Transcript Stretch Reflex

‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
Stretch Reflex
By
Dr. Abdel Aziz M. Hussein
Assist Prof. of Physiology
• The divergence function of interneurons
is involved in :•
•
•
•
•
a- temporal summation
b- spatial summation
c- reverberation
d- irradiation
• Interneuron after-discharge circuits
prolong the duration of :•
•
•
•
a- sensory input to the spinal motor centers
b- synaptic delay in central synapses
c- discharge of efferent neurons
d- conscious perception of the evoked
sensation
• The ability of stronger stimuli to produce
wider range of reflex responses depends
upon :• a- presence of reverberating circuits in reflex
pathway
• b- presence of parallel-chain circuits in reflex
pathway
• c- convergence of interneurons
• d- divergence of interneurons
• Recruitment of a reflex response is due to :• a- difference in the amount of presynaptic inputs
to the various efferent neurons initiating the reflex
• b- difference in the conduction velocity of the
various afferent neurons mediating the reflex
• c- delay at the neuromuscular junction
• d- presence of inhibitory interneurons in the
reflex pathway
• After-discharge of reflex responses :• a- increase the magnitude of the reflex
responses
• b- delays the onset of fatigue of reflex
responses
• c- involves interneuron circuits
• d- depends upon spatial summation
Stretch (Myotatic) Reflex
Def
 It is a reflex contraction of a muscle when it is
passively stretched
 Pathway
1. Stimulus:→ passive stretch
2. Receptors: → ms spindles
3. Afferents: → fast-conducting Aα (Ia) and Aβ
(II)nerve fibers
4. Centers: → alpha motor neurons in AHCs.
5. Efferents: → axons of alpha motor neurons.
6. Effector organ: → Extrafusal ms fibers
7. Response: → ms contraction.
Stretch Reflex
Ms spindle
1ry and 2ry
endings
AHCs
Extrafusal
ms fibers
Alpha motor
neurons
Muscle Spindle
Site:
• Fleshy parts of skeletal ms parallel to their fibres
(extrafusal ms fibers) .
Shape:
•It is capsulated fusiform stretch receptor.
Structure:
•Each spindle consists of several small ms fibres (4-12
fibers) called intrafusal fibres.
Muscle Spindle
Muscle Spindle
Types of intrafusal ms fibres:
• There are 2 types;
1)Nuclear bag fibres;
• Have a dilated central area filled with nuclei.
• Are 2 of these fibres per spindle.
2)Nuclear chain fibres;
• Have nuclei which are arranged as a chain in the
receptor area.
• Are 5-8 of these fibres per spindle.
Muscle Spindle
Each ms fiber consists of 2 parts;
a)Central part:
•It is non-contractile part
•constitutes the receptor areas of the spindles
•It receives sensory innervation
b)Peripheral part:
•It is a contractile part
•when contracts, it causes stretch of the central receptor
area.
• It receives motor innervation
Muscle Spindle
Muscle Spindle
Innervation of Muscle Spindle
A) Afferent (sensory) innervations
Anulospiral or 1ry
afferents
Flower spray or 2ry
afferents
Type Ia or
A alpha (16 um)
Type II or A beta
(8 um)
Supply nuclear
bag and chain
Rapidly responding
and adapting
Supply nuclear chain
on sides of 1ry
endings
Slowly Rapidly
responding and adapting
1ry endings
2ry endings
Innervation of Muscle Spindle
B) Efferent (motor) innervations
Dynamic gamma
motor neuron
Static gamma motor
neuron
A gamma (4 um)
A gamma (4 um)
Supply nuclear
bag
Supply nuclear
chain
Enhance dynamic
response
Enhance static
response
Efferent Innervation of Muscle Spindle
Stimulation of Muscle Spindle
• The adequate stimulus that excites the ms spindles is
stretching of their central part→ depolarizes it→ initiates an
AP in 1ry and 2ry endings.
• This can occur in 2 ways:
• 1) Passive stretch of the whole ms:
• It causes stretch of the ms spindle which lies parallel to ms
fibers.
Muscle spindle
Whole Muscle
Afferents
Stimulation of Muscle Spindle
• 1) Passive stretch of the whole ms:
Stimulation of Muscle Spindle
• 2)Activation of the γ-MNs:
• By supraspinal centers or reflexely
• It causes contraction of the peripheral part the
intrafusal fibres→ stretch of receptor area
Stimulation of Muscle Spindle
2)Activation of the γ-MNs:
Types of Responses of Ms spindle to Stretch
(Types of Stretch Reflex)
Dynamic Response
Stimulus
Receptors
Afferents
Center
Response
Examples
Sudden stretch
Static Response
Maintained (steady)
stretch
nuclear bag
nuclear chain
1ry endings
primary and
secondary endings
Alpha motor neurons Alpha motor neurons
Rapid contraction
Maintained
followed by rapid
subtetanic
relaxation
contraction
e.g. tendon jerk
e.g. muscle tone
Types of Responses of Ms spindle to Stretch
(Types of Stretch Reflex)
Dynamic
response
Nuclear
bag (1ry)
Basal
discharg
e
Nuclear chain
(1ry & 2ry
endings
Static
response
Dynamic stretch Reflex
1 ry endings
Nuclear
bag
Sudden
stretch
AHCs
Alpha MNs
Static stretch reflex
AHCs
1ry and 2 ry
endings
Alpha
MNs
Nuclear
chain
maintained
stretch e.g.
gravity
Functions of stretch reflex or Ms
Spindle
1. Generation of ms tone
2. Smoothing of ms contraction or
Damping function
3. Load Reflex: stabilize joint posture
4. Proprioceptive Functions
Def.
• It is a state of
Flexors of the UL.
continuous (partial or
Elevators of the lower jaw
subtetanic) contraction
of skeletal ms during
rest.
Back ms and back of neck
Distribution:
• It is present in all
Anterior abdominal wall ms.
skeletal ms but specially
in the antigravity ms
Extensors of LL
Mechanism:
• It is a static type of SR
Continuous mild stretch of skeletal ms because ms length is
shorter than distance ( ) origin and insertion
Maintained stretch of nuclear chain fibers
Continuous mild discharge along 2ry endings (flower spray)
Stimulate α-MNS of muscle
Mild continuous (partial) contraction of skeletal ms
Muscle Tone
AHCs
1ry and 2 ry
endings
Alpha
MNs
Nuclear
chain
maintained
stretch e.g.
gravity
Dynamic state of skeletal ms tone:
• The ms tone is not static but dynamic
• in upright standing the magnitude of the ms tone is
greater in the antigravity ms especially extensors of lower
limb and trunk
• If the trunk is tilted backward the ms tone ↑es in ms of
anterior abdominal wall and ↓es in extensors.
• This by modulation of ɤ-MNs activity by the supraspinal
centers which may facilitate or inhibit the muscle tone
Regulation of skeletal ms tone:
• By supraspinal centers (facilitatory or inhibitory)
Inhibitory areas
Facilitatory areas
1. Area 4
2. Neocerebellum
1. Area 6
+
-
2. Paleocerebllum
3. Excitatory RF
3. Inhibitory RF
4. Vestibular N.
4. Red N.
Functions of skeletal ms tone:
a) Postural control:
• Is the basic mechanism for control of posture and equilibrium
• By adjusting the magnitude of ms tone of different groups of
ms.
b) Help in heat production and maintain of body temperature
c) It helps both the venous return & lymph flow:
• Ms tone has a mild squeezing effect on the walls of veins and
lymphatics of skeletal ms→ help venous return to the heart.
d) Keeps viscera in position and prevents visceroptosis:
• Stretch reflex prevents oscillations or jerkiness of body
movements
• Motor signals from the motor areas are transmitted to the
ms in an unsmooth form (↑ for few Sec and ↓ for another
Sec)
• This causes irregularities or oscillations of movements
• The signals discharged from the ms spindles cause partial
activity of αMNs of the ms
• So, the motor signals find αMNs in state of partial activity,
so they cause continuous activation of them → cause
smooth ms contraction
• When the signal decrease, produces weak direct
contraction  less shortening of the ms  the stretch
reflex signals will increase  more contraction
• The resultant contraction with be sum of both direct and
reflex.
• When the signal increase, produces strong direct
contraction  more shortening of the ms  the stretch
reflex signals will decrease  less contraction.
• The resultant contraction will be the sum of both direct and
reflex
Direct
contraction)
Reflex
contraction)
• Different signal intensities produce equal, average
contraction
• This function of the stretch reflex is consequently
termed signal averaging function
• Interruption of the ms spindles sensory discharge cause
the ms contraction to become unsmooth and jerky.
• Def.,
• It is a reflex responsible for keeping the hand or foot in
position when a moderate loads are applied
• E.g. when a person holding up a cup while someone is
filling it with tea, as the load gets bigger and bigger,
the force required to keep the hand in position must be
continually increased.
• Mechanism (Coactivation of α-MNs and γ-MNs)
• The higher motor centers activate α-MNs to initiate
contraction of a ms and also activate γ-MNs discharge
→co-activation of α-MNs and γ-MNs.
• This increases the sensitivity of the ms spindles to any slight
degree of stretch produced by increasing the load
consequently
• Ms spindles provide proprioceptive information to the
brain and cerebellum for keeping them continually
informed about muscle length and changes in that
length
Gamma motor Neurons
Alpha motor neuron
70%
Gamma motor neuron
30 %
Site:
•
•
•
•
γ-MNs are small motors neurons
represent 30% of AHCs.
The axons (about 4 u)
supply the peripheral parts of intrafusal ms fibers.
Types:
• Are 2 types of γ-MNs
1. Dynamic or d- γ-MNs→ supply nuclear bag ms fibers.
2. Static or s- γ-MNs→ supply nuclear chain ms fibers.
• They adjust ms spindle sensitivity
• ↑ γ-MNs cause contraction of the peripheral parts of
intrafusal fibers → stretch of central parts of ms spindle
→ ↑es the sensitivity of the ms spindle to stretch i.e.
ms spindle needs a small amount of passive stretch to
be stimulated
• Vice versa.
• This is important in
• A) Stabilization of body position and equilibrium
(standing);
• In upright posture , there is ↑ in the descending
excitatory signals from the facilitatory RF in the brain
stem to both  and -MNs of the antigravity ms.
• These excitatory signals ↑ the contraction of the
extrafusal ms fibers and at the same time the ms
spindle sensitivity increased.
Midbrain
Left
Internal
Capsule
L
A
Facilitatory Pontine RF
Pons
Medulla
Alpha MN
Gamma MN
Spinal Cord
• A) Stabilization of body position and equilibrium
(standing);
• Also deviation of the upright attitude toward any
direction → will cause additional stretching of the
postural ms →↑ ms tone of these ms → restores
equilibrium before it is disturbed.
B. Load reflex
• B) Load reflex (coactivation of alpha and gamma
MNs)
Higher motor centers
Midbrain
Left
Internal
Capsule
L
A
Pons
Medulla
Alpha MN
Gamma MN
Spinal Cord
Facilitatory
Areas
Inhibitory
Areas
Facilitatory Areas
Inhibitory Areas
Supraspinal
facilitation is
predominant
+-
↓ ms tone
and stretch
reflex
Facilitatory
Areas
Inhibitory
Areas
Area 4
Facilitatory R.F.
Neocerebellum
Vestibular N.
Reticulospinal T.
Vestibulospinal T.
Corticospinal T.
Area 6
Basal ganglia
Paleocerebellum
Inhibitory R.F.
Reticulospinal T.
•At the cortical level, the net effect of area 4 & area 6 &
area 4s on the stretch reflex & muscle tone is inhibitory
• So, a lesion causing damage of area 4, 4s & 6 (UMNL)
leads to increase in muscle tone
•In animals, separation ( ) cerebral cortex & brain stem →
marked ↑ in ms tone due to the removal of the net inhibitory
effect of the cerebral cortical areas & leaving the facilitatory
centers in the brain stem (VN and facilitatory RF) to act.
•↑ed ms tone leads to a state known as Decerebrate Rigidity.
1) Has a short latent period:
• has a very short time between start of stretch and
start of contraction.
• It is due to ;
a)It is monosynaptic.
b)Its afferent and efferents are rapidly conducting
nerve fibers.
2)High localization:
• It is highly localized i.e. contraction occurs only in
the stretched ms.
• No divergence due to absence of interneurons
3) It has no recruitment nor after discharge:
• It is due to lack of interneurons
4)Graded response:
• The strength of ms contraction is directly proportional
to the extent of stretch.
5) Reciprocal innervation:
• In which stretch of a ms results in reflex contraction
of the stretched ms and relaxation of the antagonistic
ms.
6)Resist fatigue:
• Sustained for a prolonged period without fatigue (e.g.
in antigravity ms):
• Delayed fatigue is due to;
a)Alternation between motors units during stretch reflex
i.e. not all units contracting at the same time.
b)Antigravity ms are tonic (slow) ms which resist fatigue
because;
1. Rich in blood supply
2. Rich in mitochondria
3. Its contraction is slow.
• It is a reflex relaxation (or lengthening) of a ms in
response to excessive stretch or contraction of
that ms.
Neural pathway:
• Stimulus: ↑ed ms tension by;
1. Overstretch or
2. Severe contraction
• Receptors: Golgi tendon organs
1) Site:
• tendons of skeletal ms in series
with ms fibers
2) Structure:
• Are encapsulated sensory receptor
• 6-20 elastic fibers
3) Innervations:
• Type Ib or A alpha afferent fibres
Receptors: GTOs
• Stimulated by ↑ed ms tension caused by
passive overstretch or active contraction
of the ms
Afferents:
• A alpha or Ib
Center :
a)inhibitory interneurons→ inhibit the α-MNs
supplying the same ms
b)excitatory interneurons→ excite the αMNs supplying the antagonistic ms
Response:
• Relaxation of the same ms
• Contraction of antagonistic group of ms.
Significance GTR:
a)Physiological significance:
• Protective reaction which prevent tearing of the ms or
avulsion of its tendon from its bony attachment when the
ms is overstretched.
Significance GTR:
b)Clinical significance: (clasp knife effect)
• Demonstrated clinically by passive flexion of a spastic limb
(e.g. in upper motor neuron lesions) at its main joint.
• As the limb is flexed, an initial resistance occurs due to
contraction of this ms a result of the stretch reflex.
• With persistent flexion, at a certain point, GTR is excited→
sudden disappearance of the initial resistance → the limb
flexes easily, as occurs due closing-of a pocket knife→
clasp knife effect.
• E.g. Flexion of knee and ankle
Def.,
• Rapid contraction followed by
relaxation of a ms due to
sudden stretching of that ms
by tapping on its tendon using
a medical hammer
Mechanism:
• It is a dynamic type of the
stretch reflex
To brain
6 Primary afferent
neuron stimulates
inhibitory interneuron
4 Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
Alpha
MNs
1ry
endings
3 Primary afferent
neuron excited
Nuclear
bag
7 Interneuron inhibits
alpha motor neuron
to flexor muscle
5
Alpha motor neuron
stimulates extensor
muscle to contract
Ms
contraction
Sudden
stretch
2 Muscle spindle
stimulated
1 Extensor muscle
stretched
Flexor muscle
8 (antagonist) relaxes
• Cause of relaxation after contraction in the tendon
jerk:
a) Stoppage of discharge from the ms spindles.
b) Stimulation of the Golgi tendon organs.
c) Stimulation of the Renshaw’s Cells.
Jerk
Biceps
jerk
Triceps
jerk
Knee
jerk
Ankle
jerk
Jaw jerk
Center
Limb position
Tendon
Response
C5,6
The elbow
120°
is Tapping on biceps Flexion of
tendon
the forearm
C6,7
The
90°
is Tapping on triceps Extension of
tendon directly
the forearm
elbow
L2, 3 & 4 knee is semi Tapping
on Extension of
flexed
by patellar tendon
the knee
seating with the
leg to be tested
crossing
over
other
S1,2
feet
slightly Tapping
dorsiflexed
tendoachilles
Trigemin Mouth slightly Tapping on chin
al nerve opened
on Plantar
flexion.
Closure
mouth
of
Reinforcement of the tendon jerks
• The response of the tendon jerks can be reinforced
by facilitating the spinal centers.
• This can be done by either;
a) Jendrassik's maneuver → ask the patient to hook his
fingers or to clench his teeth→ send signals from
the contracted ms which stimulating γ-MNs.
b) Distracting patient’s attention→ prevents any
voluntary inhibition of the reflex.
1. Localization of spinal cord lesions:
• Loss of TJ means the lesion in its center e.g. ankle
jerk is lost in sacral region lesion.
2. Assessment of the ms tone :
• In hyperreflexia (exaggerated tendon jerks) →
hypertonia (↑ms tone).
• In hyporeflexia (↓ed tendon jerks) → hypotonia
(↓ms tone).
• In areflexia (lost tendon jerks) → atonia (lost ms
tone).
3. Assessment of the integrity of pathway of stretch reflex: so
areflexia or absent tendon jerk may be due to;
Site of lesion
Condition
•Afferent lesion
Tabes dorsalis
•Center (AHC) lesion
Poliomyelitis
•Efferent lesion
Trauma or neuritis
4. Assessment of the state of Supraspinal centers:
Hyperactive(exaggerated)
TJ
Hypoactive (decreased) TJ
Physiological
causes
Anxiety and nervousness
Sleep and anaesthesia
Pathological
causes
UMNL
Lesion in area 6
Tetany and
hyperthyroidism
Paleocerebellum lesion
LMNL
Lesion in area 4
Hypothyroidism
Neocerebellar syndrome
• Occurs in the neocerebellar syndrome and chorea.
• Characterized by hyporeflexia & hypotonia
• Knee jerk is weak than normal and during relaxation of the
quadriceps ms, the leg falls like a dead weight(due to
hypotonia) & swings for sometime like a pendulum before
resting.
Def.
• Alternating regular rhythmic contractions with
incomplete relaxations of a ms (its MNs is in a state
of facilitation) in response to sudden maintained
stretch.
Cause:
• UMNL
Types
1) Ankle Clonus:
• Produced by sudden maintained dorsiflexion of the
foot  leads to regular rhythmic planter flexions due
to rhythmic contractions of soleus and gastrocnemius
muscles.
2) Knee Clonus:
• Produced by the sudden downward displacement of
the patella  rhythmic oscillations of the patella.
Mechanism of clonus:
• Clonus is the result of a stretch reflex - inverse stretch
reflex sequence, which occurs as follows :
• Sudden stretch of the ms results in its contraction through the
stretch reflex.
• This is followed by relaxation due to;
a) Stoppage of impulse discharge from the ms spindles.
b) Initiation of an inverse stretch reflex due to stimulation of the
GTOs.
• As stretch is maintained, a new stretch reflex occurs (helped
by the state of excessive spinal facilitation), and the cycle is
repeated.
Test yourself
The shortest reflex time is recorded with :a- a flexor withdrawal reflex
b-an inverse stretch reflex
c- a stretch reflex
d- a scratch reflex
A tendon jerk :a- is a dynamic stretch reflex
b- is a static stretch reflex
c- is evoked by gradually stretching the muscle
d- is evoked by stimulation of tendon receptors
A reflex arc includes :a- at least two sets of sequential neurons
b- at least two sequential sets of central
synapses
c- at least two types of sensory receptors
d- at least two types of efferent neurons
Stretch reflex is characterized by the
following except :a- disynaptic reflex
b- high localization
c- shows reciprocal innervations.
d- it is of graded response
The nuclear-chain fibers of spindles are
innervated by :a- Aα and Aδ nerve fibers
b- Aδ and C nerve fibers
c- Ia and II nerve fibers
d- only type II nerve fibers
Inverse stretch reflex :a- increases the possibility of avulsion of the
excessively stretched muscle
from its bony attachments
b- has no reciprocal innervation circuits
c- is clinically manifested by lengthening
reaction
d- is clinically tested by examining the
tendon jerks
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