Lüe Sciences No. 12, pp. 948-952, 1963 . Pergamon Press, Inc . Printed in the United States .
INCREASE OF PRIMARY ENDINGS DYNAMIC SENSITIVITY BY SLOW a FIBRES INNERVATING MUSCLE SPINDLES P . BESSOU, F . E~iONET-DENAND and Y . LAPORTE Laboratory of Physiology, Faculty of Medicine, Toulouse, France (Received 18 October 1963) It has been shown by Besaou, Emonet-Déaand and Laporte that, in the cat, some slow motor fibres to the first deep lumbi~ical muscle innervate both eatrafusal and intrafusal muscle fibres . The repetitive stimulation of a single fibre xith this doubla distribution produces contraction of a motor unit with low tetanic fusion frequency, and acceleration of spindle primary endings discharges at stimulation frequencies higher than 30 - 60 per sec . Thin acceleration has been attributed to intrafusal muscle fibre contraction and not to an indirect activation of primary endings by contraction of eatrafusal muscle fibres for two reasons : 1) it increases markedly for frequencies of stimulation greater than the fusion frequency of eatrafusal fibres and 2) it persists after intravenous injection of small doss of gallamine triethiodide (Flaaedil) which are sufficient to eliminate contraction in eatrafusel muscle fibres . The effect of repetitive stimulation of these motor fibres on the response of spindle primary endings to constant velocity stretch has been studied in siz adult cats anesthetized with Nembutal . Afferent discharges were recorded from single group I fibres in dorsal root filaments (L7 and S1) . The instantaneous frequency, i .e ., the reciprocal of the time bettyeen spikes, was measured from appropriate enlargement of photographic records . The loxer lumbar and sacral ventral roots were cut, and single motor fibres were pre948
No . 12
MUSCLE SPINDLE DYNAMIC SENSITIVITY
pared for stimulation by subdivision of the St
and 52 ventral roots .
First deep lumbrical muscles were stretched 0 .7
to 1 millime-
ter at constant velocities, between 2 to 4 mm/sec ., by an electromagnetic pulley (Goodmans type V 47) . The pulley was driven by a transistor amplifier from voltage changing at a constant rate . Displacement of the shaft of the pulley was recorded by a photoelectric cell . Prior to muscle stretching, single motor unit contractions were recorded with a sensitive isometric myograph constructed with a ailicium strain-gauge (Plicrosensor type Ms 632-350) . Repetitive stimulation of single a motor fibres with intra and eztrafueal distribution consistently produced a marked increase in discharge of spindle primary endings during the dynamic phase of stretch . The spindle dynamic response, i .e . the difference between the instantaneous discharge frequency at the end of dynamic stretch and that found one-half second later (Jensen and P . Matthews, 2), was greatly augmented . On the other hand, the static response i .e ., the instantaneous frequency one-half second after the end of a stretch (Jensen and P . Matthews, 2), was much less increased . A typical example of this effect is shown by Fig . 1 . In the ezperiment illustrated the discharge of two spindle primary endings belonging to the same first deep lumbrical muscle were recorded from two L7 dorsal root filaments . Under static conditions the discharges of both endings were accelerated by the repetitive stimulation of the single a fibre in a manner consistent xith intrafusal muscle fibre contraction (see above) . The discharges of the two endings were observed during and after a stretch of 1 mm . a t a constant velocity of 2 .8 mm/second . In the absence of any fuaimotor fibre stimulation, spindle a° 1 had a dynamic response of 40 impulsea/sec . and a static response of 30 impulses/sec . (graph . 1A) ; spindle n° 2 a dynamic response of 60 impulses/sec . and a static
MUSCLE SPII~iDLE DYNAMIC SENSITIVITY
FIG . 1 soo do~ U W W
300 Zoo ioo 0 soo ao0
"ham"" ` :i .
Effect of the repetitive stimulation of a single a fibre innervating txo spindles (tat deep lumbrical muscle of the cat) on the response of their primary endings to stretch of constant velocity . Each point of the graphs represents an impulse ; the value of its ordinate given the instantaneous frequency of this impulse (see the tezt) . The muscle xas stretched by one millimeter at the cons tant velocity of 2 .8 mm/sec (continuous lines) . 1
s Discharge~ of e primary ending connected to a group I fibre of 95 m/sec conduction velocity .
s Discharge~ of a primary ending connected to a group I fibre of 98 m/sec conduction velocity .
1A - 2A = Discharges in the absence of say fuaimotor fibre stimulation . 1B - 2H ; Discharges during the a fibre repetitive stimulation (broken lines) . This fibre (conduction velocity 56 m/sec) xas stimulated at a frequency of 105 per sec . The stimulation started one second before the onset of stretch . response of 50 impulses/sec . (graph . 2A) . The single motor fibre xas then stimulated at a frequency of 105/sec, the stimulation beginning one second before the onset of stretch (graph . 1B and 2B) . This stimulation resulted in a very marked increase of the
No . 12
MUSCLE SPII~TDLE DYNAMIC SENSITIVITY
discharge frequency during the stretch . The dynamic response increaeed from 40/ eec . to 290/ sec . i n spindle n° 1, a difference of 250 impulses per sec ., and from 50/ sec . t o 265/sec in spindle n° 2, a difference of 215 impulses/sec . The static response xae augmented only by 90 impulses for spindle n° 1 and by 85 impulses for spindle n° 2 . Various frequencies of stimulation, ranging betxeen 25 and 220 per second, xere used . It xas regularly found that the increase of dynamic response augmented xith the frequency of stimulation . A mazimal effect occurred for frequencies in the vicinity of 150 180/sec ., i .e ., for frequencies xell above the fusion frequency of the eztrafueal muscle fibres innervated by these a fibres showing that the increased response to stretch was mostly, if not entirely, due to intrafueal fibre contraction . The increased dynamic sensitivity of the primary endings xae present at stimulation frequencies as lox ea 25 per sec ., xhich under static conditions resulted is a pause of the discharge of primary endings, due to the contraction of eztrafueal muscle fibres (1) . Ia addition to study of the effects produced by this a fibre, the action of five pure fusimotor fibres ; or Y fibres, which also innervated both spindles xae observed . One of them increased the dynamic response of both endings but to a lesser degree than did the a fibre . Another fibre markedly increased the static response of both endings xhile having no effect on the dynamic response . Ths three other fusimotor fibres moderately increased the static responce of both endings and had small and variable effects upon the dynamic response . P . Matthexe (3) has recently demonstrated the ezistence of fusimotor fibres xith either dynamic or static effectâ in spindles
MUSCLE SPINDLE DYNAMIC SENSITNITY
of coleus muscles . Oa the basis of their effects on primary endings, the slox a fibres innervating both eztra and iatrafusal muscle fibres can be considered as "dynamic" fibres . If ie accept the plausible hypothesis of Jensen and Mattheis (2) that the terminals surrounding the nuclear bag fibres are responsible for moat of the discharge initiated by the primary endings during phaaic stretch and that terminals surrounding the nuclear-chain fibres contribute the greatest part of the discharge initiated under static conditions, ie are led to suggest like P . Matthexe (3) did for the fibyes xith dynamic effects that these sloe a fibres innervate nuclear bag fibres . Hoxever, since the action of these fibres on spindle static responses is not negligeable, xe cannot ezclude they also innervate nuclear chain fibres . .The physiological importance of the increase in phaaic aeasitivity of the primary endings produced by the slox cx fibres ianereating both eztrafusal and intrafusal muscle fibres is difficult to assess because their number in various musolea ie not knoxn . If these fibres sere numerous, their reflez activation could have the interesting consequence of producing muscle tonic contraction xhich xould be accompanied by an increase of spindle primary ending responaiveaesa to phaaic stretches . References 1 . P . BESSOÛ, F . EMONET-DENAND and Y . LAPORTE, Mnture . G .B . , ,1~8, 594 (1963) . 2 . J .S .S . JANSE N and P .$ .C . MATTHEWS, J . Phvsiol . , 16~1, 357 (1962) . 3 . P .B .C . KATTHEWS, Quart . J . Ezy~ r . Phvsiol . , ,~, 324 (1962) .