the vestibular system is located in the inner ear and is involved in detecting head movement, maintaining balance and posture, and formation of the perception of one's physical boundaries. it is located within the petrous portion of the temporal bone and has several components: the bony labyrinth is the outer covering which contains the 3 mutually perpendicular semicircular canals and the vestibule. inside the semicircular canals are the semicircular ducts which contain endolymph and the crista receptors in the ampulla of each duct. inside the vestibule are the utricle and saccule, which contain the macula receptor.
the crista receptors inside the semicircular ducts are designed to sense angular acceleration- rotation of the head causes the fluid inside the semicircular ducts to shift due to the inertia of the fluid itself. the fluid pressing against the crista receptors causes a distortion of the cilia embedded within the crista, which causes a receptor potential. each semicircular canal is in a different orientation such that any given rotation will produce a different set of shifts in each duct, the combination of which is then integrated and processed downstream.
the macula receptors inside saccule and utricle are designed to sense linear acceleration. these are made of receptor cells with cilia embedded within a gelatinous otolith layer, on top of which there are calcium carbonate stones. when the head is tilted, the stones distort the otolith layer and therefore the cilia from the receptor cells, producing a receptor potential. occasionally the otoliths can detach and enter the semicircular ducts, causing benign paroxysmal positional vertigo.
the vestibular pathway starts at the vestibular nerve, which carries information to the vestibular nuclei in the pons. from there the vestibular nuclei projects to cranial nerves III, IV, VI in order to coordinate eye movement along with head movement (to enable tracking of an object in space- the vestibulo-ocular reflex), and also to the vestibulospinal tract, for control of muscles involved in posture and balance. damage to the vestibular pathway might result in vertigo, a loss of equilibrium and balance and a sense of falling.
information from the vestibular system is also projected to the vestibular cortex, which is located at the temporal-parietal junction, insula, somatosensory cortex, and the superior parietal cortex. in the parietal association cortex, vestibular information is integrated with other modalities such as somatosensory, visual, audition, etc. in order to form an integrated sense of one's environment and body. the vestibular cortex is also known to be involved in the sense of localization; ie, where one perceive's one's center of awareness to be. dysfunctions within the temporal-parietal junction in particular have been demonstrated to cause such hallucinations (of increasing intensity and of vestibular dysfunction) as autoscopy, where one sees one's own body in another space, heatoscopy, where one sees one's own body in another space and feels one's awareness might be located there as well, and out of body experiences, in which the person's center of awareness is completely removed from their body.
questions
anatomy and receptor physiology...
1. where is the vestibular system located?
2. the bony labyrinth is divided into...
3. what is the membranous labyrinth and what is it divided into?
4. what is endolymph secreted by? where does it drain?
5. what is perilymph secreted by? where does it drain?
6. what is meniere's disease?
7. what are the three semicircular ducts/canals? describe their orientation.
8. rotation of ducts with head causes...
9. what are crista and cupula?
10. how does the crista sense rotation?
11. describe the sensory receptor system within the saccule and utricle.
12. the macula is sensitive to what type of acceleration as opposed to the crista?
13. what happens in benign paroxysmal positional vertigo?
innervation...
14. vestibular nerve conducts to...
15. from where do the vestibular nuclei in the pons receive input?
16. where do the vestibular nuclei project to?
17. what is the vestibulo ocular reflex?
18. what is nystagmus?
19. what do the vestibulospinal tracts project to?
20. what is vertigo and what can it be caused by?
perception and sensation...
21. what does the parietal association cortex do?
22. how was the vestibular cortex mapped out?
23. where is the vestibular cortex?
24. which part of the vestibular cortex is mainly responsible for creating the sense of being in one's body?
25. what is autoscopy?
26. describe an out of body experience.
27. describe an autoscopic hallucination.
28. describe "heatoscopy".
29. which of these illusions has the most vestibular dysfunction? which has the least?
answers
1. within the petrous portion of the temporal bone.
2. the vestibule and the semi-circular canal.
3. membranous tube containing receptors for head movement; divided into semicircular ducts (in the semicircular canals) and saccule and utricle (within the vestibule).
4. secreted by cochlear duct and drains into dural sinuses via endolymphatic duct.
5. secreted by periosteum and drains into CSF via perilymphatic duct.
6. excess endolymph secretion or fluid pressure causing nausea/vomiting, vertigo/dizziness, abnormal saccadic eye movements.
7. anterior, horizontal, posterior. mutually perpendicular.
8. flow of endolymph within ducts.
9. the receptor organ in the ampula of each canal; the cupula is the gelatinous mass which the cilia of receptor cells are embedded in.
10. rotation of the head in the same alignment as the semicircular canals produces pressure against the crista because of the inertia of the endolymph.
11. maculae is the sensory device which is composed of ciliated macular receptor cells which have cilia embedded in the gelatinous otolithic membrane, which is then covered by calcium carbonate stones which produce shifts in the otolithic membrane when the head is tilted.
12. linear acceleration as opposed to angular acceleration.
13. when otoliths from utricle fall into the semicircular canals and produce "apparent motion" in the crista, resulting in dizziness, vertigo, imbalance, nausea
14. the vestibular nuclei in brain stem, cerebellum.
15. directly from the vestibular nerve or indirectly from the cerebellum.
16. CN III, IV, VI for control of eye movement.
17. a reflex that requires the cerebellum which causes movement of eyes opposite to rotation of head.
18. involuntary saccadic movements when the eyeball is at rest.
19. to the muscles that control posture and balance.
20. disorientation and a sense of spinning-- can be caused by a tumor in vestibular system or meniere's disease.
21. integrates somatosensory, proprioceptive, visual, auditory, vestibular input to form body schema- body, personal space, extra personal space.
22. by looking at brain responses to vestibular illusions in epilepsy, or to galvanic/caloric vestibular stimulation.
23. the temporal-parietal junction, insula, somatosensory cortex, superior parietal cortex.
24. the temporal-parietal junction.
25. the experience of perceiving one's own body in another place because of the lack of congruence among the sensory inputs integrated in the association cortices.
26. seeing one's own body from an elevated position, feeling that the center of awareness is located outside of the body (disembodiment).
27. seeing a double of oneself in extrapersonal space, with the center of awareness remaining inside the body.
28. seeing a double of oneself in extrapersonal space with the center of awareness being split between the body and the extrapersonal space.
29. out of body has the most TPJ dysfunction, then heatoscopy, then autoscopy.
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