Beginning in grade school, most everyone is taught about the five senses:
touch, sight, taste, smell and sound. These are responsible for our interaction
with the external world around us. Sensory nerves in our bodies pick up
on these outside stimuli and deliver the information to our brains for
processing, through our senses. However, we humans also possess several
senses that are responsible for our internal functioning. One of the most
important of these senses is called proprioception, or the sense of position.
It's defined as the conscious or unconscious awareness of joint position,
and it refers to being able to sense the position of your own limbs in
space, without seeing them move.
Indeed, proprioception allows us to recognize and control our limbs without looking at them, and it's such an integral part of human functioning that it's often taken for granted. We use this special sense every day, and without it some of the simplest tasks would become nearly, if not entirely, impossible. For example, let's look at the usually effortless task of walking. Most of us walk with our heads and eyes facing forward, watching what's ahead of us or looking at our surroundings. We don't find it necessary to look down and watch every step our feet make, and this is thanks to proprioception. Our body's internal senses already know how our feet are moving and the position they're in, without having to constantly watch them. If we did have to keep our eyes on our feet, it would make walking a very difficult task. The same concept is true of driving a car; we're able to keep our eyes on the road while our body accurately senses how our feet are pressing the gas and brake pedals and where our hands are positioned on the steering wheel.
No single internal organ provides us with proprioceptive ability, as is the case with other types of senses. Rather, it's a sense that's distributed throughout the nervous system. Inside our muscles and joints lie specialized sensory nerve endings (proprioceptors) that measure the amount of tension and degree of contraction, and it's this internally derived information that tells us where our limbs are in space. Typical proprioceptors in skeletal muscle are muscle spindles and Golgi tendon organs, which constantly transmit information on nerves traveling to the spinal cord and into the brain. With that information, the brain is able to calculate where the limbs must be at all times. For highly technical processes, the eyes serve as a backup mechanism to confirm this placement of limbs and movement.