Vestibular System

I. Functions of the Vestibular System Functions to maintain both static and dynamic (i.e., kinetic) equilibrium of the body and its parts as a result of head displacement. Receptors are located in the utricle and saccule as well as the semicircular ducts of the inner ear. Utricle and saccule detect changes in gravitational forces and angular acceleration (for static equilibrium). Semicircular ducts detect angular acceleration (for kinetic equilibrium). Through its own motor system: a. Maintains extensor antigravity muscle tone via descending pathways to spinal cord. b. Maintains equilibrium via the same descending motor tracts. c. Produces compensatory eye movements via ascending motor connections (mainly the medial longitudinal fasciculus, MLF) to extrinsic eye muscle motor nuclei (abducens, throchlear, oculomotor nuclei).

II. Vestibular Apparatus. How does it work? Macula of utricle and saccule contains specialized hair cells. Hairs are embedded in gelatinous mass that contains otoliths (statoconia). Gravity or linear acceleration causes displacement of otoliths and bending of the hairs (kinocilia) of hair cells causing the latter to depolarize. Crista of semicircular ducts contains hair cells. Hairs (kinocilia) are embedded in the gelatinous mass of the cupula which is bathed by the endolymph. Semicircular ducts work in pairs. Angular acceleration (rotatory movement) causes displacement of endolymphatic fluid and movement of cupula which bends the kinocilia resulting in a depolarizing receptor potential in the hair cell. Hair cells in macula and crista are innervated by the peripheral processes of the vestibular nerve (cell bodies in the vestibular ganglion). Receptor potentials then cause generation of action potentials in the vestibular peripheral process and conduction of these potentials towards the CNS.

III and IV. Vestibular Centers and Pathways 1 st Order Neuron Cell body in the vestibular (Scarpa’s) ganglion. Proximal process innervates the hair cells in macula and crista. Central process joins the vestibular nerve (part of 8 th cranial nerve). Vestibular nerve enters brainstem at the cerebellopontine angle. Most of these vestibular afferents terminate in the vestibular nuclear complex (4 subnuclei) at the level of the pons and rostral medulla. Fibers from the semicircular ducts terminate in the sup. and rostral parts of med. subnuclei. Fibers from utricle and saccule terminate in lat. subnucleus. Some vestibular afferents ascend to the cerebellum and terminate directly in the cortex of flocculonodular lobe and uvula (part of the archicerebellum or vestibular cerebellum).

III and IV. Centers and Pathways (Cont.) What is the Archicerebellum? The archicerebellum (or vestibular cerebellum) is the oldest (i.e., archi-) part of the cerebellum. Consists of the flocculus, nodule and uvula, three small lobules of the cerebellum. Flocculus and nodule are often paired together as the flocculo-nodular lobe. In addition the fastigial nucleus, deeply placed in the white matter of the cerebellum closed to its midline, is also part of the archicerebellum. These ancient parts of the cerebellum are phylogenetically derived together with the rest of the vestibular system. Cells in the cortex of flocculo-nodular lobe and uvula send inhibitory axons to the fastigial nucleus and to the vestibular nuclear complex. Cells in the fastigial nucleus send excitatory axons bilaterally to the vestibular nuclear complex.

III and IV. Vestibular Centers and Pathways (Cont.) 2 nd Order Neurons Cell body localized in the vestibular nuclear complex. Axons of vestibular nuclear cells project to: a. Cortex of archicerebellum and fastigial nucleus (all ipsilateral). Each fastigial nucleus projects back bilaterally to the vestibular nuclear complexes. In addition the archicerebellar cortex also projects back to the vestibular nuclear complexes. Last two are feedbacks. Note: All connections between archicerebellum and vestibular nuclear complex join the juxtarestiform body. b. Spinal cord by way of the lateral vestibulospinal (LVST) tract and the medial vestibulospinal tract. LVST acts to facilitate ipsilateral spinal extensor motor neurons that control antigravity muscles (extensors of neck, trunk and limbs). Med vestibulospinal tract is also the descending limb of the MLF (medial longitudinal fascicle). It descends to cervical and upper thoracic segments of the cord and is involved in reflex neck/head movements in response to vestibular stimuli. c. Extrinsic eye muscle motor nuclei (i.e., abducens motor nucleus, trochlear motor nucleus and oculomotor nucleus). These connections are mediated mainly by the ascending limb of the MLF.

III and IV Centers and Pathways (Cont.) How the vestibular system controls conjugate eye movements in the horizontal plane (vestibulo-ocular reflex) Horizontal rotation of the head causes both eyes to move in the same direction. Moving the head to the right causes both eyes to move to the left. Moving the head to the left causes both eyes to move to the right. Both (left and right) horizontal semicircular ducts detect horizontal rotation of the head. How are the vestibular pathways used so that both eyes move opposite to head rotation? When the head turns to the right the hair cells in the right horizontal semicircular duct are stimulated strongly causing more firing in the right vestibular nerve. In contrast cells in the left duct remain quiescent. Vestibular afferents in the right vestibular nerve simultaneously facilitate motor neurons in the left abducens nucleus and right oculomotor nucleus. Activation of right oculomotor nerve and left abducens nerve causes contraction of left lateral rectus and right medial rectus producing both eyes to move to the left. Repeat the same with turning head to the left. How do both eyes move to the right?

Clinical Correlation (not in the notes) Unilateral destructive lesion to the vestibular nerve or nuclear complex causes: a. Falling to the side of lesion. b. Patient is unable to balance properly. c. Transient nystagmus to the opposite side of the lesion. Unilateral irritating lesion to the vestibular complex (such as that originating from an intramedullary tumor in the vicinity of complex). a. Transient nystagmus to the lesion’s side. b. Abnormal righting reflexes.