve, leading to oculomotor nerve palsy.
Oculomotor Nerve: The oculomotor nerve controls most of the eye’s movements, including constriction of the pupil and maintenance of an open eyelid. When compressed by an aneurysm or vascular enlargement of the posterior communicating artery, this nerve can become dysfunctional, resulting in symptoms such as ptosis (drooping of the eyelid), diplopia (double vision), and pupil dilation.
Although the posterior communicating artery does not directly contribute to the function of the oculomotor nerve, its anatomical proximity to this nerve makes it a significant factor in certain neurological conditions.
Clinical Significance
The posterior communicating artery (PComA) is clinically significant because of its role in maintaining collateral circulation between the anterior and posterior cerebral circulations through the Circle of Willis. This connection allows for compensation in the event of arterial blockages or stenosis, such as in cases of carotid artery occlusion or vertebrobasilar insufficiency, reducing the risk of ischemic damage to the brain.
A common clinical issue involving the PComA is the formation of aneurysms at its junction with the internal carotid artery. These aneurysms are a frequent cause of subarachnoid hemorrhage and can compress the nearby oculomotor nerve (cranial nerve III), resulting in oculomotor nerve palsy, which manifests as ptosis (drooping eyelid), diplopia (double vision), and pupil dilation. Understanding the anatomy and variations of the posterior communicating artery is crucial for diagnosing and treating conditions related to cerebrovascular disease and aneurysms.
