3 parts of the ear
External ear
- Auricle
- External auditory meatus
- Tympanic membrane
- Separates external and middle ear
Middle ear
- Tympanic cavity
- Has 3 ossicles transmitting the vibrations from the tympanic membrane
- Malleus abuts the tympanic membrane,
- Incus
- Stapes abuts the oval window
- Mastoid antrum
- Communicates with pneumatized spaces of the mastoid.
- Eustachian tube
- Communicates with pneumatized spaces of the mastoid.
Inner ear
- The inner ear comprises two parts:
- Bony labyrinth
- Formed by
- Cavernous openings in the petrous portion of the temporal bone
- Membranous labyrinth
- Formed by a simple epithelial membrane
- Filled with a fluid called endolymph,
- Lines the contours of the bony labyrinth from which it is separated by a fluid called perilymph.
- Consists of the
- Hearing
- Cochlea (auditory labyrinth)
- Vestibular labyrinth
- Utricle
- Saccule
- Semicircular canals
Peripheral Vestibular System
- Function: detects movement and position of the head in space.
- Inner ear (labyrinth) is composed of 2 parts:
- Bony labyrinth:
- Formed by cavernous openings in the petrous portion of the temporal bone
- Membranous labyrinth:
- Formed by a simple epithelial membrane
- Filled with Endolymph
- Lines the contours of the bony labyrinth from which it is separated by a fluid called perilymph
- Contains the peripheral receptors of the vestibular
- Function detect
- Linear acceleration
- Position of the head in space.
- Utricle
- Horizontal
- Saccule
- Vertical
- Innervation
- Superior vestibular nerve Utricle
- Inferior vestibular never Saccule
- Structure
- Dilatations of the membranous labyrinth
- Separated (by perilymph) from the bony labyrinth.
- Filled with endolymph
- Lined by simple cuboidal epithelium except at receptor-rich regions called maculae.
- Contain the following four elements:
- Supporting cells:
- Which consist of simple columnar epithelial cells that are continuous with the simple cuboidal epithelial cells that line the utricle and saccule;
- Hair cells:
- Which are specialized receptor cells intercalated between the supporting cells
- “Hairs” = microvilli.
- Hair cell contains ~40 to 80 microvilli
- Kinocilium
- Each hair cell has ONE kinocilium
- It arises from a centriole
- One kinocilium is located in the periphery of each hair cell
- This polarizes the hair cell in the direction of the kinocilium.
- Kinocilia of the maculae of the utricle and saccule are arranged in such a way as to polarize the maculae in relation to an imaginary curved line called the striola .
- Utricular maculae are polarized toward the striola,
- Saccular maculae are polarized away from the striola.
- Weight of the otolithic membrane exerts a gravitational pull on hair cells.
- The orientation of the hair cells relative to this gravitational force determines the direction of their displacement.
- When the head is held in the horizontal plane, the vertical gravity vector is perpendicular to the hair cell, and no displacement occurs.
- When the head is tilted, say, to the left, hair cells follow, and the gravitational force of the otoliths, now at an angle to the cells, deflects them further in this direction.
- Sti vs no sti of hair cells
- Displacement of hair cells along the axis of polarization depolarizes the cells and initiates excitatory impulses that are carried in afferent fibers to the vestibular nuclei.
- Displacement of hair cells in the direction opposite to the axis of polarization hyperpolarizes the cells and initiates inhibitory impulses.
- Otolithic membrane
- A gelatinous mass embedded with crystals of calcium carbonate (Otoliths) that covers the hair cells
- Dendrites of cells in the vestibular ganglion
- Which carry afferent impulses from the hair cells to the vestibular nuclei of the brainstem.
- Function
- Provide information about the angular acceleration of the head in any direction, though they lack the ability of the utricle and saccule to detect its static position.
- Canals lie in planes oriented at right angles to one another
- Innervation
- Superior vestibular nerve
- Structure
- 3 endolymph-containing semicircular ducts formed by the membranous labyrinth,
- In direct continuation with the utricle.
- Ampullae
- Located at one end of each canal enlarges at its point of attachment to the utricle.
- Functional counterparts of the maculae of the utricle and saccule.
- Contain a thickened sensory epithelium called the ampullary crest, which is composed of columnar epithelial supporting cells with interspersed kinocilia-containing hair cells.
- Cupula
- A gelatinous mass
- Where the hair cells are embedded in
- They fills the space between the crest and the roof the ampulla.
- The cupula is the equivalent of the otolithic membrane but lacks its high-specific gravity calcium carbonate crystals.
- Angular acceleration of the head causes displacement of endolymphatic fluid and movement of the cupula, which stimulates the hair cells.
- Kinocilia of the horizontal canals are polarized in the direction of the utricle.
- Bending hair cells of these ducts in the direction of the utricle thus results in excitatory output.
- Kinocilia of the vertical ducts are polarized in the direction opposite the utricle:
- Bending hair cells in these ducts in the direction of the utricle thus results in inhibitory output.
- In a given plane, the axis of polarization of the ampullae of a pair of ducts, one in the right ear and one in the left, are mirror images of one another.
- The combined effect of excitatory and inhibitory input from paired semicircular ducts on opposite sides of the head apprises the brain of the head's rotary movements.
Utricle and saccule
Semi-circular canals
Central Vestibular System
Central projections of the vestibular system terminate in three major areas
- Spinal cord
- Cerebellum
- Oculomotor nuclei of the brainstem (III, IV, VI).
1st order (bipolar) neurons
- Input from the vestibular labyrinth is transmitted by first-order (bipolar) neurons whose cell bodies are located in the vestibular ganglion (Scarpa's ganglion), which lies at the base of the internal auditory canal.
- Vestibular nerve fibres enter the brainstem between the medulla and pons.
- Terminate
- Vestibular nuclei OR
- Inferior cerebellar peduncle → cerebellum.
2nd order neurons
- Second-order neurons cell bodies make up the vestibular nuclei
- Superior vestibular nuclei SVN
- Inferior vestibular nuclei IVN
- Medial vestibular nuclei MVN
- Lateral vestibular nuclei LVN
- Vestibular nuclei project
- Spinal cord:
- Vestibulo-spinal tract (lateral vestibular or Deiters' nucleus exclusively)
- Responsible for upright posture
- Facilitatory influence alpha and beta motor neurons, particularly those that innervate extensor muscles.
- The tonic excitation of extensor (antigravity) motor neurons provides the physiological basis for the maintenance of extensor muscle tone that is required for an upright body posture.
- Reticulospinal tract acts to balance this
- Unopposed tonic facilitatory influence on spinal motor neurons would result in decerebrate rigidity
- An exaggerated tone of upper and lower limb extensors seen in patients with massive cerebral lesions that effectively disconnect the brainstem from the cerebrum
- This can occur when there is a separation of the reticulospinal tract from its excitatory cortical input, as occurs in transection of the rostral brainstem
- 2 component
- Medial vestibulospinal tract
- Arises in the medial vestibular nucleus.
- It projects crossed and uncrossed fibers in the descending MLF as far as the cervical spinal segments.
- Lateral vestibulospinal tract
- Larger
- Arises in the lateral vestibular nucleus (Deiters' nucleus).
- It projects primarily uncrossed fibers to all levels of the spinal cord.
- Both vestibulospinal tracts terminate on interneurons in laminae VII and VIII
- Oculomotor/trochlear/abducens nuclei
- Directly OR
- Medial longitudinal fasciculus (MLF)
- Vestibulo-ocular reflex
- Leftward head rotation in the horizontal plane results in the flow of endolymphatic fluid in the direction opposite the head rotation, causing
- Stimulation of the left horizontal duct (fluid flowing toward the utricle) and
- Inhibition of the right (fluid flowing away from the utricle).
- A stimulated left horizontal duct increases the impulse frequency of its afferent output → first-order vestibular neurons → Vestibular nuclei → Second-order neurons in Contralateral abducens nucleus →
- Right lateral rectus → abduction of the right eye.
- MLF → Ipsilateral oculomotor nucleus → Left medial rectus → Adduction of the left eye.
- Ipsilateral oculomotor nucleus also receives input from the vestibular nuclei directly
- Because the labyrinths work in pairs, an increase in afferent output of the left horizontal semicircular duct is accompanied by diminished afferent output in the same receptor on the opposite side of the head.
- The combined effect of these two pathways is simultaneous contraction of the left medial rectus (adduction of the left eye) and the right lateral rectus (abduction of the right eye) associated with relaxation of the left lateral rectus and the right medial rectus.
- Caloric testing
- Left ear cold water injection → Endolymph to get heavier → endolymph sinks → as if the head is turning towards the right side → slow phase to the right → fast phase to the left
- Reciprocal (bidirectional) connection between cerebellum and vestibular system
- Cerebellar afferents from the vestibular system includes:
- First-order neurons that originate in the vestibular labyrinths (direct)
- Second-order neurons that originate in the ipsilateral vestibular nuclei (indirect)
- Inferior cerebellar peduncle
- The vestibulocerebellum, which is the part of the cerebellum that reciprocates with the vestibular system, includes the flocculonodular lobe and the vermis.
- Course
- The vestibular nerve gives rise to afferent fibers that terminate in the vestibular nuclei of the brainstem → The neurons of the vestibular nuclei in turn give rise to mossy fibers → enter cerebellum via inferior cerebellar peduncle → ipsilateral flocculonodular lobe on the same side.
- Function
- Responsible for body posture, equilibrium, and the control of eye movements.
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