Every major city has a central hub through which its citizens can access public transportation to get to their desired destination. In the case of the human body, the nervous system can be divided into the major road ways (nerves) that carry individuals (impulses) to and from the big city (the brain). The thalamus is ideally situated at the core of the diencephalon, deep to the cerebral cortices and conveniently acts as the central hub. The thalamus relays and integrates a myriad of motor and sensory impulses between the higher centres of the brain and the peripheries. The gross structure, anatomical relationships, nuclear composition and some neuronal tracts that terminate at the thalamus and its blood supply will be discussed in this article.
The thalamus is made up of two symmetrical structures formed from the diencephalon. Each half of the thalamus is elongated along the anteroposterior axis giving it an ovoid appearance. It is narrowest at the anterior end and widest at the posterior part. The thalami are made up of grey matter that is partitioned by a “Y” shaped white matter structure known as the internal medullary lamina. As a result of the location of the internal medullary lamina, each thalamus is divided into roughly three main parts: the anterior, medial and lateral thalamus. The anterior part lies between the short limbs of the internal medullary lamina, while the medial and lateral parts lie on the respective side of the main stem of the “Y”. The left thalamus communicates with the right thalamus by way of the interthalamic adhesion.
The overall appearance of the thalamus may seem unremarkable with the exception of two protuberances on the posteroventral surface. These are the medial and lateral geniculate bodies, which are responsible for the processing of auditory and visual sensory inputs, respectively. Furthermore, the thalami are each surrounded two layers of white matter. Dorsally, it is covered by a layer known as the stratum zonale; while laterally, it is covered by the external medullary lamina, which separates the lateral and ventral thalamus from the thalamic reticular nucleus and the subthalamus.
The thalamus lies at the core of the diencephalon. As a result, it is surrounded by several very important structures. Its anterior pole forms the posterior wall of the interventricular foramen of Monro, which permits communication between the lateral and the third ventricles. Additionally, there are five veins that coalesce to form the internal cerebral vein at the anterior end of the thalamus. These vessels can be remembered as STACS (superior striate, thalamostriate, anterior terminal, choroidal and septal veins). Furthermore, the medial wall of each thalamus forms the lateral walls of the aforementioned third ventricle. The dorsal surface is in close relation to the stria terminalis, the choroid plexus of the third ventricle and the body of the fornix. The internal cerebral vein courses along the dorsomedial length of the thalamus, while the superior thalamostriate vein runs along the dorsolateral surface. The posterior most aspect of the thalamus is known as the pulvinar. Each pulvinar is lateral to the pineal gland, the Habenular and posterior commissures, posterolateral to the corpora quadrigemini (superior and inferior colliculi), and superior to the medial and lateral geniculate bodies. Additionally, the posterior thalamus is deep to the splenium of the corpus callosum.
Superficial to the stratum zonale of the thalamus is the caudate nucleus. The head of the caudate nucleus lies anterosuperiorly to the thalamus with the body travelling superior and laterally to the body of the thalamus. Lateral to the external medullary lamina is the reticular nucleus, then the posterior limb of the internal capsule. Numerous neuronal tracts travel through the different limbs of the internal capsule to synapse in the thalamus. Furthermore, the internal capsule also separates the thalamus from the globus pallidus and putamen (collectively known as the lentiform nucleus). Finally, there are two major structures lying inferiorly to the thalamus. Anteroinferiorly is the hypothalamus, caudal to the hypothalamic sulcus. Directly inferior to the thalamus is the cerebral peduncle and the cerebral aqueduct of Sylvius. Also note that the inferior surface of the thalamus is continuous with the tegmentum of the (floor of the midbrain between the cerebral peduncle and the quadrigeminal plate).
Arrangement of Nuclei
In addition to being divided into anterior, medial and lateral parts by the internal medullary lamina, the nuclei of the thalamus are further subdivided into those that are dorsal and those that are ventral. Together there are sixteen nuclei that are found in the thalamus. Most of these cell bodies are located in the larger lateral part of the thalamus.
When considering the lateral group of thalamic nuclei, it helps to imagine the thalamus as a double decker bus. The lateral dorsal nucleus, lateral posterior nucleus and the pulvinar are found on the upper level of the bus (dorsal surface of the thalamus); whereas the ventral anterior, ventral lateral and the subdivisions of the ventral posterior nuclei are found on the lower level of the bus (ventral surface of the thalamus). The details surrounding the connections of the dorsal group of nuclei are uncertain. However, they are believed to communicate with the cingulate gyrus, temporal, parietal and occipital lobes, along with other thalamic nuclei. The ventral anterior and ventral lateral nuclei are believed to be involved in motor cortex activities. They both have pathways leading to the substantia nigra, premotor cortex, reticular formation and the corpus striatum. Additionally, the ventral lateral nucleus also connects to the cerebellum and red nucleus of the roof of the midbrain. The ventral posterior nucleus is further subdivided into the ventral posterior medial (VPM) and ventral posterior lateral (VPL) nuclei. These relay information to the primary somatosensory cortex (Brodmann 3,1,2) by way of the posterior limb of the internal capsule then through the corona radiate (projection fibers). The VPL receives its input from the afferent spinal and medial lemnisci; while the VPM receives its input from the afferent trigeminal lemniscus and the solitariothalamic tract.
In the medial segment of the thalamus, there are three nuclei. The median nucleus is the most medial of the three structures. The medial dorsal nucleus is superior to the medial nucleus, but both are lateral to the median nucleus and medial to the internal medullary lamina. This cluster of nuclei are responsible for integrating special (olfactory), somatic and visceral afferent information with emotions. The largest of the three medial nuclei, the medial dorsal nucleus, has both efferent and afferent connections with the nuclei of the hypothalamus as well as with the prefrontal cortex.
The anterior nucleus, found between the short limbs of the internal medullary lamina, is connected with the mammillothalamic tract (from the mammillary nucleus of the mammillary bodies to the hypothalamus) as well as with the hypothalamus and cingulate gyrus. These structures are integrated with the limbic system as they organize emotion and recent memory.
Sensory cerebral cortical fibers along with some fibers of the reticular formation (neurons of arousal and consciousness) tend to converge on a thin cluster of neurons between the posterior limb of the internal capsule and the external medullary lamina. This group of nerves is known as the reticular nucleus and is thought to aid in the cortical regulation of thalamic activity. There are smaller nuclei found in the internal medullary capsule known as the intralaminar nuclei and the midline nuclei. These nuclei receive inputs from the reticular formation. The former, however, also communicates with the trigeminothalamic and spinothalamic tracts, as well as other nuclei of the thalamus.
Finally, the medial and lateral geniculate bodies are also considered nuclei of the thalamus. Fibers directly from the lateral lemniscus, along with fibers of the ipsilateral (and some from the contralateral) inferior colliculus terminate in the medial geniculate body (MGB) via the inferior brachium. From the MGB, the fibers form the auditory radiation then terminates in the primary auditory cortex (Brodmann 41, 42). The axons of the retinal cell bodies terminate in the six layers of the lateral geniculate body (LGB). It should be noted that these fibers are not purely ipsilateral, but rather a mix of fibers from the ipsilateral temporal field and the contralateral nasal field of the respective eye. The decussation of fibers of the contralateral nasal fields occurs in the optic chiasm. The terminal fibers then end in the occipital lobe’s visual cortex (Brodmann 17, 18, 19).
In addition to the tracts mentioned above, there are several other ascending tracts that pass through the thalamus. These include:
- the ventral spinothalamic, which is responsible for conveying the sensation of light touch and pressure from Merkel’s tactile disks to the VPL
- the lateral spinothalamic, which transmits impulses of pain and temperature changes from A-delta and C fibers to the VPL and intralaminar nuclei
- and the dorsal column-medial lemniscus pathway, which carries proprioception (joint position), tactile discrimination, vibration sense and form recognition from Pacinian corpuscles, Meissner’s plexuses and Golgi tendon organs to the VPL or VPM
The primary blood supply of the thalamus is from the posterior cerebral artery. Contributing branches from the posterior communicating artery also supply the thalamus after passing through the posterior perforated substance. These arteries arise from the vertebrobasilar arterial system, which anastomoses indirectly with the carotid artery by way of the circle of Willis.
Due to the location of the thalamus, any lesion or insult to the organ will impact adjacent structures. For example, a neoplasm in the anterior part of the thalamus can obstruct the interventricular foramen of Monro. A similar neoplasm in the posteromedial thalamus may obstruct the third ventricle and more importantly, the cerebral aqueduct of Sylvius. In both instances, not only would the respective functions of the thalamus be compromised, but the patient could also develop a non-communicating hydrocephalus. However, most insults to the thalamus are ischaemic in nature. The cause of the ischaemia may either be iatrogenic (caused during a therapeutic procedure) or due to vascular compromise (thrombotic or haemorrhagic). If this damage impedes the VPM or VPL nuclei, all contralateral sensory inputs would be loss.
Vascular accidents of that thalamus can also produce ataxic choreoathetosis (uncoordinated, involuntary movements). Furthermore, the thalamus is intricately involved in relaying pain to the cerebral cortex. This is important to note since surgical cauterization of these fibers can be used to alleviate immense pain in terminal cancer patients. Additionally, a phenomenon known as thalamic pain – where the thalamus overreacts to pain impulses from the contralateral side – has been observed subsequent to a thalamic infarct.