We know that neurons are encapsulated by myelin. But what makes the myelin?


The brain contains two major classes of cells: neurons and glia. Glia are responsible for creating the myelin sheath, as well as having many other functions.

There are different kinds of glia, including Schwann cells, oligodendrocytes, astroctytes, microglia, and more. The Schwann cells and the oligodendrocytes are responsible for the myelin sheath. Schwann cells do so in the peripheral nervous system (PNS;part of nervous system that isn’t the brain or spinal cord, so, effectually, the nerves), while the oligodendrocytes are responsible for myelin in the central nervous system (CNS;brain and spinal cord).


Both Schwann cells and oligodendrocytes produce thin sheets of myelin that wrap many times around an axon. The myelin in the PNS vs the CNS are similar, but still have differences.




Major Principles of Neuronal Functioning


Neurons themselves are classified by how many processes, or extensions, they have. Neurons have extensions on one end usually called dendrites, which act as receiving cables for electrical signals. Dendrites, which mean trees, are like the rabbit ears of an old television set, receiving signals. A neuron is also made up of an axon, down which the action potential, or nerve impulse, travels. Neurons, therefore, are called either unipolar, bipolar, pseudo-unipolar or multipolar, based on the number of processes they have. Unipolar neurons have one branch that serves as the axon. Bipolar neurons, which are predominantly sensory neurons, have two processes, a dendrite and an axon. Pseudo-unipolar neurons are initially bipolar, but then the axon and dendrite fuse into a single, continuous process. The axon actually splits, one part being sensory, the other, motor. Multipolar neurons have one axon and a bunch of dendrites.

Human behavior depends not so much on the different kinds of neurons per se, but rather, more on neuronal organization into circuits that have specific functions. In other words, it is not variety that matters, it is connectivity.

A major organizational principle of the brain is that neurons with similar functions and properties can actually produce different actions based on the way they are interconnected. In other words, one dopaminergic neuron can differ from another dopaminergic neuron based on what connections it has made. Receptors are also a key issue here: what/which neurons connect to what/which receptors is important.

Another key principle of brain function is that the information conveyed by an action potential is based not so much on the signal’s form, but more on the pathway the signals travels along in the brain. Again, connectivity is the key here.

Neurons do not connect randomly with one another within the context of neuronal networks. Rather each neuron makes specific connections with certain neurons, but not with others. The connectivity has specificity, in other words.

Another major principle of the brain is the principle of dynamic polarization. This indicates that electrical signals going through a neuron does so only in one direction. This usually occurs down the axon to the next neuron. Of course, there are exceptions, but most neurons operate in this fashion.