NERVOUS SYSTEM Postural Reflexes - PowerPoint Presentation

1. NERVOUS SYSTEMPostural Reflexes

2. POSTUREEach species of animal maintains a normal orientation in space called posture / attitudeEg: All domestic animals stand on four limbsHorizontal postureHumans stand on lower limbs only vertical postureMaintenance of posture Requires continuous activity of a complex interrelated reflexesPostural Reflexes

3. POSTURAL REFLEXESSupporting ReflexesAttitudinal ReflexesLocal Supporting Reflexes(Local static reflexes)Segmental Supporting ReflexesTonic Neck ReflexesTonic labyrinthine Reflexes

4. SUPPORTING REFLEXES – Local supporting reflexes Limit their activity to the segments of spinal cord that contributed afferent signalsPrimarily myotatic reflexesSupported by reflex activity elicited by cutaneous receptorsEg: When paw is placed on the ground, the flexors of digits and carpus are stretched and the toes spread apart.This initiates myotatic reflex contraction of respective muscles and provide support to the animal body so placed on the groundIf the animal changes its centre of gravity resulting in stretch of muscles antagonistic to the earlier group, those muscles undergo reflex contraction and the animal is supported again.

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6. SUPPORTING REFLEXES – Segmental Supporting ReflexesMay produce alteration in muscular activity in all limbs even when the afferent signals originated only in cervical or lumbar regionsWhen a rear limb is extended in a local supporting response, the opposite fore limb is also extendedSimilarly if a fore limb exhibits a supporting response, contra lateral hind limb is also extendedThus afferent activity in one segment of the spinal cord alters the activity in another segment of spinal cord.

7. PATHWAYS IN SUPPORTING REFLEXESTwo pathways are usedSlow conducting multi-synaptic pathwayAscends bilaterally in the propriospinal tractsRapidly conducting pathway with less synapsesUses spino-reticulo and reticulo-spinal pathwaySpino-bulbo-spinal pathwayWithdrawal and crossed extensor reflexes are segmental and supporting in nature

8. ATTITUDINAL REFLEXESThese reflexes cause alterations in posture in response to changes in position of headServed by stretch receptors of neck muscles Tonic neck reflexesA portion of membranous labyrinth in the internal earTonic labyrinthine reflexes

9. TONIC NECK REFLEXESStretch receptors of neck muscles initiate reciprocal influences on extensor muscle tone in the limbs.An increase in tone of the extensors in fore limbs is accompanied by a decrease of tone in the extensors of hind limbs and vice-versaEg: Ventral flexion of the head causes extension of both rear limbs and relaxation of extensors in fore limbsDorsal flexion of head has the opposite effectLateral flexion of head leads to extension of ipsilateral limbs and relaxation of contra lateral limbs

10. TONIC LABYRINTHINE REFELEXESInitiated by alterations in the position of the head within earths gravitational fieldIncrease or decrease extensor tone in all the four limbs simultaneouslyEg: animal is placed on its back with its nose 45o above the horizontal, extensor tone in all the four limbs is increasedSimilarly if the head is placed 45o below the horizontal, extensor tone in all the four limbs is decreasedIf the animal is placed on its side and the head is raised to an erect position, righting of the body occurs – called righting ReflexesRighting reflexes are a series of complex reflexes initiated by tonic labyrinthine reflexesTonic labyrinthine reflexes cause righting of head, which in turn brings about tonic neck reflexes resulting in normal postureIf a blind folded cat is dropped in supine position from a reasonable height, it will turn rapidly and land on its feet. Righting reflexes involve visual and cutaneous sensory systems and vestibular system

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13. PLACING REFLEXESThese are cortical reflexesInvolve vestibular, cutaneous and visual sensory systems in their afferent limbsDivided based on the sensory system involved intoVestibular placing reflexesTactile placing reflexesVisual placing reflexes

14. VESTIBULAR PLACING REFLEXESBest observed in a blind folded animal (cat)If such an animal is rapidly lowered from a heightIts four limbs are extended, toes are spread apart in preparation to support the body on its contact with ground

15. TACTILE PLACING REFLEXESIn a blind folded animal (cat)If its paw, chin or hind limb is brought into contact with a hard surface, and the remaining body is free in airThe animal immediately prepares to stand up as in vestibular placing reflexes

16. VISUAL PLACING REFLEXESWhen a normal animal (not blind folded) is rapidly lowered from a height towards a visible surfaceThe animal prepares to stand up as in vestibular / tactile placing reflex

17. HOPPING REFLEXIf an animal is supported on three limbs only, and such an animal is displaced laterally / medially / rostrally / caudallyThe animal shall hop in the direction of displacement

18. CENTRAL MECHANISMS IN POSTURAL REFLEXESCNS structures that directly control postural reflexes:Cortex – visual & tactile placing and hopping reflexesPonsMedullaCervical spinal cord – Tonic neck reflexesCerebellum, reticular formation, red nucleus, & descending tracts alter the activity of above structuresAny abnormal function of these structures alters the excitability of spinal cord motor neuronal pools leading to disturbances in normal reflex activityVestibular placing And Tonic labyrinthine reflexes

19. VESTIBULAR ORGANImportant receptor in operation of postural reflexesBony labyrinth of internal ear contains the membranous labyrinthSurrounding membranous labyrinth and separating it from the bony labyrinth is a fluid called – PERILYMPHIn the cavities of membranous labyrinth is ENDOLYMPHMembranous labyrinth is composed of Non-acoustic portion – vestibular organAcoustic portion – cochlear duct

20. VESTIBULAR ORGANComprises of two membranous sacsSaccule and UtricleSaccule communicates with cochlear ductUtricle with semicircular canalsThree semi-circular canalsRostral, caudal and horizontalSpecialized receptor organs within SCC , Saccule & UtricleIn utricle & saccule – “macculae”Macculae - otolithsIn SCC - cristae

21. OTOLITHS IN UTRICLE AND SACCULEComprises of plaque of hair cells covered by gelatinous materialWithin the gelatinous substance and attached to hair cells are calcareous substances called otolithsOtoliths are involved in most labyrinthine reflexesAdequate stimulus is displacement of otoliths duetoGravity orPositive / negative linear acceleration of head

22. CRISTAE IN SEMICICRCULAR CANALSLocated within the enlargement (ampulla) of each of SCC Receptors in cristae are also hair cells covered by gelatinous materialGelatinous material forms a definite structure - “cupula”Cupula obstructs the ampulla of SCCSCC are oriented approx. at right angles to each otherCalled Rsotral, lateral and caudal SCCThus atleast one canal rotates in the direction of headWhen a SCC moves with the head, the endolymph lags behind due to inertiaThis causes a deflection of the cupula in the direction opposite to that of the headThis is adequate stimulus for the hair cellsIf head rotates at a continuous constant velocity or stops rotation, the inertia of the endolymph is overcome and this removes the deflection of cupulaTherefore cristae could only sense the direction of movement of head but not the final position

23. RECEPTORS IN THE SCC ARE EXCITED ONLY BY ROTATORY ACCELERATIONOTOLITHS ARE STIMULATED BY POSITION OF HEAD IN THE GRAVITATIONAL FIELD AND LINEAR ACCELERATION OF HEAD

24. NystagmusWhen the SCC are stimulated continuously for some timeTwo and fro movements of eyes are seenTwo components to nystagmusA slow deviation of eyes in the direction opposite to the direction of rotation of head andA fast deviation in the direction of rotation of headCNS structures responsible for nystagmus areVestibular nucleusMedial longitudinal fasciculusMotor nucleus of extra occular muscles (cranial III, IV & VI)Nystagmus in animals is observed commonly under anaesthesia

25. STIMULATION OF SCCAny movement that causes endolymph to move in a different direction or rate stimulates cristae in SCCDifferences in temperature between different regions of SCC also stimulate cristae through convection currentsThis is used as a clinical test to establish the integrity of vestibular apparatus

26. MOTION SICKNESSDevelops due to prolonged stimulation of cristae/otolithsCar sickness, air sickness and sea sickness are all similarCondition in dog is characterized byExcessive salivationSwallowingFall in BPVomitionCerebellum plays an important role in the development of motion sicknessRemoval of nodulus and uvula of cerebellum abolishes motion sickness

27. Nervous systemReticular Formation of the Brain Stem

28. RETICULAR FORMATIONReticular formation is so called bec. Of cyto-architectonic structures Reticulum of nerve fibers passing in all directionsExtends from spino-medullary junction , through out the length of medulla, pons and mid brainRostral to mid brain the reticular formation is continuous with medail thalamus and lateral hypothalamusReceives afferents from all sensory systemsEfferents received from CNS structuresEX: cortex, basal ganglia, thalamus, hypothalamus and cerebellum

29. FIBER SYSTEMS IN RETICULAR FORMATIONOrganized into three groupsAscending fibers - Ascending reticular formationProject to other structures of brain stem and cerebrumDescending fibers – descending reticular formationProject to spinal cordBoth ascending and descending reticular formationProject to cerebellum

30. EXCITATORY FUNCTION OF RETICULAR FORMATIONRF and Vest.nucleus function in close associationBoth are intrinsically excitable Intrinsic excitability inhibited by basal ganglia and cortexAblation of basal ganglia and cortex results in Rigidity of all antigravity muscles Ablation of brain stem just below Vest. NucleusFlacid antigravity musclesTherefore RF and Vest.Nucleus normally transmit excitatory signals into antigravity muscles

31. RECIPROCAL EXCITATION / INHIBITION OF MUSCLES BY RETICULAR FORMATIONStimulation of RF near its midline causesIpsilateral flexors to contract andIpsilateral extensors to relaxStimulation in the lateral portions of RF causesIpsilateral extensors to contract andIpsilateral flexors to relaxAs muscles on one side of the body contract, same muscles on the contra lateral side relax and antagonists contract simultaneouslyPhenomenon of reciprocal excitation is strongly expressed in RF

32. SUPPORT OF THE BODY AGAINST GRAVITY BY RETICULAR FORMATIONWhen an animal standsVestibular and other nuclei in RF send continuous signals into SC and from there to extensors in limbs to keep them contractedThe impulses are transmitted mainly viaVestibulo-spinal tract andReticulo-spinal tractBut the degree of activity in individual muscles is determined by the equilibrium mechanismThus if an animal begins to fall to one sideExtensors on that side contract more powerfullyWhile contralateral extensors tend to relaxThus Vestibular nucleus and RF together provide support against gravity

33. FUNCTIONS OF RETICULAR FORMATIONControl of skeletal muscle motor activityControl of cardio-vascular systemControl of RespirationControl of other autonomic activitiesControl of activity within sensory pathways

34. CONTROL OF RESPIRATION BY RETICULAR FORMATIONRespiratory centers are located in the RFAll these centers are bilateral

35. REGULATION OF HEART BY RETICULAR FORMATIONThrough the autonomic nerves supplying heart, RF exerts a lot of influence on heartTherefore RF can change the rate and/or force of contracftion of heartCardiac pressor and depressor centers are located in RF

36. CONTROL OF VASOMOTOR TONE BY RETICULAR FORMATIONVasomotor, dilator and constrictor centers are located within the medullary RF

37. CONTROL OF MICTURITION BY RETICULAR FORMATIONReticular formation center that control micturition was demonstrated in many sp.In general this centre inhibits micturition

38. CONTROL OF SLEEP – WAKEFULNESS CYCLES BY RETICULAR FORMATIONReticular formation is also known as Reticular Activating systemBecause it controls sleep and wakefulness cycles in all mammmalsReticulocortical pathways are used in this function