Transcript stretch reflexes

Chapter 9 Motor System - 2
Spinal Reflex, Descending Pathways
and Cerebellum
Content
• Spinal Reflexes
• Function of Brain Stem
• Function of Cerebellum
Reference
P164 - 180
P464-470
P473-475
P673-684
P691-707
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
Dynamic
intrafusal
fiber
Static intrafusal
fibers
Static
intrafusal
fibers
Afferent
axons
Ia
II
Nuclear Bag
Fiber
} Primary
ending
Secondary
}
ending
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
Muscle
spindle
Dorsal root
Muscle fiber
Tendon
Ventral root
-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 midportion 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 interneurons that inhibit motor neurons controlling the
antagonistic muscles
•Inhibition of the antagonistic muscles is called reciprocal
inhibition
•causes the antagonists to relax
The types of the Stretch Flex
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 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
innervation

Gamma
motoneurons:
– Innervate the
poles of the
fibers.
-LOOP
Descending influence (UMN)
Muscle spindle
1a

Activation of the -loop
results in increased
muscle tone

MUSCLE
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
Part 5. Spinal cord transection and spinal shoc
(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)
(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
Facilitated and inhibitory area
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.
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 TRACTS & LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN &
RED NUCLEUS
(Rubrospinal Tract)
UPPER MOTOR NEURON
(Corticospinal Tracts)
VESTIBULAR NUCLEI
(Vestibulospinal Tract)
PONS & MEDULLA
RETICULAR FORMATION
(Reticulospinal Tracts)
LOWER (ALPHA) MOTOR NEURON
THE FINAL COMMON PATHWAY
SKELETAL
MUSCLE
Properties of the Facilitated Area
•
•
•
•
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.
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 TRACTS & LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN &
RED NUCLEUS
(Rubrospinal Tract)
UPPER MOTOR NEURON
(Corticospinal Tracts)
VESTIBULAR NUCLEI
(Vestibulospinal Tract)
PONS & MEDULLA
RETICULAR FORMATION
(Reticulospinal Tracts)
LOWER (ALPHA) MOTOR NEURON
THE FINAL COMMON PATHWAY
SKELETAL
MUSCLE
•
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
• 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
•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.
Descending influence (UMN)
Muscle spindle
1a

Activation of the -loop
results in increased
muscle tone

MUSCLE
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
muscle contractions
– produce smooth, reproducible movements
Without normal cerebellar function,
movements appear jerky and uncontrolled
Functional Divisions-cerebellum
• Vestibulocerebellum (flocculonodular lobe)
The vestibulocerebellum
input-vestibular nuclei
output-vestibular nuclei
Function 1: Control of the equilibrium and
postural movements.
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 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