What is gray matter?

Gray matter, or gray matter, is a type of neural tissue that can be found in the brain and spinal cord. It is one of the main components of the central nervous system and is mainly composed of cell bodies and their dendrites.

The name is due to the fact that dead tissue has a gray color that clearly differs from the color of white matter, the other most important type of neuronal tissue. This color difference is due to the white color of the myelin, as the white matter is mainly composed of myelinated axons. In living tissues, the gray matter color is a very faint gray with yellowish and pinkish hues given by the blood capillaries.

Unlike the Central Nervous System, there is no gray matter in the Peripheral Nervous System.

Composition and distribution

Gray matter is mainly composed of cell bodies of neurons, dendrites, and glial cells (astroglia and oligodendrocytes). Axons, both myelinated and unmyelinated, and blood capillaries can also be found. The greatest amount of gray matter is found in the cerebral cortex (surface of the cerebral hemispheres) and in the cerebellar cortex (surface of the cerebellum). It can also be found deep in the cerebrum and cerebellum, as well as the spinal cord.

The location of the gray matter can be summarized as:

Cerebral cortex Cerebellar cortex Deep areas of the brain: thalamus, hypothalamus, subthalamus, basal ganglia, putamen, globus pallidus, nucleus accumbens, septal nuclei Deep areas of cerebellum: dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus Brain stem: substantia nigra , red nucleus, olivary nuclei, cranial nerve nuclei Spinal cord: The gray matter in the spinal cord is located in the center and can be seen as a butterfly in cross section. Three zones are differentiated known as the posterior horn or horn, the anterior horn and the gray commissure. In the thoracic and lumbar segment of the cord, the intermediate or lateral lateral horn is also present.
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Gray matter, being formed primarily by neuronal bodies, not myelinated axons, cannot transmit nerve impulses rapidly. This fact makes the gray matter is involved in the processing of information and not with its transmission. The amount of gray matter in a living being’s nervous system is often interpreted as proportional to its intelligence, although this has never been proven. There are even species with more gray matter than others that are supposedly more intelligent, for example dolphins have more gray matter than humans.

In the brain, most of the neuronal bodies are in the gray matter-forming regions involved, among others, in muscle control, sensory perception, memory, emotions, decision-making or self-control. the brain consumes 20% of all the oxygen that the human body consumes; Of this amount, 95% is specifically consumed by the gray matter, which can give an idea of ​​the high energy consumption of these cells in relation to the rest of the body.

The amount of gray matter is not the same in all individuals of the same species. Some research has found a relationship between a higher density of gray matter in specific areas of the brain and the development of certain skills. For example, musical skills have been linked to a larger area of ​​Broca’s, an area of ​​the brain also involved in speech and language processing.

Some changes in the structure of the gray matter may be related to some psychiatric diseases. For example, decreased gray matter volume in the left inferior parietal lobe, right superior temporal gyrus, right middle frontal gyrus, and left caudate nucleus has been associated with bipolar disorder.

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Positive correlations were also found between gray matter loss and impaired cognitive ability and short-term memory in older people. In a University of Western Australia study published in 2011, greater gray matter loss was associated with older smokers than older nonsmokers.

The gray matter in the spinal cord

It is very common to talk about processing information in the gray matter of the brain, but not so much about the ability of the spinal cord to produce nervous responses on its own to certain stimuli, responses that also involve processing of information by the gray matter of the brain. the brain, spinal cord.

Among the responses generated in the spinal cord, the most important are those known as spinal cord reflexes or reflex actions, which are defined as an involuntary motor response that appears immediately to a specific stimulus. From a sensory point of view, gray matter acts as a selection filter for stimuli directed at higher levels.

In the gray matter, several types of neurons can be distinguished, whose classification and study was pioneered by the Spanish physician Santiago Ramón y Cajal. Spinal cord neurons can be classified into:

root neurons: located in the anterior horn, they are neurons whose axon comes directly from the Central Nervous System. They are classified into: motor neurons: of the somatic nervous system. They make neuromuscular synapses (alpha motor neurons) and synapses on interfacial muscle fibers (gamma motor neurons). vegetative protoneurs: of the autonomic nervous system. Also known as preganglionic neurons. heart neurons: Distributed throughout the gray matter of the spinal cord, their axons join the cords of white matter in the spinal cord.
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Golgi type II neurons: are interneurons (its axon communicates with other neurons, they never leave the CNS) short axons that are distributed throughout the spinal gray matter. spinal ganglion: Although not in the spinal cord, the cell bodies located in the spinal ganglia make their connections to and from the cord.


Lateral dissection of a human brain spinal cord section Gray matter and white matter micrograph

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