Discovering the Brain (1992) / Chapter Skim
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2 Major Structures and Functions of the Brain
2 Major Structures and Functions of the Brain
Pages 13-33
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From page 13... ...
; if we envision it at all, we are likely to see it as a large, rounded walnut, grayish in color. This schematic image refers mainly to the cerebral cortex, the outermost layer that overlies most of the other brain structures like a fantastically wrinkled tissue wrapped around an orange.
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From page 14... ...
The brain owes its outer appearance of a walnut to the wrinkled and deeply folded cerebral cortex, which handles the innumerable signals responsible for perception and movement and also for mental processes. Below the surface of the cortex are packed a number of other specialized structures: the thalamus, an important relay station for the senses, and the hypothalamus, a meeting point between the nervous system and the endocrine system and between emotion and physical feeling.
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From page 15... ...
In the hindbrain is the fourth ventricle, continuous with the central canal of the spinal cord. A cavity in the forebrain becomes the third ventricle, which reacts further forward into the two lateral ventricles, one in each cerebral hemisphere.
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From page 16... ...
The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. The masses contain nerve cell bodies that sort information from four of the senses—sight, hearing, taste, and touch and relay it to the cerebral cortex.
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From page 17... ...
Also in the hypothalamus are neurons that monitor body temperature at the surface through nerve endings in the skin, and other neurons that monitor the blood flowing through this part of the brain itself, as an indicator of core body temperature. The front part of the hypothalamus contains neurons that act to lower body temperature by relaxing smooth muscle in the blood vessels, which causes them to dilate and increases the rate of heat loss from the skin.
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From page 18... ...
; for another, electrical stimulation of a portion of the hypothalamus has been shown to induce sleep in experimental animals, although the mechanism by which this works is not yet known. In all, the hypothalamus is a richly complex cubic centimeter of vital connections, which will continue to reward close study for some time to come.
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From page 19... ...
. The cerebellum thus is neither the sole initiator of movement nor a simple link in the chain of nerve impulses, but a site for the rerouting and in some cases refining of instructions for movement.
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From page 20... ...
RETICULAR NETWORK Some nerve fibers from the cerebellum also contribute to the reticular formation, a widespread network of neurons ("reticular" is derived from the Latin word for Ninety. This formation and some neurons in the thalamus, together with others from various sensory systems of the brain, make up the reticular activating system- "the means by which we maintain consciousness.
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From page 21... ...
~ ~ . because the amygdala is located near a common site of origin of epileptic seizures that is, in the temporal lobe of the cerebral hemispheres—epileptics sometimes experience unidentifiable or unpleasant odors or changes of mood as part of the aura preceding a seizure.
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From page 22... ...
These are bundles of nerve fibers that connect one region of the cerebral cortex with another, so that the hippocampus, as well as other parts of the limbic system, exchanges signals with the entire cerebral cortex. The hippocampus has been shown to be important for the consolidation of recently acquired information.
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From page 23... ...
In the parietal lobes, the cerebral cortex processes the signals that come from sensation; the temporal lobes are concerned with memory, hearing, and, in Wernicke's area, with the ability to understand language. The occipital lobes are specialized to manage the intricate processing of vision.
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From page 25... ...
A fifth lobe, known as the insula, is located deep within the parietal and temporal lobes and is not apparent as a separate structure on the outer surface of the cerebral hemispheres. Two noticeable bulges, the precentral gyros and the postcentral gyros, are named for their positions just in front of and just behind the central sulcus, respectively.
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From page 26... ...
of human evolution. Medical imaging shows increased activity in the association cortex after other areas of the brain have received electrical stimulation and also before the initiation of movement.
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From page 27... ...
Among its other functions, the prefrontal cortex is responsible for inhibiting inappropriate behavior, for keeping the mind focused on goals, and for providing continuity in the thought process. Long-term memory has not yet been found to reside in any exclusive part of the brain, but experimental findings indicate that the temporal lobes contribute to this function.
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From page 28... ...
This is often called Wernicke's area, after the German neurologist Karl Wernicke, who in the late ISOOs laid the basis for much of our current understanding of how the brain encodes and decodes language. A bundle of nerve fibers connects Wernicke's area directly to Broca's area.
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From page 29... ...
From the outer surface inward, these are the molecular layer, made up for the most part of junctures between neurons for the exchange of signals; the external granular layer, mainly interneurons, which serve as communicating nerve bodies within a region; an external pyramidal layer, with large-bodied "principal" cells whose axons extend into other regions; an internal granular layer, the main termination point for fibers from the thalamus; a second, internal pyramidal layer, whose cells project their axons mostly to structures below the cortex; and a multiform layer, again containing principal cells, which in this case project to the thalamus. The layers vary in thickness at different sites on the cortex; for example, the granular layers (layers 2 and 4)
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From page 30... ...
The ability of the axon to conduct nerve impulses is greatly enhanced by the myelin sheath that surrounds it, interrupted at intervals by nodes. Myelin is a fatty substance, a natural electrical insulator, that protects the axon from interference by other nearby nerve impulses.
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From page 31... ...
It is gray matter that forms sheets of cortex on the surface of the cerebral hemispheres. White matter receives its name from the appearance of the myelin enclosing the elongated region of axons.
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From page 32... ...
During this time the sodium channels remain closed; the membrane is in a refractory phase. An action potential the very brief pulse of positive membrane voltage is transmitted forward along the axon; it is prevented from propagating backward as long as the sodium channels remain closed.
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From page 33... ...
Interestingly, in the membrane's overall balance sheet, the importance of a particular synapse varies with its proximity to where the axon leaves the nerve cell body, so that numerous excitatory potentials out at the ends of the dendrites may be overruled by several inhibitory potentials closer to the some. Other kinds of synapse regulate the release of neurotransmitters into the synaptic cleft, where they go on to affect the postsynaptic channels as described above.
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