The vertebral artery is a major artery that supplies blood to the brainstem, cerebellum, and posterior parts of the brain. It is one of the primary branches of the subclavian artery.
Location
The vertebral artery originates from the subclavian artery on both sides of the body. It ascends through the neck, passing through the transverse foramina of the cervical vertebrae from C6 to C1. After exiting the transverse foramen of the first cervical vertebra (C1), it curves medially and enters the cranial cavity via the foramen magnum. Inside the skull, the vertebral arteries from both sides converge at the level of the medulla oblongata to form the basilar artery at the base of the brain. The vertebral artery runs along the vertebral column and is positioned close to the brainstem, playing a critical role in the blood supply to the posterior brain.
Structure and Anatomy
Origin
The vertebral artery arises from the subclavian artery on both the left and right sides of the body. It is one of the first branches of the subclavian artery and is responsible for supplying blood to the posterior parts of the brain. The vertebral artery ascends through the neck, passing through the transverse foramina of the cervical vertebrae.
Course
The vertebral artery has a four-part course, divided into segments known as V1 to V4:
- V1 (Pre-foraminal Segment): This segment begins at the origin of the vertebral artery from the subclavian artery. It travels upward and enters the transverse foramen of the C6 vertebra. In some anatomical variations, the artery may enter at the C5 or C7 vertebra. The V1 segment runs alongside the anterior scalene muscle and passes behind the common carotid artery.
- V2 (Foraminal Segment): In this segment, the vertebral artery ascends through the transverse foramina of the cervical vertebrae from C6 to C1. The artery runs through these bony canals, which protect it as it ascends through the neck. The V2 segment is accompanied by the vertebral veins and sympathetic nerve fibers.
- V3 (Atlantoaxial Segment): Upon exiting the transverse foramen of the C1 vertebra (atlas), the vertebral artery courses laterally around the superior aspect of the atlas. It then curves medially and travels through the suboccipital triangle, a region bordered by the suboccipital muscles, before crossing the posterior arch of the atlas. This segment is exposed as it moves through this region, making it vulnerable to compression or injury. The V3 segment runs underneath the rectus capitis posterior minor muscle and alongside the first cervical spinal nerve.
- V4 (Intracranial Segment): After crossing the posterior arch of the atlas, the vertebral artery enters the cranial cavity through the foramen magnum. Inside the cranial cavity, the V4 segment of the vertebral artery travels anteriorly along the medulla oblongata, positioned within the subarachnoid space. The two vertebral arteries from either side converge at the lower border of the pons to form the basilar artery. The basilar artery then supplies blood to the brainstem, cerebellum, and posterior parts of the brain.
Branches
Along its course, the vertebral artery gives off several important branches:
- Anterior Spinal Artery: One of the first branches to arise from the vertebral artery within the cranial cavity, it supplies the anterior portion of the spinal cord.
- Posterior Inferior Cerebellar Artery (PICA): This major branch arises from the V4 segment of the vertebral artery and supplies the inferior part of the cerebellum and parts of the medulla oblongata.
- Posterior Spinal Arteries: These small arteries arise from the vertebral artery or PICA and supply the posterior aspect of the spinal cord.
- Meningeal Branches: These small branches supply the meninges, the protective coverings of the brain and spinal cord.
Relations
- Anteriorly: In the neck, the vertebral artery lies anterior to the longus colli muscle and posterior to the common carotid artery. As it ascends, it passes behind the vertebral bodies of the cervical spine.
- Laterally: As it ascends through the cervical transverse foramina, the vertebral artery is flanked by the vertebral veins.
- Posteriorly: The vertebral artery is positioned anterior to the spinal cord as it runs through the transverse foramina.
- Superiorly: Inside the cranial cavity, the vertebral arteries merge at the base of the brain to form the basilar artery at the level of the lower pons.
Termination
The vertebral arteries on both sides converge at the level of the lower pons, where they unite to form the basilar artery. The basilar artery is a key vessel that supplies blood to the brainstem, cerebellum, and posterior portions of the brain. This convergence occurs near the pontomedullary junction, and from there, the basilar artery travels along the ventral surface of the pons.
Anastomoses
The vertebral artery contributes to the formation of the Circle of Willis, an arterial network at the base of the brain that provides collateral circulation. Its posterior communicating arteries connect with the internal carotid artery system, allowing blood to flow between the anterior and posterior parts of the brain. This anastomotic network is crucial for maintaining blood supply to the brain in case of arterial blockages or reduced blood flow.
Variations
There are several anatomical variations in the vertebral artery’s course and branching patterns. These include variations in the vertebra at which the artery enters the transverse foramina, the size of its branches, and the exact course of the artery as it travels through the suboccipital region. In some individuals, the vertebral arteries on both sides may differ in size or dominance, meaning one vertebral artery may be larger than the other.
Function
The vertebral artery plays a critical role in supplying oxygenated blood to key structures in the posterior part of the brain, including the brainstem, cerebellum, occipital lobes, and portions of the spinal cord. Its contribution to the brain’s blood supply is essential for maintaining functions such as coordination, balance, vision, and basic life-sustaining processes. Below is a detailed explanation of the specific functions of the vertebral artery.
Blood Supply to the Brainstem
The vertebral artery is essential for providing blood to the brainstem, which consists of the medulla oblongata, pons, and midbrain. These areas control fundamental bodily functions such as breathing, heart rate, and blood pressure regulation. Through its terminal branches and its eventual convergence into the basilar artery, the vertebral artery ensures that the brainstem receives a continuous supply of oxygenated blood to support these life-sustaining functions.
Medulla Oblongata: The vertebral artery and its branches supply blood to the medulla, which is responsible for autonomic functions like regulating breathing, heart rate, and digestion. The anterior spinal artery and posterior inferior cerebellar artery (PICA), which branch from the vertebral artery, nourish the medulla, ensuring the survival of these critical processes.
Blood Supply to the Cerebellum
The vertebral artery contributes to the blood supply of the cerebellum, the part of the brain that controls motor coordination, balance, posture, and fine movements. Through its branch, the posterior inferior cerebellar artery (PICA), the vertebral artery supplies the inferior part of the cerebellum.
Posterior Inferior Cerebellar Artery (PICA): This artery is one of the vertebral artery’s most important branches and provides blood to the lower parts of the cerebellum. Proper cerebellar function is essential for coordinating voluntary movements, maintaining posture, and ensuring balance, particularly during physical activities.
Blood Supply to the Spinal Cord
The vertebral artery is also responsible for supplying blood to the upper spinal cord, particularly through the anterior spinal artery and the posterior spinal arteries.
- Anterior Spinal Artery: This artery arises from the vertebral arteries and runs along the midline of the spinal cord, supplying the anterior two-thirds of the spinal cord, including the motor pathways. The health of the anterior spinal artery is critical for motor function and reflexes in the limbs.
- Posterior Spinal Arteries: These arteries, which may arise from either the vertebral artery or the PICA, supply the posterior part of the spinal cord. This area includes the sensory pathways, which transmit signals related to touch, temperature, and proprioception.
Through these arteries, the vertebral artery ensures the spinal cord receives the oxygenated blood it needs for proper sensory and motor function.
Blood Supply to the Occipital Lobes
The vertebral artery, after forming the basilar artery, contributes to the blood supply of the occipital lobes through its terminal branches, the posterior cerebral arteries. The occipital lobes are the primary centers for vision, processing visual stimuli and interpreting visual information.
Posterior Cerebral Arteries (PCA): Although the PCA branches from the basilar artery, the vertebral arteries are crucial in the formation of the basilar artery. The PCA supplies the occipital lobes, allowing for normal visual perception and processing.
Without the vertebral artery’s contribution to this vascular network, the occipital lobes would suffer from ischemia, leading to potential visual disturbances or blindness.
Contribution to the Circle of Willis and Collateral Circulation
The vertebral artery forms an important part of the Circle of Willis, a circulatory anastomosis at the base of the brain that ensures adequate blood supply to the brain even in the event of arterial occlusion or blockages. The vertebral arteries converge to form the basilar artery, which, through the posterior cerebral arteries, connects to the internal carotid artery system via the posterior communicating arteries.
Collateral Circulation: This collateral network allows blood to flow between the anterior and posterior circulations of the brain, ensuring that oxygenated blood can be rerouted to different parts of the brain if there is a blockage or narrowing in one of the major arteries. The vertebral artery’s contribution to this system is crucial for protecting the brain from ischemic events, such as strokes, by maintaining blood flow to critical areas.
Support for Autonomic and Cranial Nerve Function
By supplying the brainstem, the vertebral artery indirectly supports several cranial nerves that emerge from this region, particularly those involved in autonomic and reflexive functions.
Cranial Nerves: The vertebral artery’s supply to the brainstem ensures proper function of cranial nerves involved in basic reflexes such as swallowing (via the vagus nerve), pupil constriction (via the oculomotor nerve), and facial sensation (via the trigeminal nerve). This blood supply is essential for maintaining sensory and motor control of the face, neck, and upper body, as well as autonomic functions like breathing and heart rate regulation.
Support for Equilibrium and Balance
Through the labyrinthine artery (a branch of the vertebral artery or the basilar artery), the vertebral artery indirectly supplies the vestibular apparatus of the inner ear, which is responsible for maintaining balance and equilibrium.
Labyrinthine Artery: This artery provides blood to the cochlea and vestibular apparatus, which are essential for hearing and balance. By maintaining the function of these structures, the vertebral artery helps ensure proper sensory input related to balance and spatial orientation.
Clinical Significance
The vertebral artery is clinically significant due to its role in supplying blood to critical structures, including the brainstem, cerebellum, occipital lobes, and spinal cord. Damage or obstruction of the vertebral artery can lead to serious neurological conditions such as vertebrobasilar insufficiency, where reduced blood flow to the posterior brain can cause dizziness, visual disturbances, loss of balance, and fainting. This condition is often associated with atherosclerosis or narrowing of the vertebral arteries.
The vertebral artery is also vulnerable to injury from trauma, particularly in cases of cervical spine fractures or excessive neck movement, which can cause vertebral artery dissection. This can lead to ischemic stroke in the posterior circulation, affecting the brainstem and cerebellum, resulting in symptoms such as vertigo, double vision, and ataxia.
The vertebral artery’s contribution to the Circle of Willis is critical for maintaining collateral circulation, allowing for continued blood flow to the brain even if other major arteries are compromised. This makes it a key vessel in stroke prevention and recovery in the posterior part of the brain.
