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Surface electrode recording of platysma single motor units during speech
Journal of Phonetics (1980) 8, 169 - 173
Surface electrode recording of platysma single motor units during speech Michael McClean and Shimon Sapir University of Washingt o n, Seattle, Washington U.S. A . R eceived 18th July 1978
Abstract:
Preliminary data are presented on the voluntary activation of single motor units recorded from the neck with surface electrodes. Behavioral tests support the conclusion that these motor units are from the platysma muscle , and that this muscle functions as an antagonist to the orbicularis oris inferior muscle during speech. The patterns of motor unit activation observed during simple bilabial speech movements are consistent with the si ze principle as well as previous studies of single motor unit activatio n and movement dynamics.
Introduction It is now well established that the analysis of single motor unit activity in speech muscles can provide unique and significant information on how the central nervous system controls speech movements (MacNeilage , 1973; MacNeilage , Sussman , & Powers, 1977; Sussman , MacNeilage , & Powers , 1977) . However, obtaining analyzable single motor unit data generally requires the use of special hooked-wire or bipolar needle electrodes whose pick-up field is sufficiently restricted so that a limited number of motor unit action potentials is sampled . In the present report we will describe the recent finding that conventional surface electrodes may be used to obtain analyzable single motor unit data from the platysma muscle as it is activated during speech . We believe that this capability results from the superficial location and broad plate-like anatomy of the platysma muscle. This latter characteristic probably results in a relatively small number of motor units being distributed over a large surface area , permitting one to sample a small number of motor unit potentials with electrodes having a broad pick-up field. To our knowledge no one has previously reported electromyographic data on the activation of the platysma muscle during speech. Our preliminary research indicates that the platysma muscle functions as an antagonist to the orbicularis oris inferior muscle (001) . This finding may be of so me theoretical importance in modelling the neuromuscular control o f lip movements for speech , since the platysma muscle inserts in several other perioral muscles in a manner which would permit it to have significant effects on their resulting force vectors (Grant , 1972 ;Sieber & DuBrul, 1970 ;Kennedy, 1974, personal observations) . It is also of interest to note that phylogenetically the platysma muscle is one of the two oldest perioral muscles among the primates (Huber, J 931; Lenneberg, 1967) and that at the level of the facial nucleus and nerve it is quite distinct from the other perioral muscles (Szentagothai , 1948; Courville , 1966; Sic her & DuBrul , 1970; Carpenter , 197 6) . 0095 - 4470/80/020169+05 $02 .00/0
©1980 Academic Press Inc. (London) Ltd .
170
M. McClean and S. Sapir
Methods To date we have searched for speech activated platysma single motor units in four subjects using surface electrodes. In all four we were able to isolate at least one single motor unit which was consistently activated for inferiorly directed lower lip movements during speech. Physiological recordings were made of three of these subjects while they : (!) held the instantaneous firing frequency of particular motor units at various static levels utilizing audio-visual feedback , (2) produced I we / 20 times with a slow transition between the two vowels; (3) produced the syllable /wee / 20 times at a slow rate . The use of jure/ and /wee/ was intended to provide a wide range of lower lip velocities, since we hoped to acquire preliminary data on the relationship between parameters of single motor unit activation and labial motion. Electromyographic recording was done with either Beckman recessed miniature surface electrodes or I.M.A . Electronics miniature silver cup electrodes. Electrodes were attached to a well cleaned neck surface at interelectrode distances of 5-20 mm with Beckman doubleadhesive collars or cloth tape and interfaced to the skin with electrode transmission cream (EKG So 1). Effective elect rode placement was aided by visual palpation wherein a subject maximally contracted the neck muscles revealing gyri which course from the clavicle to the mandible and lower lip area. Longtitudinal placement of electrodes along these gyri inferior and lateral to the thyroid notch proved effective in obtaining single motor unit recordings from what we believe is the platysma muscle. Finding an electrode position that picked up a single motor unit(s) which was consistently activated during speech sometimes required considerable searching . However , once such a position was found and measured with respect to the surrounding anatomical landmarks , we were able to readily obtain recordings from what appeared to be the same single motor unit(s) in repeated electrode placements . Electromyographic signals were amplified with Honeywell 135 amplifiers and recorded along with lower lip displacement and audio on a Honeywell 5600 FM tape recorder. Lower lip displacement was transduced in the vertical and horizontal dimensions of the sagitta l plane with strain gages (see Abbs & Gilbert, I 973). All recording was done with interincisor distance fixed at approximately 6 mm with a bite block . Data to be described were displ ayed and analyzed on a storage oscilloscope . For the purposes of this preliminary report we will discuss only data obtained on one subject. This subject was selected because his recordings revealed two single motor units , whereas on ly one single motor unit was readily detected above the noise floor from single electrode place ments on the other two subjects. However , we have observed few differences across subjects in terms of characteristic firing frequency or the timing of motor unit activation in relation to lower lip movements for speech. Results and discussion Our conclusion that the single motor units sampled were from the pl atysma muscle is based primarily on the fact th at inferiorly directed lower lip movements were co nsistentl y accompanied by single motor unit activation for all four subjects. This relation was observed during speech and nonspeech lip movements. During the non-speech movements the head , jaw , and larynx were in a fixed position. Speech related motor unit activity was observed for electrode positions below the thyroid not ch and 2- 7 em off midline . This range of lateral positioning corresponds closely to the lateral extent of the platysma muscle , suggesting that the motor units sampled were not from some of the deeper neck muscles . To check for this latter possibility we had subjects perform a variety ofbehavioral manoeuvers designed to activate the cricothyroid , sternocleidomastoideus, sternothyroid. omohyoid, and sternohyoid muscles . However , these manoeuvers revealed either no activity or relatively low level interference patterns, but no activation of the single motor units o bse rved during lo wer lip movements.
Platysma single motor units
I7I
Figure I(a) shows two platysma motor units (MUI and MU2) as they were activated during the static portion of the I re I vowel in I ure I. For this type of static activation the mean and maximum instantaneous firing frequencies for MUI were 59 and 91 Hz respectively, and for MU2 these same statistics were 52 and I 00 Hz. These values are based on I 0 randomly selected I 00 ms samples from which 29 intervals were measured for MU I and 34 intervals for MU2 . The maximum values are substantially higher than have been reported for other perioral muscles (MacNeilage, 197 3; MacNeilage, et al., 1977) .
'"' ~~ ,..---, 20ms
sup . l,
(b)
I
in f.
~Omm
,---., 0 ·2 s (c)
Figure l
(a) Activation of MUL and MU2 during static portion of/<£/ in production of f ux j; (b) Top trace: Lower lip displacement in the vertical or superior-inferior dimension during a slow j ure/ tra nsition , Bottom trace: Activation of MUl and MU2 during the sa me tr a nsition ; (c) Lower lip displace ment and MU.l - MU2 activation during a rapid j ure/ transition . Calibrations are the same as in I B.
The relationship between lower lip displacement and single motor unit activation may be seen in Fig. I (b). This figure shows vertical displacement of the lower lip and activation of MU1 and MU2 during the lurel transition . It may be seen that as lower lip depression in creases there is initially a recruitment of MUI followed by an increase in firing frequency and then recruitment of MU2 . As lower lip depression continues MU2 also displays an increase in firing frequency. Thus , both frequency and re cruitment coding are illustrated in Fig. I (b). The fact that the low amplitude motor unit was the first one recruited is consistent with the size principle (Henneman , Somjen , & Carpenter , 1965) and the related assumption that the relative am plitud e of single motor unit action potentials is a good predictor of motone uron size and threshold (Olson , Carpenter, & Henneman, I968; Sussman , et al .. 1977) . Figure l(c) shows another example of motor unit activation during the lurel transition . However , in this instan ce the velocity of lower lip displacement is much greater than in the Fig . l(b) example, and the increase in velocity is accompanied by a marked decrease in the rec ruitment interval between MUJ and MU2. The patterns of motor unit activation in Figs l(b) and l(c) are consistent with some recent findings of Desmedt & Godaux (1977)
172
M McClean and S. Sapir
on m?tor unit recruitment for voluntary ballistic and ramped foot torques. We are presently studymg this app2..rent relationship between lower lip velocity and recruitment interval in greater detail. As mentioned earlier, we believe that the platysma muscle may function as an antagonist to 001 dunng speech. This point is illustrated in Fig. 2(a) which shows oscilloscopic records of OOI and platysma during repeated productions of the syllable fwre/. As mav be seen the ' activation of platy~:ma motor units is closely timed with the shutdown of OOI. · (0)
(bI
sup. [
1 '
4mm
i nf .
Figure 2
(a) Simultaneous re cording s of the orbicul aris oris inferior muscle (001) and th e plat ys ma muscle during slow rep etitions of the syllable fwre /. (b) Top trace: Plat y sma mo tor unit activation during a sing le production of fwre j, Bottom tra ce: V e rti cal displa cement of th e lowe r lip during the same produ ction of / wa::/ .
Figure 2(b) shows the activation of platysma motor units during a single production of /wre/ with an expanded time base. The very short recruitment interval seen here is similar to that observed in Fig. l(c). In this particular example MUI is the first recruited motor unit. However, in several instances of /wre/ production it was not possible to clearly distinguish the MU I and MU2 waveforms during the first 30 - 50 ms of motor unit activation. This difficulty is probably the result of several factors. The instantaneous firing frequencies of MUJ and MU2 reached their maximal values (91 and J 82 Hz respectively) during this period of time. When this is coupled with the fact that the durations of the motor units sampled were relatively long (6 - 10 ms) , it seems likely that in some instances the two motor unit potentials summated to produce interference patterns. For ballistic type lip movements as occur in /wre / one might expect coa ctivation or recruitment order reversal of MUI and MU2 resulting from differences in motor axon conduction velocities (Desmedt & Godaux, 1977) . This would also increase the likelihood of MUJ and MU2 summating to produce interference patterns. Lastly , it is possibl e that additional motor units were recruited during this 30 - 50 ms interval , since it corresponded to a period of high acceleration of lower lip displacement. These problems may be partially surmounted in future studies by optimizing electrode positions to reduce motor unit durations, thereby allowing us to distinguish the waveform s of di fferent motor unit potenti als. In Fig . 2(b) it may also be seen that MUI was the last motor unit deactivated . This was observed in almost every instance of /wre / production , and it is consistent with the si ze
Platysma single motor units
173
principle which predicts that with the removal of excitatory inputs small motoneurons should be the last ones de act ivated (Henneman , et al ., 1965). In general the platysma motor unit activity we have anal yzed so far is consistent with present knowledge of motor unit physio logy. While the relat ive impo rtance of the platysma muscle in lip motor control is uncertain, it does appear that it f un ctio ns as an antago nist to 001 during speech. These observations whe n coupled with t he capability for non-invasive recording make the platysma muscle ideally suited for laboratory demonstrations of single motor unit activity in spee ch . In future st udies we plan to examine the re lations between platysma motor unit activity . lowe r lip motion , and various classes of bilabial speech gestures in greater detail . This research was supported by gran ts from NINCDS (NS 14048 and NS 0029 1). The aut hors wish to thank Charles Larson and Jam es Till for their co mments o n an early ve rsion of thi s manus cript. References Abbs, J . & G ilb ert, B. (1973) . A strain gage transducer system for li p a nd jaw mot ion in two dim ensions. Journal of Speech and Hearing Research, 16 , 175 - 200 . Carpe nt e r, M. ( 1976) . Human Ne uroanatomy. Bait imore: Williams a nd Wilkin s Co . Co urville , J. (1966) . The nucle us of the facia l nerve; t he relat io n between ce llular groups a nd periphera l b ranches of the nerve . Brain R esearch, 1 , 33 8 -354 . Desmedt, J . & Goda ux, E. ( 1977). Ballisti c co nt ra ct io ns in man: characte rist ic recrui tme nt patt ern of single moto r units of th e tibialis a nte rior muscle. Journal of Physiology, 264 , 673 - 693 . Gra nt , J. (1972) . An Atlas of Anatomy. Ba lt im o re: Willia ms a nd Wi ll< in s Co. Henne man, E., Somjen , G. & Carpe nter , D. (1965). F un ctional sig nifican ce o f ce ll size in spinal motone urons. Journal of Ne urophy siology , 28 , 560 - 5 80. Huber , E. ( 19 3 1). l:."volution of Facia l Musculature and Facial Expression. Baltimo re: The J o hn s Hopkin s Un ive rsi t y Press. Kennedy , J . (197 4). A graphi c descript io n of t he pe rioral mus culature. Masters thesi s, Unive rsity of Washington. Lc nn eberg, E. ( 1967). Biological Fou ndations of /.anguage. New York: John Wiley and Sons, Inc. MacN eilage, P. ( 19 7 3) . Preliminaries to the study o f single moto r unit act ivit y in speech mu sc ulature . Journal of Phonetics I , 55 - 71. Ma cNe ilage, P., Suss ma n, H. & Powers , R . ( 1977). Discharge pat tern s acco mpanyi ng susta in ed activation o f motor unit s in speec h musc ulatur e. Journal of Phonetics , 5 , 135- 14 7. Olson , C., Ca rpen ter, D. , and Henneman , E. (1968). Orderly rec rui tme nt of mu scle action potentials . .4rch i vesofNu erolo~y 19,5 91 - 597. Sichcr. H. & Du Bru1, E. ( 1970). Oral Anatomy. Saint Louis: C. V. Mo sb y Co. Sussman, H ., MacNc ilagc, P. & Powers, R . (197 7). Recruitment a nd discharge patterns o f single mo tor units during speec h prod uct io n. Journal of Speech and Hearin~ R esearch, 20, 613- 630 . Sze ntagn thai. J . ( 1948). The represe ntatio n of th e facia l a nd scalp muscles in the facial nucleus. Journal of Comparative Neuro logy. 88. 207 - 22 0 .
Surface electrode recording of platysma single motor units during speech Michael McClean and Shimon Sapir University of Washingt o n, Seattle, Washington U.S. A . R eceived 18th July 1978
Abstract:
Preliminary data are presented on the voluntary activation of single motor units recorded from the neck with surface electrodes. Behavioral tests support the conclusion that these motor units are from the platysma muscle , and that this muscle functions as an antagonist to the orbicularis oris inferior muscle during speech. The patterns of motor unit activation observed during simple bilabial speech movements are consistent with the si ze principle as well as previous studies of single motor unit activatio n and movement dynamics.
Introduction It is now well established that the analysis of single motor unit activity in speech muscles can provide unique and significant information on how the central nervous system controls speech movements (MacNeilage , 1973; MacNeilage , Sussman , & Powers, 1977; Sussman , MacNeilage , & Powers , 1977) . However, obtaining analyzable single motor unit data generally requires the use of special hooked-wire or bipolar needle electrodes whose pick-up field is sufficiently restricted so that a limited number of motor unit action potentials is sampled . In the present report we will describe the recent finding that conventional surface electrodes may be used to obtain analyzable single motor unit data from the platysma muscle as it is activated during speech . We believe that this capability results from the superficial location and broad plate-like anatomy of the platysma muscle. This latter characteristic probably results in a relatively small number of motor units being distributed over a large surface area , permitting one to sample a small number of motor unit potentials with electrodes having a broad pick-up field. To our knowledge no one has previously reported electromyographic data on the activation of the platysma muscle during speech. Our preliminary research indicates that the platysma muscle functions as an antagonist to the orbicularis oris inferior muscle (001) . This finding may be of so me theoretical importance in modelling the neuromuscular control o f lip movements for speech , since the platysma muscle inserts in several other perioral muscles in a manner which would permit it to have significant effects on their resulting force vectors (Grant , 1972 ;Sieber & DuBrul, 1970 ;Kennedy, 1974, personal observations) . It is also of interest to note that phylogenetically the platysma muscle is one of the two oldest perioral muscles among the primates (Huber, J 931; Lenneberg, 1967) and that at the level of the facial nucleus and nerve it is quite distinct from the other perioral muscles (Szentagothai , 1948; Courville , 1966; Sic her & DuBrul , 1970; Carpenter , 197 6) . 0095 - 4470/80/020169+05 $02 .00/0
©1980 Academic Press Inc. (London) Ltd .
170
M. McClean and S. Sapir
Methods To date we have searched for speech activated platysma single motor units in four subjects using surface electrodes. In all four we were able to isolate at least one single motor unit which was consistently activated for inferiorly directed lower lip movements during speech. Physiological recordings were made of three of these subjects while they : (!) held the instantaneous firing frequency of particular motor units at various static levels utilizing audio-visual feedback , (2) produced I we / 20 times with a slow transition between the two vowels; (3) produced the syllable /wee / 20 times at a slow rate . The use of jure/ and /wee/ was intended to provide a wide range of lower lip velocities, since we hoped to acquire preliminary data on the relationship between parameters of single motor unit activation and labial motion. Electromyographic recording was done with either Beckman recessed miniature surface electrodes or I.M.A . Electronics miniature silver cup electrodes. Electrodes were attached to a well cleaned neck surface at interelectrode distances of 5-20 mm with Beckman doubleadhesive collars or cloth tape and interfaced to the skin with electrode transmission cream (EKG So 1). Effective elect rode placement was aided by visual palpation wherein a subject maximally contracted the neck muscles revealing gyri which course from the clavicle to the mandible and lower lip area. Longtitudinal placement of electrodes along these gyri inferior and lateral to the thyroid notch proved effective in obtaining single motor unit recordings from what we believe is the platysma muscle. Finding an electrode position that picked up a single motor unit(s) which was consistently activated during speech sometimes required considerable searching . However , once such a position was found and measured with respect to the surrounding anatomical landmarks , we were able to readily obtain recordings from what appeared to be the same single motor unit(s) in repeated electrode placements . Electromyographic signals were amplified with Honeywell 135 amplifiers and recorded along with lower lip displacement and audio on a Honeywell 5600 FM tape recorder. Lower lip displacement was transduced in the vertical and horizontal dimensions of the sagitta l plane with strain gages (see Abbs & Gilbert, I 973). All recording was done with interincisor distance fixed at approximately 6 mm with a bite block . Data to be described were displ ayed and analyzed on a storage oscilloscope . For the purposes of this preliminary report we will discuss only data obtained on one subject. This subject was selected because his recordings revealed two single motor units , whereas on ly one single motor unit was readily detected above the noise floor from single electrode place ments on the other two subjects. However , we have observed few differences across subjects in terms of characteristic firing frequency or the timing of motor unit activation in relation to lower lip movements for speech. Results and discussion Our conclusion that the single motor units sampled were from the pl atysma muscle is based primarily on the fact th at inferiorly directed lower lip movements were co nsistentl y accompanied by single motor unit activation for all four subjects. This relation was observed during speech and nonspeech lip movements. During the non-speech movements the head , jaw , and larynx were in a fixed position. Speech related motor unit activity was observed for electrode positions below the thyroid not ch and 2- 7 em off midline . This range of lateral positioning corresponds closely to the lateral extent of the platysma muscle , suggesting that the motor units sampled were not from some of the deeper neck muscles . To check for this latter possibility we had subjects perform a variety ofbehavioral manoeuvers designed to activate the cricothyroid , sternocleidomastoideus, sternothyroid. omohyoid, and sternohyoid muscles . However , these manoeuvers revealed either no activity or relatively low level interference patterns, but no activation of the single motor units o bse rved during lo wer lip movements.
Platysma single motor units
I7I
Figure I(a) shows two platysma motor units (MUI and MU2) as they were activated during the static portion of the I re I vowel in I ure I. For this type of static activation the mean and maximum instantaneous firing frequencies for MUI were 59 and 91 Hz respectively, and for MU2 these same statistics were 52 and I 00 Hz. These values are based on I 0 randomly selected I 00 ms samples from which 29 intervals were measured for MU I and 34 intervals for MU2 . The maximum values are substantially higher than have been reported for other perioral muscles (MacNeilage, 197 3; MacNeilage, et al., 1977) .
'"' ~~ ,..---, 20ms
sup . l,
(b)
I
in f.
~Omm
,---., 0 ·2 s (c)
Figure l
(a) Activation of MUL and MU2 during static portion of/<£/ in production of f ux j; (b) Top trace: Lower lip displacement in the vertical or superior-inferior dimension during a slow j ure/ tra nsition , Bottom trace: Activation of MUl and MU2 during the sa me tr a nsition ; (c) Lower lip displace ment and MU.l - MU2 activation during a rapid j ure/ transition . Calibrations are the same as in I B.
The relationship between lower lip displacement and single motor unit activation may be seen in Fig. I (b). This figure shows vertical displacement of the lower lip and activation of MU1 and MU2 during the lurel transition . It may be seen that as lower lip depression in creases there is initially a recruitment of MUI followed by an increase in firing frequency and then recruitment of MU2 . As lower lip depression continues MU2 also displays an increase in firing frequency. Thus , both frequency and re cruitment coding are illustrated in Fig. I (b). The fact that the low amplitude motor unit was the first one recruited is consistent with the size principle (Henneman , Somjen , & Carpenter , 1965) and the related assumption that the relative am plitud e of single motor unit action potentials is a good predictor of motone uron size and threshold (Olson , Carpenter, & Henneman, I968; Sussman , et al .. 1977) . Figure l(c) shows another example of motor unit activation during the lurel transition . However , in this instan ce the velocity of lower lip displacement is much greater than in the Fig . l(b) example, and the increase in velocity is accompanied by a marked decrease in the rec ruitment interval between MUJ and MU2. The patterns of motor unit activation in Figs l(b) and l(c) are consistent with some recent findings of Desmedt & Godaux (1977)
172
M McClean and S. Sapir
on m?tor unit recruitment for voluntary ballistic and ramped foot torques. We are presently studymg this app2..rent relationship between lower lip velocity and recruitment interval in greater detail. As mentioned earlier, we believe that the platysma muscle may function as an antagonist to 001 dunng speech. This point is illustrated in Fig. 2(a) which shows oscilloscopic records of OOI and platysma during repeated productions of the syllable fwre/. As mav be seen the ' activation of platy~:ma motor units is closely timed with the shutdown of OOI. · (0)
(bI
sup. [
1 '
4mm
i nf .
Figure 2
(a) Simultaneous re cording s of the orbicul aris oris inferior muscle (001) and th e plat ys ma muscle during slow rep etitions of the syllable fwre /. (b) Top trace: Plat y sma mo tor unit activation during a sing le production of fwre j, Bottom tra ce: V e rti cal displa cement of th e lowe r lip during the same produ ction of / wa::/ .
Figure 2(b) shows the activation of platysma motor units during a single production of /wre/ with an expanded time base. The very short recruitment interval seen here is similar to that observed in Fig. l(c). In this particular example MUI is the first recruited motor unit. However, in several instances of /wre/ production it was not possible to clearly distinguish the MU I and MU2 waveforms during the first 30 - 50 ms of motor unit activation. This difficulty is probably the result of several factors. The instantaneous firing frequencies of MUJ and MU2 reached their maximal values (91 and J 82 Hz respectively) during this period of time. When this is coupled with the fact that the durations of the motor units sampled were relatively long (6 - 10 ms) , it seems likely that in some instances the two motor unit potentials summated to produce interference patterns. For ballistic type lip movements as occur in /wre / one might expect coa ctivation or recruitment order reversal of MUI and MU2 resulting from differences in motor axon conduction velocities (Desmedt & Godaux, 1977) . This would also increase the likelihood of MUJ and MU2 summating to produce interference patterns. Lastly , it is possibl e that additional motor units were recruited during this 30 - 50 ms interval , since it corresponded to a period of high acceleration of lower lip displacement. These problems may be partially surmounted in future studies by optimizing electrode positions to reduce motor unit durations, thereby allowing us to distinguish the waveform s of di fferent motor unit potenti als. In Fig . 2(b) it may also be seen that MUI was the last motor unit deactivated . This was observed in almost every instance of /wre / production , and it is consistent with the si ze
Platysma single motor units
173
principle which predicts that with the removal of excitatory inputs small motoneurons should be the last ones de act ivated (Henneman , et al ., 1965). In general the platysma motor unit activity we have anal yzed so far is consistent with present knowledge of motor unit physio logy. While the relat ive impo rtance of the platysma muscle in lip motor control is uncertain, it does appear that it f un ctio ns as an antago nist to 001 during speech. These observations whe n coupled with t he capability for non-invasive recording make the platysma muscle ideally suited for laboratory demonstrations of single motor unit activity in spee ch . In future st udies we plan to examine the re lations between platysma motor unit activity . lowe r lip motion , and various classes of bilabial speech gestures in greater detail . This research was supported by gran ts from NINCDS (NS 14048 and NS 0029 1). The aut hors wish to thank Charles Larson and Jam es Till for their co mments o n an early ve rsion of thi s manus cript. References Abbs, J . & G ilb ert, B. (1973) . A strain gage transducer system for li p a nd jaw mot ion in two dim ensions. Journal of Speech and Hearing Research, 16 , 175 - 200 . Carpe nt e r, M. ( 1976) . Human Ne uroanatomy. Bait imore: Williams a nd Wilkin s Co . Co urville , J. (1966) . The nucle us of the facia l nerve; t he relat io n between ce llular groups a nd periphera l b ranches of the nerve . Brain R esearch, 1 , 33 8 -354 . Desmedt, J . & Goda ux, E. ( 1977). Ballisti c co nt ra ct io ns in man: characte rist ic recrui tme nt patt ern of single moto r units of th e tibialis a nte rior muscle. Journal of Physiology, 264 , 673 - 693 . Gra nt , J. (1972) . An Atlas of Anatomy. Ba lt im o re: Willia ms a nd Wi ll< in s Co. Henne man, E., Somjen , G. & Carpe nter , D. (1965). F un ctional sig nifican ce o f ce ll size in spinal motone urons. Journal of Ne urophy siology , 28 , 560 - 5 80. Huber , E. ( 19 3 1). l:."volution of Facia l Musculature and Facial Expression. Baltimo re: The J o hn s Hopkin s Un ive rsi t y Press. Kennedy , J . (197 4). A graphi c descript io n of t he pe rioral mus culature. Masters thesi s, Unive rsity of Washington. Lc nn eberg, E. ( 1967). Biological Fou ndations of /.anguage. New York: John Wiley and Sons, Inc. MacN eilage, P. ( 19 7 3) . Preliminaries to the study o f single moto r unit act ivit y in speech mu sc ulature . Journal of Phonetics I , 55 - 71. Ma cNe ilage, P., Suss ma n, H. & Powers , R . ( 1977). Discharge pat tern s acco mpanyi ng susta in ed activation o f motor unit s in speec h musc ulatur e. Journal of Phonetics , 5 , 135- 14 7. Olson , C., Ca rpen ter, D. , and Henneman , E. (1968). Orderly rec rui tme nt of mu scle action potentials . .4rch i vesofNu erolo~y 19,5 91 - 597. Sichcr. H. & Du Bru1, E. ( 1970). Oral Anatomy. Saint Louis: C. V. Mo sb y Co. Sussman, H ., MacNc ilagc, P. & Powers, R . (197 7). Recruitment a nd discharge patterns o f single mo tor units during speec h prod uct io n. Journal of Speech and Hearin~ R esearch, 20, 613- 630 . Sze ntagn thai. J . ( 1948). The represe ntatio n of th e facia l a nd scalp muscles in the facial nucleus. Journal of Comparative Neuro logy. 88. 207 - 22 0 .