IS ALL NERVOUS SYSTEM ACTIVITY REFLEXIVE?
The simplest human reflex, elicited by rapidly stretching a muscle, is called the tendon (or deep tendon or muscle stretch) reflex. The afferent end is the muscle spindle (Figure 6–1), and the sensory pathway is monosynaptically linked to motor neurons, resulting in contraction of the stretched muscle (Figure 6–2). Depending on your concept of free will, the most complicated motor behaviors, and even thinking itself, might be considered reflexic, with internal or external stimuli triggering an exceedingly complex reflex arc that ultimately produces an effecter response. We previously focused on such free will reflexes. In this part of the neurological examination, we focus on monosynaptic tendon reflexes and polysynaptic superficial reflexes of a simple, stereotypic nature.
The main components of the muscle spindle are intrafusal muscle fibers, sensory axon endings, and motor axon endings. The intrafusal fibers are specialized muscle fibers with central regions that are not contractile. Gamma motor neurons innervate the contractile polar regions of the intrafusal fibers. Contraction of the polar regions pulls on the central regions of the intrafusal fiber from both ends. The sensory endings spiral around the central regions of the intrafusal fibers and are responsive to stretch of these fibers. (Reproduced with permission from Kandel ER, Koester JD, Mack SH, Siegelbaum SA. 2021. Principles of Neural Science, 6th ed. New York: McGraw Hill. Adapted with permission from Hulliger M. 1984. The mammalian muscle spindle and its central control. Rev Physiol Biochem Pharmacol 101:1-110. Copyright © Springer-Verlag 1984.)
In monosynaptic pathways, sensory axons from muscle spindles make excitatory connections on two sets of motor neurons: alpha motor neurons that innervate the same (homonymous) muscle from which they arise and motor neurons that innervate synergist muscles. They also act through interneurons to inhibit the motor neurons that innervate antagonist muscles. When a muscle is stretched by a tap with a reflex hammer, the firing rate in the sensory fiber from the spindle increases. This leads to contraction of the same muscle and its synergists and relaxation of the antagonist. The reflex therefore tends to counteract the stretch, enhancing the spring-like properties of the muscles. The records on the right demonstrate the reflex nature of contractions produced by muscle stretch in a decerebrate cat. When an extensor muscle is stretched, it normally produces a large force, but it produces a very small force (dashed line) after the sensory afferents in the dorsal roots have been severed. (Reproduced with permission from Kandel ER, Koester JD, Mack SH, Siegelbaum SA. 2021. Principles of Neural Science, 6th ed. New York: McGraw Hill. Adapted with permission from Liddell EGT, Sherrington CS. 1924. Reflexes in response to stretch (myotatic reflexes). Proc R Soc London B Biol Sci 96:212-242.)