Proprioception

Have you heard about “sixth sense”? When we get a hint that something is going to happen, good or bad, we realize we have the power to sense the future, though short-term. You must be wondering  What is Proprioception? Well, Proprioception is a kind of sixth sense thing. It is also referred to as ‘kinaesthesia’ and is a sense of self-movement and body position. Our muscles, tendons, and joints have mechanosensory neurons in them, which are mediated by proprioceptors.  

These proprioceptors have the capability of encoding behavioral information. Such multiple proprioceptors activate during distinct behaviors and encode different types of information like a load on a limb, limb limit, limb velocity, and limit. Both invertebrates and vertebrates show distinct but similar types of encoding this information. Our central nervous system impacts proprioception and other sensory systems like vision and vestibular systems that represent body position, movement, and acceleration. 

Proprioception in Vertebrates and Invertebrates 

Proprioception meaning, in short, can be stated as ‘perception or awareness of the position and movement of the body.’ Limb velocity and movement (muscle length and the rate of change) are encoded by one group of sensory neurons in Vertebrates, and another type encodes the static muscle length. Muscle spindles are composed of these two groups. Whereas, in invertebrates, different sub-group of neurons of the Chordotonal organ encode limb position and velocity.

Sensory neurons determine the load on a limb in the Golgi tendon organs, type lb afferents. At given muscle forces, these proprioceptors activate, meaning the resistance experienced by that muscle. In invertebrates, limb load ‘Campaniform sensilla’ is determined by a similar mechanism. 

‘Ruffini endings’ and ‘Pacinian corpuscles’ in vertebrates decide the third role of proprioceptors when a joint is at a particular position. In invertebrates, hair plates, a row of bristles positioned along joints, detect when the limb moves. Thus proprioception meaning can be easily understood by different examples in vertebrates and invertebrates.

Mechanism of Proprioception

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So what would be the proprioception definition? It can be stated as the perception by an animal of stimuli relating to its position, equilibrium, posture, or internal condition. In humans, a differentiation is made between conscious proprioception and non-conscious proprioception;

• The dorsal column medial lemniscus pathway communicates conscious proprioception to the cerebrum. 

• Non-conscious proprioception is communicated mainly through the dorsal spinocerebellar tract and ventral spinocerebellar tract to the cerebellum. 

• A non-conscious reaction can be observed in the human proprioceptive reflex or righting reflex when the body tilts at any position. This particular action can be seen in children who have just learned to gain control of the movement of neck muscles. This control is effected by the cerebellum that affects balance.

Whenever there is limb movement, the positioning awareness is coordinated through proprioception. The skeletal muscles and tendons in the vertebrates continuously inform the brain about the position of the limbs and actions of muscles. Perception of gravity induces the awareness of changes in the equilibrium. In invertebrates, the organ ‘statocyst’ primarily determines this. Statocyst consists of a fluid-filled chamber with sensitive hairs and one or more small stone-like grains called statoliths. They could be free-moving or loosely fixed to the sense hairs resulting in the proprioceptive sense. Statocysts are found in many cnidarians and worms. 

In vertebrates, ‘otoliths’ are the saccule and utricle grains of the ear. In both cases, a change in the animal’s equilibrium or position is communicated to sense hairs due to the pressure of the statoliths. Thus the proprioception definition largely revolves around the organs that contain the sense hairs.  

The mechanically sensitive proprioceptors are distributed throughout the animal body. Vertebrates mostly have three types of proprioceptors, muscle spindles, Golgi tendon organs, and joint receptors. They are embedded at different positions and have other mechanisms, but the result is the same.

It is also estimated that there may be a temporary loss of proprioception periodically as the growth progresses. It may also happen that there will be considerable loss or gain in the bodyweight because of too many fluctuations of fat. Even when someone is tired, it triggers proprioception impairment spontaneously. 

About Proprioceptive Sense 

Proprioceptive sense, however, is a puzzle as we are not aware of how it happens. They can be differentiated from the exteroceptors like the eye and ear. It remains a mystery how we can indicate the exact position in stable or moving conditions without looking at our limbs. This might be explained based on our planned movement and hence the awareness of the brain, which knows we are in a planned position, which might result in near most accurate proprioceptive signals.

Similar actions can be seen in plants too. For example, terrestrial plants control the orientation of their primary growth through many vectorial stimuli like a light ingredient or gravitational acceleration. This control is called ‘tropism.’ The dual sensing and control by gravisensing and proprioception are formalized into a mathematical model validated on 11 species samples.