One overlooked sense, known as proprioception, is as important, if not more important as the other senses, for normal functioning. Proprioception is "the process by which the body can vary muscle contraction in immediate response to incoming information regarding external forces," by utilizing stretch receptors in the muscles to keep track of the joint position in the body.
Proprioception and kinesthesia, the sensation of joint motion and acceleration, are the sensory feedback mechanisms for motor control and posture. Theses mechanisms along with the vestibular system, a fluid filled network within the inner ear that can feel the pull of gravity and helps the body keep oriented and balanced, are unconsciously utilized by the brain to provide a constant influx of sensory information. The brain can then send out immediate and unconscious adjustments to the muscles and joints in order to achieve movement and balance.
Proprioception, also often referred to as the sixth sense, was developed by the nervous system as a means to keep track of and control the different parts of the body. An example that enables one to best understand this sensory system is one showing what happens if this sensory system is no longer there. A man lost his sixth sense along with the ability to feel light touch when a virus killed the necessary nerves. The man still had all the nerves to control muscle movement but had no feedback from the outside world about where his limbs were except that obtained by sight. A normal person is able to move a finger, knowing where and what the finger is doing, with little effort. The normal person could just volunteer the finger to move back and forth and proprioception would make this an easy task. Without proprioception, the brain cannot feel what the finger is doing, and the process must be carried out in more conscious and calculated steps. The person must use vision to compensate for the lost feedback on the progress of the finger.
The proprioception sensory system is carried out utilizing proprioceptors in the muscles that monitor length, tension, pressure, and noxious stimuli. The muscle spindles, the most complex and studied of the proprioceptors, informs other neurons of the length of the muscle and the velocity of the stretch. The density of muscle spindles within a muscle increases for muscles involved in fine movements, as opposed to those involved in larger course movements. The brain needs input from many of these spindles in order to register changes in angle and position that the muscle has accomplished. There is also more spindles found in the arms and legs, muscles that must maintain posture against gravity.
There are also proprioceptors sending information to the nervous system from joints and ligaments. Depending on the amount, where in the body, and from what proprioceptors the different input is coming from, determines if the information will be made conscious or processed unconsciously. All the input coming into the nervous system is processed, and then depending on the state of the muscle, there are commands sent back to the muscle.
Proprioception is a primary building block in the human's ability to learn, repeat, and become comfortable with so many movements throughout life.
With the sense of proprioception our lives are made less complicated, and we are given the ability to learn and utilize many movement patterns freeing us to focus awareness on new tasks and thoughts.
