It should be uncontroversial that we have a sixth sense. In fact, we have more than six senses. They are not connections to spiritual realms, though. It simply turns out that hearing, sight, smell, touch and taste don’t map to five different sensory organs. The sense of touch, for instance, is really a composite of different sensory neurons for temperature, pressure, texture, pain, and so on. In addition to this, we have specialized cells that help us maintain balance and keep track of where our body parts are in relation to each other. The most ingenious of these, I think, are located in the inner ear. The structure called the labyrinth is composed of the cochlea (hearing organ), three semicircular canals that give us a sense of three-dimensional orientation, and two otholithic organs, the utricle and saccule, which sense acceleration.
The semicircular canals are roughly orthogonal and filled with a fluid called endolymph, and “hairy” sensory cells. As we move our head in different directions, the endolymph moves within the canals, and when it rushes past the “hairs” of the motion sensors, we register movement in the horizontal or vertical plane, depending on which canal the movement is in.
The utricle and saccule work similarly, although they sense acceleration:
The otolith organs sense gravity and linear acceleration such as from due to initiation of movement in a straight line. Persons or animals without otolith organs are imbalanced. The schematic diagram above illustrates how they work. A set of hair cells are coupled to a mass of stones. When the stones accelerate, with respect to the hairs, they exert a shearing force on the hairs. This force is detected by the hair cells and sent to the brain via branches of the vestibular nerve. The utricle sends input to the brain via the superior division of the nerve, and the saccule, via the inferior division. There is considerably more complexity to the organization of the utricle and saccule, including different types of hair cells and detail to the sensory macule (patch of sensory cells) that we have omitted.
The otolithic organs sense motion according to their orientation. The utricle is largely horizontal in the head, and largely registers accelerations acting in the horizontal plane of the head (called the axial plane by radiologists). The saccule is largely vertical, actually parasagittal, in the head, and registers accelerations in the vertical plane (called parasaggital or coronal plane).
This is the sense in which our balance is located in the ear, although sight and stretch sensors in the muscles also contribute to our sense of where we are and where we’re moving.
dailymeh posted this on August 4, 2010
