Within the brain, there are a few deep structures that are essential for normal brain functioning. One of these structures is the thalamus, which is located on the midline of the brain and consists of two symmetrical halves. The word thalamus is derived from Greek, meaning “chamber” or “inner room,” as it receives almost all sensory and motor information. Thus, the thalamus is generally believed to act as a relay station between a variety of subcortical areas and the cerebral cortex.
Anatomy and Physiology
There are three main parts of the diencephalon: the hypothalamus, the subthalamus, and the thalamus. All three of these structures work closely together as a nuclear complex in many different bodily functions. The hypothalamus controls temperature, hunger, thirst, fear, anger, and the pituitary gland. The hypothalamus acts with the portion of the reticular system in the midbrain to keep the brain alert and awake. The subthalamus acts like a train depot; it carries impulses from the basal nuclei to the thalamus, and then to the hypothalamus. It is important for transporting and modulating neural impulses used for coordinating movements. Finally, the thalamus is a center that delivers or relays sensory impulses to the surface of the cerebrum, such as impulses from the cerebellum to the cerebral cortex. The thalamus provides input about ongoing movement to the motor areas of the cerebral cortex. It also contains a special part of the reticular system that helps coordinate sensory messages and helps regulate the activity of the brain.
Overall, the thalamus is the main part of the diencephalon and is the regulation of consciousness, sleep, and alertness. It is found at the most superior portion of the brainstem, near the center of the brain. It is highly connected by fiber tracts that bind it to the overlying cerebral cortex. The thalamus is about the size and shape of a walnut and is located obliquely and symmetrical on each side of the third ventricle. There are two thalami, one in each brain hemisphere, and the superior medial surface of each thalamus makes up the lateral walls of the third ventricle. The thalami are also connected to each other on the midline by the interthalamic adhesion.
The thalamus consists of six functionally distinct nuclei, which are the lateral (ventral and dorsal tiers), medial, anterior, intralaminar, midline, and reticular nuclei. The lateral, medial, and anterior groups are named by their location relative to a large collection of axons called the internal medullary lamina. The system of lamellae is made up of myelinated fibers that separate the different subparts of the thalamus. The intralaminar, midline, and reticular groups are considered nonspecific thalamic nuclei.
The lateral nuclei are further divided into two tiers: the ventral and dorsal. The ventral tier of the lateral nucleus is further divided into six nuclei, which are named for their location within the ventral tier. These consist of relay neurons that receive limited sensory and motor input and project this information to specific sensory and motor cortical regions. The dorsal tier of the lateral thalamus and the medial group project to association cortices and thus are both considered association nuclei. The anterior group receives specific neuronal impulses and relays this information to the hypothalamus and the cingulate gyrus of the cerebral cortex. The intralaminar group is made up of a cluster of nuclei that are intermixed within the internal medullary lamina. The reticular group is found on the lateral aspect of the thalamus, and the midline group is located on the dorsal wall of the third ventricle.
Connections and Functions
Because of how integrated the thalamus is in almost all neurological functions, there are several connections and functions that are involved with these structures. An easy way to understand this is first by grouping the thalamic functions and then identifying the responsible connections. The functions are grouped by limbic, motor, somatic sensation, hearing, and vision. For limbic functions, which include emotional expression, the main inputs are from the mammillary body of the hypothalamus, the cingulate gyrus, the amygdala, the hypothalamus, and the reticular formation of the brainstem. The output fibers for limbic function terminate in the cingulate gyrus, the prefrontal cortex, and the basal forebrain.
Motor information is brought to the thalamus by the globus pallidus of the basal nuclei and the dentate nucleus of the cerebellum. The major output for motor function ends in the premotor and the motor cortices. This helps modulate ongoing movement and sends this appropriate information to the motor areas of the cortex. Sensory information of the body is received from the dorsal column and medial lemniscal pathways from the spinal cord as well as the sensory nuclei of cranial nerve V. This sensory information is then relayed to the somatosensory cortex. Research studies have shown that the somatosensory cortex and its association cortices also project back to the thalamus to further integrate sensory information. This information is then projected to the temporal, parietal, and occipital lobes.
Neurons involved with hearing project to the thalamus via the inferior colliculus. The thalamus then modulates these signals and sends them to the primary auditory cortex within the temporal lobe. Vision is transferred to the thalamus via the retinal ganglion axons that make up the optic nerve and the optic tract. The thalamus then modulates these signals and sends them to the primary visual cortex within the occipital lobe. Finally, studies have also shown that the thalamus modulates its own activity. This is done through the reticular nucleus. It receives information from the cerebral cortex and the other thalamic nuclei and projects this information after modulation of the signal back out to the remaining thalamic nuclei.
Patricia A. Bloomquist
See also: Auditory System; Brain Anatomy; Discriminative Touch; Nociception; Olfactory System; Sensory Receptors; Somatosensory Cortex; Somatosensory System; Touch; Visual System
Hebb, Adam O., & George A. Ojemann. (2013). The thalamus and language revisited. Brain and Language, 126(1), 99—108.
Jones, Edward G. (2003). History of neuroscience: The thalamus. In IBRO history of neuroscience. Retrieved from http://ibro.info/wp-content/uploads/2012/12/The-Thalamus.pdf