The thalamus is one of the most complicated structures in the brain and is involved in nearly all of its activities. The body's cycles, senses, thoughts and emotions are just a small sampling of it's involvement. It's a tough topic for experts, never mind laypeople. Although it hardly meets the level of a even a brief summary of the subject, the complexity of the topic makes this page a little longer. Fortunately, it's also a fascinating structure to study. The focus of this page is about the role of atrophy (degeneration) of thalamic nuclei and their role in neurodegenerative conditions. Getting the big picture is more important than the details.click here for Key Roles of Thalamic Structures
THE DIENCEPHALON (location of the thalamus)
The red arrow in the brain scan above points to the location of the thalamus. The thalamus and hypothalamus make up the bulk of the diencephalon of the brain, which also includes the subthalamus. While it is is technically part of the diencephalon, the subthalamus is functionally more related to the basal ganglia which is located in the lobes of the brain. The lobes of the brain drape over the diencephalon located in the core, as depicted in the picture above.
The floor of the cranial vault is separated into three compartments called the anterior (front), middle and posterior (rear) fossas located on three different levels. The middle is located behind the eyes. The middle cranial fossa is complex and associated with many problems due to design that will discussed briefly below and more so in future pages. Part of the problem has to do with the location of the hypothalamus and third ventricle above the middle fossa. Downward displacement and pressure from these structures can cause problems in structures that lie beneath them, such as the pituitary gland and optic nerve.
The thalamus has been classically described as a group of structures called nuclei that will be described below. Rather than nuclei, most anatomists and neurosurgeons prefer to describe it according to zones or location of the nuclei. It is a very complicated structure and topic. A brief description of the key roles of some of the different structures and zones of the thalamus are simplified and summarized below.
KEY ROLES OF THALAMIC STRUCTURES
One of the primary purposes of the different structures of the thalamus is to relay sensory and motor (muscles and movement) information that it receives from the brainstem, cerebellum and spinal cord located below it, to the lobes of the brain that surround it. These thalamic structures also serve as decoders, or readers, similar to those used in computers, of the information they receive and process. The thalamic nuclei also receive, process, sort and send information back and forth between the different lobes of the brain. In addition to sorting and forwarding information, one of the primary jobs of the thalamic nuclei is to reduce the amount of irrelevant and redundant information that is sent to the cortex of the lobes of the brain for them to process. This prevents information overload and allows the cortex to focus on the task at hand.
Although little is known about the specific role the thalamic structures play in cognition, it is well known that they are essential to attention and memory. They are, likewise, important to consciousness and alertness. Lesions to the left or right side of the thalamus can result in attention neglect type deficits. Attention neglect deficits occur when the brain fails to pay attention to signals that are being sent to it from one side of the body, such as the left visual field for example, despite having intact eyes and optic nerves to pick up the signals. The problem is due to failure of thalamic nuclei to process and forward the signals.
In the picture above,the thalamus is the area inside the blue box indicated by the number two. The empty space in the middle of the blue box is the third ventricle. The two empty spaces above the blue box and thalamic nuclei are the left and right lateral ventricles. The ventricles are chambers inside the brain where cerebrospinal fluid (CSF) is produced. As can be seen in the picture, thalamic strutures surround and form most of the roof, walls and floor of the third ventricle, as well as the floor of the lateral ventricles.
The location of the thalamic nuclei around the third ventricle makes them susceptible to structural strains and deformation due to faulty fluid mechanics in the brain. Enlargement of the third ventricle is associated with tension, stretching and compression of surrounding structures and circulatory pathways. Thus, researchers suspect that, mechanical strains and deformation of the thalamus due to faulty fluid mechanics in the third ventricle may play a role in many signs and symptoms associated with neurodegenerative diseases such as blurriness of vision, diploplia, loss of memory, incontinence, fatigue, cog fog, heat intolerance, sleep disturbances, sensory loss, and muscle weakness.
This page is mostly about the impact the enlargement of the third ventricle has on thalmic nuclei and some related topics. Future pages will cover the impact of enlargement of the third ventricle and deformation of the thalamic nuclei on nearby structures such as the internal capsule, the sella turcica, the pituitary gland, the cavernous sinus, the optic chiasma and the optic nerve. The frequent involvement of surrounding thalamic nuclei and other nearby structures has to do with their locations in and around the third ventricle (periventricular) beneath the core of the brain.
DEEP GREY MATTER
The nuclei of the thalamus are part of what is called the deep grey matter of the brain because of what they are and their location in the core of the brain. Grey matter is comprised of unmyelinated nerve areas. The grey matter areas of the brain are for command, control, processing and memory. While atrophy of other deep grey matter areas, such as the hippocampus of the temporal lobe of the brain, have been studied in Alzheimer's and other neurodegenerative diseases, the thalamic structures haven't received as much attention. More recent studies, however, have shown that multiple sclerosis (MS) is associated with atrophy of the deep grey matter.
While MS has classically been associated with demyelination and white matter lesions, grey matter lesions are now being recognized as important components of the pathology of multiple sclerosis. In fact, demyelinating lesions are frequently being found in the deep grey matter in multiple scelerosis. Of the deep grey matter structures, the lesions are most often found in the thalamus and caudate nucleus. The caudate nucleus is part of the basal ganglia of deep grey matter that are involved in movement disorders such as found in Parkinson's. Deep grey matter lesions have also been found in the putamen and globus pallidus of the basal ganglia, the substantia nigra, the claustrum, the amygdala (limbic brain), hypothalamus, and substantia nigra. The basal ganglia and substantia nigra are involved in movement disorders such as Parkinson's. Most deep grey matter lesions in MS involve both the grey and white matter.
There are different theories as to the cause of the degeneraion of nerves in the deep grey matter. One theory is that the lesions and neuronal loss may be due to acute damage from inflammation. Another theory is that it may be caused by chronic decreases in trophic factors that support cell health such as those supplied, moved and removed by blood and CSF flow. A decrease in trophic factors can result in chronic slow degeneration of nuclei.
Some researchers further suspect that atrophy of thalmaic nuclei may be due to excess glutamate and subsequent excitotoxicity and nerve cell death caused by alterations in the uptake of glutamate. Interestingly, excitotoxicty is related to strokes and the ischemic or glutamate cascade that follows a loss of blood flow to the brain. In this regard, another possible cause of the excess glutamate, excitotoxity and subsequent atrophy in MS, may be due to sluggish blood and CSF flow and subsequent chronic ischemia. In any case, demyelination and neurodegenerative changes are common in deep grey matter in multiple sclerosis and may contribute to clinical impairment.
In addition to multiple sclerosis, degeneration of thalamic structures has been associated with a decrease in congnition in Huntington's disease and Lewy-body dementia, as well as other neurodegenerative diseases. Interestingly, Lewy-body dementia is a variant type of neurodegenerative condtion more like a hybrid that is a cross between Alzhiemer's and Parkinson's disease. As researchers continue to uncover more, it seems different neurodegenerative conditions share many features in common.
MYELINATED NERVES (white matter)
The paler whitish looking sections of the brain in the above picture are myelinated nerves. Myelin are special insulating cells that wrap themselves around nerves. The purpose of myelin is to speed up transmission of signals between the brain and spinal cord, as well as different parts of the brain. The area marked by the number one is called the crus cerebri. The crus cerebri carries signals that are sent out or received by the outer surface of the brain called the cortex.
The pathways of the crus cerebri come together and make up the outer boundary of the thalamic nuclei to form what is called the internal capsule. The internal capsule is a major communication pathway. Some researchers suspect that displacement and stretching of the internal capsule may play a role in sensory loss, muscle weakness and muscle function. I will cover the internal capsule as a separate topic. The point to be made here is that when the thalamic nuclei bulge outward due to enlargement of the third ventricle the internal capsule is similarly stretched, strained and displaced.
THE THALAMIC NUCLEI, ZONES AND ROLES
The thalamic nuclei are sometimes broken down into four basic regions including: ventral thalamic, dorsal thalamic, epithalamic and hypothalamic areas. The ventral, anterior or frontal area (the nose side) plays an important role in memory, learning and motor (muscle) control. This area is also crucial to alertness and learning. The frontal area nuclei are part of the limbic system, which I will dicuss briefly below.
The epithalamic section is in the top rear wall and contains the pineal gland, as well as the habenular nucleus and commissure. The habenular nucleus and commisure are anatomically and physiologically closely related to the pineal gland. Commissures are simply communication links between left and right side structures in the brain for the purpose of integration, overlap and redundency. The pineal gland is important to diurnal (daily) rhythms and sleep which are often disturbed in neurodegenerative diseases. Some of the dorsal thalamic nuclei will be covered below. The hypothalamic area of the diencephalon forms the lower front floor of the third ventricle and is covered elsewhere on this site. The cross over point of the optic nerve, called the optic chiasma (cross) lies next to the hypothalamic area and forms part of the floor of the third ventricle. Anatomists and researchers also refer to the nuclei in terms of their anatomical position such as in the anterior, posterior, medial and lateral, as well as internal areas such as the intralaminar nuclei.
The sketch below shows the location of some of the thalamic nuclei on the surface as seen from an angle looking down and toward the outside wall of the right half. The intrathalamic adhesion seen on the top side of the diagram links the left and right thalamic structures. It is, likewise, a communication link between the left and right halves similar to the commisures. The different thalamic nuclei are, for the most part, separated by white matter made of nerves covered with insulation called myelin mentioned above. Of all the sensory relay systems, the olfactory nerve, for the sense of smell, is the only one that bypasses the thalamic nuclei and connects directly to the cerebral cortex for interpretation.
Using the legend above, sensory signals from the body associated with touch, stretch, pressure, and movement of the joints, muscles and skin for regulation and control of upper posture and walking are carried by the spinothalamic tract in the spinal cord to the VPL. Technically, body awareness and position sense is called proprioception. Proprioception is highly sensitive and specialized. It's automatically controlled but keeps the cortex informed about activities and provides feedback during execution of motor skills for fine tuning.
The VPL also receives signals from the medial lemniscus. The medial lemniscus is another bundle of white transmission nerves that travel together similar to a raceway for electric wires used in commercial or concrete buildings. The medial lemniscus contains sensations for touch and pressure from mechanical receptors such as those used for proprioception to regulate balance, upright posture and gait. Some of the information comes from the lower body and some of it comes from the head and neck of the upper body. The lateral lemniscus is a similar pathway dedicated to the cochlear receptors in the inner ear used for position sense and angles or direction of acceleration of movement that are related to proprioception.
The VPM area similarly receives body awareness and position type signals from the trigeminal nerve of the face that travels through what is called the trigeminothalamic tract. The trigeminal nerve is the major sensory nerve of the face. It is also the motor nerve for the jaw that provides the power to run the muscles for chewing and speaking. Sensory signals for taste are also transmitted by the trigeminal nerve, cranial nerve number five, to the VPM nuclei, as are somatosensory inputs from the trigeminal system such as the jaw.
Pain signals from the lower body travel up the spinothalamic tract in the spinal cord to the ventral trigeminothalamic tracts in the brainstem that send their signals to the posterior (rear) thalamic nuclei and the intralaminar nuclei. Some pain signals are also sent to the VPL and VPM areas mentioned above. The functional organization of the posterior thalamic complex of nuclei is much more intricate and also integrates other somatosensory and auditory signals. Sensory signals for hot and cold also travel up the spinothalamic tract in the spinal cord to the VPL nuclei.
The two red areas in the rear area of the picture above, are called the lateral and medial geniculate bodies and are related to the eyes and ears. Sensory signals for sound from the ear are sent to the medial geniculate nucleus. Sensory signals for sight from the retina of the optic nerve of the eye are first sent to the lateral geniculate nucleus.
The retinal inputs to the lateral geniculate nucleus on the left side of the brain represent the right hemi-field of vision in the left and right eye, and the left hemi-field of vision for the left and right eye sends their signals to the right lateral geniculate nuclei. The shared fields creates binocular vision for three dimensions.
The visual signals are then sent from the lateral geniculate body to the inner portion of the occipital lobe in the back of the brain. At the same time the lateral geniculate body also sends some signals to other areas of the brain that are related to movement, balance and posture among other things. The ears and medial geniculate body work similarly.
It is important for predators and prey to have their visual and auditory signals connected to their head, neck and other body parts for fight or flight preparation and movement. If a hawk or an owl sees or hears something, such as a mouse's movement, it immediately turns its head toward the source of the sensory stimulation, localizes it and creates coordinates in the cortex of the brain in an instant. It then focuses its full attention and flight on the target guided by automatic piloting systems coordinated by thalamic nuclei. If the mouse is lucky and sees the bird first, it sometimes freezes its body muscles in fear, as well as for protection to prevent further detection by motion and or sound. If not, it quickly turns and runs for cover.
The pulvinar section in the posterior (rear) area primarily receives input from the visual cortex in the occipital lobe and from the superior colliculus mentioned above, which coordinates vision with head and neck movement. The pulvinar appears to integrate sight and sound with body awareness and movement probably the same as predator or prey responses mentioned above. Integration of higher sensory signals, memories and body movements are also important for fleeing from danger, such as falling trees and rocks, or to move toward rewards, such as the sparkling sights or gurgling sounds of fresh moving water to quench thirst. The pulvinar is thought to play a role in discriminating and interpreting these types of complex sensory signals, as well as cognition so that it recognizes and sends the information to the appropriate centers for further processing and decision making.
There are also thalamic nuclei that are related to the reticular system, which is too lengthy a topic to cover here. Basically the reticular activating system is the pacemaker of the brain. Among other things, it is critical to wakefullnes and alertness (focus). Damage to certain sttructures of the thalamus can cause coma. The picture below shows color coded areas of some but not all of the different thalamic nuclei and the lobes and areas of the brain they relay their information to. It's far too complicated to go into in much detail here.
What the pictures don't show is the synergy of the thalamic nuclei. For the most part, sensory signals rarely work in isolation. Instead, they work in concert. While the higher commands and programs are composed, stored and sent down from the cortex of the lobes above, the thalamus is the conducter or central processing unit of the central nervous system and the nuclei are the microchips.
Lastly, in addition to the thalamic and hypothalamic structures, the diencephalon contains all but one of the circumventricular organs of the brain. The circumventricular organs are stimulated by chemcials in the blood stream and brain. They were so named because of their location on or close to the ventricles. They are special sensory and secretory organs made of dense beds of blood vessels that have a minimal or no blood-brain barrier. The lack of a blood-brain barrier allows them to readily take in and secrete neurotransmitters, hormones and other chemicals. The circumventricular organs are involved in body fluid regulation, cardiovascular functions, immune responses, thirst, feeding and reproductive behavior. The pineal gland and the posterior pituitary glands are circumventricular organs. The one circumventricular organ that is located elsewhere it called the area postrema located on the rear wall of the of the fourth ventricle. The area post rema is a command center for vomiting. Regurgitation is a reflex that is used to expel toxins. Among other things, increased pressure in the brain can affect the area postrema and cause nausea and vomiting.
THALAMIC STRAIN AND ATROPHY
Atrophy of the thalamus, which is a decrease in its size, has been seen in neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease and its variants (multisystem atrophy, olivopontocerebellar atrophy, progressive supranuclear palsy), Huntington's disease, multiple sclerosis and amyotrophic lateral sclerosis. Atrophy of the thalamus is also seen in schizophrenia. The degree of atrophy is typically determined by the size of the third ventricle. An enlarged third ventricle, called ventriculomegaly is currently attributed to atrophy of the thalamus. On the other hand, faulty fluid mechanics in the third ventricle may be the cause of the atrophy due to tension and compression damage to tissues in the structures surrounding the ventricle
Ventriculomegaly is caused by blockage of CSF flow. CSF flow in the cranial vault is connected to CSF flow in the spinal canal as it passes through the large hole in the base of the skull called the foramen magnum. Flow through the foramen magnum is, in turn, affected by the upper and lower cervical spine. Malformations and misalignments of the upper cervical spine can affect CSF flow in and out of the cranial vault. Degeneration and alteration in the shape and design of the lower spine can affect flow in the spinal canal. In any case, regardless of the location, blockage of CSF flow can lead to enlargement of the ventricles, which can affect surrounding structures.
In addition to problems with CSF flow and ventriculomegaly, the thalamus and hypothalamus get their blood supply from the vertebral arteries. The vertebral arteries travel through the tunnels in the vertebra of the cervical spine, as well as a connective tissue tunnel between the skull and upper cervical spine called the suboccipital cavernous sinus. After leaving the suboccipital cavernous sinus, the vertebral arteries turn upward and pass through the foramen magnum on the left and right sides of the brainstem. They then join to become the basilar artery which lies between the brainstem and clivus portion of the base of the skull. The smaller size and route of the vertebral-basilar arteries make them more susceptible to compression as they pass through tunnels of bone and connective tissues on their way to the brain.
The arteries that supply the thalamic and hypothalamic structures are at the end of the line for the vertebral-basilar artery seen at the top of the picture above. Most are small penetrating arteries which makes them more susceptible to stretching and compression injury compared to larger arteries. They are also located in tight spaces between the cortex of the brain and the third ventricle called periventricular spaces. Their location makes them susceptible to faulty fluid mechanics in the ventricles in the brain that stretch them when the ventricles enlarge and compress them against the cortex of the brain. Researchers suspect that chronic faulty fluid mechanics and pressure waves in the ventricles, as well as ventriculomegaly (enlargement) may cause compression of periventricular arteries and subsequent arophy of affected tissues, such as the thalamus.
The above is just a brief summary of the functions and problems associated with atrophy of the thalamus. As mentioned, problems with CSF volume, pressure and flow in the third ventricle may play a role in many of the signs and symptoms of neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis. The cause of the disturbance can be due to different structural problems that block CSF such as tumors that compress CSF pathways. It can also be caused by faulty valves called arachnoid granulations that connect the CSF pathways to the venous drainage system of the brain. In some cases the problem is due to a narrow design in the canal that links the third to the fourth ventricle below. The narrow canal decreases the drainage capacity out of the third ventricle. Still other cases can be caused by faulty CSF outflow from the fourth ventricle or blockage in the cisterns and other subarachnoid spaces that the fourth ventricle empties into. Blockage of flow in the subarachnoid spaces are sometimes caused by infections and inflammation that cause swelling or scarring of tissues.
In addition to blockage of its pathways inside the cranial vault, CSF flow in the third ventricle can be affected by blood and CSF flow through the foramen magnum, which serves as a vent for excess CSF and blood volume during cardiac cycles, as well as for pressure. CSF flow through the foramen magnum can, in turn, be affected by structural problems outside the cranial vault such as upper cervical malformations and misalignments, as well as malformations and deformation of the lower spine such as spondylosis, scoliosis, kyphosis, and stenosis. Structural malformations, misalignment and deformation in the cervical spine can also affect blood flow through the vertebral arteries and vertebral veins.
Strains of the surrouding structures of the thalamus and internal capsule due to enlargement of the third ventricle caused by faulty CSF flow may play a role in signs and symptoms such as blurriness, loss of vision, diploplia, loss of memory, loss of sensation, behavioral changes, changes in mood, incontinence of the bowel and bladder, disturbed sleep, and muscle weakness.
Return to Brain Anatomy from Thalamus
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