**Abstract**

The proprioception is the sense of positioning and movement. It is mediate by proprioceptors, a small subset of mechanosensory neurons localized in the dorsal root ganglia that convey information about the stretch and tension of muscles, tendons, and joints. These neurons supply of afferent innervation to specialized sensory organs in muscles (muscle spindles) and tendons (Golgi tendon organs). Thereafter, the information originated in the proprioceptors travels throughout two main nerve pathways reaching the central nervous system at the level of the spinal cord and the cerebellum (unconscious) and the cerebral cortex (conscious) for processing. On the other hand, since the stimuli for proprioceptors are mechanical (stretch, tension) proprioception can be regarded as a modality of mechanosensitivity and the putative mechanotransducers proprioceptors begins to be known now. The mechanogated ion channels acid-sensing ion channel 2 (ASIC2), transient receptor potential vanilloid 4 (TRPV4) and PIEZO2 are among candidates. Impairment or poor proprioception is proper of aging and some neurological diseases. Future research should focus on treating these defects. This chapter intends provide a comprehensive update an overview of the anatomical, structural and molecular basis of proprioception as well as of the main causes of proprioception impairment, including aging, and possible treatments.

**Keywords:** proprioception, muscle spindles, mechanotransduction, ion channels, proprioceptive pathways, spinocerebellar tracts

## **1. Introduction**

Proprioception is a wider sense, that include position and movement of parts of the body relative to one another, and the force and effort associated with muscle contraction and movement. But properly the term *proprioception* applies for the sensory information contributing to sense of self position, whereas *kinesthesis* refers of sense of movement. The first one is regarded as an automatic function and unconscious in contrast with the second one considered as conscious. In the words of Kröger and Watkins [1] "*Proprioceptive information informs us about the contractile state and movement of muscles, about muscle force, heaviness, stiffness, viscosity and effort and, thus, is required for any coordinated movement, normal gait and for the maintenance of a stable posture*". This information travels to the central nervous system, but differently to other components of somatosensitivity, a great part of the proprioceptive sense does not reach consciousness. This is probably due to suppression as a consequence of the motor signals [2] or inhibitions along somatosensory pathways [3]. The precise knowledge of the pathways of proprioception, especially

those of conscious proprioception, are of capital interest to better understand this sense. The techniques of neuroimaging are providing new insights about the cerebral process of proprioception.

Proprioception originates by the activation of proprioceptors at the periphery. Proprioceptors are a subset of mechanosensory neurons that provide afferent innervation to specialized sensory organs located inside the muscles and tendons, but probably also in joint capsules and ligaments, and the skin. According to Proske and Gandevia [4, 5] the sense of "*proprioception is achieved through a summation of peripheral sensory input describing the degree of, and changes in, muscle length and tension, joint angle, and stretch of skin*". In fact, the proper definition of proprioception coined by Sherrington in 1906 ("*In muscular receptivity we see the body itself acting as a stimulus to its own receptors—the proprioceptors*") suggest that the body contains different kinds of proprioceptors. Here we have focused on muscle spindles and Golgi's tendon organs. Especial interest was done on the mechanisms of mechanotransduction and the ion channel in this process.

Proprioception is impaired in some physiological and pathological situations. It will gain interest in the coming years due to the aging of population: the deficit of proprioception is associated with the increased frequency of falls in the elderly [6–8]. Furthermore, several diseases, especially some neurodegenerative disorders, course with proprioception deficits [9, 10] which treatment require a better knowledge of the molecular aspects of proprioception and new active research.

This chapter is aimed not to perform a Review on all the different aspects of proprioception but just to review some general and recent advances in proprioception. We intend to provide the readers of this book with an up-to-date appraisal of the structural and biological basis of proprioception. There are excellent reviews on the topic [4, 5, 11, 12] and we forward the interested to them. Robert W. Banks' extraordinary paper (2015) [13] masterfully sums up the history of knowledge of muscle spindles. Likewise, the recent reviews Kröger [14] Kröger and Watkins [1] are mandatory.

### **2. Proprioceptors**

The peripheral receptors of proprioception are located in tissues around the joints, including skin, muscles, tendons, fascia, joint capsules, and ligaments [15] which contains different morphotypes of mechanoreceptors [16]. It is currently believed that proprioception is not generated by a single receptor, but by multiple of receptors. In any case proprioception has been related with sensory receptors localized in the muscles while kinesthesis has been more associated with joint and cutaneous receptors [17–19]. Nevertheless, the historically regarded as true proprioceptors are muscle spindles and Golgi's tendon organs.

The *joints mechanoreceptors* are Ruffini-like and Pacinian corpuscles which signal joint movement but not movement direction or joint position [15]. Regarding *cutaneous receptors* four kinds of mechanoreceptors are present in glabrous skin (Meissner's corpuscles, Pacinian corpuscles. Ruffini corpuscles and Merkel cellneurite complexes) [20–22]. A definitive role of cutaneous mechanoreceptors as proprioceptors has not been definitively established [23–25] although it is possible a convergence between cutaneous and muscle afferents at the spinal cord and thalamic levels.

But independently of the modest contribution of cutaneous and articular mechanoreceptors to proprioception, the main stretch-sensitive receptors are muscle spindles found in most, but not all, skeletal muscles. For instance, they are absent form most cephalic muscles [26, 27]. Interestingly, muscle spindles are more

**3**

**Figure 1.**

*Structural and Biological Basis for Proprioception DOI: http://dx.doi.org/10.5772/intechopen.96787*

**2.1 Muscle spindles**

noted [35].

threshold mechanoreceptors [5].

abundant in muscles in which the precision of movements must be accurate. On the other hand, the main tension-sensitive receptors are the Golgi's tendon organs, located at the ends of muscle fibers [28, 29]. These two sensory organs respond to changes in mechanical conditions, namely in muscle length (muscle spindles) or in actively generated force (Golgi-tendon organs) but both are contraction receptors.

Vertebrate muscle spindles are complex sensory organs that have both sensory and motor innervation. Each muscle spindle receives at least one sensory fiber that innervate specialized muscle fibers denominated intrafusal fibers. These intrafusal fibers also receive motor innervation by γ-motoneurons [30, 31]. Structurally, they are encapsulated mechanoreceptors, and functionally are slowly adapting-loth

Muscle spindles are highly variable in number from none in most cephalic muscles (see [27]) to numerous in lumbrical or deep neck muscles [32, 33]. These differences are attributed functional muscular demands of muscles but the number of muscle spindles per motor unit is rather equal [34]. Also, no topographical differences in muscle spindles between mono- and multiarticular muscles were

Within the connective capsule that delimits each muscle spindles there are the intrafusal fibers and the periaxial space filled with a fluid. Three zones can be differentiated at the muscle spindle: the central or equatorial zone, the juxtaequatorial zone, and the terminal or polar zone; small segments of the intrafusal

*The intrafusal muscle fibers.* Banks and co-workers [36] established that mammalian muscle spindles regularly contain three types of intrafusal muscle fibers. Based on their morphology and the arrangement of nuclei in the equatorial zone they fall into two main categories: nuclear bag fibers and nuclear chain fibers. Bag

*Schematic representation of a muscle spindle and a Golgi's tendon organ. Muscle spindles are capsulated mechanoreceptors that consist of intrafusal muscle fibers (bag1, bag2 and chain), a periaxial space filled with a fluid, and a connective capsule. They are supplied by Ia (blue) and II (green) afferents. Golgi's tendon organs* 

*are capsulated mechanoreceptors that consist of collagen fibers and type Ia afferents (red).*

fibers can be found outside the poles of the muscle (**Figure 1**).

abundant in muscles in which the precision of movements must be accurate. On the other hand, the main tension-sensitive receptors are the Golgi's tendon organs, located at the ends of muscle fibers [28, 29]. These two sensory organs respond to changes in mechanical conditions, namely in muscle length (muscle spindles) or in actively generated force (Golgi-tendon organs) but both are contraction receptors.
