Maintaining your correct balance is often taken for granted. From the time we rise from bed to walking down busy roads or even transitioning from a straight-walking gait to a side-stepping one is of paramount interest. However, with an impaired sense of balance, life becomes very difficult on even the most fundamental levels — mundane tasks like walking, hand-eye coordination activities, and even sitting become incredibly difficult. Before we get into why nausea is caused by spinning, let’s first try to understand what balance is and how it is achieved.
Balance can be defined as the ability to maintain our body’s center of mass over its base of support. A properly functioning balance system allows humans to see clearly while moving, identify one’s spatial orientation concerning gravity, and also help maintain postural stability when one is performing our wide range of motions. Contrary to popular belief, there is no single system responsible for balance in human beings. It is instead controlled by a myriad of various sensory inputs from different parts of the body that are integrated to give us a holistic sense of balance. The different parts of the human body that are responsible for balance include vision, proprioception (touch), and the vestibular system (motion, equilibrium, spatial orientation). Along with these inputs is the respective integration of motor outputs to inputs, as mentioned earlier, which is responsible for giving us a complete sense of balance.
Maintaining balance, as stated above, involves the interpretation of different inputs by the brain. The three main organs responsible for gathering inputs for balance include the eyes, muscles, and joints (considered as one sensory input), as well as the vestibular system. When we consider vision, the primary sensory organ is the eyes, or more specifically, the retina. The retina contains structures called rods and cones. Rods are believed to be necessary for tuning vision in low-light conditions. Cones, on the other hand, help with providing color to our vision. When light hits our rods and cones, it tells the brain how one is oriented with one subject to another.
Proprioceptive inputs come from the skin, muscles and joints, and involve sensory receptors that are sensitive to stretching or pressure in that particular skin, muscle or bone. However, the primary junction points for proprioceptive inputs in the neck and ankle are especially important. Inputs from the neck indicate the direction in which the head is turned, while cues from the ankles indicate the body’s movement or sway relative to both the standing surface and the quality of the surface.
When it comes to an intuitive sense of balance, this input comes from the vestibular system. The vestibular system is responsible for motion, spatial and equilibrium inputs. The vestibular organs lie within the ear and consist of the utricle, saccule, and three semicircular canals. The utricle and saccule detect gravity (information in a vertical orientation) and linear movement. The semicircular canals, which detect rotational movement, are located at right angles to each other and are filled with a fluid called endolymph. When the head rotates in the direction sensed by a particular canal, the endolymphatic fluid within it lags because of inertia and exerts pressure against the canal’s sensory receptor. The receptor then sends impulses to the brain regarding the movement from the specific canal being stimulated. When the vestibular organs on both sides of the head are functioning properly, they send symmetrical impulses to the brain.
Reason for Spinning
Now that we understand how the body balances itself, we can move on to answering why one feels dizzy as a result of spinning. The main reason for disorientation or dizziness is due to the vestibular organs in the ear. The three semicircular canals are tightly packed together at right angles to one another, and each of these canals is responsible for giving your head a certain type of three-dimensional spatial awareness. The canals are filled with a fluid that sloshes around as you move. Your ears sense motion by detecting the way that tiny strands of hair lining the canals wave back and forth in this moving liquid, like water plants swaying in a river current. The strands, called hair cells, are suspended in a gelatinous substance called cupula, layered below a fluid called endolymph. When movement of the head occurs, the endolymph sloshes in one direction or the other through each canal, dragging the slower cupula with it and bending the embedded hair cells to and fro. The information about which way the hair cells are swaying at any given moment gets relayed to the brain via roughly 20,000 nerve fibers and is interpreted by the brain as a movement.
Thus, when you have stopped rotating, your semicircular canals have not, due to inertia. The endolymph keeps spinning, resisting change yet again. As the fluid continues to move, it once again deflects the cupula — this time in the direction in which you were spinning moments before. As the oozing cupula bends those hair cells, a signal of movement is transmitted to the brain. You sense that you’re moving, but you’re actually not, which is why you feel dizzy whenever you spin too fast!