Continuing the postural control series I’m going to talk a little bit about the influence of the vestibular system on balance and postural control. I recommend reading the earlier post on the topic which can be found here.
The Vestibular System:
The vestibular system is a complex structure located in inner ears on both sides. Its function is to detect angular and linear movements of the head. It comprises of three semicircular canals; anterior, posterior and horizontal canals (for detection of angular motion) angled approximately 90 degrees to each other (see the picture below), as well as two otolithic structures called saccule and utricle (for detection of linear translations of the head).
Consider a hula hoop filled with water and small hair cells sticking out into the water from its inner walls. As you turn the hula hoop around, the water will stay stationary (much like a pot filled with water; as you turn it around, the water stays still). While the fluid will be relatively stationary, the hair cells that are attached to the walls of the hula hoop will bend. This is what happens in the otolithic organs (pictured above) of the vestibular system.
The small hair cells sticking out from the canal walls function as a modified nerve cells. When the cells are bent in a certain direction they will generate action potentials (as discussed in the Postural Control Part I – Proprioception) that represent the positional change of the head. These action potentials are conveyed by approximately 15 000 nerve cells to the integration centres in the central nervous system (e.g. the brainstem and the cerebral cortex) where the sensation about the particular direction of movement is perceived, based on the combination of the hair cell activation from all 6 canals and 4 otolithic organs in total on both sides of the head. We’ve all done the good old spinning chair “trick” where the person is spun around many times and then asked to stand up and watched as he/she stumbles around. As the fluid in the canals gain momentum when the person is spinning in the chair, the fluid continues to bend the hair cells once the person stands up. This leads to sensation of movement and activation of certain muscle groups (as discussed below) that make the person steer and fall to one side…
Alongside with contributing to the perception of movement and position of head/body in space, the nerve cells from the integration centres form neurological connections with both the eye- and the intrinsic spinal muscles that control spinal movement. This ensures the smooth, simultaneous control of eyes, head and neck as we turn our head to fix our eyes to a new target in our environment.
In a nutshell, the vestibular system is a mechanism which allows our central nervous system to detect angular and linear translations of our body and head in space so that appropriate stabilising responses by the musculoskeletal system can be employed in order to maintain visual gaze and tracking, upright posture and balance.
Some of the implications of the above mechanisms:
- Benign Paroxysmal Positional Vertigo (BPPV): This is a common disorder that can triggered by a head trauma. It’s characterised by episodes of positional vertigo. The impact to the head can cause some of the calcium crystals in the utricle to dislodge and float freely in the fluid inside the vestibular system. As the person moves his/her head to a certain position the crystals can float to the semicircular canals and once the gravity pulls them down the weight of the crystals will cause distortion of the small hair cells and produce sense of movement and vertigo. The condition is simple to diagnose and treat by an experienced practitioner.
- Acute otitis media/vestibular neuronitis: The active infection/inflammation of the middle ear or the nerve which transmits these signals to the brain can sometimes cause “malfunctioning” of the subtle mechanics of the vestibular system and make the person feel dizzy.
How does this all come into play for a chiropractor then? An understanding of our inbuilt reflex systems will give us greater clarity to how the spine functions. In particular, a reflex system which becomes a core part of our understanding and assessment of our patients is the vestibulo-spinal reflex (VSR). The VSR is a purely involuntary system (meaning you don’t have any conscious control over it) which receives changes in sensory input from the vestibular system and translates it into activation of spinal muscles to make the necessary changes to spinal posture. A failing of this system will lead to repetitive strain of the spinal joint tissues and eventual injury.
Our aim as chiropractors is to enhance the spinal reflex system such as this one (among several others), and we can achieve that by using spinal adjustments, vestibular treatment manoeuvres when necessary and targeted vestibular rehabilitation exercises to increase the function of this unit.
Hope you enjoyed another entry on balance and posture. I have tried to simplify the topic trying to cover only the most relevant aspects of the vestibular system.