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Previous chapters may have hinted that the parts of the nervous system are interconnected in rigid, immutable ways, but this is far from accurate. The details of the nervous system are much too complex to be completely laid out genetically. Instead, only the general layout of the nervous system is specified genetically; later stages of development are a time of great plasticity, in which neurons and…

We seldom perceive things in a completely neutral fashion. Various sights and sounds make us happy, sad, or angry; some tastes and smells are extremely gratifying, others disgusting. Such feelings engendered by sensory inputs are ultimately the result of brains wired to promote survival and reproduction, and they are variable depending on current physiological needs and the social situation. Food, for example, becomes more attractive when…

The cerebral cortex is a sheet of neurons and their interconnections. About 1800 cm 2 (2 ft 2 ) in surface area, the cortex covers the corrugated surface of the cerebral hemispheres in a layer just a few millimeters thick. This thin layer of gray matter accounts for nearly half the weight of the brain and is estimated to contain about 25 billion neurons, interconnected by more…

Photoreceptors are sensitive but slow, and all animals with image-forming eyes have mechanisms to prevent images of interest from moving across the retina too quickly to be analyzed. (Our eyes behave in many respects like cameras with a shutter speed of about of a second.) A widespread strategy is to use some combination of eye movements and head or body movements to keep the direction of…

Cerebellum means “little brain,” and in a real sense, it is: it accounts for only about 10% of the mass of the brain, but the cerebellum contains as many neurons as all the rest of the central nervous system (CNS) combined. This semidetached mass of neural tissue covers most of the posterior surface of the brainstem, tethered there by three pairs of fiber bundles called cerebellar…

In 1817 James Parkinson, an English country physician, published a brief monograph entitled “An Essay on the Shaking Palsy,” in which he described the symptoms of several individuals who had the disease that now bears his name. Patients with Parkinson's disease, as described in more detail later in this chapter, are characterized by tremor, generally increased muscle tone, and difficulty initiating voluntary movements, which are unusually…

Each of us has fewer than a million motor neurons with which to control muscles. Without them, we would be completely unable to interact with the outside world. With them, however, we are capable of an enormous range of complex activities, from automatic and semiautomatic movements such as postural adjustments to the characteristically human movements involved in speaking and writing. The way in which a wide…

It is clear from everyday experience that humans are a visually oriented species. Although it is arguable which of our senses is most important, loss of vision certainly has a greater impact on humans than loss of, for example, olfaction or taste. Partly because of its importance (and partly for anatomical and technical reasons discussed later), a great deal of research has been done on the…

The diencephalon, mostly hidden from view between the cerebral hemispheres ( Fig. 16.1A ), constitutes only about 2% of the central nervous system (CNS) by weight. Nevertheless, it has widespread and important connections, and the great majority of sensory, motor, and limbic pathways involve a stop in the diencephalon. Most motor and limbic pathways also involve telencephalic structures that are discussed in later chapters, so this…

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Hearing and balance are very different senses functionally, but they begin peripherally in very similar ways. The eighth cranial nerve carries two special sensory components, one in a cochlear division and one in a vestibular division. Both divisions innervate elaborate end-organs containing specialized mechanoreceptors (called hair cells because of their appearance), but accessory structures in the end-organs specialize the two divisions to respond to different types…

Dating back to their origins in some primordial sea, living cells have shared an ability to respond to chemicals, at a minimum detecting and absorbing nutrients. Neurons detect chemicals at synapses, but some cells in or closely associated with the nervous system go beyond this, specializing in the detection of certain classes of chemicals adjacent to their membranes and using this information to affect autonomic function,…

The caudal medulla looks somewhat similar to the spinal cord, but this similarity seems to disappear at more rostral levels of the brainstem. One of the complicating factors is the arrangement of the tracts and nuclei associated with cranial nerves III to XII. These tracts and nuclei appear discouragingly intricate on first inspection, but there is a common way of systematizing the cranial nerves so that…

The spinal cord continues rostrally into the brainstem ( Fig. 11.1 ), which performs spinal cord–like functions for the head. The brainstem contains the lower motor neurons for the muscles of the head and does the initial processing of general afferent information concerning the head. However, it does much more than this, reflecting in large part the additional functions of the cranial nerves attached to it,…