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Structure and Function of the Central Nervous System

Both the brain and spinal cord are protected by bones (the skull, or cranium, and the vertebrae, respectively) and membranes called meninges. Cerebrospinal fluid exists between layers of the meninges for further cushioning. The spinal cord functions as an intermediary between the brain and the spinal nerves, allowing bi-directional communication along its length. Neurons that exist solely within the spinal cord or brain are referred to as interneurons, and can be involved in many functions. The brain is the most important and complex organ of the nervous system, and while we are far from a complete understanding of the way it works, we can list its most important component parts and assign to each particular functions (see Figure 14.2).
  • Brainstem: The brainstem connects directly with the spinal cord. It is remarkably similar in all vertebrates, and regulates essential functions such as heartrate and respiration. Often the brainstem is referred to as the medulla oblongata, although technically the medulla oblongata is only one part of the brainstem.
  • Cerebellum: Located near the base of the brain and dorsal to the brainstem, the cerebellum is responsible for the complex coordination of muscular movements and the maintenance of balance. It is especially large and important in birds, as it plays a crucial role in the coordination of flight.
  • Diencephalon: Located just above and ventral to the brainstem, the diencephalon consists of several parts and is associated with various functions. The thalamus is involved with the selective sorting and relaying of sensory information to the cerebral cortex. The hypothalamus is important in the maintenance of homeostasis, specifically regulating body temperature, water/solute balance, and sensations of hunger and thirst, among other parameters. It is able to produce hormones to aid in its functioning, and therefore is sometimes referred to as a gland. Both the hypothalamus and the thalamus, in association with certain portions of the cerebrum, also comprise what is known as the limbic system. The limbic system seems to be associated with the generation of emotions and the control of certain basic behavioral responses. For example, by responding to thoughts or circumstances with a feeling of anger, aggressive behavior might be initiated. Emotions are thought to have evolved precisely for this purpose; for example, fear signals danger, and an organism may have a better chance of survival if it flees a threatening situation. The limbic system also plays a major role in sexual stimulation and behavior, clearly another important function if a species is to remain in existence.
Cerebrum: The cerebrum is comprised of two hemispheres connected by a structure called the corpus callosum. The cerebral cortex is a relatively thin layer of tissue that forms the outer surface of the cerebrum. In general, the cerebrum is responsible for the most complex nervous functions in an organism. In all vertebrates, this includes the integration of the sensory reception, processing, and motor response functions which we have already mentioned. The cerebral cortex is the most variable portion of the brain in different vertebrates. Fishes and amphibians have no cerebral cortex whatsoever, and even birds and reptiles possess only a rudimentary one. In lower mammals it is more apparent, culminating with the large and folded structure characteristic of primates and humans. The cortex is responsible for all of the so-called “higher brain functions” of humans, including the processes of thought and reasoning, and the capacity for complex memory storage, learning, and language.

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