Can you learn to wiggle your ears
A maze for balance
Without it, we would be driving a carousel all the time: the sense of balance steers the body through space like a three-dimensional navigation device. Its center sits deep in our ears and signals muscles and joints: keep your posture!
Scientific supervision: Prof. Dr. Hans Straka
- The vestibular system is important for the perception of the position and movement of one's own body in space.
- Three semicircular canals form the sense of rotation, two atrial sacs are responsible for linear changes in speed (acceleration).
- The vestibular system cooperates with the visual system and the sensors made up of muscles, tendons and joints: They are both the source and addressee of information about the body's position in space.
- The interconnection with the eye muscles is particularly quick: the vestibulo-ocular reflex enables a stable image despite body movement.
- Disorders of the vestibular system lead to symptoms similar to "seasickness": mainly dizziness and nausea.
Children like to turn on their own axis as fast and as long as possible. If they then brake, they often feel dizzy. This is due to the fact that the endolymph in the inner ear - actually a sluggish liquid - in the semicircular canals eventually gets going and circles along with prolonged rotation. If the rotation ends abruptly, the endolymph continues to move for a short time. This gives the impression that the environment or you suddenly turn in the opposite direction - you get “seasick”.
The giant gondola swings back and forth in sweeping movements. It accelerates jerkily, brakes again, suddenly rears up in a loop and finally stands upside down at the highest point - for seemingly endless seconds. The inmates involuntarily screech. Blood rushes to the head and the world begins to turn around.
Anyone who has ever sat in the Wild Mouse, in the Flying Circus or in a comparable amusement ride, knows it: fear paired with happiness, heart pounding mixed with dizziness and nausea. When the machine finally comes to a standstill, you stagger out of the gondola like a drunk. The world turns a little further, head and body first have to find each other again. Such adventures demand maximum performance from the sense of balance. We usually only become aware that it exists when we put it to the test. Normally, it does its work completely unobtrusively and extremely reliably.
Sensitive in all directions
The center of the sense of balance, the so-called vestibular system, is present as part of the ear and thus on each side of the head. It is located in the inner ear, a complexly shaped cavity. The vestibular apparatus itself has five components: three semicircular canals and two atrial sacs, the so-called macular organs. While the macular organs detect linear accelerations, for example when driving a car or elevator, the semicircular canals are responsible for rotational movements such as shaking the head or nodding.
Like three spirit levels in a room
The three semicircular canals are channels surrounded by membranes. Inside there is a liquid, the endolymph. The corridors are at a 90-degree angle to each other: one horizontally in the head, one perpendicular to it at an angle to the front and the third at an angle to the back. / P>
Each semicircular canal has a bulge called an ampulla. Inside it, embedded in a cushion, sit hair cells: the sensory cells of the organ of equilibrium. Each one has numerous appendages that look like fine cilia. These so-called cilia are each of different lengths and protrude into a gelatinous membrane, the cupula, which is fused with the wall of the semicircular canal. The hair cells of an ampoule are each arranged in the same direction. As a result, they are aroused or inhibited together.
If the person turns his head to the left, the wall of the semicircular canal also moves to the left. However, the endolymph in it is sluggish and does not flow with it immediately. “It's like having a cup of coffee,” compares Stefan Glasauer from the Center for Somatosensory Research at the Neurological Department of the University of Munich: If you put this on a turntable, the cup will turn at first, but not the coffee itself. The result is a relative movement of the coffee in relation to the cup. In the case of the semicircular canals, this relative movement exerts pressure on the cupula and bends the cilia of the hair cells. For example, if you turn your head to the left, the cilia in the left horizontal semicircular canal curve to the right and are thus excited. In the opposite direction, they would be inhibited.
The macular organs - the ear stones show the perpendicular
In the case of the macular organs, motion detection works a little differently. The small atrial sac, the saccule, lies vertically in the skull and responds to corresponding accelerations, for example when you go up in an elevator. The large atrial sac, the utriculus, on the other hand, reacts to horizontal acceleration forces, i.e. forwards and backwards or sideways movements. The sacs also contain hair cells, the cilia of which protrude into a gelatinous mass: the otolith membrane.
This contains the smallest calcium carbonate crystals, the otoliths, also called statoliths or, colloquially, ear stones. If the person is now exposed to acceleration, for example when the bus driver really accelerates, the gelatinous mass starts moving thanks to gravity and the weight of the otoliths: This bends the fine cilia and thus stimulates the hair cells of the utricle.
Teamwork with eyes and deep sensitivity
What happens after the stimulation of the sensory cells takes place in a similar way in the macular organs and semicircular canals: The mechanical stimulus, i.e. the turning of the cilia, is translated into an electrical signal. Ion channels are located at the tips of the cilia that protrude into the endolymphatic space. When the head is not moving, a small part of them is open so that the inflow and outflow of positively charged potassium ions from the endolymph into the hair cells is balanced. Depending on the direction in which the cilia are bent, more ion channels open and more potassium ions flow into the cell: Depolarization occurs and the cell is excited. If, on the other hand, the ion channels are closed, fewer potassium ions flow into them and hyperpolarization occurs, which has an inhibiting effect. The mechanical signal is translated into a neurochemical one.
In the event of arousal, the hair cells release more of the neurotransmitter glutamate. In this way, the information about a certain body movement in space reaches the downstream nerve cell and further into the brain. The nerve fibers from the three semicircular canals and the two macular organs bundle to form what is known as the equilibrium nerve. It joins the auditory nerve in the inner ear to form the eighth cranial nerve (nervus vestibulo-cochlearis). The fibers from the stibular organs move on to the oldest part of the brain, the brain stem, where they contact nerve cells in the vestibular nuclei and in the cerebellum. This is where all information comes together that is necessary for balance and the perception of proper movement: that of the macular organs and semicircular canals of both hemispheres. In addition, there are signals from the eyes that signal large-scale movements in the environment, as well as those from the proprioceptive sensory system, i.e. the receptors from muscles, tendons and joints. The information from the various systems is linked and passed on to the eye, arm and leg muscles.
A sharp image in seven milliseconds
The connection to the cranial nerve nuclei, which control the eye muscles, is the fastest. The so-called “vestibulo-ocular reflex” only takes about seven milliseconds. It is one of the fastest reflexes in the central nervous system and enables us to continuously see a stable picture of the environment as we move our head. For example when jogging: “Actually, joggers should perceive their surroundings like a blurred photo,” explains sensorimotor researcher Glasauer. "Thanks to the vestibulo-ocular reflex, the eye compensates for the movement by moving in the opposite direction at the same speed." If the head sinks down, the eye is directed upwards.
The motor neurons in the spinal cord, which ultimately control the muscles of the legs, are also informed of the change in position. On the other hand, they steer, for example, if you stumble, stand on a swaying ship or the subway driver suddenly brakes and thus stabilize our upright position. However, due to the large number of leg muscles, the vestibulo-spinal reflexes are much less precise than the vestibulo-ocular reflexes. That's why we sometimes stumble.
Ultimately, the interaction of the vestibular, visual and proprioceptive sensory systems enables us to perceive our body movements in space, to orientate ourselves visually and to keep our balance even with complex movements such as gymnastics. The vestibular system plays a decisive role in this. Regardless of the direction in which the head moves: the hair cells are so sensitive to direction that they specifically encode all movements and convert them into neural signals. This results in an excitation pattern that the central nervous system can clearly interpret. It knows exactly in which position the head is in space and in which direction it is moving at any moment. This system is therefore also an indispensable prerequisite for the function of our sense of direction. Together with the place cells and place cells, the discovery of which was awarded the Nobel Prize in 2014. Searched and found: Orientation cells, form the equilibrium organ with its complex interconnections, so to speak, the GPS of the brain.
Prone to interference, but capable of rehabilitation
All these processes take place largely unconsciously in everyday life - unless the perception of balance is disturbed. For example, sometimes the information from the different sensory sources does not match. A deception occurs at the train station, for example, when you are sitting in a stationary train and the train begins to roll on the neighboring track. Then there is this brief moment of uncertainty as to whether your own train or the other train will leave. The visual and vestibular systems in particular contradict each other here: the eyes report movement, while the vestibular system reports standstill.
In some people, however, the vestibular apparatus itself is disturbed - for example with circulatory disorders or inflammation in the inner ear. Those affected complain of vertigo, like after a carousel ride, and can no longer stand upright. “Such patients also report that they feel like they are outside of their body,” says Stefan Glasauer. The awareness of the position and movement of one's own body in space suffers.
Fortunately, the vestibular system is highly adaptable: if, for example, an inner ear is damaged in an accident and hair cells are lost there, the sense of balance initially works poorly. "With such permanent damage, the brain gets used to the deficit and can use the remaining information to compensate for it so that the patient's equilibrium is permanently stabilized again," explains Glasauer. Compensating for such vestibular damage does not work so well in old people. Your sense of balance and the vestibulo-spinal reflexes that stabilize your posture are no longer as fast. This is one of the reasons that falls become more common with age.
for further reading:
Cullen KE. The vestibular system: multimodal integration and encoding of self-motion for motor control. Trends Neurosci. 2012 Mar; 35 (3): 185-96. (to the text)
Publication: on August 16, 2012
Updated: on December 09, 2016
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