Vestibular Apparatus & Pathways Functions of the Vestibular System •
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Subserve perception of movement in space and tilt with respect to gravity Provide reflex balance reactions to sudden instability of gait or posture “vestibulo-spinal reflexes”. Stabilise the eyes on earth fixed targets preserving visual acuity during head movements “vestibular-ocular reflexes”. Assist control of blood pressure and heart rate during rapid up-down tilts. Assist synchronisation of respiration with body reorientations. Provokes motion sickness when stimulated in unusual motion environments Provide a reference of absolute motion in space which helps interpret the relativistic signals of the other senses in creating a perception of spatial orientation.
Vestibular Apparatus Consists of sensory organs located in the inner ear: (membranous labyrinth). Comprises: - Semicircular canals: lateral, posterior & anterior. o Kinetic labyrinth: respond to angular accelerations caused by head rotation. - Otolith organs: utricle and sacculus. o Static labyrinth: respond to static head position and linear acceleration.
The Vestibular Labyrinth: Bony labyrinth: this is the chamber formed by the petrous temporal bone. It is filled with perilymph.
Membranous labyrinth: essentially located within the utricle, saccule and semicircular canals. It is filled with endolymph and contains the sensory receptor cells.
Vestibular sensory organs • Sensory receptors:
Vestibular hair cells are mechanical transducers detecting static tilt and acceleration • Hair cell cilia embedded in a gelatinous matrix containing calcium carbonate crystals called otoconia • Hair cell synapses with the primary neurone dendrite (cell body is in Scarpas ganglion) which projects to vestibular nuclei in the brainstem. • Located in: o Macula: in utricle and saccule o Crista: of ampulla (swelling of semicircular canals) • Sensory hairs (two types) o Stereocilia: arranged in rows of varying heights o Single long kinocilium • Transduction Mechanism o Mechanoelectrical transduction mechanism similar to the cochlea o Hair cell receptor potential: Towards the kinocilium → depolarisation Away from the kinocilium → hyperpolarisation o Ganglion cell discharge Toward the kinocilium → increased frequency Away from the kinocilium → decreased frequency
Primary afferent neurons •
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Form nerve endings on hair cells in macula and ampulla: o Type I: chalice-like endings form ribbon synapses o Type II: simple nerve terminals Cell bodies located in vestibular ganglion Central process synapse in brainstem vestibular nuclei
Otolith organs: the Static Labyrinth • • • • •
Saccular and utricular maculae lie perpendicular to each other o Movement in any direction stimulates a distinct subset of cells In the layer of hair cells there are breaks called striola. These differentiate between the hair bundles of opposing polarities so that movement in any direction will stimulate a distinct subset of cells. Tilt of the head to one side has opposite effects on corresponding hair cells of the other side The otolith organs respond preferentially to static head position and linear acceleration (static labyrinth) Because the maculae in the utricle and saccule like perpendicular to one another, which ever way the head moves, a certain bundle of cells is going to move and that bundle of cells with stimulate
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action potential changes. Tilting the head to one side also has opposite effects on corresponding hair cells of the other side. Therefore: When the head is upright – discharge is tonic. Constant hair displacement imposed by gravity Vestibular ganglion neurons innervating the saccule Static head tilt – tonic discharge changes for the duration of head tilt. Linear acceleration – Linear accleration in the horizontal plane induces transient hair displacement of some cells of the utricular macula and modulation of spontaneous discharge in ganglion cells.
Semicircular canals: the kinetic labyrinth • • • • • • • • • •
The crista of the ampulla detects head rotation in 3 orthogonal planes The cilia of the hair cells are embedded in a gelatinous projection called the cupula Kinocilia are all oriented in the same direction on each side of the head Hair cells in the crista respond to the differential movement of the endolymph during rotational movements (angular acceleration) in the plane of the SCC (kinetic labyrinth) Acceleration towards the side of the receptor increases firing rate while deceleration decreases it. Rotation of the head in any plane produces change of activity of the receptors in one or more SCC’s. Head rotation deforms the hair cells of the left and right SCC in opposite directions Inertia of movement causes endolymph to move in opposite direction with respect to the canals Push-pull arrangement operates for all three pairs of canals. Therefore: Head is turned left: Firing rate of vestibular ganglion cells increases on the left side and decreases on the right side.
Vestibular Pathways -
The primary afferent fibres from the hair cells travel via the vestibulo-cochlear nerve (VIII) to the brainstem where they synapse in the vestibular nuclei:
Superior Lateral Medial Inferior Somatotopic organisation (in vestibular nuclei) o Static labyrinth (otoliths) - lateral & inferior o Kinetic labyrinth (SCC) - superior & medial From the vestibular they project to 4 places: o o o o
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1. Spinal Cord The fibres descend in the lateral and medial vestibulospinal tracts. o Lateral Vestibulospinal Tract: descends ipsilaterally in ventral funiculus to terminate in the lateral ventral horn affecting motor neurons to limb muscles. o Medial Vestibulospinal Tract: descends bilaterally in medial longitudinal fasciculus to terminate in cervical and upper thoracic spinal cord terminating in medial ventral horn and affecting motor neurons to neck and back muscles. o These are used to maintain posture ďƒ postural reflex. o Postural reflexes: o Integrate sensory information from vestibular apparatus, somatic receptors and the eyes to increase extensor muscle tone and maintain upright posture o Mediated by brainstem descending tracts: Vestibulospinal and Reticulo-spinal tracts.
2. Eyes o Superior and medial vestibular neurons project to o o
motor nuclei supplying extraocular muscles. This results in the vestibulo-ocular reflex. When the head rotates to the left, the eyes rotate in compensation to the right with repositioning saccades to the left (used to maintain gaze on a target).
Vestibular nystagmus is the involuntary movement of the eye as part of the vestibuloocular reflex. o The eye slowly drifts one way and then quickly saccades back to its centre. o It can be tested with a warm caloric test: warming the right ear and watching for a slow drift away from the stimulated side followed by a fast saccade towards the stimulated side. The direction of the nystagmus is named in accordance to the fast saccadic phase. o
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3. Cerebellum o Vestibular afferent neurons either directly from the vestibular ganglion or indirectly from the vestibular nuclei in the brainstem project to the flocculonodular lobe of the cerebellum.
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Efferent fibres from the cerebellum (fastigial nucleus) project onto all the vestibular nuclei. These control head movements, eye movements and posture.
4. Thalamus and Cortex o o o
All of the neurons from the vestibular nuclei project onto ventral posterior and ventral lateral nuclei of the thalamus. From here they project onto cortical areas 2V and 3a, in part of the somatosensory cortex (the “head” part), and also onto superior parietal cortex: ‘vestibular cortex’ concerned with spatial orientation. These may account for the feelings of dizziness (vertigo) in certain kinds of vestibular stimulation.