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,…

The spinal cord is the traditional starting point for a detailed consideration of the central nervous system (CNS). It is a uniformly organized part of the CNS and one of the simplest (in a relative sense), but many principles of cord function also apply to other levels of the nervous system. At the same time, the spinal cord is extraordinarily important in the day-to-day activities we…

The ongoing activity and output of the central nervous system (CNS) are greatly influenced, and sometimes more or less determined, by incoming sensory information. An example is our constant awareness of the position of our limbs in space and the use of this awareness in guiding movements. The basis of this incoming sensory information is an array of sensory receptors, a a Receptor is a term…

Early in the last century, Ramón y Cajal and others used the Golgi stain to demonstrate that the nervous system is a collection of individual neurons (e.g., see Fig. 1.16A ) rather than a vast syncytial network, as some had alleged. An obvious corollary of this demonstration is that neurons must have mechanisms by which they communicate with one another. Although there are some instances in…

We depend on our brains to process and convey huge quantities of information rapidly and reliably. As a biological system, the brain must do this using neurons and their axons and synapses rather than wires and transistors. This makes the task more difficult, because it is substantially harder to move electrical signals around in the aqueous medium inside and surrounding neurons than in more conventional electronic…

Turtles can walk around for hours with no oxygen supply to their brains. In contrast, our brains are absolutely dependent on a continuous supply of well-oxygenated blood. After just 10 seconds of brain ischemia, we lose consciousness. After 20 seconds, electrical activity ceases, and after just a few minutes, irreversible damage usually begins. Corresponding to this metabolic dependence, blood vessels in the central nervous system (CNS),…

The cavity of the embryonic neural tube develops into a continuous, fluid-filled system of ventricles lined with ependymal cells; each division of the central nervous system (CNS) contains a portion of this ventricular system. Cerebrospinal fluid (CSF) is formed within the ventricles, fills them, and emerges from apertures in the fourth ventricle to fill the subarachnoid space. CSF is responsible for suspension of the brain through…

Living brain is soft and mushy, despite the network of cytoskeletal proteins contained in neurons and glial cells. Without support of some kind, the central nervous system (CNS) would be unable to maintain its shape, particularly as we walk and run around and occasionally bump our heads. The brain and spinal cord are protected from outside forces by their encasement in the skull and vertebral column,…