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Acta physiol. scand. 1973. 89. 482-490 Froin the Depai tnient of Physiology, Karolinska Institutrt, Stockholni. Snedcn Contraction Properties of Cat Facial Muscles BY CHRIST ER LINDQUIST Received 3 1 January 1 Yi3 Abstract LINDQLUST, CHR. Coritractiorr propurtics of cat faciul rui~~c./c.\. Acta physiol. scantl. 1973. 89. 482-490. ‘l’lie contraction prciperties of swne facial nitiscles of the cat havr l)ern investigated. ‘l‘he con- traction time of a niaxinial isometric twitch is 8.5 111s for the orbicularis oculi. 21 ins f(ir the depressor conchae and 33 nls for the orbicularis oris. It is suggested that a slower decay of tlir active state may be one factor cxplaining the longer contraction time of thc orbiciilaris oris as compared to the orhiciilaris oculi. The duration of a Iriaxitnal cornpoiincl action potential is also longer for the orhicularis oris than for the orbiciilaris oculi (8.5 and 2.8 111s resprctivvly). hfuscle length is inore critical for the twitch anlplitude of tlie orbicularis ociili than for tliv orl)icirlaris oris. Summation of individual twitclirs occurs at a stimulus frequency of rniiiiniiirir 2OL 22 132 for the orbicularis ociili and at I(,-12 Hz for the orl)iciilaris oris. Apparent fusion occurs at 100 Hz for the orbiciilaris oculi and at 60 Hz for tlie orbicularis oris. ‘I’lir ratr of tension developrnent increases for both muscles with stiinulus freqiiencics up to 200 HZ. Averiigr tetaniis/ twitch ratios are 4.5 for the orbicularis oris and 7.0 for the orbicular.is oculi. In the cat polysynaptic reflexes can be elicited in the orbicularis oculi by tapping the skin around the eye and on the nose, arid sinlilar reflexes are evoked in the auricular muscles Iiy an innocuous mechanical stirnulation of the pinria. It is obvious that the reflex contractions of the orbicularis oculi and the auricular inuscles sene protectivr functions. Such reflexes cannot be evoked in the orbicularis oris rririsrle by nicc~haiii- cal stimulation of the face (Lindquist and MArtensson 1970). Instead. this iriusclr serves more sustained motor activity. Considering the different fuiictions it rnny I)e reasonable to assume that the orbicularis oculi and the orlkularis oris also haw dif- ferent contraction properties. This matter is investigated in this and in a following paper (Edstriiim arid Lindquist 1973). Methods 25 trarhpotomized cats anesthetized i.p. with NenibutalQ, 40 1ng;kg b.wt., werc usd. I3lirik rrflrx contractions were recorded also in cats undcr ctiloralose anestfiesia (50-70 trig/ kg b.wt. i.v. 1. Recording of contractions from individual facial muscles is vcry difficult sincc they lack tendons, are often interwoven and sometimes inscrt into the skin. Dissection is thus liziblr to injure the inuscle fibers. Most recordings have therefore bcrn rnadc with tlie skin intact. Con- 482

Contraction Properties of Cat Facial Muscles

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Page 1: Contraction Properties of Cat Facial Muscles

Acta physiol. scand. 1973. 89. 482-490 Froin the Depai tnient of Physiology, Karolinska Institutrt, Stockholni. Snedcn

Contraction Properties of Cat Facial Muscles BY

CHRIST ER LINDQUIST

Received 3 1 January 1 Y i 3

Abstract

LINDQLUST, CHR. Coritractiorr propurtics of cat faciul r u i ~ ~ c . / c . \ . Acta physiol. scantl. 1973. 89. 482-490.

‘l’lie contraction prciperties of swne facial nitiscles of the cat havr l)ern investigated. ‘l‘he con- traction time of a niaxinial isometric twitch is 8.5 111s for the orbicularis oculi. 2 1 ins f ( i r the depressor conchae and 33 nls for the orbicularis oris. I t is suggested that a slower decay o f tlir active state may be one factor cxplaining the longer contraction time of thc orbiciilaris oris as compared to the orhiciilaris oculi. The duration of a Iriaxitnal cornpoiincl action potential is also longer for the orhicularis oris than for the orbiciilaris oculi (8.5 and 2.8 111s resprctivvly). hfuscle length is inore critical for the twitch anlplitude of tlie orbicularis ociili than for tliv orl)icirlaris oris. Summation of individual twitclirs occurs at a stimulus frequency of rniiiiniiirir 2 O L 2 2 132 for the orbicularis ociili and a t I(,-12 Hz for the orl)iciilaris oris. Apparent fusion occurs at 100 Hz for the orbiciilaris oculi and at 60 Hz for tlie orbicularis oris. ‘I’lir ratr o f tension developrnent increases for both muscles with stiinulus freqiiencics up t o 200 HZ. Averiigr tetaniis/ twitch ratios are 4.5 for the orbicularis oris and 7.0 for the orbicular.is oculi.

In the cat polysynaptic reflexes can be elicited in the orbicularis oculi by tapping the skin around the eye and on the nose, arid sinlilar reflexes are evoked in the auricular muscles Iiy an innocuous mechanical stirnulation of the pinria. I t is obvious that the reflex contractions of the orbicularis oculi and the auricular inuscles s e n e protectivr functions. Such reflexes cannot be evoked in the orbicularis oris rririsrle b y nicc~haiii- cal stimulation of the face (Lindquist and MArtensson 1970). Instead. this iriusclr serves more sustained motor activity. Considering the different fuiictions i t rnny I)e reasonable to assume that the orbicularis oculi and the orlkularis oris also h a w dif- ferent contraction properties. This matter is investigated in this and in a following paper (Edstriiim arid Lindquist 1973).

Methods 25 trarhpotomized cats anesthetized i.p. with NenibutalQ, 40 1ng;kg b.wt., werc u s d . I3lirik r r f l rx contractions were recorded also i n cats undcr ctiloralose anestfiesia (50-70 trig/ kg b.wt. i.v. 1 . Recording of contractions from individual facial muscles is vcry difficult sincc they lack tendons, are often interwoven and sometimes inscrt into the skin. Dissection is thus liziblr to injure the inuscle fibers. Most recordings have therefore bcrn rnadc with tlie skin intact. Con-

482

Page 2: Contraction Properties of Cat Facial Muscles

FACIAL MUSCLE PHYSIOI.OGY 483

I:ig. 1. Isometric twitch con- tractions of the orbicularis oculi ( A ) , the depressor conchae ( R ) and the orbi- cularis oris ( C ) in rrsponsc* to suprarria..inial stiiiiulation of thc-ir respectiw motor iicrvc. ’I’inie bar 20 i n s .

traction ciirves f rom tlie ~ ~ r l ~ i c i i l ~ i i is nris were obtained by ;I string OI ‘I I i r c . t n l I ~ o o k attached t o the angle of the iiioutli and tlic transducer. I n a siniiliir wiiy tlic contractions f l . o n 1 tlie orhi- cularis oculi were recorded ironi the upper or I C J W ~ Y lid. ’ lhe 111~1sc1rs did probably not p1111 CIII

the rrcording instrument with niasinial force, but tliis sllould 11avr no bc*:iring on the int r r - pretation o f the rrsiilts. I n the niajoritj of the esperiliients tlic ~ i i i i ~ l r s were activated in- directly b y stiiiiulation of their individual m o t o r nerves. I n tliis way it i h rc.;lsod)ly safe to con- clude that the stiltiulation activated a single iririsclc, wherc;is direct 1111isc1e stiniiilation increasrtl the possibility of coxt iva t ion of otlier facial ~nusclcs. ‘l’lic peripheral end of wverrd nerve\ was stiiii~ilated witli h i p i h r chloridcd silver wire electrodes. Stiiiiiilu:, pulses oi 0.1 i n s duration were deliverid b y a Grass S4 stiniulator. U‘lirn needed, reescitation of i i iotor n r r \ v mdings \V:IS

prevcntrd by giving double closely spiccd stilniilus pii~scs (rf. Brown and hf;itthews 1960). Near-isoiiietric iiiiihcle coiitractiniis ; i t optiinal iiiuscli. length were recorded with ;I Grass F’l’

03 transducer ( self-resc:nance frcqiit,ncy 100 200 H z ) o r with ;I specixlly constructed h;ii.iiiiii titanate transducer ( sclfresoniince frequency 2000 Hz) . l ‘ h e fornicr W:IS i i s r d when thc. lowest freqiieiicy o f nerve stiniulation gi\,ing iiiasiitial ci)ntr~ictile tension \V;LS so1lght. ‘I’his irc-qii(.ricy ir, rciciwd to a s th r “apparent itision” irecliiency j (1. Buller and 1,ewis l!Ni5 1 . ‘I’hc Iiigh-sc.11- sitivity hai i i i r i i t itanate t randucc i . was iisetl to f i r i t l the freque!icy o f i i c r \ ~ stinuilation giving fusion of inusclr contractions even \vlieii rctcordrd a t high ainplification. ‘I‘his freqiirncy is rrferred to as the “absolute fusion frrcliitmcy”. ’I‘he recil~rocal o f thr ;rtjsolutc fusion frcqucncy shoiild be a fair estiniate o f the duration of tlie n i ~ ~ x i n i a l intensity of the active state in a singlc twitch (cf. hl&tcnsson and Skoglund I!464,. ’l’his v;ilue m a y also be rstiiiiated by tlic i i i e t h o c l of hlacl‘herson and Wilkic [ 1954 I and by the nrethod of Gabel, Carson and Vance ( 1968 1 . By the forniei- niethod the tmsion cii ives resulting froiii one and two stiriiuli are co:iipircd and the shortest nirchanical separation tiinr observed indicates the onset o f decay of the acti\,c statc- “plateau”. Thi, lattcr niethod is based on the opinion h a t tlie titlie f r o i n tht. beginning of the twitch to p w k tension acceleration coincides with the \~alur for the durat ion of the activc stat(‘ “plateau”. ‘The tension acceleration is obtained by electronic doiible difft.reIitiation of t l i t , twitcli tension curve. l ’his was acconil~lishrd b y passing thr signal f r o i i i [lie transducrr tliroiigti ii tirtw constant of 100 ! t s . ’l’lie transducer was connrcted t o a catliodc follower and a C;i-;iss P 6 d(, ainplifier. ‘The recorded signals were displayed on a ‘l‘ektronis 50‘1 di in l - l~c; r i i i oscilliscope.

hfiiscle action potrntials w c w i.ecorded with gross uninsulated steel n c d l e s 01’ with bipolar DIS.4 concentric needle clrctrodi~s and displayed on the oscilloscope.

1 he ternperattire in t h c , Licc wiis controlled by a n infrared heating l a i i i p and kept within 34 --39” C: which gavr consistent \ . a l r i r a o f t tic contraction timias.

,.

Results Twitch coritrtictioti rlra,.cic.tc~ri.,tic..r. Isometric t \ \ itches at optimal muscle lctigth of the orbicularis oculi ( A ) , the dcprcssor conchae (H j arid the orbicularis oris ( C i i i

response to supramaxirnal stimulation of their respective rriotor nerve are shown i t 1

Fig. 1. The contraction tirnc is shortest Tor the orbicularis oculi (8.5 ins) and longest for the orbicularis oris (33 111s). whereas an intermediate value is typical o f thc depressor conchae (24 111s). ‘Table I surnrriarizes the experiiiierital findings as reg-arch the contraction times of the three rnusrles. Conipoiind action potentials rccorded 011

supramaxitrial motor nerve stirnulation in the niuscle with thc longest contractiott time (orbicularis oris) were of longer duration than those rrrorded in thc Iriusclc with the shortest contraction time (orbicularis oculi j . T h e range of potential dura-

Page 3: Contraction Properties of Cat Facial Muscles

484 CHRI STICK LI NDQLIS‘I _ _ 1 . \ R I . I ; I. Contraction titrirs of facial muscles

.\I\lsclc Kangc of iivcragc Nuniher Of

contraction tiirirs contraction tirric experirnrnts Ills Ills

Orbicularis oculi 7.5-1 0.0 8.5 6 Ohiciilaris oris “7- 35 3 3 8 1)vprrssor conchat. 24 3

rioiis for the foniicr iriiiscle was 6.1-9.4 1x1s arid the average duration 8.5 n i b . w l i tw-

a s for the latter muscle the range was 2.4-3.0 ins and the average 2.8 Ins. \;;iriations in muscle length have very small cffects on the twitch cotit r x t i o t i

times of the orbicularis ork and oculi. This is Iiorne out Iiy Fig. 2 showing the con- traction tiriics of a rnaxiiiial twitch (squares) o f the orliritlaris oculi ( A ) arid thc orhicmlaris oris ( U ) at lengths hhorter and longer than the optirnal length (LO’ ) .

‘Hie ariiplitudcs o f the twitches in relation to the muscle length are also plotted i t i

the same diagrams (triangles). In A it can be seen that niaxiirial contractioti atnpli- t u c k is ol)tained in the orbicularis oculi on ly i f the muscle length is within ;I few tmths c f a iri i i i froin the optirnal length. On the otlicr hand, virtu;illy iiiaxiiiial coil-

tractiotis in the orhicularis oris are obtained when the Iiiuscle length is within a n i i n

from the optitrial length. I t is obvious that the twitch ainplitude of the orlkularis oculi is much more dependent on the muscle length than that of the orlicularis oris atid this difference has to be taken into account when comparing the contraction properties of the two iiiuscles. T h e changes i r i passive tension (circles in Fig. 2 ) prod~iced in the two muscles h y length variations are coiiiparable.

Hornogcvrr~ity of tho m o t o r urrit j ! ~ o j ! ~ / / l a t z o r ~ ~ . l h e r e arc usually differences i t i coii-

traction tirnes between niotor units supplied by nerve fibers of different clia t i i r l ( s r s

(see c.g. Wuerker, hlcPhedran and Henriemari 1965 ) . Sirice thresholds for (~Iwtrical stiinulatioti are related to fiber size it is sometimes possilde to distinguish niotor units with different contraction times by studying rcxcords frorii muscles obtainetl when stitiiiili of different ititctisitics are applied to their n i o t o r IICITCS ( c f . , ~.g., Atltlct.zc~li

m d Scars 1964). i\ti expcriitietit with this tcchiiic~iic~ is illustrated in Fig. 3 .i ; i t i d 13 w h i h s h o w s several superitripwed twitch contractions of the orl)iciilaris ocitli ;it id

oris twpectively obtained a t cliflcretit strengths of stimulation of their r i~rves . Con- tractions obtained by stimulation of motor fibers with low electrical thresholds have a timr course similar to those obtained by activation of high-threshold fibera. ‘I’his ohzcmratioii indicates that there arc’ no iiiajor differences I)etween the contraction times of the individual inotor units in the orbirularis oculi and those in the orl~iciil~iris oris.

Blink rcflc~.v c‘orifrnc.t iotr tznir. Tapping on the cat’s facial skill elicits two sep;iratc reflex discharges in the orbicularis oculi (Lindquist and Mirterissori 1970). h t i t is tiot known whether the same types of motor units discharge in the early ;iriti the h t c coniponrnt. Knowing the capacity of the orticLtlaris oculi for qtiick tetrsiori

Page 4: Contraction Properties of Cat Facial Muscles

F A C I A L MUSCLE PHYSIOLOGY

40-

30-

20-

10 -

485

ms - 35 A

0

.. 30 0 0 0 0

O O O O

0

0

0 0

tension 9

6

5

4

3

2

1

A

B

A A

A A A

A A

A A A A

A A 0

A A

A A A A

0 contraction time

10

tension

0

A L A

A A 50

A contract ion time

25

Fig. 2. T h e contraction time (squares), active isometric twitch tension ( triangles) and passi\,e tension (circles) of the orbicularis oculi ( A ) and the orbicularis oris ( H ) plotted against niuacle length. L,, Iirarks the length giving the largest twitch ainplitude.

Page 5: Contraction Properties of Cat Facial Muscles

(:HKIS‘IER LINDQUIS I

Fig. 3. Siiperiinliosrd recordings o f twitch contractions elicited in the orbiciil;itis oculi ( A ) and orbicularis oris ( H ) by sti~nirlntion of tlirir motor nerves a t various stirniilus - strengths. l’inie bars 10 ins.

A

Fig. 4. Reflex action potentials ( lower beans) and isometric contractions (upper beanis J elicited i n tlir orbiciilaris oculi by tap stimuli applied to the ipsilateral medial angle of the eyr. I n A only i i short-latency rt-flex is produced, in R also a reflex of longer latrncy. ‘Tinic bar I5 111s.

de\doprnent it was o f interest to see how this is utilized i r i the blink reflex. I n several expcriments recordings were therefore made of the EMG and the contractile response of the orbicularis oculi to periorbital tap stirnulation (cf. Lindquist and Mirtenssori 1 9 7 0 ) . Fig. 1 is from a typical experiment. The tap stinidation applied in ‘4 gives

;I short-latency reflex action potential (lower beam). ‘The contraction resultitig from this discharge (upper bcarn) reaches a peak in around 15 ms. I t is evident from this value that the short contraction time of the orbicularis oculi is taken ac1varit;ige o f for rapid protection of the eye. The tension produced by a. reflex contraction as the one in A was often 3--3 times larger than a single tuitch elicited by supraniaxinial stinnilation of the orbicularis oculi muscle nerve, which implies that some motor units discharge repetitively in the early reflex response.

Fig. 4 H illustrates a situation when, in addition to an early response. also x Iatc reflex discharge results. Ttir second reflex discharge produces a hump i n t h c record and a prolongation of the contractile response. The time to peak of the reflex con- traction is now 30 Ins. Since the interval between the two reflex discharges is around I0 tns. alm the motor units taking part in the longcr-latency reflex discharge must h a w short contraction times. This is in good agreement with the results prcscnted a1xn.e showing that the orbicularis oculi is homogeneous a s regards contractiori tinirs of its motor units.

Rcspo i iw to r r p d t i u c stirnitlation. Stimulus frequencics giving suniiiiation and fu- sion of contractions vary among muscles in relation to their respective contraction timc (Cooper and Eccles 1930). I t was therefore considered pertinent to study the responses of the orbicularis oculi arid oris to repetitive stirnulation of diffcrcnt fre- quencies.

Page 6: Contraction Properties of Cat Facial Muscles

FACIAL MUSCLE PHYSIOLOGY 487

.o 5 e

Fig. 5. l h e relation between stimulus frequency and tension developed in the orbicularis oculi (triangles) and the orbi- cularis oris (circles). Inset de- scribed in text.

A

A

!'Y 55s

I .1 i

P

* l l , , . I , I , . I I .

50 I00 150 Stimulus f r q u m c y (/s 1

The inset of Fig. 5 shows muscle contractions elicited in the orbicularis oculi (Ieft- hand column I and the orbicularis oris (right-hand column) by stimulation of their respective motor nerves a t 15, 25, 55 and 85 stimuli per s in a typical experinient. At 15 Hz there is a clearly visible sunimation of contractions in the orbicularis oris whereas separate twitches are recorded in the orbicularis oculi. ,4 sIriall suminatiori is seen in the orbicularis oculi at 25 Hz and the summation in the orbicularis oris is now considerable. At 55 Hz two-thirds of the maxirnal tetariic tension is produced in the orbicularis oculi and close t o maximal tension is attained in the orbicularis oris. At 85 H z , finally. there is apparcnt fusion ( ( . I . Methods) of orbicularis oris contrac- tions but still clearly visible oscillations in the tension output of the orhicularis oculi. In Fig. 5 isometric tension for the two muscles is plotted against frequency of nerve stiniulatiori. It appears that in the frequency range 15-40 Hz the orbicularis oris (circles) responds almost linearly and maxinially to changes in stirnulus frequency. For the orbicularis oculi (triangles) this range is larger arid shifted to higher fre- quencies, rir. 35-80 Hz. The findings indicate the possibility of a more precise nervous control of orbicularis oculi contractions.

Tetanus/twitch ratios were estiniated in six cats and the values in Fig. 5 were typical. The range of this ratio for the orbicularis oris was found to be 4-5 with a n average of 4.5. For the orbicularis oculi the range was 5.5--8.5 arid the average tetanus/twitch ratio 7.0.

Apparent fusion (cf. Methods) of orbicularis ocrili muscle coritractioiis occurs at around 100 Hz. I n man discharge frequencies up to 180 Hz have been recorded in single units from this muscle arid it has been suggested that impulse frequencies higher than the fusion frequency give an increased rate of tension development (Gordon 1951). Fig. 6 illustrates an experiment in which 100, 150 and 200 stimuli per second were given to the orbicularis oculi muscle nerve. There is a definite but

Page 7: Contraction Properties of Cat Facial Muscles

488 CHRISTER I.INDQUIST

Fig. 6 . Suprriniposed recordings of the rising pli;isc. o f contractions in t h r orbicularis oculi elicited on rrprti- tive stimulation a t 100, 150 and 200 Hz.

not dramatic increase in the rate of tension development with the higher frequenciras of stimulation. The maximal rate of tension development should be obtained whcw the interval between successive stimuli is short enough to keep the contractile elc- inents at their maximal intensity of the active state. The duration of the latter may he estimated either by the double stirnulation method of MacPhersori and Wilkie (1954) or by double derivation of the twitch tension curve (Gabel. Carson and Vance 1968) and should be equal to the reciprocal of the highest value of the ahso- lute fusion frequency ( c f . Methods) recorded with a high-sensitivity transducrr (Mirtensson and Skoglurid 1964). (For technical reasons the double derivation method could only be used in the case of the orbicularis oris.) These methtds gave values of the duration of the inaximal active state intensity in the range of 3.5-5 ins for the orbicularis oculi as well as for the orbicularis oris when the twitch contrac- tions were elicited by supraniaximal nerve stimuli. T h e rate of tension development in these muscles may therefore be expected to increase with stimulus frequencies u p to 200-300 Hz.

The absolute refractory period of the nerve-niotor endplate-muscle niemlxme system may however limit the frequency response of the muscle. I t was therefore detcrniined with double nerve stimulation and EMG-recording and found t o he just below 1 ms for the orbicularis oculi and between 1.2 and 1.5 ms for the orbicularis oris. These values are comparable to those obtained for the flexor hallucis longus (1.03 Ins) and the soleus (1.61 ms) (Buller and Lewis 1965). The absolute fre- quency limit for activation of the contractile elements should therefore exceed 300 Hz if it was set by the absolute refractory period of the nerve-motor endplate-muscle membrane system alone.

Following repetitive stimulation twitch contractions of striated muscles are usually potentiated. The degree of this post-tetanic potentiation ( P T P ) is higher for fast-twitch than for slow-twitch muscles (Close 1972). The question was raiscd whether there are differences also in this respect between the orbiciilaris ociili and the orbicularis oris muscles. P T P was studied following conditioning stimuli of many different frequencies. It was found that in both muscles the maximal twitch potentia- t i o n is produced by conditioning stimuli of 200-300 Hz and that the PTP is twice as large in the orbicularis oculi at all frequencies.

Page 8: Contraction Properties of Cat Facial Muscles

FAC:IAI. ntusc:LE PHYSIOLOGY 489

Discussion The corltraction time recorded for the orbicularis oculi is in the same range as the contraction time of the thyroarytenoid (9-13 ms according to M2rtensson and Skoglund 1964) and of certain extra-ocular muscles (7.5-10 111s for the internal rectus according to Cooper and Eccles 1930). T h e orbicularis oris is coniparable to the cricothyroid (30-35 ms according to Martensson and Skoglund 1964) and to some of the fast extremity muscles (40 ms for the gastrocnemius according t o Cooper and Eccles 1930). The contraction time of a muscle is given by the intrinsic speed of shortening of the contractile material, the duration of the active state and by the properties of the series-elastic elements (Close 1965). T h e results ohtained in this series of experiments may justify a discussion about the factors respon.sible for the difference in contraction time between the orbicularis oculi and oris. T h e estimated duration of the maximal intensity of the active state was the same in the two muscles. According to Hill (1949) the falling phase of the active state passes through the peak of the twitch contraction. Since the maximum tension recorded i n tetanic contraction corresponds to the maximal intensity of the active state ( c f . Goffart and Ritchie 1952). it is possible to estimate the rate of decay of the acti\re state if the tetanus/twitch ratio' and the contraction time are known ( cf. M%rtensson arid Skoglund 1964). The tetanus/twitch ratio was lower for the orbicularis oris than for the orbicularis oculi and the contraction time longer; the rate of fall of the active state in the orbicularis oris is thus slower than in the orbicularis oculi and hence the total duration of the active state is longer in the orbicularis oris. This difference may well be one factor determining the difference in contraction tirnr between the two muscles.

The cause of the longer duration of the compound action potential recorded in the orbicularis oris has riot been specifically investigated. Factors such as the propagation velocity of the muscle fibers and the distance over which the potential is propagated may be involved (c f . , c.g., Buchthal, Guld and Rosenfalck 1955). .i\ longer duration of the action potential may imply a slower activation of the contractile elements of the muscle. I t does not, however. seem likely that this is responsible for the longer contraction time of the orbicularis oris, since the compound action potential is prolonged both in the orbicularis oculi arid oris during long-lasting repetitive stirnu- latiori without any significant alteration in the contraction times o f the muscles (cf. Edstrom and Lindqiiist 1973).

The experimental results indicate that the motor unit population in the orbicularis oculi as well as in the orbicularis oris is homogeneous as regards contraction times of the motor units. T h e agrees well with the fact that a large majority of muscle fibers within the orbiscularis oculi and within the orbicularis oris have a similar activity of niyofibrillar ATPase (cf. Edstrom and Lindquist 1973 ) which determines the speed of contraction.

In his experiments on nian Gordon (1951 ) recorded irripulses of frc-qllencies up to 180 Hz i n single motor units of the orbicularis oculi during blinking. He suggested that these very high discharge frequencies served the purpose of increasing the

Page 9: Contraction Properties of Cat Facial Muscles

490 CHRI STER LINDQL'I ST

speed of tension development. The experiments described above show that stimulus frequencies above those necessary for producing apparent fusion of muscular contra('- tions do increase this speed. Similar observations have been made also for extrmiity muscles of the cat (Buller and Lewis 1965). However, when comparing the time to peak of the blink reflex in Fig. 4 A and the contraction time of the niaxirrial isometric twitch of the orbicularis oculi illustrated in Fig. 3 A no marked difference is apparent in the rate of tension development. I t is therefore unlikely that high- frequency n e n e impulses to the orbicularis oculi muscle of the cat serve the pirpose of increasing the speed of eye closure in the blink reflex.

References . P. and T. A. SEAKS, "The mechanical properties and innervation of fast and slo\s nits i n the intercostal nitiscles of the cat. J. Physiol. (Lond.) 1964. 173. 1 I t - ~ 129.

BROWN. M. C:. and P. B. C. Mxr'riIEwS, The effect on a niiiscle twitch of the back-rrsponse of its motor nerve fibres. J. Physiol. (Lond.) 1960. 150. 332-346.

B u c w x x , F., C:. GULD and P. ROSENFALCK, Innervation zone and propagation velocity in hunian muscle. Actn physiol. scand. 1955. 35. 174-190.

I ~ U L L E K . A. J . and D. M. LEWIS, 'The rate of tension development in isometric. tetanic coriti'iic- tions of ~narnmalian fast and slow skeletal muscle. J. Physiol. (Lond.) 1965. 176. 337-354.

CLOSE. R.. The relation between intrinsic speed of shortening and duration of the active state of niwcle. J. Physiol. (Lond.) 1965. 180. 542-559.

CLOSE, R.. Dynamic properties of mammalian skeletal niuscles. Physiol. RPU. 1972. 5?. 1 ?<I- 197.

COOPER. S. and J. C . ECCLES, The isometric responses of niarninalian muscles. J. I ' / i ~ ~ . ~ i u l .

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