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ب سم الله الرحمن الرحيم. ﴿و ما أوتيتم من العلم إلا قليلا﴾. صدق الله العظيم الاسراء اية 58. Physiology of CNS. Stretch Reflex. By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology. Gamma Motor Neurons. Alpha motor neuron. Gamma motor neuron.
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بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58
Physiology of CNS Stretch Reflex By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology
Alpha motor neuron Gamma motor neuron
Gamma MNs 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 • Dynamic or d- γ-MNs→ supply nuclear bag ms fibers. • Static or s- γ-MNs→ supply nuclear chain ms fibers.
Functions of Gamma MNs 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.
Functions of Gamma MNs 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 Facilitatory Pontine RF L Pons A Medulla Alpha MN Gamma MN Spinal Cord
Functions of Gamma MNs B) Load reflex (coactivation of alpha and gamma MNs)
Inverse Stretch Reflex • 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
Golgi Tendon Organs (GTOs) 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.
Inverse Stretch Reflex 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.
Inverse Stretch Reflex 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.
Properties of Stretch Reflex 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
Properties of Stretch Reflex 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.
Properties of Stretch Reflex 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; • Rich in blood supply • Rich in mitochondria • Its contraction is slow.
Properties of Stretch Reflex 7) It is controlled by many supraspinal centers:
Area 4 Area 6 and 4s Basal ganglia Paleocerebllum Vestibular N. Neocerebellum Pontine R.F. Medullary R.F. Medial Reticulospinal T. Corticospinal T. Lateral 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 4 Area 6 and 4s Basal ganglia Paleocerebllum Vestibular N. Neocerebellum Pontine R.F. Medullary R.F. Medial Reticulospinal T. Corticospinal T. Lateral Reticulospinal T.
The Tendon Reflex Primary afferent neuron stimulates inhibitory interneuron 6 To brain Primary afferent neuron stimulates alpha motor neuron to extensor muscle 4 Interneuron inhibits alpha motor neuron to flexor muscle 7 Alpha motor neuron stimulates extensor muscle to contract 5 Primary afferent neuron excited 3 Muscle spindle stimulated 2 Flexor muscle (antagonist) relaxes Extensor muscle stretched 8 1
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
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; 4. Assessment of the state of Supraspinal centers:
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
List or enumerate; • Functions of Stretch reflex • Functions of Inverse stretch reflex • Properties of stretch reflex • Supraspinal centers regulating stretch reflex • Functions of muscle tone • Properties of polysynaptic reflexes • Properties of synaptic transmission • Properties of EPSPs or IPSPs
Effect of (explain the mechanism) ; • Tapping of tendon of muscle by a medical hammer • Excessive stretch of a muscle • Sudden maintained stretch of muscle in UMNL • Binding of neurotransmitter to postsynaptic G-protein coupled receptors • Application of painful stimulus to one limb during standing
