Vestibular system
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The vestibular system senses motion and orientation of the head. It contains two classes of sensory structures: the otolith organs (utricle and saccule) which detect linear acceleration and the semicircular canals which detect rotational acceleration. Sensory hair cells in these structures transduce motion signals into neural impulses. The vestibular nuclei integrate this information and send signals to control eye movements via the vestibulo-ocular reflex and posture/balance via vestibulospinal pathways. Damage to the vestibular system can cause vertigo, nystagmus and loss of balance/orientation.
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Vestibular system
- 1. Sensation and Sensory Processing Vestibular System
- 2. Function of Vestibular System Static head position Linear acceleration Rotational acceleration Otolith organs Semicircular canals
- 3. Vestibular Labyrinth Embryology: arises from the otic placode together with cochlea Morphology: represents set of interconnected channels Histology: sensory hair cells able to transduce motion signals into neural impulses Anatomy: two classes of sensory structures 1) the otolith organs: the utricle and sacculus; 2) three semicircilar canals. Canals are filled with endolymph (high K+ / low Na+) and surrounded by perilymph (low K+ / high Na+)
- 4. Sense of the right to left head tilting and rolling action in the horizontal plane. Utricle horizontal arrangement of the hair cells Sacculus vertical arrangement of the hair cells Sense of flexion and extension of the head, elevation and depression (as in an elevator). Otolith organs The hair cells are innervated by the vestibular division of cranial nerve VIII (cell bodies reside in Scarpa’s ganglia)
- 7. Otolith Organ Otolithic membrane – gelatinous layer upon the hair cells in otolith organ. Otoconia – crystals of calcium carbonate upon the otolithic membrane Gravity causes the gelatinous membrane to shift Tonic depolarization and hyperpolarization of hair cells.
- 8. Organization of Semicircular Canals Only one axis of depolarization Horizontal Inferior (posterior) Superior (anterior)
- 9. Functional pairs of SC • left horizontal and right horizontal • left anterior (superior) and right posterior • left posterior and right anterior (superior)
- 10. Central Vestibular Processing Ganglion cells 2.“Vestibulocerebellum” flocculonodular lobe of the cerebellum 3. Reticular formation of the brainstem 1. Vestibular nuclei (S, M, L) in the rostral medulla and caudal pons
- 11. Histology of Vestibular Nuclei (VN) • Level of mid-pons • Medulla-pons junction Lat VN Sup VN Med VN Med VN Spinal VN Spinal trigeminal tract + nucleus
- 13. Vestibulo-Ocular Reflex (eye movements opposite to the direction of rotation) Impulses from the left semicircular canal Activation of right lateral rectus and left medial rectus (right CN VI and left CN III) Reduction of tone of the left lateral rectus and right medial rectus (left CN VI, and right CN III)
- 15. Vestibular Nystagmus = rhythmic form of reflexive eye movements composed of slow component in one direction interrupted repeatedly by fast saccadic-like movements in opposite direction. It is normally driven by persistent rotation of the head. The slow component is driven by the vestibulo-ocular reflex. The fast component is the eye reposition in the orbit – result of sensory-motor integration reflecting proprioceptors from eye muscles.
- 16. Lat. vestibulo- spinal truct Regulation of Posture cerebellum LVn MVn Spinal moto- neurons smn mm. of neck and upper limbs mm. of lower limbs (ipsilateral) Med. vestibulo- spinal truct Vestibulo- cervical reflex (bilateral)
- 17. Higher vestibular processing • Parietal cortex (proprioceptive representation ‘face zone’= sense of spacial orientation of the head; • Reticular formation (coordination of viscero-motor activity = sickness and vomiting)
Editor's Notes
- : processing sensory information underlying motor responses to (and perceptions of) self-motion, head position, and spatial orientation relative to gravity, helping to stabilize gaze, head, and posture.
- otolith organs macula: sensory epithelium of the otolith organs (i) hair cells in the maculae of the utricle and sacculus are arranged into two populations with opposing orientations along an axis of mirror symmetry (ii) motion in one direction will depolarize one subpopulation of hair cells and hyperpolarize the other
- deformation of stereocilia toward the largest stereocilium leads to depolarization and increased release of neurotransmitter on peripheral endings of afferent fibers deformation away from the largest stereocilium leads to hyperpolarization and decreased release of neurotransmitter
- deformation of stereocilia toward the largest stereocilium leads to depolarization and increased release of neurotransmitter on peripheral endings of afferent fibers deformation away from the largest stereocilium leads to hyperpolarization and decreased release of neurotransmitter
- mechanism of otolith organ function: consists of a hair cells and supporting cells, and an overlying gelatinous layer (the otolithic membrane) upon which are embedded crystals of calcium carbonate, called otoconia. the hair cells protrude into this gelatinous layer, which is heavier than the surrounding epithelium and fluids. When the head is tilted, gravity causes the gelatinous membrane to shift relative to the underlying epithelium; this displaces the hair cell stereocilium and leads to tonic depolarization and hyperpolarization.
- Ampulla – is a bulbous expansion at the base of the semicircular canals that contains the sensory epithelium (called the crista) and an overlying gelatinous mass (called the cupula) into which the stereocilium of the hair cells protrude. The cupula creates a barrier for the flow of endolymph around the semicircular canal. When the head is rotated in the plane of the semicircular canal, the inertia of the endolymph produces a transient force that distends the cupulla away from the direction of rotation. Distension of the cupulla deflects the stereocilia of the hairs cells, which leads to depolarization or hyperpolarization of the hair cells within any given crista.
- semicircular canals are paired on the two sides of the head: left horizontal and right horizontal left anterior (superior) and right posterior left posterior and right anterior (superior)
- central processes of ganglion cells project via the vestibular division of the eighth cranial nerve to vestibular nuclei in the rostral medulla and caudal pons, and directly to the “vestibulocerebellum” (flocculonodular lobe of the cerebellum) one major function of the vestibular nuclei is to coordinate movements of the eyes and head to allow for stable visual fixation during head or whole body movements
- Stroke of upper region of medulla (posterior inferior cerebellar artery) = complex of symptoms (trigeminal system /via spinal trigeminal nucleus and tract/+ spinocerebellar system/inferior cerebellar peduncle/ + vestibular system / vestibular nuclei.
- Afferent: Efferent: A few Purkinje cell axons pass directly out of the cerebellum to the lateral vestibular nucleus.
- vestibulo-ocular reflex rotational movements of the head induce eye movements opposite to the direction of rotation, thus allowing maintained visual fixation of both eyes pathway input from the horizontal canals reaches the vestibular nuclei on the two sides of the medulla/pons each vestibular nucleus in turn sends a excitatory projections to the contralateral abducens nucleus the abducens nucleus directly innervates the ipsilateral lateral rectus muscle, which pulls that eye toward the lateral side (i.e., abduction) other cells in the abducens nucleus cross the midline again and project to the opposite oculomotor nucleus, which innervates the ipsilateral medial rectus causing that eye to turn toward the midline (i.e., adduction) to facilitate this excitatory action, there are also inhibitory projections from the vestibular nuclei to the ipsilateral abducens nucleus this inhibitory projection turns off the excitatory output of the cranial nerve nuclei (opposite abducens and oculomotor nuclei) that drive the antagonistic muscles
- The nystagmus is an indication of balanced activity of the vestibular system. The hypofunction of vestibular apparatus on one side could lead to the appearance of this sigh (symptom) without actual head movement. =pathological alteration in the balance of activity b/w the two sides The direction of the eye movement during slow component is toward the side with hypofunction of semicircular canals. If the superior-inferior semicircular canals are misbalanced the patient will have vertical nystagmus.
- Another major function of the vestibular nuclei is to make reflexive adjustments of posture that compensate for movements of the head descending projections from the vestibular nuclei reach the medial aspect of the ventral horn of the spinal cord via medial and lateral vestibulospinal pathways medial vestibulospinal projection to the upper cervical cord that regulates head and neck position “lateral” vestibulospinal projection to motor neurons in the medial ventral horn that excite extensor muscles in the trunk and limbs; this pathway mediates balance and maintenance of an upright posture