Commissural and Associative Pathways

Transverse or commissural fibers unite the two hemispheres; association fibers connect different structures in the same hemisphere. Most commissural and associative pathways are well visible on cross-sectional MRI cuts. In this chapter, the main pathways are drawn on 3D MRI reformatted images. MR tractography can nowadays directly identify many of them.

The corpus callosum is the biggest interhemispheric commissure, containing fibers that reach all aspects of the cerebral cortex. It has a central portion, the body or trunk, an anterior part, the genu, and a posterior one, the splenium. The genu bends dorsally and downward to form the rostrum. This becomes progressively thinner and goes toward the anterior commissure. This thinner part is called the "lamina terminalis". The genu's fibers curve forward and lateral to the frontal horns. This structure is called the "forceps minor". The passage between the body and the splenium is often narrow. This portion is also known as "isthmus". The corpus callosum delimitates the roof of the lateral ventricles. The splenium's fibers cover the lateral ventricles posteriorly and extend laterally to cover the lateral aspect of the temporal horn. They are termed the "tapetum", while the most posterior fibers, which curve backwards to the occipital lobes, are called the "forceps major".

• Anterior Commissure

The anterior commissure is a bundle of myelinated fibers about 3 mm thick, located above the rostrum of the corpus callosum, anterior to the interventricular foramen of Monro. Its fibers connect the medial aspects of the temporal lobes and course laterally, producing a groove on the anterior inferior part of the lentiform nucleus.

• Superior Longitudinal Fasciculus

The superior longitudinal fasciculus, or arcuate fascicle, extends from the frontal to the parietal and occipital lobes. It goes laterally to the corona radiata. Among its fibers, the pathway between the speech areas of Broca and the Wernicke's are included.

• Fasciculus Occipito-Frontalis

It extends from the occipital pole to the frontal lobe, passing laterally to the caudate nucleus. Two portions are distinguished: the fasciculus occipito-frontalis superior and the inferior.

• Inferior Longitudinalis Fasciculus

This fascicle starts from the occipital pole and extends to the temporal lobe passing laterally to the lateral ventricle.

• Uncinate Fasciculus

It connects the orbital gyri of the frontal lobe and the speech area with the cortex of the temporal lobe, passing below the claustrum, the external and the extreme capsules.

• Posterior Commissure

This structure is located immediately below the pineal gland, on the dorsal aspect of the midbrain, in correspondence with the beginning of the cerebral aqueduct. It is related to the habe-nular commissure, which is located above, adjacent to the pineal body.

• Habenular Commissure

This commissure is specifically devoted to connecting the two habenular nuclei with each other. These nuclei are located on the dorsomedial aspect of the thalami.

The fur nix is the main hypothalamic commissure. Its morphology is described in Chapter 1 (page 24).

• Mammillothalamk Tract

The fibers of the mammillothalamic tract extend from the mammillary body, across the thalamus, to end in the anterior thalamic nuclei. It is adjacent to the third ventricle wall, in the hypothalamus.

• Short Arcuate Fibers

These fibers connect adjacent gyri. They are also called "U" Fibers.

II. Pituitary Gland

The pituitary gland is median, and strongly connected to the hypothalamus by means of the stalk. The gland varies in size and approximately reaches the adult shape and volume by the age of 4. The height of the gland, however, varies over time and in consequence of the hormonal state. In males, it measures about 4 ± 1 mm, in females 4,4 ± 1.4 mm. In females, however, the height is more variable, being more pronounced in fertile life (about 6-7 mm), reaching 10 mm during adolescence and 12 mm during pregnancy.

The gland is composed of two lobes: the anterior, embry-ologically derived from the I branchial arch (liathke's pouch), and the posterior, which derives from the hypothalamus. In between is a thin layer, the pars intermedia, that sometimes takes a cystic aspect. It is located inside the sella turcica, covered by the tentorium sellae, a dura mater plica perforated in the middle to leave a passage for the stalk. Laterally, the gland is delimited by the cavernous sinuses. There is no dural wall to delimitate the medial aspect of the cav ernous sinus, while the lateral wall is thick, formed by a dural plica. The dura mater of the lateral wall contains the cranial nerves III, IV, VI and V2. The cranial nerve VI passes inside the sinus.

III. Vascularization

The hypothalamus is richly supplied by several small arteries coming from both carotids and Willis' circle.The anterior lobe of the pituitary gland is supplied by superior pituitary arteries coming from the internal carotid and by posterior communicating arteries. They wrap the upper stalk, where they form an initial capillary network and a group of draining veins. These reach the anterior pituitary lobe, where a second capillary plexus is formed (pituitary portal system). This system receives anterior pituitary hormones. Finally, some principal venous ducts drain the blood toward the cavernous sinus.

The posterior lobe of the gland receives supply directly from internal carotid arteries, by means of posterior pituitary arteries. The venous drainage is to the cavernous sinus.

Neuroendocrine System

This system is essentially composed of the hypothalamus and the pituitary gland. The system has the particular task of producing hormones. Therefore, a very particular circulation is present.

I. Hypothalamus

The hypothalamus is situated below the thalamus and the basal ganglia, and forms the lateral wall of the lower aspect of the third ventricle, separated from the thalamus by the hypothalamic sulcus. It contains several complex nuclei, the mammillary bodies, anterior to the interpeduncular fossa, the tuber cinereum (a protuberance located behind the infundibulum), the infundibulum, the pituitary stalk, the lamina terminalis and the anterior commissure.

Area 46 [Motor speech marginal area) Area 45 [Broca's speech area) Area 44 (Broca's speech area) Area 41 (Primary auditory area) Area 42 (Primary auditory area) Area 22 (Wernicke's speech area) Area 21 (Wernicke's speech area) Area 37 (Sensory speech marginal area) Area 40 (Sensory speech marginal area) Area 39 (Sensory speech marginal area)

Speech production area (Broca)

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