Chapter 9 Motor System - 2

Spinal Reflex, Descending Pathways and Cerebellum

Content

• Spinal Reflexes

• Function of Brain Stem

• Function of Cerebellum

Reference

P164 - 180 P464-470P473-475

P673-684P691-707

Section 1. 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 I Stretch Reflex

1 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 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 spindle

Static intrafusal fibers

Dynamic intrafusal fiber

Afferentaxons

IaII

Static intrafusal

fibers

Primaryending

Secondaryending}

}

Nuclear Bag Fiber

Nuclear Chain Fiber

Anatomy of Muscle Spindle• Primary sensory endings

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

response)

Anatomy of Muscle Spindle• Secondary sensory endings

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

Anatomy 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 Reflex

Dorsal root

Ventral root

Muscle spindle

Tendon

Muscle fiber

-mn

The 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 Reflex

• Whatever the stimulus, when the spindles are activated

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

The Stretch Reflex

• The reflexive muscle contraction that follows resists further stretching of the muscle

The Stretch Reflex

• 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

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

• 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 response

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

Activation of the -loopresults in increased

muscle tone

Functional significance of gamma 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.

Part 2. The Deep Tendon Reflex

Structure and Innervation of Golgi 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 properties

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

passive stretch or active contraction

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 Reflex

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

The 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 golgi

Compare spindle and golgi

Part 3. The Crossed Extensor 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

Part 4. Superficial Reflexes

• elicited by gentle cutaneous stimulation• dependent upon functional upper motor

pathways – Babinski reflex

Babinski reflex - an UMN sign• Adult response - plantar flexion of the big toe and

adduction of the smaller toes• Pathological (Infant) response - dorsoflexion (extension) of

the big toe and fanning of the other toes• Indicative 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)

Part 5. Spinal cord transection and spinal shock

(2) During spinal shock:

complete loss of all reflexes,

no tone, paralysis

complete anesthesia,

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 on continual tonic excitation from higher centers (the reticulospinal-, vestibulospinal- corticospinal tracts).

(4) The recovery of spinal neurons excitability.

Section II. 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• Extend the entire extent to the medulla, lying ventrally and medially

near the middle.• Transmit inhibitory signals to the same antigravity anterior motor

neurons (medullary reticulospinal tract).

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)

• Receive collaterals from the corticospinal tract; the rubrospinal tracts; and other motor pathways.

• activate the medullary reticular inhibitory system to balance the excitatory signals from the P.R.T.,

• Maintain normal tense of the body M. under normal conditions.

1. motor cortex; 2. 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 Decerebrate Rigidity: transection of the brainstem at midbrain level (above vestibular nuclei and below red midbrain level (above vestibular nuclei and below red nucleus)nucleus)

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

limbs

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

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

limbs

Decerebrate RigidityDecerebrate RigidityDecerebrate 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.

1a1a

Descending influence (UMN)Descending influence (UMN)

MUSCLE

Muscle spindle

Activation of the -loopresults in increased

muscle tone

Activation of the -loopresults in increased

muscle tone

Section III. The cerebellum and its motor functions

Cerebellar Input/Output Circuit

Function 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, movements 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.

Method

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 (feedforward control)

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

Function 1: Control of the equilibrium and postural movements.

Function 2 regulate the eye movement

– 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: 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

•Spinocerebellum (vermis & intermediate)

•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 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 (functions)

• Planning and programming of sequential movements– Panning: begins in the sensory and promotor

area of the cortex and transmitted to the cerebrocerebellum

– Programming: Cerebrocerebellum is involved with what will be happening during the next sequential movement a fraction of the second later.

•Vestibulocerebellum (flocculonodular lobe)

Balance and body equilibrium

•Spinocerebellum (vermis & intermediate)

Rectify voluntary movement

•Cerebrocerebellum (lateral zone)

Plan voluntary movement

Clinical Abnormalities of the Cerebellum

• 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 • Hypotonia

Dysmetria and Ataxia • in the absence of the cerebellum• the subconscious motor control

system cannot predict how far movements will go.

• The movements ordinarily overshoot their intended mark

• the conscious portion of the brain overcompensates in the opposite direction for the succeeding compensatory movement

Past Pointing

• in the absence of the cerebellum, • a person ordinarily moves the hand or

some other moving part of the body considerably beyond the point of intention.

• This results from the fact that normally the cerebellum initiates most of the motor signal that turns off a movement after it is begun

• if the cerebellum is not available to do this, the movement ordinarily goes beyond the intended mark.

Failure of Progression

• Dysdiadochokinesia– The inability to perform rapid alternating

movement such as ronation and supination of the hand

• Dysarthria – speech deficits– Articulation problems

Intention Tremor

• Present during reaching movement

• Not at rest

Cerebellar Nystagmus

• Horizontal oscillating eye movement