Chapter 6. The Central Nervous System
6.3 Brain Structure
Learning Objectives
By the end of this section, you will be able to:
- differentiate between white and gray matter;
- differentiate between (1) a nucleus and a ganglion and (2) a nerve and a tract; and
- label gyrus and sulcus on a diagram.
As we start learning about the central nervous system, a few terms and concepts will be used frequently. We will explore them in this section.
Gray and White Matter
Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure 6.3.1 demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin. Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.
Cell Bodies and Axons
Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the central nervous system (CNS) is referred to as a nucleus. In the peripheral nervous system (PNS), a cluster of neuron cell bodies is referred to as a ganglion.
Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract, whereas the same thing in the PNS would be called a nerve. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 6.3.2). A similar situation outside of science can be described for some roads. Imagine a road called “Broad Street” in a town called “Anyville.” The road leaves Anyville and goes to the next town over, called “Hometown.” When the road crosses the line between the two towns and is in Hometown, its name changes to “Main Street.” That is the idea behind the naming of the retinal axons. In the PNS, they are called the optic nerve, and in the CNS, they are the optic tract. Table 6.1 helps to clarify which of these terms apply to the central or peripheral nervous systems.
| Category | CNS | PNS |
|---|---|---|
| Group of neuron cell bodies (i.e., gray matter) | Nucleus | Ganglion |
| Bundle of axons (i.e., white matter) | Tract | Nerve |
Gyrus and Sulcus
The surface human brain is not a smooth structure. It contains ridges and grooves. A gyrus (plural gyri) is the ridge of one of those wrinkles, and a sulcus (plural sulci) is the groove between two gyri. A fissure is a deeper sulcus. A good way to remember them is that sulcus is similar to the word sulking, which can mean being or feeling down.
Section Review
Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.
Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found, and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.
Review Questions
Glossary
- ganglion
- localized collection of neuron cell bodies in the peripheral nervous system
- gray matter
- regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter
- gyrus
- ridge formed by convolutions on the surface of the cerebrum or cerebellum
- nerve
- cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system
- nucleus
- in the nervous system, a localized collection of neuron cell bodies that are functionally related; a “center” of neural function
- sulcus
- groove formed by convolutions in the surface of the cerebral cortex
- tract
- bundle of axons in the central nervous system having the same function and point of origin
- white matter
- regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin
Glossary Flashcards
This work, Human Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.
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