5Gamma MNs Site: Types: 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-MNsDynamic or d- γ-MNs→ supply nuclear bag ms fibers.Static or s- γ-MNs→ supply nuclear chain ms fibers.
7Functions 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 stimulatedVice versa.7
10Functions of Gamma MNsThis is important inA) 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 msAlso 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.10
13Functions of Gamma MNsB) Load reflex (coactivation of alpha and gamma MNs)13
14Inverse 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 contractionReceptors: Golgi tendon organs
15Golgi Tendon Organs (GTOs) Site:in the tendons of skeletal ms.Structure:Are encapsulated sensory receptor6-20 elastic fibersInnervations:type Ib or A alpha afferent fibresMechanism of stimulation:stimulated by ↑ed ms tension caused by passive overstretch or active contraction of the ms.
16Inverse Stretch Reflex Neural pathway:Afferents:A alpha or IbCenter :a)inhibitory interneurons→ inhibit the α-MNs supplying the same msb)excitatory interneurons→ excite the α-MNs supplying the antagonistic msResponse:Relaxation of the same msContraction of antagonistic group of ms.
17Inverse 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.
19Properties 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
20Properties of Stretch Reflex 3) It has no recruitment nor after discharge:It is due to lack of interneurons4)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.
22Properties 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 supplyRich in mitochondriaIts contraction is slow.
23Properties of Stretch Reflex 7) It is controlled by many supraspinal centers:
24Medial Reticulospinal T. Lateral Reticulospinal T. Area 4Area 6 and 4sBasal gangliaPaleocerebllumVestibular N.NeocerebellumPontine R.F.Medullary R.F.Medial Reticulospinal T.Corticospinal T.Lateral Reticulospinal T.
25Supraspinal 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 toneIn 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).
26Medial Reticulospinal T. Lateral Reticulospinal T. Area 4Area 6 and 4sBasal gangliaPaleocerebllumVestibular N.NeocerebellumPontine R.F.Medullary R.F.Medial Reticulospinal T.Corticospinal T.Lateral Reticulospinal T.
29The Tendon Reflex 6 Primary afferent neuron stimulates inhibitory interneuronTo brain4Primary afferentneuron stimulatesalpha motor neuronto extensor muscle7Interneuron inhibitsalpha motor neuronto flexor muscle5Alpha motor neuronstimulates extensormuscle to contract3Primary afferentneuron excited2Muscle spindlestimulatedFlexor muscle(antagonist) relaxes1Extensor musclestretched8
30Tendon 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
31Examples of Tendon Jerks CenterLimb positionTendonResponseBiceps jerkC5,6The elbow is 120°Tapping on biceps tendonFlexion ofthe forearmTriceps jerkC6,7The elbow is 90°Tapping on triceps tendon directlyExtension of the forearmKnee jerkL2, 3 & 4knee is semi flexed by seating with the leg to be tested crossing over otherTapping on patellar tendonExtension of the kneeAnkle jerkS1,2feet slightly dorsiflexedTapping on tendoachillesPlantar flexion.Jaw jerkTrigeminal nerveMouth slightly openedTapping on chinClosure of mouth
32Tendon 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.
33Clinical 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).
34Clinical 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:Site of lesionConditionAfferent lesionTabes dorsalisCenter (AHC) lesionPoliomyelitisEfferent lesionTrauma or neuritisHyperactive(exaggerated) TJHypoactive (decreased) TJPhysiological causesAnxiety and nervousnessSleep and anaesthesiaPathological causes-UMNL-Lesion in area 6-tetany and hyperthyroidism-lesion of paleocerebellum-LMNL-Lesion in area 4-hypothyroidism-neocerebellar syndrome
35Pendular Knee Jerk It occurs in the neocerebellar syndrome and chorea. It is characterized by hyporeflexia & hypotoniaKnee 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.
36Clonus●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
37ClonusTypes1- 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.
38Clonus ●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.
39Test yourselfThe 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
40A reflex arc includes :- a- at least two sets of sequential neuronsb- at least two sequential sets of central synapsesc- at least two types of sensory receptorsd- at least two types of efferent neurons
41Stretch reflex is characterized by the following except :- a- disynaptic reflexb- high localizationc- shows reciprocal innervations.d- it is of graded response
42The nuclear-chain fibers of spindles are innervated by :- a- Aα and Aδ nerve fibersb- Aδ and C nerve fibersc- Ia and II nerve fibersd- only type II nerve fibers
43Inverse stretch reflex :- a- increases the possibility of avulsion of the excessively stretched musclefrom its bony attachmentsb- has no reciprocal innervation circuitsc- is clinically manifested by lengthening reactiond- is clinically tested by examining the tendon jerks
44Q1: IPSPs and EPSPs EPSPs State Neurotransmitters Duration Amplitude Mechanism
45Q2: Cation and Anion ion channels NeurotransmittersInner lininge.g.PSPs
46Q3: Compare Muscle Spindle and Golgi Tendon organs Ms SpindlereceptorsGolgi tendon receptorsSiteInnervationStimulusFunctions
47Q4: stretch reflex and inverse stretch reflex StimulusReceptorsAfferentsSignificance
48List or enumerate;Functions of Stretch reflexFunctions of Inverse stretch reflexProperties of stretch reflexSupraspinal centers regulating stretch reflexFunctions of muscle toneProperties of polysynaptic reflexesProperties of synaptic transmissionProperties of EPSPs or IPSPs
49Effect of (explain the mechanism) ; Tapping of tendon of muscle by a medical hammerExcessive stretch of a muscleSudden maintained stretch of muscle in UMNLBinding of neurotransmitter to postsynaptic G-protein coupled receptorsApplication of painful stimulus to one limb during standing