The third ventricle is surrounded by some of the most critical structures in the brain. Because of its location and relationship to surrounding structures, abnormal pressure waves in the third ventricle may play a role in some of the common signs and symptoms seen in neurodegenerative diseases such as: memory disorders, cog fog, sleep disorders, frequent urination, incontinence (loss of bladder and bowel control), heat intolerance and more. One possible cause may be obstruction to cerebrospinal fluid (CSF) outflow that increases pressure on the surrounding structures similar to a water hammer effect. The cause of the obstruction to CSF flow may be due to increased resistance in the CSF pathways and drainage outlets, which empty into the venous dural sinuses of the brain. Consequently, increased resistance in the venous drainage system can effect CSF flow, as well as pressure waves in the brain.
The third ventricle is surrounded by the structures of the diencephalon of the brain. The diencephalon is part of the central processing unit or motherboard of the brain. It connects and coordinates the lobes of the brain on its rostral (top) end, to the brainstem, cerebellum and spinal cord on its caudal (lower) end. The structures of the diencephalon are made of dense areas of grey nerves called nuclei similar to the grey nerves of the outer cortex of the brain. The grey nerve areas are for initiating, receiving and processing information. The white nerves are for rapid transmission of signals. The diencephalon has four brain regions which are: the thalamus, hypothalamus, pineal gland (keeps track of length of daylight and secretes melatonin for sleep) and pituitary gland. The diencephalon connects the lobes of the brain above it called the telencephalon to the midbrain portion of the brainstem that lies below.
The midbrain is generally considered to be the top of the brainstem because the diencephalon is, structurally speaking, tightly knit with the fibers of the attached lobes, making it difficult to separate them during dissection. Some experts, however, include parts of the diencephalon with the brainstem because of their control and close functional connection to the autonomic nervous system of the brainstem and spinal cord. In any case, the third ventricle is located in the middle of some of the most fundamental and critical parts of the brain and brainstem. Atrophy of the surrounding structures such as the thalamus, or obstruction to CSF flow, can cause the this ventricle to enlarge. Furthermore, some researchers suspect that obstruction of CSF flow can lead to atrophy of the thalamus due to compression of surrounding blood vessels resulting in decreased blood flow called ischemia.
The arrow in the image below points to a white sphere shaped structure, which is the thalamus. A fuzzy white semicircular crescent shaped area, called the fornix arches over the left top and side of the third ventricle. The fornix forms the roof of the third ventricle. The fornix starts in the hippocampus toward the rear and floor of the third ventricle. It then arches over and connects to the hypothalamus toward the front and bottom of the third ventricle. The hippocampus plays an important role in memory as does the hypothalamus. The mammillary bodies will be discussed further below.
The walls of the third ventricle are formed by the inner structures of the thalamus. The left and right halves of the thalamus are connected by bands of gray matter called the interthalamic adhesion inside the third ventricle. Their function is unknown. The front wall is formed by the anterior commissure, the lamina terminalis, and the optic chiasma. The anterior commissure contains myelinated (white) nerves that connect the left and right thalamus. The optic chiasma is where the optic nerves for the left and right eye come together, as well as cross over to opposite sides before going into the thalamus and occipital lobe in the back of the brain where vision is interpreted. The posterior wall is formed by the pineal gland and habenular commissure. The habenular commissure is also a myelinated structure that connects the right and left thalamus.
The floor of the third ventricle is formed by the mammillary bodies, tuber cinereum, hypothalamus, subthalamus, infundibulum, posterior perforated substance and the upper part of the midbrain called the tectum, which means roof. The mammillary bodies are closely related to the hypothalamus and memories, especially memories related to smells. They are also connected to the hippocampus by the fornix mentioned above, which are also important to memory. As far as the mamillary bodies are concerned, remembering smells is important to all animals that forage for food. The tuber cinereum is a prominence of the base of the hypothalamus extending ventrally (forward) into the infundibulum or pituitary stalk to the pituitary gland below. The posterior perforated substance is the area of the floor of the third ventricle that is penetrated by many blood vessels.
The subthalamus portion of the floor of the third ventricle is located beneath the thalamus toward the rear and flanks of the hypothalamus. The subthalamus is closely related to the hypothalamus. It also contains nerves that connect it to and allow for communication with the caudate nucleus and putamen of the basal ganglia. The basal ganglia are gray matter nerve control centers located deep inside the white matter in the lobes of the brain. The subthalamus also connects and communicates with the substantia nigra located in the midbrain at the top of the brainstem. The substantia nigra produces an important neurotransmitter called dopamine. The basal ganglia, the substantia nigra and dopamine are involved in Parkinson's disease and other movement disorders.
The narrow roof over the third ventricle is formed by the fornix, mentioned previously. The fornix is lined by the telea choroidea. The telea choroidea is a thin dense network of capillary blood vessels that are supplied by the medial posterior choroidal arteries. The posterior choroidal arteries are branches of the posterior cerebral artery. The posterior cerebral arteries are the last branches of the vertebral-basilar artery. The vertebral arteries pass through tunnels in the cervical spine and foramen magnum to get to the brain. The telea choroidea attach to the top middle portion of the thalamus called the tenia thalamia because of its sharp tenticle-like edges.
The telea choroidea provides the blood to produce the CSF. CSF is produced from blood that has been pushed or pulled through an extra fine filter in the wall of the ventricles called the blood-brain and CSF barrier. What remains after filtering through the blood-brain barrier is mostly water with sugar and a few other elements mixed in. The pushing force comes from circulatory pressures generated by the heart and the subsequent pulsations it causes in the brain. The pulling pressure comes from salt secreted into the ventricles by skin-like cells called ependyma that cover the telea choroidea. Pulling pressure also comes from the increased pressure gradient caused by upright posture.
It is well documented now, that cognitive impairment in multiple sclerosis is associated with a decrease in the size of the brain due to atrophy. Earlier studies, however, focused mostly on the overall loss of the size of the brain. More recent studies focused more on the size of the ventricles and atrophy of the central parts of the brain that surround the ventricles called the periventricular areas. One study suggested that enlargement of the third ventricle was more frequently associated with neurologically based psychological problems.
The enlarged size of the ventricles is attributed to atrophy (shrinking) of adjacent gray matter structures, such as the thalamus, which is part of the grey matter (nerves) of the brain mentioned above. This is interesting because typically, multiple sclerosis is thought of as a neurodegenerative condition that affects the white matter of the brain. White matter (nerves) contain an insulating, non-conducting coat called myelin. Myelin coated nerves are for rapid transmission of nerve signals. In earlier studies, enlarged lateral ventricles were mostly associated with Alzheimer’s disease, dementia, and a condition called normal pressure hydrocephalus (NPH). Although much less frequently, enlarged ventricles were also seen in some cases of Parkinson’s disease as well. They have now been associated with multiple sclerosis.
Enlarged lateral ventricles seen in Alzheimer’s disease are attributed primarily to ischemic atrophy due to decreased arterial blood flow. Certain cases however, are due to NPH. The mechanism by which normal CSF pressure can enlarge the ventricles, however, remains a mystery to engineer’s and researchers alike. Improvement in MRI technology and computer modeling are getting close to solving the mystery. Theories, thus far, suggest that the ventricles enlarge due to damage done to the tissues that surround them that are exposed to compression and shear stresses. Shear stresses stretch the tissues and cause tension strains. Excess tension can cause tissues to tear. The enlarged ventricles can also cause compression of surrounding CSF and vascular spaces. This damage is the result of decreased blood and CSF circulation.
Cerebrospinal fluid flows into the third ventricle through openings in its roof called the foramen of Monroe. The two foramen of Monroe are connected to the left and right lateral ventricles above. The third ventricle is connected to the fourth ventricle below by the cerebral aqueduct (canal) of Sylvius, which passes through the midbrain. The fourth ventricle is located in front of the cerebellum (hind brain) and behind the pons of the brainstem. The pons looks like a big bulge on the front side of the brainstem. It serves as a connecting structure that links the midbrain above to the medulla or the lower brainstem below. It also connects the brainstem to the cerebellum behind it. CSF leaves the third ventricle and flows through the cerebral aqueduct of Sylvius and into the fourth ventricle. CSF flows out of the fourth ventricle through four openings and into large wells in the subarachnoid space called the prepontine cistern and cisterna magna. The subarchnoid space is part of the protective membranes that surround the brain called meninges. The CSF in the cisterna magna and prepontine and cisterns surround the brainstem and cerebellum. (See picture at top of page for cisterns.)
Obstructed CSF Flow
Because of its location between the two lateral and the fourth ventricle, the third ventricle takes the brunt of problems caused by obstruction to CSF flow. Obstructive non-communicating hydrocephalus occurs when the cerebral aqueduct of Sylvius is blocked, such as from a tumor, or in cases where it is congenitally absent. In contrast to obstructive non-communicating hydrocephalus, non-obstructive “communicating” hydrocephalus is associated with an open canal. One cause of communicating hydrocephalus is due to a narrow undersized cerebral aqueduct of Sylvius. An undersize cerebral aqueduct decreases the drainage capacity of the third ventricle.
Communicating hydrocephalus can be caused by inadequate absorption and delivery of CSF into the venous drainage system of the brain. It can also be caused by problems from further below, such as tumors in or around the fourth ventricle, narrowing or blockage of the fourth ventricle's outlets, or problems in the cisterns the fourth ventricle empties into. As I discuss elsewhere on this site, problems in the cisterns can be caused by obstruction of CSF flow through the foramen magnum and spinal canal. The management of hydrocephalus requires diversion of CSF by either an extra-cranial (internal to external) shunt from the brain to the body, or by an intra-cranial (internal) shunt that bypasses the obstruction, such as an endoscopic third ventriculostomy (ETV). I will cover ETV as a separate topic.
In any case, obstruction to CSF flow, as well as potential turbulence and inversion flows caused by the obstruction, may also play a role in causing the third ventricle to enlarge due to shear forces, compressive forces, ischemia or a combination of stresses. Consequently, the third ventricle may play a role in many of the common signs and symptoms seen in neurodegenerative diseases due to atrophy of surrounding tissues such as the thalamus and possibly pressure from CSF inversion flows. Due to its important role in neurodegenerative diseases the thalamus will be described on a separate page.
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DR. MICHAEL FLANAGAN'S BOOK: THE DOWNSIDE OF UPRIGHT POSTURE - THE ANATOMICAL CAUSES OF ALZHEIMERS, PARKINSONS AND MULTIPLE SCLEROSIS
A MUST READ TO GET THE BIG PICTURE ABOUT NEURODEGENERATIVE DISEASES OF THE BRAINThis book analyzes the causes of how and why some people are afflicted with neurodegenerative disease of the brain. To see more about the book click here for information and/or to make a purchase.