Thalamus

2021-02-12 12:00 AM

The thalamus, or the dorsal and ventral thalamus collectively, are two oval structures made up of grey matter at the base of the cerebrum. This structure’s primary function is as a relay center through which sensory nerves transmit signals from the spinal cord and brainstem on the way to the cerebral cortex.

Sensory nerves receive information from a stimulus, and pass that information onto the thalamus, and then onto the cerebral cortex where the information is further processed to determine the location, type, and duration of the sensation. The thalamus also plays a significant role in sensory perception and movement.

Certain areas of the thalamus are dedicated to specific parts of the body and where the sensations are meant to travel toward the cerebral cortex. Still, the thalamus isn’t limited to just the cerebral cortex – it also directly communicates to many other regions of the brain and is believed to be integral to cerebral, cerebellar, and brainstem functions.

Anatomy of the Thalamus

The thalamus has two ends, the anterior and posterior poles, and four surfaces: medial, lateral, superior, and inferior. Nuclei in a given pole or surface regulate specific functions or processing of sensory information and maintain particular connections with parts of the nervous and limbic system.

Understanding the anatomy of the thalamus will help you in comprehending the specific regulatory mechanisms of this structure.

Medial Surface

The medial surface of the thalamus comprises the upper portion of the lateral wall of the third ventricle of the brain and is lined by ependyma (remember that ependyma is the layer of ependymal cells that create cerebrospinal fluid, CSF). The medial surface serves to connect the two thalami by an interthalamic adhesion.

On its inferior (bottom) portion, it is connected to the hypothalamus by a hypothalamic sulcus, which extends from the upper part of the cerebral aqueduct (another cerebral ventricle) to the interventricular foramen (tract through which CSF flows).

A bundle of fibers called the stria medullaris thalami are located near the junction of medial and superior (upper) surfaces.

Lateral Surface

The lateral surface of the thalamus is covered by a layer of myelinated fibers called the external medullary lamina which separates the lateral surface from the reticular nuclei.

Superior Surface

This surface of the thalamus is coated by white matter (remember white and grey matter: white matter contains nerve fibres, axons, that extend from their individual neurons.

They are covered in myelin sheaths. Grey matter, on the other hand, is composed of the neuronal cell bodies and unmyelinated axons). This white matter is called the stratum zonal. (Note that the stratum zonal is also composed of grey matter, however, the surface is what makes up the white matter.)

The medial (inner, toward the center of the body) region of the superior surface is separated from the fornix by the choroid fissure (an attachment site for the choroid plexus, the structure which contains ependymal cells).

The superior surface of the thalamus also forms part of the floor of the lateral ventricles.

The lateral region of the superior surface of the thalamus contains the stria terminalis, a structure that plays a role in the regulation of emotions and behaviors related to stress. Another layer of white matter called the external medullary lamina divides the lateral region of the superior surface from the reticular nucleus.

Inferior Surface

The inferior surface of the thalamus is connected to the anterior portion of the hypothalamus and the posterior portion of the subthalamus. The subthalamus is what separates the thalamus from the tegmentum of the midbrain.

Anterior Pole

The anterior pole of the thalamus constitutes the posterior boundary of the interventricular foramen.

Posterior Pole

Also known as the pulvinar, the posterior pole of the thalamus extends past the third ventricle and over the superior colliculus (a small elevation on each side of the posterior region of the midbrain). Reticular nuclei are located laterally to the primary mass of nuclei here.

Nuclei of the midline are connected to either the ependyma of the lateral walls of the third ventricle or are adjacent to the interthalamic adhesion.

Functions of the Thalamus

The thalamus has numerous connections to many, many parts of the brain and so has several different functions. It is considered to be an organ of the limbic system in addition to its part in the central nervous system (CNS), as it connects specific portions of the cerebral cortex to parts of the brain and spinal cord which control the processing of sensory information and movement.

Specifically, the thalamus works to send neuronal transmissions to the brain for the regulation of the Circadian rhythm in order to suppress the body’s response to sensations such as sound during sleep.

Moving functions

The thalamus plays a part in moving control by providing positive reinforcement of movements initiated in the moving cortex. There are three specific nuclei associated with the thalamic role of moving control:

  • Ventrolateral: coordination and cadence of movement
  • Ventral anterior: planning and initiation of movement
  • Ventrointermedial: coordination of movement

At present, scientists are still a bit unclear on how exactly the thalamus functions in moving control, but it is an area that is being extensively studied. So far, what is for sure is that the connections of the thalamus, specifically to the cerebral cortex, cerebellum, and basal ganglia, in addition to a few other subcortical structures.

One hypothesis is that areas of the thalamus that are connected to the aforementioned nervous system structures are activated by glutamatergic inputs from either the cortex or the cerebellum. Inputs from the basal ganglia are not strong enough to influence the thalamus in terms of moving control.

Another idea is that the thalamus receives inputs that are equally influential from basal ganglia and cerebellar networks, and integrates them based on target and moving program.

Face and Body Sensory Information

The ventral posteromedial nucleus of the thalamus is responsible for receiving sensory information for certain areas of the face.

It also regulates the sensation of taste, receiving neuronal signals from many different parts of the gustatory system. (The gustatory system is the sensory system that focuses on information gathered by the taste buds, or as their officially called, lingual papillae, on the dorsal surface of the tongue. The sensation of taste is governed by three of twelve pairs of cranial nerves.)

The ventral posterolateral nucleus of the thalamus is responsible for receiving and transmitting sensory information from the body.

This sensory information is sent up the spinothalamic tract, which is a nerve pathway that extends up from the spinal cord to the thalamus. Sensory information regarding temperature, pain, itching, and touch is sent up and down this track.

Specifically, the sensations of touch and pressure are sent up the anterior, or ventral, spinothalamic tract, and the lateral spinothalamic tract carries information about pain and temperature.

Limbic System

The thalamus, although quite literally central to the CNS (it is located in the very center of the brain on top of the brainstem, haha), is also an integral component of the limbic system, which regulates emotions. Specifically, the thalamus plays a part in more than just emotions, though, and regulates the functionality of “higher” cognitive functions as well.

The anterior, mediodorsal, and Centro median nuclei of the thalamus are the primary parts that play a role in this emotional regulation:

  • Anterior: involved in the storage of memory and emotion.
  • Mediodorsal: responsible for motivation, enthusiasm, and emotions related to inspiration.
  • Centro median: governs the emotional component of pain.

Scientists have observed that, upon reception of pain sensory information, there are consequential changes in the biochemistry, genetic expression, and blood flow of many thalamic neurons. From this, they have continued to hypothesize that the thalamus plays a central role in the modulation of pain.

It is important to note that it is not the thalamus in and of itself that regulates emotional activity, but the neuronal connections that allow the thalamus to carry out its function. Specific nuclei in the thalamus are connected to other limbic system structures including the mammillary bodies and hippocampus.

Miscellaneous Functions of the Thalamus

The thalamus, due to its many neuronal connections to parts of the limbic, endocrine, and nervous system, is also involved in many more bodily functions than emotions and sensory information. It is also a part of what gives us consciousness. Specifically, the thalamo-corticothalamic circuit is integral to arousal, the physiology of being awake, alertness, and activity.

In fact, attention and focus disorders such as obsessive-compulsive disorder are specifically attributed to damage or physiological malfunctions of this circuit due to its role in regulating task-dependent activities during a state of rest. Additionally, it is central to the process of impulse inhibition. So, damage to this pathway manifests in attention deficit disorder as well.

It is also hypothesized that the thalamus is not only limited to information gathered during consciousness but that it has access to the regulation and storage of information gathered during unconsciousness as well. Interpretation of this information, however, is limited to a conscious state. You can almost think of this as your awareness of your dreams during deep sleep.

Some have said that our dreams are made up of an amalgamation of information that our brain has gathered subconsciously over time – so think of the dream you had most recently. During the dream, you may have felt that certain aspects of it were familiar. It may have been very difficult to recall this information after you woke up, but there may have been details here and there that pop up in your mind that make you say – Oh! Now, I remember!

Your thalamus has stored information outside of your consciousness, and, despite your thalamus remembering this information, you’re not quite able to recall it on your own or when you’re awake. (Note, this is just an example. I am not currently aware of any scientific evidence of the information recalled during the dream state.)

Secondly, the thalamus is responsible for filtering information traveling throughout the nervous system. Remember the numerous connections the thalamus has to structures in the nervous and limbic systems? Well, because of the countless networks the thalamus has, it has actually been regarded as the most “wired” part of the brain by many researchers around the world.

The thalamus allows for the filtering out of information that is rendered “insignificant,” enabling the individual to focus their attention on a specific target or task of interest. This is done by allowing information that is directly related to that stimulus to pass through to the higher cortical areas and brainstem.

This is why you’re able to sleep through the sounds of rain and maybe even a particular TV show but wake to the sound of your alarm (hopefully).

Lastly, the thalamus isn’t restricted to just the sensations of touch, itch, taste, and pain. Its capabilities extend past these few – with the exception of olfactory sensations. However, with the other sensations and general awareness, the thalamus is unable to specify the location or intensity of the sensations outside of those primaries few.

It is said that because the thalamus is so deeply connected to consciousness and general awareness of the human nervous system, damage or poor functioning of the thalamus can lead not only to attention disorders but can even put someone into a coma, or worse, render them brain dead.

The thalamus is extremely important to the regulation of the human nervous system. It is the center of information processing, and is what maintains consciousness, organizes subconscious information, and regulates the very survival of the human being. There is still much to be learned about this structure and it poses quite the challenge due to its countless neuronal connections to structures within the central nervous system, limbic system, and more.