Sherrington's Law of Reciprocal Innervation
AGONIST, ANTAGONIST, ANTAGONIST, AND YOK !"S#LS$ !"S# LS$ As As a rule, every every contraction contraction of a muscle brings about a movement. Considered as the mover producing that movement, the muscle is called an agonist . A movement in the direction opposite that produced by the agonist is caused by its antagonist . antagonist . Thus, the medial rectus muscle adducts the globe, and the lateral rectus muscle abducts it. The medial and lateral rectus muscles are antagonists. Two muscles moving an eye in the same direction are synergists. synergists. The superior oblique muscle and the inferior rectus muscle are both depressors of the eye. As such they are synergists. However, the superior oblique muscle causes an incyclorotation and the inferior rectus muscle, an excyclorotation. In respect to cyclorotation, they are antagonists. ynergistic muscles in the two eyes!muscles that cause the two eyes to move in the same direction !are "nown as yoke muscles. ome readers may not be familiar with the word ##yo"e.$$ In fact, in correcting examination papers and even in the ophthalmic literature we have seen this word not infrequently misspelled ##yol".$$ The reader who grew up in an urban society or in a country where draft animals have been replaced by agricultural machines may not "now that yo"e refers to a ##wooden bar or frame by which two draft animals %e.g., oxen& are 'oined at the head or nec"s for wor"ing together.$$()) *hen one yo"ed animal moves in a certain direction, so must the other, and the same applies to the cooperation between a pair of yo"e muscles. In passing we note that although there are words for yo"e in many languages, this vivid and didactically useful allegory in explaining the functional lin"age between the extraocular muscles of the two eyes is to our "nowledge limited to the +nglish ophthalmic literature %see also Chapter -&. -&. The right medial rectus and the left lateral rectus muscles cause levoversion of the eyes. They are yo"e muscles. As pointed out earlier in this chapter, a pair of muscles in one eye can be yo"ed with a pair in the other eye. or instance, the elevators of one eye %superior rectus and inferior oblique muscles& are yo"ed as a unit to the elevators of the fellow eye, and the two pairs of depressors are similarly yo"ed. Thus, antagonistic muscles act on the same eye, and yo"e muscles act on both eyes. /o"ing may change according to the different type of eye movements. or instance, the medial rectus muscle of one eye is yo"ed with the lateral rectus muscle of the other eye in lateroversion, but the medial rectus muscles in both eyes are yo"ed during convergence. The term contralateral antagonist , used in connection with so0called inhibitional palsy %see Chapter -&, &, is contradictory and should be avoided. This term refers to the antagonist antagonist of the yoke muscle. muscle . or example, a patient with paralysis of the right superior oblique muscle who habitually fixates with the right eye will have an apparent paresis of the left superior rectus muscle. The explanation for this is that in the fixating eye the innervation of the pairs of elevators %superior rectus and inferior oblique muscles& is below normal because of loss of the opposing action of the paraly1ed superior oblique muscle. Thus, in accordance with Hering$s law %see p. 23&, equally diminished innervation will flow to the elevators of the left eye and the left eye does not elevate fully. +levation will become normal, however, when the left eye ta"es up fixation. S%RRINGTON&S LA$ *henever LA$ *henever an agonist receives an impulse to contract, an equivalent inhibitory impulse is sent to its antagonist, which relaxes and actually lengthens. This is Sherrington’s law of reciprocal innervation, innervation ,(( which implies that the state of tension in the agonist exerts a regulatory influence on the state of tension in the antagonist and vice versa. The finely graded interplay between opposing eye muscles ma"es movements of the globe smooth and steady.
*hether there are actually active centrifugal inhibitory neural impulses flowing to the antagonist as the agonist contracts or whether there is merely an absence of innervation is not clear. herrington$s law applies to all striated muscles of the body and is not limited to the extraocular muscles. The basic mechanism underlying agonistic and antagonistic muscle action was clearly understood by 4escartes, )- more than 5- years before herrington$s classic experiment ((, ( in which he demonstrated reciprocal innervation of the extraocular muscles. *hen herrington severed cranial nerves III and I6 intracranially on one side %e.g., on the right&, a paralysis occurred in all the extraocular muscles except the lateral rectus muscle, which was innervated by cranial nerve 6I. The right globe was now in divergent position. After allowing time for the motor nerves to the extraocular muscles to degenerate, herrington electrically stimulated the right cortical area, eliciting a con'ugate deviation of the eyes to the left. The left eye turned all the way to the left, but the right eye moved only to the midline. This behavior of the right eye gave evidence that the original divergent position was the result of a contraction of the right lateral rectus muscle, which was unopposed by the tonus of its antagonistic right medial rectus muscle. *hen a levoversion impulse was induced, the reciprocal innervation to the right lateral rectus muscle caused it to relax to the point where the globe could return to the midline. The validity of herrington$s law of reciprocal innervation now has been established in intact human eyes by means of electromyography . igure 37(- demonstrates electrical silence in the left lateral rectus muscle with the eyes in extreme dextroversion and in the right medial rectus in extreme levoversion. A comparable result is achieved when recordings are made from hori1ontal rectus muscles during calorically induced nystagmus.
FIGURE 4–10. Electromyographic
evidence for reciprocal innervation of extraocular muscles. Upper tracing from left lateral rectus muscle (LLR); lower tracing from left medial rectus muscle (LMR). n extreme right lateral ga!e (RL") the LLR is electrically silent and the LMR is electrically active. n extreme left lateral ga!e (LL") the LMR is electrically silent and the LLR is electrically active. (#ourtesy of $r. "oodwin M. %reinin.)
8eciprocal innervation is physiologically and clinically important. It explains why strabismus occurs following paralysis of an extraocular muscle. 8eciprocal innervation must be considered when surgery on the extraocular muscles is performed. Co0contraction of antagonistic muscles instead of relaxation of the antagonist, for instance, as demonstrated by electromyography in the retraction syndrome %see Chapter -&, is said to be always abnormal in eye muscles, although it does occur in s"eletal muscles. ome clinical applications of herrington$s law are shown in igure 37((.
FIGURE 4–11. &herrington's
law of reciprocal innervation. A n levoversion increased contraction () of the right medial rectus (RMR) and left lateral rectus (LLR) is accompanied *y decreased tonus (+) of the an tagonistic right lateral (RLR) and left medial rectus (LMR) muscles. B ncreased activity of *oth medial rectus muscles and decreased tonus of *oth lateral rectus muscles during convergence. C #ontraction and relaxation of opposing muscle groups on dextrocycloversion when the head is tilted to the left shoulder. R& right superior o*li,ue; R&R right superior rectus; L& left superior o*li,ue; L&R left superior rectus; R right inferior o*li,ue; RR r ight inferior rectus; LL left inferior o*li,ue; LR left inferior rectus. (-rom oorden "/ von0 1tlas of &tra*ismus ed 2. &t. Louis Mos*y34ear %oo5 6789.)
