Understanding the Proprioceptors
Understanding the Proprioceptors
Specialized sensory cells in the nervous system give the brain information about the outside world. Some of the most important receptors are sensitive to sensations such as heat, pain, or pressure. Another very important category of sensory receptors are the proprioceptors, which are specialized cells that give the brain information about movement and the body’s position in space. They play a major role in many of the therapeutic techniques and approaches we use, so let’s do a quick review of these important sensors.
The proprioceptors are specialized nervous system sensors whose primary function is to manage crucial facets of movement. They are highly complex and transfer tremendous amounts of information. We often take for granted just how important this information is. However, without proprioceptive input, it would be difficult, if not impossible, to move accurately at all. Watching an infant who is just starting to gain control over his/her motor system gives you a clue as to what our movement would be like if we did not have a highly developed proprioceptive system.
There are several specialized proprioceptors including muscle spindles, golgi tendon organs, ruffini endings, and pacinian corpuscles. The ruffini endings and pacinian corpuscles are skin and joint proprioceptors that provide feedback about degrees of pressure and the angle or position of joints. The muscle spindles and golgi tendon organs are designed for transmitting information about the length and tension relationships in muscle tissue. There is often confusion about the function of these last two proprioceptors so this discussion focuses on their roles.
Muscle Spindles: A Key to Function
Understanding muscle spindle function is important for those working with musculoskeletal disorders. The muscle spindles are spread throughout the contractile regions of the muscle belly. They can both contract and elongate just like muscle fibers. The muscle spindle fibers are also called intrafusal fibers in order to differentiate them from the contractile muscle (extrafusal) fibers.
The muscle spindle is mostly responsive to two types of stimulation. The first is the amount of change in muscle length (called the tonic response). The second is the rate of changing muscle length (called the phasic response). The primary responsibility of the muscle spindles is to keep the muscle from elongating too far and causing a tensile stress injury, such as a strain.
A muscle is naturally elastic and has the ability to elongate. However, if that muscle is stretched beyond its capability to elongate, it can tear. The muscle spindle’s role is to prevent this from happening. The muscle spindle senses danger if the muscle lengthens too far (the tonic response) or if it elongates too rapidly (the phasic response). Even if it is not near the end of the muscle’s range, a rapid elongation of muscle tissue can set off the phasic response and stimulate the muscle spindles. In this case the muscle spindle is ‘sensing’ that if the current rapid rate of elongation continues, it will bypass the muscle’s ability to withstand stretching and fibers may be damaged.
The muscle spindle’s protective reaction in this situation is to cause a muscle contraction in that same muscle to offset the excessive lengthening of the muscle fibers. This reactive contraction is called the stretch (or myotatic) reflex. It is likely that you have had your stretch reflex tested by a physician tapping on your patellar tendon, causing a rapid elongation of the quadriceps muscle group. This rapid elongation causes the muscle spindles to initiate a muscle contraction in the quadriceps, which forces your knee to rapidly extend - the ‘knee-jerk’ reflex. This principle is illustrated in above (Figure 1). If this individual’s hamstring muscles are lengthened either too far or too fast in the direction of the arrow, the stretch reflex will kick in and there will be a contraction of the hamstrings.
Improper tension levels in the muscle spindle cells can lead to biomechanical imbalance and various soft-tissue problems. The effectiveness of many massage and stretching techniques is directly related to how the muscle spindles are functioning. For example, if a muscle stays in a shortened position for some length of time, the muscle spindles can change their threshold of stimulation. Therefore, returning the muscle to its normal length may be perceived by the muscle spindle as a significant stretch. In contrast, when the muscle is stretched past its normal resting length, the muscle spindles become overly active. Massage techniques which are aimed at normalizing the tension in the muscles and helping to enhance muscular elongation help restore the proper threshold levels in the muscle spindles.
Golgi Tendon Organ
The other proprioceptor of primary concern is the golgi tendon organ. Its function is almost opposite that of the muscle spindle. Where the muscle spindle is responsible for determining if there is too much length on the muscle, the golgi tendon organ is responsible for determining if there is too much contraction stimulus on the muscle fibers. If the muscle fibers are trying to contract with more force than they are capable of, an injury is likely. A simple example of this is if someone tries to pick up a weight that is too heavy for them. Or perhaps they might be trying to catch a heavy object that is falling. When the muscle contraction stimulus is more than the muscle can handle, muscle fibers may tear.
The golgi tendon organ is located in the musculotendinous junction. Its primary role is to determine if the muscle is contracting with too much effort. Since the belly of the muscle is elastic and the tendon fibers are not, the location of the golgi tendon organ at the musculotendinous junction is ideally suited for its role. If these cells determine that there is too much effort required of the muscle for its strength capability, it will shut off the contraction stimulus. The golgi tendon organ does not have as sensitive a threshold as the muscle spindle, so it is not activated as easily. It is a good thing this is true, because it would make strong muscular effort very difficult if the contraction stimulus was constantly being shut off.
There is a type of massage technique (sometimes called muscular approximation) that advocates attempting to push the two ends of a muscle toward each other. The idea is that this movement stimulates a muscle contraction and creates a golgi tendon response that can help a muscle relax. This does not happen for several reasons.
First, the force required to stimulate the golgi tendon organ needs to be close to that of the muscle’s maximum capability to generate a contraction. Unless you are moving limbs toward each other you are only grasping the skin and sliding it over the very slippery underlying fascia, so you aren’t shortening the muscle at all. In addition, the golgi tendons are not very active during passive muscular engagement, so even if you or the client shorten the muscle through limb movement, the golgi tendon organ is not likely to be stimulated because there is no active muscle contraction.
There are a number of techniques that use proprioceptor function with great advantage. The various forms of facilitated stretching - which go by names such as active isolated stretching, PNF, or muscle energy technique (MET) - all take advantage of the muscle spindle’s function for their effectiveness. Other techniques, such as active engagement methods that include client movement along with massage, also take advantage of proprioceptive information to help reduce muscular tightness. The more you understand about how proprioceptors function, the more effective will be your use of these techniques in different situations.