White matter in the context of Cerebellar cortex

The human brain is the central organ of the nervous system, and with the spinal cord, comprises the central nervous system. It consists of the cerebrum, the brainstem and the cerebellum. The brain controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sensory nervous system. The brain integrates sensory information and coordinates instructions sent to the rest of the body.

The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres. Each hemisphere has an inner core composed of white matter, and an outer surface – the cerebral cortex – composed of grey matter. The cortex has an outer layer, the neocortex, and an inner allocortex. The neocortex is made up of six neuronal layers, while the allocortex has three or four. Each hemisphere is divided into four lobes – the frontal, parietal, temporal, and occipital lobes. The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought, while the occipital lobe is dedicated to vision. Within each lobe, cortical areas are associated with specific functions, such as the sensory, motor, and association regions. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side, such as language in the left and visual-spatial ability in the right. The hemispheres are connected by commissural nerve tracts, the largest being the corpus callosum.

In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to another). Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus. Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.

In the hippocampus, there are neural pathways involved in its circuitry including the perforant pathway, that provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1), and the subiculum.

The cerebellum (pl.: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or even larger. In humans, the cerebellum plays an important role in motor control and cognitive functions such as attention and language as well as emotional control such as regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination, precision, and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, and integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium, posture, and motor learning in humans.

Anatomically, the human cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres. Its cortical surface is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex. These parallel grooves conceal the fact that the cerebellar cortex is actually a thin, continuous layer of tissue tightly folded in the style of an accordion. Within this thin layer are several types of neurons with a highly regular arrangement, the most important being Purkinje cells and granule cells. This complex neural organization gives rise to a massive signal-processing capability, but almost all of the output from the cerebellar cortex passes through a set of small deep nuclei lying in the white matter interior of the cerebellum.

The cerebrum, or the largest part of the vertebrate brain, is made up of two cerebral hemispheres. The deep groove known as the longitudinal fissure divides the cerebrum into the left and right hemispheres, but the hemispheres remain united by the corpus callosum, a large bundle of nerve fibers in the middle of the brain whose primary function is to integrate sensory and motor signals between the hemispheres. In eutherian (placental) mammals, other bundles of nerve fibers like the corpus callosum exist, including the anterior commissure, the posterior commissure, and the fornix, but compared with the corpus callosum, they are much smaller in size.

Broadly, the hemispheres are made up of two types of tissues. The thin outer layer of the cerebral hemispheres is made up of gray matter, composed of neuronal cell bodies, dendrites, and synapses; this outer layer constitutes the cerebral cortex (cortex is Latin for "bark of a tree"). Below that is the larger inner layer of white matter, composed of axons and myelin.

Grey matter (gray matter in American English) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distinguished from white matter in that it contains numerous cell bodies and relatively few myelinated axons, while white matter contains relatively few cell bodies and is composed chiefly of long-range myelinated axons. The colour difference arises mainly from the whiteness of myelin. In living tissue, grey matter actually has a very light grey colour with yellowish or pinkish hues, which come from capillary blood vessels and neuronal cell bodies.

The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mammals. It spans part of the longitudinal fissure, connecting the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about 10 cm (3.9 in) in length and consisting of 200–300 million axonal projections.

A number of separate nerve tracts, classed as subregions of the corpus callosum, connect different parts of the hemispheres. The main ones are known as the genu, the rostrum, the trunk or body, and the splenium.

In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain controls body temperature, reproductive functions, eating, sleeping, and the display of emotions.

Vesicles of the forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary brain vesicles during the early development of the nervous system. At the five-vesicle stage, the forebrain separates into the diencephalon (thalamus, hypothalamus, subthalamus, and epithalamus) and the telencephalon which develops into the cerebrum. The cerebrum consists of the cerebral cortex, underlying white matter, and the basal ganglia.

The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neurons in the corticospinal tract, and they become myelinated usually in the first two years of life.

The corticospinal tract is one of the pyramidal tracts, the other being the corticobulbar tract.

There are four paired deep cerebellar nuclei embedded in the white matter centre of the cerebellum. The nuclei are the fastigial, globose, emboliform, and dentate nuclei.

In lower mammals the emboliform nucleus appears to be continuous with the globose nucleus, and these are known together as the interposed nucleus.

The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In all but five species of mammal the great majority of fibers connecting the two hemispheres travel through the corpus callosum, which in humans and all non-monotremes is more than 10 times larger than the anterior commissure. Other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.

The anterior commissure plays a key role in pain sensation, more specifically sharp, acute pain. It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalae and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior.

Gyrification is the process of forming the characteristic folds of the cerebral cortex. The peak of such a fold is called a gyrus (pl. gyri), and its trough is called a sulcus (pl. sulci). The neurons of the cerebral cortex reside in a thin layer of gray matter, only 2–4 mm thick, at the surface of the brain. Much of the interior volume is occupied by white matter, which consists of long axonal projections to and from the cortical neurons residing near the surface. Gyrification allows a larger cortical surface area, and hence greater cognitive functionality to fit inside a smaller cranium.

In most mammals, gyrification begins during fetal development. Primates, cetaceans, and ungulates have extensive cortical gyri, with a few species exceptions, while small rodents such as the rat, and mouse have none. Gyrification in some animals, for example the ferret, continues well into postnatal life.

Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent anatomical region of neuropil is the brain which, although not completely composed of neuropil, does have the largest and highest synaptically concentrated areas of neuropil in the body. For example, the neocortex and olfactory bulb both contain neuropil.

White matter, which is mostly composed of myelinated axons (hence its white color) and glial cells, is generally not considered to be a part of the neuropil.

Oligodendrocytes (from Greek 'cells with a few branches'), also known as oligodendroglia, are a type of neuroglia whose main function is to provide the myelin sheath to neuronal axons in the central nervous system (CNS). Myelination gives metabolic support to, and insulates the axons of most vertebrates. A single oligodendrocyte can extend its processes to cover up to 40 axons, that can include multiple adjacent axons. The myelin sheath is segmented along the axon's length at gaps known as the nodes of Ranvier. In the peripheral nervous system the myelination of axons is carried out by Schwann cells.

Oligodendrocytes are found exclusively in the CNS, which comprises the brain and spinal cord. They are the most widespread cell lineage, including oligodendrocyte progenitor cells, pre-myelinating cells, and mature myelinating oligodendrocytes in the CNS white matter. Non-myelinating oligodendrocytes are found in the grey matter surrounding and lying next to neuronal cell bodies. They are known as neuronal satellite cells, and their presence is not understood.

A connectome (/kəˈnɛktoʊm/) is a comprehensive map of neural connections in the brain, and may be thought of as its "wiring diagram". These maps are available in varying levels of detail. A functional connectome shows connections between various brain regions, but not individual neurons. These are available for large animals, including mice and humans, are normally obtained by techniques such as MRI, and have a scale of millimeters. At the other extreme are neural connectomes, which show individual neurons and their interconnections. These are usually obtained by electron microscopy (EM) and have a scale of nanometers. They are only available for small creatures such as the worm C. elegans and the fruit fly Drosophila melanogaster, and small regions of mammal brains. Finally there are chemical connectomes, showing which neurons emit, and are sensitive to, a wide variety of neuromodulators. As of 2025, only C. elegans has such a connectome.

The significance of the connectome stems from the realization that the structure and function of any brain are intricately linked, through multiple levels and modes of brain connectivity. There are strong natural constraints on which neurons or neural populations can interact, or how strong or direct their interactions are. Indeed, the foundation of human cognition lies in the pattern of dynamic interactions shaped by the connectome.

Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It allows the mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively. Molecular diffusion in tissues is not random, but reflects interactions with many obstacles, such as macromolecules, fibers, and membranes. Water molecule diffusion patterns can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state. A special kind of DWI, diffusion tensor imaging (DTI), has been used extensively to map white matter tractography in the brain.

In neuroanatomy, a nucleus (pl.: nuclei) is a cluster of neurons in the central nervous system, located deep within the cerebral hemispheres and brainstem. The neurons in one nucleus usually have roughly similar connections and functions. Nuclei are connected to other nuclei by tracts, the bundles (fascicles) of axons (nerve fibers) extending from the cell bodies. A nucleus is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up as a region of gray matter, often bordered by white matter. The vertebrate brain contains hundreds of distinguishable nuclei, varying widely in shape and size. A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers.

The term "nucleus" is in some cases used rather loosely, to mean simply an identifiably distinct group of neurons, even if they are spread over an extended area. The reticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus.