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The Thalamus and Internal Capsule: Getting to and From the Cerebral Cortex
The diencephalon, mostly hidden from view between the cerebral hemispheres ( Fig. 16.1A ), constitutes only about 2% of the central nervous system (CNS) by weight. Nevertheless, it has widespread and important connections, and the great majority of sensory, motor, and limbic pathways involve a stop in the diencephalon. Most motor and limbic pathways also involve telencephalic structures that are discussed in later chapters, so this chapter provides only a general overview of the connections of diencephalic nuclei. These connections are discussed in more detail in terms of functional systems in subsequent chapters, and a series of sections demonstrating major structures of the diencephalon and the telencephalon is provided in Chapter 25 . Nearly all the connections between the cerebral cortex and subcortical structures, prominently including the diencephalon, travel through the internal capsule, so an overview of this structure is provided here as well.
Three-dimensional reconstructions of the diencephalon. (A) The entire CNS, showing the diencephalon (green) almost completely surrounded by the cerebral hemispheres. (B) The diencephalon, brainstem, and cerebellum, with the cerebral hemispheres removed.
The Diencephalon Includes the Epithalamus, Subthalamus, Hypothalamus, and Thalamus
The diencephalon (see Fig. 16.1B ) is conventionally divided into four parts, each of which includes the term thalamus (from a Greek word meaning “inner chamber”) as part of its name. a
a A few other structures, most notably the globus pallidus, are derived embryologically from the diencephalon but usually are not considered part of it in discussions of the adult brain.
These parts are the (1) epithalamus, which includes the pineal gland and a few nearby neural structures, (2) dorsal thalamus, which is usually referred to simply as the thalamus, (3) subthalamus, and (4) hypothalamus.
The only part of the diencephalon that can be seen on an intact brain is the inferior surface of the hypothalamus (see Figs. 3.16 and 3.17 ), which includes the mammillary bodies and the infundibular stalk. However, the entire medial surface of the diencephalon, much of which forms each wall of the third ventricle, can be seen on a hemisected brain ( Fig. 16.2 ). Superiorly, the diencephalon borders the body of the lateral ventricle; inferiorly, it is exposed to subarachnoid space; laterally, it is bordered by the internal capsule ( Fig. 16.3 ). The caudal boundary of the diencephalon is a plane through the posterior commissure; the rostral boundary is the anterior commissure. These rostral and caudal boundaries are approximate and somewhat arbitrary and are used only for purposes of discussion, as they are functionally continuous with bordering structures.
As a consequence of the cephalic flexure, the axis of the diencephalon is inclined about 80 degrees with respect to the axis of the brainstem (see Fig. 3.1 ). This means that sections cut in a plane similar to that used in the last few chapters (i.e., perpendicular to the long axis of the brainstem) are at a peculiar angle to the diencephalon. Therefore, in this and subsequent chapters, sections cut in axial and coronal planes are shown (see Fig. 16.1 ). b
b The axial sections are oriented with the anterior portion at the top of the picture, because this is the way computed tomography scans and magnetic resonance images are conventionally oriented. One result that can sometimes cause confusion is that anterior parts of the brainstem are situated toward the top of the picture; this is upside down relative to the way the brainstem is pictured in Chapter 11 , Chapter 12 , Chapter 13 , Chapter 14 , Chapter 15 .
The Epithalamus Includes the Pineal Gland and the Habenular Nuclei
The pineal gland is a midline, unpaired structure situated just rostral to the superior colliculi. It resembles a pinecone in shape, which is how it got its name. Because each of us has only one pineal gland, which is located deep within the brain, it was once thought that this organ might be the seat of the soul. This now seems unlikely, because pineal tumors do not cause the changes expected with distortion of the soul; rather, these tumors compress the midbrain and cause the changes expected with distortion of this part of the brainstem. Early findings may include hydrocephalus (because the cerebral aqueduct gets squeezed shut) and various deficits in eye movements and pupillary reactions (because of damage to the oculomotor and trochlear nuclei and pathways ending in them). In addition, pineal tumors may cause changes in sexual development, giving a clue to at least one of its possible functions. The pineal gland arises as an evagination from the roof of the diencephalon; in fish, amphibians, and many reptiles, it contains photoreceptor cells similar to retinal cones. In these species it is suspected of monitoring day length and season and participating in the regulation of circadian and circannual rhythms (although there are probably other functions as well). The pineal gland of birds and mammals contains no photoreceptors and consists of a collection of secretory cells (pinealocytes), some glial cells, and a rich vascular network. Nevertheless, it still receives a light-regulated input by way of a circuitous pathway that begins in the retina and, after one or more relays in the hypothalamus, reaches the intermediolateral cell column of the spinal cord. Preganglionic sympathetic fibers from the spinal cord then synapse on postganglionic neurons of the superior cervical ganglion, which in turn send their axons to the pineal.
The mammalian pineal is an endocrine gland involved in seasonal cycles (e.g., reproductive cycles) and other functions and has no known neural output. Instead it secretes a hormone derived from serotonin, called melatonin, at relatively high rates during darkness. In many species melatonin has an antigonadotropic effect, and light, by way of the neural pathway just described, causes a decrease in melatonin production. As days get longer in the spring, melatonin production declines, which in turn causes an increase in gonadal function. This system is of considerable importance in mammals with prominent seasonal sexual cycles, but its effects in humans are not as clear. It has been reported, however, that nonparenchymal pineal tumors, which presumably destroy pinealocytes, tend to be associated with precocious puberty, as though the production of some antigonadotropic substance had been halted. The converse has been reported as well—that parenchymal pineal tumors tend to be associated with hypogonadism. These tumors are relatively rare, however, and in humans the pineal is probably more important in the regulation of circadian rhythms, including sleep-wake cycles (see Chapter 22 ). The routine clinical importance of the pineal arises from the fact that after the age of about 17 years, calcareous concretions accrue in it. This makes it opaque to x-rays and therefore a useful radiological landmark ( Fig. 16.4 ). Because it normally lies in the midline, slight shifts in pineal position can be indicative of expanding masses of various types.
The pineal gland is attached to the diencephalon by a stalk. Caudally at the base of the stalk is the posterior commissure; rostrally is a small swelling on each side called a habenula (see Figs. 16.2 and 16.12 ). Underlying each habenula are the habenular nuclei. Each habenula receives one major input bundle, the stria medullaris (“white stripe”) of the thalamus, and gives rise to one major output bundle with the awesome name of habenulointerpeduncular tract (or fasciculus retroflexus ). The habenulointerpeduncular tract, as its name implies, extends from the habenula to the interpeduncular nucleus, located between the cerebral peduncles, and to other parts of the midbrain reticular formation. The fibers of the stria medullaris originate in the globus pallidus and some limbic structures, so the pathway through the habenula is one route through which the basal nuclei and limbic system can influence the brainstem reticular formation. The habenula has been shown to regulate the release of the biogenic amines from the brainstem reticular formation and is thought to play a role in assigning “reward value” to stimuli. In other words the habenula nuclei will send messages to the dopamine and serotonin cells of the brainstem, increasing their activity based on how well an individual “enjoyed” a certain stimulus. In addition, studies have shown that the lack of activity of the habenula and its projections to the brainstem reticular formation may play a role in depression.
The Subthalamus Includes the Subthalamic Nucleus and the Zona Incerta
Parts of the midbrain tegmentum continue into the diencephalon as the subthalamus. This area is completely surrounded by neural tissue and is located inferior to the thalamus, lateral to the hypothalamus, and medial to the cerebral peduncle and internal capsule (see Figs. 16.11 and 16.12 ). The subthalamus contains rostral portions of the red nucleus and substantia nigra and is traversed by somatosensory pathways on their way to the thalamus, as well as by several pathways involving the cerebellum and basal nuclei (the latter pathways are discussed in Chapters 19 and 20 ). In addition, the subthalamus contains the subthalamic nucleus and zona incerta (see Fig. 16.11 ). The subthalamic nucleus is a lens-shaped, biconvex structure located just medial and superior to parts of the cerebral peduncle and internal capsule. This nucleus is interconnected with the basal nuclei, as discussed in Chapter 19 . The zona incerta is a small mass of gray matter intervening between the subthalamic nucleus and the thalamus. It appears to be a rostral continuation of the midbrain reticular formation and has very widespread connections (including direct projections to the cerebral cortex), although its function is largely unknown.
The Thalamus Is the Gateway to the Cerebral Cortex
The thalami are a pair of large, egg-shaped, nuclear masses with a posterior appendage ( Figs. 16.1B and 16.5 ); together they make up about 80% of the diencephalon. Each thalamus extends anteriorly to the interventricular foramen, superiorly to the transverse cerebral fissure and the floor of the lateral ventricle, and inferiorly to the hypothalamic sulcus; posteriorly it overlaps the midbrain (see Fig. 16.13 ). The thalamus is part of a remarkably large number of pathways; all sensory pathways (other than olfaction) relay in the thalamus, and many of the anatomical circuits used by the cerebellum, basal nuclei, and limbic structures also involve thalamic relays. These various systems use more or less separate portions of the thalamus, which has therefore been subdivided into a series of nuclei.
Thalamic nuclei can be distinguished from one another by their topographical locations within the thalamus and by the patterns of their inputs and outputs.
The Thalamus Has Anterior, Medial, and Lateral Divisions, Defined by the Internal Medullary Lamina
The topographical organization of the thalamus is shown in Fig. 16.6 and Table 16.1 . A thin, curved sheet of myelinated fibers, the internal medullary lamina, divides most of the thalamus into medial and lateral groups of nuclei ( Figs. 16.7 and 16.8 ). Anteriorly, the internal medullary lamina splits and encloses an anterior group of nuclei, usually referred to collectively as the anterior nucleus, which borders on the interventricular foramen. The medial group similarly contains a single large nucleus, the dorsomedial (DM) nucleus (also commonly called the medial dorsal [MD] nucleus).
TABLE 16.1
Topographical Subdivisions of the Thalamus and Their Principal Nuclei
| Subdivision | Principal Nuclei | Common Abbreviation |
|---|---|---|
| Anterior division | Anterior | |
| Medial division | Dorsomedial (medial dorsal) | DM (MD) |
| Lateral division | Dorsal tier | |
| Lateral dorsal | LD | |
| Lateral posterior pulvinar | LP | |
| Ventral tier | ||
| Ventral anterior | VA | |
| Ventral lateral | VL | |
| Ventral posterior | VP | |
| Ventral posterolateral | VPL | |
| Medial geniculate | MGN | |
| Lateral geniculate | LGN | |
| Intralaminar nuclei | Centromedian | CM |
| Parafascicular | PF | |
| Others | ||
| Reticular nucleus | Reticular nucleus |
The lateral group of nuclei composes the bulk of the thalamus and is further subdivided into a dorsal tier and a ventral tier. The dorsal tier consists of the lateral dorsal (LD) nucleus (see Fig. 16.11 ), the lateral posterior (LP) nucleus (see Fig. 16.12 ), and the large pulvinar (see Fig. 16.13 ). The lateral posterior nucleus is continuous with the pulvinar; both nuclei have similar connections, so the two together are sometimes referred to as the pulvinar-LP complex. The bulk of the ventral tier consists of three nuclei arranged in an anterior-posterior sequence: the ventral anterior (VA) nucleus ( Fig. 16.9 ), the ventral lateral (VL) nucleus ( Figs. 16.10 and 16.11 ), and the ventral posterior (VP) nucleus (see Figs. 16.11 and 16.12 ). The ventral posterior nucleus is customarily subdivided into the ventral posterolateral (VPL) nucleus and the ventral posteromedial (VPM) nucleus. VPL is the somatosensory relay nucleus for the body, and VPM serves the same function for the head. VA and VL are involved in motor control circuits that include the cerebellum and basal nuclei. The lateral geniculate nucleus (visual system) and medial geniculate nucleus (auditory system) are located posterior to these ventral tier nuclei and inferior to the pulvinar, and they protrude posteriorly alongside the midbrain ( Fig. 16.13 ).
Intralaminar Nuclei Are Embedded in the Internal Medullary Lamina
The internal medullary lamina splits at other locations within the thalamus and encloses additional groups of cells collectively called the intralaminar nuclei. The two largest of these are the centromedian (CM) and parafascicular (PF) nuclei (see Fig. 16.12 ). The centromedian nucleus is a relatively large, round nucleus located medial to VPL/VPM. The parafascicular nucleus is located medial to the centromedian nucleus and received its name from the fact that the habenulointerpeduncular tract (fasciculus retroflexus) passes through it.
The Thalamic Reticular Nucleus Partially Surrounds the Thalamus
The lateral surface of each thalamus is covered by a second curved sheet of myelinated fibers called the external medullary lamina, a layer in which fibers sort themselves out on their way into and out of the thalamus. The thin shell of cells that intervenes between the external medullary lamina and the internal capsule is the thalamic reticular nucleus c
c The thalamic reticular nucleus and the brainstem reticular formation were named for their reticulated appearance, but they are distinct in terms of anatomical location and patterns of connections.
(see Figs. 16.7 to 16.13 ). The reticular nucleus seems to be continuous inferiorly with the zona incerta (see Fig. 16.11 ), but this continuity is of no apparent functional significance.
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