A Labyrinth for Balance

The giant gondola swings back and forth in sweeping movements. It accelerates jerkily, brakes again, suddenly rears up into a loop and finally comes to a halt upside down at the highest point – for what seems like endless seconds. The passengers scream involuntarily. Blood rushes to their heads and the world around them begins to spin.

Anyone who has ever sat in the Break Dance, the Flying Fury, or a similar ride knows the feeling: fear mixed with happiness, your heart pounding, dizziness and nausea. When the machine finally comes to a stop, you stagger out of the gondola like a drunk. The world continues to spin a little longer, and your head and body have to find each other again. Adventures like this demand the utmost from your sense of balance. We usually only become aware of its existence when we put it to the test in this way. Normally, it does its job completely unobtrusively and with the utmost reliability.

The center of the sense of balance, the vestibular system, is part of the ear and is therefore present on both sides of the head. It is located in the inner ear, a complex cavity. The vestibular apparatus itself has five components: three semicircular canals and two vestibular sacs, known as the macula organs. While the macula organs detect linear acceleration, such as when traveling in a car or elevator, the semicircular canals are responsible for rotational movements such as shaking or nodding the head.

The three semicircular canals are channels surrounded by membranes. Inside them is a fluid called endolymph. The canals are at 90-degree angles to each other: one is horizontal in the head, one runs perpendicular to it at an angle toward the front, and the third runs at an angle toward the back.

Each semicircular canal has a bulge called the ampulla. Embedded in a cushion inside the ampulla are hair cells: the sensory cells of the vestibular system. Each hair cell has numerous extensions that look like fine cilia. These cilia vary in length and protrude into a gelatinous membrane called the cupula, which is fused with the wall of the semicircular canal. The hair cells in an ampulla are all arranged in the same direction. As a result, they are excited or inhibited together.

When a person turns their head to the left, the wall of the semicircular canal also moves to the left. However, the endolymph inside is sluggish and does not flow immediately. “It's like a cup of coffee,” compares Stefan Glasauer from the Center for Somatosensory Research at the Department of Neurology at the University of Munich: If you place it on a turntable, the cup otates with it, but the coffee itself does not. The result is a relative movement of the coffee relative 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, when you turn your head to the left, the cilia in the left horizontal semicircular canal bend to the right and are thus stimulated. In the opposite direction, they would be inhibited.

Motion detection works slightly differently in the macula organs. The small sacculus is located vertically in the skull and responds to corresponding accelerations, for example when traveling upward in an elevator. The large utriculus, on the other hand, reacts to horizontal acceleration forces, i.e., forward, backward, or even sideways movements. The sacs also contain hair cells whose cilia protrude into a gelatinous mass: the otolith membrane.

This contains tiny calcium carbonate crystals, the otoliths, or colloquially known as ear stones. When a person is exposed to acceleration, for example when a bus driver steps on the gas, gravity and the weight of the otoliths cause the gelatinous mass to move, bending the fine cilia and stimulating the hair cells of the utricle.

What happens after the sensory cells are stimulated is similar in the macula organs and semicircular canals: the mechanical stimulus, i.e., the bending of the cilia, is translated into an electrical signal. Ion channels are located at the tips of the cilia, which protrude into the endolymphatic space. When the head is still, a small portion of these channels are 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 stimulated. If, on the other hand, the ion channels are closed, fewer potassium ions flow in and hyperpolarization occurs, which has an inhibitory effect. The mechanical signal is thus translated into a neurochemical one.

In the event of excitation, the hair cells release increased amounts of the neurotransmitter glutamate. This is how information about a specific body movement in space reaches the downstream nerve cell and is transmitted to the brain. The nerve fibers from the three semicircular canals and the two macula organs bundle together to form the vestibular nerve. In the inner ear, it joins the auditory nerve to form the VIII cranial nerve (nervus vestibulo-cochlearis). The fibers from the vestibular organs continue 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 the information necessary for balance and perception of one's own movement comes together: that from the macula organs and semicircular canals of both hemispheres of the brain. This is joined by signals from the eyes, which 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 together and forwarded to the eye, arm, and leg muscles.

The fastest connection is to the cranial nerve nuclei that control the eye muscles. The so-called “vestibulo-ocular reflex” takes only about seven milliseconds. It is one of the fastest reflexes in the central nervous system and enables us to see a stable image of our surroundings continuously while we move our head. Take jogging, for example: “Joggers should actually perceive their environment as a blurred photo,” explains sensorimotor researcher Glasauer. “Thanks to the vestibulo-ocular reflex, however, the eye compensates for the movement by moving in the opposite direction at the same speed.” If the head drops down, the eye moves up.

The motor neurons in the spinal cord, which ultimately control the muscles in the legs, are also informed of the change in position. They counteract this when, for example, we stumble, stand on a swaying ship, or the subway driver suddenly brakes, thus stabilizing our upright position. However, due to the large number of leg muscles, vestibulo-spinal reflexes are much less accurate than vestibulo-ocular reflexes. This is why we sometimes stumble.

Ultimately, the interaction of the vestibular, visual, and proprioceptive sensory systems enables us to perceive our body movements in space, orient ourselves visually, and maintain our balance even during complex movements such as gymnastics. The vestibular system plays a crucial role in this. No matter which direction the head moves in, the hair cells are so directionally sensitive that they specifically encode all movements and convert them into neural signals. This results in a pattern of excitation that the central nervous system can clearly interpret. It knows exactly where the head is in space and in which direction it is moving at any given moment. This system is therefore also an indispensable prerequisite for the functioning of our sense of orientation. Together with the place cells and spatial cells, whose discovery was awarded the Nobel Prize in 2014 ▸ Sought and found: orientation cells, the vestibular system with its complex interconnections forms the GPS of the brain, so to speak.

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. This deception occurs at the train station, for example, when you are sitting in a stationary train and the train on the neighboring track starts to move. This creates a brief moment of uncertainty as to whether your own train or the other train is moving. In this case, the visual and vestibular systems in particular contradict each other: the eyes report movement, while the vestibular system reports stillness.

In some people, however, the vestibular apparatus itself is disturbed – for example, due to circulatory disorders or inflammation in the inner ear. Those affected complain of vertigo, like after a ride on a carousel, and can no longer stand upright. “Such patients also report that they feel as if they are outside their bodies,” says Stefan Glasauer. Their awareness of the position and movement of their own body in space suffers.

Fortunately, however, the vestibular system is highly adaptable: if, for example, the inner ear is damaged in an accident and hair cells are lost, the sense of balance initially functions poorly. “In the case of such permanent damage, the brain gets used to the deficit and can compensate for it with the remaining information in such a way that the patient's balance is permanently stabilized again,” explains Glasauer. However, compensating for such vestibular damage does not work as well in older people. Their sense of balance and the vestibulo-spinal reflexes that stabilize their posture are no longer as quick. This is one of the reasons why falls become more frequent with age.

First published on August 16, 2012
Last updated on October 17, 2025