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.