The nervous system informs the body about itself and the world around it and enables the body to react to this information. To accomplish this, the nervous system actively identifies, integrates and interprets incoming sensory stimuli, and produces electrochemical impulses that are distributed via peripheral nerves to generate responses to the environment and internal conditions.
The human nervous system is perhaps the most complex thing we know. This complexity is apparent at all levels of its structure, from the creases and folds of its outward appearance, to the dizzying number of connections made by the multitude of cells at the microscopic level. The study of this structure is called “neuroanatomy.”
In contrast to neurophysiology, neuroanatomy stresses form over function, the “what” over the “how” and “why.” This distinction arose because for most of the history of neuroscience, the intricate structure of the nervous system could only be studied after it was preserved and removed from the body. Thus, structure was preservable but function was not.
It is our great fortune to live in an age where new techniques are making it possible to observe anatomy and physiology simultaneously, enabling scientists to observe these intertwined aspects as they perform the tasks for which they were designed over hundreds of millions of years of evolution. However, this is not to say that we should minimize all of the previous anatomical work that painstakingly laid out the details of the structure of this system. Quite to the contrary, we must begin there, before the newer techniques can have any significant meaning for us.
Neuroanatomy, like all anatomy, can sometimes seem like a pointless exercise in the memorization of obscure names. To make matters worse, researchers have studied the nervous system from several points of view, and often they are inclined to create names for structures that reflect their particular point of view. Consequently, many structures have multiple names, and clear delineation between what constitutes a single structure is not always agreed upon in the various naming schemes.
This rich history makes neuroinformatics, the study of how to organize what we know about the nervous system, an extremely challenging field. The trick is to be as precise as possible, but at the same time, to realize that biology is inherently messy and full of variation. Imagine borders between two countries; some are sharply defined as by a river, and some lie somewhere within a more rugged terrain such as a mountain range. For the latter case, it is easy to draw a line on a map, but that doesn’t make the border any clearer when you’re down on the ground lost among the peaks.
Nevertheless, we will plunge ahead. One way to begin subdividing the nervous system is to break it into the central nervous system, and the two components of the peripheral nervous system: the visceral, or autonomic nervous system, and the voluntary, or somatic nervous system. The central nervous system, and specifically the brain, is our focus in this article. As its name implies, the central nervous system is the center to which the other two systems feed their sensory information and from which they derive their motor output. The somatic nervous system obtains sensory information from the external world and provides motor output to the voluntary muscles that allow us to move within it. The visceral nervous system receives input from, and sends output to, our internal organs, thus controlling involuntary functions such as blood flow, breathing, digestion, and so on.
The two main structures of the central nervous system are the brain and the spinal cord. As mentioned earlier, the brain is studied from several points of view, and each view gives some insights into how its structures can reasonably be parsed. From the point of view of development and how the brain forms within the embryo, the brain can be subdivided into the prosencephalon or forebrain, the mesencephalon or midbrain, and the rhombencephalon or hindbrain.
In humans, most of the mass of the brain is within the forebrain. The forebrain consists of the diencephalon, which includes structures such as the thalamus and hypothalamus. The hypothalamus is a center for hormonal control as well as an integral part of the limbic system which governs various emotional states. Nuclei within the hypothalamus have been carefully studied with respect to various behaviors including drug addiction.
The thalamus contains a number of nuclei that receive input from the various specific sensory receptors such as those for vision, hearing and touch. In fact, only our sense of smell, which is governed through an old, more primitive part of the brain, bypasses the thalamus on the way to the cortex. In the past, researchers minimized the role that the thalamus plays and even called it simply a relay station. More recently, it has been realized that the role of the thalamus is more complex, and that its function can not be isolated from that of the cortex because information is flowing continuously in both directions between the two structures.
When we look at a whole brain, what we mostly see are the convolutions of the cerebral cortex which, developmentally, is the telencephalon of the forebrain. It is to this structure that we attribute not only perception of the world and initiation of action, but higher order thought as well. The outer, visible regions of these convolutions are the gyri (singular: gyrus). The grooves are referred to as sulci (singular: sulcus), the deepest of which are sometimes called fissures.
The longitudinal fissure runs down the middle of the cortex and provides a prominent landmark for separating the brain into the left and right hemispheres. Two other prominent landmarks are the central sulcus which runs from the midline down the side of the brain and delineates the frontal lobe from the rest of the cortex, and the lateral fissure which delineates the temporal lobe. The remaining lobes, the occipital and the parietal, are separated by less prominent landmarks including the parieto-occipital sulcus and the occipital notch.
Each lobe can be divided into many regions, many of which we will discuss in future articles. Again, these regions are named according to the point of view of their namer. For example, the occipital lobe which is mostly devoted to vision, has a single, small area that has at least four names; the V1 or primary visual cortex, because it receives the earliest information from the eyes by way of the thalamus; the Brodmann area 17, because it was the 17th separate region identified by an anatomist who was looking at differences in cell densities in the different layers; and the striate cortex, because in cross section, it has a distinct band of white myelin within the cell layer.
This example illustrates the types of names we will encounter as we continue to explore the cortex. The Brodmann areas are particularly useful and commonly used because they subdivide the entire cortex into a distinct set of named structures. Function is not always known, but if the function of a particular area does become apparent, then it is often given one or more additional names either for the function itself, such as primary auditory cortex, or for the person who discovered the function, such as Wernicke’s or Broca’s areas. Areas of cortex are also named according to their anatomical location and gross appearance such as superior temporal gyrus or angular gyrus.
Neuroanatomy is challenging, as may be apparent in even this very superficial introduction. We can get lost in all of the names. But the names help researchers and clinicians identify and share the locations of their findings, and, as we progress, will help us to create a more coherent view of how the brain’s structure relates to its function.