Nervous Systems. Chapter 49. Lecture Outline. Overview: Command and Control System

Transcription

1 Chapter 49 Nervus Systems Lecture Outline Overview: Cmmand and Cntrl System The human brain cntains an estimated (100 billin) neurns. The circuits that intercnnect these brain cells are enrmusly cmplex. Recent technlgies such as functinal magnetic resnance imaging (fmri) can recrd brain activity frm utside a persn s skull. The brain is scanned with electrmagnetic waves, and changes in bld xygen levels at sites f neurnal activity generate a signal. A cmputer then uses the data t cnstruct a three-dimensinal map f the subject s brain while the subject des varius tasks. Scientists lk fr crrelatins between particular tasks and activity in specific regins f the brain. The ability t sense and react riginated billins f years ag with prkarytes that culd detect changes in their envirnment and respnd in ways that enhanced their survival and reprductive success. Fr example, bacteria cntinue t mve in a particular directin as lng as they encunter increasing cncentratins f a fd surce. Later, mdificatin f simple recgnitin and respnse prcesses prvided multicellular rganisms with a mechanism fr cmmunicatin between cells f the bdy. By the time f the Cambrian explsin, mre than 500 millin years ag, systems f neurns that allwed animals t sense and mve rapidly were present in essentially their current frms. Cncept 49.1 Nervus systems cnsist f circuits f neurns and supprting cells. In mst animals with nervus systems, clusters f neurns perfrm specialized functins. Such clustering is absent in the cnidarians, the simplest animals with nervus systems. In cnidarians, a series f intercnnected nerve cells frm a diffuse nerve net that cntrls the cntractin and expansin f the gastrvascular cavity. In mre cmplex animals, the axns f multiple nerve cells may be bundled t frm nerves. Nerves channel and rganize infrmatin that flws alng specific rutes thrugh the nervus system. Sea stars have a set f radial nerves cnnecting t a central nerve ring. Within each arm, the radial nerve is linked t a nerve net frm which it receives input and t which it sends signals cntrlling mtr activity. Such an arrangement allws fr the cntrl f elabrate mvements. Animals with elngated, bilaterally symmetrical bdies have mre specialized nervus systems. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-1

2 Such animals exhibit cephalizatin, the clustering f sensry neurns and interneurns at the anterir end. One r mre nerve crds extending tward the psterir end cnnect these structures with nerves elsewhere in the bdy. In nnsegmented wrms like the planarian, a small brain and lngitudinal nerve crds make up the simplest clearly defined central nervus system (CNS). The entire nervus system f such animals can be cnstructed frm nly a small number f cells. In the nematde Caenrhabditis elegans, an adult wrm cntains exactly 302 neurns, n mre and n fewer. In mre cmplex invertebrates, such as annelids and arthrpds, the number f neurns is much greater. Behavir is regulated by mre cmplicated brains and by ventral nerve crds cntaining ganglia, segmentally arranged clusters f neurns. Within an animal grup, nervus system rganizatin ften crrelates with lifestyle. Sessile and slw-mving mlluscs, such as clams and chitns, have relatively simple sense rgans and little r n cephalizatin. Active predatry mlluscs, such as ctpuses and squids, have the mst sphisticated nervus systems f any invertebrate. In vertebrates, the brain and the spinal crd frm the CNS; the nerves and ganglia make up the peripheral nervus system (PNS). Reginal specializatin is a hallmark f bth systems. The brain and spinal crd f the vertebrate CNS have tightly crdinated activities. The brain integrates the cmplex behavir f vertebrates. The spinal crd cnveys infrmatin t and frm the brain and generates basic patterns f lcmtin. The spinal crd als acts independently as part f the simple nerve circuits that prduce reflexes, the bdy s autmatic respnses t stimuli. A reflex prtects the bdy by triggering a rapid, invluntary respnse t a particular stimulus, such as pulling yur hand away frm a ht stve. In vertebrates, the spinal crd runs alng the drsal side f the bdy. Segmental ganglia are present just utside the spinal crd. An underlying segmental rganizatin is apparent in the arrangement f neurns within the spinal crd. The brain and spinal crd f vertebrates derive frm the chrdate characteristic f a hllw drsal embrynic nerve crd. During develpment, the hllw cavity f the embrynic nerve crd is transfrmed int the narrw central canal f the spinal crd and the ventricles f the brain. The central canal and the fur ventricles are filled with cerebrspinal fluid, frmed in the brain by filtratin f the bld. The cerebrspinal fluid circulates slwly thrugh the central canal and the ventricles and then drains int the veins, bringing nutrients and hrmnes t the brain and clearing wastes. In mammals, the cerebrspinal fluid cushins the brain and spinal crd by circulating between layers f cnnective tissue that surrund the CNS. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-2

3 In additin t these fluid-filled spaces, the brain and the spinal crd cntain gray matter and white matter. Gray matter cnsists f mainly neurn cell bdies, dendrites, and unmyelinated axns. White matter cntains bundled axns with myelin sheaths, making them whitish. White matter is n the utside f the spinal crd, linking the CNS t the sensry and mtr neurns f the PNS. White matter in the brain is n the inside, signaling between neurns in learning, emtin, prcessing f sensry infrmatin, and generating cmmands. A variety f glia are present thrughut the vertebrate brain and spinal crd. Ependymal cells line the ventricles and have cilia that circulate cerebrspinal fluid. Micrglia prtect the nervus system frm invading micrrganisms. Oligdendrcytes and Schwann cells functin in axn myelinatin, a critical activity in the vertebrate nervus system. Astrcytes have the mst diverse set f functins: They prvide structural supprt fr neurns and regulate the extracellular cncentratins f ins and neurtransmitters. Astrcytes can respnd t activity in neighbring neurns by facilitating infrmatin transfer at synapses and, in sme instances, releasing neurtransmitters. Astrcytes adjacent t active neurns cause nearby bld vessels t dilate, increasing bld flw t the area and enabling the neurns t btain xygen and glucse mre quickly. During develpment, astrcytes induce cells that line the capillaries in the CNS t frm tight junctins. The result is the bld-brain barrier, which restricts the passage f mst substances int the CNS and permits tight cntrl f the extracellular chemical envirnment f the brain and spinal crd. Radial glia play a critical rle in the embrynic develpment f the nervus system, frming tracks alng which newly frmed neurns migrate frm the neural tube, the structure that gives rise t the CNS. Bth radial glia and astrcytes can als act as stem cells, generating neurns and additinal glia and ffering a ptential way t replace neurns and glia that are lst t injury r disease. The PNS transmits infrmatin t and frm the CNS and regulates a vertebrate s mvement and internal envirnment. Sensry infrmatin reaches the CNS alng afferent PNS neurns. After infrmatin is prcessed within the CNS, instructins travel t muscles, glands, and endcrine cells alng efferent PNS neurns. The vertebrate PNS cnsists f left-right pairs f cranial and spinal nerves and their assciated ganglia. The cranial nerves riginate in the brain and terminate mstly in rgans f the head and upper bdy. The spinal nerves riginate in the spinal crd and extend t parts f the bdy belw the head. Mst f the cranial nerves and all f the spinal nerves cntain bth afferent and efferent neurns. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-3

4 The lfactry nerve is afferent, dedicated t cnveying sensry infrmatin fr lfactin, the sense f smell. The efferent branch f the PNS is made up f the mtr system and the autnmic nervus system. The mtr system cnsists f neurns that carry signals t skeletal muscles in respnse t external stimuli. Althugh the mtr system is subject t cnscius cntrl, much skeletal muscle activity is actually cntrlled by the brain stem r by reflexes mediated by the spinal crd. The autnmic nervus system regulates the internal envirnment by invluntary cntrl f smth and cardiac muscles and the rgans f the digestive, cardivascular, excretry, and endcrine systems. Three divisins sympathetic, parasympathetic, and enteric make up the autnmic nervus system. The sympathetic and parasympathetic divisins functin antagnistically in regulating rgan functin. The sympathetic divisin is respnsible fr arusal and energy generatin (the fight-r-flight respnse). With stimulatin f this divisin, the heart beats faster, the liver cnverts glycgen t glucse, digestin is inhibited, and secretin f epinephrine frm the adrenal medulla is stimulated. Activatin f the parasympathetic divisin generally causes ppsite respnses that prmte calming and a return t self-maintenance functins ( rest and digest ). Increased activity in the parasympathetic divisin lwers heart rate, increases glycgen prductin, and enhances digestin. The verall functins f the sympathetic and parasympathetic divisins are reflected in the lcatin f neurns in each divisin and the neurtransmitters that these neurns release. The enteric divisin cnsists f netwrks f neurns in the digestive tract, pancreas, and gallbladder. Neurns f the enteric divisin cntrl secretin and smth muscle cntractin t prduce peristalsis. The enteric divisin is regulated by the sympathetic and parasympathetic divisins. The smatic and autnmic nervus systems cperate t maintain hmestasis. Fr example, if bdy temperature drps, the hypthalamus signals the autnmic nervus system t cnstrict surface bld vessels, reducing heat lss. At the same time, the hypthalamus signals the smatic nervus system t cause shivering, increasing heat prductin. Cncept 49.2 The vertebrate brain is reginally specialized. In all vertebrates, three bilaterally symmetrical, anterir bulges f the neural tube the frebrain, midbrain, and hindbrain frm as the embry develps. By the fifth week f embrynic develpment in humans, the three primary bulges have frmed five brain regins. Three f these regins, derived frm the midbrain and the hindbrain, give rise t the brain stem, a set f structures that frm the lwer part f the brain. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-4

5 The hindbrain als gives rise t anther majr brain center, the cerebellum. As embrygenesis prceeds, the mst prfund changes in the human brain ccur in the telencephaln, the regin f the frebrain that gives rise t the adult cerebrum. Rapid grwth f the telencephaln causes the uter prtin f the cerebrum, called the cerebral crtex, t extend ver and arund much f the rest f the brain. Majr centers that develp frm the diencephaln are the thalamus, hypthalamus, and epithalamus. The brain stem functins in hmestasis, crdinatin f mvement, and cnductin f infrmatin t and frm higher brain centers. The adult brain stem cnsists f the midbrain, the pns, and the medulla blngata r medulla. The medulla and the pns transfer sensry infrmatin and mtr instructins between the PNS and the midbrain and frebrain. They als help crdinate large-scale bdy mvements, such as running r climbing. In carrying instructins abut the mvement frm cell bdies in the midbrain and frebrain t synapses in the spinal crd, mst axns crss in the medulla frm ne side f the CNS t the ther. As a result, the right side f the brain cntrls much f the mvement f the left side f the bdy, and vice versa. The midbrain cntains centers that receive and integrate different types f sensry infrmatin, sending cded sensry infrmatin alng neurns t specific regins f the frebrain. Sensry axns invlved in hearing terminate in r pass thrugh the midbrain n their way t the cerebrum. In nnmammalian vertebrates, prtins f the midbrain frm prminent ptic lbes that may be the nly visual centers. In mammals, visin is integrated in the cerebrum, nt the midbrain. The mammalian midbrain crdinates visual reflexes, such as the peripheral visin reflex, in which the head turns t an appraching bject withut frming an image f it. Signals frm the brain stem affect attentin, alertness, appetite, and mtivatin. The medulla cntains centers that cntrl several autmatic, hmestatic functins, including breathing, heart and bld vessel activity, swallwing, vmiting, and digestin. The pns als participates in sme f these activities; fr example, it regulates the breathing centers in the medulla. Axns frm the brain stem reach many areas f the cerebral crtex and cerebellum, releasing neurtransmitters such as nrepinephrine, dpamine, sertnin, and acetylchline. The brain stem and cerebrum cntrl arusal and sleep. Arusal is a state f awareness f the external wrld; sleep is a state in which external stimuli are received but nt cnsciusly perceived. Centers in the brain stem cntrl arusal and sleep. The reticular frmatin, a diffuse netwrk f neurns in the cre f the brain stem, acts as a sensry filter t chse which infrmatin reaches the cerebral crtex. The brain ften ignres certain stimuli while actively prcessing ther inputs. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-5

6 The pns and medulla cntain centers that cause sleep when stimulated, and the midbrain has a center that causes arusal. All birds and mammals shw characteristic sleep/wake cycles regulated by melatnin, a hrmne prduced by the pineal gland. Peak melatnin secretin ccurs at night. Melatnin has been prmted as a dietary supplement t treat jet lag, insmnia, seasnal affective disrder, and depressin. Melatnin is synthesized frm sertnin, which itself may be the neurtransmitter f the sleep-prducing centers. Sertnin in turn is synthesized frm the amin acid tryptphan. Because tryptphan levels are relatively high in milk, drinking milk befre bedtime may prmte sleep by increasing the prductin f sertnin and melatnin. Sleep is essential fr survival. Furthermre, sleep is an active state, at least fr the brain. EEG recrdings shw that the frequency f brain waves changes as the brain prgresses thrugh the distinct stages f sleep. Sleep and dreams may be invlved in the cnslidatin f learning and memry. Regins f the brain activated during a learning task becme active again during sleep. Sme animals display evlutinary adaptatins that allw fr substantial activity during sleep. Bttlense dlphins, which sleep with ne eye pen and ne eye clsed, swim while sleeping, rising t the surface t breathe air. EEG recrdings frm the hemispheres f sleeping dlphins shw that dlphins sleep with nly ne brain hemisphere at a time. The cerebellum crdinates mvements and balance. The cerebellum receives sensry infrmatin abut jint psitin and muscle length, infrmatin frm the auditry and visual systems, and input abut mtr cmmands issued by the cerebrum. Infrmatin frm the cerebrum passes first t the pns and frm there t the cerebellum. The cerebellum integrates this sensry and mtr infrmatin as it carries ut crdinatin and errr checking during mtr and perceptual functins. Hand-eye crdinatin is an example f cerebellar cntrl; if the cerebellum is damaged, the eyes can fllw a mving bject, but they will nt stp at the same place as the bject. The cerebellum is als invlved in learning and remembering mtr skills. The embrynic diencephaln evlved int three adult brain regins: the thalamus, hypthalamus, and epithalamus. The embrynic diencephaln is the frebrain divisin that evlved earliest in vertebrate histry. The thalamus and hypthalamus are majr integrating centers that act as relay statins fr infrmatin flw in the bdy. The epithalamus includes the pineal gland, the surce f melatnin, and als cntains ne f several clusters f capillaries that generate cerebrspinal fluid frm bld. The thalamus is the main input center fr sensry infrmatin ging t the cerebrum. Incming infrmatin frm all the senses is srted in the thalamus and sent t the apprpriate cerebral centers fr further prcessing. The thalamus als receives input frm the cerebrum and ther parts f the brain that regulate emtin and arusal. The thalamus is frmed by tw masses, each rughly the size and shape f a walnut. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-6

7 The hypthalamus is ne f the mst imprtant brain regins fr the cntrl f hmestasis. The hypthalamus cntains the bdy s thermstat as well as centers fr regulating hunger, thirst, and ther basic survival mechanisms. The hypthalamus is the surce f psterir pituitary hrmnes and f releasing hrmnes that act n the anterir pituitary. Hypthalamic centers play a rle in sexual and mating behavirs, the fight-r-flight respnse, and pleasure. The hypthalamus regulates the bilgical clck. Specialized nerve cells within the hypthalamus regulate circadian rhythms, daily cycles f bilgical activity. Organisms frm bacteria and fungi t plants, insects, birds, and humans display these rhythms. In mammals, the cycles cntrlled by the hypthalamus influence a number f prcesses, including sleep, bdy temperature, hunger, and hrmne release. Circadian rhythms in mammals rely n a bilgical clck, a mlecular mechanism that directs peridic gene expressin and cellular activity. Bilgical clcks can maintain a 24-hur cycle even in the absence f envirnmental cues. Humans kept in a cnstant envirnment exhibit a cycle length f 24.2 hurs, with little variatin. In mammals, circadian rhythms are crdinated by a pair f hypthalamic structures called the suprachiasmatic nucleus, r SCN. In respnse t visual infrmatin, the SCN acts as a pacemaker t synchrnize the bdy s bilgical clcks t the natural cycles f day length. Animals whse SCN are remved lack rhythmicity in behavirs and brain activity. In 1990, Michael Menaker transplanted brain tissue between nrmal and mutant hamsters with faulty circadian rhythms and demnstrated that the SCN determines the circadian rhythm f the whle animal. In mammals, infrmatin prcessing is largely centered in the cerebrum. The cerebrum develps frm the embrynic telencephaln, an utgrwth f the frebrain that arse early in vertebrate evlutin as a regin supprting lfactry receptin as well as auditry and visual prcessing. The cerebrum is divided int right and left cerebral hemispheres. Each hemisphere cnsists f an uter cvering f gray matter, the cerebral crtex; internal white matter; and grups f neurns cllectively called basal nuclei lcated deep within the white matter. The basal nuclei are imprtant centers fr planning and learning mvement sequences. Damage in this brain regin during fetal develpment can result in cerebral palsy, a defect that disrupts the issuance f mtr cmmands t the muscles. The cerebral crtex is particularly extensive in mammals, where it is vital fr perceptin, vluntary mvement, and learning. In humans, the cerebral crtex accunts fr abut 80% f ttal brain mass and is highly cnvluted. Due t its cnvlutins, the cerebral crtex has a large surface area but still fits inside the skull: Less than 5 mm thick, it has a surface area f apprximately 1000 cm 2. The cerebral crtex is divided int right and left sides, each respnsible fr the ppsite half f the bdy. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-7

8 The left side f the crtex receives infrmatin frm, and cntrls the mvement f, the right side f the bdy, and vice versa. A thick band f axns knwn as the crpus callsum enables cmmunicatin between the right and left cerebral crtices. The cerebrum is very plastic, especially early in develpment, and areas f the brain can take n nvel functins. A dramatic example f this phenmenn results frm a treatment fr the mst extreme cases f epilepsy, a cnditin causing episdes f electrical disturbance, r seizures, in the brain. Infants with severe epilepsy may have a cerebral hemisphere surgically remved. Amazingly, recvery is nearly cmplete, as the remaining hemisphere assumes mst f the functins nrmally prvided by the entire cerebrum. Even in adults, damage t a prtin f the cerebral crtex can trigger the develpment r use f new brain circuits, leading in sme cases t recvery f functin. Sme vertebrates demnstrate cgnitin. In humans, the utermst part f the cerebral crtex frms the necrtex, six parallel layers f neurns arranged tangential t the brain surface. Such a large, highly cnvluted necrtex was thught t be required fr advanced cgnitin, the perceptin and reasning that frm knwledge. Bth primates and cetaceans pssess an extensively cnvluted necrtex. Birds lack a necrtex and were thught t have substantially lwer intellectual capacity. In fact, birds are capable f sphisticated infrmatin prcessing. Scrub jays can remember the relative perid f time that has passed since they stred and hid specific fd items. New Calednian crws are highly skilled at making and using tls, an ability therwise well dcumented fr nly humans and great apes. African gray parrts understand relatinal cncepts that are numerical r abstract, distinguishing between same and different and grasping the cncept f nne. The sphisticated cgnitive ability f birds is based n an evlutinary variatin n the architecture f the pallium, the tp r uter prtin f the brain. Whereas the human pallium the cerebral crtex cntains flat sheets f cells in six layers, the avian pallium cntains neurns clustered int nuclei. This rganizatin, likely ancestral in vertebrates, was transfrmed int a layered ne early in mammalian evlutin. Cnnectivity was maintained during this transfrmatin s that, fr example, the pallium f bth mammals and birds receives sensry input sights, sunds, and tuch frm the thalamus. The result was tw different arrangements, each supprting cmplex and flexible brain functin. Cncept 49.3 The cerebral crtex cntrls vluntary mvement and cgnitive functins. Each side f the cerebral crtex is made up f the frntal, tempral, ccipital, and parietal lbes (each named fr a bne f the skull). Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-8

9 Each lbe cntains a number f functinal areas, including primary sensry areas, which receive and prcess a specific type f sensry infrmatin, and assciatin areas, which integrate the infrmatin frm varius parts f the brain. The increased size f the cerebral crtex during mammalian evlutin resulted frm an expansin f the assciatin areas. A rat s cerebral crtex cntains mainly primary sensry areas, whereas a human cerebral crtex cnsists largely f assciatin areas respnsible fr mre cmplex behavir and learning. The cerebral crtex prcesses infrmatin. The cerebral crtex receives sensry input frm dedicated sensry rgans, such as the eyes and nse, as well as receptrs in the hands, scalp, and elsewhere. Smatsensry receptrs prvide infrmatin abut tuch, pain, pressure, temperature, and the psitin f muscles and limbs. Sensry infrmatin cming int the crtex is directed via the thalamus t primary sensry areas within the brain lbes. Different types f input are directed t distinct lcatins: visual infrmatin t the ccipital lbe, auditry input t the tempral lbe, and smatsensry infrmatin t the parietal lbe. Infrmatin abut taste ges t the parietal lbe, but t a regin separate frm the area fr smatsensry input. Olfactry infrmatin is first sent t regins f the crtex that are similar in mammals and reptiles and then via the thalamus t an interir part f the frntal lbe. Infrmatin received at the primary sensry areas is passed t assciatin areas, which prcess particular features in the sensry input. In the ccipital lbe, sme grups f neurns in the primary visual area are specifically sensitive t rays f light riented in a particular directin. In the visual assciatin area, infrmatin related t such features is cmbined in a regin dedicated t recgnizing cmplex images, such as faces. Integrated sensry infrmatin is passed t the frntal assciatin area, which helps plan actins and mvement. The cerebral crtex then generates mtr cmmands that cause particular behavirs. These cmmands cnsist f actin ptentials prduced by neurns in the mtr crtex, which lies at the rear f the frntal lbe. The actin ptentials travel alng axns t the brain stem and spinal crd, where they excite mtr neurns, which in turn excite skeletal muscle cells. In bth the smatsensry crtex and the mtr crtex, neurns are distributed in an rderly fashin accrding t the part f the bdy that generates the sensry input r receives the mtr cmmands. Neurns that prcess sensry infrmatin frm the legs and feet are lcated in the regin f the smatsensry crtex that lies clsest t the midline. Neurns that cntrl muscles in the legs and feet are lcated in the crrespnding regin f the mtr crtex. The crtical surface area devted t each bdy part is nt prprtinal t the size f the part. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 49-9

10 Instead, the surface area crrelates with the extent f neurnal cntrl needed fr muscles in a particular bdy part (fr the mtr crtex) r with the number f sensry neurns that extend axns t that part (fr the smatsensry crtex). Fr example, the surface area f the mtr crtex devted t the face is very large, reflecting the extensive invlvement f facial muscles. Language and speech are lcalized in the brain. Damage t particular regins f the crtex by injuries, strkes, r tumrs can prduce distinctive changes in a persn s behavir. French physician Pierre Brca cnducted pstmrtems n patients wh culd understand language but culd nt speak and fund that many f them had defects in a small regin f the left frntal lbe. That regin, knwn as Brca s area, is in frnt f the part f the primary mtr crtex that cntrls muscles in the face. The German physician Karl Wernicke fund that damage t a psterir prtin f the left tempral lbe, nw called Wernicke s area, ablished the ability t cmprehend speech but nt the ability t speak. Mre than a century later, studies f brain activity using fmri and psitrn-emissin tmgraphy have cnfirmed that Brca s area is active during the generatin f speech and Wernicke s area is active when speech is heard. Brca s area and Wernicke s area are part f a much larger netwrk f brain regins invlved in language. Reading a printed wrd withut speaking activates the visual crtex; reading a printed wrd alud activates bth the visual crtex and Brca s area. Frntal and tempral areas becme active when meaning must be attached t wrds, such as when a persn generates verbs t g with nuns r grups related wrds r cncepts. Crtical functin is lateralized. The tw human cerebral hemispheres d nt have identical functins. The left side f the cerebrum plays a dminant rle with regard t language, as reflected in the lcatin f bth Brca s area and Wernicke s area in the left hemisphere. The left hemisphere is mre adept at math and lgical peratins, while the right hemisphere is dminant in the recgnitin f faces and patterns, spatial relatins, and nnverbal thinking. These differences in hemisphere functin in humans are termed lateralizatin. Sme lateralizatin relates t handedness, the preference fr using ne hand fr certain mtr activities. Rughly 90% f individuals in all human ppulatins are right-handed. fmri studies have revealed that language prcessing differs in relatin t handedness. When subjects thught f wrds withut speaking ut lud, brain activity was lcalized t the left hemisphere in 96% f right-handed subjects but in nly 76% f left-handed subjects. The tw hemispheres nrmally wrk tgether, exchanging infrmatin thrugh the fibers f the crpus callsum. Patients with extreme frms f epilepsy wh have their crpus callsum surgically severed exhibit a split-brain effect. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

11 When they see a familiar wrd in their left field f visin, they cannt read the wrd: The sensry infrmatin that travels frm the left field f visin t the right hemisphere cannt reach the language centers in the left hemisphere. Each hemisphere in such patients functins independently f the ther. The generatin and experience f emtins invlve many regins f the brain. The limbic system, which includes the amygdala, the hippcampus, and parts f the thalamus, is a grup f structures surrunding the brain stem in mammals. Structures within the limbic system have diverse functins, including emtin as well as mtivatin, lfactin, behavir, and memry. Parts f the brain utside the limbic system als play a rle in the generatin and experience f emtin. Emtins that manifest themselves in behavirs such as laughing and crying invlve an interactin f parts f the limbic system with sensry areas f the cerebrum. Structures in the frebrain attach emtinal feelings t basic, survival-related functins cntrlled by the brain stem, including aggressin, feeding, and sexuality. Emtinal experiences may be stred separately frm the system that supprts explicit recall f events. The fcus f emtinal memry is the amygdala, which is lcated in the tempral lbe. Adults wh learn t avid an aversive situatin, such as an image that is always fllwed by a mild electrical shck, remember the image and underg autnmic arusal as measured by increased heart rate r sweating when they see the image again. Peple with brain damage cnfined t the amygdala d nt exhibit autnmic arusal but can recall the image because explicit memry is intact. The prefrntal crtex, a part f the frntal lbes critical fr emtinal experience, is als imprtant in temperament and decisin making. In a famus 19th-century case, Phineas Gage was injured by a meter-lng rd that penetrated int his frntal lbe. Gage recvered, but his persnality changed drastically. He became emtinally detached, impatient, and erratic in his behavir. Tumrs that develp in the frntal lbe smetimes cause the same cmbinatin f symptms that Gage experienced. Intellect and memry seem intact, but decisin making is flawed and emtinal respnses are diminished. In the 20th century, the same prblems were bserved as a cnsequence f frntal lbtmy, a discredited surgical prcedure that severs the cnnectin between the prefrntal crtex and the limbic system. Behaviral disrders are nw treated instead with drug therapy. Cnsciusness is an emergent prperty f the brain. Over the past few decades, neurscientists have studied cnsciusness using brain-imaging techniques such as fmri and PET scans. Neurscientists can cmpare activity in the human brain during different states f cnsciusness (fr example, befre and after a persn is aware f seeing an bject). Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

12 Imaging techniques can als be used t cmpare the cnscius and uncnscius prcessing f sensry infrmatin, prviding an increasingly detailed picture f hw neural activity crrelates with cnscius experiences. There is a grwing cnsensus amng neurscientists that cnsciusness is an emergent prperty f the brain, ne that recruits activities in many areas f the cerebral crtex. Several mdels pstulate the existence f a srt f scanning mechanism that repetitively sweeps acrss the brain, integrating widespread activity int a unified, cnscius mment. Cncept 49.4 Changes in synaptic cnnectins underlie memry and learning. During embrynic develpment, regulated gene expressin and signal transductin establish the verall structure f the nervus system. Tw prcesses dminate the remaining develpment f the nervus system. In the first prcess, neurns cmpete fr grwth factrs, which are prduced in limiting quantities by target tissues. Cells that dn't reach their targets fail t receive factrs and underg prgrammed cell death. In fact, half f the neurns frmed in the embry are eliminated. The secnd prcess is synapse eliminatin. A develping neurn frms many synapses, mre than it needs. The activity f the neurn stabilizes sme synapses and destabilizes thers. By the end f embrygenesis, mst neurns have lst mre than half f their initial synapses, leaving behind the cnnectins that survive int adulthd. Tgether, neurn and synapse eliminatin set up the netwrk f cells and cnnectins within the nervus system required thrughut life. Nervus systems are plastic and can be remdeled. Althugh the basic architecture f the CNS is established during embrynic develpment, it can change after birth. The capacity fr the nervus system t be remdeled in respnse t its wn activity is called neural plasticity. Much f the reshaping f the nervus system ccurs at synapses. When the activity f a synapse crrelates with the activity f ther synapses, changes may ccur that reinfrce that synaptic cnnectin. When the activity f a synapse fails t crrelate with the activity f ther synapses, the synaptic cnnectin weakens. These prcesses can result in either lss r additin f a synapse. These changes result in increased signaling between particular pairs f neurns and decreased signaling at ther sites. Changes can als strengthen r weaken signaling at a synapse. Remdeling and refinement f the nervus system ccur in many cntexts as we develp the ability t sense ur surrundings r recver frm injury r disease. Shrt- and lng-term memries are rganized differently in the brain. Sme infrmatin is held in shrt-term memry lcatins and released as it becmes irrelevant. Fr knwledge that is retained lnger, the mechanisms f lng-term memry are activated. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

13 Infrmatin can be fetched frm lng-term memry and returned t shrt-term memry. Bth types f memry are stred in the cerebral crtex. Shrt-term memries are accessed via temprary assciatins frmed in the hippcampus. Fr lng-term memries, the links in the hippcampus are replaced by mre permanent cnnectins within the cerebral crtex. The hippcampus is essential fr acquiring new lng-term memries, but nt fr maintaining them. Peple wh suffer damage t the hippcampus cannt frm any new lasting memries, but they can recall past events frm befre their injury. What evlutinary advantage is ffered by different rganizatins fr shrt-term and lng-term memries? The delay in frming cnnectins in the cerebral crtex enables lng-term memries t be integrated int the existing stre f knwledge and experience, allwing meaningful assciatins. Cnsistent with this idea, the transfer f infrmatin frm shrt-term t lng-term memry is enhanced by the assciatin f new data with data previusly learned and stred in lngterm memry. Mtr skills are usually learned by repetitin. Yu can perfrm mtr skills withut cnsciusly recalling the individual steps required t d these tasks crrectly. The learning f skills and prcedures invlves cellular mechanisms very similar t thse respnsible fr brain grwth and develpment, as neurns actually make new cnnectins. In cntrast, the memrizatin f phne numbers, facts, and places which can be very rapid and may require nly ne expsure t the relevant item may rely mainly n changes in the strength f existing neural cnnectins. Lng-term ptentiatin (LTP) leads t a lasting increase in the strength f synaptic transmissin. LTP, first characterized in tissue slices frm the hippcampus, invlves a presynaptic neurn that releases the excitatry neurtransmitter glutamate. Fr LTP t ccur there must be a brief, high-frequency series f actin ptentials in this presynaptic neurn, which must arrive at the synaptic terminal at the same time the pstsynaptic cell receives a deplarizing stimulus. LTP invlves tw types f glutamate receptrs, each named fr a mlecule NMDA r AMPA that artificially activates that receptr. The set f receptrs present n the pstsynaptic membranes changes in respnse t an active synapse and a deplarizing stimulus. The result is lng-term ptentiatin a stable increase in the size f the pstsynaptic ptentials at the synapse. LTP is ne f the fundamental prcesses by which memries are stred and learning takes place. Cncept 49.5 Nervus system disrders can be understd in mlecular terms. Disrders f the nervus system, including schizphrenia, depressin, Alzheimer s disease, and Parkinsn s disease, result in mre hspitalizatins in the United States than d heart disease r cancer. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

14 Many disrders that alter md r behavir can nw be treated with drugs. Scietal attitudes are changing as awareness grws that nervus system disrders result frm chemical r anatmical changes in the brain. Majr research effrts are under way t identify genes that cause r cntribute t disrders f the nervus system. Identifying such genes ffers hpe fr identifying causes, predicting utcmes, and develping effective treatments. Genetic cntributins nly partially accunt fr mst f these disrders, hwever. The ther significant cntributin cmes frm envirnmental factrs. T distinguish between genetic and envirnmental variables, scientists ften carry ut family studies, tracking hw family members are related, which individuals are affected, and which family members grew up in the same husehld. These studies are especially infrmative when ne f the affected individuals has a genetically identical twin r an adpted sibling wh is genetically unrelated. Schizphrenia has a very strng genetic cmpnent. Abut 1% f the wrld s ppulatin suffers frm schizphrenia, a severe mental disturbance characterized by psychtic episdes in which patients lse the ability t distinguish reality. Schizphrenics typically suffer frm hallucinatins (such as vices that nly they can hear) and delusins (fr example, the idea that thers are pltting t harm them). Schizphrenics suffer fragmentatin f nrmally integrated brain functins. Tw lines f evidence suggest that schizphrenia affects neurnal pathways that use dpamine as a neurtransmitter. 1. Amphetamine, which stimulates dpamine release, can prduce the same symptms as schizphrenia. 2. Many f the drugs that alleviate the symptms f schizphrenia blck dpamine receptrs. Schizphrenia may als alter glutamate signaling because the street drug angel dust r PCP blcks glutamate receptrs and induces strng schizphrenia-like symptms. New medicatins are relatively safe and can alleviate the majr symptms f schizphrenia. Depressin is a disrder characterized by depressed md and abnrmalities in sleep, appetite, and energy level. Individuals affected by majr depressive disrder experience perids lasting weeks during which they have n interest in and take n pleasure frm nearly any activity. One f the mst cmmn nervus system disrders, majr depressin affects abut ne in every seven adults at sme pint. Wmen are affected twice as ften as men. Biplar disrder invlves swings f md frm high t lw and affects abut 1% f the wrld s ppulatin. The manic phase is characterized by high self-esteem, increased energy, a flw f ideas, vertalkativeness, and increased risk taking. The depressive phase cmes with lwered ability t feel pleasure, lss f mtivatin, sleep disturbances, and feelings f wrthlessness, smetimes leading t suicide. Bth biplar disrder and majr depressin have genetic and envirnmental cmpnents. Majr depressive and biplar disrders are amng the nervus system disrders fr which available therapies are mst effective. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

15 Many drugs, including Przac, that are used t treat depressive illness increase the activity f bigenic amines in the brain. Depressive disrders are als smetimes treated with anticnvulsant drugs r lithium. Drug addictin is assciated with the brain reward system. Drug addictin is a disrder characterized by cmpulsive cnsumptin and a lss f cntrl in limiting intake. A number f drugs that vary cnsiderably in their effects n the CNS can be addictive. Fr example, ccaine and amphetamine act as stimulants, whereas herin is a pain-relieving sedative. All f these drugs, as well as alchl and tbacc, are addictive fr the same reasn: Each increases the activity f the brain s reward system, neural circuitry that nrmally functins in pleasure, incentive mtivatin, and learning. In the absence f drug addictin, the reward system prvides mtivatin fr activities that enhance survival and reprductin, such as eating in respnse t hunger, drinking when thirsty, and engaging in sexual activity when arused. In addicted individuals, mtivatin is directed tward drug cnsumptin. Scientists have learned hw the brain's reward system wrks and hw particular drugs affect its functin, largely thrugh the use f experimental animals. Rats prvide themselves with ccaine, herin, r amphetamine when given a dispensing system linked t a lever in their cage. Rats may exhibit addictive behavir, cntinuing t self-administer the drug rather than seek fd, even t the pint f starvatin. These and ther studies have led t the identificatin f bth the rganizatin f the reward system and its key neurtransmitter, dpamine. Neural inputs are received by neurns in a regin near the base f the brain called the ventral tegmental area (VTA). When activated, these neurns direct actin ptentials alng axns that synapse with neurns in specific regins f the cerebrum and release dpamine. Addictive drugs affect the reward system. Each drug has an immediate effect that enhances the activity f the dpamine pathway. As addictin develps, lng-lasting changes in the reward circuitry prduce a craving fr the drug, independent f any pleasure assciated with cnsumptin r distress assciated with abstinence. Alzheimer s disease is prgressive and age related. Alzheimer s disease is a dementia characterized by cnfusin and memry lss. Its incidence is age related, rising frm abut 10% at age 65 t abut 35% at age 85. The disease is prgressive, as patients becme less able t functin ver time. There are als persnality changes, almst always fr the wrse. Patients ften lse their ability t recgnize peple, including family. Alzheimer s disease invlves the death f neurns in many areas f the brain, including the hippcampus and cerebral crtex, leading t massive shrinkage f brain tissue. Many symptms f Alzheimer s disease are shared with ther frms f dementia, and the disease is difficult t diagnse while the patient is alive. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

16 A pstmrtem finding f tw features amylid plaques and neurfibrillary tangles in remaining brain tissue cnfirms a diagnsis f Alzheimer s disease. The plaques are aggregates f -amylid, an insluble peptide cleaved frm a membrane prtein fund in neurns. Membrane enzymes, called secretases, catalyze the cleavage, causing -amylid t accumulate in plaques utside the neurns. It is these plaques that appear t trigger the death f surrunding neurns. The neurfibrillary tangles bserved in persns with Alzheimer's disease mainly cnsist f the prtein tau. Tau nrmally helps regulate the mvement f nutrients alng micrtubules in neurns. In persns with Alzheimer s disease, tau underges changes that cause it t bind t itself, resulting in neurfibrillary tangles. Parkinsn s disease is a mtr disrder. Parkinsn s disease is characterized by difficulty in initiating mvements, slwness f mvement, and rigidity. Patients have muscle tremrs, pr balance, a flexed psture, and a shuffling gait. Rigidity f the facial muscles decreases the ability t vary facial expressins. Parkinsn s disease is an age-related, prgressive brain illness. The incidence f Parkinsn s disease is abut 1% at age 65 and abut 5% at age 85. In the U.S. ppulatin, apprximately 1 millin peple are afflicted. The symptms f Parkinsn s disease result frm the death f neurns in the midbrain that nrmally release dpamine at synapses in the basal nuclei. As with Alzheimer s disease, there is an accumulatin f prtein aggregates. A rare frm f the disease that appears in relatively yung adults has a clear genetic basis. Mlecular studies f mutatins linked t this early-nset Parkinsn s disease reveal the disruptin f genes required fr certain mitchndrial functins. Whether mitchndrial defects cntribute t the mre frequent frm f the disease in lder patients is a subject f active research. At present, there is n cure fr Parkinsn s disease. Symptms may be managed by brain surgery, deep-brain stimulatin, and drugs such as L-dpa, a mlecule that can crss the bld-brain barrier and be cnverted t dpamine within the CNS. One ptential cure is t implant dpamine-secreting neurns, either in the midbrain r in the basal nuclei. Studies with animals make this a prmising strategy: In rats with an experimentally induced cnditin that mimics Parkinsn s disease, transplantatin f dpamine-secreting neurns can lead t a recvery f mtr cntrl. Neural stem cells may ffer a cure fr damaged r diseased brain tissue. Unlike the PNS, the mammalian CNS cannt fully repair itself when damaged r diseased. Surviving neurns in the brain can make new cnnectins and smetimes cmpensate fr damage, as in the remarkable recveries f sme strke victims. Mst brain and spinal crd injuries, strkes, and disrders that destry CNS neurns, such as Alzheimer s disease and Parkinsn s disease, have devastating and irreversible effects. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

17 In 1998, Fred Gage, at the Salk Institute in Califrnia, and Peter Erickssn, at the Sahlgrenska University Hspital in Sweden, annunced that the adult human brain can prduce new neurns. The evidence that new neurns frm in the brains f adults came frm a grup f terminally ill cancer patients wh had agreed t dnate their brains fr research upn their death. T mnitr tumr grwth in this grup, dctrs had given these patients brmdexyuridine (BrdU), an altered nucletide that is incrprated int DNA during replicatin. DNA cntaining BrdU can be readily detected and thus marks cells that grw and divide after BrdU enters the bdy. Gage and Erickssn reasned that BrdU wuld mark nt nly the grwing tumr but als any cells in the brain that had recently divided. When the patients were examined pstmrtem, there was evidence f newly divided neurns in the hippcampus f each brain. The discvery f dividing neurns in the adult brain indicated the presence f stem cells, which retain the ability t divide indefinitely. Althugh sme prgeny f stem cells remain undifferentiated, thers differentiate int specialized cells. In the brain, the stem cells are called neural prgenitr cells and are cmmitted t becming either neurns r glia. One f the gals f further research is t find a way t induce the bdy s wn neural prgenitr cells t differentiate int specific types f neurns r glia when and where they are needed. Anther bjective is t transplant cultured neural prgenitr cells int a damaged CNS and have them restre functin. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc