Anatomy and Physiology of Balance

1. Anatomy and Physiology of Balance The Balance System depends on a complex set of interactions between a variety of systems: Inner ears Brainstem and cerebellum Eyes Spinal cord Postural MusclesCortices The Balance System depends on a complex set of interactions between a variety of systems: Inner ears Brainstem and cerebellum Eyes Spinal cord Postural MusclesCortices

2. Within the temporal bone lies the bony labyrinth a maze of tunnels lined with very dense and highly calcified bone. These we know as the cochlea, vestibule, and semicircular canals. Superior/Anterior Posterior Lateral/Horizontal These approximate (although not eactly) right angles to each other. This will allow for the analysis of curvilinear motion in terms of its components within each of these planes.Within the temporal bone lies the bony labyrinth a maze of tunnels lined with very dense and highly calcified bone. These we know as the cochlea, vestibule, and semicircular canals. Superior/Anterior Posterior Lateral/Horizontal These approximate (although not eactly) right angles to each other. This will allow for the analysis of curvilinear motion in terms of its components within each of these planes.

3. Within the bony labyrinth are spaces defined by soft tissue: the so-called membranous labyrinth. These divide the labyrinth into perilymphatic spaces and endolymphatic spaces. Just as in the cochlea, the endorgans for the balance half of the inner ear are contained within the endolymphatic spaces. Semicircular canals form partial circles whose diameter (if completed) would be on the order of 6.5 mm. The interior diameter of the endolymphatic tubes in the semicircular canals is up to 0.4 mm. Each of these canals has one swollen or ampulated end, which contains the endorgans. The posterior and superior canals share their non-ampulated ends in what is called the common crus. Within the bony labyrinth are spaces defined by soft tissue: the so-called membranous labyrinth. These divide the labyrinth into perilymphatic spaces and endolymphatic spaces. Just as in the cochlea, the endorgans for the balance half of the inner ear are contained within the endolymphatic spaces. Semicircular canals form partial circles whose diameter (if completed) would be on the order of 6.5 mm. The interior diameter of the endolymphatic tubes in the semicircular canals is up to 0.4 mm. Each of these canals has one swollen or ampulated end, which contains the endorgans. The posterior and superior canals share their non-ampulated ends in what is called the common crus.

4. Another view-- membranous labyrinth. These divide the labyrinth into perilymphatic spaces and endolymphatic spaces. Here we have labelled the utricle and saccule, the remaining endorgans for your sense of head motion. These endorgans are found in: the Ampullae--swollen end of each semicirc the Utricle the Saccule Also visible here is the endolymphatic sac, the subject of some surgical procedures. Another view-- membranous labyrinth. These divide the labyrinth into perilymphatic spaces and endolymphatic spaces. Here we have labelled the utricle and saccule, the remaining endorgans for your sense of head motion. These endorgans are found in: the Ampullae--swollen end of each semicirc the Utricle the Saccule Also visible here is the endolymphatic sac, the subject of some surgical procedures.

6. Vestibular Hair Cells Type I (aka inner) Type II (aka outer) With Kinocilium Within each of the endorgans, the sensory transducers are hair cells just like we found in the cochlea. And they still come in two varieties, only here there is no �inner� or �outer� placement for reference. So they are labelled as Type and Type II. And they have a large cilium in addition to the sterecilia. This is known as the kinocilium. What breaks off to leave the basal body in the cochlea.Within each of the endorgans, the sensory transducers are hair cells just like we found in the cochlea. And they still come in two varieties, only here there is no �inner� or �outer� placement for reference. So they are labelled as Type and Type II. And they have a large cilium in addition to the sterecilia. This is known as the kinocilium. What breaks off to leave the basal body in the cochlea.

7. Here they are (in an example from the otolithic organ).Here they are (in an example from the otolithic organ).

8. The Semicircular Canals posterior canal shares plane with contralateral anterior canal. horizontal canals share plane.

9. Stimulated by Angular Acceleration greatest when fulcrum is within head induces relative motion of endolymph crista is displaced by fluid motion

11. Responses of the Cristae All kinocilia are oriented in the same direction Crista in each pair of canals respond inversely to each other

12. The Otolithic Organs Saccule: roughly vertical orientation, responds to acceleration components within saggital plane Utricle: horizontal (+ 30 deg.) orientation

13. Excitation Patterns in the Utricle The sensory organ in both the saccule and utricle is the maccula. hair cells sit with their cilia embedded in a gelatinous membrane containing otoliths/otoconia/ stratoconia (calcium carbonate crystals). There is a curved stripe within each maccula that contains only type I (read "inner") hair cells, called the striola. The kinocilia on all of the hair cells are oriented toward the striola in the utricle and away from the striola in the saccule. The sensory organ in both the saccule and utricle is the maccula. hair cells sit with their cilia embedded in a gelatinous membrane containing otoliths/otoconia/ stratoconia (calcium carbonate crystals). There is a curved stripe within each maccula that contains only type I (read "inner") hair cells, called the striola. The kinocilia on all of the hair cells are oriented toward the striola in the utricle and away from the striola in the saccule.

14. Cranial Nerve VIII To Review: The vestibular system consists of the two otolithic organs, the utricle and the saccule (which are located in the vestibule, or central portion of the labyrinth), and the semicircular canals which run off of the vestibule. superior (anterior vertical) posterior (posterior vertical) horizontal (lateral)  The vertical canals share a common opening to the vestibule, the common crus, at their non-ampullated ends. Their ampullar ends as well as both ends of the horizontal canal also open into the vestibule. The Saccule: roughly vertical orientation, responding toacceleration components within a saggital plane. The Utricle: horizontal (+ 30 degrees) orientation.  The macculae are primarily sensitive to linear acceleration and gravity. To Review: The vestibular system consists of the two otolithic organs, the utricle and the saccule (which are located in the vestibule, or central portion of the labyrinth), and the semicircular canals which run off of the vestibule. superior (anterior vertical) posterior (posterior vertical) horizontal (lateral)

15. Vestibular Portion of C.N. VIII superior division: utricle, anterior part of saccule, and horiz & anterior canals inferior division: posterior part of saccule, and posterior canal to vestibular nuclei to cerebellum

16. Vestibulocochlear Nerve Exits temporal bone near its medial edge. Enters lateral face of brainstem at the level of the lower pons. Synapsing in (Cochlear and) Vestibular Nuclei

17. Responses of Vestibular Neurons: To changes in acceleration, but onset and fade slowly For most normal head movements firing rates are in phase with head VELOCITY.

18. This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.

19. Other inputs to vestibular nuclei: Cerebellum: primarily inhibitory Spinal cord Pontine reticular formation Contralateral vestibular nuclei

20. From the Vestibular Nuclei: Vestibulo-Oculomotor Pathways: Direct: to oculomotor nuclei. Indirect: via reticular formation to oculomotor nuclei (III IV and VI) Vestibulo-Spinal Pathways: Lateral V-S-throughout spinal cord Medial V-S-cervical & thoracic Reticulospinal tract-via brainstem reticular formation

21. Median Longitudinal Fasciculus A tract linking Vest. Nuclei to nuclei of CN III, IV, & VI; Supports conjugate eye movement during movement of the head. Continuous with the medial vestibulospinal tract. The mlf runs near midline ventral to ventricle IV and the periaqueductal gray matter of the midbrain

24. In the brainstem Vestibular inputs undergo integration Integrated signal is combined with original (velocity driven) signal Processing to reset spatial map for eye musculature

25. Integration & �Leaky� Integration

26. This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.

27. The VOR

31. Central Nervous System Will Adapt to Peripheral Damage

32. Eye Movements Saccades�rapid shift in gaze Pursuit�stabilize image of moving object Fixation�stabilize image of still object VOR�stabilize image during head motion OKN�backup for when VOR decays to cont�d head rotation Vergent movements�change depth of focus Smooth Pursuit: Slower, for keeping target on the foveawhen there is relative movement. Controlled by the occipital cortex and are based on inputs from the retina via visual cortex. Saccadic: Fast, up to 70 degrees per .1 sec, ballistic movements which can come under voluntary control with input from the pre-frontal cortex. The pontine gaze center (within reticular formation) is involved with the initiation of these movements in the horizontal direction. VOR: to move eyes relative to head movements. Mediated via connections from the vestibular system. Accommodating Reflex: to keep target that is moving closer or farther away in focus. Mediated through the occipital cortex. Smooth Pursuit: Slower, for keeping target on the foveawhen there is relative movement. Controlled by the occipital cortex and are based on inputs from the retina via visual cortex. Saccadic: Fast, up to 70 degrees per .1 sec, ballistic movements which can come under voluntary control with input from the pre-frontal cortex. The pontine gaze center (within reticular formation) is involved with the initiation of these movements in the horizontal direction. VOR: to move eyes relative to head movements. Mediated via connections from the vestibular system. Accommodating Reflex: to keep target that is moving closer or farther away in focus. Mediated through the occipital cortex.

33. Pause cells inhibit Burst Neurons which stimulate: III & VI (horizontal) or III & IV (vertical) Saccades

34. This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.This chart represents the basic organization of the vestibular system. We see that the semicirc�s are primarily responding to head rotation or angular acceleration. The Saccule and Utricle are responsive to Linear acceleration and the pull of gravity. Together they form one of the three primary inputs to the central nervous system components dedicated to balance: visual// proprioception & tactile To foreshadow, the outputs of this system include: the occulomotor muscles, the spinal cord and cerebellum, and the forebrain.

43. Functional Balance: Navigating in our Environment

44. A Quiz Vestibular Pathways Quiz