Myelinogenesis is the process of sequential myelination or development of a myelin

Myelin

Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 sheaths around nerve fibres of the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

.

Function

The myelination process allows brain signals (neural signals) to propagate faster and with less signal loss. This enables better tissue connectivity within specific brain regions and also improves broader neural pathways connecting spatially separate regions required for many sensory, cognitive, and motor functions.

Some scientists consider myelination to be a key human evolutionary advantage, enabling faster processing speeds and leading to further brain specialization. Myelination is considered to be a key developmental stage in the human brain, continuing for at least another 10 to 12 years after birth "before even a general development is completed". Therefore, the rate of development of these brain structures will determine the rate of development of related brain functions.

While the rate at which individual children develop varies, the sequence of development is the same for all children (with a range of ages for specific developmental tasks to take place).

Process

Paul Flechsig

Paul Flechsig

Paul Emil Flechsig was a German neuroanatomist, psychiatrist and neuropathologist. He is mainly remembered today for his research of myelinogenesis.-Biography:...

 spent most of his career studying and publishing the details of the process in the cerebral cortex

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 of humans. This takes place mostly between two months before and after birth

Birth

Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. The time of human birth is defined as the time at which the fetus comes out of the mother's womb into the world...

. He identified 45 separate cortical areas and, in fact, mapped the cerebral cortex by the myelination pattern. The first cortical region to myelinate is in the motor cortex

Motor cortex

Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...

 (part of Brodmann

Brodmann

Brodmann may refer to:* Korbinian Brodmann, German neurologist* Brodmann area, a region in the brain cortex...

's area 4), the second is the olfactory cortex and the third is part of the somatosensory cortex (BA 3,1,2).

The last areas to myelinate are the anterior cingulate cortex (F#43), the inferior temporal cortex (F#44) and the dorsolateral prefrontal cortex (F#45).

"In the cerebral convolutions, as in all other parts of the central nervous system, the nerve-fibres do not develop everywhere simultaneously, but step by step in a definite succession, this order of events being particularly maintained in regard to the appearance of the medullary substance. In the convolutions of the cerebrum the investment with medullary substance (myelinisation) has already begun in some places three months before the maturity of the foetus, whilst in other places numerous fibres are devoid of medullary substance even three months after birth. The order of succession in the convolutions is governed by a law identical with the law which I have shown holds good for the spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

, the medulla oblongata

Medulla oblongata

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

, and the mesocephalon, and which may be stated somewhat in this way- that, speaking approximately, equally important nerve-fibres are developed simultaneously, but those of dissimilar importance are developed one after another in a succession defined by an imperative law (Fundamental Law of Myelogenesis). The formation of medullary substance is almost completed in certain convolutions at a time when in some it is not even begun and in others has made only slight progress..."

Stages

Although the mechanisms and processes of myelination are yet to be fully understood, especially complex gene function, some specific stages in this process have become clear:

• Stage 1: Axon
  Axon
  An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
  
   contact
• Stage 2: Glial cell
  Glial cell
  Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...
  
  gene production
• Stage 3: Axon ensheathment
• Stage 4: Maturation.

Many key questions about the myelination process still remain:

• Why axons of similar size seem to myelinate at the same time;
• Whether proteins, molecules and genes are involved in determining axon size;
• If axonal activity influences the process, if there is a causal relationship, or if there is a more complex combined effect;
• Why certain processes seem to be lost if myelination does not occur.