The human ear does not only provide human beings the sense of hearing (auditory Sense), but this structure is also responsible for a very important sense. This is the sense of balance and motion, which is called equilibrium. To simply put, the ear contains certain organs that are fluid filled. These organs detect the change in the morph of the fluid to determine the direction of gravity and also when we are in the state of motion the movement of this fluid is recorded by our brain and associated nervous structures to reach our desired destination without tripping here and there or hurting ourselves. This is also interesting to know that in addition to the fluid which is present in these structures there are also small stones that help in detection and stimulation regarding the motion, position, and posture of the body. These such organs are called otolith organs.
In short, the existence of such organs is the reason for human beings to be such efficient bipedal organisms. These otolith organs include Utricle, Saccule, and Semi-circular canals. These three structures in unity make our vestibular system, which is present inside the ear of vertebrates, and as a whole responsible for the sense of equilibrium. Both of these structures are connected to the brain or Central Nervous System through nerves. The efferent nerves provide them nerve impulses coming from the brain, and the afferent nerves are those which take the messages from the utricle and saccule and transfer them to the Central Nervous System. In this article, we will discuss in detail the basic biology, anatomy, physiology, development, and pathology of Utricle and Saccule.
Functional Difference between Utricle and Saccule
As briefed above that both these structures are the parts of the vestibular system which is responsible for the equilibrium of the body and the basic functioning of both structures is somewhat the same, but there is a little difference in how each of them perceives motion. The function of the utricle is more leaned towards detection and perception of horizontal motion whereas the saccule is more leaned towards the perception and detection of vertical motion. When they work simultaneously, they create the perception of angular motion which helps us feel and perceive the height and the distance you have covered on an escalator. It is interesting to know that people who rarely use an escalator, when using it, they feel weird because they are not used to the feeling of perception of angular motion which is created due to simultaneous working of utricle and saccule, which rarely happened in their lifetime. That is why some people feel dizzy or even confused when they use an escalator, they become unable to figure out how to step off it.
Anatomy and physiology of Utricle
The utricle, in comparison to morphology, is larger than the saccule. It has a distinct oblong shape. It is observed to be transversely compressed. The utricle holds the upper back location in the vestibule. It can also be seen contacting the elliptical recess, which is a structure perforated with holes and adjoins the vestibule. The utricle has a macula, which in anatomical terms is a spot that can be used to distinguish a structure from the tissues which are present in the surrounding or vicinity of the organ. The macula of the utricle can be observed as a thick spot on the inside surface or boundaries of the utricle. On this spot, the epithelium consists of hair cells that are responsible for the perception of gravity and motion across the latitude. The layer which gives the gelatinous appearance is associated with statoconia ( which is structurally calcium carbonate and generally referred to as otolith). These combined are called the otolithic membrane. And here the stereocilia ( these are sensory organelles of hair cells and associated with the sensation of motion) and kinocilium ( these are a special type of cilia present on the tip of hair cells). These organelles and structures play a key role in the perception of position and motion. For instance, when you move your head as in tilting, the cilia and hair cells move and also pull the layers causing motion in them in direction of gravity. Which makes you feel your head tilted. Inside the utricle the spot, which was referred to as macula, is present. It is situated on the bottom of the utricle horizontally. There are three layers that make up the macula. This patch is roughly 3mm in size and on it, hair-like structures are present, which are called hair cells. The hair cells are embedded in the bottom layer and act like mechanoreceptors with similar functioning organelles which are 40 to 70 in number, as discussed earlier these are called the stereocilia. On the middle layer, there are calcium carbonate granules which are characteristic of statoconia. Furthermore, on the tip of the hair cell, there is one prominent cilium which is called kinocilium. This structure is responsible for the polarization of the hair cell.
Due to the polarization of hair cells, the layer giving gelatinous appearance in which the hair cells are embedded increases the weight of the otolithic membrane due to the presence of calcium carbonate in the statoconium. The weight increase is due to the formation of calcium carbonate and protein conjugated granules due to the polarization of hair cells. Thus, the increased weight creates a distinct stimulation of gravitation pull on the layers by an increase in the inertia of hair cells. This causes prominent detection of motion and orientation of head which then helps in maintaining body posture and coordination.
Moreover, the hair cells also detect the degree of the angle it made along the horizontal axis to determine how much head is tilted and how much the orientation is disturbed from the normal. And the hair cells and the associated structures on it weigh less as compare to the otolithic membrane. Due to this difference, there comes a difference in inertia, and the hair cell stop but the otolithic layer continues its motion. This lag or pattern creates stimulation of perception of linear motion. Sometimes when a person wakes up or stands up after lying down after an extensive period, he might still feel like lying down or leaning backward due to delayed motion of the otolithic membrane compared to the hair cells, due to difference in the inertia.
Anatomy and physiology of Saccule
This is comparatively shorter in size structure while considering other parts of the vestibular system. Unlike the utricle, which is oblong, the saccule gives more of a globular shape. Considering its location, it seems to be found in the Spherical recess, closer to where the cochlea opens through the vestibular duct. It is interesting to know that even though the functions of utricle and saccule are quite similar, but they are neither attached nor having physical contact. Simply put, they do not have any direct communication. On the saccule, the macula is observed to be on the anterior part, as there is prominent thickening. The overall microanatomy of the saccule is similar to that of the utricle; however, the branches of the vestibulocochlear nerve can be observed in the form of saccular filaments.
Just like the utricle, there are hair cells present on the macula. The hair cells having ion-gated channels to bring about polarization to conjugate the calcium carbonate granules present on the statoconia with the protein present in the gelatinous layer in the form of glycoproteins. This produces calcium carbonate protein granules, which as described earlier, causes differential inertia. A canal emerges from the posterior of the saccule, this canal is called the endolymphatic duct. Further ahead, this duct is found to be contacting another duct which is called the utricosaccularis duct, which then leads to the end in a pouch-like structure called endolymphatic sac, by going along vestibuli aquaeductus. At the end of the sac, it makes indirect contact with the dura matter having a part of the temporal bone in between. The canalis reuniens of hensen is a short duct that arises from the bottom of the saccule.
The saccule is more oriented towards the detection of velocity in the linear plane, and the overall posture and orientation of the body in space. However, the utricle is more focused on the interpretation of head orientation to assist in sense of sight while the saccule helps to keep the overall body posture and also keeps track of the motion and position of the head. In short, saccule and utricle mostly enhance each other’s functioning by complementing some functions or reinforcing some functions. The saccule may not be a very well understood organ, but it is interesting to know the function it plays in some birds during their mating season. The saccule appears to be possessing a greater number of hair cells on the macula in the female birds when the mating season approaches. This increases their ability to hear the mating calls made by the male birds and also be more perceptive of the surroundings which help them in their survival.
During the stages of embryonic development, the external ear is formed by the ectodermal groove present between the first and second branchial arches. Meanwhile, the internal ear structures like the tympanic cavity are formed by the sac present between the first and second branchial arches. The sac is called the endodermal pouch and when it comes in contact with the ectodermal groove the formation of the eardrum takes place. The ectoderm from the hindbrain thickens and results in the internal ear’s epithelium and a plate-like structure is observable which lays the foundation of a sac-like structure that develops into the otocyst. The otocyst further develops into a snail shell-like shape which becomes the cochlear duct and the foundation of hearing organs. The middle portion of the otocyst forms two chambers like structures, which are called utricle and saccule. Being derived from the otocyst, which is an important developmental structure of the ear, the utricle and saccule are termed as otolith organs. In brief, the internal ears structures i.e vestibular system and associated organs, and cochlea are developed from the common structure which is called the otic vesicle. Evolutionary Biologists believe that these structures are not as per se the organs of necessity but the organs of convenience, as they assist in many ways that are pretty much ignorable yet important in daily life.
General Pathology and Diseases of Utricle and Saccule
This condition affects the utricle and saccule. It is characterized by the sudden deformation of both. It can occur by physical trauma or inflammation of the organs. This causes hypersensitivity of otolith organs and the patient losses the ability to stand as he cannot balance his body. This is accompanied by pain and severe ringing in the ear ( acute tinnitus). Usually, gentamicin is used, as it has a destructive effect on the internal ear structures.
This is caused due to viral infection in the inner ear. As the name suggests, there is inflammation of the nerves. Those nerves that act as a connection between the vestibular system and associated organs get damaged and this might result in an impaired connection of the brain with the utricle. As a result, the utricle loses its functionality and vertigo manifests along with the inability to focus the sight on an object.
Benign Paroxysmal Positional Vertigo
As described in the anatomical potion of the article, that the macula of the utricle and saccule has layers, and one of those statoconia, which contain calcium carbonate crystals. In some cases, these crystals get dislodged from the layer. BPPV is commonly observed in old people because the layers start to degenerate in old age and increase the risk of crystal dislodging. However, it can also occur in young one and adult due to nutritional deficiencies or most likely by physical trauma to the head. This leads to irritation as well as infection in the ear, but most importantly this causes imbalance, dizziness, and vertigo which lead to nausea. This happens due to faulty stimulation of the otolithic membrane.
- Saladin, Kenneth S. (2011). Anatomy & Physiology: The Unity of Form and Function. New York: McGraw-Hill. ISBN 978-0-07-337825-1. OCLC 799004854
- Boulpaep, Emile L.; Boron, Walter F. (2005). Medical physiology: a cellular and molecular approach. St. Louis, Mo: Elsevier Saunders. ISBN 978-1-4160-2328-9. OCLC 56963726