Hockenbury Discovering Psychology 5th txtbk

56 CHAPTER 2 Neuroscience and BehaviorFigure 2.7 The Nervous System Thenervous system is a complex, organizedcommunication network that is dividedinto two main divisions: the centralnervous system (shown in blue) and theperipheral nervous system (shown inyellow).system and the peripheral nervous system (see Figure 2.7). In orderfor even simple behaviors to occur, such as curling your toesor scratching your nose, these two divisions must function as asingle, integrated unit. Yet each of these divisions is highlyspecialized and performs different tasks.The neuron is the most important transmitter of messagesin the central nervous system. In the peripheralnervous system, communication occurs along nerves.Nerves and neurons are not the same thing. Nerves aremade up of large bundles of neuron axons. Unlike neurons,many nerves are large enough to be seen easily withthe unaided eye.nervesBundles of neuron axons that carry informationin the peripheral nervous system.central nervous system (CNS)Division of the nervous system that consistsof the brain and spinal cord.spinal reflexesSimple, automatic behaviors that areprocessed in the spinal cord.Nerves and Neurons Are Not the SameA cross section of a peripheral nerve isshown in this electron micrograph. Eachblack circle represents the end of oneaxon. As you can see, a nerve is actuallycomposed of bundles of neuron axons.The Central Nervous SystemThe central nervous system (CNS) includes the brainand the spinal cord. The central nervous system is so criticalto your ability to function that it is entirely protectedby bone—the brain by your skull and the spinal cord byyour spinal column. Surrounding and protecting the brainand the spinal cord are three layers of membranous tissues,called the meninges. As an added measure of protection, thebrain and spinal cord are suspended in cerebrospinal fluid toprotect them from being jarred. There are four hollow cavitiesin the brain, called ventricles, which are also filled withcerebrospinal fluid. The inner surfaces of the ventricles arelined with neural stem cells, specialized cells that produceneurons in the developing brain.The central nervous system is aptly named. It is centralto all your behaviors and mental processes. And it is thecentral processing center—every action, thought, feeling,and sensation you experience is processed through the central nervous system. Themost important element of the central nervous system is, of course, the brain, whichacts as the command center. We’ll take a tour of the human brain in a later section.The spinal cord handles both incoming and outgoing messages. Sensory receptorssend messages along sensory nerves to the spinal cord, then up to the brain. Toactivate muscles, the brain sends signals down the spinal cord, which are relayed outalong motor nerves to the muscles.Most behaviors are controlled by your brain. However, the spinal cord can producespinal reflexes—simple, automatic behaviors that occur without any brain involvement.For example, the withdrawal reflex occurs when you touch a painfulstimulus, such as something hot, electrified, or sharp. As you can see in Figure 2.8,this simple reflex involves a loop of rapid communication among sensory neurons,which communicate sensation to the spinal cord; interneurons, which relay informationwithin the spinal cord; and motor neurons, which signal the muscles to react.Spinal reflexes are crucial to your survival. The additional few seconds that itwould take you to consciously process sensations and decide how to react couldresult in serious injury. Spinal reflexesare also important as indicators thatthe neural pathways in your spinalcord are working correctly. That’swhy physicians test spinal reflexesduring neurological examinations bytapping just below your kneecap forthe knee-jerk spinal reflex or scratchingthe sole of your foot for the toecurlspinal reflex.