020: Proprioception, Proprioceptive System, Proprioceptors

Aug 28, 2021

Many people regard the human body as the most complex organism in the universe. Of course, such a belief flows from a highly anthropometric perspective of reality. Nevertheless, the body is indeed the most complex organism in humanity's universe. But to get an idea of just how small that is on the cosmic scale, watch the video above. It's the opening scene from one of my favourite films,  Contact, based upon the book of the same name by the inimitable Carl Sagan.

Astronomy has a beautiful way of injecting humility into a person's attitude toward life. To paraphrase Ellie in Contact, there are 400 billion stars in our galaxy alone. If only one out of a million of those had planets, and just one out of a million of those had life, and just one out of a million of those had intelligent life, there would be literally millions of civilisations out there. So, there's a chance our bodies are not the most complex organism in the universe. But we can say, "at least for now it is."

Given how profoundly sophisticated the body is, understanding it is no simple task. But the veil hiding its deeply-held secrets is even more impenetrable when the language we use to talk about it is itself perplexing. One of the most significant barriers to learning in all branches of science is the convoluted vocabulary. So it may come as no surprise that many of the questions I receive stem from difficulty interpreting the language. Today's post will address three terms submitted for clarification by a reader of this blog: proprioception, proprioceptive system, and proprioceptors.

Proprioception is a sensation within the body that allows it to gather data about the state of its parts relative to each other and the external environment. It was first conceptualised and introduced to the scientific community by Sir Charles Sherrington, the British physiologist who discovered the stretch reflex. The strict definition of the term refers to the body's ability to sense movement and joint angles. However, depending upon the source, 'proprioception' may be applied to describe the body's general sense of self-awareness, incorporating vision, the vestibular system (structures within the ears aiding in balance and spatial awareness), interoception (internal body feelings), and even intuition.

The sensation of proprioception is provided by the proprioceptive system, which is the collective name for highly specialised sensory organs located throughout the musculoskeletal system. All proprioceptors belong to a class of neural structures called mechanoreceptors, which means they convert mechanical stimuli like compression and stretch into membrane action potentials. When a joint or several joints move, the surrounding tissues are distorted. Such distortions include shortening and lengthening in different directions and magnitudes, leading to contrasting patterns of contraction and stretching of the tissues relative to the direction and range of the joint motion. For example, flexing the elbow causes the tissues on the anterior of the upper arm to shorten while the tissues on the posterior lengthen. The opposing mechanical states are detected by proprioceptors, which subsequently relay information to the central nervous system about the amount and direction of joint motion.

Proprioceptors are the sensory organs that each respond to a very specific type of stimulus. Important proprioceptors include muscle spindles (detect speed and amount of muscle stretch), Golgi tendon organs (detect amount of muscle tension), joint receptors (detect motion only at the extremes of physiological motion), free nerve endings (detect potentially harmful stimuli), and cutaneous receptors in the skin (detect pressure, vibration, skin stretch, temperature, and pain). The structure and function of these receptors are too elaborate to discuss in this post, and thus each will receive its own entry soon.

To summarise, proprioceptors are highly specialised nervous system structures located in muscles, tendons, joints, ligaments, and skin. They relay important information to the central nervous system about what the body is doing and where it is in space. They are collectively known as the proprioceptive system. Proprioception is the sensation that arises from the nervous system processing their combined inputs.