Addis Ababa UniversityCollege of Health ScienceDepartment of Medical

Physiology

Presentation on physiology of vestibular apparatus and clinical

correlates

By Girmay F. 104/12/2023

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Presentation Out line

1. Objectives

2. Introduction

3. Anatomy of Vestibular apparatus

3.1 The otolithic organs

3.2 Semicircular canals

3.3 Structures of hair cell and genesis action potential

4.Function of the utricle and saccule

5. Function of semicircular ducts.

6. Neural connection of vestibular nuclei

7. vestibule-ocular reflex.

8. Vestibular dysfunction

9. References

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1.Objectives

At the end of presentation ,students expected to

1.Describe the anatomy and function of the vestibular apparatus

2.Explain the major input and out put of vestibular nuclei

3.Discuss the importance of vestibule ocular reflex.

4.List some clinical disorders of body equilibrium.

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2.Introduction

• Gravity or acceleration tends to cause displacement of the body from equilibrium.

• Posture is back ground position which helps in carrying out a specific movement to desired target.

• Postural reflexes are integrated at various levels of the neural axis.

• The vestibular system has important sensory functions, contributing to the perception of self-motion, head position, and spatial orientation relative to gravity.

• It help to stabilize gaze head, and posture.

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3.Anatomy of Vestibular apparatus

• located in the petrous portion of the temporal bone, called Bony labyrinth.

• Membranous labyrinth• The membranous labyrinth is

the functional unit of vestibular apparatus.

• Compose of cochlea

semicircular canals two otolith organs.

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Fig.1.membranous labyrinth

3.1 Otolith organs

• Membrane bound sac like structure ,Utricle and saccule.

Detect linear acceleration Detect orientation of head

with respect to gravity. Macula is the sensory organ Located on the inside

surface of each utricle and saccule

Macula 2 mm in diameter.

fig.2. location of macula

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otolithic organs Cont’d

• Containing

- sustentacular cells

- numerous hair cells • the Cilia embedded in

gelatinous layer called statoconia.

• Impregnated with calcium carbonate crystals which are called otoconia or ear dust

• Base and side of hair cells vestibular nerve.

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Fig.5.structure of macula

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Otolithic organs Cont’d

The macula of utricle• Mainly in horizontal plane.• Hair cells oriented vertically up

ward • Determining orientation of the

head when the head is in up right position.

Respond to:-

1.changes in head position fore and aft lift

2.Linear acceleration in horizontal plane running

8Fig.3 anatomical orientation of macula

• Saccule

• Utricle

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Otolithic organs cont’d

oriented in vertical plane Hair cells arising in

horizontal plane• Respond to:- changes in head position

supine positionLinear acceleration in

vertical plane. jumping down

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• Saccule

• Utricle

Fig.4.anatomical orientations of maculae

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3.2 Semicircular canals

• Detect angular acceleration • There are 3 canals

SuperiorHorizontal(lateral) posterior

• Right angles to each other• Each canal is a continuous

endolymph-filled .     • Enlargement at one of its

end called ampulla.  

10Fig.6.parts of SCC.

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Semicircular canals Cont’d

• Ampulla containing a receptor organ called crista ampullaris.

• The projecting cilia are embedded in gelatinous mass called cupula.

• The cupula extends right across the lumen up to the opposite wall of ampulla.

11Fig.7.structure of crista ampullaris

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3.3 Structure of hair cells and genesis of action potential

• Each hair cell has about 50-70 stereocilia and one kinocilium.

• Arising from its apical end.• Stereocilia and the

kinocilium are connected by tip links

Fig.8 structure of hair cell

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Structure of hair cells Cont’d

13Fig.9.A,B,C

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structure of hair cells Cont’d

• When stereocilia bend towards kinocilium membrane receptor depolarization the impulse traffic increases

• When stereocilia bend away from kinocilium membrane receptor repolarization decreases the impulse traffic

• Orientation of the head in space changes and the weight of the statoconia bend the cilia, appropriate signals are transmitted to the brain to control equilibrium.

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4.The function of utricle and saccule

1.Detection of Linear acceleration • When the body suddenly thrust forward-(i.e. the body

accelerates)• The statoconia,fallbackward on the hair cell cilia, and

information of dysequilibrium is sent into the nervous centers, causing the person to feel as though he or she were falling backward.

• This automatically causes the person to lean forward until the resulting anterior shift of the statoconia exactly equals the tendency for the statoconia to fall backward because of the acceleration.

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5.The function of Utricle and Saccule cont’d

2.Maintenance of static equilibrium

• The hair cells are all oriented in different directions in the maculae of the utricles and saccules, so that with different positions of the head, different hair cells become stimulated.

• The “patterns” of stimulation of the different hair cells apprise the brain of the position of the head with respect to the pull of gravity.

• In turn, the vestibular, cerebellar, and reticular motor nerve systems of the brain excite appropriate postural muscles to maintain proper equilibrium.

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The function 0f Semicircular Ducts

Detect angular acceleration• Rotational acceleration

stimulates crista ampullaris.

• The endolymph is displaced in a direction opposite to the direction of rotation.

• The fluid pushes on the cupula, deforming it.

Fig.10.movement of SCC with body

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The function of semicircular ducts cont’d

• this bends the processes of the hair cells.

• When a constant speed of rotation is reached

• the fluid rotate at the same rate as the body and the cupula swings back into the upright position .

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The function of semicircular canals Cont’d

When rotation is stopped, deceleration produces

displacement of the endolymph in the direction of the rotation, and the cupula is deformed in a direction opposite to that during acceleration.

It returns to mid position in 25 to 30 seconds.

Fig.11.mov’t of endolymph with respect to SCCs

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The function of semicircular canals Cont’d

• Simultaneously, the cilia at the top of the hair cell are bent. If the direction of bending is towards the kinocilium, depolarization takes place.

• Bending of the cilia in the reverse direction causes hyperpolarization; this alters impulse traffic along the nerves innervating the hair cells.

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6.Neuronal connection of the vestibular nuclei

Vestibular nuclei

• Four major vestibular nuclei, some minor subgroups.

Input

vestibular receptorsCerebellum, Brain stem Spinal cord cerebra cortical areas. Fig.12.inputs of vestibular nuclei

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Neuronal connection of vestibular nuclei cont’d

Ganglion of Scarpa, which is situated in the upper part of the outer end of the internal auditory meatus.

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Fig.13.pathways for equilibrium sensation

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Neural connection of vestibular nuclei cont’d

• Primary cortical center for equilibrium located in the parietal lobe.

• Flocculonodular lobe of cerebellum concerned with dynamic equilibrium.

23Fig.14 Inputs and out puts of vestibular nuclei

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Neural connection of vestibular nuclei cont’d

Fig.14.

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Neural connection of vestibular nuclei cont’d

Fig.15.descending projection of vestibular nuclei to spinal cord

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Neural connection of vestibular nuclei cont’d

Descending projections from the medial and lateral vestibular nuclei to the spinal cord.

• The MVN project bilaterally in the MLF to reach the medial part of the ventral horns and mediate head reflexes in response to activation of semicircular canals.

• The LVN sends axons via the LVT to contact anterior horn cells innervating the axial and proximal limb muscles.

• Neurons in the LVN receive input from the cerebellum, allowing the cerebellum to influence posture and equilibrium.

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7.Vestibulo ocular reflex

• Stabilizes images on retina during head movement .

• When rotation starts, the eyes move slowly in a direction opposite to the direction of rotation, maintaining visual fixation.

• When the limit of this movement is reached, the eyes quickly snap back to a new fixation point and then again move slowly in the other direction.

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Fig.16. VOR

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Cont’d

o For example, horizontal movement of the two eyes toward the right requires contraction of the left medial and right lateral rectus muscles.

Fig.17.neural projection associated with VOR

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8.Vestibular dysfunction

• Peripheral vestibular disorder• Central vestibular disorder• Loss of equilibrium and postural adjustments.• PVD = neuritis, labyrinthitis,meniere’s disease, BPPV,and

following surgery .

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8.1 Benign paroxysmal positional vertigo

• the most common vestibular disorder

• characterized by episodes of vertigo that occur with particular changes in body position.

• Possible cause is that otoconia from the utricle separate from the otolith membrane and become lodged in the cupula of the posterior semicircular canal.

• This causes abnormal deflections when the head changes position relative to gravity.

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8.2 Central vestibular disorder

• Primarily involve the vestibular nuclei complex and the cerebellum, as well as structures of the reticular activating system, midbrain, and higher centers of cortical function.

• Affect integration and processing of sensory input from the vestibular, visual, and somatosensory systems.

• Most common cause of CVD includes brainstem strokes, head trauma, migraine-related

vestibulopathymultiple sclerosis, and cerebellar degeneration.

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8.3 Nystagmus

• jerky movement of the eye observed at the start and end of a period of rotation.

• Back and forth or rotational eye movements. The movements will be slower in one direction.

• The direction of eye movement is identified by the direction of the quick component.

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8.4 Motion sickness

• Excessive vestibular stimulation

• The symptoms are probably due to reflexes mediated via vestibular connections in the brain stem and the flocculonodular lobe of the cerebellum.

• symptom

Nausea, blood pressure changes, sweating, pallor

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9.References

• Berne and levy physiology 6th edition,bruce m koeppen and Bruce A. Stanton.

• Guyton and hall text book of medical physiology,12th edition, Philadelphia: Elsevier Inc.

• Ganong’s review of medical physiology 23th edition. Mc Graw Hill

• Neuroscience exploring the brain Third Edition. Dale Purves George J. Augustine, Sinauer Associates, Inc. Publishers Sunderland, Massachusetts U.S.A.

• Internet websites

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Thank you !!!

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