The Labyrinth

A. Kinetic labyrinth

1. Three semicircular ducts lie within the three semicircular canals (i.e., superior, lateral, and posterior).

2. These ducts respond to angular acceleration and deceleration of the head.

a. They contain hair cells in the crista ampullaris. The hair cells respond to en-dolymph flow.

Vestibular Nerve

Cerebellopontine angle

Nodulus

Vestibular nuclei

Inferior cerebellar peduncle -

Inferior olivary nucleus

Semicircular ducts v.

and macula

Ampulla and crista

Utricle

Endolymphatic I duct

Medial lemniscus

Pyramid

- Vestibular nerve and ganglion in internal auditory meatus

Cochlear duct

Saccule and macula

Figure 12-1. Peripheral connections of the vestibular system. The hair cells of the cristae ampullares and the maculae of the utricle and saccule project, through the vestibular nerve, to the vestibular nuclei of the medulla and pons and the flocculonodular lobe of the cerebellum (vestibulocerebellum). ML F = medial longitudinal fasciculus.

b. Endolymph flow toward the ampulla (ampullopetal) or utricle (utriculopetal) is a stronger stimulus than is endolymph flow in the opposite direction.

B. Static labyrinth

1. The utricle and saccule respond to the position of the head with respect to linear acceleration and the pull of gravity.

2. The utricle and saccule contain hair cells whose cilia arc embedded in the otolithic membrane. When hair cells are bent toward the longest cilium (kinociU ium), the frequency of sensory discharge increases.

III. THE VESTIBULAR PATHWAYS (Figures 12-1 and 12-2) consist of the following structures.

A. Hair cells of the semicircular ducts, saccule, and utricle are innervated by peripheral processes of bipolar cells of the vestibular ganglion.

Vestibulothalamic tracts

Vestibular area of cerebral cortex

Abducent nucleus of CN VI of pons

Ventral posterior inferior nucleus

Midbrain

Trochlear Nucleus

Nodulus of cerebellum

Lateral vestibulospinal (Deiters') tract

Thalamus

Oculomotor nucleus of CN III Trochlear nucleus of CN IV

Vestibular ganglion Cochlea

Juxtarestiform body

Vestibular nuclei

Figure 1.2-2. The major central connections of the vestibular system. Vestibular nuclei project, through the ascending medial longitudinal fasciculi (MLF), to the ocular motor nuclei and subserve vestibulo-ocular reflexes. Vestibular nuclei also project, through the descending MLFand lateral vestibulospinal tracts, to the ventral horn motor neurons of the spinal cord and mediate postural reflexes. CN = cranial nerve.

Nodulus of cerebellum

Lateral vestibulospinal (Deiters') tract

Vestibular area of cerebral cortex

Vestibulothalamic tracts

Abducent nucleus of CN VI of pons

Ventral posterior inferior nucleus

Thalamus

Midbrain

Oculomotor nucleus of CN III Trochlear nucleus of CN IV

Juxtarestiform body

Vestibular nuclei

Vestibular ganglion Cochlea

Figure 1.2-2. The major central connections of the vestibular system. Vestibular nuclei project, through the ascending medial longitudinal fasciculi (MLF), to the ocular motor nuclei and subserve vestibulo-ocular reflexes. Vestibular nuclei also project, through the descending MLFand lateral vestibulospinal tracts, to the ventral horn motor neurons of the spinal cord and mediate postural reflexes. CN = cranial nerve.

B. The vestibular ganglion is located in the fundus of the internal auditory meatus.

1. Bipolar neurons project through their peripheral processes to the hair cells.

2. Bipolar neurons project their central processes as the vestibular nerve [cranial nerve (CN) VIII] to the vestibular nuclei and to the flocculonodular lobe of the cerebellum.

C. Vestibular nuclei

1. These nuclei receive input from:

a. The semicircular ducts, saccule, and utricle b. The flocculonodular lobe of the cerebellum

2. The nuclei project fibers to:

a. The flocculonodular lobe of the cerebellum b. CN III, IV, and VI through the medial longitudinal fasciculus (MLF) C. The spinal cord through the lateral vestibulospinal tract d. The ventral posteroinferior and posterolateral nuclei of the thalamus, both of which project to the postcentral gyrus

IV. VESTIBULO-OCULAR REFLEXES are mediated by the vestibular nuclei, MLF, ocular motor nuclei, and CN III, IV, and VI.

A. Vestibular (horizontal) nystagmus

1. The fast phase of nystagmus is in the direction of rotation.

2. The slow phase of nystagmus is in the opposite direction.

B. Postrotatory (horizontal) nystagmus

1. The fast phase of nystagmus is in the opposite direction of rotation.

2. The slow phase of nystagmus is in the direction of rotation.

3. The patient past-points and falls in the direction of previous rotation.

C. Caloric nystagmus (stimulation of horizontal ducts) in normal subjects

1. Cold water irrigation of the external auditory meatus results in nystagmus to the opposite side.

2. Warm water irrigation of the external auditory meatus results in nystagmus to the same side.

Caloric Irrigations

Figure 12-3. Cold caloric responses in the unconscious patient. When the brain stem is intact, the eyes deviate toward the irrigated side; with bilateral transection of the medial longitudinal fasciculi (MLF), the eye deviates to the abducted side. Destruction of the caudal brain stem results in no deviation of the eyes. Double-headed arrows indicate nystagmus; single-headed arrows indicate deviation of the eyes to one side.

Figure 12-3. Cold caloric responses in the unconscious patient. When the brain stem is intact, the eyes deviate toward the irrigated side; with bilateral transection of the medial longitudinal fasciculi (MLF), the eye deviates to the abducted side. Destruction of the caudal brain stem results in no deviation of the eyes. Double-headed arrows indicate nystagmus; single-headed arrows indicate deviation of the eyes to one side.

3. Remember the mnemonic COWS: Cold Opposite, Warm Same. D. Test results in unconscious subjects (Figure 12-3)

1. No nystagmus is seen.

2. When the brain stem is intact, there is deviation of the eyes to the side of the cold irrigation.

3. With bilateral MLF transection, there is deviation of the abducting eye to the side of the cold irrigation.

4. With lower brain stem damage to the vestibular nuclei, there is no deviation of the eyes.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment