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Proprioceptors are receptors located in your muscles, tendons, joints and the inner ear, which send signals to the brain regarding the body's position allowing proprioception. An example of a "popular" proprioceptor often mentioned by aircraft pilots, is the "seat of the pants".
Proprioceptors respond to stimuli generated by muscle movement and muscle tension. Signals generated by exteroceptors and proprioceptors are carried by sensory neurons or nerves and are called electrochemical signals. When a neuron receives such a signal, it sends it on to an adjacent neuron through a bridge called a synapse. A synapse "sparks" the impulse between neurons through electrical and chemical means. These sensory signals are processed by the brain and spinal cord, which then respond with motor signals that travel along motor nerves. Motor neurons, with their special fibres, carry these signals to muscles, which are instructed to either contract or relax.
In other words, these sensors present a picture to your brain as to where you are in space as external forces act on your body. For example, picture yourself sitting at a red traffic light in your car. The light changes to green and your foot presses the accelerator. As you accelerate away from the traffic light, you will "feel" yourself being pushed back in to the seat. That experience is transmitted to your brain via the proprioceptors, in particular, through the sensors in your backside and back. The brain interprets this information as an acceleration in the forward sense. If you now slam on the brakes to stop suddenly, you will find different proprioceptors at work. The deceleration will be felt through your hands and feet and your backside will now be trying to slide forward in the seat. This information is again presented to your brain and thus it interprets the deceleration taking place. In turn, the brain now signals the muscles in your arms and legs to contract and stop you from sliding forward in the seat. A similar sensation will take place when you turn a corner. If you turn left, your body will slide across the seat toward the right and vice versa for a turn to the right.
The downfall with our internal motion sensors is that once a constant speed or velocity is reached, these sensors stop reacting. Your brain now has to rely on visual cues until another movement takes place and the resultant force is felt. In motion simulation, when our internal motion sensors can no longer detect motion, a “washout” of the motion system may occur. A washout allows the motion platform occupant to think they are making a continuous movement when actually the motion has stopped. Since there are restrictions on the range of motion for any motion platform, there are some movements which it cannot physically complete. When the craft is turning around completely, for instance, the motion system completes the first part of the turn and then slides the platform back into the neutral platform position. The old position data is thus "washed out". In other words, washout is where the simulator actually returns to a central, home, or reference position in anticipation of the next movement. This movement back to neutral must occur without the occupant actually realising what is happening. This is an important aspect in motion simulators as the human feel sensations must be as close to real as possible.