The basilar artery is a major blood vessel that supplies the brainstem and posterior parts of the brain, particularly the cerebellum and occipital lobes. It is formed by the convergence of the two vertebral arteries at the base of the skull.
Location
The basilar artery is located at the base of the brain, within the pontine cistern, running along the ventral (anterior) surface of the pons, a structure in the brainstem. It forms from the union of the two vertebral arteries at the level of the lower border of the pons and ascends along the midline of the brainstem. The basilar artery terminates by bifurcating into the posterior cerebral arteries, which supply the posterior part of the brain. Along its course, it gives rise to several branches, including the anterior inferior cerebellar arteries (AICA), pontine arteries, and superior cerebellar arteries.
Structures and Anatomy
Origin
The basilar artery forms from the union of the two vertebral arteries at the lower border of the pons. The vertebral arteries arise from the subclavian arteries and ascend through the transverse foramina of the cervical vertebrae. Upon entering the cranial cavity through the foramen magnum, the two vertebral arteries converge at the base of the brainstem, specifically at the pontomedullary junction, to form the basilar artery.
Course
The basilar artery ascends along the midline of the ventral (anterior) surface of the pons in the pontine cistern, a part of the subarachnoid space. It runs in close proximity to the brainstem’s structures and is positioned between the pons and the clivus, a sloped area of the skull base. As it travels upward, it remains centered along the pons until it reaches the upper border, where it bifurcates.
Termination
The basilar artery terminates at the upper border of the pons by bifurcating into the two posterior cerebral arteries, which supply the posterior regions of the brain, including the occipital lobe and parts of the temporal lobe. These arteries are part of the Circle of Willis, a circulatory anastomosis that provides collateral blood supply to the brain.
Relations
- Anteriorly: The artery lies on the clivus, a bony slope on the skull base.
- Posteriorly: It is closely related to the pons, the main brainstem structure that the artery runs along.
- Laterally: The basilar artery gives rise to several lateral branches that supply the brainstem and cerebellum.
- Superiorly: It bifurcates into the posterior cerebral arteries near the midbrain, completing its course at the upper border of the pons.
Branches
The basilar artery gives off several important branches along its course:
- Pontine Arteries: These small branches arise directly from the basilar artery and supply blood to the pons and other parts of the brainstem. They can be divided into paramedian, short circumferential, and long circumferential branches.
- Anterior Inferior Cerebellar Arteries (AICA): These arteries branch off near the lower part of the basilar artery and supply the anterior inferior surface of the cerebellum as well as parts of the medulla oblongata and pons.
- Labyrinthine Artery: In some individuals, a small branch, the labyrinthine artery (also known as the internal auditory artery), may arise from the basilar artery or the AICA. This artery supplies the inner ear, specifically the cochlea and vestibular apparatus, which are responsible for hearing and balance.
- Superior Cerebellar Arteries (SCA): These arise from the basilar artery near its termination. They supply the superior surface of the cerebellum, parts of the midbrain, and the superior vermis (a structure that connects the two hemispheres of the cerebellum).
- Posterior Cerebral Arteries (PCA): These are the terminal branches of the basilar artery, formed at its bifurcation. The posterior cerebral arteries supply the occipital lobes, parts of the temporal lobes, and the thalamus. They also form part of the Circle of Willis.
Anastomoses
The basilar artery forms part of the Circle of Willis, which is an important circulatory anastomosis at the base of the brain. The posterior cerebral arteries, which arise from the basilar artery, connect to the internal carotid system via the posterior communicating arteries. This network allows for collateral blood flow to the brain in case of blockage or reduced blood supply in one of the major arteries.
Variations
There are anatomical variations in the course and branching of the basilar artery. These can include differences in the size or origin of its branches, particularly the labyrinthine artery and the AICA. In some cases, the basilar artery may bifurcate slightly higher or lower than usual, or the artery itself may be more tortuous or show minor differences in length.
Function
The basilar artery plays a critical role in the blood supply to the brainstem, cerebellum, and posterior parts of the brain, including the occipital lobes. It is responsible for delivering oxygenated blood to essential structures involved in vital functions such as coordination, motor control, balance, and consciousness. Below is a detailed explanation of the functions served by the basilar artery through its various branches.
Blood Supply to the Brainstem
The basilar artery is the primary vessel that supplies blood to the brainstem, which includes the pons and parts of the medulla oblongata and midbrain. These areas are vital for basic life functions, including control of heart rate, breathing, and blood pressure. Through its branches, the basilar artery delivers blood to the brainstem, allowing it to maintain these essential autonomic functions.
Pontine Arteries: The basilar artery gives off multiple small pontine arteries as it travels along the ventral surface of the pons. These arteries supply the pons with oxygenated blood, ensuring its ability to regulate breathing, facial movements, and sensory information transmission. Without an adequate blood supply from these arteries, the function of the pons could be impaired, leading to life-threatening conditions.
Blood Supply to the Cerebellum
The cerebellum is responsible for coordinating voluntary movements, maintaining posture, and ensuring balance. The basilar artery supplies large portions of the cerebellum through the anterior inferior cerebellar arteries (AICA) and superior cerebellar arteries (SCA).
- Anterior Inferior Cerebellar Arteries (AICA): The AICA branch from the lower part of the basilar artery and provide blood to the anterior inferior part of the cerebellum. They also supply portions of the brainstem, particularly the medulla and pons.
- Superior Cerebellar Arteries (SCA): These arteries arise near the termination of the basilar artery and supply the superior aspect of the cerebellum, as well as parts of the midbrain. The blood delivered by these arteries supports the cerebellum’s role in motor coordination and balance. Proper cerebellar function is essential for maintaining smooth, precise movements and balance.
Blood Supply to the Inner Ear
The basilar artery indirectly supports hearing and balance by supplying blood to the inner ear through the labyrinthine artery (or internal auditory artery), which often arises from the basilar artery or the AICA. This small artery provides oxygenated blood to the cochlea and vestibular apparatus, structures essential for hearing and balance. Any compromise in the blood supply from the labyrinthine artery can lead to hearing loss or balance disorders, such as vertigo.
Blood Supply to the Occipital and Temporal Lobes
The basilar artery terminates by bifurcating into the posterior cerebral arteries (PCA), which supply the occipital lobes, parts of the temporal lobes, and portions of the thalamus. These regions of the brain are responsible for processing visual information and certain aspects of memory and sensory integration.
Posterior Cerebral Arteries (PCA): These arteries deliver blood to the occipital lobe, which processes visual stimuli. They also supply the inferior part of the temporal lobe, which is involved in memory storage, and portions of the thalamus, which integrates sensory information. An adequate blood supply to these regions ensures proper visual perception and cognitive functions.
Contribution to the Circle of Willis and Collateral Circulation
The basilar artery is a key contributor to the Circle of Willis, a critical arterial network that provides collateral circulation to the brain. The posterior cerebral arteries (PCA), which are the terminal branches of the basilar artery, connect to the internal carotid system via the posterior communicating arteries. This connection allows for collateral blood flow to the brain, meaning that if one of the main arteries in the brain becomes blocked or narrowed, blood can still be delivered to critical brain areas through alternate pathways.
This redundancy in blood supply is vital in preventing brain ischemia and stroke, particularly in regions supplied by the posterior cerebral arteries, such as the occipital lobes and brainstem.
Support for Brainstem Reflexes and Cranial Nerves
By supplying blood to the brainstem, the basilar artery supports important reflexes and functions controlled by the cranial nerves. The brainstem is home to multiple cranial nerve nuclei, including those responsible for controlling eye movements, facial muscles, swallowing, and hearing.
- The basilar artery ensures that the brainstem receives adequate oxygen and nutrients to support reflexive actions such as blinking, pupil constriction, and swallowing.
- It also nourishes areas involved in the control of autonomic functions such as breathing and cardiovascular regulation.
Without the proper blood supply from the basilar artery and its branches, the brainstem would not be able to carry out these essential functions.
Clinical Significance
The basilar artery is crucial due to its role in supplying blood to vital brain structures, including the brainstem, cerebellum, and posterior parts of the brain. Damage or blockage of the basilar artery can lead to serious, often life-threatening conditions such as basilar artery thrombosis or brainstem stroke, which can result in locked-in syndrome, coma, or death due to the impairment of basic functions like breathing and heart rate regulation.
Disorders involving the basilar artery, such as vertebrobasilar insufficiency, can lead to symptoms like dizziness, vision disturbances, and difficulties with balance and coordination. These symptoms arise from inadequate blood flow to the posterior brain regions supplied by the basilar artery, including the cerebellum and occipital lobes.
Given its proximity to critical areas of the brainstem, damage to the basilar artery can severely impact cranial nerve function, leading to motor and sensory deficits, hearing loss, or facial paralysis. The basilar artery’s involvement in the Circle of Willis also makes it an essential component of the brain’s collateral circulation, helping protect against ischemic damage in the event of other arterial blockages.
