Video: Vestibular sensations

Cowabunga! Ever wonder how your body knows when you're moving, tilting, or just standing still? As you shift your weight or turn your head, your body somehow keeps you balanced -- even without a wave to knock you off. That balancing act? It's all thanks to your inner ear’s hidden surfboard team, also known as the vestibular system. The different parts of our vestibular system sense every turn and tilt, keeping you steady as you navigate life's waves.

So let's take a deep dive into understanding our vestibular sensations.

Vestibular sensation, which helps us with our sense of balance and spatial orientation, is made possible by the vestibular system. This system is located in the inner ear and consists of two key components: the semicircular canals and the otolith organs. Together, these structures detect changes in head position and movement, enabling us to stay balanced and oriented in space.

In more technical terms, they help us maintain both static and dynamic equilibrium. So let's start by exploring static equilibrium first.

Static equilibrium deals with maintaining balance when we are stationary. For example, it helps us stay upright when we stand still with our head tilted. Critical for detecting static equilibrium are the otolith organs, which include the utricle and the saccule. These otolith organs are membranous cavities filled with a clear fluid called endolymph, located in the bony vestibule of the inner ear.

Lining the epithelium of the walls within each of the two otolith organs is a structure known as the macula. The macula contains sensory receptor cells called hair cells that detect head position relative to gravity, essential for maintaining static equilibrium.

Now the ends of these hair cells contain the stereocilia, which are hair-like extensions, and one much longer cilium called the kinocilium. These stereocilia and kinocilia lie within the otolithic membrane, a gelatinous layer with calcium carbonate crystals embedded within. These crystals are referred to as statoconia, or otoliths.

So how do these otolith organs sense head position relative to gravity?

When our head is tilted, the otolith crystals shift with the aid of gravity, pulling on the gelatinous layer which then bends the stereocilia of the hair cells beneath. This generates signals which are sent to the brain, allowing us to sense the position and orientation of our head.

Now it should be noted that information about the position of our head is constantly sent to the brain, whether we're moving or not. This is possible because the hair cells continuously release the neurotransmitter glutamate which generates action potentials in the vestibulocochlear nerve.

When the stereocilia are bent because of a change in head orientation, the amount of glutamate released changes depending on which direction the hair cells bend. If they bend towards the kinocilium, the hair cells depolarize and more glutamate is released, leading to an increase in signaling in the vestibular nerve. If they bend away from the kinocilium, the hair cells are hyperpolarized and less glutamate is released, reducing the activity of the vestibular nerve.

Now because the maculae in the left and right ears are paired, tilting your head in one direction will depolarize the hair cells in one ear and hyperpolarize the hair cells in the other. This allows the brain to sense in which direction the head is tilted.

Hair cells help identify head posture both in the saccule and utricle; however, there's a slight difference between what the two structures sense. In an upright position, the macula of the utricle is oriented horizontally with the stereocilia pointing vertically. This makes the hair cell sensitive to when the head is tilted sideways, or to the front or back. In the saccule, the macula is oriented vertically with the stereocilia pointing horizontally.

In terms of static equilibrium while in an upright position, the hair cells in the saccule detect the vertical position of the head relative to gravity. The saccule provides more information about head orientation when the head is not in an upright position; for example, when we lie down.

Now let's have a closer look at dynamic equilibrium.

Whereas static equilibrium dealt with maintaining balance while stationary, dynamic equilibrium refers to maintaining balance when the head or body is undergoing either rotational motions, such as when you shake your head side to side when saying no, or linear acceleration, such as when riding in a car or elevator.

To maintain dynamic equilibrium, our vestibular system measures both linear and angular acceleration.

As seen for static equilibrium, linear acceleration is detected by the utricle and saccule. When acceleration begins, the hair cells move with the rest of the body; however, both the otolithic membrane and otoliths lag behind due to inertia. This lag causes the stereocilia to bend in the opposite direction, changing the activity of the vestibular nerve.

When acceleration stops, the hair cells also stop with the body, but the otolithic membrane continues moving for a short period until it catches up to the hair cells. This bends the stereocilia in the opposite direction back to neutral, returning the activity in the vestibular nerve back to baseline levels, which your brain perceives as no movement. This explains why, if riding in a car with your eyes closed, you are able to sense the car taking off and braking, but you may find it harder to realize if the car is moving at all when it is being driven at a steady speed.

Angular acceleration, or rotational movement, is detected by the semicircular canals. There are three of these canals, each oriented in a different plane at approximately 90 degrees with each other. These differences in orientation allow us to detect head rotations in any direction.

The anterior, also known as the superior semicircular canal, detects rotations in the sagittal plane, such as when nodding your head up and down to say yes. The lateral, also known as the horizontal semicircular canal, detects rotations in the transverse plane, such as when shaking your head side to side to say no. The posterior, also known as the inferior semicircular canal, detects rotations in the coronal plane, such as when tilting your head toward your shoulder.

To understand how these canals function, let's take a look at their structure.

Found at the terminal end of each canal is a dilated structure called the ampulla. At the base of the ampulla is a sensory region known as the ampullary crest or crista ampullaris, which is akin to the maculae found in the otolith organs.

Similarly, the ampullary crest contains sensory hair cells that are embedded in a gelatinous structure called the cupula, while the canals are also filled with endolymph.

So, how do they function? Well, they function pretty much the same as the utricle and saccule. The main difference is how the head motion is transmitted to the cilia. When the head rotates, inertia causes the endolymph to lag behind relative to the motion of the semicircular canal. This lag creates a relative movement of the endolymph, bending the cupula and displacing the hair cells.

Depending on the direction of stereocilia deflection relative to the kinocilium, the hair cells either increase or decrease glutamate release, altering vestibular nerve activity and conveying information about the direction and speed of rotation. When rotation stops, the continued movement of the endolymph bends the cupula back in the opposite direction, signaling to the brain that the head is no longer rotating.

Now, let's break down the pathway of how information about head position and movement reaches the brain.

So we've seen that in both the otolith organs and the semicircular canals, hair cells act as sensory receptors. We know that when these cells bend, the amount of glutamate released changes, effectively turning physical movement into electrical signals. These nerve signals then travel along the vestibular nerve, a branch of the vestibulocochlear nerve, to reach the vestibular nuclei in the brainstem.

From the vestibular nuclei, information is sent to several areas of the central nervous system, including the thalamus and parietal cortex for conscious awareness, the cerebellum and the spinal cord for balance control and posture adjustments, and the extraocular muscles that control eye movement.

By integrating signals from this pathway with vision and proprioception, vestibular sensations help us walk, run, and even stand still without losing our balance. It's why we can turn our heads while keeping our eyes focused on one spot and why we can adjust posture in response to movement.

And that concludes our tutorial on the vestibular sensations. Check out our further study units and content on neurophysiology.