Comparison of Treeshrew and Primate Electromyography WB Ratios

Compared to anthropoids, strepsirrhines with unfused symphyses tend to have higher average W/B ratios for their jaw muscles during chewing (Hylander et al., 2000, 2002). The deep masseter clearly shows the most

tfi BAT O

Figure 4. Plot of average peak jaw-muscle activities during chewing in treeshrews. On average, most treeshrew jaw muscles peak in two separate groups. The working-side anterior temporalis (WAT), working-side posterior temporalis (WPT), balancing-side superficial masseter (BSM), and balancing-side deep masseter (BDM) tend to peak before the remaining muscles. The balancing-side anterior temporalis (BAT), balancing-side posterior temporalis (BPT), and working-side superficial masseter (WSM) (represented by the dot on the solid line at 0 on the x-axis) peak later in the chewing cycle. The working-side deep masseter is intermediate between these two groups. The dot represents the average peak activity for a muscle, while the bar passing through the dot demonstrates the 95% CI of the mean.

tfi BAT O

Figure 4. Plot of average peak jaw-muscle activities during chewing in treeshrews. On average, most treeshrew jaw muscles peak in two separate groups. The working-side anterior temporalis (WAT), working-side posterior temporalis (WPT), balancing-side superficial masseter (BSM), and balancing-side deep masseter (BDM) tend to peak before the remaining muscles. The balancing-side anterior temporalis (BAT), balancing-side posterior temporalis (BPT), and working-side superficial masseter (WSM) (represented by the dot on the solid line at 0 on the x-axis) peak later in the chewing cycle. The working-side deep masseter is intermediate between these two groups. The dot represents the average peak activity for a muscle, while the bar passing through the dot demonstrates the 95% CI of the mean.

Table 4. Summary comparisons of average jaw muscle W/B ratios among treeshrews and primates

Species

Superficial masseter

Deep masseter

Anterior temporalis

Posterior temporalis

Treeshrew

2.7

2.8

2.1

2.O

Galago

2.2

3.9

4.4

2.4

Lemur

1.7

2.4

1.S

2.O

Baboon

1.9

1.O

1.2

1.O

Macaque

1.4

1.O

1.2

1.2

Owl monkey

1.4

1.3

1.4

1.4

Callitrichid

1.9

1.3

1.3

1.2

differences in W/B ratios between these strepsirrhines and anthropoids. The W/B ratios for the remaining muscles, in particular the superficial masseter, are more similar between strepsirrhines and anthropoids. Treeshrew average W/B ratios are more similar to those for ring-tailed lemurs and greater gala-gos as compared to the lower W/B ratios found in the four anthropoid groups (Table 4). Thus, treeshrews appear more like strepsirrhines with unfused symphyses in showing lower levels of balancing-side jaw-muscle activity relative to the working-side muscles (cf Hylander et al., 2000).

Jaw-muscle firing patterns

The posterior temporalis and superficial masseters in strepsirrhines and anthropoids both generally follow the Triplet hypothesis with Triplet I jaw muscles peaking before Triplet II muscles during a chewing cycle (Weijs, 1994). The vertically oriented primate anterior temporalis fires along with its same-side posterior temporalis (Hylander et al., 2005) rather than peaking prior to Triplets I and II as suggested by Weijs (1994). Here we follow Hylander et al. (2005) by including the primate working-side anterior tem-poralis with Triplet I and the balancing-side anterior temporalis as part of Triplet II. Platyrrhines may be an exception to this generalized Triplet pattern given that their balancing-side superficial masseter tends to peak at about the same time as their working-side superficial masseter on average and hence is not firing with the working-side temporalis as part of Triplet I (Table 5).

The firing patterns of the deep masseter show the most variation among primate jaw-closing muscles. Anthropoids differ from those strepsirrhines with mobile, unfused symphyses by peaking their balancing-side deep masseter (BDM) after their working-side superficial masseter (WSM) (Hylander et al.,

Table 5. Summary comparisons of average timing differences between peak EMG activity relative to the working-side superficial masseter for treeshrews and primates

Species

WDM

BSM

BDM

WAT

BAT

WPT

BPT

Treeshrew

3

10

8

12

-2

12

-4

Galago

11

21

23

13

-3

14

-3

Lemur

37

14

24

23

6

17

0

Baboon

47

17

-6

16

-6

20

-19

Macaque

65

17

-20

8

-6

7

-16

Owl monkey

13

-1

-11

13

-14

13

-15

Callitrichid

10

-5

-12

3

-13

11

-15

Positive values indicate that peak EMG activity of the muscle precedes peak EMG activity of the working-side superficial masseter. Negative values indicate the reverse condition.

Positive values indicate that peak EMG activity of the muscle precedes peak EMG activity of the working-side superficial masseter. Negative values indicate the reverse condition.

2000, 2002) (Table 5). In contrast, the BDM peaks well before the WSM in greater galagos and ring-tailed lemurs. The working-side deep masseter (WDM) is also quite variable in its timing of peak activity among primates. The WDM is the first muscle to peak, at a time well before the muscles in Triplet I, in lemurs, macaques and baboons (Table 5). Alternatively, the WDM peaks with the other muscles of Triplet I in galagos and platyrrhines (Table 5).

Treeshrews share similar firing patterns for their jaw-closing muscles during chewing with strepsirrhines, and are arguably most similar to galagos. Treeshrews also appear to fire their jaw-closing muscles in two triplets; with the working-side anterior temporalis (WAT), working-side posterior temporalis (WPT) and balancing-side superficial masseter (BSM) firing as an initial group followed by the working-side superficial masseter (WSM), balancing-side anterior temporalis (BAT) and balancing-side posterior temporalis (BPT) (Tables 3, 5). Given that we do not have EMG data from the medial pterygoids during chewing (i.e., the third muscle in both Triplet I and Triplet II), we cannot unequivocally state that treeshrew jaw-muscle activity patterns fire in two Triplets. The main difference between galagos and treeshrews for Triplet I is that the BSM and the balancing-side deep masseter (BDM) tend to peak before the working-side temporalis in galagos as compared to the reverse condition in treeshrews. Compared to anthropoids, treeshrews are similar to strepsirrhines in that their BDM peaks before their WSM. This difference in the timing of the peak activity of the BDM clearly distinguishes treeshrews and strepsirrhines with mobile, unfused symphyses from anthropoids (Table 5).

0 0

Post a comment