Our brain needs information to determine which signals it must send to the muscles, and that information comes from the sensory organs. For staying upright, especially the vestibular or balance organ (for our sense of balance) and the tactile receptors (for our sense of touch) are important. Our eyes also play a role in staying upright, but in healthy persons this role is less important. The visual information that we collect using our eyes is essential for determining the direction in which we steer, and that topic will be discussed separately from staying upright.Our vestibular organ registers displacements and rotations of our head. This organ comes as a pair, with one member of the pair behind every ear. The vestibular organ is a part of the so-called internal ear, which is shown in the figure to the right: the vestibular organ is the set of tubes on top of the cochlea, our hearing organ. From the signal of the vestibular organ, the brain can determine whether our body is still in balance and, if not, in which direction a correction is required to regain balance. The lower the speed of the bicycle, the less stable it is, and the more the cyclist depends on his senses (among which the vestibular organ) to stay upright.
We know a lot about the function of the vestibular organ, and in this video this is explained in detail. This video not only covers the vestibular organ itself, but also the reflexes that originate from it. Here, you can find the content of this video as an animation.
The second sensory input that is important for staying upright comes from our sense of touch. In its broadest sense, our sense of touch involves both perception as a result of touching external objects, as well as the perception of our own body (also called “proprioception”). Proprioception pertains to information about the position of our joints and the tension of our muscles. We are rarely aware of our proprioception, but its importance is immediately obvious when we loose it, as is nicely demonstrated in this video:
The disease history of this patient teaches us two things:
- Without our sense of touch (which includes our proprioception) it is almost impossible to walk.
- If we loose our sense of touch, by extensive training we can learn to replace the lost information by information from another sensory organ (in the video, the eyes).
The enormous difficulty of walking without our sense of touch, teaches us that the brain must connect motor output (the signals to the muscles) and sensory input (vestibular and proprioceptive information). Here lies an important role for the cerebellum, the master controller and automator of our brain. The cerebellum is so important that it deserves a separate page.