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Stretch Reflex - Copy.pptx
1.
2. 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.
5. Ms spindle
1ry and 2ry
endings
AHCs
Alpha motor
neurons
Extrafusal
ms fibers
6.
7. Muscle Spindle
Site:
• Fleshy parts of skeletal muscle parallel to their fibres
(extrafusal muscle fibers) .
Shape:
•It is capsulated fusiform stretch receptor.
Structure:
•Each spindle consists of several small muscle fibres (4-
12 fibers) called intrafusal fibres.
9. Muscle Spindle
Types of intrafusal muscle fibres:
• There are 2 types:
1)Nuclear bag fibres:
• Have a dilated central area filled with nuclei.
• There are 2 of these fibres per spindle.
2)Nuclear chain fibres:
• Have nuclei which are arranged as a chain in the receptor
area.
• There are 5-8 of these fibres per spindle.
10.
11. Muscle Spindle
Each muscle fibre 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
15. A) Afferent (sensory) innervations
Innervation of Muscle Spindle
Annulospiral or 1ry
afferents
Type Ia or
A alpha (16 um)
Supply nuclear
bag and chain
Rapidly responding
and adapting
Flower spray or 2ry
afferents
Type II or A beta
(8 um)
Supply nuclear chain
on sides of 1ry
endings
Slowly responding and
adapting
19. • 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.
Stimulation of Muscle Spindle
Whole Muscle
Muscle spindle
Afferents
20. • 1) Passive stretch of the whole ms:
Stimulation of Muscle Spindle
21. • 2)Activation of the γ-MNs:
• By supraspinal centers or reflexly
• It causes contraction of the peripheral part the intrafusal
fibres→ stretch of receptor area
Stimulation of Muscle Spindle
23. Types of Responses of Ms spindle to Stretch
(Types of Stretch Reflex)
Dynamic Response Static Response
Stimulus Sudden stretch Maintained (steady)
stretch
Receptors nuclear bag nuclear chain
Afferents 1ry endings primary and secondary
endings
Center Alpha motor neurons Alpha motor neurons
Response Rapid contraction
followed by rapid
relaxation
Maintained subtetanic
contraction
Examples e.g. tendon jerk e.g. muscle tone
24.
25. Nuclear
bag (1ry)
Nuclear chain
(1ry & 2ry
endings
Basal
discharge
Types of Responses of Ms spindle to Stretch
(Types of Stretch Reflex)
Dynamic
response
Static
response
29. 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
30.
31. Def.
• It is a state of continuous
(partial or subtetanic)
contraction of skeletal ms
during rest.
Distribution:
• It is present in all skeletal
ms but specially in the
antigravity ms
Extensors of LL
Flexors of the UL.
Back ms and back of neck
Elevators of the lower jaw
Anterior abdominal wall ms.
32. 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
34. 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
35. Regulation of skeletal ms tone:
• By supraspinal centers (facilitatory or inhibitory)
Facilitatory areas Inhibitory areas
1. Area 4
2. Neocerebellum
3. Excitatory RF
4. Vestibular N.
1. Area 6
2. Paleocerebllum
3. Inhibitory RF
4. Red N.
+
-
36.
37. 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:
38.
39.
40. • 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
41. • 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
43. • 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.
44.
45.
46. • 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.
47.
48. • 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
49.
50.
51. • Ms spindles provide proprioceptive information to the
brain and cerebellum for keeping them continually
informed about muscle length and changes in that length
54. 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.
55. • 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.
56.
57.
58. • 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.
61. • 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.
71. •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.
72.
73. 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
74.
75. 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.
76.
77. 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.
78.
79. • It is a reflex relaxation (or lengthening) of a ms in
response to excessive stretch or contraction of
that ms.
80.
81.
82.
83. 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
84. 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 α-
MNs supplying the antagonistic ms
Response:
• Relaxation of the same ms
• Contraction of antagonistic group of ms.
87. 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
88.
89. 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
90. To brain
4
3
5
8
7
6
2
1
Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
Primary afferent
neuron excited
Muscle spindle
stimulated
Extensor muscle
stretched
Flexor muscle
(antagonist) relaxes
Alpha motor neuron
stimulates extensor
muscle to contract
Interneuron inhibits
alpha motor neuron
to flexor muscle
Primary afferent
neuron stimulates
inhibitory interneuron
Sudden
stretch
Nuclear
bag
1ry
endings
Alpha
MNs
Ms
contraction
91.
92. • 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.
93. Jerk Center Limb position Tendon Response
Biceps
jerk
C5,6 The elbow is
120°
Tapping on biceps
tendon
Flexion of
the forearm
Triceps
jerk
C6,7 The elbow is
90°
Tapping on triceps
tendon directly
Extension of
the forearm
Knee
jerk
L2, 3 & 4 knee is semi
flexed by
seating with the
leg to be tested
crossing over
other
Tapping on
patellar tendon
Extension of
the knee
Ankle
jerk
S1,2 feet slightly
dorsiflexed
Tapping on
tendoachilles
Plantar
flexion.
Jaw jerk Trigemin
al nerve
Mouth slightly
opened
Tapping on chin Closure of
mouth
94.
95.
96.
97.
98.
99. 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.
100.
101. 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).
102. 3. Assessment of the integrity of pathway of stretch reflex: so
areflexia or absent tendon jerk may be due to;
4. Assessment of the state of Supraspinal centers:
Site of lesion Condition
•Afferent lesion Tabes dorsalis
•Center (AHC) lesion Poliomyelitis
•Efferent lesion Trauma or neuritis
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
103. • 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.
104. 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
105. 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.
106.
107. 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.
109. 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
110. 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
111. Stretch reflex is characterized by the
following except :-
a- disynaptic reflex
b- high localization
c- shows reciprocal innervations.
d- it is of graded response
112. 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
113. 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
114. • The divergence function of interneurons
is involved in :-
• a- temporal summation
• b- spatial summation
• c- reverberation
• d- irradiation
115. • 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
116. • 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
117. • 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
118. • 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
