‫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
Lecturer of Physiology
Member of American Society of Physiology
Alpha motor
neuron
Gamma motor
neuron
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:
• There 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. the 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 the MNs discharge to antigravity
ms is increased →↑ magnitude of stretch reflex and
ms tone in these ms
• 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.
Midbrain
Left
Internal
Capsule
L
A
Facilitatory Pontine RF
Pons
Medulla
Alpha MN
Gamma MN
Spinal Cord
• B) Load reflex (coactivation of alpha and gamma
MNs)
• 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
overstretch or severe
contraction
• Receptors: Golgi tendon
organs
Site:
• in the tendons of skeletal ms.
Structure:
• Are encapsulated sensory receptor
• 6-20 elastic fibers
Innervations:
• type Ib or A alpha afferent fibres
Mechanism of stimulation:
• stimulated by ↑ed ms tension caused
by passive overstretch or active
contraction of the ms.
Neural pathway:
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.
Significance GTR:
a)Physiological significance:
• It is a protective reaction which prevent tearing of the ms or
avulsion of its tendon from its bony attachment when the ms is
overstretched.
b)Clinical significance: (clasp knife effect)
• It is 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.
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:
• It can be sustained for a prolonged period without
fatigue (e.g. in antigravity ms):
• The 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.
7) It is controlled by many supraspinal centers:
Area 6
and 4s
Area 4
Basal
ganglia
Paleocerebllum
Vestibular N.
Neocerebellum
Pontine R.F.
Medullary R.F.
Medial
Reticulospinal T.
Lateral
Corticospinal T.
Reticulospinal T.
Supraspinal Control of Stretch Reflex
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 the separation between the cerebral cortex &
brain stem leads to marked increase in muscle tone due to
the removal of the net inhibitory effect of the cerebral
cortical areas & leaving the facilitatory centers in the brain
stem to produce their excitatory effect.
•The increased muscle tone leads to a state known as
(Decerebrate Rigidity).
Area 6
and 4s
Area 4
Basal
ganglia
Paleocerebllum
Vestibular N.
Neocerebellum
Pontine R.F.
Medullary R.F.
Medial
Reticulospinal T.
Lateral
Corticospinal T.
Reticulospinal T.
To brain
6 Primary afferent
neuron stimulates
inhibitory interneuron
4 Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
7 Interneuron inhibits
alpha motor neuron
to flexor muscle
5
3 Primary afferent
neuron excited
Alpha motor neuron
stimulates extensor
muscle to contract
2 Muscle spindle
stimulated
1 Extensor muscle
stretched
Flexor muscle
8 (antagonist) relaxes
Tendon Jerks
●Def.,
• It is a brief contraction of
a skeletal ms to sudden
stretch produced by
tapping its tendon sharply
& strongly (using a reflex
hammer) .
●Mechanism:
• It is a dynamic type of the
stretch reflex
Examples of Tendon Jerks
Jerk
Biceps
jerk
Center
C5,6
Triceps C6,7
jerk
Limb position
Tendon
Response
The elbow
120°
is Tapping on biceps Flexion of
tendon
the forearm
The
90°
is Tapping on triceps Extension of
tendon directly
the forearm
elbow
Knee
jerk
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
Ankle
jerk
S1,2
feet
slightly Tapping
dorsiflexed
tendoachilles
Jaw
jerk
Trigeminal Mouth slightly Tapping on chin
nerve
opened
on Plantar
flexion.
Closure
mouth
of
Tendon Jerks
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.
Clinical Significance of TJ
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).
Clinical Significance of TJ
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
•-lesion of paleocerebellum
•-LMNL
•-Lesion in area 4
•-hypothyroidism
•-neocerebellar syndrome
Pendular Knee Jerk
• It occurs in the
neocerebellar syndrome
and chorea.
• It is 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 be resting.
Clonus
●Def.,
• This is 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
Clonus
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.
Clonus
●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
Q1: IPSPs and EPSPs
EPSPs
State
Neurotrans
mitters
Duration
Amplitude
Mechanism
EPSPs
Q2: Cation and Anion ion channels
Cation
Neurotrans
mitters
Inner lining
e.g.
PSPs
Anion
Q3: Compare Muscle Spindle and Golgi Tendon
organs
Ms Spindle
receptors
Site
Innervation
Stimulus
Functions
Golgi tendon
receptors
Q4: stretch reflex and inverse stretch reflex
Stretch
reflex
Stimulus
Receptors
Afferents
Significance
Inverse stretch
reflex
• List or enumerate;
1. Functions of Stretch reflex
2. Functions of Inverse stretch reflex
3. Properties of stretch reflex
4. Supraspinal centers regulating stretch reflex
5. Functions of muscle tone
6. Properties of polysynaptic reflexes
7. Properties of synaptic transmission
8. Properties of EPSPs or IPSPs
• Effect of (explain the mechanism) ;
1. Tapping of tendon of muscle by a medical
hammer
2. Excessive stretch of a muscle
3. Sudden maintained stretch of muscle in UMNL
4. Binding of neurotransmitter to postsynaptic Gprotein coupled receptors
5. Application of painful stimulus to one limb
during standing
THANKS