Part 4 Motor Functions of the Nervous System I Motor Unit and Final Common Pathway

Part 4

Motor Functions of the Nervous System

I Motor Unit and Final Common Pathway

1. Motor Unit

•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 motor unit is a single motor neuron ( motor) and all (extrafusal) muscle fibers it innervates

Motor units are 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

organization of motor subsystems

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

II Motor Functions of the Spinal Cord – Spinal Reflex

Spinal ReflexesSpinal Reflexes

Somatic reflexes mediated by the spinal cord are called spinal reflexes

These reflexes may occur without the involvement of higher brain centers

Additionally, the brain can facilitate or inhibit them

1. Stretch Reflex

(1) Anatomy of Muscle Spindle(1) Anatomy of Muscle Spindle The muscle spindles detect cha

nge in the length of the muscle-- stretch receptors that report the s

tretching of the muscle to the spine.

Each spindle consists of 3-10 intrafusal muscle fibers enclosed in a connective tissue capsule

These fibers are less than one quarter of the size of extrafusal muscle fibers (effector fibers)

Anatomy of Muscle SpindleAnatomy of Muscle Spindle

The central region of the intrafusal fibers which lack myofilaments are noncontractile,

serving as the receptive surface of the spindle (sensory receptor)

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

that send sensory inputs to the CNS Primary sensory endings

– Type Ia fibers– Innervate the center of the spindle

Secondary sensory endings– Type II fibers– Associated with the ends of the spindle

Components of muscle spindleComponents of muscle spindle

Static intrafusal fibers

Dynamic intrafusal fiber

Afferentaxons

IaII

Static intrafusal

fibers

Primaryending

Secondaryending}

}

Anatomy of Muscle SpindleAnatomy of Muscle Spindle Primary sensory endings

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

Anatomy of Muscle SpindleAnatomy of Muscle Spindle Secondary sensory endings

– Type II fibers stimulated only by degree of stretch

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

limited to their ends as only these areas contain actin and myosin filaments

These regions are innervated by gamma () efferent fibers

Muscle stretch reflex

(2) 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 Proprioceptor 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

Dorsal root

Ventral root

Muscle spindle

Tendon

Muscle fiber

-mn

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


At spinal cord sensory neurons synapse directly (mono- synaptically) with the motor neurons which rapidly excite the extrafusal muscle fibers of stretched muscle


The reflexive muscle contraction that follows (an example of serial processing) 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•In essence, the stretch stimulus causes the antagonists to relax so that they cannot resist the shortening of the “stretched” muscle caused by the main reflex arc

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 M.;A 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


The stretch reflex is 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

(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.

WHAT IS THE -LOOP?WHAT IS THE -LOOP?

1a1a

Descending influence (UMN)Descending influence (UMN)

MUSCLE

Muscle spindle

Activation of the -loopresults in increased

muscle tone


muscle tone

Functional significance of Functional significance of gamma impact on spindle activitygamma impact 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.

2. The Deep Tendon Reflex

(1) Structure and Innervation of Golgi Organ

Golgi tendon organ: structureGolgi tendon organ: structure

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

(2) Golgi tendon organ: (2) Golgi tendon organ: response propertiesresponse properties

Less frequent than muscle spindle.

Golgi tendon organ: response Golgi tendon organ: response properties (cont)properties (cont)

Sensitive to the change of tension caused by the passive stretch or active contraction

(3) The Deep Tendon Reflex(3) The 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 Upon reaching the

spinal cord, information is sent to the cerebellum, where it is used to adjust muscle tension

Simultaneously, motor neurons in the spinal cord supplying the contracting muscle are inhibited and antagonistic muscle are activated (activation)

The Deep Tendon ReflexThe Deep Tendon Reflex

Deep tendon reflexes cause muscle relaxation and lengthening in response to the muscle’s contraction

This effect is opposite of those elicited by stretch reflexesGolgi tendon organs help ensure smooth onset and termination of

muscle contraction Particularly important in activities involving rapid switching

between flexion and extension such as in running

Compare spindle and golgiCompare spindle and golgi

Compare spindle and golgiCompare spindle and golgi

3. The Crossed 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 ), while the contralateral response activates the

extensor muscles of the opposite leg (contralateral extensor reflex)

support the weight shifted to it

4. Superficial Reflexes4. Superficial Reflexes

Superficial reflexes are elicited by gentle cutaneous stimulation

These reflexes are dependent upon functional upper motor pathways and spinal cord reflex arcs

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

(1) 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)

5. Spinal cord transection and spinal shock

(2) During spinal shock:

complete loss of all reflexes,

no tone, paralysis,

complete anaesthesia,

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

no sweating

arterial blood Pressure decrease（ 40mmHg） ,

(3) the reason: The normal activity of the spinal cord neurons depends to a great extent on continual tonic excitation from higher centers (the reticulospinal-, vestibulospinal- corticospinal tracts).

(4) The recovery of spinal neurons excitability.

III. 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 M. 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 M. of the body (the M. of vertebral column and the extensor M. 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 M 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).


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 P.R.S.,

so that under normal conditions, the body M. are normally tense.


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 li

mbs

• 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 li

mbs

Decerebrate RigidityDecerebrate Rigidity

•Results from:

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

–active facilitation from pontine RF (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

IV. The cerebellum and its motor functions

Cerebellar Input/Output Circuit

to produce smooth, reproducible to produce smooth, reproducible movementsmovements

Based on cerebral intent and Based on cerebral intent and external conditionsexternal conditions

The cerebellum tracks and modifies The cerebellum tracks and modifies millisecond-to-millisecond muscle millisecond-to-millisecond muscle contractions,contractions,

Without normal cerebellar function, moveWithout normal cerebellar function, movements appear jerky and uncontrolledments appear jerky and uncontrolled

Functional Divisions-cerebellum• Vestibulocerebellum (flocculonodular lobe)

input-vestibular nuclei

output-vestibular nuclei

The vestibulocerebellum

Function:

The control of the equilibrium and postural movements.

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

Method

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

The results of these calculations are the key to the brains’s progression to the next sequential movement.

The vestibulocerebellum

•Spinocerebellum (vermis & intermediate)


–input-periphery & spinal cord:

–output-cortex

Functions:

-- Provide the circuitry for coordinating mainly 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 in the anterior horn of spinal cord that control the distal parts of the limbs (hands and fingers)

--Provides smooth, coordinate movements of the agonist and antagonist M. of the distal limbs for the performance of acute purposeful patterned movements.


•Cerebrocerebellum (lateral zone)input-pontine N. output-pre & motor cortex

• Cerebrocerebellum (lateral zone)

Receives all its input from the motor cortex, adjacent pre-motor and somatic sensory cortices of the brain. Transmits its output information back to the brain.

Functions in a “feedback” manner with all of the cortical sensory-motor system to plan sequential voluntary body and limb movements,

Planning these as much as tenths of a second in advance of the actual movements (mental rehearsal of complex motor actions)

•Vestibulocerebellum (flocculonodular lobe)

Balance and body equilibrium


Rectify voluntary movement

•Cerebrocerebellum (lateral zone)

Plan voluntary movement

V The motor functions of basal ganglia

Putamen

Caudate

GPi

GPe

1. Corpus Striatum1. Corpus Striatum

Striatum ----- Caudate Nucleus & PutamenStriatum ----- Caudate Nucleus & Putamen

Pallidum ----- Globus Pallidus (GP)Pallidum ----- Globus Pallidus (GP)

Components of Components of Basal GangliaBasal Ganglia

2. Substantia Nigra2. Substantia Nigra

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 CHOREASYDENHAM’S CHOREASYDENHAM’S CHOREASYDENHAM’S CHOREA

- 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 (esp. caudate nucleus)Corpus Striatum (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

HEMIBALLISMHEMIBALLISMHEMIBALLISMHEMIBALLISM

- - Usually results from CVAUsually results from CVA (Cerebrovascular Accident)(Cerebrovascular Accident) involving subthalamic nucleusinvolving subthalamic 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

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 and prefrontal areas

•Project to primary motor cortex and spinal cord

•For planning and coordination of complex planned movements

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Part 4 Motor Functions of the Nervous System I Motor Unit and Final Common Pathway