Gray and White Matter

Gray and White Matter in Our Brain

Neurons are connected to one another in their billions. Building on this, we must add that the cell bodies tend to group together, rather like debris on the surface of an expanse of water. When cell bodies clump together like this, the resultant tissue appears somewhat grayish The stringy connections between gray tissues, formed principally by the axons interconnecting the cell bodies, appear white by contrast (mainly because axons are surrounded by a sheath of fatty tissue, and fat has a white appearance). This is the basis of the famous distinction between gray matter and white matter. Collections of cell bodies are gray; the fiber connections between them are white.

The cell bodies forming the gray matter group together in one or two ways – either as nuclei or in layers. The nuclei are simply balls of cell bodies, lumped together. The layers are more complicated. They are formed when the cell bodies line up in rows. The resultant sheets of cells are typically found on the outer surface of the brain – and form its cortex (“cortex” means outer layer). There is a shortage of space in the human cranium, because the amount of cortex has expanded dramatically in recent evolution; so the brain saves space by folding the layers in upon themselves, in a wavelike pattern. This  is what gives the outer surface of the brain its well-known convoluted appearance. The nuclei lie deeper within the brain, underneath these layers of cortex – and the white matter is located between the two. The white matter – principally axons – thereby connects the cell bodies of the nuclei and cortical layers with one another. The precise anatomy of the resultant systems is enormously complex, but these basic principles are easier to understand.

Mind and brain are entwined like yin and yang. Illustration by Elena.

Brainstem and forebrain

A further basic division of the brain is that between the brainstem and the forebrain. This is a distinction of great importance for understanding some of the psychological functions. These two structures are, in turn, intricately subdivided. There are innumerable terms for the various regions within them – often (and quite confusingly) more than one term for the same structure. The terms we mention here form the standard (or most widely accepted) terminology.

The brainstem is a direct extension of the spinal cord into the skull, and it is phylogenetically (i.e., in evolutionary terms) the most ancient part of the brain. The best way to depict it is by slicing the brain down the midline to produce a medial view. The lowest portion of the brainstem, the part immediately adjoining the spinal cord, is the medulla oblongata (Latin for “oblong core”) - a structure that has little to do with what is traditionally conceived of as “the mind” (the medulla contains nuclei that govern heartbeat, breathing, etc.) Above the medulla oblongata is the pons (Latin for “bridge”). Hanging behind the pons is the cerebellum (“little brain”). The top of the brainstem proper is the midbrain.

Above this region are structures that are not technically part of the brainstem (opinions on this point have changed over the years), but they are very closely connect in functional terms to the medulla oblongata, pons, and midbrain. These structures are referred to as the diencephalon. There is no generally accepted English word for this region of the brain, although at one time it was called the “twixt-brain,” which conveyed the essential fact that it lies between the brainstem and the forebrain. There are two main parts to the diencephalon. The largest, the upper portion, is the thalamus. Below the thalamus lies the hypothalamus, which is directly connected to the pituitary gland. All of these brainstem and diencephalic structures contain nuclei that are connected to one another (and to the forebrain structures) in intricate patterns.

The forebrain is phylogenetically younger than the brainstem. It consists principally of the two great cerebral hemispheres that fill the vault of the cranium. The outer surface of these hemispheres is the cerebral cortex, made up of folded layers of gray matter. Within the cerebral hemispheres, and hidden from view, are various forebrain nuclei.

Each hemisphere is divided into four lobes. At the back of the head is the occipital lobe; in the center is the parietal lobe (situated above and slightly behind the ears); below and in front of the parietal lobe is the temporal lobe (at the temples); the reminder of the hemisphere is the large frontal lobe, which lies over the eyes and is perhaps our greatest (and, in parts, uniquely human) phylogenetic acquisition. Buried between these lobes, if one pulls the temporal lobe down and lifts the frontal and parietal lobes up, lies a further region of cerebral cortex known as the insula.

Inside the cerebral hemispheres are the forebrain nuclei. The most substantial such nuclei are the basal ganglia. Close to the basal ganglia, nestled within the lower half of the frontal lobe, are the basal forebrain nuclei. Behind them, inside the anterior ((I.e.front) part of the temporal lobe, is the amygdala (Latin for “almond,” which this group of nuclei resembles in shape).

The Brain and the Inner World, Introduction to Basic Concepts. Mark Solms, Oliver Turnbull.

Although neuroscience and psychiatry/psychology have struggled to illuminate mind and brain in its own way, it is now clear that both need to work together intimately for any comprehensive understanding to emerge. Illustration by Elena.