Grey Matter and White Matter

Introduction

The central nervous system has a very complex anatomical structure. It has been divided into several parts, hemispheres, and lobes. The two structures, the grey matter, and the white matter, also refers to a classification of the central nervous system. As it is obvious from their names, the grey matter is a greyish looking substance in the brain and the spinal cord while the white matter appears white in the brain and also in the spinal cord. 

Yet, it is not that simple as both structures contain specific anatomical and functional areas that have a complex and diverse role in the body. In this article, we will discuss the structure, location, function, and diseases of both structures. 

Grey matter

The grey matter consists of the soma or the cell bodies of neurons that are present in the brain and the spinal cord. It also contains dendrites, unmyelinated axons, glial cells: the astrocytes and the oligodendrocytes, and some capillaries. The grey matter actually has a pinkish and yellowish hue in its grey color, which comes from cell bodies of neurons and blood capillaries. 

Structure and Location

The chief contents of grey matter are unmyelinated neurons and neuronal cell bodies which are present in the brain, the brainstem, and the spinal cord. In the brain, the grey matter is present at the outer side of the cerebrum and the cerebellum which are called the cerebral cortex and cerebellar cortex, respectively. Grey matter is also present in the deeper parts of the brain in the form of nuclei embedded in the white matter. In the midbrain, it consists of several structures including the thalamus, hypothalamus, globus pallidus, and basal ganglia. 

It is also present in the cerebellum as deep cerebellar nuclei which include the dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus. The important nuclei in the brainstem are substantia nigra, red nucleus, and olivary nuclei which are embedded in the white matter of the brainstem. 

Conversely, in the spinal cord, the grey matter is present on the inner side in the form of a characteristic “butterfly” shape. It is present throughout the spinal cord and is referred to as the grey column. The grey column is divided into three separate columns: anterior grey column, posterior grey column, and lateral grey column. Together these columns make an “H” shape or “butterfly” shape as previously described. The middle part is called grey commissure which connects both sides of the grey matter and it consists of interneurons and projection neurons.

The anterior grey column is directed forward and has a rounded or quadrangular shape. The posterior grey column, also called the dorsal horn, is subdivided into six layers. It contains the marginal nucleus, substantia gelatinosa, and the nucleus proprius. The lateral grey column is present in the form of a triangle in the thoracic and upper lumbar regions of the spinal cord. It is mainly composed of sympathetic preganglionic neurons. 

Function

The grey matter is involved in a lot of important body functions because it contains almost all of the neuronal cell bodies. In the brain, the grey matter regions are involved in muscle control and sensory perception such as hearing, touching, and seeing. The higher brain functions such as decision-making, memory, learning, and self-control are also performed in grey matter regions of the brain. The grey matter of the cerebellum is involved in balance, motor control, coordination, proprioception, and precision. 

The grey matter of the spinal cord is involved in muscle movement, sensory functions, and sympathetic activity. The cell bodies of alpha motor neurons present in the anterior grey column control the activity of skeletal muscles. The posterior grey column receives sensory information from the sensory receptors present in the skin, bones, and joints. 

The lateral grey column is involved in the sympathetic activity of the central nervous system. The axons of cell bodies in lateral grey column synapse with sympathetic ganglia. The upper thoracic segments control sympathetic activity in the head and thorax, while neuronal cell bodies of the lower thoracic and upper lumbar regions affect abdominal organs, pelvic organs, and lower extremities. Through the lateral grey column, the sympathetic nervous system makes changes in cardiac, pulmonary, and GIT systems to prepare the body for emergency situations.

Clinical Significance

Long-term cannabis use and high alcohol consumption has been associated with reductions in the grey matter volume of the hippocampus, amygdala, and prefrontal cortex. However, short-term cannabis use does not produce changes in the grey matter. Cannabis use is also associated with an increase in grey matter volume of the cerebellum. According to some studies, 3D platformer games are associated with increased grey matter. Meditation and mind fullness techniques are also associated with changes in the grey matter structure. 

Diseases

Grey matter diseases are neurodegenerative diseases which cause significant dysfunction of many systems including motor and sensory system. The two common degenerative diseases of grey matter are Alzheimer’s disease and Parkinson’s disease.

  1. Alzheimer’s disease 

Alzheimer’s is a neurodegenerative disease that starts with the most common symptom of difficulty in remembering things. The disease gradually worsens over time and no significant treatment options are available.

  1. Parkinson’s Disease

Parkinson’s is a neurodegenerative disorder caused by genetic and environmental factors. In this disease, the cell death of grey matter in substantia nigra, a region in the midbrain, occurs and leads to a deficiency of dopamine. The most common symptom are tremors, rigidity, and walking difficulty.

  1. Frontotemporal Dementia

This is a term for a group of uncommon disorders of the grey matter that primarily occur due to atrophy of frontal and temporal lobes. The common signs of frontotemporal dementia are extreme behavioral changes. These may include inappropriate behavior, lack of judgment, lack of interest, and compulsive behavior. Speech related disorders and movement disorders such as tremors, rigidity, and muscle spasms are also encountered. 

White matter

White matter refers to an area of the central nervous system made up of myelinated axons. It appears light because of the high lipid content of a waxy coat made up of myelin. White matter is involved in many brain functions such as coordinating between different areas of the central nervous system and acting as a relay. 

Structure

White matter is made up of a bundle of axons of neuronal cell bodies present in the grey matter area of the brain and the spinal cord. The axons are cytoplasmic projections of the cell bodies which transfer the action potential from the cell body to the effector organ and tissues or the dendrites of other neurons. The total length of myelinated axons has been estimated between 150,000 to 180,000 km. Usually, the axons of males have a longer length than females. A decline of 10% in each decade is seen in the length of axons. 

Some other cells are also present in the white matter such as oligodendrocytes, and astrocytes. Oligodendrocytes are neuroglial cells present in the central nervous system whose main function is creating myelin sheath. In the peripheral nervous system, the myelination of axons is done by another specific type of cells called Schwann cells. The astrocytes are star-shaped cells whose main function is to provide nutrients to nervous tissues and maintain extra cellular ion balance. 

The waxy coat or the myelin sheath which covers the axons is very crucial for the nervous system. It acts as an insulator, reduces ion leakage, and decreases the capacitance of the cell membrane. This helps in increasing the nerve impulse speed. Rapid signal conduction occurs when a saltatory nerve impulse jumps on the unmyelinated points on axons called nodes of Ranvier. 

Location

In the brain, white matter is present in the deeper parts, while the grey matter is present in the outer superficial part. In the white matter of the brain, many important grey matter structures are embedded such as the thalamus, hypothalamus, and basal ganglia. The white matter of the cerebellum is also present in its deep part. In the cerebellum, the white matter contains four cerebellar nuclei: dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus. 

In the spinal cord, the white matter is present on the superficial side and the grey matter is in the center of white matter. The bundles of axons of white matter run as tracts in the spinal cord. These bundles are classified into sensory or ascending, and motor or descending tracts. The ascending tracts include the medial lemniscus system, spinocerebellar tracts, and anterolateral tracts. The descending tracts include pyramidal and extrapyramidal tracts. 

Function

White matter plays a very important role in coordinating between different areas of the brain and the spinal cord. It works as a relay and transfers the action potential from one part of the nervous system to another. It also plays an important role in modulating the distribution of action potential. Some studies have shown that it also affects the learning process. 

In the spinal cord, the white matter is present in the form of tracts. The spinal cord receives information from different parts of the body and sends it to the brain through the ascending tracts or the sensory tracts. In the higher brain centers, the decision-making process takes place, and the brain sends signals through descending tracts to the spinal cord. The neuron in the spinal cord takes this information to the effector gland and tissues. 

Diseases

White matter diseases are caused due to the inflammatory process and loss of myelin sheath. Small unnoticeable strokes in the brain may also damage the white matter and cause white matter disease. The most common symptoms of white matter disease include issues with balance, difficulty in multitasking, and mood changes. 

  1. Multiple sclerosis

Multiple sclerosis is an inflammatory disorder in which lymphocytic infiltrate destroys the myelin sheath and axons. In the early stages, remyelination occurs leading to the normal functioning of the white matter. But as time passes, chronic degeneration occurs which then leads to permanent damage. The common symptoms of multiple sclerosis include tremors, unsteady gait, numbness and weakness in limbs, and lack of coordination. Some vision problems such as double vision, blurry vision, and even partial or complete loss of vision may also occur. 

  1. Psychiatric Conditions

Changes in white matter are also linked with some psychiatric conditions. Some common examples are depression, schizophrenia, post-traumatic stress disorder, attention-deficit hyperactivity disorder, autism, and obsessive-compulsive disorder. 

Summary

The white matter and the grey matter are special areas in the brain and the spinal cord. The grey matter is made up of the cell bodies of neurons, some unmyelinated axons, and glial cells. In the brain, grey matter is present on the outer side and many grey matter nuclei are embedded in the white matter in the deeper area of the brain. In the spinal cord, it is present in the center in the form of a characteristic “H” shape. It is divided into three grey columns known as anterior grey column, posterior grey column, and lateral grey column. 

The grey matter is involved in muscle control and sensory perception. It is involved in learning, memory, decision-making, emotions, and self-control. The use of cannabis and alcohol is associated with a decrease in grey matter volume while 3D games and meditation are associated with an increase in grey matter. Neurodegenerative diseases disrupt the grey matter and may result in Alzheimer’s disease, Parkinson’s Disease, and dysfunction of motor and sensory systems. 

The white matter consists of myelinated axons of the neuronal cell bodies present in the grey matter and it appears white because of the myelin sheath. Some oligodendrocytes and astrocytes are also present in it. Oligodendrocytes protect the axons by creating myelin sheath while astrocytes provide nutrients to nervous tissue. The myelin sheath acts as an insulator and increases the speed of nerve impulses.

In the cerebrum and cerebellum, the white matter is present in deeper areas and many nuclei are embedded in it. In the spinal cord, it is present on the outer side. White matter acts as a relay and ensures coordination in the brain and spinal cord. White matter diseases such as systemic sclerosis is caused by inflammatory processes that destroy the myelin sheath and axons.

References