The brain is an immensely complex structure, but there are ways we can divide up its anatomical structure into more discrete parts; the left and right hemisphere, parietal, temporal, occipital and frontal lobes. Another common divider is to separate the brain’s gray and white matter. But what are these two structures? How different are they from each other? How significant, and physiologically relevant, is this divide? Read on to find out!
Differences between Gray and White Matter
What is gray matter?
Gray matter consists primarily of neuronal cell bodies, or soma. This a spherical structure that houses the neuron’s nucleus.
What is white matter?
White matter areas of the brain mainly consist of myelinated axons, which are long relays that extend out from the soma, and which are whiteish in color due to the relatively high lipid fat content of the myelin protein that sheathes them, These form connections between brain cells, and white matter is typically distributed into bundles called tracts.
Is it really that simple?
Not quite. Whilst the above division is physiologically accurate on a system level, there are a mix of cell types present in both gray and white matter.
Gray matter also contains:
- Axon tracts
- Glial cells
- Capillary blood vessels
- Neuropil – a mix of dendrites, unmyelinated axons, and glia
White matter also contains:
- Oligodendrocytes – glial cells which produce myelin
How are gray matter and white matter arranged in the CNS?
Both gray and white matter are spread throughout the human central nervous system – the brain and spinal cord
Figure 1: The arrangement of white and gray matter in the brain.
Gray Matter Location
Neuronal cell bodies are abundant in the cerebrum, brain stem and cerebellum. This latter structure, which makes up just 10% of brain volume, contains more neurons than the rest of the brain put together. In the spinal cord, gray matter forms a “butterfly” structure, which can be visualized below in figure 2.
So which regions of the central nervous system have an external layer of gray matter?
We can think of the cerebrum and cerebellum as the brain regions which have an external layer of gray matter (see figure 1). The brainstem’s gray matter is located in groups of neurons called nuclei which are embedded within white matter tracts.
White matter location
As previously mentioned, white matter is organized into tracts of axons. In the cerebrum and cerebellum, white matter is predominantly found in deeper areas – with gray matter coating the white matter - see figure 1. Other gray matter structures, like the basal ganglia, are embedded within this white matter core. The brain’s fluid-filled ventricles are also found within the white matter.
In the spinal cord, things are largely reversed – the white matter is distributed around the central gray matter “butterfly”.
Figure 2: The arrangement of white and gray matter in the spinal cord.
Gray matter and white matter: function
What is the function of gray matter?
Gray matter-heavy brain regions include those that control muscular and sensory activity.
- Cerebral cortex - The outer layer of the brain, the cerebral cortex, consists of columns of gray matter neurons, with white matter located underneath. This area is essential to many facets of higher learning, including attention, memory, and thought.
- Cerebellum - The cerebellum is essential for motor control, coordination, and precision.
What is the function of white matter?
Neuron-rich brain regions wouldn’t count for much without the rich veins of axonal connections contained within white matter to join them up.
The white fatty myelin that gives this tissue its name is also essential to its function – myelin insulates axons, letting the signal within travel far faster, enabling the nerve cell function that is essential to normal motor and sensory function.
Diseases of gray and white matter
Gray matter disease
Diseases that cause the loss of the neurons that make up gray matter are primarily called neurodegenerative diseases. These diseases, which include dementias like Alzheimer’s disease and frontotemporal dementia, affect millions of people worldwide. White matter alterations are often present in these diseases, but physiological hallmarks such as amyloid plaques and tau neurofibrillary tangles are located in the gray matter. As might be expected, the region where neurons are lost largely dictates disease progression – the motor symptoms of Parkinson’s disease are directly related to the loss of dopamine-producing neurons in the substantia nigra.
White matter disease
As James Balm phrases it in his article for the BMC, white matter tracts are the subways of the brain, vital connections that ensure the smooth operation of our nervous system. As such, disease that affects our white matter can disrupt this nerve signal transit and can be a serious issue.
- Multiple sclerosis – In multiple sclerosis (MS) the white fatty myelin coating around axons is destroyed, leading to motor or sensory disruption. In relapsing-remitting multiple sclerosis, this myelin can be repaired and lost multiple times. In progressive multiple sclerosis, axonal damage is followed by neuron death, which is irreversible.
- White matter disease – responsible for roughly a fifth of strokes worldwide, white matter diseases affect blood vessels buried within white matter. These harden, preventing oxygen and other nutrients from reaching the white matter.
- Spinal cord injury – When the axon bundles in the spinal cord are damaged, the connection between the brain and spinal cord gray matter is lost. This can cause paralysis and sensory issues, which are often permanent if neuronal bodies are damaged.