Thus it controls the rate of the heart, the passage of food through the alimentary canal, and contraction of the bladder. At the autonomic level, the hypothalamus stimulates smooth muscle (which lines the blood vessels, stomach, and intestines) and receives sensory impulses from these areas. The hypothalamus, despite its relatively small size (roughly that of a thumbnail), controls a number of drives essential for the functioning of a wide-ranging omnivorous social mammal. (Only the sense of smell sends signals directly to the cortex, bypassing the thalamus.) Sensations of pain, temperature, and pressure are also relayed through the thalamus, as are the nerve impulses from the cerebral hemispheres that initiate voluntary movement. 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. The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. Nerves in the midbrain also control some movements of the eyeball, pupil, and lens and reflexes of the eyes, head, and trunk.ĭeep in the core area of the brain, just above the top of the brainstem, are structures that have a great deal to do with perception, movement, and the body's vital functions. What had been the middle bulge in the neural tube develops into the midbrain, which functions mainly as a relay center for sensory and motor nerve impulses between the pons and spinal cord and the thalamus and cerebral cortex. Together with nerves of the medulla, nerves from the pons also control breathing and the body's sense of equilibrium. Many nerves for the face and head have their origin in the pons, and these nerves regulate some movements of the eyeball, facial expression, salivation, and taste. In addition, nerve fibers running through the pons relay sensations of touch from the spinal cord to the upper brain centers. Nerve impulses traversing the pons pass on to the cerebellum (or "little brain"), which is concerned primarily with the coordination of complex muscular movement. In addition to being the major site of crossover for nerve tracts running to and from the brain, the medulla is the seat of several pairs of nerves for organs of the chest and abdomen, for movements of the shoulder and head, for swallowing, salivation, and taste, and for hearing and equilibrium.Īt the top of the brainstem is the pons-literally, a bridge-between the lower brainstem and the midbrain. Thus, the left brain controls movement of the right side of the body, and the right brain controls movement of the left side. Through the medulla, at the lower end of the brainstem, pass all the nerves running between the spinal cord and the brain in the pyramids of the medulla, many of these nerve tracts for motor signals cross over from one side of the body to the other. This region is also an important junction for the control of deliberate movement. The brainstem, at the top of the spinal cord, controls breathing, the beating of the heart, and the diameter of blood vessels. The hindbrain contains several structures that regulate autonomic functions, which are essential to survival and not under our conscious control. The human brain actually has its beginnings, in the four-week-old embryo, as a simple series of bulges at one end of the neural tube. In the growth of the individual embryo, as well as in evolutionary history, the brain develops roughly from the base of the skull up and outward. Indeed, in strictly biological terms, these structures can claim priority over the cerebral cortex. But underneath this layer reside many other specialized structures that are essential for movement, consciousness, sexuality, the action of our five senses, and more-all equally valuable to human existence. The cortex contains the physical structures responsible for most of what we call ''brainwork": cognition, mental imagery, the highly sophisticated processing of visual information, and the ability to produce and understand language. The preponderance of the cerebral cortex (which, with its supporting structures, makes up approximately 80 percent of the brain's total volume) is actually a recent development in the course of evolution. 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. We do not experience our brain as an assembly of physical structures (nor would we wish to, perhaps) if we envision it at all, we are likely to see it as a large, rounded walnut, grayish in color. Outside the specialized world of neuroanatomy and for most of the uses of daily life, the brain is more or less an abstract entity.
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