DIENCEPHALON

The components of the diencephalon are distributed symmetrically on both sides of the third ventricle. The hypothalamus consists of specific nuclei that are located lateral and ventral to the ventral portion of the third ventricle, ventral to the interthalamic adhesion (Fig. 19-1). The subthalamus is between the thalamus and substantia nigra of the mesencephalon. It is composed of the subthalamic body and zona incerta. The thalamus is composed of a plethora of nuclei that make up the major part of the diencephalon located dorsal to the ventral portion of the third ventricle and medial to the internal capsule. The metathalamus consists of the medial and lateral geniculate nuclei. The epithalamus includes the habenular nuclei and their connections and the pineal gland. For teaching purposes, this text considers the metathalamic nuclei to be part of the thalamus. This chapter describes the major components of the thalamus and hypothalamus. The epithalamus was previously considered with the limbic system and the subthalamus with the extrapyramidal system.

Figure 19-1 Divisions of the diencephalon. IA, Interthalamic adhesion; IVF, interventricular foramen; LT, lamina terminalis.

THALAMUS

Anatomy

The thalamus is related to the hypothalamus ventrally and to the internal capsule laterally (see Fig. 19-1; see also Fig 2-3 through 2-5).^7,^29-^32,³⁵ Leptomeninges (pia and arachnoid trabeculations) and cerebrospinal fluid (CSF) cover most of its dorsal surface. It is composed of numerous nuclei partly separated by medullary laminae and arranged in a bilaterally symmetric pattern on either side of the third ventricle. The internal medullary lamina divides each side of the thalamus into medial and lateral halves, and it splits rostrally to enclose a rostral portion, which includes the rostral thalamic nucleus (Fig. 19-2). A dorsal plane through the lateral half separates the thalamus into dorsal and ventral tiers of nuclei, and two transverse planes through the ventral tier further define these nuclei from rostral to caudal. The thin external medullary lamina forms the external boundary of the lateral half of the thalamus and is separated from the internal capsule by a narrow nuclear area, the thalamic reticular nucleus. These divisions result in the following groups of nuclei with a listing of their specific nuclei.

1 Rostral thalamic group: rostral thalamic nucleus (limbic system)

2 Medial thalamic group: medial dorsal nucleus

3 Lateral thalamic group:

Ventral rostral nucleus (extrapyramidal)

Ventral lateral nucleus (cerebellum)

Ventral caudal group:

Ventral caudal medial nucleus (cranial nerves)

Ventral caudal lateral nucleus (spinal nerves)

4 Caudal thalamic group (metathalamus)

Medial geniculate nucleus (auditory, vestibular)

Lateral geniculate nucleus (vision)

5 Intralaminar-midline thalamic group:

Central medial nucleus

Paraventricular nucleus

6 Thalamic reticular nucleus (ascending reticular activating system [ARAS])

Figure 19-2 Thalamic nuclear groups. LGN, Lateral geniculate nucleus; MGN, medial geniculate nucleus; VCL, ventral caudal lateral; VCM, ventral caudal medial; VL, ventral lateral; VR, ventral rostral.

Function

On a functional basis, the thalamic nuclei can be grouped into three major systems: (1) the direct cortical projection system, (2) the diffuse cortical projection system, and (3) the thalamic reticular system (Fig. 19-3).

Figure 19-3 Functional organization of the thalamus. ARAS, Ascending reticular activating system.

Ascending Reticular Activating System

The reticular formation is defined in Chapter 8, and the ascending component, the ARAS, is introduced at that time. The ARAS is part of the reticular formation, which, you may recall, consists of a network of neurons in the central portion or core of the brainstem from the medulla through the pons and midbrain and into the diencephalon.²¹ The ARAS receives afferents from all the conscious projection pathways of the sensory systems, which includes interoception, exteroception, and proprioception. As these conscious projection pathways (spinal and cranial) course rostrally through the brainstem to the primary relay nuclei in the thalamus, collateral axons are given off into the reticular formation (Fig. 19-4). Most of these collaterals synapse in the reticular formation. Neurons of the reticular formation that project rostrally (the ARAS) continue the impulse flow rostrally in a multisynaptic pattern to one of two areas in the diencephalon, either the thalamic reticular system or the hyposubthalamic reticular system. A few collaterals project rostrally as part of the ARAS without synaptic interruption to terminate on one of these two diencephalic areas. The diencephalic portion of the ARAS stimulates the entire cerebral cortex diffusely. The thalamic reticular system affects the cerebral cortex by way of the diffuse telencephalic connections of the thalamic association system.