Any movement of the musculoskeletal system involves both acceleration and deceleration phases. If acceleration is not precisely controlled, or if deceleration is delayed or not strong enough, the resulting overacceleration of the body part can cause rupture of the muscles, tendons, and even bones. Two types of peripheral nerve cells are involved in this coordination to protect a muscle against unnecessary injury: muscle spindles and tendon spindles. Muscle spindles prevent overstretching of the muscle fibers, and tendon spindles prevent overcontraction. Tendinitis, a common injury in athletes, is usually the result of ignoring warnings from these two spindles.
The muscle spindles are located throughout the muscle between regular skeletal muscle fibers (Fig. 4-6). A muscle spindle consists of 4 to 20 small, specialized muscle fibers called intrafusal fibers (inside the spindles), and certain sensory and motor nerve endings are associated with these fibers. A sheath of connective tissues surrounds the muscle spindle and attaches to the endomysium of the extrafusal fibers. The intrafusal fibers are controlled
by specialized spinal motor neurons: the y-motor neurons. Regular muscle fibers are controlled by the larger a-motor neurons of the spinal cord.
The central region of an intrafusal fiber contains no or only a few actin and myosin filaments; therefore, it cannot contract but can only stretch. Because the muscle spindle is attached to the extrafusal fibers, any time those fibers are stretched, the central region of the muscle spindle is also stretched. In other words, when the muscle fibers stretch, so do the muscle spindles.
Sensory nerve endings wrapped around this central region of the muscle spindle transmit information to the spinal cord when this region is stretched, informing the central nervous system about the muscle length. In the spinal cord a sensory neurons synapses with an a-motor neuron, which triggers reflexive muscle contraction in the extrafusal fibers to resist further stretching. y-motor neurons excite the intrafusal fibers, prestretching them slightly. Although the central region of the intrafusal fibers cannot contract, the ends can. The y-motor neurons cause a slight contraction of the ends of these fibers, which stretches the central region slightly. This pre-stretch makes the muscle spindle highly sensitive to even small degrees of stretch.
If the muscle stretches enough that there is a risk of rupture, the spindle responds by sending a signal to the muscle to contract. This keeps the muscle from being injured. In response to that stretch, the sensory neurons send action potentials to the spinal cord, which then activates the a-motor neurons of the motor unit in the same muscles to increase the force of contraction to overcome stretching.
After the information is sent to the spinal cord from the sensory neurons associated with muscle spindles, the same signals continue to travel up to higher parts of the central nervous system, supplying the brain with continuous feedback about the exact length of the muscle and the rate at which that length is changing. This information is essential for maintaining muscle tone and posture and for executing movements. The muscle spindle functions as a servo mechanism to provide continuous correction to motion. The brain is simultaneously aware of errors in the intended movement, and so it sends descending commands to correct the muscle contraction at the spinal cord level.
If the muscles are fatigued or injured, as with overtraining, the muscles become shortened to resist physical stretching, the coordination between intra-fusal and extrafusal fibers disintegrates, and central commands cannot be executed. If the fatigued or injured muscles are forced to work, worse injury results.
Unlike muscle spindles, the tendon spindles give an inhibiting signal, which prevents the muscle from contracting. Tendon spindles are encapsulated sensory receptors and located just proximal to where the tendon fibers attach to the muscle fibers (see Fig. 4-6). Usually, 5 to 25 muscle fibers are connected to each tendon spindle. Whereas muscle spindles monitor the length of muscle fibers, tendon spindles are sensitive to tension in the muscle-tendon complex and serve as a strain gauge, a way of sensing changes in tension. The tendon spindles are so sensitive that they can respond to the contraction of even a single muscle fiber. These inhibitory sensory receptors perform a protective function by reducing the potential for injury. When stimulated, tendon spindles inhibit the contracting (agonist) muscles and excite the antagonist muscles.
Coordination between agonist and antagonist muscles is crucial in maintaining normal mechanical balance in the joint, as well as in protecting the muscles from overstretching.
When muscles are fatigued or overtrained, they become shorter and less flexible, with soreness or sensation of pain. If these symptoms are ignored and the muscles are forced to work, the stiff muscles will transfer the stress to the tendon, resulting in tendinitis. This indicates that in the treatment of tendinitis, both muscles and tendons should be treated simultaneously.
Figure 4-6 illustrates a situation in which reflexes protect the muscles and tendon from injury. If a person moves one arm backwards quickly toward its outermost position to do a powerful throwing action, and if the arm moves backwards too far or with too much speed, there is a risk of muscle or tendon rupture. Normally, the muscle spindles send a warning signal and the muscle contracts; the arm will then stops and turns back before it reaches
the critical position. If the muscles are fatigued or injured, this movement will not be controlled precisely, and it is possible to tear or rupture the muscles or tendons.
When a muscle contracts, the tension in the tendons increases. Figure 4-7 depicts the direction of the forces sustained in the tendon. This shows how both conflicting forces contribute to tendinitis.
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