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ميظعلا الله قدص 58 ةيا ءارسلاا

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.

Left Internal Capsule

L A

Midbrain 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:

organs Golgi tendon

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 I b

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 Vestibular N.

Pontine R.F. Medullary R.F. Medial Reticulospinal T.

Paleocerebllum Neocerebellum Lateral Reticulospinal T.

Corticospinal 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 Vestibular N.

Pontine R.F. Medullary R.F. Medial Reticulospinal T.

Paleocerebllum Neocerebellum Lateral Reticulospinal T.

Corticospinal T.

4 Primary afferent neuron stimulates alpha motor neuron to extensor muscle To brain 6 Primary afferent neuron stimulates inhibitory interneuron 7 Interneuron inhibits alpha motor neuron to flexor muscle 5 Alpha motor neuron stimulates extensor muscle to contract 3 Primary afferent neuron excited 2 Muscle spindle stimulated 1 Extensor muscle stretched 8 Flexor muscle (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 Triceps jerk

C6,7

Knee jerk Ankle jerk Jaw jerk Center

C5,6 L2, 3 & 4 S1,2

Limb position Tendon Response

The elbow is 120 ° Tapping on biceps tendon Flexion of the forearm The elbow is 90 ° Tapping on triceps tendon directly Extension of the forearm knee is semi flexed by seating with the leg to be tested crossing other over Tapping patellar tendon feet slightly dorsiflexed Tapping tendoachilles on Extension of the knee on Plantar flexion.

Trigeminal nerve Mouth opened slightly Tapping on chin 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) ( ↑ms tone).

→ hypertonia 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

•Afferent lesion •Center (AHC) lesion •Efferent lesion

Condition

Tabes dorsalis Poliomyelitis Trauma or neuritis

4. Assessment of the state of Supraspinal centers: Hyperactive(exaggerated) TJ

•Anxiety and nervousness

Hypoactive (decreased) TJ

•Sleep and anaesthesia

Physiological causes 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 b Aα and Aδ nerve fibers 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 EPSPs State Neurotrans mitters Duration Amplitude Mechanism

Q2: Cation and Anion ion channels

Cation Anion Neurotrans mitters Inner lining e.g. PSPs

Q3: Compare Muscle Spindle and Golgi Tendon organs Site Innervation Ms Spindle receptors Stimulus Functions Golgi tendon receptors

Q4: stretch reflex and inverse stretch reflex Stimulus Receptors Afferents Significance Stretch reflex 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 G protein coupled receptors 5. Application of painful stimulus to one limb during standing

THANKS