Unit 1: Anatomy and Physiology of the Vestibular System PDF

Unit 1: Anatomy and Physiology of the Vestibular System Mr. Jim Saroj Winston Assistant Professor  Peripheral vestibular system including semicircular canals, utricle and saccule Central vestibular pathway (vestibular nerve, brainstem, cerebellum and cortex) Reflexes i...

Unit 1: Anatomy and Physiology of the Vestibular System Mr. Jim Saroj Winston Assistant Professor  Peripheral vestibular system including semicircular canals, utricle and saccule Central vestibular pathway (vestibular nerve, brainstem, cerebellum and cortex) Reflexes involving vestibular system like vestibuloocular reflex, vestibulospinal reflex and vestibulo colicreflexadvise Other systems involved in maintenance of balance like proprioceptive system,visual systemetc.  VESTIBULAR SYSTEM PERIPHERAL VESTIBULAR SYSTEM CENTRAL VESTIBULAR SYSTEM  Development In the 5th week the ovoid otocyst undergoes elongation in both dorsal and ventral directions.  Ventral part represents the future cochlear duct In the dorsal portion develops into SCC the developing semicircular ducts. The intermediate region is destined to subdivide into the utricle and the saccule  In the 8th week the endolymphatic and semicircular ducts are well represented. The intermediate sac has divided into utricle and saccule.  Early in the 9th fetal month the general adult form of the internal ear is nearly attained.  a) Peripheral vestibular system including semicircular canals, utricle and saccule  Consists of the membranous and bony labyrinths The bony labyrinth is housed within the petrous portion of the temporal bone Membranous labyrinth is within the bony labyrinth  The bony labyrinth is filled with perilymphatic fluid Perilymphatic fluid communicates via the cochlear aqueduct with cerebrospinal fluid. Because of this communication, disorders that affect spinal fluid pressure (such as lumbar puncture) can also affect inner ear function.  Membranous Labyrinth The membranous labyrinth is suspended within the bony labyrinth by perilymphatic fluid and supportive connective tissue. The membranous labyrinth is filled with endolymphatic fluid It contains five sensory organs: – the membranous portions of the three SCCs – And the two otolith organs, the utricle and saccule  Semi circular Canals Semicircular canals are curved structures that enter the utricle at each of their ends. Based on the orientations, there is a – Horizontal (HC) canal and – two vertical canals, referred to as the anterior (AC) and posterior (PC) canals.  The three canal planes are nearly orthogonal to one another.  The ducts of the two vertical canals fuse to form the crus commune (“common arm”)  One end of each SCC is widened in diameter to form an ampulla The crista ampullaris (CA) is the sensory organ of rotation. They are found in the ampullae of each of the semicircular canals of the inner ear, meaning that there are 3 pairs in total.  Each vestibular sensory (CA) organ has a neuroepithelium composed of hair cells covered by a gelatinous substance (Cupula) and supporting cells Specialized hair cells contained in each ampulla and otolith organ are biological sensors that convert displacement due to head motion into neural firing  Vestibular Hair Cells (Wersall (1956)) Type I – flask shaped – More efferent connections Type II – cylinder shaped – multiple efferent and afferent boutons With one Kinocilia and multiple sterocilia  Lindeman (1969) observed that there were regional differences in the sizes and spacing of both types of hair cells and in their hair bundle morphology. Based on these differences, the crista can be divided into three concentric zones of approximately equal areas. Physiology of SSC Ampulopetal vs Ampulofugal Anterior canal (AC) and posterior canal (PC) hair cells are oriented to be excited for ampullofugal endolymph displacement (toward the slender duct) Lateral canal (LC) hair cells are oriented to be excited by ampullopetal endolymph displacement. Nerve supply  The vestibular nerve originates in the vestibular ganglion, which is located within the internal auditory meatus (IAC).  The vestibular nerve leaves the inner ear through the IAM. It crosses the CPA and enters the brainstem in the medullopontine sulcus. The vestibular nerve separates from the cochlear nerve before reaching the vestibular nuclear complex. The vestibular nerve ends in the vestibular nuclear complex, which consists of four nuclei: medial, lateral, superior, and inferior. Vestibular Nuclei The vestibular nuclei are located in the medulla and pons of the hindbrain, under the floor of the fourth ventricle. The vestibular nuclei are a complex of four major nuclei that work together to integrate information from the cerebellum, somatosensory organs, contralateral nuclei, and primary vestibular afferents. The four main vestibular nuclei are: Superior nucleus: Receives input from the superior and posterior semicircular ducts and is involved in the vestibulo-ocular reflex Medial nucleus: Receives input from the semicircular canals and gives rise to the medial vestibulospinal tract Lateral nucleus: Also known as Deiter's nucleus Inferior nucleus: One of the four 2nd order vestibular nuclei  Cells of SVN also known as the nucleus of Bechterew,  Cells of IVN The inferior vestibular nucleus (DVN) is made up of large diameter neurons and has a distinctive checkered appearance due to the fiber bundles that run rostro-caudally through it  Cells of LVN The LVN, also known as Deiter’s nucleus, extends along the lateral column of the vestibular complex. The SVN, MVN, and DVN border it. Lateral to the LVN are the sensory fibers of CN VIII. It is distinguishable from the other nuclei by its giant cells in the dorsocaudal region and intermediate- sized cells in the rostroventral region. The LVN aids the vestibulospinal reflex to maintain proper posture and balance via the limbs' paravertebral and proximal extensor muscles.  Cells of MVN The MVN, also known as the nucleus of Schwalbe, is the largest of the 4 nuclei in total cell volume and, unlike the others, runs mostly in the medial column. It is bordered by the SVN and DVN, posteriorly by the fourth ventricle, and inferiorly by the dorsal motor vagal and hypoglossal nuclei. The MVN has a dorsal collection of small parvocellular neurons and a ventral collection of larger magnocellular neurons. It mediates the vestibulo-ocular reflex (VOR) along with the SVN, which ensures clear vision by rotating the eyes opposite to the head during horizontal rotation.[ Inputs to V Nucleus Central vestibular Connections Vestibular primary afferents bifurcate as they enter the brainstem – To cerebellum – To vestibular nuclei  The primary afferents that are destined for the cerebellum bypass the vestibular nuclei  Goes directly through the juxtarestiform body through the inferior cerebellar peduncle ending up predominantly in the ipsilateral flocculus, nodulus, and anterior uvula of the cerebellum (Shinoda and Yoshida, 1975).  Neurons in all four vestibular nuclei also send axons to the cerebellum as secondary vestibulocerebellar projections. These axons end in the flocculonodular lobe, the uvula, the immediately adjacent portions of the paraflocculus, and the fastigial and dentate nuclei of the cerebellum  The semicircular canals project to the flocculus, nodulus, and uvula Whereas those of the utricle and saccule only to the nodulus and uvula (Purcell and Perachio, 2001) Therefore, the flocculonodular lobe plays an important part in regulating eye movement through the vestibulo-ocular reflex  Cortical connections of vestibular system the superior and lateral vestibular nuclei send axons to the ventral posterior nuclear complex of the thalamus, which projects to cortical areas relevant to vestibular sensations  There are four main cortical areas that respond to vestibular stimulation First are areas 2v and 3a- In the primary somatosensory cortex Integrate motor control of the head and body  Second is area 7 of the parietal cortex.  A third cortical region responding to vestibular motion signals lies in the retroinsular and granular insula areas of the lateral sulcus, together termed the parietoinsular vestibular cortex (PIVC)  Fourth, in the prefrontal cortex, area 6 and the superior frontal gyrus receive vestibular signals and are related to the frontal eye field (FEF). The FEF is involved in the control of saccades and smooth pursuit eye movements Vestibular commissural projections Three vestibular nuclei (SVN, MVN and DVN) have homologous and reciprocal connections. The LVN do not The vestibular nuclei also have divergent connections to non- homologous contralateral vestibular nuclei (c)Reflexes involving vestibular system Vestibulo Occular Reflex Vestibulo Collic Reflex (VOR) (VCR) Vestibulo Spinal Reflex Cervico Occular Reflex (VSR) (COR)  Vestibulo Occular Reflex (VOR) The vestibular–ocular reflex (VOR) is a response that permits the ocular fovea to remain on a target while the head moves. Allows the individual to recognize faces or read while moving, such as while walking or, even, after high- frequency head movements. It essentially responds to head (angular) accelerations, which can reach up to several thousands of degrees per square second. If this system is not working properly and accurately, either due to a velocity or amplitude deficiency, the shift in gaze causes a retinal slip that is perceived as an image movement, designated by oscillopsia.  VOR Pathway  Each of the six pairs of oculomotor muscles has to be coordinated to produce the desired response. The vertical SCCs and the saccule are sensible for controlling vertical eye movements The horizontal canals and the utricle regulate horizontal eye movements. Torsional eye movements are essentially controlled by the vertical SCCs as well as by the utricle  The eye response to a head rotation resides in a slow phase until the eye reaches the edge of the outer canthus and a fast phase as a reset to the initial position. This pattern repeats itself as long as the stimulus continues, and these two types of repeated movements, as a saw-tooth pattern, characterize the vestibular nystagmus variety. The direction of the nystagmus is denominated by the fast phase, as it is the easiest to perceive. d) Other systems involved in maintenance of balance like proprioceptive system and visual system  Vestibulo Spinal Reflex (VSR) & Vestibulo Collic Reflex (VCR) Helps maintain body posture and balance by adjusting muscle tone in response to changes in head position. Two functional categories of vestibulo-spinal reflexes can be distinguished: – Those acting on the limb muscles, which stabilize the position of the trunk in space (VSR) – Those acting on the neck muscles (vestibulo-colic reflexes), which stabilize the position of the head in space  The two most important vestibular descending pathways involve the – Lateral vestibulo-spinal tract (LVST) – Medial vestibulo-spinal tract (MVST). Vestibulo Spinal Tract  MVST Reflexive control of head and neck muscles arises through the neurons in the MVST. These neurons comprise the rapid vestibulo-colic reflex Role is to stabilize the head in space and to participate in gaze control MVST neurons receive input from both the vestibule and the cerebellum, as well as somatosensory information from the spinal cord. These neurons carry both excitatory and inhibitory signals to innervate neck flexor and extensor motor neurons Detected by the cervical vestibular evoked myogenic potentials (cVEMP)  LVST The lateral vestibulospinal tract projects ipsilaterally (on the same side) throughout the length of the spinal cord and is primarily involved in controlling antigravity muscles—muscles that resist the pull of gravity and keep us standing upright Key functions: Postural Control: Facilitates the activation of extensor muscles to counteract gravitational forces. This helps in maintaining an upright posture. Balance: It integrates signals from the vestibular system regarding head position and movement to adjust the body's posture, especially in response to changes in balance, such as when standing or walking. Unconscious Motor Control: The tract is involved in reflexive motor responses, such as quickly adjusting the position of the body after a loss of balance, making it part of the body's automatic postural responses.  Cervico-Occular Reflex (COR) The cervico-ocular reflex (COR) is a reflex that stabilizes vision based on sensory input from the neck muscles, helping to control eye movement in response to neck position changes. Unlike the vestibulo-ocular reflex (VOR), which is driven by vestibular inputs from the inner ear, the COR uses proprioceptive feedback from the cervical spine (neck muscles). This reflex supplements the VOR, particularly when the vestibular system's input is reduced or absent. (d) Other systems involved in maintenance of balance like proprioceptive system,visual system etc. Vision Visual Input Pathway  The dorsal stream (the "where" pathway) in the visual cortex, which processes motion and spatial orientation — key for maintaining balance Role of the occipital lobe, parietal lobe, and their integration with the cerebellum. Visual information is routed to the parietal cortex to inform spatial awareness and postural control Visual-Postural Control Mechanisms The afferent motion perception consists of two visual systems: – Focal – Ambient Focal system also known as central vision, specializes in object motion perception and object recognition Ambient or peripheral vision is sensitive to movement scene and is thought to dominate both perception of self-motion and postural control. The retinal slip, a part of the afferent motion perception, is related to a person’s displacement by the central nervous system (CNS), and is used as feedback for compensatory sway (Guerraz & Bronstein, 2008)  Vision is one of the primary sensory system used in balance (Poole, 1991; Merla & Spaulding, 1997; Uchiyama & Demura, 2009) Spontaneous lateral body oscillations are largely reduced when standing objects fixate a small light emitting diode (LED) in an otherwise darkened environment (Guerraz & Bronstein, 2008)  The peripheral vision rather than the central vision plays an essential role in maintaining stable quiet stance Berencsi, Ishihara, & Inanaka (2005), showed visual stimulation of the peripheral visual field decrease postural sway in the direction of the observed visual stimulus to the antero -posterior rather than medial-lateral Role of Proprioception in balance Proprioception is the body’s ability to sense its position, movement, and spatial orientation. It provides feedback from muscles, tendons, and joints about the body's position relative to itself and its environmentf Proprioception – Muscle Spindles: Detect changes in muscle length and tension. – Golgi Tendon Organs: Monitor force exerted by muscles. – Joint Receptors: Provide information about joint position and movement.  Information Processing:Proprioceptive signals are transmitted to the central nervous system (CNS), where they are integrated with visual and vestibular inputs. The primary somatosensory cortex processes proprioceptive feedback, contributing to body awareness and posture control. Proprioceptive feedback is crucial for adjusting posture and movement in response to changes in the environment (e.g., walking on uneven surfaces)  Proprioception plays a vital role in balance by providing essential feedback for postural adjustments. Its integration with visual and vestibular systems is crucial for effective balance control.  Inflow & Outflow theory (Guerraz & Bronstein, 2008) There are two hypotheses that attempt to explain how individuals maintain stability despite eye movements: inflow and outflow theory The inflow theory proports proprioceptive receptors (e.g., muscle spindles) of the extraocular muscle provide the information about the position and displacement of the eyes in the orbit Whereas, the outflow theory states the branches of the neural outflow (e.g., corollary discharge) or an efference copy (e.g., signals about the eye movements) informs the CNS to maintain visual consistency  The Somatosensory System To maintain normal quiet, stance and to safely accomplish the majority of activities of daily living, individuals rely primarily on proprioceptive and cutaneous input The CNS processes multimodal afferent input and integrates it at various levels, resulting in efferent processing for coordinated firing of multi alpha motoreurons and their corresponding muscle fibers (Shaffer & Harrison, 2007)  The muscle spindles play an important role in proprioception. It is mechanoreceptors that provide the nervous system with information about the muscle’s length and velocity of contraction, thus contributing to the individual’s ability to discern joint movement and position sense (Shaffer & Harrison, 2007). The muscle spindles also provide afferent feedback that translates it to appropriate reflexive and voluntary movements.  GTO Another organ that contributes to proprioceptive information is the golgi tendon organ (GTO). The GTO located at the muscle tendon interface relays information about tensile forces, and is sensitive to very slight changes (Shaffer & Harrison, 2007)  When GTO is activated, the afferent neuron synapses in the spinal cord interneurons, which inhibit the alpha motoneuron of the muscle resulting in decreased tension within the muscle and the tendon  Predominance of the use of proprioceptive information in the control of postural orientation with the absence of vision; postural perturbations applied below the vestibular canal detection threshold did affect upright stance in healthy young subjects (Vaugoyeau et al., 2007) Proprioceptive input helped to stabilize the body rather than the vestibular system.