16 The Thalamus and Internal Capsule: Getting to and from the Cerebral Cortex
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 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 major components of the diencephalon, the thalamus and hypothalamus, are active in practically everything we do. The thalamus is the gateway to the cerebral cortex and the principal subject of this chapter. The hypothalamus regulates autonomic functions and drive-related behavior and is discussed further in Chapter 23.
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 (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. 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
