What does the cochlear division of CN VIII carry information about?
The vestibular division?
What receptor type do both divisions of CN VIII innervate?
While functionally hearing and balance are different senses, information for both is conveyed from the outside world via cranial nerve VIII, the vestibulocochlear nerve.
The cochlear division carries information about sound, while the vestibular division carries information about movements and static position of the head.
Both divisions of CN VIII will innervate end organs which contain specialized mechanoreceptors called “hair cells”.
What is vestibular sense essential for (what is its function)?
What do sensory receptors of the vestibular system respond to?
Where is information of the vestibular system transmitted to?
Vestibular sense is different from our other senses in that it is not prominent in our consciousness. Although normally we aren’t aware of it, it is essential for coordination of motor responses, eye movements, and postures. When the vestibular system malfunctions, the effects become extremely prominent in our consciousness and can affect all aspects of life (e.g. “motion sickness”).
The sensory receptors of the vestibular system, in general, respond to accelerated movement of the head (such as spinning) or to changes in accelerative forces acting on the head resulting from altered positions of the head, such as tilting the head (remember gravity is an accelerative force acting on us all the time).
This information is then transmitted to the brainstem and highly specific connections are made between the vestibular system and motor nuclei of extraocular muscles as well as lower motor neurons in the spinal cord.
The end result is that the whole vestibular apparatus functions to keep the body balanced by coordinating head and body movements and enabling the eyes to remain fixed on a point in space even when the head is moving or when the head is tilted.
Note that the visual system and proprioception also function in maintaining balance.
What is the general function of the outer ear?
What is the general function of the middle ear?
What is the general function of the inner ear?
Generally, where does information conducted in CN VIII go to?
Like our other senses, hearing that a specific sensory stimulus, in this case sound waves, induce changes in specialized receptors, called “hair cells” in this system, which in turn generate action potentials in neurons, here associated with CN VIII. This information is then transmitted through a specific pathway to the central nervous system where the information is “processed” and meaning is associated with the sensation.
In the case of hearing, multiple parts of the auditory system work together to allow us to hear. The outer ear directs sound waves to the ear drum. The ear drum (tympanic membrane) and the ossicles convert sounds waves moving through the air into mechanical energy which will move through the fluid filled environment of the inner ear. The inner ear is the place where sensory transduction occurs and action potentials are generated in CN VIII via special connections to the hair cells. This information is then directed toward the cerebral cortex through specific pathways in the brain stem.
Both divisions of CN VIII are centrally directed fibers of what kind of neuron? Where are their cell bodies located?
Neurons in both ganglia send peripheral fibers to what cells? Where are these cells located?
Both divisions of the CN VIII are centrally directed fibers of bipolar neurons whose nerve cell bodies are located in ganglia within the temporal bone.
Neurons in both ganglia then send peripheral fibers to specialized receptor cells, called hair cells.
These specialized receptor cells (hair cells) are housed within a duct or membranous labyrinth which is suspended within the temporal bone.
T or F: The ductile networks (labyrinths) of the auditory and vestibular systems are continuous; however, the portions specific to the auditory and vestibular systems have specific names. For both systems the membranous ductile system is suspended in bone which has a similar shape. This system is housed within the petrous portion of the temporal bone and thus is not visible when looking into the cranial cavity.
There are bony and membranous labyrinths within the vestibular and auditory systems.
State the names for these portions of both systems.
The cochlear and cochlear duct refer to the bony and membranous labyrinths of the auditory system respectively.
The semicircular canals and vestibule refer to the bony portion of the vestibular system while the terms semicircular ducts, utricle and saccule are the names for the vestibular portions of the membranous labyrinth.
What is the bony labyrinth (or outer tube) portions of each of the systems filled with?
What is contained within the membranous labyrinth of each system?
For both senses, this creates an arrangement of a “tube within a tube”.
- The bony labyrinth (or outer “tube”) is filled with fluid called perilymph.
- The membranous labyrinth (or inner “tube”) is the membranous tube contained within the bony labyrinth. The membranous labyrinth houses the receptor cells and contains another type of fluid called endolymph.
State the specific locations of hair cells within the auditory and vestibular systems.
As mentioned previously, sensory transduction requires interaction between specialized receptors, “hair cells” and peripheral processes of CN VIII. This interaction is physically located in the inner ear (membranous labyrinth), but the hair cells are only found in specific regions of the membranous labyrinth (i.e. not everywhere throughout).
In the cochlear duct, hair cells are associated with the Organ of Corti and rest on a membrane called the basilar membrane. In the vestibular system, hair cells are clustered in specific spots called maculae in the utricle and saccule or cristae in the ampulla of the semicircular ducts (canals).
What is transferred through the outer ear to the tympanic membrane? What ar the ossicles?
What is the major task of the external and middle ear?
After going through the middle ear? What is the pathway to get to the inner ear?
Sound-induced vibrations are transferred through the outer ear to the tympanic membrane (ear drum) and then along a chain of three small bones, the ossicles. It is the major task of the external and middle ear to transfer sound vibrations as efficiently as possible from the air to the fluid filled inner ear.
The foot-plate of the stapes (bone) occupies a small hole in the temporal bone, the oval window. On the other side of the oval window lies the perilymph filled vestibule. Rocking of the stapes disturbs fluid in the inner ear. The vestibule leads to the cochlea which contains the organ of Corti (the location of hair cells on the basilar membrane).
Explain how information is transmitted after it reaches hair cells. Explain the specific role of hair cells and the mechanical/electrical phenomena that occur.
Hair cells are so named because of the specialized apical projections, stereocilia, that project into the endolymphatic interior of the membranous labyrinth. (In the semicircular ducts, the utricle, and saccule, one kinocilium is also present but there are no kinocilia in the cochlea).The other end of the hair cell synapses on the peripheral processes of CN VIII, which in turn convey auditory or vestibular information to the CNS. Hair cells are clustered in discrete locations within the membranous labyrinth and are associated with a gelatinous mass. It is movement of this gelatinous mass relative to the hair cells which causes deflection of the hair cells and results in a receptor potential. In response to mechanical deformation, stereocilia pivot at their bases which results in opening of a mechanically gated ion channel. Deflecting the bundle of hair cells toward the tallest stereocilia increases the probability that this channel will open. Deflecting the bundle of hair cells away from the tallest sterocilia decreases the probability of opening of the channel.
K+ ions prevalent in the endolymph pass through ion channels into the interior of the hair cell, causing volatge gated Ca+ channels to open and releases excitatory neurotransmitters onto the nerve ending of CN VIII and increases firing in CN VIII.
How is intensity (loudness) of sound coded to the brain?
How is frequency (pitch) encoded? Where do hair cells sit (in relation to the basilar membrane) that code high frequency sound? Low frequency sound?
Intensity (Loudness of sound): Is encoded by the rate of action potential generation and the number of nerve fibers responding.
Frequency (Pitch of sound): Is determined by which region of the basilar membrane is stimulated. Pressure delivered to the scala vestibuli by movement of the stapes causes a wave of depolarization across the basilar membrane. Where the wave peaks depends on frequency of the stimulus. High frequency sounds are encoded at the base and low frequency sounds are encoded at the apex of the basilar membrane. The apex of the basilar membrane is wider and more flexible.
Explain the central auditory pathway. Note places of synapse and decussation.
- x Primary auditory fibers (from CN VII), whose cells bodies are in the spiral ganglion (within the inner ear), send their centrally directed processes to enter the brainstem at the pontomedullary junction.
- There each fiber bifurcates and synapses in the cochlear nuclei.
- While some of the projections from the cochlear nuclei will immediately join the lateral lemniscus of each side of the brainstem, many will synapse in the superior olivary nucleus and play a role in localization of sound. Decussation of fibers from one cochlear nucleus to the contralateral superior olivary nucleus occurs through the trapezoid body. Note: Some fibers from the cochlear nuclei cross midline and ascend in the contralateral lateral lemniscus. Also note that a lesion in the trapezoid body would not cause hearing loss but an issue with localization of sound.
- Others do not cross midline but rather join the ipsilateral lateral lemniscus.
- Fibers arising from the superior olivary nucleus will also project to the inferior colliculus via the lateral lemniscus.
Virtually all fibers of the lateral lemniscus will synapse in the inferior colliculus.
- Fibers arising from the inferior colliculus then pass through the brachium of the inferior colliculus and travel to the medial geniculate nucleus of the thalamus.
- From the thalamus, fibers project to the primary auditory area of the cortex located in the transverse temporal gyri. These gyri are found on the superior surface of the temporal lobe, mostly buried in the lateral sulcus.
see pg 102 of course notes for picture
What are the two types of hearing loss?
Define each type of hearing loss and explain causes for each type.
What is a cochlear implant? What type of hearing loss is it used to correct?
Conductive Hearing loss- where something prevents sound from reaching the inner ear. This type of hearing loss is usually correctable and may arise because of fluid, infections, excess earwax, foreign bodies, etc.
Sensorineural Hearing loss- where there is damage to CN VIII, the hair cells, or the cochlear nuclei. This is the most common type of permanent hearing loss and typically cannot be medically corrected. Some causes include illness, ototoxic drugs, head trauma, exposure to loud sounds, aging.
A cochlear implant is a device used to treat sensorineural hearing loss by bypassing damaged hair cells to directly stimulate CN VIII. This therapy does not cure hearing loss but rather allows some perception of sound sensations. It can be used in adults and children depending on a number of factors.
What is a brainstem auditory evoked potential (BAEP) used to assess?
From the BAEP a neurologist is able to determine the time it takes for an aural stimulus to travel from the point at the inner ear where the physical sound is translated into a bioelectrical impulse, to the brainstem. From these readings the neurologist can get an idea whether the auditory nerve is functioning properly.
Remember the eColi mnemonic for pathway of the auditory nerve within the brainstem
e= ear of eigth nerve
c= cochlear nuclei
o= superior olivary nucleis
l= lateral lemniscus
The utricle and saccule each contain a macula where hair cells are located. What is the function of this region?
What is the function of crista contained within ampulla of semicircular ducts?
1) The utricle and the saccule (housed in the bony vestibule) each contain a macula (location of the hair cells). The purpose of this region is to provide information about the static position of the head in space and allow for compensatory eye movements and postural adaptations. (sense static positions, linear accelerated forces. when walking, running, in straight line, in an elevator, etc.)
2) The three semicircular ducts (housed in the bony semicircular canals) communicate at both ends with the utricle. The ampulla of each duct contains a crista (location of the hair cells). The function of the semicircular ducts is to sense rotational movements of the head to allow for compensatory eye movements and postural adaptations. (stimulated when have angular force: tilting head forward and backward, spinning, etc.)
Note: For the vestibular system, it is important that the 2 ears are working together. When we tilt our heads, for example, one ear is stimulated more than the other and this is how we perceive that head is tilted.
In attached pic, note that there are 3 semicircular ducts, 3 crista, one utricle, and one saccule.
What is the otolithic membrane? What is embedded in this membrane?
What is associated with the otolithic membrane?
One important distinction between the maculae and cristae is that the stereocilia and the kinocilia associated with the maculae are embedded in a gelatinous matrix called the otolithic membrane. Calcium carbonate crystals called, otoconia are embedded in this membrane.
Explain how sensory transduction is initiated in hair cells of the vestibular system (maculae and cristae). What stimulus is required?
What physically happens to hair cells to cause depolarization?
Linear acceleration of the head produces relative movement between the otolithic membrane and the hair cells causing bending of the hair cells. This bending results in changes in ion currents in the hair cells and thus the process of sensory transduction is initiated (as it was for the auditory system). Note that otoconia being out place may cause dizziness.
Hair cells along the macula exhibit polarity (deflection of stereocilia toward the kinocilium results in depolarization while deflection away from the kinocilium results in hyperpolarization of the hair cell).
What allows us to sense tilting of the head in multiple directions?
Hair cells are oriented differently along the macula. Thus, when the head is tilted in certain directions (e.g. left versus right) some hair cells are excited while others are inhibited. This allows us to sense tilting of the head in many directions.
Compare the anatomic features of the cristae to the maculae of the utricle and saccule. How are signals initiated?
*The cristae possess the same basic features as the maculae, except that there are no otoconia.
*A graduated array of stereocilia are present as well as a kinocilium.
*In the case of the semicircular ducts the gelatinous mass is called the cupula. Deflection of the cupula results in movement of the hair cells as previously described for maculae.
*The cristae are sensitive to angular acceleration of the head.
*Angular acceleration occurs during motions such as nodding “yes” and “no”.
What semicircular duct is stimulated with movement in a horizontal plane? Flexion? Extension?
The arrangement of the semicircular canals allows rotation to be detected in any plane.
Note that the right and left semicircular canals are paired bilaterally in the following ways:
Horizontal: Stimulated by rotation in a horizontal plane. e.g. sitting in a chair and spinning around).
Superior (Anterior): stimulated by flexion and inhibited by extension
Posterior (Inferior): stimulated by extension and inhibited by flexion
The easiest way to stimulate a semicircular duct is to rotate it about an axis _____ to it.
The easiest way to stimulate a semicircular canal is to rotate it about an axis perpendicular to it.
Explain what occurs in semicircular ducts when rotation begins, during rotation, and at the end of rotation of the head.
The easiest way to stimulate a semicircular canal is to rotate it about an axis perpendicular to it:
1. As rotation begins, endolymph lags behind due to inertia, and the motion of the duct and endolymph relative to each other deflects the cupula.
2. As rotation continues, the endolymph “catches up” and deflection of the cupula ceases.
3. At the end of rotation, the endolymph continues to move, again because of inertia, and the cupula is deflected in the opposite direction. This is why after one spins in circles, they feel a sense of going in the opposite direction.
Thus the semicircular canals respond best to changes in rotation in a given plane (i.e. angular acceleration).
See slides 24-26 of notes
Where are primary cell bodies of primary vestibular fibers (from CN VIII) located?
Where do they enter the brainstem?
Where do these fibers project upon entrance into the brainstem?
Primary vestibular fibers (from CN VIII) with cell bodies are in the vestibular ganglion, send their centrally directed processes to enter the brainstem at the pontomedullary junction. While some of these fibers project to the cerebellum, most will synapse in the collection of vestibular nuclei located in the rostral medulla and caudal pons (labeled “S”, “L”, “M”, and “I” for superior, lateral, medial, and inferior nuclei in image attached to next notecard)
What are the other inputs to the vestibular nuclei?
Other inputs to the vestibular nuclei include the cerebellum, the contralateral vestibular nuclei (to assure coordinated function of the vestibular system as a whole), and brainstem nuclei that convey information of objects moving in the peripheral aspects of the visual fields (to help determine whether images moving across the retina are due to head movements or object movements). There is some direct input to the vestibular nuclei from the spinal cord; however, most of the inputs from the spinal cord relay first through the cerebellum or nuclei of the reticular formation. Inputs from the spinal cord provide information about the current position of the body to allow the vestibular system to initiate compensatory postures in response to changes in head position.
What are the outputs of vestibular nuclei? If applicable, state pathways involved.
Since the primary role of the vestibular system is to coordinate eye and head movements and initiate postural responses related to head position, the primary projections from the vestibular system are:
1. to the spinal cord (to elicit postural responses in the body)
2. to the brainstem nuclei which control the action of the extraocular muscles (nuclei of CN III, IV, and VI).
- The vestibulospinal tract is the pathway for projections to the spinal cord and the medial longitudinal fasciculus (MLF) conveys projections to the nuclei of CN III, IV, and VI.
-The cerebellum is also very involved in postural responses and thus there are extensive connections from the vestibular nuclei to the cerebellum (and vice versa as described above).
-Since we do have conscious awareness of vestibular sense, there must be projects from the vestibular nuclei to the cerebral cortex and these projections (as they do for other senses) will relay (synapse) in the thalamus.
-Lastly, there are connections between the vestibular nuclei and visceral nuclei located in the brainstem and spinal cord. These connections help regulate blood pressure and heart rate changes in response to changes in posture.
What artery supplies the vestibular nuclei?