Chapter 9: Diencephalon

INTRODUCTION

The diencephalon includes the thalamus and its geniculate bodies, the hypothalamus, the subthalamus, and the epithalamus (Fig 9–1). The third ventricle lies between the halves of the diencephalon.

A small groove on the lateral wall of the third ventricle—the hypothalamic sulcus—separates the thalamus dorsally and the hypothalamus and subthalamus inferiorly.

THALAMUS

Landmarks

Each half of the brain contains a thalamus, a large, ovoid, gray mass of nuclei (Fig 9–2). Its broad posterior end, the pulvinar, extends over the medial and lateral geniculate bodies. The rostral thalamus contains the anterior thalamic tubercle. In many individuals, there is a short interthalamic adhesion (massa intermedia) between the thalami, across the narrow third ventricle (see Fig 9–1).

White Matter

The thalamic radiations are the fiber bundles that emerge from the lateral surface of the thalamus and terminate in the cerebral cortex. The external medullary lamina is a layer of myelinated fibers on the lateral surface of the thalamus close to the internal capsule. The internal medullary lamina is a thin vertical sheet of white matter that bifurcates in its anterior portion and divides the thalamus into lateral, medial, and anterior portions (Fig 9–3).

Thalamic Nuclei

There are five major groups of thalamic nuclei, each with specific fiber connections (Figs 9–3 and 9–4; Table 9–1).

A. Anterior Nuclear Group

This group of clusters of neurons forms the anterior tubercle of the thalamus and is bordered by the limbs of the internal lamina. It receives fibers from the mammillary bodies via the mamillothalamic tract and projects to the cingulate cortex.

B. Nuclei of the Midline

These groups of cells are located just beneath the lining ...