The vertebral arteries typically branch off of the aortic arch and subclavian arteries in the thoracic cavity. They ascend along the sides of the neck through holes in the lateral aspect of the cervical vertebra (see picture below). The lateral portion of the vertebra are called the transverse processes and the holes are called intertransverse foramina.
When they get to the first cervical vertebra, also known as atlas, located just beneath the skull, they exit the spine and migrate backwards toward the middle of the rear aspect (the posterior arch) of atlas/C1 passing through a soft tissue tunnel, called the atlantooccipital membrane or more appropriately, the suboccipital cavernous sinus. This is shown in the picture below. It is the membrane between the skull and the first vertebra.
When they reach the middle of the posterior arch of atlas they turn inward and upward and pierce the outer membrane of the brain as they pass through the foramen magnum, which is the large hole in the base of the skull through which the brainstem and spinal cord connect. After passing through the foramen magnum they enter the cranial vault within the brain. Shortly after enterning the cranial vault the vertebral arteries unite and become the basilar artery. This is depicted in the lower third of the picture below.
The vertebral-basilar arteries supply about twenty percent of the blood flow to the brain. Eighty percent comes from the carotid arteries. The smaller amount, however, doesn’t diminish their importance. In fact, they supply some of the most important structures of the brain that make up the motherboard of the entire nervous system.
The structures the vertebral-basilar arteries supply include: the upper cervical cord, the brainstem and cerebellum, most of the thalamus and hypothalamus, the cortex and posterior deep white matter of the parietal lobes, the lower and middle temporal and occipital lobes and the rear part of the corpus callosum. The remaining lobes or eighty percent of the brain are supplied by the carotid arteries and are the equivalent of hard drives that attach to the motherboard in computer terms.
It is further important to note that the vertebral arteries also give off two branches before uniting called the anterior spinal arteries seen in the picture above. Additionally, the posterior spinal arteries arise as branches off of the anterior spinal arteries. Sometimes they arise from the posterior inferior cerebellar artery.
The anterior spinal arteries unite inside the vault and decend through the foramen magnum as one artery to supply the front side of the entire cord. The posterior spinal arteries stay divided and likewise descend back down through the foramen magnum along the rear of each side of the cord.
The location of the vertebral-basilar arteries make them susceptible to compression at several points along their course. To begin with, the vertebral arteries pass through the holes or tunnels in the bones of the cervical spine. Next they pass through a soft tissue tunnel in the upper cervical spine. Then they pass through the foramen magnum. Numerous types of mechanical problems in the cervical spine can distort these tunnels.
The vertebral-basilar arteries then travel upward along the belly of the brainstem next to the clivus of the base of the skull. The design of the bent base of the human skull makes the vertebral-basilar arteries susceptible to compression. It depends on the angle of the base of the skull. Different people have different angles.
The anterior and posterior spinal arteries face a double jeopardy situation as their blood flow is affected by the vertebral arteries passing through the upper cervical spine and foramen magnum. If blood flow through the vertebral arteries is decreased as it passes through the upper cervical spine and foramen magnum it will subsequently decrease flow through the anterior and posterior spinal arteries inside the cranial vault. The reduced flow through the anterior and posterior spinal arteries would then be further reduced as they pass back down through the point of constriction in the foramen magnum and upper cervical spine.
Reduced blood flow is called ischemia. The signs and symptoms of ischemia of the vertebral-basilar arteries could involve any of the structures of the brainstem, as well as the thalamus and hypothalamus. It is possible that the bulged shape of the pons could make the pontine branches more susceptible to compression.The pontine branches supply the cranial nerves five through eight. These include the accoustic and trigeminal nerves.
Most notably, chronic ischemia of the vertebral arteries can affect blood flow to the cerebellum. The cerebellum is a little brain and far to complex to cover here. It will be covered separately on this site. For now, suffice it to say that decreased blood flow to the cerebellum can affect many things but especially balance, coordination and gait. It can also affect posture. Conversely, posture and misalignments of the spine can affect the cerebellum by decreasing blood flow through the vertebral arteries.
Other symptoms would include cranial nerve signs such as trigeminal neuralgia, ringing in the ears (tinitus) and loss of hearing. Other symptoms could include autonomic signs related to the hypothalmus which sits on top of the brainstem. The hypothalamus also controls the pituitary gland also known as the master gland. Autonomic symptoms can include incontinence, sweating and sleep problems among other things.
The thalamus is the sensory switchboard, or router in computer terms, for relaying sensory information. But it also plays an important role in relaying information necessary for muscle coordination and function.
In addition to its impact on nerves via blood flow, ischemia is a primary suspect in demyelination. Compression of the vertebral arteries may, therefore, explain the location of hyperintensity signals seen in the brainstem, particularly the cerebellum, as well as in the cord such as in transverse myelitis.