C H A P T E RC O N T E N TO U T L I N EBasic Neurological Principles thatSupport PerformanceThe Affective SystemsThe Sensory SystemsEmotional SystemMotivational SystemThe Motor SystemsSummaryInteraction Among SensorimotorSystems t o Support PerformanceThe Cognitive SystemsThis chapter reviews the basic systems and functions of the central nervous system. After providing overviews of each system, the chapter also provides methodsfor interpreting the meaning of behaviors from a neuroscience point of view.Autogenic hcilitationCompensatory action of the CNSAutogenic inhibitionBalance of powerCentrifugal controlCorporal tionSensitizationPoor registrationSensitivity t o stimuliConvergenceDivergencePlasticityKelease ory integrationSensation seekingSensation avoidingHomonymous hemianopia
Neuroscience Principlesto Support Habilitationand RecoveryWinnie l ui n,PhD, OTR, FAOTAOBJECTIVESThe i y % m a t i o in this chaptcr is iutended to help the vende c1. appreciate the importance o f various brain fimctions in the performance oflife tasks2. identifp the basic neurological mechanisms which support performance3. ciescribe thc similarities and differences a m o n g syste nswithin the centralIIer\ OlISSYStClll4. understand the roles o f attcntional, motivational and emotional systems insupport o f performance5. recognize examples of release plienomcn lwithin the CNS6 . [email protected] a range of contextually rclcvant interventions t o support performance in persons with bmin involvement in their disability7. interpret the iicaningof behaviors from a neuroscience point of view, as acontributing factor in intervention p l n n i n g .
BASICNEUROLOGICALPRINCIPLESTHAT SUPPORTPERFORMANCENeurological principles enable us to understand how thenervous system operates to support performance. They areimportant because they subserve all the functions of the centralnervous system (CNS) and are one of the key intrinsic factorsthat support occupational performance (see PersonEnvironment Occupational Performance Model in Chapter 3).The occupational therapist can select and design activities compatible with central nervous system functions, and take fulladvantage of their impact to support or facilitate performance.Centrifugal ControlCentrijirgal control, the most basic principle of CNS operations, is the brain's ability to regulate its own input. The CNSensures that the information it receives and processes is themost useful for its own functioning. Four types of centrifugalcontrol: suppression, balance of power, divergence and convergence (Noback & Demerest, 1987) will be discussed.SuppressionSuppression is the CNS' ability to screen certain stimuliso that other stimuli receive more carefill attention. Personsare bombarded with an array of sensory stimuli throughoutthe day; through suppression, the brain determines whichstimuli warrant attention and response and which stimuli canbe ignored safely. People who are distractible are likely tohave poor suppression: they have difficulty engaging in purposeful behavior because they are constantly attending to allthe other stimuli available in the environment. Those withpoor suppression often have difficulty screening out appropriate stimuli. If the wrong stimuli are screened out or if stimuliare screened based on rigid or ritualistic patterns rather thanin response to specific environmental demands, the personmay engage in inappropriate or even dangerous behaviors.Balance of PowerThe balance of power refers to the complementary functions of the various parts of the brain. For example, certainparts of the brain are responsible for increasing activity level,while other parts provide inhibitory control. Some parts ofthe brain initiate movement and other parts stop or controlthe amount of movement that occurs. Normally, there is abalance of power. enabling the CNS to finely tune responsesto meet all environmental demands.When the balance of power is disrupted as in brain injury,Bibliographic citation of this chapter: Dunn. W. (1977). Implementing neuroscience principles to support habilitation and recovery. In C. Christiansen& C. Baum (Eds.), Occ.r pu/iorlclltherulq: E i i u b l i n g i i r c t i mcioi w / / - b r i n g(2nd ed.). Thorofare. NJ: SLACK Incorporated.the CNS experiences a release phenomenon. With injury inhigher cortical centers, arousal mechanisms are releasedfrom inhibitory control, which can lead to hyperactivity anddistractibility. These behaviors do not reflect dysfunction inthe arousal mechanisms themselves (i.e., the reticular activating system). but rather represent a disassociation ofarousal systems from control usually provided by modulatingcenters (i.e., a release). A release phenomenon can occur inany part of the CNS related to any function. Many of theabnormal behaviors observed and documented by occupational therapists can be attributed to a release phenomenon,or poor maintenance of the balance of power in the CNS.DivergenceDivergence is the CNS's ability to send information itreceives from one source to many parts of the CNS simultaneously. For example, if the brain receives a stimulus signaling potential harm, that information needs to get to manyareas sin ultaneouslyto generate "fight or flight" responses.Divergence also ensures that an entire muscle is engaged inaction when a movement is required, rather than a smallnumber of muscle fibers.ConvergenceConvergent neurons require input from a variety ofsources before a response will be generated. For example. aspecific neuron may activate only if it receives three or moretypes of input. Convergent neurons enable the CNS to temper responsiveness to specific stimuli thus preventing a person from reacting inappropriately when only partial stimuliare available. Without this convergent neuronal network, theindividual would respond to every stimulus, demonstratingpoor integration of the convergent neuronal network.Balance of Excitation and InhibitionExcitationExcitation is the depolarization of neurons that movesthem closer to the activation threshold. Temporal patternsoccur when a neuron is repeatedly stimulated, enabling theneuron to either be slowly depolarized or to continue sendinga message over and over again as is required to maintain muscle tone for postural control. Spatial neuronal patterns occursimultaneously over many areas of the brain when engagingmany CNS components at once is important, such as whennoticing a sound, recognizing it ah a baby's cry, determiningit is a hungry cry, and proceeding to fill that need.In hibitionInhibition is the hyperpolarization of neurons that makesit more difficult for the neurons to activate. Inhibition is the
CNS's ability to decrease its responsiveness to specific stimuli at any given moment. Although neuroscientists frequently discuss descending inhibition (i.e., higher centers havingan effect on lower centers), the CNS is rich with interneuronnetworks that can inhibit neighboring cells (i.e., lateral inhibition). Lateral inhibition helps make target stimuli strongerand clearer, which contributes to organization and integrationof information for the CNS.Feedback and Feedforward MechanismsFeedback is the CNS's ability to send information backto itself as a check and balance. This enables the person tojudge whether actions already initiated need to be modifiedfor the future. Nohack & Dernerest ( 198 I ) describe two general types of feedback: local feedback and reflected feedback. Local feedback inhibition occurs when small interncurons within a neighboring circuit form connections that stopa stimulus that has been occurring in a large neuron. Thismechanism helps keep activity froin continuing beyond itsiiseful period. For example, a local feedback circuit can stopthe ongoing firing of a niotor neuron by sending an inhibitory signal to the motor nerve, thus allowing a pcrson to relaxthe muscle and stop the movement. This inhibition, however,occurs only if the local circuit neuron information is strongerthan other messages on that large motor neuron. Reflectedfeedback occurs when higher CNS centers send descendingfibers to influence the sensory or niotor neurons. These higher centers can send excitatory or inhibitory messages makingit either easier or more difiicult to activate the neurons.Feedforward circuits exert influence in a forward direction coinciding with the information flow of the neurons.This most frequently occurs in ascending sensory systempathways to either alert higher centers about incoming information or inhibit some areas in order to strengthen the focusof more important parts of the environmental stimuli. Feedforward inhibition is critical for task performance, becausepeople always are confronted with more stimuli than areneeded to complete the task successfully.Intersensory IntegrationIntersensory integration is a critical feature of the CNS.Interneurons in the spinal cord participate in primitive intersensory integration, however the brain stem is a primary sitefor this activity. Nuclei in the brain stem receive input fromseveral sensory sources, allowing organization and integration of information at this level. For example, the vestibularnuclei receive input from the vestibular organ, the visual system and the proprioceptors. This intersensory integrationsupports the development of a multidimensional map of selfand environment and a map of how self and environmentinteract appropriately.PlasticityPlasticity is the CNS's ability to adapt structurally orfunctionally in response to environmental demands.Previously, neuroscientists believed that plasticity was mostevident during the prenatal stage and childhood; nowresearchers have found that there are various types of internal and external environmental alterations that support orinhibit the manifestations of plasticity (Lund, 1978). Whenindividuals participate in interventions that require morefunctional patterns of movement, they receive organized patterns of sensory feedback that may alter the internal environment, creating opportunities for axon reorganization, alteredsynaptic activity, or dendritic branching, just as enrichedenvironments support these actions during the developmentalperiods (Bach-y-Rita, 1980; Moore, 1980). Such findingschallenge innovative rehabilitation practice.Compensatory Action of the CNSParts of the CNS are interdependent (Moore, 1980).When damage occurs to one or more portions of the system,the interdependency relationships are disrupted; the resultingobservable behaviors and performance are hypothesized tobe compensating for the loss of information from one of itsmembers. This compensatory model suggests that the CNScenters d o not function on their own to initiate action, butwork as part of a network in the CNS to enable a particularaction. When data are not available to the system because ofdamage, the network is disrupted, making it difficult for aparticular action to occur. Destructive behavior is the resultof an incomplete and compensating CNS.SummaryThe principles of centrifugal control, balance of excitation and inhibition, plasticity, and compensatory action arebasic and must be used to design interventions that supportoccupational performance. Interventions that conflict withthese basic principles compromise the person by notenabling them to profit from the therapeutic experience.The olfactory (smell) and gustatory (taste) systems areprimitive, chemically-based sensory systems signaling theCNS about odors and tastes. The somatosensory, proprioceptive and vestibular systems enable an individual to developan accurate map of self and how one interacts with the environment. Finally, the visual and auditory systems are responsible for mapping environmental variables so that interactionwith the environment can he accurate and reliable.
General Principles of Sensory SystemOperationsAlthough each sensory system is unique, there are somebasic similarities in the way all sensory systems develop andfunction:1 . Input tnechrrnisms. Each sensory system is responsiblefor bringing information from the environment to the nervous system for processing.2 . Processing levels. Each sensory system processes its ownspecial brand of information at a variety of central nervous system levels including the receptor site, spinalcord, brain stem, thalamus (except for olfactory), andhigher cortical centers.3 . Multidimensiot ulitlfi,rn trtiotr.The sensation from eachsensory system is complex and multidimensional.4. Purposes , f i r processiilg infortntrtiotr. Each sensory system processes information for two primary purposes: a )to identify stimuli in the environment, making the CNSaware of these stimuli, determining which require attention and which are potentially harmful (i.e., arousal oralerting mechanisms) and b) gathering information toconstruct maps of self and environment to bc used by theCNS for organization and planning (i.e., discriminationor mapping).When the CNS is functioning normally, the arousallalerting and discriminationlmapping components complementeach other to form a balance of power. This allows the individual to interact with the environment and gather inforniation for discrimination and mapping under most conditions,while always having the capability to notice potentiallyharmful stimuli. This balance of power is delicate andrequires constant assessment of environmental stimuli so thatall potentially important stimuli are noticed without interfering with ongoing purposeful activity.The components of arousallattention and discriminalionlmapping will be discussed in each sensory system section. Asa guide to understanding the sensory systems, Tables 8- 1,8-2. and 8-3 summarize key information that can be used thcrapeutically. Table 8-1 lists the descriptors for each sensorysystem; the descriptors are divided into columns to correspond to the arousallalerting and discriminationlmappi