Proprioception, from Latin proprius (one’s own) and capere (to take or grasp), is the sense of your own body in space, in time. The vestibular system, mainly responsible for the body’s sense of balance, plays a huge role in conveying information about equilibrium and the body’s orientation in space to the proprioceptive system. Kinesthesia is monitored by mechanoreceptors, neural receptors triggered by a mechanical deformation, and provides information about the body’s motion in space. Together the vestibular and kinesthetic systems make up proprioception. Additionally, proprioception can be divided into two subcategories: interoception—or the sense of the body’s internal physiology; and exteroception—the sense of the body’s external physiology. Proprioception plays a large role in everyday life as it is what provides motor control to the body and allows you to move without directly looking at each of your limbs as they create the motion.
Joint Receptors and Joint Position Sense
Receptors within the joint have historically been associated with proprioception and with providing information to the brain about where the joint is in relation to the body, as well as the joint’s angle relative to itself. Research is moving away from this idea that joint receptors are the main receptor for joint position sense and proprioception. Part of the reason for this is that joint receptors are not as specific as was once thought and cannot provide information about the exact degree of bend. However, they do play a larger role in joint position sense in certain joints over others. Joint position sense is thought to be one of the main ways the brain is able to determine the location of body parts relative to space and itself. Research will continue to refine what is accepted as the main receptors of joint position sense.
Research is pointing more and more in the direction of spindle fibers and their associated neurons as the main sensory receptors for joint position sense. Muscle spindles are enclosed in a capsule and innervated by three neurons: alpha motor neurons (α-MN), gamma motor neurons (γ-MN), and muscle spindle afferent neurons. These capsules are found throughout the muscle body and are partly responsible for maintaining muscle tone. The motor neurons co-activate each other in a process called alpha-gamma co-activation. This stimulus causes the muscle fiber to contract in order to maintain a specific spindle length to allow the muscle spindle afferents to remain responsive. When the spindle lengthens during muscle movement, it is the muscle spindle afferent that is triggered, sending a response to the brain about movement in that muscle. Spindle fibers mainly help to relay information about muscle length and relative velocity to the brain.
Golgi Tendon Organs
Golgi tendon organs are another large category of receptors for joint position sense. However, rather than being located within the muscle body itself, as spindle fibers are, Golgi tendon organs are located within muscle tendons. These organs are innervated by neurons called Golgi tendon afferents. These neurons are activated when the tendon is stretched through muscle contraction. This then produces a signal relaying muscle tension to the brain. Overall, Golgi tendon organs relay a sense of force on the muscle and are responsible for the sensation of “heaviness” that can occur within the muscle.
Loss of Proprioception
Proprioception loss is associated with a variety of causes. These include muscle fatigue, sudden gain or loss of weight (especially muscle weight), overstimulation of the parietal cortex due to migraines and epilepsy, and loss of limb as in phantom limb pain. Some common neurological diseases are also associated with proprioception errors. One example of this is multiple sclerosis (MS). These errors in MS occur mainly because of the loss of myelination of nerves that is caused by and characteristic of the disease. The alpha motor neurons are specifically damaged as MS progresses, leading to proprioceptive ataxia (or loss of proprioception). Damage to the spinal cord can also result in a loss of proprioception by preventing neurons from being able to relay their information to the brain.
One main disorder of the proprioceptive system is proprioception deficit disorder (PDD) also known as Sacks’s syndrome or Descartes’s disease. PDD is associated with the complete loss of proprioception resulting in the feeling of being disembodied. Individuals with PDD have limited control of their bodies and often do not realize that the body part they feel touching them is indeed their own, as they cannot feel their limb in space at that location. Overall the partial loss or complete loss of proprioception can cause difficulties for afflicted individuals as they cannot move properly without an extreme amount of visual focus on the body part.
Proprioception is a huge area of research, as it is still not well understood. Some research has proposed that proprioception also includes a large amount of anticipation of where your limbs will be in space before moving them. Joint position sense cannot entirely explain this phenomenon. Additionally, while it is known that proprioception is processed partially in the cortex, exact processes have not been described.
Riannon C. Atwater
See also: Golgi Tendon Organs; Proprioception Deficit Disorders
Proske, Uwe, & Simon C. Gandevia. (2012). The proprioceptive senses: Their roles in signaling body shape, body position and movement, and muscle force. Physiological Reviews, 92(4), 1651—1697. Retrieved from http://physrev.physiology.org/content/92/4/1651