Muscle Sense Organs
There are several types of sense organs in muscle. The pain resulting from exercising too vigorously after long disuse (muscle soreness) or from torn muscle fibers are good examples of muscle sense organs at work. These pain receptors, which are few in number, are found not only in the muscle fibers themselves but also in blood vessels (arteries, but not veins) that supply the muscle cells and in the connective tissues that surround the fibers.
Other kinds of sense organs found within the muscles and joints are called proprioceptors. The function of proprioceptors is to conduct sensory reports to the CNS (central nervous system) from (1) muscles, (2) tendons, (3) ligaments, and (4) joints. These sense organs are concerned with kinesthesia or kinesthetic sense, that in general, unconsciously tells us where our body parts are in relation to our environment. Their contributions enable us to execute a smooth and coordinated movement, no matter whether we are putting a golf ball, hitting a home run, or simply climbing an unfamiliar flight of stairs without stumbling. They also help us to maintain a normal body posture and muscle tonus. The tendency for the lower jaw to drop, the head to droop forward, and the knees to buckle because of the effects of gravity are all counterbalanced by the so-called antigravity muscles, which relay information regarding position in space.
How do these sense organs or proprioceptors function? We can begin to answer this question by first describing how each type of sense organ sends specific sensory information to the CNS. There are three important muscle sense organs concerned with kinesthesia: muscle spindles, Golgi tendon organs, and joint receptors.
The Muscle Spindle
Muscle spindles are perhaps the most abundant type of proprioceptor found in muscle. Briefly, muscle spindles (also called stretch receptors) send information to the CNS concerning the degree of stretch of the muscle in which they are embedded. This provides the muscles with information, for example, as the exact number of motor units necessary to contract in order to overcome a given resistance ; the greater the stretch, the greater the load and the greater the number of motor units required. The spindles are important in the control of posture and, with the help of the gamma system in voluntary movements.
Structure of the Spindle
It is nothing more than several modified muscle fibers contained in a capsule, with a sensory nerve spiraled around its center. These modified muscle cells are called intramural fibers to distinguish them from the regular or extramural fibers. The center portion of the spindle is not capable of contracting, but the tow ends contain contractile fibers. The thin motor nerves innervating the ends are of the gamma type and are thus called gamma motor nerves or fusimotor nerves. When they are stimulated, the ends of the spindle contract and pull against the center region. The larger motor nerves innervating the regular or extramural fibers are called alpha motor nerves. When they are stimulated the muscle contracts in the usual sense.
Function of the Spindle
As mentioned before, the spindle is sensitive to length or stretch. Therefore, because the spindle fibers are found throughout the muscle and lie parallel to the regular fibers, when the whole muscle is stretched, the center portion of the spindle is stretched also. This stretching activates the sensory nerve (annul spiral nerve) located there, which then sends impulses to the CNS. In turn, these impulses can activate the alpha motor neurons that innervate the regular muscle fibers, and the muscle contracts. If the muscle shortens when it contracts, the spindle also shortens, thus stopping its flow of sensory impulses; the muscle then relaxes.
The spindle is sensitive to both the rate of change in length and to the final length attained by the muscle fibers. The functional significance of these two types of sensitivity can be illustrated by a muscle engaged in a steady contraction, as when the elbow is flexed steadily against a load (for example, when holding a book). The type of stretch placed on the muscle because of the load is called tonic stretch and is concerned with the final length of the muscle fibers. If the load is light, the fibers will be stretched only moderately, and the frequency discharge of the sensory impulses from the spindle will be low. Thus, only a few motor units are called on in keeping the load steady.
If there is and unexpected increase in the load being held, such as by adding another book, the muscle will be stretched again. This is evidenced by the fact that the forearm will be lowered owing to the added load. The ensuing reflex contraction initiated by the spindle will reposition the forearm to its original level. However, there will be some overcompensation; that is, at first the contraction will be greater than needed. The greater and more abrupt the increase in load, the greater the frequency of discharge of the spindle, the greater the contraction, and the greater the overcompensation. In other words, with this type of stretch, called phasic stretch, the spindle is responding to the rate or velocity of the change in length and not to the length per se.
The Gamma System
There is one other way in which the spindle can be stretched. Contractile ends of the spindle fibers are supplied with motor nerves from gamma neurons. These gamma neurons can be stimulated directly by the motor centers located in the cerebral cortex of the brain via their pyramidal tract nerve connections to the spinal cord. When stimulated in this manner, the ends of the spindle contract, thus stretching the center portion and stimulating the sensory nerve. In other words, the muscle spindle can be activated by itself, apart from the rest of the muscle. This special neural arrangement is called the gamma system or gamma loop. This kind of setup provides a very sensitive system for the execution of smooth, voluntary movements. Furthermore, it has been suggested that the gamma neurons have a recruitment order much the same as the motor neurons of alpha motor neurons. Although all the functional interrelationships in producing precise voluntary movements are not completely understood, this combined recruitment is called alpha gamma coactivation. When thinking about voluntary movements and alpha-gamma coactivation, considers that gamma firing occurs just a bit prior to alpha activation. This puts an initial stretch bias on the sensory system resulting in some firing from the annul spiral nerve. One way to stop this backflow of sensory impulses is to contract (shorten) the whole muscle to precisely the proper amount, making a perfect matchup of ìgamma-alphaî activation (note reversal of terms to emphasize order of firing) and a turnoff of backflow. If the matchup is imperfect, the initial bias would not be completely removed. In this event, the continued backflow of afferent impulses would signal the alpha motor neurons to send additional impulse volleys. The muscle would undergo further contraction with a concomitant decrease in sensory backflow. This process would be continued until all the initial bias is removed. Remember that these processes occur very rapidly but, at the same time, do require time.
For an example of how the gamma system works, let us go back to the person voluntarily holding a book in a fixed position, elbow flexed at 90 degrees. I stated that the tonic stretch on the entire muscle created the load provides information that keeps the load (book) in a relatively fixed position. However in addition, the gamma neurons are stimulated by impulses sent down directly from the motor cortex. The ends of the spindle contract, the sensory nerve sends impulses back to the CNS and additional information is provided concerning the number of motor units that is required to maintain the original voluntarily initiated position. This additional information provides the refinement that is needed for a smooth rather than jerky movement.
In conclusion, there are three ways that the muscle spindle can activate the alpha motor neurons that cause the muscle to contract: (1) by tonic stretch,, (2) by phasic stretch, (3) by the gamma system or gamma loop. All these controls work together to provide for effective, coordinated, and smooth movement.