Proprioception (from Latin proprius, meaning "one's own") is the sense of the position of parts of the body, relative to other neighbouring parts of the body. Unlike the five exteroception human senses of sight, taste, smell, touch, and hearing, that advise us of the outside world, proprioception is a sense that provides feedback solely on the status of the body internally. It is the sense that indicates whether or not your body is moving with required effort as where the various parts of the body are located in relation to each other.
Kinesthesia is another term that is often used interchangeably with proprioception. Some users differentiate the kinesthetic sense from proprioception by excluding the sense of equilibrium or balance from kinethesia. An inner ear infection, for example, might impact the sense of balance. This would impact the proprioceptive sense, but not the kinesthetic sense. The infected person would be able to walk, but only by using their sense of sight to maintain balance; they would be unable to walk with their eyes closed.
The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the joints and muscles (stance). There are specific nerve receptors for this form of perception, just like there are specific receptors for pressure, light/dark, temperature, sound, and other sensory experiences.
Proprioception is tested by police officers using the field sobriety test where the subject is required to touch his nose with his eyes closed. People with normal proprioception may make an error of no more than 2 cm. People with severely impaired proprioception may have no clue as to where their hands (or noses) are without looking.
Proprioception is what allows someone to learn to walk in complete darkness without bumping into the furniture. During the learning of any new skill, sport, or art, it is usually necessary to become familiar with some proprioceptive concerns specific to that activity. Without the appropriate integration of proprioceptive input, an artist would not be able to brush paint onto a canvas without looking at the hand as it moved the brush over the canvas; it would be impossible to drive an automobile because a motorist would not be able to steer or use the foot pedals while looking at the road ahead; we could not touch type or perform ballet; and one would not even be able to walk without literally "watching where you put your feet".
The proprioceptive sense can be sharpened through study of many disciplines. The Alexander Technique and related methods use the study of mannerisms to directly enhance kinesthetic judgment of effort and location. Juggling trains reaction time and spatial location.
Oliver Sacks once reported the case of a young woman who lost her proprioception due to a viral infection of her spinal cord. At first she was not able to move properly at all. Later she relearned by using her sight (watching her feet) and vestibulum (or inner ear) only. She eventually acquired a stiff and slow movement, which is believed to be the best possible in the absence of this sense.
Apparently, temporary loss or impairment of proprioception may happen periodically during growth, mostly during adolescence. Possible experiences include: suddenly feeling that feet or legs are missing from your mental self-image; the need to look down at arms, hands, legs, etc. to convince yourself that they are still there; falling down while walking, especially when attention is focused upon something other than the act of walking (e.g., looking at a person who started talking or reading a billboard).
The proprioceptive sense can become confused because humans will adapt to a continuously-present stimulus; this is called habituation or desensitization. The effect is that it seems as though proprioceptive sensory impressions disappear, just as a scent seems to disappear when a person smells it for a prolonged period of time. One practical advantage of this is that unnoticed actions or sensation continue in the background while an individual's attention can move to another concern. Alexander Technique addresses these issues.
People who have a limb amputated may still have a sense of that limb; this is termed a phantom limb. This phenomenon is not limited to one sensation, however. Phantom sensations can occur that are perceived as movement, pressure, pain, itching, or hot/cold as well. (Note: The work of V. S. Ramachandran indicates that despite popular belief, the phantom limb phenomena is actually the result of neural signal bleed through the brain's sensory maps, rather than from stimulation of nerves.)
Temporary impairment has also been known to occur due to an overdose of vitamin B6 (pyridoxine and pyridoxamine). Most of the impaired function discontinues shortly after the intake of vitamins returns to normal. Impairment can also be caused by cytotoxic factors such as chemotherapy.