Muscle Spindle: A Servo In Its Own Right

Your refrigerator or air conditioner automatically switches off their compressors when their preset thermal set points are reached. They resume again on their own when the entropy (heat) rises inside. This is an example of servo mechanism: a sensor arm, processing unit and an actuator arm. The apparatus senses heat (more specifically temperature), uses the information and compares it with the preset value (error correction) and then sends signals to the motor (the actuator) to turn it on (or off). This error sensing feedback to optimize a mechanism is what servo is all about. The picture depicts how a scanning tunneling microscope uses this servo mechanism technique.

Such controls are abundant in our bodies. The muscles, for example, contain miniature 'sensors' called intrafusal fibers. They reside in the interior belly of the muscle bulk. These fibers run parallel to the rest of the muscle fibers, the extrafusal fibers. Intrafusal fibers are of two types: nuclear bag fibers (bag, sincethe middle of this fiber is swollen) and nuclear chain fibers (these fibers end on the previously mentioned nuclear bag fibers). The number of these intrafusal fibers in a muscle are scanty and there are few contractile elements in them, which are located at the ends. The center portions of these fibers are devoid of contractile elements.

If you stretched a muscle by pulling it or by tapping on the patellar tendon, the quadriceps muscle would become elongated. The intrafusal fibers would lengthen too. The central portion of these fibers would stretch just as a balloon does when stretched. The central portions of Intrafusal fibers are supplied by sensory nerve fibers, called type 1a nerve fibers. Since the nerve fibers are wrapped intimately around (annulospiral or primary endings in both bag and chain type; and flower spray or secondary endings in case of chain fibers) these portions, the nerve endings stretch too along with the muscle fibers themselves. This opens up stretch sensitive ion channels, allowing cations (positively charged ions like sodium ions) to enter the cells interior. The cells depolarize and fire. The information is carried (as shown) to the spinal cord which springs into action by sending action potential via alpha motor neurones, through a monosynaptic pathway. These nerves innervate extrafusal fibers, fibers rich in contractile proteins, the muscle contracts as a result: stretch reflex.

Although the intrafusal muscles which constitute the muscle spindle (a sketch is shown alongside) are poorly endowed with contractile machinery, they do have some at their ends. These elements are supplied with motor nerves called gamma efferent of Leksell. As the muscle contracts in the said example, we can visualize that the central portions are no longer stretched, rather they are relaxed. How will the muscle know how much work it has yet to do when its informing machinery is limp and no longer in a state to give its valuable feedback? This is achieved by the super intelligent computer, our body, which energizes the gamma efferents each time alpha motor neurons are activated: alpha gamma coactivation. Now as gamma motor neuron discharges, the contractile elements of the intrafusal fibers contract. Thus you can see that the central portion is stretched again. So each time the muscle contract, intrafusal fibers contract too, making sure that type 1a fibers were able to process and send information on muscle length and the rate of change of this length. The muscle is doing differential calculus by computing ds/dt (rate of change of length)!

(Our good old speedometer also does calculus using a magnet (mechanically coupled to the wheel) and a coil (or a metal foil) to compute rate of change of flux as a function of time. The magnet of a car also rotates (RPM= a function of time) as its wheel rotates. This rotating magnet induces current (change of flux) in a metal disk with a needle, which then indicate as a result of magnetic repulsion. Calculating distance is rather easy: 2*pi*r)

Gamma efferents also maintain muscle tone by firing continuously. Gamma discharge-->intrafusal fibers contract-->stretches the central portion-->1a fibers activated-->alpha motor discharge-->extrafusal fibers contract. Muscle tone is defined as the resistance the muscle offers to passive stretch. Another mathematician that works silently is the Golgi tendon organ. These sensors are located in the tendinous ends of muscles. In contrast to the muscle spindle which are parallel to the extrafusal fiber (hence information on length), the tendon organ are in series with the muscle fibers (hence info on tension). The Golgi tendon organ uses Newton's third law to calculate the tension and the rate of change of it (force=tension, suppose, A is in series with B is pulled by C, tension in B will be the same as the force in A). The output from this sensor is inhibitory, helping the muscle to relax when the tension is great, preventing muscular tear or bone avulsion. As far as I know, tendon organs used to do their task from pre-Newtonian times!! And it doesn't really give a damn who invented calculus; Newton or Leibniz.

The above mentioned concept is important if we are to understand spasticity, rigidity, actions of muscle relaxants and various ailments.

Last modified: March 20, 2009
Reference: Textbook of medical physiology, 11e, Guyton & Hall, pp 673-684