The living brain is pale pink. "Preserved in formalin, it becomes firm and loses its color," writes Thomas B. Czerner in What Makes You Tick? The Brain in Plain English (2001). In this section we will discuss the brain cells of which the brain is structured. These cells are organized as either a nucleus or as cortex.
The preserved brain is largely gray in color, and therefore is referred to as gray matter. Gray matter is packed with billions of neurons. It has the "mushy consistency of cooked oatmeal," writes Czerner.
Since there are so many of them, it is tempting to visualize neurons as being tiny in terms of their reach; in reality, some neurons have axons that are very long. When multitudes of axons are grouped together, they appear as white matter. Czerner describes white matter as "a collection of long, insulated axons, outgoing branches of neurons that connect various parts of the brain to each other," owing the white color to "myelin, a glistening, lipid, insulating material wrapped around those long axons."
Antonio R. Damasio provides a succinct description of gray and white matter in Descartes' Error: Emotion, Reason, and the Human Brain (1994). "The gray matter corresponds largely to collections of nerve cell bodies, while the white matter corresponds largely to axons, or nerve fibers, emanating from cell bodies in the gray matter." Neurons are "supported by glial cells," that are "essential for brain activity," writes Damasio.
Czerner writes: "Each neuron is surrounded by ten to fifty glial cells, which make up the bulk of the brain. Glial cells serve the neuron in several ways. They provide structural support, insulate longer neural branches with myelin to speed electric signals down their axons, maintain nutrition, aid in waste removal and even lay down the markers that lead the branches of a developing neuron to its proper destination. A study of Albert Einstein's brain showed that he may have had an overabundance of glial cells."
"The gray matter comes in two varieties," Damasio writes. "In one variety the neurons are layered as in a cake and form a cortex. Examples are the cerebral cortex which covers the cerebral hemispheres, and the cerebellar cortex which envelops the cerebellum. In the second variety of gray matter the neurons are not layered and are organized instead like cashew nuts inside a bowl. They form a nucleus."
The best example of neurons organized as cortex is the neocortex, often called the cerebral cortex. The intact brain pictured above with all its convolutions (image links to source) is a good illustration. The neocortex is folded in a way that allows a larger surface area to fit within the confines of the human skull. Anatomists call each cortical fold a sulcus, and the smooth area between folds a gyrus. At the base of the specimen pictured above, where it would be situated at the lower rear of one's head, you can see the cerebellar cortex that Damasio discusses. The cerebellum's three lobes are situated adjacent to the brain stem.
When neurons are not layered as in cortex, as we discuss above, they are grouped into a nucleus. The plural of nucleus, as you may know, is nuclei. Understanding the two major ways in which neurons are organized in the brain—as cortex and as nuclei—is most important to making sense of the brain's complex anatomy. In the close-up of the dissected brain to the right, I have drawn a red oval around recessed nuclei (image links to source), which I will refer to as subcortical nuclei. Surrounding these subcortical nuclei is the neocortex. We discuss both subcortical nuclei and the neocortex in greater detail later in Part 1 of MyBrainNotes.com.
Regarding the image above and many others on this site, I would like to thank John A. Beal from the Department of Cellular Biology and Anatomy, Louisiana State University Health Sciences Center in Shreveport. Professor Beal has made these excellent images accessible through Wikipedia.
The major components of a neuron are detailed in the illustration to the left. The length of the axon can be very much longer, however, than that depicted in the illustration. Also, in his video course, Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, Robert M. Sapolsky illuminates the complexity of a single neuron. He explains that an average neuron may have 10,000 dendritic spines and 10,000 axon terminals. Communication between neurons is both chemical and electrical, so the term electrochemical is used to describe the overall process.
Very simplistically stated, chemical molecules called neurotransmitters carry messages from one neuron to the next. The vesicles on a neuron's axon terminals release neurotransmitter molecules. From a chemical perspective, a neurotransmitter molecule "fits like a key into a lock," says Sapolsky, as it contacts a receptor on a second neuron's dentritic spines. In Brainscapes: An Introduction to What Neuroscience Has Learned about the Structure, Function, and Abilities of the Brain (1995), Richard M. Restak explains that "Receptors are large dynamic protein molecules that exist along and within the cell membrane." Receptors can, explains Restak, "increase in number and avidity for their neurotransmitter according to circumstances. Large and prolonged intakes of certain substances, for instance, lead to an increase in the number of receptors for these substances—the basis for the withdrawal response in addiction…. Later, if the addicted person stays away from the drugs the receptors eventually die off…."
The union of neurotransmitter and receptor is either inhibitory or excitatory. "Each neurotransmitter influences its own receptor independent of the action of other receptors," writes Restak. "While some neurotransmitters decrease the voltage between the inside and the outside of the nerve cell and thus stimulate the cell into action (an excitatory neurotransmitter), others increase it and thus inhibit the cell from firing (an inhibitory neurotransmitter."
Once the neurotransmitter and receptor lock together, Restak describes their action as "dynamic" and "exquisitely sensitive." There are two families of receptors. Restak explains that ion channel receptors influence the activity of ion channels for sodium, potassium, calcium, and chloride, "principally, directly, or indirectly, via biochemical intermediates." He writes: "Ion channel receptors … bring about changes in membrane permeability and thus govern the flow of ions through their channel. When a neurotransmitter reacts with its receptor, their interaction results in a change in the shape of the receptor so that ions can then flow across the membrane from the point of high concentration toward the point of lower concentration." A second family of receptors "do not contain ion channels within their structures," explains Restak. Rather, they act through intermediaries, the G proteins, located inside the nerve cell. Restak writes: "Basically, G proteins function as coupling factors that serve as links between the receptor on the outside surface of the nerve cell membrane and a vast number of interlinked cellular processes within the cell."
Sufficient neurotransmitter excitation of ion channel receptors in dendritic spines creates a wave of ionic change that prompts the neuron's axon hillock to reach its action potential. An electrical signal thereby bursts down the neuron's axon (and axonal branches called collaterals) and into axonal terminals to prompt vesicles to release a chemical neurotransmitter. In other words, the neuron "fires." As vesicles release neurotransmitter molecules, each molecule floats through the microscopic synapse with the potential to make contact with a receptor. Neurons communicating with each other in one direction develop a circuit. Often, there are reciprocating circuits between brain structures.
Sapolsky explains that "neurotransmitters can be taken back up into the presynaptic terminal and repackaged into vesicles or an enzyme can rip up the neurotransmitter" in a process of degradation.
Local neuronal circuits in the neocortex constitute cortical regions. Not only do cortical regions connect with each other, but cortical regions and subcortical nuclei interconnect as well. We will discuss cortical-subcortical circuits in more detail in Parts 2 and 3 of MyBrainNotes.com. The photograph of neurons to the right is from the laboratory of Te-Won Lee.
Damasio explains that a "bundle of axons from a known source to a given target is often referred to as a projection, because the axons project to a particular collection of neurons. A sequence of projections across several target stations is known as a pathway." You can often see a pathway (white matter) with the naked eye. As I mentioned earlier, Czerner describes white matter as "a collection of long, insulated axons, outgoing branches of neurons that connect various parts of the brain to each other …."
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