The images presented within this chapter are from the same single healthy 25-year-old female subject that was presented in Chap. 1. Voxel dimensions are 1 mm in all dimensions (isotropic), with reconstructions created in the true axial and true coronal planes. Selected images were chosen for labeling that best represented the local anatomy in a given region. Images were magnified to focus on the brainstem and cerebellum and were adjusted to accentuate difference between the gray and white matter so that labeling is more obvious.

Note that these images appear blurry; while 1 mm isotropic pixel dimensions are adequate for imaging the cerebral hemispheres, this resolution is insufficient to capture the detailed anatomy of the small brainstem and cerebellar structures. Such anatomy is better imaged with a 7 tesla MRI scanner (see Chap. 10). However, most MRI facilities have 1.5T and 3T scanners; thus we chose to label images as the radiologist would see them. Furthermore, we chose to reconstruct images in the planes most commonly viewed by radiologists; as such, our labels are more relevant to routine clinical MRI than the labels presented in traditional histological atlases, which are typically aligned perpendicular to the long axis of the brainstem [3–7].

Each page contains the labeled images on the left-hand side. In order to keep the labels small, label numbers are specific to each brain region (cerebellum, brainstem). A small image on the top right of the page documents the locations of the slices, and a key in the lower right-hand side of the page lists the individual structures.

1 Cerebellum Overview

Embryologically, the cerebellum develops from the dorsal aspect of the rhombencephalon, specifically the dorsal metencephalon. Once formed, the cerebellum then develops into a midline portion known as the vermis and two more lateral cerebellar hemispheres. Three primary lobes develop: the primary fissure separates the more cranial anterior lobe from the caudal posterior lobe, and the posterolateral fissure separates the posterior lobe from the flocculonodular lobe. The posterior lobe has two fissures, a horizontal fissure that further divides the posterior lobe between the folium/superior semilunar and tuber/inferior semilunar lobules and a prepyramidal fissure that divides the posterior lobe between the tuber/inferior semilunar and pyramid/biventral lobules. The cerebellar tonsils consist of the uvula and flocculonodular lobe. These lobes contain a total of 18 smaller lobules, 9 in the vermis and 9 in the cerebellar hemispheres. The general structure and function of the cerebellum are detailed in Table 7.1.

Table 7.1 General structure and function of the cerebellum
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The vermis receives input from the spinal cord. It is important in the control of the muscle tone and axial limb movement, maintaining posture of the antigravity muscles. The cerebellar hemispheres receive input from the brain through the pontine nuclei. These areas are responsible for the non-motor functions of the cerebellum such as cognition, language and emotion processing, and modulation. The flocculonodular lobe is heavily connected with the vestibular nuclei and brainstem nuclei for the important head and eye movement coordination.

The complex anatomy of the cerebellum has been categorized in many different ways. From the literature, two separate but similar naming systems have become more prominent than the others, specifically, those of Ito [1] and Larsell [2]. A comparison of these nomenclatures is presented in Tables 7.2, 7.3, and 7.4.

Table 7.2 Comparison of Ito’s and Larsell’s nomenclature for the cerebellar vermis
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Table 7.3 Comparison of Ito’s and Larsell’s nomenclature for the cerebellar hemispheres
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Table 7.4 Nomenclature of the cerebellar fissures
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The nomenclature of Larsell [2] was used to label the cerebellar figures (Figs. 7.1–7.14) in this chapter. For the sake of brevity, only additional structures not found in Tables 7.2, 7.3, and 7.4 will be listed in the key for each cerebellar page.

2 Brainstem Overview

The brainstem develops from caudal two of the three embryological vesicles: the more rostral mesencephalon (a.k.a. midbrain) and the more caudal rhombencephalon. The rhombencephalon further subdivides into the metencephalon (a.k.a. pons) and the myelencephalon (a.k.a. medulla oblongata). All nuclei and tracts respect a columnar organization along the rostral-caudal axis of the brainstem. Thus, it is important to remember that some cranial nerve nuclei and most white matter tracts run through the entire length of the brainstem. It is a combination of symptoms that helps to localize pathology in one of the three parts of the brainstem.

In the developing central nervous system, a basal plate and an alar plate are formed delimited by the sulcus limitans. The alar plate is the dorsal part of the neural tube, whereas the basal plate is the ventral portion of it. The alar plate continues caudally into the sensory dorsal part of the spinal cord, and the basal plate continues to form the motor part of the spinal cord. In the brainstem the alar plates move out laterally and contain the general somatic, general and special visceral afferents of the cranial nerves. Also ascending sensory tracts are seen more laterally. The basal plate contains the motor axons and contain the general somatic and general and special visceral efferents. Also the motor descending fibers are mostly found in the medial part of the brainstem.

The brainstem contains most of the encephalic reticular centers. The brainstem reticular nuclei are divided into three longitudinal columns: median, central (or medial), and lateral. Median nuclei are the raphe nuclei and are cholinergic, locus coeruleus contain noradrenergic fibers, and the PAG and the reticular nuclei are mostly serotoninergic.

In this section, the gray matter contents of each brainstem region are reviewed. In Chap. 8, the white matter tracts in each brainstem region are discussed. The functions of the cranial nerves are separately detailed in Chaps. 12–23 and will not be referred to in this chapter. Finally, the functions of the gray matter structures and white matter tracts will be discussed in Chap. 11.

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