Proprioception and proprioceptive sensitivity

Edited by Dr. Davide Sganzerla

What is proprioceptive sensitivity?

Proprioceptive sensitivity is a very sophisticated mechanism, which aims to provide the Central Nervous System with information of the highest precision, in real time, about:

1) parameters of biomechanical movement (speed, force, direction, acceleration);
2) physiological parameters on the state and on the biological changes that occur in the muscles, tendons and joints as a consequence of the movement performed.

Most of the proprioceptive information never reaches the level of consciousness, being delegated to the control of the elaboration of the motor project and its execution.

The control and execution of the motor project take place both in the transmission phase, in which the motor project elaborated in the brain is transmitted to the motor neurons, and in the execution phase, in which the motor neurons activate the locomotor system that faithfully carries out the orders received .
At this level, proprioception is very important, both for a control mechanism on the correct execution of the movement, and for a mechanism of possible correction in the event that unforeseeable external phenomena disturb the strategically programmed motor projects. It can therefore be said that proprioception is controlled by negative feedback circuits: the action performed by a system is compared with the programmed action and any difference (error) is reported to the system so that it activates the appropriate corrections.
The proprioceptive sensitivity, and in particular the proprioceptors, are also the basis of medullary reflexes: defense reactions designed to maintain the integrity of the body in the face of potentially harmful situations. In this task, the receptors activate some circuits, exclusively medullary , able to provoke reactive movements of defense of the organism.

Beyond these numerous tasks, the proprioceptive system, as a whole, also provides information to the nervous structures capable of processing the processes of awareness and consciousness. That is, it also sends information to the cerebral cortex.
Conscious proprioceptive perception, which we all possess, is a construction elaborated by the cerebral cortex on the basis of information coming from peripheral proprioceptive receptors.
In this system the complex integration of the peripheral afferents coming from the different perceptive channels is combined with the information coming from memory and experience. Memory brings a wealth of information on past experiences, while experience is the tool through which each of us colors the sensations coming from the outside world, making them their own through the attribution of personal values.
The synthesis of the three types of information reported above gives rise to what is commonly called "body image", ie awareness of the "existence, position and movement of our body. Apart from the common sensory channels (sight, hearing, touch ), each one forms this awareness through proprioceptive information.
Proprioceptivity therefore describes the sensory inputs that originate, in the course of centrally guided movements, from particular structures: proprioceptors. Their main function is to provide feedback information on the organism's own movements, in other words to signal, moment by moment, what are the movements that the organism itself is carrying out; on the basis of this information the higher centers are able to correct or modify the movement in progress.

The proprioceptors

The proprioceptors are specialized sensory organs, which give information both on the positions assumed by the body in conditions of rest (proprioception proper), and on the dynamic parameters of movement (kinesthesia). The role of these messages can be very important, simultaneously and at different levels, for many structures of the Central Nervous System.

There are three main systems in which the codes coming from the sensitive pathways are processed.
A first system, unconscious, is responsible for controlling and reacting to dangerous situations. This system guarantees immediate protection from any physical damage and is mainly controlled by spinal cord.
A second system is responsible for controlling the execution of motor projects and automatisms. This system guarantees maximum precision and adherence between motor design and movement in all movements, in order to obtain the greatest possible advantage. The system is mainly controlled by the cerebellum and it too is completely unconscious.
A third system uses sensory information from peripheral receptors for conscious purposes. Through this system each of us forms and controls his own body image. This third system is predominantly controlled by the cerebral cortex, which processes, integrating them in various ways, all the information coming from the periphery.
The proprioceptive sense organs can be divided into three main groups:

the muscle receptors, which include the neuromuscular spindles, the Golgi tendon organs, the muscle-placed Pacini receptors, and the free muscle terminations of the muscle, perimysium and epimysium;
joint receptors;
the cutaneous mechanoreceptors, which include the corpuscles of Merkel, the corpuscles of Meissner, the corposculi of Ruffini and the corpuscles of Pacini.

The spindles and Golgi organs are receptors sensitive to the stretched state of the muscle, particularly useful in defining the parameters for the unconscious proprioceptive system (headed by the cerebellum) or for reflex responses (spinal cord). They play a role of primary importance both in proprioception and in the mechanisms of motor control. Furthermore, they seem to be more suitable for informing about the mechanical characteristics of the movement than about the resting state of the locomotor system sense of movement it would therefore be the type of information they preferably encoded.
The articular and cutaneous receptors, although fundamental at the level of unconscious proprioception, play a more important role in static sensations, therefore in the sense of position (conscious proprioception). Studies have been carried out (Gandevia and Burke 1992) in which the afference from each of the cutaneous mechanoreceptors and joint receptors was artificially stimulated and differences were noted. The stimulation of the fibers coming from Merkel's corpuscles gives the sensation of skin pressure or indentation of the skin. The stimulation of the fibers coming from the Meisssner corpuscles gives the sensation of localized vibration, strictly adhering to the duration and frequency of the stimulus. The stimulation of the fibers coming from the Ruffini corpuscles gives the occasional sensation of joint movement. The stimulation from the fibers coming from the Pacini corpuscles gives the sensation of a diffuse vibration.The sensation coming from the articular receptors gives the sensation of deep focused pressure, movement or joint stress.