Your Brain’s “Miraculous” Proprioception: You wouldn’t want to be without it
Imagine having to watch your feet with each step to ensure that your leg muscles position them most efficiently for painless walking, and in order to avoid putting unnecessary wear and tear on your knee, hip, and ankle joints.
Thankfully, this conscious attention to your gait is unnecessary for most people. This is the case because your brain continually processes sensory information about proprioception, the virtually automatic sense of the position of your body parts with respect to one another and to the earth’s surface and its gravitational attraction. It starts in your eyes and ears, but less well-known is key information coming from your muscles and joints and zinging its way up the most backward-pointing (posterior) portion of your spinal cord to your brain. When that information reaches your brain, it’s processed by several brain areas in concert. The upshot: for your typical casual or sports-related movement actions — walking, running forward, backward, or laterally; kicking, throwing, or striking a ball — your brain automatically adjusts the position of your extremities with respect to what we’ll think of as the center plane of the trunk of your body; and, if relevant, adjusts the position and balance of your body with respect to a static or moving external object, such as a tennis ball in flight.
Your brain does this to properly position your various joints for a well-executed movement and, to whatever extent possible, one that produces optimal efficiency of muscle action and minimal wear and tear on your joints. Playing tennis can exemplify the type of four-limbed movement for which precise proprioception is essential — sprinting forward and laterally to a spot, then stopping quickly to position your body, while simultaneously positioning your racket head and turning your shoulders to hit a backhand. (Readers will hopefully forgive the focus on tennis, a game I played for many years.) Competition skateboarding and gymnastics would be more extreme examples, of the necessity for excellent proprioception.
In contrast, a comparatively much simpler action, touching an index finger to the tip of your nose with your eyes closed, requires this non-visual awareness of your arm and hand position with respect to the center of your face. The fact that this action is part of sobriety tests offers a clue to the understanding that alcohol intoxication has effects on impairing brain processing of proprioception. Since your automobile or motorbike can roughly be considered an extension of your body (see “Extended Physiological Proprioception”, below), proprioceptive impairment isn’t an appropriate condition in which to operate a motor vehicle.
Ultimately processed in the somatosensory area of the cerebral cortex, anatomically correlated to the post-central gyrus, the transition from this sensory aspect of proprioception to actual efficient movement involves adjacent areas of the cerebral cortex, as well as areas of the brainstem and the cerebellum, where movement-program information is stored.
Muscle Stretch Receptors Inside Muscles
Although balance organs like the semicircular canals in your inner ear appear to be an important sensory-receptor input to the brain’s proprioceptive sense, they may not be the most important. That distinction may belong to your mechanoreceptors. Buried inside your muscles, among the thousands of muscle fibers that shorten to exert force on your bones and joints, are muscle spindles. These are curiously modified muscle fibers distributed throughout your muscles that act more or less as adjustable-sensitivity (technically, “biasable”) stretch receptors, sending their information via nerves into your spinal cord. Imagine: from each and every muscle in your body, a continual stream of this information pours into your central nervous system, with data about the situation for every body muscle: is the muscle currently shortened, putting a force on the bones/joints to which it’s attached, or is it relaxed and stretched because other muscles are acting oppositely on those bones/joints? This is quantitative, of course, registering not simply a binary shortened/stretched answer, but also to what extent each is occurring. The moment-to-moment adjustments to leg and hip muscles that enable us to maintain our standing balance in a rocking and bouncing train or bus exemplify how automatic these adjustments can often be. We tend to be aware of them, while they are occurring or just afterward, but they are not really what we would call “consciously initiated” movements.
Tendon-tension and Joint-situation Receptors
To further refine this type of information, the brain also receives mechanoreceptor information from the tendons that connect muscles to their bones and from the joints between the bones that are either moving or held stable. Finally, because areas of your skin stretch with movement, skin stretch-receptors are also thought to contribute to this body-sense information. Muscle stretch, force on muscle tendons against gravity, how much each joint is being moved or not: all this sensory information travels through sensory nerves and into the spinal cord, where, in organized fashion, it travels upward in what are known as the posterior columns of the cord. These columns are composed of major cables of white matter (thousands of fast-transmitting myelinated nerve fibers). While there are separate nerve-fiber cables in the cord for information coming from the lower body and that coming from the upper body, this need not confuse the issue. Ultimately, this information is organized in the lower brain and sent to the “somatosensory” cerebral cortex on the opposite side of the brain, This means that your right-side proprioceptive information is processed by your left hemisphere and vice-versa.
Just beneath the level of direct conscious experience, this flood of body position information is computationally processed at every moment by your brain — how shortened or stretched are each of your body’s muscles; how much force is each applying to its tendons and beyond to bones (against gravity); what is the status of each of your joints. Somehow, using stored movement (motor)-program, information in your cerebellum, the somatomotor part of each hemisphere — adjacent to the somatosensory — works with other neurons in the Basal Ganglia to send barrages of movement (motor) information back down separate sets of cables in the spinal cord and, ultimately, signals go out to the muscles from the cord to initiate actions such as the sprint to, rapid stop, and almost simultaneous striking of the tennis ball. Moreover, additional motor information is sent to the muscle spindles themselves, in order to adjust their sensitivity. Naturally, the muscles, tendons, and joints are continually monitored during all movements, thereby quickly updating proprioceptive input to the brain.
Now Comes the Real “Miracle”
Based on this information and stored movement programs you’ve taught your brain’s cerebellum over time, you semi-automatically prepare to perform your next movement. For example, depending on how well you hit your backhand, you might be edging toward the net (or retreating toward the baseline). Once you decide, this intent can evoke a motor program that prepares all your muscles for this next sequence of body moves by adjusting the sensitivity of those (biasable) muscle spindles inside all those muscles to some optimal level that facilitates that next set of movements on the court. The motor output is continued through to the end of the movement and into subsequent movements depending upon how well you’ve programed different muscle-sequence behaviors into your brain. Throughout all this, your proprioceptive system is working overtime.
Extended Physiological Proprioception
This concept was formulated because when you use a tool like a manual screwdriver, you’re able able to properly locate and maintain the position/pressure combination with the screwdriver’s tip on a slotted screw head. Conceptually, you extend the proprioceptive guidance of your arm and hand muscles to the business end of that screwdriver — as an extension of your arm and hand. A tennis or squash racket offers another example. Similarly, skating extends your lower-limb proprioception into your skates. The concept can roughly be extended to any tool that can be envisioned as an extension of your body, even to driving a motor vehicle.
Can you Improve Your Proprioception? Yoga
The impact of yoga poses on proprioception has been studied with apparently conflicting results, with some studies showing improvement of proprioception after short-term yoga training, while others assert balance improvements as separate from a failure to improve proprioception. It may well be that resolving this uncertainty may require the difficult assessment of initially naïve individuals compared with themselves after long-term yoga practice.
Living With Diminished Proprioception
There are two common ways that folks suffer losses in proprioception. A serious back injury (or sometimes a mishap in surgery) that damages the most posterior part of the spinal cord can block proprioceptive information from body parts below the injury level from reaching the brain. The transience or permanence of the situation will be determined by the severity of that spinal cord injury. Usually, more severe and often permanently disruptive are strokes that damage the somatosensory cerebral cortex of one brain hemisphere. These commonly lead to proprioceptive deficits on the opposite side of the body. Having suffered such an event in my right somatosensory cortex during middle age, I sadly stayed away from tennis for years afterward because I had difficulty properly positioning myself to hit balls from the left side of my body with my backhand. Even after several years of practicing, I never entirely recovered; relying on my vision to position and orient my body never completely substituted. In contrast, hitting from my right side (my forehand) was relatively unaffected. An even more basic loss was the eventual wear and tear on my left hip from the somewhat inaccurate placement of my left foot and faulty weight distribution on that hip and leg during tennis, walking or hiking. The foot plants outside what was previoiusly the optimal distance from the center plane of my body (prior to the stroke). Although ongoing visual vigilance of foot placement can be attempted as a substitute, it never fully compensates for the proprioceptive loss and is far from automatic, interrupting the normal smooth flow of movements.
The lesson is clear: living with diminished proprioception is no fun; your proprioceptive ability is, in this sense, a quasi-miraculous invention of evolution that really becomes evident when you no longer have its full effects working in your behalf. I hope readers will never have to do without that full functionality.