According to Schelling venous back jets into the brain and shear stresses in the cord combined with cerebrospinal fluid reflux are likely causes of demyelination. Other researchers have suggested that chronic edema and normal pressure hydrocephalus can damage myelin simply by stretching it. Zamboni suggests that venous drainage problems lead to iron accumulation, which initiates neurodegenerative processes. There is still another theory that suggests that demyelination may be due to chronic ischemia, that is, decreased blood flow to the brain and cord. For now, I will focus on the blood supply to the brain and ignore the cord.
As depicted in the picture above, blood is delivered to the brain via the anterior and posterior blood supply routes. The anterior blood supply routes are the internal carotid arteries. They are called anterior because they are in the front of the neck. The posterior blood supply routes are the vertebral-basilar arteries in the back of the neck. The carotid arteries travel along either side of the trachea (windpipe) and enter the brain through an opening in the base of the skull called the foramen lacerum where it passes through the cavernous sinus of the drainage system of the brain. (Click on any picture to enlarge it and get a closer view).
As shown in the picture above, the vertebral arteries pass through holes in the transverse processes of the cervical spine. They exit the spine at the first cervical vertebra and enter into a memebrane between the base of the skull and the first cervical vertebra. The membrane is called the suboccipital cavernous sinus or atlantooccipital membrane as seen in the picture on the right.
After traveling through the suboccipital cavernous sinus for a short distance the vertebral arteries turn in and head upward. They next pass through the foramen magnum as shown in the picture on the left. Shortly after passing through the foramen magnum the vertebal arteries unite to become the basilar artery. Before uniting the vertebral artery gives off two branches. One is called the posterior inferior cerebellar artery because it supplies the lower rear aspect of the cerebellum. The other artery is called the anterior spinal artery.
The two anterior spinal arteries then unite and head back down through the foramen magnum to supply the entire length of the anterior aspect of the spinal cord. Another artery called the posterior spinal artery branches off either the posterior inferior cerebellar arteries or the anterior spinal arteries to become the posterior spinal artery. The posterior spinal artery supplies the posterior aspect of the entire length of the cord. The cord also gets its blood supply from what are called radicular arteries. For now we will overlook the possible role of the anterior and posterior spinal arteries in chronic ischemia of the spinal cord and focus on the vertebral-basilar arteries and ischemia of the brain. In brief, chronic ischemia of the cord may play a role in amyotrophic lateral sclerosis and primary lateral sclerosis.
The vertebral-basilar arteries supply most of the the motherboard of the brain, which is the brainstem and cerebellum. Except for the nose and eyes it also supplies all the cranial nerves, which control the muscles and senses of the head and neck down to the shoulders, as well as autonomic function of the internal organs via the vagus cranial nerve. In this regard, the vertebral-basilar arteries also supply the hypothalamus, which is the control center for autonomic vegetative functions in the body, and the thalamus, which is the sensory switchboard or router of the brain.
In additon to the brainstem and cerebellum, the vertebral-basilar arteries supply the inner parts of the temporal, and the inner parts and rear pole of the occipital lobes of the brain. Among other things, the inner temporal lobe is important to memory. The occipital lobe is important for interpreting visual signals which is likewise dependant upon memory.
The areas located at the tail ends of the anterior and posterior blood supply routes are called the watershed areas of the brain. The watershed areas of the brain are the weakest point in the blood supply routes and oftentimes are the ones most affected by decreases in blood flow. Like the brain the cord also has watershed areas which I will leave out of this discussion. One of the watershed areas in the brain is around the periventricular areas where MS lesions are often found. Alzhiemer’s and migraine headaches also have a predilection for periventricular white matter lesions. Many researchers believe that the white matter lesions in MS, migraines and Alzheimer’s disease are caused by chronic ishemia.
While the lesions are typically found higher up in the brain above the covering over the cerebellum, many of the symptoms associated with MS come from cranial nerves supplied by the vertebral-basilar arteries. Symptoms such as dizziness and fatigue are mostly likely signs of ischemia. Likewise, the lesions seen in certain cases of MS, especially those associated with migranous symptoms are most likely caused by chronic ischemia. In addition, the vertebral-basilar arteries are the ones most commonly involved in ministrokes called transient ischemic attacks. The design of the skull, spine and circulatory system of the brain predisposes humans to upper cervical strains and subsequent compression and back pressure against the posterior blood supply routes, which are the vertebral-basilar arteries. The outcome is chronic ischemia in the areas of the brain they supply.
In my next post I will discuss upper cervical strains and deformation of neurovascular tunnels. Upper cervical strains and deformation of critical neurovascular tunnels are most likely one of the major causes of chronic ischemia and edema in the brain. They may also play a role in normal pressure hydrocephalus, but that’s an entirely different story more related to Parkinson’s and Alzheimer’s disease.
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