Functional anatomy of the balance function of the inner ear

Welcome! This learning tool is developed to bring you an understanding of the functional anatomy of the balance function of the inner ear.

Instructions:

1. Click "start" on one of the modules on the home page and navigate your way through them by scrolling down each page and clicking "next" when it appears.

2. To go back a page just click the arrow beside the "page numbers e.g. 1/6" at the upper right hand side of the page.

3. Answer true or false, drag and drop, multiple choice, fill in the blanks and many other types of questions along the way!

4. Look out for the:picture and attempt what is asked - it will help your understanding!

5. You can stop at any time by clicking the "take a break" icon in the top right corner or go back to the home page by clicking the "home" icon in the top left corner.

By the end of this learning tool you should:

When you are ready, click "Start course" at the top right hand side of this page and remember to HAVE FUN!

1. Basic anatomy of the internal ear

What is the internal ear?

The internal ear is the innermost compartment of the ear and is contained within the petrous temporal bone of the skull. The internal ear contains the vestibulocochlear organs that are responsible for the reception of sound and maintenance of balance.

The internal ear consists of the sacs and ducts of the endolymph fluid-filled membranous labyrinth (fig. 1).

The membranous labyrinth is suspended by delicate filaments in a perilymph-filled bony labyrinth.

These fluids are involved in stimulating the end organs of hearing and balance.

 

 

(Gray, 1995; Moore et al., 2009)

The bony labyrinth

The bony labyrinth is a series of cavities contained within the otic capsule of the petrous part of the temporal bone. It has 3 main regions:

  • cochlea
  • vestibule of the bony labyrinth
  • semicircular canals

Figure 2 depicts the otic capsule. The otic capsule is made of bone denser than petrous temporal bone. It is often wrongly illustrated and identified as the bony labyrinth.

The bony labyrinth is in fact the FLUID-FILLED SPACE that is surrounded by the otic capsule.

Cochlea

The cochlea is the shell shaped component of the bony labyrinth and contains the organs concerned with hearing.

It contains the cochlear duct and features a round window (fig. 2) that is closed off by a secondary tympanic membrane.

Vestibule

The vestibule is a small oval chamber that contains the utricle and the saccule  (fig. 3) and other parts of the balancing apparatus (otherwise known as the vestibular labyrinth).

It features an oval window on the lateral wall.

The vestibule is continuous with the cochlea anteriorly, semi-circular canals posteriorly and the posterior cranial fossa by the vestibular aqueduct which transmits the endolymphatic duct and expands into the endolymphatic sac (fig. 3).

Semicircular canals

There are 3 semicircular canals that lie at right angles to one another: anterior (superior), posterior and lateral (horizontal) (fig. 3). Within the semicircular canal is a semicircular duct.

The semicircular canals open into the vestibule of the bony labyrinth at the ampullae (which are just expansions of the bone or duct).

 

 

(Gray, 1995; Moore et al., 2009)

The membranous labyrinth

The membranous labyrinth consists of a series of communicating sacs and ducts suspended in the bony labyrinth.

It has 2 divisions:

  • vestibular labyrinth
  • cochlear labyrinth

This course will only consider the vestibular labyrinth as this is the component of the inner ear responsible for maintaining balance.

The vestibular labyrinth, otherwise known as the vestibular apparatus is composed of the:

  • utricle
  • saccule
  • 3 semicircular ducts

Utricle

The utricle communicates with the saccule through the utricosaccular duct, from which the endolymphatic duct arises (fig. 4).

Saccule

The saccule is continuous with the cochlear duct through the ductus reuniens (fig. 4).

Semicircular ducts

The semicircular ducts open into the utricle through five openings that are reflective of the way the surrounding semicircular canals open into the vestibule.

(Gray, 1995)

 

In the walls of the membranous labyrinth, five areas of sensory epithelium upon which terminal fibres of the vestibular division of the vestibulocochlear nerve (cranial nerve VIII) exist (fig. 5).

  • Maculae
    • The macula of the utricle is in the floor of the utricle.
    • The macula of the saccule is vertically placed on the medial wall of the saccule.
  • Cristae ( within the ampullae walls near the utricular openings of each semicircular duct)
    • Each semicircular duct has an ampullary crest - the hair cells within these, like the maculae, stimulate primary sensory neurones whose cell bodies are in the vestibular ganglion.

Hover your mouse over the information icons ("i")  in figure 5 for more detail!

(Gray, 1995)

Test your understanding! Fill in the blanks.

The , and make up the vestibular apparatus.

The  is on the lateral wall of the vestibule of the bony labyrinth.

The is the fluid filled space that is immediately surrounded by the otic capsule.

There are two each located within the saccule and utricle.

There are three areas of specialised epithelium within the semicircular ducts named .

The saccule and the cochlear duct are continuous by the .

Test your reasoning! True or false?

  • The membranous semicircular ducts contain perilymph.
  • The cochlea contains the organs for the balance function.
  • The membranous labyrinth is contained within the bony labyrinth.
  • The ampullae are expansions of the semicircular canals that open to the saccule.
  • The bony labyrinth consists of the cochlea, semicircular canals and otic capsule.

Test your knowledge! Label the diagram of the right vestibular system:

  • Cochlea
  • Cochlear duct
  • Utricle
  • Saccule
  • Oval window
  • Round window
  • Endolymphatic sac
  • Endolymphatic duct
  • Ampulla of semicircular duct
  • Ampulla of semicircular canal

2. How does the inner ear control balance?

The vestibular system

The vestibular system senses acceleration and gives rise to our sense of balance.

RECAP:

The vestibular system consists of the utricle, saccule and 3 semicircular ducts!

The utricle and saccule are otolith organs that contain  hair cells, which are attached to calcium carbonate crystals called otoconia (fig. 6). The hair cells are embedded in an overlying gelatinous matrix.

 

 

 

 

 

 

 

 

 

 

At the base of the ampulla of the semicircular ducts, innervated hair cells of the crista project mechanosensing stereocilia into an overlying gelatinous cupula that is displaced in response to head movement by the fluid in the ducts (fig. 7).

Together the inner ear organs signal to the central nervous system and with visual input and proprioception allow an individual to be aware of their  orientation in space.

(Armstrong et al., 2015)

Mechanical transduction in the hair cells

Stereocilia emerge from the superior aspect of hair cells.

The stereocilia increase in length along a consistent axis to the longest kinocilium. Small connecting threads exist that travel from the superior part of one cilium to an ion channel on the lateral aspect of the cilium beside it.

The connecting threads function like a string attached to a hinged hatch.

When the cilia are bent towards the kinocilium, the ion channels open to allow an influx of potassium ions (K+) which depolarise the cell and open calcium ion channels (Ca2+) resulting in the release of neurotransmitters from the inferior aspect of the hair cell (fig. 8). This elicits an action potential in the dendrite of the vestibulocochlear nerve.

This mechanism converts mechanical energy into neural impulses.

 

 

(Meredith et al., 2016)

Function of the semicircular ducts in balance

The semicircular ducts detect angular acceleration.

The ability of the head to pivot on the neck subjects it to angular velocity and acceleration forces.

The 3 semicircular ducts and canals lie on 3 orthogonal planes which match up to an x, y and z axis (fig.9).

Rotation along a plane as the head moves on the neck results in angular acceleration forces working upon the fluid in the ducts. The endolymph movement lags relative to the head movement and causes the cupula to deviate and thus bend the inner sensory hair cells. 

The bending of hair cells results in the modulation of neurotransmitter release, initiating  action potentials in the associated afferent nerve fibres.

The activity rate of these fibres encode the strength of the acceleration: the stronger the acceleration, the more activity arises in the afferent fibres.

The semicircular ducts work in pairs to detect head movements. Movement in one direction will excite the receptors in the ampulla on one side of the head and inhibit receptors on the other side of the head - this is commonly known as the "push-pull phenomenon".  

(Highstein et al., 2005)

Function of the otolith organs in balance

The otolith organs detect linear acceleration and head positioning

The head and body are subjected to linear forces including tilt and gravity.

The otolith organs lie at right angles to each other so in any position gravity has an effect on the otoconia. The otoconia pull and bend the cilia they are attached to and initiate activity  through the vestibulocochlear nerve (fig. 10).

The utricle is most sensitive to tilt when the head is verticle.

 The saccule is most sensitive to tilt when head is horizontal.

Since the otolith organs are attached to the skull, head acceleration, such as moving from standing to lying, moves the otoconia because they are heavier than the surrounding gelatinous matrix. The otoconia lag behind the head movement  because of inertia forces acting upon them. The relative movement of the otoconia bends the hair cells and initiates the transduction of linear acceleration.

 

(Armstrong et al., 2015; Highstein et al., 2012)

Test your knowledge! The otolith organs consist of the:

  • utricle
  • ampulla
  • semicircular ducts
  • saccule
  • cochlea

Test your understanding! Describe the events that occur once the stereocilia bend towards the kinocilium.

Fill in the blanks:

The semicircular ducts detect acceleration.

The  are sensitive to tilt and gravity.

The is the tallest cilia.

3. Did someone say SCIENCE?

Semicircular canal evolution and human bipedalism

Day and Fitzpatrick (2005)

Day and Fitzpatrick (2005) deduced a rule of thumb that is consistent across various land mammals including birds and prosimians. The radius of the curvature of the semicircular canal arcs are generally related to the size of the animal. Therefore:

the bigger the animal, the larger the semicircular canals will be

This has a functional significance:

Larger semicircular canals are thought to be more sensitive than smaller canals. This is because of a greater diameter of canal available for an increased volume of fluid to exert a higher pressure on the hair cells. As a result, heavier and bulkier animals preserve their rotational sensitivity despite their slower movements.

Spoor et al. (1994)

There is strong evidence to support long-term changes in the semicircular canals and relate them to human bipedalism.

Spoor et al. (1994) examined the vestibular apparatus of living primates and early hominids using high resolution computed tomography (CT) to determine the locomotive behaviour of species  based on their bony labyrinth. This paper disagrees with the general canal-radius to body-mass rule by Day and Fitzpatrick (2005). In humans, the anterior and posterior semicircular canals are larger than the horizontal canal whereas in primates and early hominids, all three canals are of similar size. 

Now what is the functional significance of this?

The anterior and posterior canals are set in such a manner to detect vertical acceleration changes which are important in regulating balance in a vertical orientation. Fossils of early primates that were thought to be obligatory bipedal species (like Homo erectus) show  semicircular patterns resembling that of the modern Homo sapiens, whereas the skulls of non-obligatory bipedal species like the Australopithecus africanus, show canals similar to the existing non-human primates. Spoor et al. (1994) suggested that this adaptation of larger vertical canals accompanied the evolution of bipedalism and highlighted the significance of the anterior and posterior canals in particular, in bipedal balance.

BE CAREFUL!

The bipedalism of the Australopithecus africanus species was thought to be as a result of a postural component absent of complex movements such as running and jumping, therefore mainly affecting the utricle and saccule and not the semicircular canals as suggested by Spoor et al. (1994).

Also, another species involved in the study (Homo habilis) had semicircular canal proportions not like any other fossil or surviving hominid thus any functional interpretation was only speculative and not factual.

Spoor et al. (1994) mentioned that  large sample sizes were available because the petrous pyramids containing an intact labyrinth are common in hominid fossil collections, but they only used 42 samples in their own study to draw population sized conclusions.

 

The role of the vestibular system

The vestibular system is successful in maintaining head and eye coordination, controlling balance, sustaining a vertical posture and producing an awareness of spatial orientation.

There is a greater understanding of the semicircular canals and their function than the otolith organs and thus reflexes associated with the semicircular canals are used more prominently in assessment of function (Armstrong et al., 2015).

Vestibulocollic Reflex

(Wilson et al., 1995)

The vestibulocollic reflex is a reflex that adjusts the orientation of the head when it diverts from a normal vertical position.

The vestibular system initiates the reflex by sensing linear acceleration or a gravitational force through the otolith organs or angular acceleration through the semicircular ducts and taking steps to stabilise the head back into an appropriate position.

This reflex is functionally important for motor function and maintaining balance whilst walking, sitting and standing!

Vestibuloocular Reflex

(Spoor, 2003)

The vestibuloocular reflex is an important reflex stimulated by the vestibular system that functions to stabilise an image on the retina as the head moves by producing eye movements in the opposite direction  of head movement (fig. 11).

Why is this functionally important?

Preventing blurring of vision during movement is important in animals that rely on visual clues to manoeuvre through their surroundings (like birds and primates) and also for walking and even reading in humans!

A quick correction of vision related to head movement is required or else we end up with an image equivalent to that of a photograph taken with an unsteady hand!

Vestibulospinal Reflex

(Spoor, 2003)

The vestibular system is associated with the vestibulospinal tract located in the central nervous system. This reflex is dedicated to maintaining upright posture and head stabilisation by activating spinal muscles to counteract unstable body movements. It is thought that balance corrections are modulated by vestibular input but triggered by proprioceptive signals.

How is this functionally significant?

  • The cat righting reflex is partly due to the vestibulospinal reflex which allows them to orientate themselves to land on their feet after a fall from a height!
  • Newborn babies need this reflex to allow them to master head and neck control and contend with gravity.

Give it a go!

 

Read the following sentence whilst turning your head to the right or left and notice what happens to your eyes:

As I am reading this sentence and turning my head, I am able to continue reading this sentence because my eyes are moving in the opposite direction of my head.

That was your vestibuloocular reflex at work!

If an individual is falling asleep whilst sitting upright in a chair and their head begins to fall forwards, what reflex is put to work to correct this and move the head back into an appropriate position?

  • Vestibuloocular reflex
  • Vestibulocollic reflex
  • Vestibulospinal reflex

Give it a go!

What happens when the vestibular system works on its own?

1. Sit on an office chair that has wheels and raise your feet off the ground.

2. Close your eyes (to remove visual input) and swivel  the chair to the left or right (or ask someone to turn you).

3. Open your eyes!

4. You should've had a sensation of turning in the same direction as the chair despite your body still facing forwards and not physically turning to the left or right

That was your vestibular system working ALONE to give you a degree of spatial awareness and balance.

 

 

4. Summary

What have you learned?

So now you should be a vestibular system expert!

We have completed all our learning outcomes:

  • Know the basic anatomy of the inner ear
  • Know the action of the different components of the inner ear that are related to balance
  • Understand how the vestibular components of inner ear function to detect spatial orientation

We have learned that the otolith organs (the utricle and saccule) detect linear acceleration and head movement and are most sensitive to tilt and gravity. They cause signals to be sent to the brain to analyse our body movements.

We know that the semicircular ducts are most important in angular rotation and through inertia of fluid, the cupula deflects and bends the hair cells fixed inside it. This open channels to cause a chain reaction of ions that ultimately send action potentials through the vestibulocochlear nerve to give the brain information on our body's orientation.

Further research is required to fully understand the actions that occur in the ampulla and otolith organs but it is important to remember that the vestibular apparatus does not work alone but in tandem with visual and proprioceptive inputs to maintain balance!

References

Armstrong, P.A., Wood, S.J., Shimizu, N., Kuster, K., Perachio, A. and Makishima, T. (2015) 'Preserved otolith organ function in caspase-3-deficient mice with impaired horizontal semicircular canal function'. Experimental Brain Research. 233(6): pp.1825-1835

Day, B.L. and Fitzpatrick, R.C. (2005) 'The vestibular system'. Current Biology. 15(15): pp.R583-R586

Gray, H. (1995) ‘Gray’s Anatomy’. 39th ed. New York: Churchill Livingstone

Highstein, S.M. and Holstein, G.R. (2012) 'The anatomical and physiological framework for vestibular prosthesis'. Anatomical Record. 295(11): pp.2000-2009

Highstein, S.M., Rabbitt, R.D., Holstein, G.R. and Boyle, R.D. (2005) 'Determinants of spatial and temporal coding by semicircular canal afferents'. Journal of Neurophysiology. 93(5): pp.2359-2370 

Meredith, F.L. and Rennie, K.J. (2016) 'Channeling your inner ear potassium: K+ channels in vestibular hair cells'. Hearing Research. 2016: pp.1-12

Moore, K.L., Dalley, A.F. and Agur, A.M.R. (2009) 'Clinically oriented anatomy'. 6th ed. Philadelphia: Lippincott, Williams and Wilkins

Spoor, F. (2003) 'The semicircular canal system and locomotor behaviour, with special reference to hominin evolution'. Courier Forschungsinstitut Senckenberg. 243:pp.93-104

Spoor, F., Wood, B. and Zonneveld, F. (1994) 'Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion'. Nature. 369(6482): pp.645-648

Wilson, V.J., Boyle, R., Fukushima, K., Rose, P.K., Shinoda, Y., Sugiuchi, Y. and Uchino, Y. (1995) 'The vestibulocollic reflex'. Journal of Vestibular Research. 5(3): pp.147-170