Friday, May 25, 2007

Unilateral vs bilateral biting

Well, I haven't really had anything to post for awhile but I came up with an idea today.

Another one of my introductions into bite force analyses. This time, it's a relatively old but very well cited study by Jeff Thomason et al. (1990). This study is usually cited for the isometric stress values in jaw adductor muscles. What this means, is that when muscles contract, the individual fibres exert a certain amount of stress (force over a unit area). Thomason et al. (1990) showed that the mean muscle stress was 317 kPa which was well within the range of 147 - 392 kPa obtained for other vertebrates in a previous study (Carlson and Wilkie 1974).

The significance of these results is that they allow us to calculate muscular contractile forces from the physiological cross-sectional areas (PCSA) of muslces. Since stress is force over unit area, if the total area is known, then force can be calculated. Now, since stress value seems to be within a known range, we can multiply the stress value (N/m^2) with PCSA (m^2) and get muscle force (N).

However, there is another important aspect to Thomason et al. (1990) that seems to be overlooked. They actually observe the difference in bite forces between unilateral and bilateral muscular contractions. The jaw adductor muscles in anaesthetized opossums were experimentally stimulated and the resulting bite forces were recorded with a devise clamped between the upper and lower 2nd and 3rd premolars on one side of the jaw (unilateral biting condition). There are two notable experiments, one that stimulated only the muscles on the working side of the jaw (ipsilateral) and another that stimulated both the working and balancing sides (contralateral) of the jaws (bilateral stimulation). The results are very interesting (see figure).

The top graph shows approximately the pattern observed in a unilateral bite force from ipsilateral stimulations of the jaw adductor muscles, again, that's the muscles on the same side of the jaw as the biting point. Two distinct plateaus can be observed. The first is known to be the maximum bite force attained from an ipsilateral muscle stimulation from other experiments. Then what of the second plateau? Now, that's caused by the supposedly unstimulated muscles on the other side actually activating to add to the bite force.

This is really cool as this shows two things. First, it shows that muscle stimulations on the two sides of the skull are somehow linked, and the overstimulation of the ipsilateral muscles actually jumped across and started stimulating the contralateral muscles. This is recognised from the second graph, which shows the pattern in bite force from bilateral stimulations. Note that the value at which bite force plateaus is nearly identical to the second plateau of the top graph.

Second, it shows that bilateral contractions of the muscles actually nearly doubles the unilateral bite force. At least under experimental conditions, it has been shown that when biting on one side of the jaw, as you do when you're trying to crack open some pistachios, you can get twice as much bite force when you use muscles on both sides of the head rather than just the one. This seems really obvious, but it has been a topic of much debate - do muscles on both sides contract in a unilateral bite? At least in some mammals, it has been suggested that the muscles on the balancing side will only contribute to about 30% of the total contractile force (I can't remember the exact reference for this). On the other hand, Compton and Hylander (1986) mention that 'when the tenrec eats hard food, such as bone, there is little or no differential between the active and balancing sides'.

Thomason et al. (1990) also show that some voluntary bites by opossums can reach values obtained as maximal in experiments. This suggests to me that at least in opossums (and tenrecs), unilateral bite forces can employ bilateral muscular contractions to attain maximum bite force. In other words, a unilateral bite can be just as forceful as a bilateral bite.

References:
Carlson, F. D., and Wilke, D. R. 1974. Muscle physiology. Prentice-Hall, Englewood Cliffs.

Crompton, A. W., and W. L. Hylander. 1986. Changes in mandibular function following the acquisition of a dentary-squamosal jaw articulation. Pp. 263-282. In N. I. Hotton, P. D. MacLean, J. J. Roth, and C. Roth, eds. The ecology and biology of mammal-like reptiles. Smithsonian Institute Press, Washington.

Thomason, J. J., A. P. Russell, and M. Morgeli. 1990. Forces of Biting, Body Size, and Masticatory Muscle Tension in the Opossum Didelphis-Virginiana. Canadian Journal of Zoology-Revue Canadienne De Zoologie 68(2):318-324.