The cerebrum is the largest and most obvious portion of the human brain. It forms from the embryonic structure called the telencephalon. The cerebrum is the center of voluntary motor control and complex mental processes.

The word cerebellum means “little brain,” and the cerebellum is indeed a much smaller structure than the cerebrum, weighing approximately one eighth as much. The cerebellum is located at the rear of the brain, below and at the base of the cerebrum (Fig. 2-15). It resembles a small orange wedged in the juncture of the attachment of the spinal cord to the melon-shaped cerebrum. The cerebellum as it is understood is a relatively recent evolutionary addition to the nervous system. Initially it was found in fish and was almost solely related to vestibular functioning. As movement on four legs evolved, the cerebellum developed a rich mass of connections to the spinal cord. As upright posture developed and human beings continued to learn new physical skills, the cerebellum, particularly the posterior lobes, developed many linkages with the cerebrum.

Similar to the cerebrum, the cerebellum consists of two hemispheres. Each is primarily concerned with coordination of movements ipsilaterally, providing fine coordination of movement. The cerebellum plays an important role in postural stability and fixation, as well as in learning a novel motor act. Coordination of the extremely rapid and precise movements of normal articulation of speech also depends on intact cerebellar functioning. Damage may result in a particular type of motor speech disorder, one of the classic dysarthria types called ataxic dysarthria (see Chapter 8). The cerebellum may also have a role in cognitive processing with linkages found with the lateral prefrontal cortex. Cerebellar anatomy and function are discussed in Chapter 6.

The cerebrum represents one of the largest regions of the brain as seen in Fig. 3, and its functions are critical for survival. It is responsible for processing information associated with movement, smell, sensory perception, language, communication, memory, and learning. The left and right symmetrical hemispheres present in the cerebrum are responsible for a different set of tasks. This division of labor is where the terms “left brained,” meaning a person is more analytical and logical, and “right brained” where someone is more intuitive, arise—despite the lack of convincing scientific evidence to support such claims [7]. The cerebral cortex serves as the outer layer of the cerebrum and it consists of mostly of gray matter, which is a type of tissue labeled on the basis of its color [8]. Four lobes make up the cerebral cortex: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Each lobe has a distinct function. For example, the frontal lobe processes information associated with problem solving, speech, and emotions. The parietal lobe senses stimuli and movement, while the temporal lobe deals with processing auditory stimuli and speech. The occipital lobe processes visual information. Generally, this region controls voluntary action by working in coordination with the region of the brain known as the cerebellum, which is part of the brainstem.

The hippocampus, basal ganglia, and olfactory bulb are located in the deeper regions of the cerebrum, and these structures play unique roles in the brain function. The structure of the hippocampus resembles a seahorse, and accordingly it is named after the Greek word meaning seahorse. This region plays an important role in long-term memory [9]. It consists of two sections: the hippocampus proper region and the dentate gyrus. The dentate gyrus holds particular interest as it is one of the regions of the brain where adult neural stem cells are found, as well as a site of neurogenesis, which is the process of forming new neurons from stem cells [10]. This region of the brain becomes dysfunctional in patients suffering from Alzheimer's disease, and neuroscientists have been looking for connections between neurogenesis and Alzheimer's disease [11]. The basal ganglia consist of the nuclei (the command center of a cell) located laterally in a coronal section from a structure called the thalamus, which is found in the diencephalon region of the brain. These two structures work together to coordinate movement through signaling by the molecule glutamate. More on cell-to-cell signaling will be discussed in Section 4, which details the cells of the nervous system and their functions. The main functional cellular units of the nervous system are neurons. These cells rely on different types of signaling to transmit a variety of messages throughout the body. Multiple diseases and disorders are associated with improper basal ganglia function, including Parkinson's disease, attention deficit hyperactivity disorder (ADHD), and schizophrenia [12]. Some of the symptoms of these diseases manifest as disordered movement, which is consistent with the function of this region in healthy tissue.

As its name implies, the olfactory bulb plays a critical role in maintaining the sense of smell. This region contains several receptors that enable the body to sense and filter stimuli detected through olfaction [13]. This information is then transmitted to other regions of the brain where it is processed accordingly. The olfactory bulb also contains multipotent stem cells to replenish cells lost during the sensing process [14]. These neural stem cells migrate to the olfactory bulb from a region called the subventricular zone, which will be discussed later in this chapter. Interestingly, the loss of the ability to smell is observed in many neurodegenerative diseases, including dementia and Alzheimer's disease. This observation suggests a potential common link between these diseases caused by an inability of the brain to perform neurogenesis, the development of new neural tissue.

The cerebellum is the largest structure of the hindbrain and located in the back portion of the skull below the temporal and occipital lobes and behind the brainstem. It has a midline vermis, seen when the brain is hemisected, and a larger, lateral hemisphere on each side. Each hemisphere has thin, transverse, and parallel folds called folia. These are separated by shallow sulci. The surface cortex of the cerebellum is made up of gray matter. There is a deeper layer of white matter. A sagittal section shows the white matter branching into fernlike patterns called the arbor vitae. Each hemisphere has four gray matter masses inside the white matter. These are called the deep nuclei. All input to the cerebellum moves to the cortex, and all output comes from the deep nuclei. The cerebellum coordinates many functions, including skilled voluntary movements, posture, gait, and muscular tone. It may be involved in modulating emotional states and cognition. The organization of the cerebellum is very complex, along with its afferent and efferent connections. There are several functional networks in the cerebral cortex. Connections between the cerebrum and cerebellum may be indirect, with cortical association systems or brainstem nuclei that exclude the pontine nuclei. The networks mirror each other in the anterior and posterior cerebellums.

The cerebellum continuously monitors body movements in comparison to brain information, sending out control messages to make fine corrections. Injury to the cerebellum, therefore, results in ataxia. The individual cannot sit or stand without assistance. Alcohol also has a temporary effect. The cerebellum is additionally involved in emotions, language, and thinking. It is thought to analyze these events in comparison to the brain's “intentions,” and make adjustments as needed. There is a lot that is not fully understood about the cerebellum and its effects upon nonmotor functions.

This chapter provides an overview of cerebellum. Cerebellum is a little brain only in respect to its volume. The surface area and cellular content of its cortex are little inferior to those of the cerebrum. The cerebellum was developed in primitive fish for the processing of information derived from the lateral line receptors and their specialized development in the vestibules of the inner ear. The early phylogenetic development of the supravestibular grey matter into a cerebellum is associated with integration of vestibular and spinal cord afferent impulses required for posture. The long loop reflexes through the intermediate cerebellum and/or cerebral cortex may be supposed to provide a brake for flexion responses in a similar manner to the vermisfastigial loops for extension responses.

By the end of the nineteenth century (Sherrington, 1900), the cerebellum was recognized as an important structure for motor control, and there was considerable interest in the fibers to and from the cerebellum and their possible role in the regulation of movement. Like the cerebrum, the cerebellum is a large brain structure composed of a highly convoluted and extensive cortex which surrounds a central area of white matter. The cerebellar nuclei are contained deep within the white matter and their axons constitute the principal efferent fibers from the cerebellum. All of the cerebellar nuclei project directly or indirectly to motor structures. One of the surprising facts about the cerebellar cortex is its great surface area. If the human cerebellar cortex were to be flattened, the unrolled cortical sheet would be over 1 m long and about 1/6 m across (Braitenberg and Atwood, 1958).

What kinds of afferents does the cerebellar cortex receive? Does visual information reach it? Fibers which enter the cerebellum do so by way of one of its three stalks or peduncles, the largest of which is the middle cerebellar peduncle. Nearly all of the axons in the middle cerebellar peduncle have their cell bodies in the pontine nuclei and the pons has long been recognized as the largest single source of afferent fibers to the human cerebellum. Cajal (1909) described clearly the structure of the pontine nuclei and recognized the role of pontine cells in relaying information from the cerebral cortex to the cerebellum. But Cajal's discussion dealt principally with the input to the pontine nuclei from the motor cortex; the widespread connections to the pontine nuclei from other parts of the cerebral cortex and from the midbrain were, as yet, unrecognized.

It began to be clear that the cerebellum must receive several different kinds of sensory fibers when Snider and Stowell (1944) discovered that flashes of light, clicks, and tactile stimulation all could evoke potentials in the cerebeller cortex. At first, the visual input was thought to be relayed only by way of the superior colliclus and to project only to a restricted portion of the vermis of the cerebellar cortex. Subsequent experiments, however, have shown that there are visual inputs in addition to that from the colliculus, and that visual information is widely distributed to the cerebellar hemispheres as well as the vermis (Fadiga and Pupilli, 1964). It is the study of these visual pathways via the pons to the cerebellum that has been our principal interest for a number of years.

Lower down the neuraxis, we see the cerebellum. Its bulk is partly hidden by the overlying cerebral hemisphere. The cerebellum is a sort of computer; it regulates the rate, range and force of movements. It works in concert with the cerebrum, as well as with the spinal cord and other structures. Without the cerebellum, we would still enjoy all our sensations. But its loss would seriously impair the dexterity and smooth execution of our movements, while noticeably diminishing our strength and muscular tone.

Below the cerebellum is the pons, a bridge of nerve fibers which appears to cross from one side of the cerebellum to the other. It seems to strap the cerebellum to the brain stem like a backpack around someone's waist. It is a key link between the cerebrum and cerebellum, a massive cable through which the cerebral motor region “plugs in” to the cerebellar computer. This connection allows volitional movements to be carried out in a coordinated, flowing and well-directed manner.

Nestled underneath the tentorium in the posterior cranial fossa, the cerebellum is a remarkable brain structure. It plays an essential role in motor coordination [1] and cognitive function [2]. The cerebellum is divided into two hemispheres and a midline zone called the vermis. Further subdivision is based on hierarchical folds known as lobes (anterior, superior posterior, inferior posterior, and flocculonodular) and lobules (identified by Roman numerals I–X). Brodmann areas have been used to subdivide the cerebral cortex for more than a century [3]. However, unlike the cerebrum, the cerebellum has a rather homogeneous cytoarchitecture, and although some evidence is increasingly challenging this assumption [4], the cerebellar lobules are still the preferred way to subdivide the structure. Different groups have advanced the application of computational techniques to segment the cerebellum with the objective of developing a fully automatic algorithm. From atlas-based approaches to the newest deep learning models, each novel software alternative becomes more accurate and requires less human intervention. In a data-driven world, such methods are essential for advancing our understanding of the cerebellum and its role in brain function and dysfunction. It is worth noting that different methods have their advantages and limitations, requiring investigators to choose wisely to maximize the possible insights of their research.