Keywords
relay nuclei, association nuclei, anterior limb, posterior limb, sublenticular limb, retrolenticular limb
Chapter Outline
The Diencephalon Includes the Epithalamus, Subthalamus, Hypothalamus, and Thalamus, 103
The Thalamus Is the Gateway to the Cerebral Cortex, 103
The Thalamus Has Anterior, Medial, and Lateral Divisions, Defined by the Internal Medullary Lamina, 103
Patterns of Input and Output Connections Define Functional Categories of Thalamic Nuclei, 104
Small Branches of the Posterior Cerebral Artery Provide Most of the Blood Supply to the Thalamus, 105
Interconnections Between the Cerebral Cortex and Subcortical Structures Travel Through the Internal Capsule, 105
Small Branches of the Middle Cerebral Artery Provide Most of the Blood Supply to the Internal Capsule, 106
The diencephalon is a relatively small, centrally located part of the cerebrum that, like the spinal cord and brainstem, is functionally important way out of proportion relative to its size. It is subdivided into four general regions, each with the term “thalamus” as all or part of its name.
The Diencephalon Includes the Epithalamus, Subthalamus, Hypothalamus, and Thalamus
The epithalamus includes the pineal gland.
The subthalamus includes the subthalamic nucleus.
The epithalamus and subthalamus are located where their names imply—above and below the thalamus, respectively. The major constituent of the epithalamus is the pineal gland , an endocrine gland near the posterior commissure and the midbrain-diencephalon junction. It secretes melatonin, a hormone involved in the regulation of circadian rhythms and seasonal cycles. The major constituent of the subthalamus is the subthalamic nucleus , an important part of the basal ganglia (see Chapter 19 ).
The Thalamus Is the Gateway to the Cerebral Cortex
Some collections of chemically coded fibers, such as serotonergic fibers from the raphe nuclei and noradrenergic fibers from the locus ceruleus, reach the cerebral cortex directly. However, the vast majority of the afferents to the cerebral cortex arise either in the cortex itself or in the thalamus. Thalamocortical afferents include fibers representing all the specific sensory, motor, and limbic pathways (with the exception of previously discussed pathways of olfaction). In contrast, efferents from the cerebral cortex to sites like the spinal cord, brainstem, and basal ganglia reach their targets directly. (Although there are also many cortical projections back to the thalamus, these do not form a link in any descending pathway.) This large collection of thalamocortical afferents and cortical efferents travels through the internal capsule ( Fig. 16.1 ).
The Thalamus Has Anterior, Medial, and Lateral Divisions, Defined by the Internal Medullary Lamina
A thin sheet of myelinated fibers, the internal medullary lamina , subdivides the thalamus into nuclear groups. The internal medullary lamina bifurcates anteriorly and so defines anterior , medial , and lateral nuclear groups ( Fig. 16.2 ).
The anterior and medial subdivisions have only one major nucleus each (the anterior and dorsomedial nuclei, respectively). The lateral division, in contrast, contains an array of four major nuclei or nuclear groups. From anterior to posterior these are the ventral anterior nucleus ( VA ), the ventral lateral nucleus ( VL ), the ventral posterolateral and ventral posteromedial nuclei ( VPL and VPM , respectively), and the pulvinar ( Fig. 16.3 ). In addition, the lateral and medial geniculate nuclei ( LGN and MGN , respectively) form two bumps posteriorly and inferiorly on the main bulk of the thalamus.
Intralaminar Nuclei Are Embedded in the Internal Medullary Lamina.
Clumps of cells embedded in the internal medullary lamina collectively constitute the intralaminar nuclei , which have a distinctive pattern of connections. Functionally similar collections of neurons on the ventricular surface of the thalamus are called the midline nuclei .
The Thalamic Reticular Nucleus Partially Surrounds the Thalamus.
A thin shell of neurons, called the reticular nucleus , covers the lateral and anterior surfaces of the thalamus. The reticular nucleus is spoken of as part of the thalamus because of its location, but in terms of development and connections it is actually a separate structure. (It shares the name “reticular” with the reticular formation of the brainstem because of its reticulated appearance, but these two are actually distinct entities.)
Patterns of Input and Output Connections Define Functional Categories of Thalamic Nuclei
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