Myelin

Myelin is a critical, fatty substance that encases the axons of nerve cells, enabling the swift and efficient transmission of electrical impulses throughout the nervous system^[1]. Functioning like the insulation around an electrical wire, myelin protects the signal from interference and enhances the speed of impulse propagation along the myelinated fibre, which is essential for the intricate and rapid communication required for optimal brain function^[2].

Phospholipids are a key component of the myelin sheath and contribute significantly to its function and structure. ^[3]. A phospholipid molecule consists of a hydrophilic (water-attracting) head and one or more hydrophobic (water-repelling) tails. These properties make them ideal for forming biological membranes such as the myelin sheath^[4]. The phospholipids' hydrophobic tails face inward towards each other, while the hydrophilic heads face outward, interacting with the cell's aqueous environment, thereby forming a stable, protective, and insulating layer around the axon^[5].

The whitish appearance of myelin gives the "white matter" in our brain its name. This is due to the high lipid content of the myelin sheath. Phospholipids, along with other lipids like cholesterol and glycolipids, contribute to this white appearance due to their light refracting and reflecting properties^[6]. Areas of the brain dense with myelinated axons - the neural "highways" connecting different regions - have a whitish hue^[7]. It's this network of myelinated fibers, appearing white in contrast to the "gray matter" (comprised of cell bodies of neurons, dendrites, and unmyelinated axons), that is referred to as white matter.^[8].

A common way to visualise myelin is using the Luxol Fast Blue stain, a basic copper phthalocyanine dye used in histology^[9]. An example is in the header image above. This stain reacts with the lipid component of the myelin sheath, staining it blue or blue-green, allowing visualization of myelinated fibers' distribution in tissue samples^[10]. If the myelin is damaged or depleted, as in conditions like multiple sclerosis, this will be evident in the staining^[11].

Myelin is produced by two types of cells: oligodendrocytes in the central nervous system (CNS, which includes the brain and spinal cord) and Schwann cells in the peripheral nervous system (PNS, all the other nerves in the body). Oligodendrocytes can form myelin sheaths around several axons simultaneously, while each Schwann cell typically myelinates a single axon^[13].

The process of myelination, or laying down this fatty sheath around neurons, isn't something that happens all at once. It begins in utero and continues throughout childhood and adolescence. Some research suggests it can continue into the third decade of life, particularly in the frontal lobes, which are associated with executive functions like decision-making and self-control ^[15]. This protracted development may help explain why these functions continue to mature well into early adulthood.

In humans, the process of myelination follows a general pattern from back to front, and from sensory and motor regions to higher cognitive regions ^[16]. This means that regions involved in basic body functions and sensory processing become myelinated before areas involved in complex cognition and social-emotional processing. The latter includes the speech and language centres of the brain, like Broca's and Wernicke's areas, which are found in the left hemisphere in most right-handed individuals ^[17]. As these areas become more fully myelinated, children and adolescents become better able to produce, understand, and interpret speech and language, which is critical for successful social communication.

There's a lot of similarity in the structure of myelin across different species. However, the thickness of the myelin sheath and the periodicity (distance between adjacent layers of wrapped myelin) can differ. For example, in humans, the periodicity is typically around 11-12 nm, but in other mammals, like cats or mice, it can be slightly less ^[18]. These small differences might have a significant impact on the speed of neural transmission.

Disruptions to myelin can lead to various illnesses, many of which can be severely debilitating. Multiple sclerosis (MS) is one of the most well-known of these. In MS, the immune system mistakenly attacks the myelin sheaths in the CNS, causing a range of symptoms from fatigue and difficulty walking to problems with speech and swallowing ^[19]. Other myelin disorders include leukodystrophies, a group of rare, genetic disorders that primarily affect children.