Proprioception
[1] Most animals possess multiple subtypes of proprioceptors, which detect distinct kinesthetic parameters, such as joint position, movement, and load.
To determine the load on a limb, vertebrates use sensory neurons in the Golgi tendon organs:[7] type Ib afferents.
Proprioceptors can form reflex circuits with motor neurons to provide rapid feedback about body and limb position.
Many invertebrates, such as insects, also possess three basic proprioceptor types with analogous functional properties: chordotonal neurons, campaniform sensilla, and hair plates.
[citation needed] Members of the transient receptor potential family of ion channels have been found to be important for proprioception in fruit flies,[17] nematode worms,[18] African clawed frogs,[19] and zebrafish.
[21] Humans with loss-of-function mutations in the PIEZO2 gene exhibit specific deficits in joint proprioception,[a] as well as vibration and touch discrimination, suggesting that the PIEZO2 channel is essential for mechanosensitivity in some proprioceptors and low-threshold mechanoreceptors.
[28] For instance, for a person to walk or stand upright, they must continuously monitor their posture and adjust muscle activity as needed to provide balance.
Similarly, when walking on unfamiliar terrain, or even tripping, the person must adjust the output of their muscles quickly based on estimated limb position and velocity.
After the last cell division, proprioceptors send out axons toward the central nervous system and are guided by hormonal gradients to reach stereotyped synapses.
This transfer can be modeled mathematically, for example to better understand the internal workings of a proprioceptor[34][35][36] or to provide more realistic feedback in neuromechanical simulations.
Poppele and Bowman [43] used linear system theory to model mammalian muscle spindles Ia and II afferents.
They found that the following Laplace transfer function describes the firing rate responses of the primary sensory fibers for a change in length:
Specifically, they find that the firing rate of a Golgi tendon organ receptor may be modeled as a sum of 3 decaying exponentials:
Proprioception, a sense vital for rapid and proper body coordination,[46] can be permanently lost or impaired as a result of genetic conditions, disease, viral infections, and injuries.
For instance, patients with joint hypermobility or Ehlers–Danlos syndromes, genetic conditions that result in weak connective tissue throughout the body, have chronic impairments to proprioception.
[49] In regards to Parkinson's disease, it remains unclear whether the proprioceptive-related decline in motor function occurs due to disrupted proprioceptors in the periphery or signaling in the spinal cord or brain.
However, both individuals regained some control of their limbs and body by consciously planning their movements and relying solely on visual feedback.
The impact of losing the sense of proprioception on daily life is perfectly illustrated when Ian Waterman stated, "What is an active brain without mobility".
It has been proposed that even common tinnitus and the attendant hearing frequency-gaps masked by the perceived sounds may cause erroneous proprioceptive information to the balance and comprehension centers of the brain, precipitating mild confusion.
The subject is required to touch his or her nose with eyes closed; people with normal proprioception may make an error of no more than 20 mm (0.79 in)[citation needed], while people with impaired proprioception (a symptom of moderate to severe alcohol intoxication) fail this test due to difficulty locating their limbs in space relative to their noses.
[citation needed] Oliver Sacks reported the case of a young woman who lost her proprioception due to a viral infection of her spinal cord.
[63] Moreover, there are specific devices designed for proprioception training, such as the exercise ball, which works on balancing the abdominal and back muscles.
In 1847, the London neurologist Robert Todd highlighted important differences in the anterolateral and posterior columns of the spinal cord, and suggested that the latter were involved in the coordination of movement and balance.
[citation needed] In 1880, Henry Charlton Bastian suggested "kinaesthesia" instead of "muscle sense" on the basis that some of the afferent information (back to the brain) comes from other structures, including tendons, joints, and skin.
[75][76] Terrestrial plants control the orientation of their primary growth through the sensing of several vectorial stimuli such as the light gradient or the gravitational acceleration.
A quantitative study of shoot gravitropism demonstrated that, when a plant is tilted, it cannot recover a steady erected posture under the sole driving of the sensing of its angular deflection versus gravity.
An additional control through the continuous sensing of its curvature by the organ and the subsequent driving an active straightening process are required.
This dual sensing and control by gravisensing and proprioception has been formalized into a unifying mathematical model simulating the complete driving of the gravitropic movement.
This model has been validated on 11 species sampling the phylogeny of land angiosperms, and on organs of very contrasted sizes, ranging from the small germination of wheat (coleoptile) to the trunk of poplar trees.
[75][76] Further studies have shown that the cellular mechanism of proprioception in plants involves myosin and actin, and seems to occur in specialized cells.
