1. Motor Units and Muscle Receptors
Part 3 of 4
James Knierim, Ph.D.
Muscle Receptors and Proprioception
The motor system requires sensory input in order to function properly. In addition to sensory information about the external environment, the motor system also requires sensory information about the current state of the muscles and limbs themselves. Proprioception is the sense of the body’s position in space based on specialized receptors that reside in the muscles and tendons. The muscle spindle signals the length of a muscle and changes in the length of a muscle. The Golgi tendon organ signals the amount of force being applied to a muscle.
Muscle spindles are collections of 6-8 specialized muscle fibers that are located within the muscle mass itself (Figure 1.7). These fibers do not contribute significantly to the force generated by the muscle. Rather, they are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle. Because of the fusiform shape of the muscle spindle, these fibers are referred to as intrafusal fibers. The large majority of muscle fibers that allow the muscle to do work are termed extrafusal fibers. Each muscle contains many muscle spindles; muscles that are necessary for fine movements contain more spindles than muscles that are used for posture or coarse movements.
Figure 1.7 |
Muscle spindle and Golgi tendon organ |
Sensory Innervation of Muscle Spindles
Because the muscle spindle is located in parallel with the extrafusal fibers, it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialized sensory fibers that innervate the intrafusal fibers. These sensory fibers have stretch receptors that open and close as a function of the length of the intrafusal fiber.
- Group Ia afferents (also called primary afferents) wrap around the central portion of all 3 types of intrafusal fibers; these specialized endings are called annulospiral endings. Because they innervate all 3 types of intrafusal fibers, Group Ia afferents provide information about both length and velocity.
- Group II afferents (also called secondary afferents) innervate the ends of the nuclear chain fibers and the static nuclear bag fibers at specialized junctions termed flower spray endings. Because they do not innervate the dynamic nuclear bag fibers, Group II afferents signal information about muscle length only.
Because of their patterns of innervation onto the three types of intrafusal fibers, Group Ia and Group II afferents respond differently to different types of muscle movements. Figure 1.9 shows the responses of each type of afferent to a linear stretch of the muscle. Initially, both Group Ia and Group II fibers fire at a certain rate, encoding the current length of the muscle. During the stretch, the two types differ in their responses. The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the muscle length; at the end of the stretch, its firing decreases, as the muscle is no longer changing length. Note, however, that its firing rate is still higher than it was before the stretch, as it is now encoding the new length of the muscle. Compare the response of the Group Ia afferent to the Group II afferent. The Group II afferent increases its firing rate steadily as the muscle is stretched. Its firing rate does not depend on the rate of change of the muscle; rather, its firing rate depends only on the immediate length of the muscle.
Figure 1.9 |
Responses of muscle spindles. The Group Ia afferent responds at a highest rate when the muscle is actively stretching, but also signals the static length of the muscle because of its innervation of the static nuclear bag fiber and the nuclear chain fiber. The Group II afferent signals only the static length of the muscle, increasing its firing rate linearly as a function of muscle length. |
Although intrafusal fibers do not contribute significantly to muscle contraction, they do have contractile elements at their ends that are innervated by motor neurons.
Figure 1.10 |
Alpha-gamma coactivation. The muscle starts at a certain length, encoded by the firing of a Ia afferent. When the muscle is stretched, the muscle spindle stretches and the Ia afferent fires more strongly. When the muscle is released from the stretch and contracts, the muscle spindle becomes slack, causing the Ia afferent to fall silent. The muscle spindle is rendered insensitive to further stretches of muscle. To restore sensitivity, gamma motor neurons fire and cause the spindle to contract, thereby becoming taut and able to signal the muscle length again. |
Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers, which contract only slightly. The function of intrafusal fiber contraction is not to provide force to the muscle; rather, gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch, over a wide range of muscle lengths. This concept is illustrated in Figure 1.10. If a resting muscle is stretched, the muscle spindle becomes stretched in parallel, sending signals through the primary and secondary afferents. A subsequent contraction of the muscle, however, removes the pull on the spindle, and it becomes slack, causing the spindle afferents to cease firing. If the muscle were to be stretched again, the muscle spindle would not be able to signal this stretch. Thus, the spindle is rendered temporarily insensitive to stretch after the muscle has contracted. Activation of gamma motor neurons prevents this temporary insensitivity by causing a weak contraction of the intrafusal fibers, in parallel with the contraction of the muscle. This contraction keeps the spindle taut at all times and maintains its sensitivity to changes in the length of the muscle. Thus, when the CNS instructs a muscle to contract, it not only sends the appropriate signals to the alpha motor neurons, it also instructs gamma motor neurons to contract the intrafusal fibers appropriately; this coordinated process is referred to as alpha-gamma coactivation.
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