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Overview of the bony and membranous structures of the labyrinth.
Hello everyone! This is Megan from Kenhub, and welcome to another anatomy tutorial. Today, we will be discussing the structures of the bony labyrinth of the inner ear.
Before we discuss in detail the anatomical features of the labyrinth, let me remind you of some important points about the structure. The labyrinth is the most internal part of the ear and, for this reason, we usually refer to it as the inner ear. The two other parts of the ear are the external ear which extends from the auricle to the eardrum and the middle ear which extends from the eardrum up to the oval window. All three parts of the ear – the external ear, the middle ear and the inner ear – are designed in such a way as to act as a transducer that converts the mechanical energy of the incoming sound wave into neuronal impulses that can be interpreted as sound. In addition, the inner ear also plays pivotal roles in maintaining postural balance and visual focus on a single object. For the purpose of this tutorial, we will be focusing on the labyrinth and we will look at the different anatomical landmarks of this part of the ear.
The labyrinth is located in the petrous part of the temporal bone and consists of a series of interlinked cavities that can be divided into three general parts. First, we have the cochlear part, which is the auditory component and is concerned with hearing. Then we have the vestibular component, which deals with balance while stationary. And finally, we have the semicircular component, which regulates balance while in motion. So, in this image, we can see the cochlear component here, we can see the vestibular part here, and the semicircular part here.
So, now, let's look at the auditory component of the inner ear – the cochlea. The name cochlea comes from the Greek word for snail, cochlos, and as you can see, the structure is spirally coiled like the shell of a snail. This spiral-shaped, hollow structure makes up the auditory part of the inner ear. It is comprised of specialized structures including the scala vestibuli, the scala tympani, the cochlear duct, the membranes that separates the two scala and the cochlear duct from each other - Reissner's membrane and the basilar membrane, helicotrema, the organ of Corti, and specialized sensory hair cells of the organ of Corti.
In the next slides, we're going to explore these structures. The function of the cochlea is to propagate sound waves from the base which is situated near the middle ear and the oval window to the apex. The most important structure that we see inside the cochlea is the cochlear duct, also known as the scala media, and is seen here highlighted in green. The cochlear duct – as you can see in this image – has a central position in the cochlea and separates the cochlea into two canals – the scala tympani and the scala vestibuli. It is separated from the scala tympani by the basilar membrane and from the scala vestibuli by the Reissner's membrane. In the basilar membrane of the cochlear duct rests the spiral organ of Corti which projects into the cochlear duct and is a sensory organ for hearing.
The cochlear duct that we just looked at is significantly thinner than the cochlear canal and divides the canal into two chambers as we saw previously. The most apical of the two chambers is the vestibular duct or the scala vestibuli which is seen here highlighted in green. This perilymph-filled chamber is situated superior to the cochlear duct and borders on the oval window extending from the vestibule of the inner ear to the helicotrema. It functions to conduct sound vibrations to the cochlear duct and is separated from this duct by a double-layered epithelial membrane known as Reissner's membrane.
The second chamber that see inside the cochlear canal is the tympanic duct or the scala tympani. Like the scala vestibuli, it is also filled with perilymph and is located inferior to the cochlear duct and terminates at the round window. The tympanic duct is separated from the cochlear duct by the basilar membrane.
The two chambers of the cochlea that we've just described – the scala vestibuli and the scala tympani – communicate at the apex of the cochlea via a narrow slit known as the helicotrema. We can see the helicotrema more clearly in the next image.
The presence of this structure is a result of the termination of the cochlear duct before it reaches the apex of the cochlea. Hair cells situated near the helicotrema can best detect low frequency sound. At the very base of the cochlea, there is an opening called the round window which is the passage through which the cochlea communicates with the middle ear. This opening is situated at the end of the scala tympani below the oval window and is closed off from the middle ear by a secondary tympanic membrane known as the round window membrane. In the next illustration, we zoom in on this structure and again you can see that it's highlighted in green.
So far, we have covered the cochlear part of the labyrinth which, as we said in the beginning of the tutorial, is responsible for hearing. Now, it's time to move on and see the next part – the vestibule.
The vestibule is a hollow cavity located between the cochlea and the semicircular canal. It is situated medial to the tympanic cavity posterior to the cochlea and anterior to the semicircular canals. Taking a closer look at the vestibule, we see that it communicates with the middle ear via this opening seen here highlighted in green which is known as the window of the vestibule or the oval window. This membrane-covered opening is normally closed by the base of the stapes and the annular ligament.
The membranous parts of the vestibule are the saccule and the utricle. The saccule is a two- to three-millimeter pouch situated in the inner ear that contains sensory cells. Located near the scala vestibuli, this structure is responsible for gathering sensory innervation about linear movement in the vertical plane with respect to the movement of the head for spatial orientation of the body. Along with the utricle and the semicircular canals, the saccule is important for maintaining balance. The saccule that we just looked at is connected to the cochlear duct via a small duct seen here in green known as the canalis reuniens of Hensen. The canalis reuniens of Hensen is a small duct found in the inner ear that connects the lower part of the saccule to part of the cochlear duct near its vestibular extremity.
Like the saccule, the utricle is situated in the inner ear at the superior-posterior part of the vestibule and it is the second membranous sac of the vestibule. It is slightly larger than the saccule measuring approximately 2.5 to 3.5 millimeters in diameter and it communicates with the semicircular ducts. As one of the parts of the balancing apparatus of the inner ear, the utricle detects linear acceleration and the tilting of the head in the horizontal plane. It contains both stereocilia and kinocilia that detect these movements.
Now, let's look at the third component of the labyrinth – the semicircular component. This part of the labyrinth is composed of three semicircular bony structures that project from the vestibule in three different directions. Each of these three bony canals contains perilymph and encloses a membranous semicircular duct filled with endolymph. The first of the three semicircular canals that we'll look at is the one here highlighted in green – the anterior semicircular canal. The function of the vertically-orientated anterior semicircular canal is to detect rotations of the head in the sagittal plane. It is connected with the posterior semicircular canal via the common crus which we will be discussing later on in this tutorial.
Each of the three semicircular canals contains a membranous semicircular duct. The anterior semicircular duct is situated perpendicular to the temporal bone. At the end of each semicircular canal, there is a dilation of the canal which is called the bony ampulla. In the image on the right, you can see the anterior bony ampulla. Inside each bony ampulla, there is a corresponding membranous ampulla which contains a sensory organ for the angular acceleration and deceleration known as the crista ampullaris. So, in the right image, you can see the anterior membranous ampulla highlighted in green.
The lateral semicircular canal is the smallest of the three semicircular canals. It is situated in the horizontal direction and functions to detect rotations of the head in the transverse plane, for example, when we turn our heads looking to our left or looking to our right. Inside the lateral semicircular canal, there is a corresponding membranous duct called the lateral semicircular duct seen here highlighted in green.
In the next slide, we can see the lateral bony ampulla and, in the slide after that, we can see the lateral membranous ampulla highlighted in green. As I said previously, inside this ampulla there is the crista ampullaris which is the sensory organ for the angular acceleration and deceleration.
Finally, the posterior semicircular canal is the third semicircular canal of the inner ear. It is connected to the anterior semicircular canal via the common crus. Its function is to detect rotation of the head around the coronal plane or the anterior-posterior axis. Inside the posterior semicircular canal is the membranous posterior semicircular duct. This membranous duct that is enclosed by the posterior semicircular canal runs parallel to the longitudinal axis of the petrous part of the temporal bone. And, finally, we can see the posterior bony ampulla and the posterior membranous ampulla.
The ampullae of the three canals open into the vestibule independently. However, the non-ampullated ends at the anterior and posterior semicircular canals fuse to form the common bony crus. Therefore, we can say that the common bony crus is formed by the anterior semicircular canal and the posterior semicircular canal joining to form a common limb. This posteriorly situated limb can be seen here in our illustration highlighted in green.
Similarly, the anterior and posterior membranous ducts fuse to form a membranous crus known as the common membranous crus. The common membranous crus opens into the utricle of the inner ear. The non-ampullated end of the lateral semicircular canal is known as the simple bony crus which can be seen here highlighted in green. The simple bony crus opens into the wall of the vestibule.
Now that we've looked at the membranous and bony structures of the labyrinth, let's move on to look at the nerves that are located in this part of the inner ear. The cochlear nerve is the branch of the vestibulocochlear nerve that caries the auditory sensory information from the cochlea directly to the brain. The cochlear nerve enters the base of the cochlea and is connected with specific receptors of the spiral organ of Corti. The vestibular nerve is the other part of the vestibulocochlear nerve. It carries spacial information from the semicircular canals and the vestibule to the brain. The vestibular nerve particularly transmits sensory information from vestibular hair cells found on the utricle, saccule and the semicircular canals via the vestibular ganglion.
After branching off from the eighth cranial nerve, the vestibular division branches again giving off a superior and inferior division, each with their own vestibular ganglion. The vestibular ganglion is situated at the floor of the internal acoustic meatus. It contains bipolar neurons associated with the vestibular nerve whose peripheral processes form synapses with the hair cells of the vestibular sensory organ. The superior part of the vestibular ganglion supplies the utricle, the anterior part of the saccule, the anterior semicircular canal, and the lateral semicircular canal. Whereas, the inferior vestibular ganglion supplies the posterior semicircular canal and part of the saccule.
The anterior ampullary nerve is a branch of the vestibular nerve. This nerve arises from the superior part of the vestibular ganglion to supply the ampullary crest of the anterior semicircular canal. Similar to the anterior ampullary nerve, the lateral ampullary nerve arises from the superior vestibular ganglion to supply the ampullary crest of the lateral semicircular canal. The posterior ampullary nerve branches from the inferior vestibular ganglion to supply the ampullary crest of the posterior semicircular canal.
Another nerve that arises from the superior vestibular ganglion is the utricular nerve. This nerve – as you can see here – innervates the macula of the utricle. The saccular nerve – as you can see in this illustration – branches from the inferior vestibular ganglion to supply the macula of the saccule.
The final nerve that we will discuss in this tutorial is the facial nerve, cranial nerve VII. This nerve emerges from the brainstem and its motor and sensory parts pass together through the posterior cranial fossa and enter the petrous part of the temporal bone passing through the internal acoustic meatus on its way to the facial canal. This nerve does not innervate any structures in the inner ear but is located really close to the labyrinth. For more on the facial nerve, please check out our tutorial on the superficial nerves of the face and our tutorial on cranial nerves which can be found on our website.
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