Introduction to Sensory Systems
In the previous atoms we have covered the five basic sensory systems of the human body, which include the visual, auditory, gustatory, olfactory, and somatosensory systems. Recent advances in science have expanded this classical list of five sense systems to include two more systems: proprioception or kinesthesia, which is the sense of position of parts of the body, and vestibular, which senses gravity and provides balance.
Proprioception and kinesthesia
Proprioception is the sense of the relative position of neighboring parts of the body and the strength of effort being employed in movement. It is distinguished from exteroception, by which one perceives the outside world, and interoception, by which one perceives pain, hunger, and the movement of internal organs. A major component of proprioception is joint position sense (JPS), which involves an individual's ability to perceive the position of a joint without the aid of vision.
Kinesthesia is the awareness of the position and movement of the parts of the body using sensory organs, which are known as proprioceptors, in joints and muscles. Kinesthesia is a key component in muscle memory and hand-eye coordination. The discovery of kinesthesia served as a precursor to the study of proprioception.
While the terms proprioception and kinesthesia are often used interchangeably, they actually have many different components. Often the kinesthetic sense is differentiated from proprioception by excluding the sense of equilibrium or balance from kinesthesia. An inner ear infection, for example, might degrade the sense of balance. This would degrade the proprioceptive sense, but not the kinesthetic sense. The affected individual would be able to walk, but only by using the sense of sight to maintain balance; the person would be unable to walk with eyes closed. Another difference in proprioception and kinesthesia is that kinesthesia focuses on the body's motion or movements, while proprioception focuses more on the body's awareness of its movements and behaviors. This has led to the notion that kinesthesia is more behavioral, and proprioception is more cognitive.
The vestibular system is the sensory system that contributes to balance and the sense of spatial orientation. Together with the cochlea (a part of the auditory system) it constitutes the labyrinth of the inner ear in most mammals, situated in the vestibulum in the inner ear (Figure 1). The vestibular system comprises two components that assist with our movements and sense of balance: the semicircular canal system, which indicate rotational movements; and the otoliths, which indicate linear accelerations. Some signals from the vestibular system are sent to the neural structures that control eye movements and provide us with clear vision, a process known as the vestibulo-ocular reflex. Other signals are sent to the muscles that control posture and keep us upright.