Muscle senses tell us about the position of our bodies in space and how much force we are currently using in any muscle. For a fun review of muscle spindle action in the stretch reflex, go to this neat site, or the one we used in class. For a nice (but still a bit too detailed) description of muscle spindles, go to this Northeastern Univ PT site and click on "the Muscular Spindles."
These sensory receptors are especially interesting (not just because my dissertation was on an insect version of one!). You see, the sensory neurons in the spindle wrap themselves around specialized muscle fibers. When the entire muscle contracts or relaxes, these specialized muscle fibers are adjusted so that their central region does not change length-- that way, the sensory neurons around them do not change in length. The end result of that is that muscle spindle sensory neurons are NOT activated by voluntarily generated movement. If you plan a movement and move it correctly, these spindles will remain silent.
The beauty of the spindle is that they only respond if the movement you make is not the one you plan. They are perfect error detectors for movement. Have you ever gone to lift something you thought was light and it turned out that item was heavy? Isn't it a weird feeling you experience when you realize that more force is necessary? That weird feeling is a proprioceptive feeling generated by your muscle spindle sensory neurons. Another term for muscle spindle sensory neurons is "stretch receptors." This term makes it sound like the function of these sensory neurons is only to tell when you stretch, so I prefer muscle spindle receptors.
You may think that I have given you a very complicated version of muscle spindles. However, they are much more complicated than I even began to explain! If you are interested in a bit more detail, you can visit this great muscle spindle site.
Golgi Tendon Organ
As a little aside, I found a short historical re-cap of the person, Camillo Golgi, for whom this organ is named...
This is a simpler structure than the muscle spindle. The Golgi tendon organ sensory receptors are squeezed in within the collagenous fibers of a muscle tendon. When the muscle contracts, the tendons are stretched. The stretching of the tendons compresses them, squeezing down tightly on the Golgi tendon organ sensory neurons. This squeezing of the sensory receptors is what activates them.
The GTOs therefore tell the CNS when muscles contract. The more active the GTO receptors, the more contraction that occurs. Their input into the CNS can, if it gets strong enough, lead to inhibition of the motor neurons that contracted their muscle to begin with. This negative feedback loop prevents us from contracting our muscles so hard that we rip them right off the bone! Yuck! But our muscles are certainly powerful enough to do that!
© 2011 STCC Foundation Press