One approach to an understanding of the nervous system is to conceptualize that it is composed of a number of functional modules, starting with simpler ones and evolving in higher primates and humans to a more complex organizational network of cells and connections. The function of each part is dependent upon and linked to the function of all the modules acting in concert.
The basic unit of the CNS is the spinal cord (see Figure 1 and Figure 2), which connects the CNS with the skin and muscles of the body. Simple and complex reflex circuits are located within the spinal cord. It receives sensory information (afferents) from the skin and body wall, which are then transmitted to higher centers of the brain. The spinal cord receives movement instructions from the higher centers and sends motor commands (efferents) to the muscles. Certain motor patterns are organized in the spinal cord, and these are under the influence of motor areas in other parts of the brain. The autonomic nervous system, which supplies the internal organs and the glands, is also found within the spinal cord.
As the functional systems of the brain become more complex, new control "centers" have evolved. These are often spoken of as higher centers. The first set of these is located in the brainstem, which is situated above the spinal cord and within the skull (in humans). The brain-stem includes three distinct areas — the medulla, pons, and midbrain (see Figure OA, Figure OL, Figure 6, and Figure 7). Some nuclei within the brainstem are concerned with essential functions such as pulse, respiration, and the regulation of blood pressure. Other nuclei within the brainstem are involved in setting our level of arousal and play an important role in maintaining our state of consciousness. Special nuclei in the brainstem are responsible for some basic types of movements in response to gravity or sound. In addition, most of the cranial nerves and their nuclei, which supply the structures of the head, are anchored in the brainstem (see Figure 8A and Figure 8B). Many nuclei in the brainstem are related to the cerebellum.
The cerebellum has strong connections with the brainstem and is situated behind the brainstem (inside the skull) in humans (see Figure OA, Figure OL, and Figure 9A). The cerebellum has a simpler form of cortex, which consists of only three layers. Parts of the cerebellum are quite old in the evolutionary sense, and parts are relatively newer. This "little brain" is involved in motor coordination and also in the planning of movements. How this is accomplished will be understood once the input/output connections of the various parts of the cerebellum are studied.
Next in the hierarchy of the development of the CNS is the area of the brain called the diencephalon (see Figure OA, Figure OL, and Figure 11). Its largest part, the thalamus, develops in conjunction with the cerebral hemi spheres and acts as the gateway to the cerebral cortex. The thalamus consists of several nuclei, each of which projects to a part of the cerebral cortex and receives reciprocal connections from the cortex. The hypothalamus, a much smaller part of the diencephalon, serves mostly to control the neuroendocrine system via the pituitary gland, and also organizes the activity of the autonomic nervous system. Parts of the hypothalamus are intimately connected with the expression of basic drives (e.g., hunger and thirst), with the regulation of water in our bodies, and with the manifestations of "emotional" behavior as part of the limbic system (see below).
With the continued evolution of the brain, the part of the brain called the forebrain undergoes increased development, a process called encephalization. This has culminated in the development of the cerebral hemispheres, which dominate the brains of higher mammals, reaching its zenith (so we think) in humans. The neurons of the cerebral hemispheres are found at the surface, the cerebral cortex (see Figure 13 and Figure 14A), most of which is six-layered (also called the neocortex). In humans, the cerebral cortex is thrown into ridges (gyri, singular gyrus) and valleys (sulci, singular sulcus). The enormous expansion of the cerebral cortex in the human, both in terms of size and complexity, has resulted in this part of the brain becoming the dominant controller of the CNS, capable, so it seems, of overriding most of the other regulatory systems. We need our cerebral cortex for almost all interpretations and actions related to the functioning of the sensory and motor systems, for consciousness, language, and thinking.
Buried within the cerebral hemispheres are the basal ganglia, large collections of neurons (see Figure OA, Figure OL, and Figure 22) that are involved mainly in the initiation and organization of motor movements. These neurons affect motor activity through their influence on the cerebral cortex.
A number of areas of the brain are involved in behavior, which is characterized by the reaction of the animal or person to situations. This reaction is often termed "emotional" and, in humans, consists of both psychological and physiological changes. Various parts of the brain are involved with these activities, and collectively they have been named the limbic system. This network includes the cortex, various subcortical areas, parts of the basal ganglia, the hypothalamus and parts of the brainstem. (The limbic system is described in Section D of this atlas.)
In summary, the nervous system has evolved so that its various parts have "assigned tasks." In order for the nervous system to function properly, there must be communication between the various parts. Some of these links are the major sensory and motor pathways, called tracts (or fascicles). Much of the mass of tissue in our hemispheres is made up of these pathways (e.g., see Figure 33 and Figure 45).
Within all parts of the CNS there are the remnants of the neural tube from which the brain developed; these spaces are filled with cerebrospinal fluid (CSF). The spaces in the cerebral hemispheres are actually quite large and are called ventricles (see Figure OA, Figure OL, Figure 20A, Figure 20B, and Figure 21).
The CNS is laced with blood vessels as neurons depend upon a continuous supply of oxygen and glucose. This aspect will be discussed further with the section on vasculature (e.g., see Figure 58).
Early studies of the normal brain were generally descriptive. Brain tissue does not have a firm consistency, and the brain needs to be fixed for gross and microscopic examination. One of the most common fixatives used to preserve the brain for study is formalin, after which it can be handled and sectioned. Areas containing predominantly neuronal cell bodies (and their dendrites and synapses) become grayish in appearance after formalin fixation, and this is traditionally called gray matter. Tracts containing myelinated axons become white in color with formalin fixation, and such areas are likewise simply called the white matter (see Figure 27 and Figure 29).
We have learned much about the normal function of the human CNS through diseases and injuries to the nervous system. Diseases of the nervous system can involve the neurons, either directly (e.g., metabolic disease) or by reducing the blood supply, which is critical for the viability of nerve cells. Some degenerative diseases affect a particular group of neurons. Other diseases can affect the cells supporting the myelin sheath, thereby disrupting neurotransmission. Biochemical disturbances may disrupt the balance of neurotransmitters and cause functional disease states.
The recent introduction of functional imaging of the nervous system is revealing fascinating information about the functional organization of the CNS. We are slowly beginning to piece together an understanding of what is considered by many as the last and most important frontier of human knowledge, an understanding of the brain.
Certain aspects of clinical neurology will be included in this atlas, both to amplify the text and to indicate the importance of knowing the functional anatomy of the CNS. Knowing where a lesion is located (the localization) often indicates the nature of the disease (the diagnosis), leading to treatment and allowing the physician to discuss the prognosis with the patient.
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