How Does the Brain Produce Movement?

1 PThe Hierarchical Control of MovementThe Forebrain and movement InitiationThe Brainstem and Species-Typical MovementFocus on disorders : AutismThe Spinal Cord and movement ExecutionFocus on disorders : ParaplegiaThe Organization of the Motor SystemThe Motor CortexThe Corticospinal TractsThe Motor NeuronsThe Control of MusclesThe Motor Cortex and SkilledMovementsInvestigating Neural Control of Skilled MovementsThe Control of Skilled Movements in Other SpeciesHow Motor Cortex Damage Affects Skilled MovementsThe Basal Ganglia and the CerebellumThe Basal Ganglia and movement ForceFocus on disorders : Tourette s SyndromeThe Cerebellum and movement SkillThe Organization of theSomatosensory SystemSomatosensory Receptors and Sensory PerceptionDorsal-Root Ganglion NeuronsThe Somatosensory Pathways to the BrainSpinal-Cord Responses to Somatosensory InputThe vestibular System and BalanceExploring the SomatosensorySystemThe Somatosensory HomunculusThe Effects of Damage to the Somatosensory CortexThe Somatosensory Cortex and Complex Movement354 How does the BrainProduce movement ?

2 CHAPTER10 Kevork Djansezian/AP PhotoMicrograph: Dr. David Scott/PhototakepKamala is a female Indian elephant that lives at the zoo in Calgary, Canada. Her trunk, which is really just a greatly extended upper lip and nose, con-sists of about 2000 fused muscles. A pair of nostrils runs itslength and fingerlike projections are located at its tip. Theskin of the trunk is soft and supple and is covered sparselywith sensory hairs. Like all elephants, Kamala uses hertrunk for many purposes. It can gather food, scratch an ear,rub an itchy eye, or caress a baby. It can also be used to ex-plore. Kamala raises it to sniff the wind, lowers it to exam-ine the ground for scents, and sometimes even pokes it intoanother elephant s mouth to investigate the food there. She,like other elephants, can inhale as much as 4 liters of waterinto her trunk, which she can then place in her mouth todrink or squirt over her body to bathe.

3 She can also inhaledust or mud for bathing. Kamala s trunk is a potentialweapon, too. She can flick it as a threat, lash out with it inaggression, and throw missiles with it. Her trunk is bothimmensely strong and very agile. With it, Kamala can liftobjects as large as an elephant calf, sometimes uprootingentire trees, yet this same trunk can grasp a single peanutfrom the palm of a proffered one way, however, Kamala uses this versatile trunkvery unusually for an elephant (Onodera & Hicks, 1999).She is one of only a few elephants in the world that paintswith its trunk (Figure 10-1). Like many artists, she paintswhen it suits her, but nevertheless she has commemoratedmany important zoo events, such as the arrival of newspecies to the zoo. An elephant artist is not as far-fetched asthe idea may at first seem. Other elephants, both in thewild and in captivity, pick up small stones and sticks anddraw in the dust with them.

4 But Kamala has gone well be-yond this simple doodling. When given paints and a brush,she began to Produce works of art, many of which havebeen sold to art collectors. Kamala, in fact, has achieved aninternational reputation as an defining feature of animals is their ability to the example of Kamala illustrates, even very skilledmovements are not limited to humans. Although we hu-mans display the most skilled motor control of all animals,members of many species have highly dexterous move-ments. This chapter explores how the Brain producesmovement. We begin by considering how the control ofmovement is organized. Then, we examine the variouscontributions of the neocortex, the brainstem, andthe spinal cord to movement . Of particular interestis how neurons of the motor cortex take part in pro-ducing skilled movements. Next, we investigatehow the basal ganglia and the cerebellum help tofine-tune our control of movement .

5 Finally, we turnto the role of the somatosensory system. Althoughother senses, such as vision, play a part in enablingmovement, body senses play a special role, as youwill soon discover. Figure 10-1 Kamala (her name means lotus flower ) was born in 1975 in SriLanka s Yala National Park and orphaned shortly thereafter. Shewas adopted by the Calgary, Alberta, Zoological Society. Elephantswere first observed to paint with sticks or rocks in the dust, andsome have become accomplished artists when given paints and abrush. Kamala began painting as part of an environmentalenrichment program and her paintings are widely sold tocollectors. 355 The Calgary Zoological SocietyThe Calgary Zoological SocietyTHE HIERARCHICAL CONTROL OF MOVEMENTWhen Kamala paints a picture, her behaviors are sequentially organized. First, she looksat her canvas and her selection of paints; then, she considers what she wants to paint;and, finally, she executes her painting.

6 These sequentially organized behaviors are dic-tated by the hierarchical organization of Kamala s nervous system. The major compo-nents of this nervous system hierarchy are the neocortex, the brainstem, and the spinalcord. All contribute to controlling the behaviors required to Produce her the same way, your hierarchically organized nervous system controls everymovement that you make. Figure 10-2 shows the sequences of steps taken when thehuman nervous system directs a hand to pick up a coffee mug. The visual system mustfirst inspect the cup to determine what part of it should be grasped. This informationis then relayed from the visual cortex to cortical motor regions, which plan and initiatethe movement , sending instructions to the part of the spinal cord that controls themuscles of the arm and hand. As the handle of the cup is grasped, information fromsensory receptors in the fingers travels to the spinal cord, and from there messages aresent to sensory regions of the cortex that control touch.

7 The sensory cortex, in turn, in-356 CHAPTER 10pMotor neuronSenory neuronFrontal-lobe motor areas plan the reach and command the receptors on the fingers send message to sensory cortex saying that the cup has been neurons carry message to muscles of the hand and cord carries sensory information to cord carries information to information required to locate cortex receives the message that the cup has been ganglia judge grasp forces, and cerebellum corrects movement 10-2 The Brain tells the hand to reach, and thehand tells the Brain that it has suc-ceeded. Movements such as reaching fora cup require the participation of wideareas of the nervous system. The motorregions of the frontal lobe formulate theplan and command the movements re-quired to reach for the cup. The messageto the muscles is carried by pathwaysfrom the frontal lobe to the spinal neurons of the spinal cord carrythe message to the muscles of the handand arm.

8 Sensory information from thevisual system is required to direct thehand to the cup, and sensory informa-tion from sensory receptors in the handis required to confirm that the cup has been grasped. The basal gan-glia participate in the movement by estimating the forces required to make the grasp, and the cere-bellum participates by correcting errors in the movement as it is the motor cortex that the cup is now being held. Other regions of the Brain alsoparticipate in controlling the movement , such as the basal ganglia, which help to pro-duce the appropriate amount of force, and the cerebellum, which helps to regulatetiming and corrects any errors in movement . Although at this point you probably willnot remember all these various steps in controlling a movement , refer to Figure 10-2when you reach the end of this chapter as a way of reviewing what you have important concept to remember right now is simply the hierarchical organizationof the entire idea that the nervous system is hierarchically organized originated with theEnglish neurologist John Hughlings-Jackson.

9 He thought of the nervous system asbeing organized into a number of layers, with successively higher levels controllingmore complex aspects of behavior by acting through the lower levels. The three majorlevels in Hughlings-Jackson s model are the same as those just mentioned for Kamala:the forebrain, the brainstem, and the spinal cord. Hughlings-Jackson also proposedthat, within these divisions, further levels of organization could be adopted the concept of hierarchical organization from evolu-tionary theory. He knew that the chordate nervous system had evolved in a series ofsteps: the spinal cord had developed in worms; the brainstem in fish, amphibians, andreptiles; and the forebrain in birds and mammals. Because each level of the nervous sys-tem had developed at different times, Hughlings-Jackson assumed that each must havesome functional independence. Consequently, if higher levels of the nervous systemwere damaged, the result would be regression to the simpler behaviors of lower ani-mals, a phenomenon that Hughlings-Jackson called Brain -damagedperson would still possess a repertoire of behaviors, but they would be more typical ofanimals that had not yet evolved the destroyed Brain hierarchically organized structure such as the mammalian nervous system, how-ever, does not operate piece by piece.

10 It functions as a whole, with the higher regionsworking through and influencing the actions of the lower ones. In the control ofmovement, many parts of the nervous system participate, with some regions engagedin sensory control, others in planning and commanding the movement , and still oth-ers in actually carrying the action out. To understand how all these various regionswork together to Produce even a simple movement , we will consider the major compo-nents of the hierarchy one by one, starting at the top with the Forebrain and movement InitiationComplex movements, such as painting a work of art, include many components. Forinstance, your perceptions of what is appearing on the canvas must be closely coordi-nated with the brush strokes that your hand makes to achieve the desired effect. Thesame high degree of control is necessary for many other complex behaviors. Considerplaying basketball.