47

Chapter 4

Motor control

Bernhard Haas

CHAPTER CONTENTS

Introduction 47

Information transmission 48Receptors 49

Motor control 52Controlling ‘simple’ movements 52Postural control and balance 52Gait 53The structures of the nervous system forcontrolling movement 53Control processes of voluntary movement 58

LEARNING OUTCOMES

When you have completed this chapter, you should beable to:1. outline the roles of the various nervous system

centres in the control of movement2. explain how movements are planned, generated and

controlled3. apply understanding to specific movements (e.g.

balance and gait)4. demonstrate knowledge of information transmission

within the nervous system.

INTRODUCTION

The purpose of this chapter is to help you withunderstanding the human nervous system andhow it participates in motor control. This chapterwill give you an overview that will help you getthe whole picture, and you should refer to specifictexts to obtain detailed information. Traditionally,this has been done by reducing the function of thiscomplex system to the properties of its individ-ual elements: the neurons and control centres.Although it is essential to understand the languageused to describe the system and to have a basicknowledge about which elements contribute tothe complex affair of human movement, thisreductionist view is not sufficient, and sometimesyou will have to ignore the individual elements inorder to better understand the system as a whole.

One of the traditional views of the nervous sys-tem has been that it is purely hierarchical, with a

top down approach of control. According to thisview, the cortical areas of the brain would exert ahigher level of control and organize voluntaryskilled movement. On the other end, the spinal cordwould be fairly low down in the control stakes andmainly execute plans designed and refined above.A number of textbooks, such as Tortora and Derrick-son (2008),Ganong (2003),Kiernan (2008) andMartiniand Nath (2008), will provide you with furtherdetails. The section in this chapter on The structuresof the nervous system for controlling movement will alsogive you an overview of the individual parts of themovement control system. In reality, there is, how-ever, no real separation between voluntary move-ments and the background of postural control thatmaintains the body in an upright position with theaid of automatic reflexes and responses. See alsoChapter 5 for more information on posture and bal-ance. Therefore parallel systems of control, with inte-gration of all levels rather than just a serial hierarchy,may be a more appropriate description. All levels ofcontrol, from the spinal cord up to the cerebral cor-tex, are necessary and integrated to provide the baseof axial stability for more normal distal mobility andskilled or refined coordinated limb movements(Kandel et al. 2000). In addition, the environmentalcontext and the movement task itself will influencehow the nervous system organizes movement.

ACTIVITY 4.1

Chris reports that one of his friends at university hasbeen involved in a car accident. He injured his legduring the accident, but his bones are now healed andhe is back at university. Unfortunately, he still finds itdifficult to move his foot because one of his nerves inhis leg sustained an injury.

Work in small groups or on your own. Find a physiologytextbook that has a diagram of peripheral nerves and findthe common fibular nerve. Identify the function of thisnerve and explain why Chris’s friend may find it difficultto move his foot. He also cannot feel touch or pressureover the dorsum of his foot. Explain why.

INFORMATION TRANSMISSION

The vast numbers of neurons in the human ner-vous system need to communicate with each other,often very rapidly. Information within neuronsand between neurons is carried by electrical and

chemical signals. The rapid transmission of signals,which is vital for human movement, is a functionof the action potential. This action potential isachieved by temporary changes of current flow inand out of cells, which then propagate a signalalong the nerve axon. A necessary precondi-tion for action potentials is the creation of a mem-brane potential, the resting potential (Tortora andDerrickson, 2008). Please note that no movement ispossible if the action potential is completely inter-rupted, and that movement will be impaired ifthe signal propagation is abnormal. This may bethe case if the myelin sheath that surrounds nerveaxons is damaged, such as in multiple sclerosis. Fig-ure 4.1 shows the concentrations of ions inside andoutside the nerve cell during the resting potential.Figure 4.2 shows the changes in membrane poten-tial during the action potential. Box 4.1 sum-marizes the key facts about the action potential.

Information transmission from one cell to anotheroccurs at the synapse. The most important compo-nents of a synapse are the presynaptic membrane,the synaptic cleft and the postsynaptic membrane(Latash, 2008). An action potential arrives at the pre-synaptic membrane. This leads to the influx of Ca2þ,which in turn facilitates the fusion of neurotransmittervesicles to the membrane for the release of the neuro-transmitter into the synaptic cleft. Neurotransmittermolecules diffuse across the synaptic cleft and bindat specialist receptor sites in the postsynaptic neuron.This changes the potential in the postsynaptic neuronas ion channels are opened and the voltage across thecell membrane changes. Depending on the particulartype of channel that is being activated, either depolar-ization or hyperpolarizationmay occur. This explainshow an action potential in the presynaptic neuron cancause either excitation or inhibition of the postsynap-tic membrane. Opening of Naþ channels would leadto depolarization and therefore excitation, whereasopening of the Cl� channels would hyperpolarizethe postsynaptic neuron and lead to inhibition. Fig-ure 4.3 summarizes the events that occur at a synapse.

ACTIVITY 4.2

Work in small groups or on your own. Find a physiologytextbook that has a diagram of peripheral nerves andanswer the following question: chemical synapses suchas the one shown in Figure 4.3 transmit a signal inonly one direction – why?

48 Human Movement

RECEPTORS

The central nervous system (CNS) needs to receivecontinuous feedback about movement. It receivesthis information in the form of the status of mus-cles, i.e. length, instantaneous tension, and rate ofchange of length and tension (Shumway-Cookand Woollacott, 2007). Muscle spindles detect therate and changes in the length of a muscle, whereasGolgi tendon organs detect degree and rate ofchange of tension. Signals from these sensory

receptors operate at an almost subconscious level,transmitting information into the spinal cord, cere-bellum and cerebral cortex, where they assist in thecontrol of muscle contraction.

The muscle spindle has both a static and adynamic response. The primary and secondaryendings respond to the length of the receptor, soimpulses transmitted are proportional to thedegree of stretch and continue to be transmittedas long as the receptor remains stretched. If thespindle receptors shorten, the firing rate decreases.

Cl –

Extracellular fluid

Na+

A–

Intracellular fluid

Plasma membrane

Cl –

Cl –

Cl –

Cl –

Na+ Na+ Na+ Na+ Na+

K +

A– A– A– A– A–

A–

Cl –

Na+

K +

K +

K +

K +

Ion Concentration intracellular fluid Concentration in extracellular fluid

150 mmol/l

12 mmol/l

5 mmol/l

150 mmol/l

5 mmol/l

150 mmol/l

125 mmol/l

–

K + – Potassium

Na+ – Sodium

Cl – – Chlorine

A– – Organic anions

Membranechannel

Sodium–potassiumpump

Figure 4.1 Distribution of ions across the cell membrane during the resting potential.

Chapter 4 Motor control 49

30

0

–55

–70

Depolarization

Repolarization

Stimulus

Hyperpolarization

Threshold

Time in milliseconds

Mem

bran

e po

tent

ial i

n m

illivo

ltsResting membranepotential

Figure 4.2 The action potential.

Ca2+ Ca2+

Presynaptic end bulb

Synaptic cleft

Postsynaptic cell

Na+A B C

Figure 4.3 Synaptic events associated with the action potential.

BOX 4.1 Summary of important facts associated with the action potential

n A necessary precondition for the creation of anaction potential is the resting potential.

n The resting potential creates the excitability of thecell.

n The resting potential is the unequal distribution of ionsacross the cell membrane, with a negative charge of�70 mV in the cytosol (the intracellular fluid).

n The action potential emerges if a stimulus is largeenough to take the membrane potential above thethreshold (�55 mV).

n When the threshold level is reached, voltage-gatedNaþ channels open and Naþ rushes into the cell,which produces the depolarization period.

n Voltage-gated Kþ channels open to allow Kþ to flowout of the cell and produce the repolarization period.

n Another action potential cannot be generated duringthe depolarization period and during most of therepolarization period.

n The action potential propagates along the axonsegment by segment until it reaches the synaptic endbulb at the end of the axon.

n Propagation is more rapid in myelinated axons, wherethe signal leaps from node to node. Large-diameteraxons also propagate signals faster than small-diameter axons.

n Axons of sensory neurons transmitting informationabout touch, pressure and movement, as well as theaxons of motor neurons transmitting movementinstructions to the skeletal muscles, are all large andmyelinated.

50 Human Movement

Only the primary endings respond to suddenchanges of length by increasing their firing rate,and then only while the length is actually increas-ing. Once the length stops increasing, the dischargereturns to its original level, although the staticresponse may still be active. If the spindle recep-tors shorten, then the firing rate decreases.

Control of the static and dynamic response is bythe gamma motor neuron. Normally, the musclespindle emits sensory nerve impulses continuously,with the rate increasing as the spindle is stretched(lengthened) or decreasing as the spindle shortens.

The spinal reflexes associated with the musclespindle and Golgi tendon organ are the stretchreflex and the tendon reflex, respectively. Stimula-tion of the stretch reflex leads to a reflex contrac-tion of the muscle that has been stretched,whereas the tendon reflex will lead to a reflexrelaxation of the muscle if there is tension build-up. Both reflexes have a protective function.

The stretch reflex also has the ability to preventsome types of oscillation and jerkiness of bodymovements even if the input is jerky, i.e. a damp-ing function (Hall, 2005).

When the motor cortex or other areas of thebrain transmit signals to the alpha motor neurons,the gamma motor neurons are nearly always sti-mulated simultaneously, i.e. a coactivation of thealpha and gamma systems so that intra- and extra-fusal muscle fibres (usually) contract at the sametime. This stops the muscle spindle opposing themuscle contraction and maintains a proper damp-ing and load responsiveness of the spindle

regardless of change in muscle length. If the alphaand gamma systems are stimulated simultaneouslyand the intra- and extrafusal fibres contractequally, then the degree of stimulation of the mus-cle spindle will not change. If the extrafusal fibrescontract less because they are working against agreat load, the mismatch will cause a stretch onthe spindle, and the resultant stretch reflex willprovide extra excitation of the extrafusal fibres toovercome the load (Cohen, 1998).

The gamma efferent system is excited or con-trolled by areas in the brainstem, with impulsestransmitted to that region from the cerebellum,basal ganglia and cerebral cortex.

The Golgi tendon organ, as a sensory receptor inthe muscle tendon, detects relative muscle tension.Therefore it is able to provide the CNS with instan-taneous information on the degree of tension ofeach small segment of each muscle. The Golgitendon organ is stimulated by increased tension.When the increase in tension is too great, thetendon reflex response is evoked in the same mus-cle, and this response is entirely inhibitory. Thebrain dictates a set point of tension beyond whichautomatic inhibition of muscle contraction pre-vents additional tension. Alternatively, if the ten-sion decrease is too low, then the Golgi tendonorgan reacts to return the tension to a more normallevel. This leads to a loss of inhibition, so allowingthe A-alpha motor neuron to be more active andincrease the muscle tension.

Box 4.2 lists key facts about receptors andreflexes.

BOX 4.2 Summary of important facts associated with receptors and reflexes

n Sensory feedback for movement control is mainlyprovided by receptors inside the muscle and betweenthe muscle and tendon.

n The receptors are the muscle spindle and the Golgitendon organ.

n The muscle spindle provides information aboutmuscle length changes.

n The Golgi tendon organ provides information abouttension changes.

n Both of these receptors are also closely linked tospinal reflexes.

n The stretch reflex relies on muscle spindleinformation and is triggered when a muscle is

lengthened. It is designed to prevent overstretchingof a muscle by causing a reflex contraction of thelengthened muscle.

n The tendon reflex relies on Golgi tendon informationand is triggered when tension is building up at theinterchange of muscle and tendon. It is designed toprevent tearing of a muscle by causing a reflexrelaxation of the muscle.

n Stretch reflex and tension reflex therefore haveopposite effects on a muscle.

Chapter 4 Motor control 51

MOTOR CONTROL

CONTROLLING ‘SIMPLE’ MOVEMENTS

Human movement is anything but simple. Thereis infinite variability, and any attempt to describea complex system in simple terms is likely to tellyou only part of the story. However, if you under-stand the ‘simple’, then you are more likely tograsp the more complex.

Most human voluntary movements require thedesign and planning of that movement by a controlcentre. This control centre will use previous experi-ences in the planning of movements. Once a move-ment plan has been designed, it will be supplied asa signal by the control centre in a feed forwardman-ner to an execution centre responsible for activatingmuscles to produce the movement. Feed forwardimplies that the signal is independent of the outputor any other variable (Latash, 2008). The feed for-ward signal uses knowledge of the dynamics ofthe musculoskeletal system and the environment itis in (Stroeve, 1999). Once the movement hasstarted, receptors will be able to provide feedbackabout the movement. The controller may then beable to alter the signal according to this feedback.The addition of a comparator centre provides amechanism to speed up the refinement of move-ment according to its feedback. Over a period oftime, the feedback will in turn influence the feedforward signal designed by the control centre andmotor learning will have taken place (Houk et al.1997). It may be worth visiting Chapter 6 (MotorLearning) before you move on. Figure 4.4 showssuch a simple movement control system using feedforward and feedback mechanisms.

POSTURAL CONTROL AND BALANCE

Postural control and balance involve controlling thebody’s position in space for stability and orientation(Shumway-Cook andWoollacott, 2007). The nervoussystem participates in postural control by design-ing command signals and by providing feedbackthrough a number of receptors. The interpretationand integration of all the feedback signalswould alsobe undertaken by the nervous system. The posturalcontrol requirements vary with the task. For exam-ple, sitting in a chair and watching televisionrequires minimal stability control, whereas standingon one leg and watching television is a lot moredemanding on the postural control system.

Therefore, we need to have a flexible controlsystem that can adapt to these varying demands.Like the simple movement system above, posturalcontrol requires the production of movements ormuscular contractions that help keep the bodyupright in space. Like the simple movement sys-tem above, postural control is also achieved by acombination of feed forward and feedbackmechanisms. The control centre for posture willutilize previous experiences. These previousexperiences will contribute to an internal represen-tation of the body or body schema (Massion, 1994).This body schema provides reference points forbody alignment, movement and orientation inspace. The aim of the nervous system is then tomaintain this body schema during changes in theenvironment or during movement. The feedbackmechanisms for posture and balance involve more

Control centre

Design of movement signal,using previous experiences.Following planning andrefinement a movement signalis sent to execution centre

Comparator centre

Receives a copy of movementsignal from control centre andfeedback from execution centreabout current output. Makescomparison between plan andoutput and can changemovement signal if there is adiscrepancy

Execution centre

Receives movement signal fromcommand centre and instructsappropriate muscles to contract.Movement generates feedbackon output. This is sent tocomparator centre to check fordiscrepancies. Receives alteredmovement signal fromcomparator centre if there wasa discrepancy.

Figure 4.4 Feed forward and feedback system of a ‘simple’voluntary movement.

52 Human Movement

than just the receptors for movement in the mus-cles. In addition, there will be feedback aboutmovements of the head through the vestibular sys-tem in the inner ear, visual feedback, and feedbackabout pressure changes through the support sur-faces of the body (Kandel et al. 2000). The feed for-ward mechanisms will have to include signals thatare able to anticipate disturbances to the posturalcontrol system that will arise as a consequence ofmovement (Aruin et al. 2001). Figure 4.5 shows thesystem of feed forward and feedback for the controlof posture and balance. Please read Chapter 5 formore detailed exploration of these issues.

GAIT

Walking requires the cooperation of a large num-ber of muscles and joints. Research on animalshas shown that a neural network in the spinal cordis responsible for regulating the stepping motionsduring gait. There is controversy about whethersuch a network also exists in humans (Vilenskyand O’Connor, 1997; Guadagnoli et al. 2000).

The brainstem, together with the spinal cord, couldprovide such a network or central pattern genera-tor to coordinate locomotion. The impulse forwalking may come from higher cortical centres,but these central pattern generators could providethe motor pattern for walking. Figure 4.6 providesa proposed diagram of a central pattern generatorfor movement in the lamprey fish (Grillner et al.1995). A similar neural network may also exist inhumans. It may be worth also visiting Chapter 11.

Box 4.3 lists key facts about the nervous systemand movement.

THE STRUCTURES OF THE NERVOUSSYSTEM FOR CONTROLLING MOVEMENT

This chapter has so far given you an overview ofhow the nervous system controls all types of move-ment and how the necessary signals for this controlare generated and propagated. This has been thedifficult part of the chapter, and once you haveunderstood that, you should move on to the nextpart. This part will add some more detail about

Postural correction

Feedback

Vision Proprioception Equilibrium Pressure

Control centre

Design of movement signal,using body schema

Execution centre

Receives movement signals from command centreand instructs appropriate muscles to contract.Some of these muscles have postural function(e.g. maintain posture) while others have focalmovement function (e.g. move limb)

Feed-forward signalincludes anticipatorypostural adjustment todeal with focal movement

The focal movementproduces a posturalperturbation which issensed through feedbackmechanisms

Figure 4.5 Feed forward andfeedback system for posturalcontrol.

Chapter 4 Motor control 53

I E

L M

E I

M L

Feedback from muscle spindles

Higher control centre

Design of movement signal, usingprevious experiences. Following planningand refinement a movement signal is sentto the execution centre in brainstem andspinal cord

Lower control centre in brainstem

and spinal cord

Signal bursts excite all interneurons (E) within the box. That means they excite the motor neuron (M) formuscle contraction, the inhibitory interneurons (I), which cross the midline to inhibit the activity in thecontralateral side, and the lateral interneuron (L), which then in turn inhibits the interneuron (I). Feedbackfrom muscle spindles provides feedback of movement which can provide further excitatory stimuli ipsilaterallyor inhibitory signals contralaterally.

Figure 4.6 Hypothetical model of a central pattern generator for locomotion. (After Grillner et al. 1995, with permission.)

BOX 4.3 Key factors relating to the nervous system and movement

n The nervous system as a whole controls all types ofmovement.

n These movements can differ in complexity andcharacteristics. They can range from a relativelysimple contraction of a muscle over one joint tomultijoint and whole body movements such as thoseused during walking.

n Voluntary movements are designed utilizing previousexperiences and use feed forward signals towards themuscles.

n In addition to the feed forward signals, there will alsobe feedback about movement and the body inrelation to the environment.

n All voluntary movements, including posture, balanceand gait, are based on these feed forward andfeedback principles.

n The feedback generated through movementexperience also provides for the possibility of motorlearning.

54 Human Movement

the individual parts of the nervous system, whichhave been described only in very broad terms upuntil now. For example, you will find that the com-parator centre described in Figure 4.4 is in realitycalled the cerebellum.

Cerebral cortex

The cerebral cortex is the main centre for the con-trol of voluntary movement. It uses the informationit receives from the cerebellum, basal ganglia andother centres in the CNS, as well as the feedbackfrom the periphery, to bring movements under vol-untary control.

The cerebral cortex, or more specifically the asso-ciation areas of the cerebral cortex, provides theadvanced intellectual functions of humans, havinga memory store and recall abilities along with otherhigher cognitive functions. The cerebral cortex is,therefore, able to perceive, understand and inte-grate all the various sensations. This provides thetransition from perception to action (Shumway-Cook and Woollacott, 2007). Its primary movementfunction is in the planning and execution of manycomplex motor activities, especially the highlyskilled manipulative movements of the hand. Thisfact becomes clear when one considers the size ofa cortical area for a particular part of the body.

The motor cortex occupies the posterior half ofthe frontal lobes. It is a broad area of the cerebralcortex concerned with integrating the sensationsfrom the association areas with the control ofmovements and posture. It is closely related toother motor areas, including the primary motorarea and the premotor or motor association area.The primary motor area contains very large pyra-midal cells that send fibres directly to the spinalcord and anterior horn cells via the corticospinalpathways. In contrast, the premotor area has afew fibres connecting directly with the spinal cord,but it mainly sends signals into the primary motorcortex to elicit multiple groups of muscles, i.e. sig-nals generated here cause more complex muscleactions usually involving groups of muscles thatperform specific tasks, rather than individual mus-cles. This area connects to the cerebellum and basalganglia, which both transmit signals back, via thethalamus, to the motor cortex. Projection fibresfrom the visual and auditory areas of the brainallow visual and auditory information to beintegrated at cortical level to influence the activityof the primary motor area.

Each time the corticospinal pathway transmitsinformation to the spinal cord, the same informa-tion is received by the basal ganglia, brainstemand cerebellum. Nerve signals from the motor cor-tex cause a muscle group to contract. The signalthen returns from the activated region of the bodyto the same neurons that caused the contraction,providing a general positive feedback enhancementif the movement was successful and recording it forfuture use. The role of the cerebral cortex and itssubdivisions is described in Figure 4.7.

Basal ganglia

The basal ganglia consist of five nuclei deep insidethe brain (putamen, caudate nucleus, globus palli-dus, subthalamic nucleus and substantia nigra).They serve as side loops to the cerebral cortex,because they receive their input from the cerebralcortex and project exclusively back to the cerebralcortex. The basal ganglia are involved in all typesof movement but have a predominant role in theprovision of internal cues for the smooth runningof learned movements (Morris and Iansek, 1996).

It is believed that the basal ganglia play anessential role in the selective initiation of mostactivities of the body as well as the selective sup-pression of unwanted movements. A number ofdistinct loops have been described, and the inter-connections of inhibitory or excitatory neurotrans-mitters explain the variety of symptoms thatemerge in disorders of the basal ganglia. The directpathway is responsible for the facilitation of move-ment, whereas the indirect pathway is moreinvolved in the inhibition of unwanted movements(Rothwell, 1994). Figure 4.8 shows the direct loopthrough the basal ganglia.

Cerebellum

The cerebellum is vital for the control of very rapidmuscular activities such as running, talking, typ-ing, playing sport or playing a musical instrument.Loss of the cerebellum leads to incoordination ofthese movements such that the actions are stillavailable but no longer rapid or coordinated.This is caused by the loss of the planning function.

The cerebellum makes comparisons between themovement plan and output and can change move-ment signal if there is a discrepancy.

Extensive input and output systems operate toand from the cerebellum. Input pathways to thecerebellum from the cerebral cortex, carrying both

Chapter 4 Motor control 55

motor and sensory information, pass through vari-ous brainstem nuclei before reaching the deepnuclei of the cerebellum. Likewise, output fromthe cerebellum exits via the deep nuclei to the cere-bral cortex to help coordinate voluntary motoractivity initiated there.

The cerebellum does not initiate motor activitiesbut plays an important role in planning, mediating,correcting, coordinating and predicting motoractivities, especially for rapid movements. It isvitally important for the control of posture andequilibrium, when it works in close relationshipwith the brainstem. Working with the basal ganglia

and thalamus, the cerebellum helps to control vol-untary movement by utilizing feedback circuitsfrom the periphery and the brain. The distal partsof the limbs are controlled by information from themotor cortex and from the periphery, and this infor-mation is integrated in the cerebellum. This pro-vides smooth, coordinated movements of agonistsand antagonistic muscle groups, allowing the per-formance of accurate, purposeful intricate move-ments, which are especially required in the distalpart of the limbs. This is achieved by comparingthe intentions of the higher centres of the motor cor-tex with the performance of respective parts of thebody. Overall, the cerebellum serves as an error-correcting device for goal-directed movements.It receives information on body position and move-ments in progress and then computes and deliversappropriate signals to the brainstem effector centresto correct posture and smooth out movements. Thecerebellum is also important in the process oflearning and acquisition of motor skills (Houket al. 1997). Figure 4.5 showed the position of thecomparator centre in the control of movement.

Brainstem

The principal role of the brainstem in control ofmotor function is to provide background contrac-tions of the postural muscles of the trunk, neck

Cortical association areas

Interpretation and integration of all sensory information for the designand planning of movements. Higher cortical functions.Long-term memory

Supplemental and

premotor area

Involved in theperformance of complexand sequentialmovement tasks

Primary visual cortex

Receives visualinformation and sendsthis to correspondingassociation areas forinterpretation andmemory storage

Primary

somatosensory cortex

Receives information onmovement, touch,pressure, equilibriumand sends this tocorrespondingassociation areas forinterpretation andmemory storage

Motor commands tobrainstem and spinal cord

Motor commands tospinal cord

Primary motor cortex

Sends final motorcommand to spinal cordfollowing planning anddesign stages involvingcortical associationareas, cerebellum, basalganglia, supplementaland premotor areas

Figure 4.7 The cerebral cortex in movement control.

Cerebral cortex

Thalamus Striatum

Globus pallidus (internal)

Substantia nigra

Figure 4.8 Hypothetical model of the direct loop in thebasal ganglia.

56 Human Movement

and proximal parts of limb musculature, soproviding support for the body against gravity.The relative degree of contraction of these individ-ual antigravity muscles is determined by equilib-rium mechanisms, with reactions being controlledby the vestibular apparatus, which is directlyrelated to the brainstem region.

The brainstem connects the spinal cord to thecerebral cortex. It is comprised of the midbrain,pons and medulla oblongata. The central core ofthis region is often referred to as the reticular for-mation. This region of the CNS comprises all themajor pathways connecting the brain to the spinalcord in a very compact, restricted space. It is alsothe exit point of the cranial nerves from the CNS.

It is through the integration of the informationreaching the reticular formation that axial posturalcontrol and gross movements are controlled. Inputto the reticular formation is from many sources,including the spinoreticular pathways, collateralsfrom spinothalamic pathways, vestibular nuclei,cerebellum, basal ganglia, cerebral cortex andhypothalamus. The smaller neurons make multipleconnections within the area, whereas the largerneurons are passing through, being mainly motorin function.

The vestibular nuclei are very important for thefunctional control of eye movements, equilibrium,support of the body against gravity, and the grossstereotyped movements of the body. The direct con-nections to the vestibular apparatus of the inner earand cerebellum, as well as the cerebral cortex, enablethe use of preprogrammed, background attitudinalreactions to maintain equilibrium and posture.Workingwith the pontine portion of the reticular for-mation, the vestibular nuclei are intrinsically excit-able; however, this is held in check by inhibitorysignals from the basal ganglia (Hall, 2005). Overall,the motor-related functions of the brainstem are tosupport the body against gravity; generate gross, ste-reotyped movements of the body; and maintainequilibrium. This is achieved in association with thecerebellum, basal ganglia and cortical regions.

Spinal cord

The grey matter of the spinal cord is the integrativearea for the spinal reflexes and other automaticmotor functions. As the region for the peripheralexecution of movements, it also contains the cir-cuitry necessary for more sophisticated movementsand postural adjustments.

Sensory signals enter the cord through the sen-sory nerve roots and then travel to two separatedestinations:

1. same or nearby segments of the cord, wherethey terminate in the grey matter and elicit localsegmental responses (excitatory, inhibitory,reflexes etc.)

2. higher centres of the CNS, i.e. higher in the cord,and brainstem cortices, where they provideconscious (and unconscious, i.e. cerebellum)sensory information and experiences.

Each segment of the cord has several million neu-rons in the grey matter, which include sensoryrelay neurons, anterior motor neurons andinterneurons.

Interneurons are small and highly excitable,with many interconnections either with each otheror with the anterior motor neurons. They have anintegrative or processing function within the spinalcord, as few incoming sensory signals to the spinalcord or signals from the brain terminate directly onan anterior motor neuron. This is essential for thecontrol of motor function. One specific type ofinterneuron is called the Renshaw cell, located inthe anterior horn of the spinal cord. Collateralsfrom one motor neuron can pass to adjacentRenshaw cells, which then transmit inhibitory sig-nals to nearby motor neurons. So stimulation ofone motor neuron can also inhibit the surroundingmotor neurons. This is termed recurrent or lateralinhibition. This allows the motor system to focusor sharpen its signal by allowing good transmis-sion of the primary signal and suppressing the ten-dency for the signal to spread to other neurons(Rothwell, 1994). Together with the brainstem, thespinal cord contains a network of neurons that con-trol walking. Figure 4.9 shows the basic compo-nents of a spinal reflex pathway.

ACTIVITY 4.3

Agnes tells Chris that she has a friend who hasdifficulties moving her left arm and also her left leg.She wonders if this may be similar to the problemChris’s friend has.

Work in small groups or on your own. Find aneurology textbook and identify the symptomsfollowing a stroke. Explain why Agnes’s friend hasmovement problems on the left side of her body.Which part of the nervous system has beenaffected?

Chapter 4 Motor control 57

CONTROL PROCESSES OF VOLUNTARYMOVEMENT

Figure 4.10 summarizes in a simplistic diagram thecontrol processes for voluntary movement. Followthe arrows and boxes from the design to the execu-tion and then the return of feedback, which isfinally stored as memory traces.

1. The cortical association areas play the key rolein the design and planning of voluntarymovements. Action potentials from the corticalassociation areas project to the basal ganglia forrefinement and selective activation ofmovements and/or inhibition of unwantedmovements.

2/3. The thalamus here is part of the basalganglia loops and sends impulses to the motorcortex, which is seen as the final commonpathway.

4. Impulses from the motor cortex are almostsimultaneously sent to the cerebellum, thebrainstem and the spinal cord. The cerebellumwill use this information to compare it with themovement sensory information received fromthe periphery (6). The brainstem will play a rolein maintaining background postural control,while impulses to the spinal cord are more of a

focal nature for the activation of individualmuscles or groups of muscles.

5. Alpha motor neurons cause muscle contraction.

6. The sensation of movement, together withother relevant feedback information, is senttowards the CNS. This sensory information isneeded by various centres. The spinal cord willuse it in its integration of spinal reflexes andthe control of walking patterns. The brainstemutilizes sensory feedback mostly for posturalcontrol and balance. Sensory feedback is alsosent to the thalamus. The cerebellum comparesthe movement as it occurs with the originalmovement instruction sent by the motorcortex.

7. If there is a discrepancy between the intendedmovement and the actual movement, correctingsignals can be sent directly to the executioncentres.

8. The thalamus distributes sensory feedbackto its appropriate location on the sensorycortex.

9. Sensory experiences are interpreted by thecortical association areas, and memorizedmovements are stored for future use in thedesign and planning of movements.

1. Sensory organ

A muscle is lengthened passively andthis is sensed by the muscle spindle

5. Effector organ

The muscle which was originallylengthened contracts in order toprevent over-lengthening

4. Motor neuron

An instruction to contract a muscleis sent via a motor neuron

2. Sensory neuron

The sensory neuron carries theaction potential to the posterior hornin the spinal cord

3. Integrating centre

The spinal cord acts as integrating centre, passing theinformation either directly, or indirectly via interneurons,to the motor parts of the reflex pathways

Figure 4.9 The components of a basic spinal reflexpathway, using the stretch reflex as an example.

58 Human Movement

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Cortical association areas

Design of movement signal, using body schema; interpretation of movement sensation and storage of movement and sensory engrams for future use

Basal ganglia

Refinement, selective activation and/or inhibition

Motor cortex

Final common pathway, sends movement command

Cerebellum

Error corection, coordination, motor learning

Brainstem

Posture, balance

Spinal cord

Execution of movement by sending signals for contraction to muscles, integration of spinal reflexes and gait patterns through central pattern generators

Muscles

Contract for movement; movement in turn creates the sensation of movement

Sensory cortex

Receives ‘conscious’ information of movement, touch, pressure and pain

1

3

8

2

4

9

Thalamus

Distribution

7

6

5

Figure 4.10 Control processes forvoluntary movement.

Chapter 4 Motor control 59

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60 Human Movement