The brainstem is the axial part of the brain; it continues in the caudal area forming the spinal cord, at the level of the occipital foramen, and, upward, it continues into the diencephalon and the telencephalon. It is formed, from the bottom to the top, by the medulla oblongata, or bulb, the pons of Varolii, and the mesencephalon.

In the brainstem, we find the emergence of the cranial nerves, from the third to the twelfth, and the relevant nucleus, except for the spinal part of the spinal accessory nerve (ninth cranial nerve, CN9; Fig. 1.6).

The medulla oblongata is located between the spinal cord, below, and the pons, above, and it is delimited by the bulbopontine sulcus. It is connected to the cerebellum through the inferior cerebellar peduncles. In the medulla oblongata, the motor nerves coming from the cerebral cortex, and going towards the spinal cord (corticospinal tract), form the pyramids that, in their inferior part, on the front side, cross each other and form the so-called pyramidal decussation.

The limit with the spinal cord is conventionally located in the space between where the last pair of cranial nerves emerge (twelfth, or hypoglossal nerve, CN12) and the first pair of cervical nerves.

The medulla oblongata also includes the nucleus responsible for the ocular reflexes of protection (eyelid reflex and tear production), of the superior respiratory reflexes (sneeze and cough), and feeding reflexes (suction and swallowing).

The pons is limited by the bulb, below, and the mesencephalon, above, that separate it, respectively, from the bulbopontine and the pontomesencephalic sulci. It looks like a ventral thickening in the brainstem and superficially it is streaked with transversal fibres, from one cerebral hemisphere to the other (the pontocerebellar fibres) that form on each side the middle cerebellar peduncles. On the front side of the pons, we find the basilar sulcus, which contains the basilar artery and the pontine cistern.

The mesencephalon is the portion of brainstem that connects the pons and the cerebellum to the diencephalon and the telencephalic hemispheres. It consists of the cerebral peduncles, in the front, and of the quadrigeminal plate, behind. The left and right cerebral peduncles are cylindrical trunks going outwards and upwards, delimiting the interpeduncular fossa, and, therefore, penetrating into the corresponding cerebral hemisphere. The quadrigeminal plate is a super axial centre that consists of four colliculi, two superior and two inferior, related to the visual and hearing system respectively, and connected to the lateral and the medial geniculate nucleus, which are diencephalic structures. The mesencephalon is also connected to the cerebellum through the superior cerebellar peduncles.

The internal organisation of the brainstem differs from that of the spinal cord with regard to the different location of the centres and of the pathways; the grey matter, in fact, is not gathered together, it is rather divided in smaller structures (cranial nerve nuclei and the typical nuclei of the brainstem) interposed with the ascending and descending ways: it is not therefore possible to clearly distinguish between white and grey regions, as it is in the spinal cord.

In the bulb, the pons, and the mesencephalon, we can distinguish between a ventral area, mainly occupied by white matter, and a dorsal area, where the grey matter is located (nuclei).

The brainstem regulates several essential functions; in particular, the reticular system (a group of neuronal populations that do not correspond to specific anatomical entities) plays an essential role, either directly or indirectly, in the control of the somatic motor functions, visceral motor functions or homeostasis (cardiac, vessels, respiratory, and visceral secretion functions), with the alternation of sleep–wakefulness, in the processes of addiction, adaptation to the environment, and transmission of general visceral afferents to the superior centres.

The cerebellum is localised behind the medulla oblongata and the pons, and it is separated from them by the interposition of the fourth ventricle. It is situated in the posterior cranial fossa, contained in the cerebellar fossa of the squama occipitalis, and is separated by the tentorium cerebelli from the superior occipital lobes of the telencephalic hemisphere. The cerebellum has an elliptic shape with a large transverse axis, where there is a clear distinction between a central section – narrow and with a sagittal prolongation, which was called vermis by the first anatomists, because of the silkworm shape – and two lateral sections, most highly developed and corresponding to the cerebellar hemispheres.

On the bottom there is a depression, corresponding to the cerebellar hilus, where the superior, medial and inferior cerebellar peduncles join, and establish a connection with the mesencephalon, the pons and the medulla oblongata respectively. The cerebellum surface is transversely crossed by wide primary sulci, and we can distinguish between three lobes: anterior, posterior and flocculonodular. These lobes are divided into secondary sulci, consisting of thin circular convolutions called folia.

The cerebella tonsils are two lobules on the bottom side of the cerebellum, close to the bulb. They are on the lateral outline of the foramen magnum and may partly come out into the spinal canal. Their position is similar to that of the palatine tonsils, and this is the reason for their name.

A section of the brain shows a superficial mantle of grey matter, the cerebellar cortex, with a white matter underneath, named corpus medullare cerebelli, containing grey matter formations, corresponding to the intrinsic nuclei.

There are four cerebelli nuclei. On the medial axis, moving outwards: the fastigial, globose, emboliform and lateral or dentate. The first three nuclei are situated in the vermis, while the larger one, the dentate, is in the cerebellar hemisphere.

Systems of fibers reach the cerebellar cortex, bringing the stimuli coming from the peripheral system (from muscles, tendons, joints, sense organs). The cerebellar cortex is in fact a great integration centre for the stimuli that regulate and coordinate the muscular functions; the cerebellum, through the interposition of the ventral–lateral nucleus of the thalamus, sends impulses to the primary and secondary motor areas of the cerebral cortex to balance the voluntary movements. The cerebellum is also responsible for the body posture, ensuring the required tone. It also provides the required synergy and eumetria of the synergistic and antagonistic muscles, increasing the tone of the former, and decreasing the tone of the latter.

The diencephalon is derived from the primitive vesicle of the forebrain, it is situated in the front end of the mesencephalon, and merges into it without interruption. It is laterally and dorsally surrounded by the cerebral hemispheres; the ventral section only is not coated by other nervous structures and, therefore, it can be seen from the outside. The median sagittal portion of the diencephalon consists of a thin fissure, the third ventricle, also called the diencephalic ventricle, which is characterised by a caudal narrowing that continues in the aqueduct of Sylvius. The existence, and the shape, of the third ventricle divide the diencephalon into two symmetrical parts, the right and the left. In turn, each of the said parts consists of a dorsal section, which is more substantial, and a ventral part, which is smaller.

The dorsal part of the diencephalon includes the dorsal thalamus (or thalamus proper) and a group of structures toward the dorsal–superior part of the same, forming the epithalamus.

The dorsal thalamus is the more substantial part of the diencephalon and the most important integration centre, intercalated by the sub-cortical structures and the cerebral cortex. All the different sensory functions, except for the olfactory one, convey into it before being projected to the cerebral cortex. The thalamus is therefore a sensitive station between the brainstem and cortex. The two thalami are connected through a bridge of grey matter called an intermediate or interthalamic commissure. The medial and lateral geniculate nuclei are linked to each thalamus. The lateral geniculate nucleus is larger and it is connected to the superior colliculus, while the medial geniculate nucleus is connected to the inferior colliculus. Retinal fibers convey into the lateral geniculate, from where the nerve fibers, projected into the visual cortex, originate; the medial geniculate is instead the commuting diencephalic station of the auditory pathway and its fibres establish a connection with the auditory cortex of the temporal lobe.

The epithalamus is a thin strip of tissue in the posterior–superior part of the dorsal diencephalon and it consists of the medullary stripe of the dorsal thalamus, the habenular trigone, the habenular commissure, the epiphysis (or pineal body) and the posterior commissure of the diencephalon. The ventral portion of the diencephalon is normally divided into the hypothalamus and the ventral thalamus (or subthalamus).

The hypothalamus is below the hypothalamic Monro sulcus and includes the plate of the third ventricle and the periventricular area (part of the base of the infundibulum, the tuber cinereum and mammillary bodies). The hypothalamus contains some of the most important integration centres of the autonomic nervous system and, therefore, takes part in the homeostatic regulation of some complex visceral functions (such as the body temperature), emotional expression, and the function of several endocrine glands. The hypothalamus, generally, has the important function of coordinating the endocrine glands, through a strong system of anatomical and functional connections with the hypophysis (hypothalamus–hypophyseal system). In the hypothalamus there are several groups of cells that constitute actual nuclei; among those, the paraventricular and supraoptic nuclei, which contain cells with secreting function (neurosecretion) and that produce oxytocin and vasopressin respectively, or antidiuretic hormone (ADH). These hormones are carried by the neurites, forming the supraoptic and the paraventricular nuclei, up to the neurohypophysis, where they are stored, to be released into the blood when the organism requires them. Through the hypophysis (or pituitary gland) the central nervous system in connected to the endocrine system.

The hypophysis consists of a glandular part, the adenohypophysis, embryologically derived from an invagination of the oral ectoderm (Rathke’s pouch), and a neural part derived from the diencephalon plate (neurohypophysis). Topographically, we can distinguish between a proximal tuberal part, smaller and located in the suprasellar area, and an intrasellar part, which is contained in a small bony cavity, the sella turcica, situated in the sphenoid bone, in the middle cranial fossa, at the base of the brain. The sella turcica is coated inside by the dura mater and it is covered by a dural fold, the diaphragma sellae, crossed by the pituitary stalk. The hypophysis has an ovoid shape, with a transverse diameter of approximately 1.5 cm, a sagittal diameter of 0.7–1.0 cm and a vertical diameter of 0.7–0.9 cm. In adult humans the hypophysis usually weighs less than 1 g, and it is normally bigger in men. The hypothalamus and hypophysis are closely connected through the hypothalamic–hypophyseal portal system, a set of vessels (Fig. 1.7). Inside such an anastomotic system, the hypothalamus, through the release of specific neurohormones (release factors or release-inhibiting factors), regulates the endocrine function of the hypophysis and of the organism.

The subthalamus (or ventral thalamus), that in caudal direction continues straight into the mesencephalic tegmentum, is on a side of the hypothalamus and below the grey matter of the dorsal thalamus. It forms an important subcortical station interposed with the pathways that regulate the activity of the voluntary muscles.

The telencephalon originates from the telencephalic vesicle, which is the rostral part of the primitive proencephalic vesicle. It has an ovoid shape and occupies the largest part of the cranial cavity. The two hemispheres are separated by a deep fissure, the interhemispheric fissure, into which the falx cerebri penetrates; they are caudally separated from the brainstem by the transverse fissure.

Each hemisphere has three faces: the lateral, the medial, and the inferior, penetrating into each other corresponding to the margins, which may be more or less rounded. The lateral face is convex and connected to the internal surface of the cranial vault; the medial face is flat, vertically orientated, marking the hilus of the cerebral hemisphere, and therefore the area where the corpus callosum and the cerebral peduncle enter the cerebral hemisphere. The inferior face is connected to the cranial bones of the base, and, namely, from the front to the back, with the vaults of the orbits, the greater wings of the sphenoid bone, the sella turcica, the temporal bone pyramids, the occipital bone and the cerebellar tentorium.

Each hemisphere, from outside to inside, is composed of a thin layer of grey matter, the cerebral cortex or mantle; a central mass of white matter between the cortex and the basal nuclei; grey subcortical nuclei or basal nuclei (Fig. 1.8).

The external surface of the telencephalic hemispheres is characterised by several sulci and prominences that amplify their extension. They can be distinguished in primary fissures, dividing the hemispheres’ surface into lobes, and secondary fissures that cause in each lobe a series of circular convolutions or cerebral gyri.