Anatomy and Physiology of Balance
The vestibular system is part of the team which helps us maintain our balance. The team consists of the vestibular system, the visual system, the somatosensory system, along with the brain and nervous system. When one system malfunctions the two remaining systems may compensate for the third. Compensation can take some time to occur but until then, the body may have trouble balancing itself in space.
Vision greatly influences our balance regarding posture and points of visual reference for orientation. Change in visual stimuli sends a message to our central nervous system causing reflexes in muscles and tendons to adjust our body’s position. Have you ever been stopped or parked in the car when you see you are moving backwards and instantly press harder on the brake only to find it was just the car next to you moving forward? You can thank your eyes for that! Specifically, vision contributes by what we call the vestibular ocular reflex, saccadic system, smooth pursuit system, and the optokinetic reflex. Our visual system helps us to focus clearly, even when we are moving, via the vestibular ocular reflex or the VOR with eye movements that are equal but opposite to head movements. Therefore if the head moves right, the eyes move equally to the left. If you look close in the mirror at your eye, pick a point of reference such as a vein. Tilt your head and watch your eye stay where it is, pretty interesting! With slow movement, we use what is called the smooth pursuit system to create slow, smooth eye movements to follow an object. The saccade system allows the eye to acquire new objects. You can see this in use by watching someone’s eyes while they are spinning in a chair. Similarly, but more involuntary, is the optokinetic reflex which allows the eye to follow moving objects when the head is still such as when you attempt to look at every telephone pole as you drive along the highway. These reflexes are important to feed information to our brain about our surroundings.
Our proprioception system, part of the somatosensory system, is the internal sense of body movement as well as the relationships of body parts to one another. Sensors, or proprioceptors, in muscles, joints, and tendons measure joint movement, muscle length and force. These receptors sense movement triggering an automatic reflex for change in posture. There are two reflexes, one helps to sustain posture and the other aids in coordinates movement in order to maintain posture.
The vestibular system contains two types of sensory organs within the inner ear known as the cochlea, the otoliths and semicircular canals. The otoliths are the portion responsible for maintaining position with respect to gravity and linear acceleration. There are two otoliths, the utricle which is mostly horizontal, and the saccule which is mainly vertical. The otoliths consist of sensory cells, called the macula, with little cilia that stick up into a gelatinous mass containing carbonate crystals, similar to fruit in jello. When you move a plate with jello on it the mass moves in that direction and vice versa. The same thing occurs with the otolith organs, when we tilt our head forward the mass moves in that direction which is detected by the cells and the message is sent to our brain.
The three semicircular canals respond to angular motion of the head. Semicircular canals are interconnected, fluid filled tubes named the anterior, posterior and horizontal canals. These canals account for horizontal head movement: the yaw (shaking head no motion), as well as the two vertical head movements: pitch (nodding head in a yes motion) and the roll (ear-to-shoulder to ear-to-shoulder). Each canal has their own sensory cell, crista ampullaris, embedded in a gelatinous fan-like structure called the cupula. When we move our head in one direction the fluid moves in the opposite direction which pushes on the cupula. This is
detected by the cells and a message is sent to the brain. The interaction between the two ears is critical. When we move our head, one organ is stimulated while the other organ is suppressed, so there is an agreement between the two cochleae. With this information the brain then sends a message to the muscles and tendons in order to maintain balance.
As stated previously, these systems work together in order to keep the body’s center of gravity over the base of support to maintain balance. A disorder in any one of the systems can cause problems with balance. Peripheral (cochlea) or central (brain stem, cerebellum, brain) damage to the vestibular system can lead to a severe sense of imbalance until compensation occurs. If you feel you are having a problem with balance you should seek help from an otolaryngologist and audiologist.