Grey matter in the context of "Myelin"

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 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.

A cell type is a classification used to identify cells that share morphological or phenotypical features. A multicellular organism may contain cells of a number of widely differing and specialized cell types, such as muscle cells and skin cells, that differ both in appearance and function yet have identical genomic sequences. Cells may have the same genotype, but belong to different cell types due to the differential regulation of the genes they contain. Classification of a specific cell type is often done through the use of microscopy (such as those from the cluster of differentiation family that are commonly used for this purpose in immunology). Recent developments in single cell RNA sequencing facilitated classification of cell types based on shared gene expression patterns. This has led to the discovery of many new cell types in e.g. mouse grey matter, hippocampus, dorsal root ganglion and spinal cord.

Animals have evolved a greater diversity of cell types in a multicellular body (100–150 different cell types), comparedwith 10–20 in plants, fungi, and protists. The exact number of cell types is, however, undefined, and the Cell Ontology, as of 2021, lists over 2,300 different cell types.

The olfactory bulb (Latin: bulbus olfactorius) is a neural structure in the forebrain of vertebrates that is involved in olfaction, or the sense of smell. It transmits olfactory information to the other brain regions including the amygdala, orbitofrontal cortex (OFC) and hippocampus where it contributes to emotion, memory and learning.

The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway.

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.

The line of Gennari (also called the "band" or "stria" of Gennari) is a band of myelinated axons that runs parallel to the surface of the cerebral cortex on the banks of the calcarine fissure in the occipital lobe. This formation is visible to the naked eye as a white strip running through the cortical grey matter, and is the reason the V1 in primates is also referred to as the "striate cortex." The line of Gennari is due to dense axonal input from the thalamus to layer IV of visual cortex. It is the name given to the enlarged external band of Baillarger. The structure is named for its discoverer, Francesco Gennari, who first observed it in 1776 as a medical student at the University of Parma. He described it in a book which he published in 1782. Although non-primate species have areas that are designated primary visual cortex, some (if not all) lack a stria of Gennari.

Vicq d’Azyr published the stripes in Traité d'anatomie (1786), and for a while it was known as the stripe of Vicq d’Azyr.

Amyloid plaques (also known as neuritic plaques, amyloid beta plaques or senile plaques) are extracellular deposits of amyloid beta (Aβ) protein that present mainly in the grey matter of the brain. Degenerative neuronal elements and an abundance of microglia and astrocytes can be associated with amyloid plaques. Some plaques occur in the brain as a result of aging, but large numbers of plaques and neurofibrillary tangles are characteristic features of Alzheimer's disease.

The plaques are highly variable in shape and size; in tissue sections immunostained for Aβ, they comprise a log-normal size distribution curve, with an average plaque area of 400–450 square micrometers (μm). The smallest plaques (less than 200 μm), which often consist of diffuse deposits of Aβ, are particularly numerous. Plaques form when Aβ misfolds and aggregates into oligomers and longer polymers, the latter of which are characteristic of amyloid.