Chapter 32 Control of Motor Function by Nervous System.

Chapter 32Chapter 32

Control of Motor Function by Nervous System

ContentsContents

Motor Unit and Final Common PathwaySpinal ReflexesFunction of Brain Stem Function of the Basal GanglionFunction of CerebellumFunction of the Cortex

organization of motor subsystems

Section 1. Motor Unit and Section 1. Motor Unit and Final Common PathwayFinal Common Pathway

•Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles.

•Extrafusal and intrafusal fibers

The extrafusal muscle fibers are innervated by Alpha motor neuron

The intrafusal muscle fibers are innervated by Gamma motor neurons

Motor units Motor units

A single motor neuron ( motor) and all (extrafusal) muscle fibers it innervates

The physiological functional unit in muscle (not the cell) All cells in motor unit contract synchronously

Motor units and innervation ratio

Purves Fig. 16.4

Innervation ratio

Fibers per motor neuron

Extraocular muscle 3:1

Gastrocnemius 2000:1

•The muscle cells of a motor unit are not grouped, but are interspersed among cells from other motor units

•The coordinated movement needs the activation of several motors

Overview - organization of motor systems

Motor CortexMotor Cortex

Brain StemBrain Stem

Spinal CordSpinal Cord

Skeletal muscle

-motor neuron

Final common pathway

Final common path - -motor neuron(-)

musclefibers

(+)

(-)

(+)

axon hillock

motor nerve fiber

NM junction

Schwanncells

Receptors? acetylcholineesterase

Transmitter?

Final Common PathwayFinal Common Pathway, ,

a motor pathway consisting of the motor neurons by which nerve impulses from many central sources pass to a muscle in the periphery.

Section 2. Spinal ReflexesSection 2. Spinal Reflexes

Somatic reflexes mediated by the spinal cord– May occur without the involvement of higher brain

centers– Was facilitated or inhibited by brain

For example– Stretch reflex– Deep tendon reflex– Crossed extensor reflex– Superficial reflex

Part 1 Stretch ReflexPart 1 Stretch Reflex

1 Anatomy of Muscle Spindle1 Anatomy of Muscle Spindle

3-10 intrafusal muscle fibers detect change in the length

of the muscle-- stretch receptors that report

the stretching of the muscle to the spine.

The central region and peripheral region of the intrafusal fibers

Anatomy of Muscle SpindleAnatomy of Muscle Spindle Intrafusal fibers are wrapped by two types of

afferent endings– Primary sensory endings

Type Ia fibers Innervate the center of the spindle

– Secondary sensory endings Type II fibers Associated with the ends of the nuclear chain fiber

Components of muscle spindleComponents of muscle spindle

Afferentaxons

IaII

Primaryending

Secondaryending}

}

Nuclear Bag Fiber

Nuclear Chain Fiber

Anatomy of Muscle SpindleAnatomy of Muscle Spindle Primary sensory endings

– Type Ia fibers Stimulated by both the rate and amount of stretch (dynamic

response)

Anatomy of Muscle SpindleAnatomy of Muscle Spindle Secondary sensory endings

– Type II fibers stimulated only by degree of stretch (static response)

Anatomy of Muscle SpindleAnatomy of Muscle Spindle The contractile region of the intrafusal muscle

fibers are limited to their ends – only these areas contain actin and myosin filaments– are innervated by gamma () efferent fibers

2. Muscle stretch reflex

Muscle stretch reflex

Definition: Whenever a muscle is stretched, excitation of the spindles causes reflexive contraction of the same muscle from which the signal originated and also of closely allied synergistic muscle.

The basic circuit: Spindle Ia or II nerve fiber dorsal root of the spinal cord synapses with anterior motor neurons -motor N. F. the same M. from whence the M. spindle fiber originated.

Circuit of the Strength ReflexCircuit of the Strength Reflex

The Stretch ReflexThe Stretch Reflex Exciting a muscle spindle

occurs in two ways– Applying a force that

lengthens the entire muscle– Activating the motor

neurons that stimulate the distal ends of the intrafusal fibers to contact,

thus stretching the mid-portion of the spindle (internal stretch)

The Stretch ReflexThe Stretch Reflex

Whatever the stimulus, when the spindles are activated

their associated sensory neurons transmit impulses at a higher frequency to the spinal cord


The reflexive muscle contraction resists further stretching of the muscle


Branches of the afferent fibers also synapse with inter- neurons that inhibit motor neurons controlling the antagonistic muscles

•Inhibition of the antagonistic muscles is called reciprocal inhibition•causes the antagonists to relax

1) Tendon reflex (dynamic stretch reflex) Caused by rapid stretch of the muscle, as knee-jerk reflex Transmitted from the IA sensory ending of the M. S. Causes an instantaneous, strong reflexive contraction of the same muscle; Opposing sudden changes in length of the MA monosynaptic pathway being over within 0.7 ms

The types of the Stretch Flex

2) Muscle tonus (static stretch reflex): Caused by a weaker and continues stretch of the muscle, Transmitted from the IA and II sensory ending of the M. S.

Multiple synaptic pathway, continues for a prolonged period. Non-synchronized contraction, M. C. for at least many seconds or minutes, maintaining the posture of the body.

The types of the Stretch Flex


most important in large extensor muscles which sustain upright posture

Contractions of the postural muscles of the spine are almost continuously regulated by stretch reflexes (Muscle tonus )

3 Gamma impact on afferent 3 Gamma impact on afferent responseresponse

Muscle spindle: motor Muscle spindle: motor innervationinnervation

Gamma motoneurons:– Innervate the

poles of the fibers.

-LOOP-LOOP

1a1a

Descending influence (UMN)Descending influence (UMN)

MUSCLE

Muscle spindle

Activation of the -loopresults in increased

muscle tone


muscle tone

Functional significance of gamma Functional significance of gamma impact on spindle activityimpact on spindle activity

The tension of intrafusal fibers is maintained during active contraction by gamma activity.

The system is informed about very small changes in muscle length.

Part 2. The Deep Tendon Reflex

Structure and Innervation of Structure and Innervation of Golgi OrganGolgi Organ

Located in the muscle tendon junction.

Connective tissue encapsulating collagen fibers and nerve endings.

Attached to 10-20 muscle fibers and several MUs.

Ib afferent fiber.sensitive to tension

Golgi tendon organ: response Golgi tendon organ: response propertiesproperties

Less frequent than muscle spindle Sensitive to the change of tension caused by the

passive stretch or active contraction

The Deep Tendon ReflexThe Deep Tendon Reflex When muscle

tension increases moderately during muscle contraction or passive stretching,

GTO receptors are activated and afferent impulses are transmitted to the spinal cord

The Deep Tendon ReflexThe Deep Tendon Reflex

motor neurons in the spinal cord supplying the contracting muscle are inhibited

antagonistic muscle are activated

The Deep Tendon ReflexThe Deep Tendon Reflex cause muscle relaxation and

lengthening in response to

the muscle’s contraction

– opposite of those elicited

by stretch reflexes

help ensure smooth onset

and termination of muscle

contraction

important in activities

involving rapid switching

between flexion and

extension such as in running

Compare spindle and golgiCompare spindle and golgi

Part 3. The Crossed Part 3. The Crossed Extensor ReflexExtensor Reflex

The reflex occur when you step on a sharp object

There is a rapid lifting of the affected foot (ipsilateral withdrawal reflex )

the contralateral response activates the extensor muscles of the opposite leg (contralateral extensor reflex)– support the weight shifted

to it

Part 4. Superficial ReflexesPart 4. Superficial Reflexes

elicited by gentle cutaneous stimulationdependent upon functional upper motor

pathways – Babinski reflex

Babinski reflex - an UMN signBabinski reflex - an UMN signAdult response - plantar flexion of the big toe and adduction

of the smaller toesPathological (Infant) response - dorsoflexion (extension) of

the big toe and fanning of the other toesIndicative of upper motor neuron damage

Concept: When the spinal cord is suddenly transected in the upper neck, essentially all cord functions, including the cord reflexes, immediately become depressed to the point of total silence.

(spinal animal)

Part 5. Spinal cord transection and spinal shock

During spinal shock:

complete loss of all reflexes

no muscle tone, paralysis

complete anesthesia,

no peristalsis, bladder and rectal reflexes absent (no defecation and micturition )

no sweating

arterial blood Pressure decrease （ 40 mmHg ）

the reason: The normal activity of the spinal cord neurons depends on continual tonic excitation from higher centers

(the reticulospinal-, vestibulospinal- corticospinal tracts).

The recovery of spinal neurons excitability.

Section 3. Role of the brain stem

Support of the Body Against Gravity – Roles of the Reticular and Vestibular nuclei

Areas in the cat brain where stimulation produces facilitation (+) or inhibition (-) of stretch reflexes. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.

Facilitated and inhibitory area

1. Facilitated area—roles of the reticular and vestibular nuclei.：(1) The pontine reticular nuclei Located slightly posteriorly and laterally in the pons and extending to the mesencephalon Transmit excitatory signals downward into the cord (the pontine reticulospinal tract)

1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular

inhibitory area;5. Reticular

facilitated area; 6. Vestibular nuclei.

(2) The vestibular nuclei selectively control the excitatory signals to the different antigravity muscle to maintain equilibrium in response to signals from the vestibular apparatus.

1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular

inhibitory area; 5. Reticular

facilitated area; 6. Vestibular

nuclei.

MOTOR CORTEX

MOTOR TRACTS & LOWER MOTOR NEURON

SKELETALMUSCLE

MIDBRAIN &RED NUCLEUS

(Rubrospinal Tract)

PONS & MEDULLARETICULAR FORMATION

(Reticulospinal Tracts)

VESTIBULAR NUCLEI(Vestibulospinal Tract)

LOWER (ALPHA) MOTOR NEURONTHE FINAL COMMON PATHWAY

UPPER MOTOR NEURON(Corticospinal Tracts)

Terminate on the motor neurons that exciting antigravity muscle of the body (the muscle of vertebral column and the extensor muscle of the limbs).

Have a high degree of natural (spontaneous) excitability.

Receive especially strong excitatory signals from vestibular nuclei and the deep nuclei of the cerebellum.

Cause powerful excitation of the antigravity muscle throughout the body (facilitate a standing position), supporting the body against gravity.

1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.

Properties of the Facilitated Area

2. Inhibitory area –medullary reticular system (1) Extend the entire extent to the medulla, lying ventrally and medially near the middle. (2) Transmit inhibitory signals to the same antigravity anterior motor neurons (medullary reticulospinal tract).

1. motor cortex 2. Basal ganglia 3. Cerebellum4. Reticular inhibitory area5. Reticular facilitated area 6. Vestibular nuclei.

MOTOR CORTEX

MOTOR TRACTS & LOWER MOTOR NEURON

SKELETALMUSCLE

MIDBRAIN &RED NUCLEUS

(Rubrospinal Tract)

PONS & MEDULLARETICULAR FORMATION

(Reticulospinal Tracts)

VESTIBULAR NUCLEI(Vestibulospinal Tract)

LOWER (ALPHA) MOTOR NEURONTHE FINAL COMMON PATHWAY

UPPER MOTOR NEURON(Corticospinal Tracts)

(3) Receive collaterals from the corticospinal tract, the rubrospinal tracts, and other motor pathways.

These collaterals activate the medullary reticular inhibitory system to balance the excitatory signals from the pontine reticular system.

1. motor cortex2. Basal ganglia 3. Cerebellum 4. Reticular inhibitory area 5. Reticular facilitated area 6. Vestibular nuclei.

Areas in the cat brain where stimulation produces facilitation (+) or inhibition (-) of stretch reflexes. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.

• Decerebrate Rigidity: transection of the brainstem at midbrain level (above vestibular nuclei and below red nucleus)

• Symptoms include:– extensor rigidity or posturing in both upper and lower

limbs

• Decerebrate Rigidity: transection of the brainstem at midbrain level (above vestibular nuclei and below red nucleus)

• Symptoms include:– extensor rigidity or posturing in both upper and lower

limbs

Decerebrate RigidityDecerebrate Rigidity

•Results from:

–loss of input from inhibitory medullary reticular formation (activity of this center is dependent on input from higher centers).

–active facilitation from pontine reticular formation (intrinsically active, and receives afferent input from spinal cord).

•The extensor rigidity is -loop dependent

–section the dorsal roots interrupts the -loop, and the rigidity is relieved. This is -rigidity.

THE -LOOP?THE -LOOP?

1a1a

Descending influence (UMN)Descending influence (UMN)

MUSCLE

Muscle spindle


muscle tone


muscle tone

Section 4. The Cerebellum and its Motor Functions

Cerebellar Input/Output Circuit

Function of the cerebellumFunction of the cerebellum

Based on cerebral intent and external conditions– tracks and modifies millisecond-to-millisecond tracks and modifies millisecond-to-millisecond

muscle contractionsmuscle contractions– produce smooth, reproducible movementsproduce smooth, reproducible movements

Without normal cerebellar function, Without normal cerebellar function, movements appear jerky and uncontrolledmovements appear jerky and uncontrolled

Functional Divisions-cerebellum• Vestibulocerebellum (flocculonodular lobe)

input-vestibular nuclei

output-vestibular nuclei

The vestibulocerebellum

controlling the balance between agonist and antagonist M. contractions of the spine, hips, and shoulders during rapid changes in body positions.

Function 1: Control of the equilibrium and postural movements.

During running

Receive the signals from the periphery how rapidly and in which directions the body parts are moving

Calculate the rates and direction where the different parts of body will be during the next few ms.

anticipatory correction (feed-forward control)

the key to the brain’s progression to the next sequential movement.

Function 1: Control of the equilibrium and postural movements. （ Cont. ）

Function 2 regulate the eye Function 2 regulate the eye movementmovement

– Through vestibulo-ocular reflex keep the eyes still in space when the head moves

– Damage of the flocculonofular lobe result in positional nystagmus ( 位置性眼震颤）

The VOR: DefinitionThe VOR: Definition

A eye movement reflex Stimulated by head movements Moves the eyes opposite of the head Helps keep the retinal image stabilized

The VOR contributes to clear vision during head movements

Cerebellar NystagmusCerebellar NystagmusHorizontal oscillating eye movement

•Spinocerebellum (vermis & intermediate)


–input-

–somatic sensory information via spinocerebellar tracts

–Branch from corticospinal tract

–Output

–Thamalus – motor cortex

–-fastigial ( 顶） and interposed （中间核） nuclei → vestibular nuclei, reticular formation and red nucleus → vestibulospinal tract, reticulospinal tract and rubrospinal tract → motor neurons of anterior horn

Function of spinocerebellumFunction of spinocerebellum

Provide the circuitry for coordinating the movements of the distal portions of the limbs, especially the hands and fingers– Compared the “intentions ” from the motor cortex and red

nucleus, with the “performance” from the peripheral parts of the limbs

– Send corrective output signals to the motor neurons– Provides smooth, coordinate movements– Feedback control

•Cerebrocerebellum (lateral zone)

input- from the cerebral cortex via a relay in pontine nuclei

output- to dentate nucleus → dorsal thalamus and red nucleus→ primary motor cortex → corticospinal tract → motor neurons of anterior horn

Cerebrocerebellum Cerebrocerebellum (functions)(functions)

Planning and programming of sequential movements– Panning (计划形成） : begins in the sensory

and promotor area of the cortex and transmitted to the cerebrocerebellum

– Programming （运动程序编制） : what will be happening during the next sequential movement a fraction of the second later….

•Vestibulocerebellum (flocculonodular lobe)

Balance and body equilibrium


Rectify voluntary movement

•Cerebrocerebellum (lateral zone)

Plan voluntary movement

Clinical Abnormalities of the Clinical Abnormalities of the CerebellumCerebellum

Dysmetria （辨距障碍） and Ataxia ( 共济失调）

Past pointing: （过指） Failure of progression

– Dysdiadochokinesia ( 轮替运动障碍）– Dysarthria （构音障碍）

Intention tremor

Dysmetria Dysmetria （辨距障碍）（辨距障碍） and Ataxia and Ataxia (( 共济失调）共济失调）

Past PointingPast Pointing

Dysdiadochokinesia (Dysdiadochokinesia ( 轮替运动障轮替运动障碍）碍）

Intention Tremor Intention Tremor

Present during reaching movement

Not at rest

Section 5 The motor functions of basal ganglia

Putamen

Caudate

GPi

GPe

1. Corpus Striatum(1. Corpus Striatum( 纹状体）纹状体）Caudate Nucleus Caudate Nucleus （尾状核）（尾状核） Putamen (Putamen ( 壳核）壳核）

Globus Pallidus (Globus Pallidus ( 苍白球，苍白球， GP)GP)

Components of Components of Basal GangliaBasal Ganglia

2. Substantia Nigra 2. Substantia Nigra （（ SNSN））

Pars Compacta (SNc)Pars Compacta (SNc)

Pars Reticulata (SNr)Pars Reticulata (SNr)

Components of Components of Basal GangliaBasal Ganglia

3. Subthalamic Nucleus (STN)3. Subthalamic Nucleus (STN)

STN

SN (r & c)

Basal Ganglia Basal Ganglia ConnectionsConnections

•Circuit of connections–cortex to basal ganglia to thalamus to cortex–Helps to program automatic movement sequences (walking and arm swinging or laughing at a joke)

•Output from basal ganglia to reticular formation

–reduces muscle tone–damage produces rigidity of Parkinson’s disease

excitation

inhibition

directindirect

D1

D2

D1 & D2 Dopamine receptors

somatosensory cortices

Thalamus

Putamen

GPe

GPi

STN

SNc

motor cortices

cortex to basal ganglia to thalamus to cortex

GPe/i: Globus pallidus internal/external

STN: Subthalamus Nucleus

SNc: Pars Compacta Pars Compacta (part of substantia Nigra)(part of substantia Nigra)

• Direct Pathway: – Disinhibition of the thalamus facilitates cortically mediated

behaviors

excitation

inhibition

directindirect

D1

D2



Thalamus

Putamen

GPe

GPi

STN

SNc

motor cortices



SNc: Pars Compacta (part Pars Compacta (part of substantia nigra))of substantia nigra))

•Indirect pathway:

–Inhibition of the thalamus inhibits cortically mediated behaviors

excitation

inhibition

directindirect

D1

D2



Thalamus

Putamen

GPe

GPi

STN

SNc

motor cortices



SNc: Pars Compacta (part Pars Compacta (part of substantia nigra)of substantia nigra)

Medical Remarks

• Hypokinetic disorders result from overactivity in the indirect pathway.

example: Decreased level of dopamine supply in nigrostriatal pathway results in akinesia, bradykinesia, and rigidity in Parkinson’s disease (PD).

excitation

inhibition

directindirect

D1

D2



Thalamus

Putamen

GPe

GPi

STN

SNc

motor cortices



SNc: Pars Compacta Pars Compacta (part of substantia (part of substantia nigra)nigra)

Muhammad Ali in Alanta OlympicMuhammad Ali in Alanta Olympic

Parkinson’s Parkinson’s DiseaseDisease

Disease of mesostriatal Disease of mesostriatal dopaminergic systemdopaminergic system

PDPD

normalnormal

Substantia Nigra, Substantia Nigra, Pars Compacta (SNc)Pars Compacta (SNc)

DOPAminergic DOPAminergic

NeuronNeuron

Slowness of MovementSlowness of Movement- - Difficulty in Initiation and Cessation Difficulty in Initiation and Cessation of Movementof Movement

Clinical Feature (1)Clinical Feature (1)

Parkinson’s DiseaseParkinson’s Disease

Clinical Feature (2)Clinical Feature (2)

Resting TremorResting TremorParkinsonian PostureParkinsonian PostureRigidity-Cogwheel RigidityRigidity-Cogwheel Rigidity

Parkinson’s DiseaseParkinson’s Disease

•Hyperkinetic disorders result from underactivity in the indirect pathway.

example: Lesions of STN result in Ballism. Damage to the pathway from Putamen to GPe results in Chorea, both of them are involuntary limb movements.

excitation

inhibition

directindirect

D1

D2



Thalamus

Putamen

GPe

GPi

STN

SNc

motor cortices



SNc: Pars Compacta Pars Compacta (part of substantia nigra)(part of substantia nigra)

SYDENHAM’S CHOREA SYDENHAM’S CHOREA （风湿性（风湿性舞蹈病）SYDENHAM’S CHOREA SYDENHAM’S CHOREA （风湿性（风湿性舞蹈病）

- Fine, disorganized , and - Fine, disorganized , and random movements ofrandom movements of extremities, face andextremities, face and tonguetongue- Accompanied by - Accompanied by Muscular HypotoniaMuscular Hypotonia- Typical exaggeration of- Typical exaggeration of associated movements associated movements during voluntary activityduring voluntary activity- Usually recovers- Usually recovers spontaneously spontaneously in 1 to 4 monthsin 1 to 4 months

Clinical FeatureClinical Feature

Principal Pathologic Lesion: Principal Pathologic Lesion: Corpus StriatumCorpus Striatum


Principal Pathologic Lesion:Principal Pathologic Lesion:

Corpus Striatum Corpus Striatum (esp. caudate nucleus)(esp. caudate nucleus) and Cerebral Cortexand Cerebral Cortex

- - Predominantly Predominantly autosomal dominantlyautosomal dominantly inherited chronic fatal diseaseinherited chronic fatal disease (Gene: chromosome 4)(Gene: chromosome 4)- Insidious onset: Usually 40-50- Insidious onset: Usually 40-50- Choreic movements in onset- Choreic movements in onset- Frequently associated with- Frequently associated with emotional disturbancesemotional disturbances- Ultimately, grotesque gait and sever- Ultimately, grotesque gait and sever dysarthria, progressive dementiadysarthria, progressive dementia ensues.ensues.

HUNTINGTON’S CHOREAHUNTINGTON’S CHOREA亨廷顿舞蹈症

HEMIBALLISM HEMIBALLISM （（半侧投掷症）HEMIBALLISM HEMIBALLISM （（半侧投掷症）

- - Usually results from CVAUsually results from CVA (Cerebrovascular Accident)(Cerebrovascular Accident) involving involving subthalamic nucleussubthalamic nucleus- sudden onset- sudden onset- - Violent, writhing, involuntaryViolent, writhing, involuntary movements of wide excursionmovements of wide excursion confined to confined to one half of the bodyone half of the body- The movements are continuous- The movements are continuous and often exhausting but ceaseand often exhausting but cease during sleepduring sleep- Sometimes fatal due to exhaustion- Sometimes fatal due to exhaustion- Could be controlled by- Could be controlled by phenothiazines and stereotaxicphenothiazines and stereotaxic surgery surgery


Lesion: Lesion: Subthalamic NucleusSubthalamic Nucleus

•Two principal components

–Primary Motor Cortex

–Premotor Areas

Section VI Control of muscle function by the motor cortex

The primary motor cortex

The topographical representations of the different muscle areas of the body in the primary motor cortex

Characteristics of the PMC:

1, It has predominant influence on the opposite side of the body (except some portions of the face)

2. It is organized in a homunculus pattern with inversed order

3. The degree of representation is proportional to the discreteness (number of motor unit) of movement required of the respective part of the body. (Face and fingers have large representative)

4. Stimulation of a certain part of PMC can cause very specific muscle contractions but not coordinate movement.

•Projects directly

–to the spinal cord to regulate movement

–Via the Corticospinal Tract

–The pyramidal system

•Projects indirectly

–Via the Brain stem to regulate movement

–extrapyramidal system

Descending Spinal PathwaysDescending Spinal Pathways pyramidal system

Direct Control muscle tone

and conscious skilled movements

Direct synapse of upper motor neurons of cerebral cortex with lower motor neurons in brainstem or spinal cord

Descending Spinal PathwaysDescending Spinal Pathwaysextrapyramidal system

Indirect coordination of head &

eye movements coordinated function of

trunk & extremity musculature to maintaining posture and balance

Synapse in some intermediate nucleus rather than directly with lower motor neurons

• Premotor area composed of supplementary motor area and lateral Premotor area

Premotor Areas•Receive information from parietal （顶） and prefrontal （前） areas

•Project to primary motor cortex and spinal cord

•For planning and coordination of complex planned movements

Chapter 32 Control of Motor Function by Nervous System.

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Transcript of Chapter 32 Control of Motor Function by Nervous System.