Nervous System Organization Achievement Print this pagemrwolman.weebly.com/uploads/3/8/0/3/38036553/nervous_system... · involved in all voluntary movements and can even initiate

2/20/2015 Nervous System Organization

http://edugen.wileyplus.com/edugen/courses/crs2878/huffman9110/huffman9110c02/aHVmZm1hbjkxMTBjMDJfM18wLnhmb3Jt.enc?course=crs2878&id=ref 1/13

Print this page

Nervous System OrganizationAchievement

Objective 2.4:

Describe the nervous systemʹs two major divisions, and explain their respective functions.

To fully comprehend the complex intricacies of the nervous system, it helps to start with an overall map or “bigpicture.” Look at Figure 2.4. Note the organization of the nervous system as a whole, and how it is divided andsubdivided into several branches. Now, look at the drawing of the body in the middle of Figure 2.4, and imagine it asyour own. Visualize your entire nervous system as two separate, but interrelated, parts—the central nervous system(CNS) and the peripheral nervous system (PNS). The first part, the CNS, consists of your brain and a bundle ofnerves (your spinal cord) that runs through your spinal column. Because it is located in the center of your body (withinyour skull and spine), it is called the central nervous system (CNS). Your CNS is primarily responsible for processingand organizing information.



Figure 2.4 Organizational and functional divisions of the nervous system When attempting to learn and memorize a largeset of new terms and concepts, like those in this chapter, organization is the best way to master the material and get it“permanently” stored in longterm memory (Chapter 7). A broad overview showing the “big picture” helps youorganize and file specific details. Just as you need to see a globe of the world showing all the continents to easilyplace individual countries, you need a “map” of the entire nervous system to effectively study the individual parts.Note how the nervous system is divided and subdivided into various subsystems according to their differing functionsin this figure. It will help orient you as you study upcoming sections.

Now, picture the many nerves that lie outside your skull and spine. This is the second major part of your nervoussystem—the PNS. Because it carries messages (action potentials) between the central nervous system and the peripheryof the body, it is known as the peripheral nervous system (PNS). Your PNS links your CNS to your bodyʹs sensereceptors (skin, eyes, ears, etc.), muscles, and glands.

Having completed our quick overview of the complete nervous system, we can go inside each of these two divisionsfor a closer look. Letʹs begin with the central nervous system (CNS).

Central Nervous System (CNS): Brain and Spinal Cord

Although we seldom think about it, our central nervous system (CNS) is what makes us unique and special. Mostother animals can smell, run, see, and hear far better than we can. But thanks to our CNS, we can process informationand adapt to our environment in ways that no other animal can. Unfortunately, our CNS is also incredibly fragile.Unlike neurons in the PNS, which require less protection because they can regenerate, damage to neurons in the CNSis usually serious, permanent, and sometimes fatal. However, the brain may not be as “hard wired” as we oncebelieved.

Scientists long believed that after the first two or three years of life, humans and nonhuman animals are unable torepair or replace damaged neurons in the brain or spinal cord. We now know that the brain is capable of lifelongneuroplasticity and neurogenesis.

Achievement

Objective 2.5:



Discuss neuroplasticity, neurogenesis, and stem cells.

Neuroplasticity

Rather than being a fixed, solid organ, our brains are flexible and capable of changing their structure and function as aresult of usage and experience (Deller et al., 2006; Romero et al., 2008; Rossignol et al., 2008). The basic brainorganization (cortex, cerebellum, etc.) is irreversibly established before birth, but thanks to neuroplasticity someneural tissue can reorganize and change its structure and function throughout the life span. As weʹre learning a newsport or foreign language, for example, our brains change and “rewire.” New synapses form and others disappear.Some dendrites grow longer and sprout new branches, whereas others are “pruned” away. This is what makes ourbrains so wonderfully adaptive.

Remarkably, this rewiring has even helped remodel the brain following strokes. For example, psychologist EdwardTaub and his colleagues (2002, 2004, 2007) have had unusual success restoring function in stroke patients (Figure 2.5).Note that recovery from brain damage varies considerably, depending in large part on the age and health of theindividual, as well as the extent of the damage.

Figure 2.5 A breakthrough in neuroscience By immobilizing the unaffected arm or leg and requiring rigorous andrepetitive exercise of the affected limb, psychologist Edward Taub and colleagues “recruit” stroke patients' intactbrain cells to take over for damaged cells. The therapy has restored function in some patients as long as 21 yearsafter their strokes. Courtesy Taub Therapy Clinic/UAB Media Relations

Neurogenesis

In addition to the amazing process of neuroplasticity, our brains also continually replace lost cells with new cells, aprocess called neurogenesis. One type of neurogenesis involves the transplantation of immature, “uncommitted”stem cells into a patientʹs brain. Once inside the brain, these stem cells then develop into new healthy cells, whichreplace those that have died or degenerated. In addition, to transplantation into the brain, stem cells also have beenused for bone marrow transplants, and clinical trials using stem cells to repopulate or replace cells devastated by



injury or disease have helped patients suffering from Alzheimerʹs, Parkinsonʹs, diabetes, epilepsy, stress, strokes, anddepression (Chang et al., 2005; Hampton, 2006, 2007; Fleischmann & Welz, 2008; Leri, Anversa, & Frishman, 2007).

Does this mean that people paralyzed from spinal cord injuries might be able to walk again? At this point,neurogenesis in the brain and spinal cord is minimal. However, one possible bridge might be to transplant embryonicstem cells in the damaged area of the spinal cord. Researchers have successfully transplanted mouse embryonic stemcells into a damaged rat spinal cord (?Jones, Anderson, & Galvin, 2003; McDonald et al., 1999). When the damagedspinal cord was viewed several weeks later, the implanted cells had survived and spread throughout the injuredspinal cord area. More important, the transplant rats also showed some movement in previously paralyzed parts oftheir bodies. Medical researchers have also begun human trials using nerve grafts to repair damaged spinal cords(Lopez, 2002; Saltus, 2000).

Spinal Cord

Achievement

Objective 2.6:

What are the major functions of the spinal cord?

Now that we have discussed the remarkable adaptability of the central nervous system (CNS), letʹs take an even closerlook at its components—the brain and spinal cord. Because of its central importance for psychology, weʹll discuss thebrain in detail in the next major section. But the spinal cord is also very important. It is a great highway of informationinto and out of the brain. But it is much more than a simple set of cables relaying messages. Your spinal cord isinvolved in all voluntary movements and can even initiate some automatic behaviors on its own. These involuntary,automatic behaviors are called reflexes, or reflex arcs. The response to the incoming stimuli is “reflected” back—automatically.

Think back to your most recent physical exam. Did your physician tap your knee with a special hammer to check yourreflexes? Did your lower leg automatically kick out following this tap? This knee‑jerk reflex, like the one that pulls yourhand away from a painful fire, occurs within the spinal cord, without any help from the brain (Figure 2.6). Your brainlater “knows” what happened a fraction of a second after the tap on the knee because neural messages are also sentalong to the brain. The immediate, automatic response of the spinal cord, however, provides a faster reaction time.

We are all born with numerous reflexes (see Figure 2.7), many of which fade over time. But as adults we still blink inresponse to a puff of air in our eyes, gag when the back of our throat is stimulated, and urinate and defecate inresponse to pressure in the bladder and rectum. Even our sexual responses are somewhat controlled by reflexes. Justas a puff of air produces an automatic closing of the eyes, certain stimuli, such as the stroking of the genitals, can leadto arousal and the reflexive muscle contractions of orgasm in both men and women. However, in order to have thepassion, thoughts, and emotion we normally associate with sex, the sensory information from the stroking or orgasmmust be carried to the brain.

Assessment

VISUAL QUIZ



Figure 2.6 How the spinal cord operates In a simple reflex arc, a sensory receptor responds to stimulation andinitiates a neural impulse that travels to the spinal cord. This signal then travels back to the appropriatemuscle, which then contracts. The response is automatic and immediate in a reflex because the signalonly travels as far as the spinal cord before action is initiated, not all the way to the brain. The brain islater “notified” of the action when the spinal cord sends along the message.

What might be the evolutionary advantages of the reflex arc?

Answer:

If messages had to travel all the way to the brain before they could be acted upon, the animal (humanand nonhuman) might be fatally wounded.

Application

Try This Yourself

Testing for Reflexes

If you have a newborn or young infant in your home, you can easily (and safely) test for these simplereflexes. (Note: Most infant reflexes disappear within the first year of life. If they reappear in later life, it



generally indicates damage to the central nervous system.)

(a) Rooting reflex. Lightly stroke the infantʹs cheek or side of the mouth, and watch how he or she willautomatically (reflexively) turn toward the stimulation and attempt to suck.

(b) Grasping reflex. Place your finger in the infantʹs palm and note the automatic grasp.(c) Babinski reflex. Lightly stroke the sole of the infantʹs foot, and the toes will fan out and the foot will

twist inward.

Figure 2.7 Infant reflexes

Peripheral Nervous System (PNS): Connecting the CNS to the Restof the Body

Achievement

Objective 2.7:

What are the subdivisions of the peripheral nervous system, and what are their functions?



The peripheral nervous system (PNS) is just what it sounds like—the part that involves nerves peripheral to (or outside of)the brain and spinal cord. The chief function of the peripheral nervous system is to carry information to and from thecentral nervous system. It links the brain and spinal cord to the bodyʹs sense receptors, muscles, and glands.

Scientists further divide the PNS into the somatic nervous system and the autonomic nervous system. The somaticnervous system (SNS) (also called the skeletal nervous system) consists of all the nerves that connect to sensoryreceptors and skeletal muscles. The name comes from the term soma, which means “body,” and the somatic nervoussystem plays a key role in communication throughout the entire body. In a kind of “two‑way street,” the SNS firstcarries sensory information to the CNS and then carries messages from the CNS to skeletal muscles (Figure 2.8).

Figure 2.8 Sensory and motor neurons In order for you to be able to function, your brain and body must communicatewith one another. This is the job of the somatic nervous system (SNS), which receives sensory information, sends itto the brain, and allows the brain to direct the body to act. Messages (action potentials) within the nervous systemcan cross the synapse in only one direction. Sensory neurons carry messages inward from other body areas to theCNS. In contrast, motor neurons carry messages away from the CNS. Interneurons internally communicate andintervene between the sensory inputs and the motor outputs. Most of the neurons in the brain areinterneurons.Parasympathetic nervous system—“calming response”

Joe McBride/Stone/Getty Images

Sympathetic nervous system—“fight or flight”





Digital Vision/Getty Images

Parasympathetic nervous system—“calming response”

Figure 2.9 Actions of the autonomic nervous system (ANS) The ANS is responsible for a variety of independent(autonomous) activities, such as salivation and digestion. It exercises this control through its two divisions—thesympathetic and parasympathetic branches.

Study Tip

One way to differentiate the two subdivisions of the ANS is to imagine skydiving out of an airplane. When youinitially jump, your sympathetic nervous system has “sympathy” for your stressful situation. It alerts andprepares you for immediate action. Once your “para” chute opens, your “para” sympathetic nervous systemtakes over, and you can relax as you float safely to earth.

The other subdivision of the PNS, the autonomic nervous system (ANS), is responsible for involuntary tasks, such asheart rate, digestion, pupil dilation, and breathing. Like an automatic pilot, the ANS can sometimes be consciouslyoverridden. But as its name implies, the autonomic system normally operates independently (autonomic means“autonomous”).

The autonomic nervous system is itself further divided into two branches, the sympathetic and parasympathetic, toregulate the functioning of target organs like the heart, intestines, and lungs (Figure 2.9). These two subsystems tendto work in opposition to each other. A convenient, if somewhat oversimplified, distinction is that the sympathetic branchof the autonomic nervous system arouses the body and mobilizes it for action—the “fight‑or‑flight” response. Incontrast, the parasympathetic branch calms the body and conserves energy—the relaxation response. Keep in mind thatthese two systems are not an “on/off?” or either/or arrangement. Like two children on a playground teeter‑totter, onewill be up while the other is down. But they essentially balance each other out. In everyday situations, the sympatheticand parasympathetic nervous systems work together to maintain a steady, balanced, internal state.

When you hear a question from your instructor and want to raise your hand, your somatic nervous system will report toyour brain the current state of your skeletal muscles. It then carries instructions back to your muscles, allowing you tolift your arm and hand. But the somatic nervous system does not make your pupils dilate or your heartbeat quicken ifa dangerous snake comes sliding into the classroom. For this, you need your sympathetic nervous system. It wouldsimultaneously shut down your digestive processes and cause hormones, such as cortisol, to be released into thebloodstream. The net result of sympathetic activation is to get more oxygenated blood and energy to the skeletalmuscles, thus allowing us to “fight or flee.”

In contrast to the sympathetic nervous system, which arouses the body, the parasympathetic nervous system calms andreturns your body to its normal functioning. It slows your heart rate, lowers your blood pressure, and increases yourdigestive and eliminative processes. Now, we understand why arguments during meals often cause stomachaches!



Strong emotions, like anger, fear, or even joy, put the sympathetic system in dominance and prevent digestion andelimination. Strong emotions and sympathetic dominance also explain many sexual problems (Figure 2.10).

Rob McEwan/Everett Collection, Inc.

Parasympathetic dominance Sexual arousal and excitement require that the body be relaxed enough to allow increased bloodflow to the genitals—in other words, the nervous system must be in parasympathetic dominance. Parasympathetic nerves carrymessages from the central nervous system directly to the sexual organs, allowing for a localized response (increased blood flow andgenital arousal).

Rick Rusing/Stone/Getty Images

Sympathetic dominance When a person experiences strong emotions, such as anger, anxiety, or fear, the body shifts tosympathetic dominance, which can disrupt the sexual arousal that occurs under parasympathetic dominance. During sympatheticdominance, blood flow to the genitals and other organs decreases as the body readies for a “fight or flight.” As a result, the personis unable (or less likely) to become sexually aroused. Any number of circumstances—for example, performance anxiety, fear ofunwanted pregnancy or disease, or tensions between partners—can trigger sympathetic dominance.

Figure 2.10 Why Study Psychology? The complexities of sexual interaction—and in particular, the difficulties that couplessometimes have in achieving sexual arousal or orgasm—illustrate the balancing act between the sympathetic andparasympathetic nervous systems.



PictureQuest

The sympathetic nervous system in action The fightorflight response of the sympathetic nervous system mobilizes the body foraction in the face of perceived danger.

It is important to remember, however, that the sympathetic nervous system does provide adaptive, evolutionaryadvantages. At the beginning of human evolution, when we faced a dangerous bear or aggressive human intruder,there were only two reasonable responses—fight or flight. This automatic mobilization of bodily resources still hassignificant survival value in our modern life. But today our sympathetic nervous system is often activated by less life‑threatening events. Instead of bears and intruders, we have chronic stressors like full‑time college classes mixed withpart‑ or full‑time jobs, dual‑career marriages, daily traffic jams, and rude drivers on high‑speed freeways—not tomention terrorist attacks and global pollution. Unfortunately, our body responds to these sources of stress withsympathetic arousal. And, as you will see in Chapter 3, chronic arousal to stress can be very detrimental to our health.

Assessment

C H E C K & R E V I E W

Nervous System Organization

Objective 2.4: Describe the nervous systemʹs two major divisions, and explain their respective functions.

The nervous system is divided into two major divisions: the central nervous system (CNS), composed of thebrain and spinal cord, and the peripheral nervous system (PNS), including all nerves connecting the CNS tothe rest of the body. The CNS processes and organizes information, whereas the PNS carries information toand from the CNS.

Objective 2.5: Discuss neuroplasticity, neurogenesis, and stem cells.

Neuroplasticity is the brainʹs ability to reorganize and change its structure and function throughout the lifespan. Neurogenesis is the process by which new neurons are generated. Stem cells are immature(uncommitted) cells that have the potential to develop into almost any type of cell depending on thechemical signals they receive.

Objective 2.6: What are the major functions of the spinal cord?

The spinal cord is the communications link between the brain and the rest of the body, and it is involved inall voluntary and reflex responses.

Objective 2.7: What are the subdivisions of the peripheral nervous system, and what are their functions?



The two major subdivisions of the PNS are the somatic nervous system and the autonomic nervous system.

The somatic nervous system includes all nerves carrying incoming sensory information and outgoing motorinformation to and from the sense organs and skeletal muscles. The autonomic nervous system includes thenerves outside the brain and spinal cord that maintain normal functioning of glands, heart muscle, and thesmooth muscle of bloodvessels and internal organs.

The autonomic nervous system is further divided into two branches, the parasympathetic and the sympathetic,which tend to work in opposition to one another. The parasympathetic nervous system normally dominateswhen a person is relaxed. The sympathetic nervous system dominates when a person is under physical ormental stress. It mobilizes the body for fight or flight by increasing heart rate and blood pressure andslowing digestive processes.

Questions

1 The nervous system is separated into two major divisions: the _________ nervous system, whichconsists of the brain and spinal cord, and the _________ nervous system, which consists of all the nervesgoing to and from the brain and spinal cord.

Answer:

centralperipheral.

2 The autonomic nervous system is subdivided into two branches called the _________ and _________systems.

(a) automatic, semiautomatic(b) somatic, peripheral(c) afferent, efferent(d) sympathetic, parasympathetic

Answer:

sympatheticparasympathetic

3 If you are startled by the sound of a loud explosion, the _________ nervous system will becomedominant.

(a) peripheral(b) somatic(c) parasympathetic(d) sympathetic

Answer:

sympathetic

4 What is the major difference between the sympathetic and parasympathetic nervous systems?

Answer:



The sympathetic nervous system arouses the body and mobilizes energy stores to deal withemergencies. The parasympathetic nervous system calms the body and conserves the energy stores.

5 Fill in the blank lines.

Answer:

Compare your answers with Figure 2.4.

Copyright © 2010 John Wiley & Sons, Inc. All rights reserved.

Documents

Nervous System Organization Achievement Print this pagemrwolman.weebly.com/uploads/3/8/0/3/38036553/nervous_system... · involved in all voluntary movements and can even initiate