Innervation of the Structures of the Ear (Not Including Audition)

The topic of this learning tool is "Innervation of the structures of the ear (not including audition)". Section one contains instructions on how to navigate this learning tool effectively and my justification for creating this tool in the way that it is presented. This is followed by three sections which cover the innervation of different regions of the ear. Throughout each section, material is assessed to reinforce learning and the final section is a selection of questions from throughout the module to provide anoverall assessment. It is advised that you work through the module in the order it has been set out, but you are free to navigate it as seen fit. 

1. How to Use this Learning Tool

Instructions for Use: Images and Navigation

Interacting with the Images in this Module
This feature is the first to be described as understanding it will be important in order to successfully navigate the remainder of these instructions. All of the images in this module have intentionally been labelled using "hot spots", which require the user to interact with the image to reveal the appropriate information. Labels will be made available simply by hovering the cursor over the flashing "I" on each image. An example (Fig. i) is shown below, try to reveal the hidden information by interacting with the "hot spot". 

Figure i: How to interact with "hot spots"

Navigating the Home Page:
By now you will have already navigated the home page. The main features of the home page are demonstrated below (Fig. ii). 

Figure ii: How to navigate the home page. 

Navigating Sections
Each section contains a number of pages. Instructions on how to navigate through sections, such as this one, are demonstrated below (Fig. iii). 

Figure iii: How to navigate each section

Instructions for Use: Questions

There are a large number of questions, of many different varieties contained within this learning tool. There are some questions after the introduction of a new topic within each section. Many of these questions are single answer, and simply require the user to click the appropriate box and then submit their answer. When the question requires a different approach, instructions will be detailed in the relevant place. Navigation of the question pages (Fig. iv) is very similar to navigation of the content pages. 

Figure iv: Navigation of question pages

Instructions for Use: Sections

The module has been divided into three sections to adequately address the remit. These sections are external ear, middle ear and internal ear and the structures considered within each section are shown below (Fig. v). 

Figure v: Brief description of how the information has been subdived within this module

Justification of Approach

* This section has been included as a note to the marker regarding the creation of the tool and is not relevant for the learner *

This tool is aimed at third year university students enrolled in a degree involving teaching of anatomical sciences. The typical student in this demographic will have grown up around technology for their entire life, and will likely be well versed in using it. As demonstrated over the previous pages, this tool is easy to navigate and should pose no problem for the target audience, especially when supplemented with the instructions. However, the intended demographic also have a number of other traits which should be addressed in order to deliver effective learning. The main points are described below, along with a description of how these factors have been addressed. 

Interaction
The typical attention span of the intended demographic is short, and as such interaction is key to maintain engagement. The strategies to address this are that after a new point is introduced there will be one or two questions within the section to engage the learner, whilst also ensuring adequate recall of the information. At the end of the module there are interactive questions about the entire module to engage, assess and reinforce the teaching delivered. 

Visual Learning
The target audience thrive in image rich environments and are severely deterred by large blocks of text. To address this, separate ideas are on individual pages with text blocks kept as concise as possible. Furthermore, wherever possible, the text is supported with a diagram or media source. 

Structure and Immediacy
There is a strong desire to be told what is required of the participant before embarking on a task. Similarly, whilst completing of a task this demographic thrive on immediate gratification. These two traits have been addressed in tandem, with the introduction of the progress bar. In the "instructions for use section" the learner is told that their progress advances whilst moving through the content of the module and answering the interactive questions. As they work through the module they can visually see their progress advancing. This grants the user the structured approach they desire, and gives them a small reward as they work towards their goal. The structured approach is augmented by an "aims" page at the start of each section. 

Inductive Discovery and Experiential
These two related ideas state that this particular generation learn better by discovering the information for themselves, and by learning this material through experiencing it. This idea is harder to address within the confines of this learning tool. However, instead of putting in labelled diagrams, "hot-spots" have been incorporated into the image. These require the user to interact with each image to reveal the information. This allows the student to discover the information for themselves, whilst also increasing engagement. 

Educational Literature Cited

Brown, J.S. (2000) Growing up digital, Change , 32, 10– 11

Oblinger, D.G., Oblinger, J.L. (2005) Is it age or IT: first steps towards understanding the net generation. In: Oblinger, D.G., Oblinger, J.L. (eds), Educating the Net Generation, 1st ed, Educase

Prensky, M. (2001) Digital natives, digital immigrants, part II: do they really think differently?, On the Horizon, 9, 15–24

 

Acknowledgements

Thanks to all those who gave their time and allowed their photographic images to be included within this module. 

All digital images are original content and have been edited using Adobe Photoshop CS6 software, .psd files are available upon request. The original sketches have been maintained and are also available upon request as either hard copies or .jpeg files. 

Two videos have been included within this module. Both videos are original content and have been used with the permission of the subject. However, without paying a subscription fee these videos cannot be uploaded to the easygenerator module. Instead these videos have been hosted on "Dropbox" and "YouTube" to ensure compatibility. Hyperlinks are included in the relevant places, and both links will lead the user to the same video. These videos are only available via these hyperlinks, and have not been published to the public domain. 

2. External Ear

Aims

The aims of this section are to: 

  1. Revise the basic gross anatomy of the external ear 
  2. Detail the sensory innervation of the external ear and the anatomy of the relevant nerves
  3. Briefly discuss the possible motor function of the external ear

Overview of Anatomy

Basic Anatomy of the External Ear: 
The external ear is composed of two parts: the auricle (pinna) and the external acoustic meatus. The function of the auricle is to collect sound waves and focus them towards the external acoustic meatus. The external acoustic meatus will then conduct these sound waves towards the tympanic membrane.  

The shape and size of the auricle is subject to a large degree of variation between individuals. It would not be possible, or of benefit, to detail all the variations possible within this module. Some of the main features of the auricle are demonstrated in order to aid orientation when discussing innervation (Fig. 2.1). 

Figure 2.1: The basic anatomy of the visible aspect of the external ear. 

Can you identify the features of the external ear?

Drag the labels to their appropriate location on the image below.
  • Anti-helix
  • Lobule
  • Conchae
  • Tragus
  • External acoustic meatus
  • Helix

Overview of Innervation of the External Ear

The main innervation of the external ear involves sensory supply from five nerves:

  • Great auricular nerve
  • Lesser occipital nerve
  • Auriculotemporal nerve
  • Vagus nerve
  • Facial nerve

The gross anatomy of the relevant nerves and their branches will be described in order to aid appreciation of the pattern of sensory innervation of the external ear. 

Great Auricular Nerve


Figure 2.2: The course of the great auricular nerve shown in relation to the sternocleidomastoid muscle

Origin: C2 and C3 anterior rami from the cervical plexus.

Course: The great auricular nerve is the largest ascending branch from the cervical plexus. It emerges posterior to the sternocleidomastoid muscle in the posterior triangle of the neck and wraps around this muscle before running superiorly on its surface (Fig. 2.2). It bifurcates into anterior (to face) and posterior branches. The posterior branch courses over the medial surface of the auricle, which is not shown in the above image.

Supply: The posterior division supplies a large portion of the auricle. 

Questions: Great Auricular Nerve

  • The great auricular nerve runs on the surface of the trapezius muscle
  • The root value of the great auricular nerve is C2 and C3
  • The anterior division of the great auricular nerve course over the auricle of the ear
Select whether the following statements are true or false:

Lesser Occipital Nerve

Figure 2.3: Course of the Lesser occipital nerve

Origin: C2 anterior rami from the cervical plexus.

Course: Wraps around Cranial nerve XI (accessory nerve) before travelling superiorly along the posterior border of the sternocleidomastoid. The nerve perforates the connective tissue near the cranium, then ascends the scalp travelling posteriorly to the auricle (Fig. 2.3.). 

Supply: Tends to supply some of the superior aspect of the auricle, but main supply is to the scalp. 

Question: Lesser Occipital Nerve

  • Travels superiorly, ascending anterior to the auricle
  • Travels horizontally, superior to the auricle
  • Travels superiorly, ascending posterior to the auricle
  • Travels superiorly, ascending over the surface of the auricle
  • Travels horizontally, inferior to the auricle
Which of the following best describes the course of the lesser occipital nerve? 

Auriculotemporal Nerve

Figure 2.4: The origin of the auriculotemporal nerve from the mandibular division of the trigeminal nerve 

Origin: Branch of the mandibular nerve (third division of the trigeminal nerve - CN V3) (Fig. 2.4).

Course: Tends to arise as either a single or double root from the mandibular nerve, and these root(s) are closely related to the marginal mandibular artery (Fig. 2.4). The auriculotemporal nerve receives a branch from the glossopharyngeal nerve early in their course that contains parasympathetic fibres travelling to the parotid gland. The auriculotemporal nerve travels laterally coursing behind the temperomandibular joint. As it emerges at the posterior aspect of the joint it makes a sharp turn to ascend superiorly. Its superior course runs superficial to the posterior aspect of the zygoma and anterior to the external ear (Fig. 2.5). 

Supply: External ear and skin of temporal region 

Question: Auriculotemporal Nerve

  • Arises from the mandibular division of the trigeminal nerve
  • Passes in close relation to the marginal mandibular artery
  • Courses laterally, passing posterior to the temperomandibular joint
  • Makes a sharp superior turn
  • Courses anterior to the external ear
Rank these statements in order to describe the course of the auriculotemporal nerve. The first, or most proximal aspect, should be placed at the top. 

Auricular Branch of the Vagus Nerve

Origin: From the superior ganglion of the vagus nerve within the jugular foramen. 

Course: Leaves the jugular foramen within the mastoid canaliculus (bony canal that allows the nerve to pass from jugular foramen through the mastoid process). The auricular branch of the vagus nerve then runs transversely along the tympanomastoid foramen and bifurcates. One branch communicates with auricular branch of the facial nerve and the other courses to the external acoustic meatus. 

Supply: External ear 

Arnold's Ear Cough Reflex

Mechanical stimulation of the external ear can be a cause of chronic cough, as innervation from the auricular branch of the vagus nerve means that the vagus innervates both the external ear and the pharynx. In a process similiar to referred pain, stimulation of the ear can manifest as symptoms in the pharynx. However, this is uncommon, and included for interest and to highlight how knowledge of ear innervation can be of importance. 

Question: Auricular branch of the Vagus Nerve

  • The auricular branch of the vagus nerve travels within the mastoid cannaliculus
  • The entire auricular branch of the vagus nerve communicates with the facial nerve
  • Irritation of the auricular branch of the vagus nerve can manifest as a cough

Read the following statements concerning the auricular branch of the vagus nerve and select whether they are true or false. 

Auricular Branch of the Facial Nerve

Prevalence of this nerve varies from between 40-100% and is more common in people of European descent. It is suggested that, when present, the auricular branch of the facial nerve will innervate small regions over the conchae. 

Origin: Branch of the facial nerve within the facial canal, proximal to the chorda tympani. 

Course: Courses through the petrous part of the temporal bone to the external acoustic meatus. 

Supply: External ear

NB: The auricular branch of the facial nerve should not be confused with the posterior auricular nerve which arises from the facial nerve distal to the stylomastoid foramen. 

Question: Auricular branch of the Facial Nerve

  • Branches from the facial nerve within the internal acoustic meatus
  • Branches from the facial nerve within the facial canal, proximal to the chorda tympani
  • Branches from the facial nerve within the facial canal, distal to the chorda tympani
  • Branches from the facial nerve distal to its emergence through the stylomastoid foramen

Select the correct statement concerning the origin of the auricular branch of the facial neve

Innervation of the Lateral Auricle

Terminology note: the auricle is described here as lateral and medial parts. The lateral auricle is the surface that tends to be visible, whilst the medial surface is the relatively featureless surface that lies in contact with the cranium. 

Figure 2.5: The most common pattern of sensory innervation of the lateral auricle. However, the boundaries will not be as well defined as demonstrated in the image and there will be considerable variation between individuals.

Great Auricular Nerve: Sensory innervation to the majority of the inferior, and much of the posterior aspects of the lateral auricle (Fig. 2.5). 

Auricular Branch of the Vagus Nerve: Sensory innervation to the concha and some of the tragus (Fig. 2.5). It is very common to find overlap with the great auricular nerve over the tragus and inferior aspect of the concha. The auricular branch of the facial nerve may also supply scattered innervation within this region (Fig. 2.5). 

Auriculotemporal Nerve: Supplies the majority of the anterosuperior aspect of the lateral auricle (Fig. 2.5). 

Lesser Occipital Nerve: May supply a small aspect of the superior lateral auricle (Fig. 2.5).

Question: Innervation of the Lateral Auricle

Select the image which represents the cutaneous region of the lateral auricle supplied by the auriculotemporal nerve:

Innervation of the Medial Auricle

The medial surface of the auricle is considered less often in anatomical texts, and like the lateral surface, it receives only sensory innervation. It can be divided into upper, middle and lower thirds depending on its cutaneous innervation, but there are large amounts of overlap. 

Upper Third: Often supplied by the lesser occipital nerve in isolation, or sometimes a combination of the lesser occipital and great auricular nerves (Fig. 2.6). 

Middle Third: Most often innervated by the great auricular and auricular branch of the vagus nerve (Fig. 2.6). 

Lower Third: Tends to be innervated by the great auricular nerve in isolation (Fig. 2.6). 

Figure 2.6: Pattern of cutaneous sensory innervation of the medial surface of the auricle.

Question: Innervation of the Medial Auricle

  • Great auricular nerve
  • Lesser occipital nerve
  • Auriculotemporal nerve
  • Auricular branch of the vagus
  • Auricular branch of the facial
Which nerve most often supplies sensory innervation to the lower one third of the medial surface of the auricle? 

Innervation of the External Acoustic Meatus

The external acoustic meatus is the canal which runs between the auricle and the tympanic membrane. Its function is to conduct sound to the tympanic membrane. As such, the external acoustic meatus only receives sensory innervation. 

The pattern of sensory innervation of the external acoustic meatus: 

  • Anterior and Superior walls:      Auriculotemporal nerve
  • Posterior and inferior walls:      Auricular branch of the vagus nerve

Question: Innervation of the External Acoustic Meatus

Match the nerve to the correct description of its sensory supply to the external acoustic meatus: 
  • Supplies sensory innervation to the anterior and superior walls
    Auriculotemporal nerve
  • Supplies sensory innervation to the posterior and inferior walls
    Auricular branch of the vagus nerve

Auricular Mucles

Available on: Dropbox OR YouTube

Video 2.1: Function of the auricular muscles. Click either hyperlink to access the video. 

There are a number of small intrinsic, and three slightly larger extrinsic muscles of the auricle. These muscles have the capacity to alter the shape and the position of the external ear, but their function is minimal or absent in most people. However, some people are able to utilise these muscles to move the auricle (Video 2.1). These muscles can be classified as muscles of facial expression, and as such they receive motor innervation from the branches of the facial nerve distal to the stylomastoid foramen. 

Question: Auricular Muscles

  • Great auricular nerve
  • Lesser occipital nerve
  • Auricular branch of the vagus nerve
  • Auricular branch of the facial nerve
  • Auriculotemporal nerve
  • Facial nerve
Which nerve supplies motor innervation to the auricular muscles? 

3. Middle Ear

Aims

The aims of this section are: 

  1. Revise the basic anatomy and function of the middle ear
  2. Describe the innervation of the tympanic membrane
  3. Describe the motor and reflex control of the middle ear muscles 
  4. Describe the tympanic plexus, including sensory and autonomic innervation of the middle ear

Overview: Middle Ear Anatomy

The middle ear, or tympanic cavity, is found within the temporal bone. It contains three ossicle bones (malleus, incus and stapes) and two muscles (tensor tympani and stapedius). The middle ear is connected to the nasopharynx by the eustachian tube, and to the mastoid air cells by the aditus (Fig. 3.1). The cavity is lined by a mucosal layer that also extends along the eustachian tube. 

Figure 3.1: The middle ear. The image is orientated as if the observer is looking from lateral to medial, i.e. the tympanic membrane has been removed in order to look inside the cavity. Some features are not labelled in this image, but will be described later in this section

Eustachian Tube: 
The eustachian tube connects the middle ear to the nasopharynx. Its function is to allow air from the nasopharynx to enter the middle ear, which equalises the pressure either side of the tympanic membrane. 

Ossicles: 
The three ossicle bones (Fig. 3.2) form a chain which connects the tympanic membrane to the oval window of the cochlea. Their function is to transmit and amplify vibrations of the tympanic membrane to the perilymph of the cochlea. 

Malleus:    Attached to the tympanic membrane and articulates with the incus 
Incus:        Articulates with both the malleus and stapes, connecting the two bones 
Stapes:      Has a footplate which articulates with the oval window of the cochlea

Figure 3.2: The ossicle bones of the middle ear

Facial Canal and Chorda Tympani

Figure 3.3: The relation of the facial nerve and chorda tympani with the middle ear

The course of the facial nerve is closely related to the middle ear. The facial nerve passes through the internal acoustic meatus with the vestibulocochlear nerve, before entering the facial canal. The facial canal extends along the medial wall of the middle ear, superior to the oval window (Fig. 3.3). It takes a sharp turn to run along the posterior wall as it courses towards the stylomastoid foramen. Despite its proximity, the facial nerve only has a minor role in the innervation of the middle ear. 

Chorda Tympani
The chorda tympani is a branch of the facial nerve which supplies taste to the anterior two thirds of the tongue, and parasympathetic to salivary glands. It tends to branch from the facial nerve within the facial canal and courses through the middle ear. The chorda tympani will cross the tympanic membrane, running between the middle and internal mucosal layers, before joining the lingual nerve.  

Questions: Middle Ear Anatomy

  • The middle ear is contained within the temporal bone
  • The aditus acts to connect the middle ear with the nasopharynx
  • The stapes has an articulation with the tympanic membrane
  • The facial nerve is closely related to the middle ear
  • The chorda tympani runs within the tympanic membrane
Read the following statements and determine whether they are true or false. 

Tympanic Membrane

The tympanic membrane (ear drum) is found at the medial aspect of the external acoustic meatus and forms the lateral wall of the middle ear. The tympanic membrane is a thin oval structure that vibrates in response to sound waves. Vibrations of the tympanic membrane will move the malleus, and therefore the subsequent ossicle bones, allowing transmission of sound to the internal ear. 

Sensory Innervation
It has been suggested that only nociceptive (pain) sensory stimuli are transmitted from the tympanic membrane. This means that any mechanical stimuli will cause pain, rather than a sensation of touch. 

There are no detailed accounts demonstrating the precise innervation pattern of the tympanic membrane, but this is probably due to it being highly variable. The main sensory supply to the external (lateral) aspect of the tympanic membrane is from the auriculotemporal nerve. It has long been assumed that this is augmented by the vagus, facial and glossopharyngeal nerves, but this is without support from the literature. The relationship with the facial nerve is interesting, as the chorda tympani runs within the tympanic membrane, but there is no evidence suggesting it supplies sensory innervation to it. The internal aspect is supplied by the glossopharyngeal nerve, and is described later in this section to more detail.  

Question: Tympanic Membrane

The  is a thin structure which vibrates in response to sound. The main modality of innervation to this structure is . The lateral surface, in communication with the external acoustic meatus, is mainly supplied by the  nerve. The internal surface, within the middle ear, is supplied by the  nerve. 

Control of Middle Ear Pressure

It is important that air pressure on either side of the tympanic membrane remains equal to prevent distortion or rupture of this structure. A neural feedback loop within the middle ear allows control of pressure. Mechanoreceptors in the tympanic membrane monitor its shape and stiffness, whilst baroreceptors within the middle ear directly monitor pressure. These sensory inputs form a neural arc that controls the opening of the eustachian tube, and thus middle ear pressure. 

Middle Ear Muscles

The middle ear has two very small skeletal muscles; the tensor tympani and the stapedius (Fig. 3.4). The main function of these muscles are to protect the structures of the internal ear from loud sounds by reducing movement of the ossicles. 

Figure 3.4: The muscles of the middle ear

Tensor Tympani: 
Attachments: Cartilaginous portion of the eustachian tube to the malleus 
Function:         Pulls the malleus anteromedially and therefore tenses the tympanic membrane 
Innervation:   Nerve to tensor tympani from the mandibular division of the trigeminal nerve 

Stapedius:
Attachments:   Posterosuperior wall of the middle ear to stapes bone
Function:          Contraction result in stiffening of the stapes at the oval window
Innervation:     Stapedial branch of the facial nerve 

Middle Ear Muscle Reflex

The middle ear muscle reflex refers to the contraction of the stapedius and tensor tympani in response to intense auditory stimuli. Contraction of the middle ear muscles slows the movement of the stapes at the oval window, described as increasing impedance of the middle ear. This helps prevent over-stimulation of the delicate inner ear structures in response to loud noises. 

Interestingly, unlike most other skeletal muscle reflex arcs, there are no afferent fibres to the stapedius or tensor tympani muscles. Instead, afferent input is derived from the auditory signals interpreted at the cochlea. 

Bell's palsy is a medical condition in which there is paralysis of the facial nerve. As part of this condition patients may be hypersensitive to sound. This symptom can be explained by loss of function of the stapedius muscle, and therefore inability to implement the middle ear muscle reflex. 

Question: Middle Ear Muscles

Match the corresponding text boxes. 
  • Attaches between the eustachian tube and malleus
    Tensor tympani
  • Nerve that supplies the stapedius muscle
    Facial nerve
  • Nerve that supplies the tensor tympani
    Mandibular division of the trigeminal nerve
  • Attaches between the wall of the middle ear and the stapes
    Stapedius

Tympanic Plexus

The tympanic nerve, a branch of the glossopharyngeal nerve, runs along the promontory, an indent on the medial wall of the middle ear formed by the cochlea. On the surface of the promontory the tympanic nerve divides into a number of branches which form the tympanic plexus (Fig. 3.5). The branches from the tympanic plexus supply: 

  • Sensory innervation to the tympanic membrane and mucosa of the middle ear and eustachian tube 
  • Parasympathetic supply to the lesser petrosal nerve, which ultimately joins the auriculotemporal nerve to supply the parotid gland 
  • A branch to the greater petrosal nerve

As the glossopharyngeal nerve also supplies sensory innervation of the pharyngeal mucosa, it is common to have pain from the oropharynx (e.g. Tonsilitis) refer to the middle ear. 

Figure 3.5: Nerves innervating the middle ear

Autonomic Innervation

The mucosal lining of the middle ear also receives autonomic innervation. Sympathetic fibres run superiorly with the internal carotid artery and anastamose with the branches of the tympanic plexus. From the tympanic plexus they are distributed throughout the middle ear mucosa. These fibres are believed to influence blood flow and glandular secretions, but the role of this action is not entirely clear. It is thought that they may contribute to the pathological process of middle ear effusion. 

Question: Tympanic Plexus

  • Vagus
  • Facial
  • Auriculotemporal
  • Mandibular division of the trigeminal
  • Glossopharyngeal
The tympanic nerve is a branch from which nerve? 

4. Internal Ear

Aims

The aims of this section are: 

  1. To revise the anatomy and function of the internal ear 
  2. To describe how afferent signals are generated and propagated in the vestibular system
  3. To describe the vestibulo-ocular reflex, an important reflex requiring vestibular afferent input 

Internal Ear Overview

Figure 4.1: The bony labyrinth with the three segments labelled as "hot spots". 

The internal ear consists of the bony labyrinth (Fig. 4.1) which is found within the petrous temporal bone. The bony labyrinth contains perilymph, in which the membranous labyrinth is suspended. The bony labyrinth consists of three parts:  the vestibule, semicircular canals and the cochlea. The cochlea is the end organ of audition, and is not considered further within this module. The vestibule and three semicircular canals are components of the vestibular system which helps maintain balance and stability of gaze. 

The Vestibule

Figure 4.2: The membranous labyrinth (pink) shown in relation to the bony labyrinth. The utricle and saccule of the membranous labyrinth are demonstrated by "hot spots". 

The vestibule is the central portion of the bony labyrinth. It is found immediately posterior to the cochlea and anterior to the semicircular canals. The utricle and saccule are regions of the membranous labyrinth (Fig. 4.2) contained within the vestibule and within these are sensory regions called maculae. The maculae of the utricle and saccule are organs of the vestibular apparatus. 

Semicircular Canals

Figure 4.3: The membranous labyrinth (pink) shown in relation to the bony labyrinth with demonstration of the ampullae of the semicircular ducts. 

Each labyrinth consists of a superior, lateral and posterior semicircular canal and contained within these are the equivalent semicircular ducts of the membranous labyrinth. The semicircular ducts are arranged at 90° to each other. Furthermore, they are orientated in similar planes to their contralateral counterparts as follows: 

  • Right Superior with Left Posterior 
  • Right Posterior with Left Superior 
  • Right Horizontal with Left Horizontal. 

The semicircular ducts communicate with the utricle, and at this communication are terminal swellings called ampullae (Fig. 4.3). Contained within each ampulla is a region of sensory neuroepithelium called the crista ampullaris. The three cristae are organs of the vestibular apparatus. 

Question: Anatomy of the Internal Ear

  • Suspended in perilymph
    membranous labyrinth
  • Central portion of the bony labyrinth
    Vestibule
  • Have terminal swellings termed ampullae
    Semicircular ducts
  • Contains a macula
    Saccule
  • Contains a crista ampullaris
    ampulla of the superior semicircular duct

Vestibulocochlear Nerve

The vestibulocochlear nerve is the 8th cranial nerve. It arises from the brainstem at the junction between the pons and the cerebellum, before exiting the posterior cranial fossa in the internal acoustic meatus. It travels posterolaterally to the facial nerve within the internal acoustic meatus, and divides into the vestibular and cochlear nerves near its lateral aspect.

Vestibular Nerve
The main function of the vestibular nerve is to transmit afferent signals from the organs of the vestibular system. The vestibular nerve has superior and inferior divisions. The superior division is the larger of the two, and supplies the utricle and the anterior and lateral semicircular canals. The inferior vestibular nerve supplies the saccule and the posterior semicircular canal. Terminal branches of the vestibular nerve synapse with the hair cells (described further later in this section) of the maculae and cristae.

It has been demonstrated that the vestibular nerve has a small number of motor fibres. These efferent fibres form a reflex arc with the afferent division, acting to reduce the firing rate, and therefore sensitivity, in response to prolonged sensory stimulation. 

Question: Vestibulocochlear Nerve

  • Vestibular
  • Cochlear
  • Facial
  • auriculotemporal
Which nerve supplies afferent innervation to the vestibular apparatus? 

Vestibular Apparatus: Maculae

The maculae of the utricle and saccule form the static labyrinth which detects changes in head orientation due to gravity or linear acceleration. The macula of the utricle is orientated horizontally, whilst that of the saccule is vertical. This arrangement means the utricle is most sensitive when the head is upright, with the saccule having the major role when the head is horizontal. 

Hair Cells
Embedded within the epithelium of the maculae are specialised mechanoreceptors called hair cells. Each hair cell has a single large kinocilium, and this is associated with approximately 70-100 stereocilia. These are organised so that the large kinocilium is followed by rows of progressively shortening stereocilia (Fig. 4.4). The hair cells project into a gelatinous membrane which contains otoconia (calcium crystals). Gravity or head movement is able to deflect this membrane and therefore bend the hair cells. 

Figure 4.4: Hair cell organisation

Afferent Signal Generation
When the hair cells bend towards the kinocilium depolarisation occurs. Depolarisation increases firing in the vestibular nerve and the afferent signal is propagated centrally. Bending of the stereocilia away from the kinocilium results in hyperpolarisation of hair cells and inhibits transmission in the vestibular nerve. These responses are graded, which means that the rate of neuronal firing increases/decreases depending on the degree of hair cell movement. During rest there is a small degree of signal transmission in the vestibular nerve, this allows the brain to differentiate between resting conditions and hyperpolarisation. 

Due to the orientations of the utricle and saccule and its relation with the contralateral ear, each head movement will depolarise, hyperpolarise and not affect various regions. This pattern of afferent signal generation can be interpreted by the brain to allow determination of head position. 

Vestibular Apparatus: Cristae

The cristae (sing. = crista ampullaris) contained within the ampulla of the semicircular ducts form the kinetic labyrinth. The kinetic labyrinth detects rotatory movements of the head. The perpendicular arrangement of the semicircular ducts, and their alignment with those of the contralateral ear allows three dimensional interpretation of rotatory movements. 

Figure 4.5: Cupular deflection in response to acceleration of the head

Afferent Signal Generation
The cristae have hair cells in a similiar organisation as the maculae. The kinocilia and stereocillia project into the cupula, but unlike in the maculae, the cupula does not contain otoconia. Rotation of the head results in the flow of endolymph through the semicircular ducts, which deflects the cupula and bends the hair cells. 

Acceleration of the head causes the deflection of the cupula in the opposite direction of movement as it has yet to overcome inertia. Head rotation at a constant speed allows the cupula to return to its normal position and afferent signal generation ceases. Deceleration of the head results in the cupula displacing in the same direction as movement (Fig. 4.5). 

As in the maculae, when hair cells bend toward the kinocilia depolarisation occurs, whilst hyperpolarisation occurs with bending away from the kinocilia. These responses increase or decrease the rate of firing in the vestibular nerve respectively. Depolarisation is always coupled with hyperpolarisation of the hair cells in the paired contralateral semicircular duct. For example, depolarisation in the right superior semicircular duct would occur simultaneously with hyperpolarisation in the left posterior semicircular duct. This allows interpretation of the direction of rotatory movement. 

Adaptation

Once a stimulus has propagated for a short period of time, the membrane potential will return to its resting state which lowers the rate of discharge in the vestibular nerve. This property explains why people do not experience the sensation of movement when the head is maintained in a static position, such as lying down. Adaptation is due to the efferent function of the vestibular nerve.

Question: Afferent Signal Generation in the Vestibular System

The maculae of the utricle and saccule form the  labyrinth, which detect  head movements. The cristae of the ampullae form the  labyrinth, which respond to  head movements. Both contain hair cells which bend in response to their respective movements. Bending of the stereocilia towards the kinocilium results in  of the hair cell. Bending of the stereocilia away from the kinocilium results in  of the hair cell. Prolonged activation of the sensory hair cells is actively dampened by a process called .

Vestibulo-ocular Reflex

Available on: Dropbox OR YouTube

Video 4.1: The Vestibulo-ocular reflex. As seen in the video, rotatory head movements occur, but gaze remains static (looking straight ahead)

The vestibulo-ocular reflex acts to stabilise images produced on the retina in response to rotation of the head. This is of particular importance during movement as it maintains a steady gaze. Afferent signals from the vestibular nerve are relayed to the extraocular muscles, resulting in co-ordinated contraction of these muscles to move the eye in the opposite direction of head rotation. 

5. End of Module Quiz

Innervation of the External Ear

Select the image that corresponds to the course of the great auricular nerve

Innervation of the External Ear

Select the image that corresponds with the course of the lesser occipital nerve

Innervation of the External Ear

  • C2
  • C2 and C3
  • C3
  • C3 and C4
  • C4
What is the root value of the great auricular nerve? 

Innervation of the External Ear

Match the appropriate nerves to the following statements:
  • Ascends along the posterior border of the sternocleidomastoid muscle
    Lesser occipital nerve
  • Ascends along the superficial surface of the sternocleidomastoid muscle
    Great auricular nerve
  • Not present in everybody
    Auricular branch of the facial nerve
  • Closely associated with the middle meningeal artery at its origin
    Auriculotemporal nerve
  • Irritation can present as a cough
    Auricular branch of the vagus nerve

Innervation of the External Ear

Select the region of the lateral auricle that is supplied by the auriculotemporal nerve: 

Innervation of the External Ear

Select the region of the lateral auricle that is supplied by the lesser occipital nerve: 

Innervation of the External Ear

Select the region of the lateral auricle that is supplied by the vagus and facial nerves: 

Innervation of the External Ear

Select the region of the lateral auricle that is supplied by the great auricular nerve: 

Innervation of the External Ear

  • Great auricular nerve
  • Lesser occipital nerve
  • auriculotemporal nerve
  • auricular branch of the vagus nerve
  • Facial nerve
Select the two nerves that supply sensory innervation to the region of the medial auricle shown on the image below (you can select more than one option): 

Innervation of the External Ear

  • Great auricular nerve
  • Facial nerve
  • Chorda tympani
  • Lesser occipital nerve
  • Auriculotemporal nerve
Which nerve supplies motor innervation to the auricular muscles?

Innervation of the Middle Ear

  • Courses between the layers of the tympanic membrane
    Chorda tympani
  • Supplies the majority of the external (lateral) surface of the tympanic membrane.
    Auriculotemporal nerve
  • Supplies the internal (medial) surface of the tympanic membrane
    Glossopharyngeal nerve

Innervation of the Middle Ear

  • Allow sound transmission from the tympanic membrane by moving the ossicle bones
  • To increase impedance of the middle ear
  • To open and close the eustachian tube
  • To open and close the aditus
What is the main function of the tensor tympani and stapedius muscles of the middle ear? 

Innervation of the Middle Ear

  • Facial
  • Vagus
  • Mandibular division of the trigeminal nerve
  • Auriculotemporal
  • Glossopharyngeal

Which nerve innervates the muscle demonstrated by the blue arrow in the image?

Innervation of the Middle Ear

  • Great auricular nerve
  • Auriculotemporal nerve
  • Chorda tympani
  • Facial nerve
  • Mandibular division of the trigeminal nerve
Which nerve innervates the stapedius muscle? 

Innervation of the Middle Ear

  • Afferent fibres to the middle ear muscle
  • Auditory stimuli interpreted by the cochlea
  • Mechanoreceptors in the tympanic membrane
  • Baroreceptors in the middle ear
What supplies the afferent input during the middle ear muscle reflex? 

Innervation of the Middle Ear

  • The tympanic nerve is a branch of the glossopharyngeal nerve
  • The tympanic nerve branches to form the tympanic plexus
  • The middle ear receives sensory and autonomic fibres from the tympanic plexus
  • The greater petrosal nerve is the source of referred pain between the pharynx and middle ear
Select whether the following statements are true or false: 

Innervation of the Internal Ear

Click on the utricle on the image below: 

Innervation of the Internal Ear

Click on an ampulla on the image below

Innervation of the Internal Ear

Match the corresponding statements
  • Contains maculae
    Utricle and saccule
  • Contains cristae
    Ampullae
  • Arranged at 90 degrees to each other
    Semicircular ducts

Innervation of the Internal Ear

  • Vestibular nerve
  • Cochlear nerve
  • Facial nerve
  • Glossopharyngeal nerve
Which nerve transmits afferent signals from the maculae and cristae? 

Innervation of the Internal Ear

  • The macula of the utricle is orientated horizontally
  • Each hair cell of the macula has one stereocilium and many kinocilia
  • Kinocilia and stereocilia project into the cupula in the maculae
  • Deflection of the cilia towards the kinocilia depolarises the hair cell
Select whether these statements regarding the maculae are true or false: 

Innervation of the Internal Ear

  • The cristae are the components of the static labyrinth
  • Movement of the head anteriorly will universally depolarise the hair cells of the static labyrinth
  • The static labyrinth acts to sense the orientation of the head in response to linear movement
Select the correct statement concerning the static labyrinth

Innervation of the Internal Ear: Kinetic Labyrinth

The sensory regions of the kinetic labyrinth are the . The kinetic labyrinth acts to convey afferent signals regarding head position in response to  movements. The cilia of the hair cells in project into the , which deflects due to the flow of endolymph. When the cilia bend towards the kinocilium the hair cell and discharge in the vestibular nerve . When the cilia displace away from the kinocillium the hair cell  and discharge in the vestibular nerve .  Prolonged stimulation of the vestibular nerve triggers a motor response that inhibits transmission of afferent fibres, this process is called .

 

Innervation of the Internal Ear

  • Afferent signals from the vestibular system can result in contraction of the extra-ocular muscles
  • The vestibulo-ocular reflex moves the eyes in the same direction as head movement
Determine whether the following statements are true or false concerning the vestibulo-ocular reflex:

References

Reference List

 

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