JAWS: anatomical correlates of feeding behavior - Case Study Example

Case Studies in Human Anatomy and Physiology Q1 This case study focuses on the biomechanics of jaw anatomy.What is the relationship between the length of the sarcomere and the tension generated by a muscle? What is the relationship between the length or thickness of the whole muscle and the force produced by the bite? What is the relationship between the length of the mandible, height of the ramus, and the angle of the jaw and the force produced by the bite?A1The overlapping between the thick and thin filaments of sarcomeres along the myofibrils in skeletal muscle such as the masseter determines the number of cross bridges formed.

These cross bridges in turn determine the amount of tension generated during a contraction event in a masticatory muscle fiber. During contraction activity myosin heads in the zone of overlap binds to active sites and produce stretch. Therefore the stretch or strain generated by the entire muscle fiber can thus be linked to the morphology of an individual sarcomere.There is reduced tension generated by muscles fibers with short sarcomeres when stimulated. This is attributed to the fact that there is disruption of physiology of thick and thin filaments where the thick filaments fill to capacity against the Z lines.

The thin filaments extending across the sarcomere overlap the filaments on the other side. Despite the fact that cross-bridge binding takes place, the myosin heads cannot pivot hence no stretch produced. The extent at which muscle stretches is controlled by connective tissue surrounding the muscle and the titin filaments that bind the thick filaments to Z lines in the muscle fiber. In the case study experiments any further increase in sarcomere length reduces the tension generated by reducing the number of potential cross-bridge interactions and size of the zone of overlap .

When this zone of overlap is at its lowest level, there is no interaction between the thin and thick filaments. The muscle generates no tension, and cannot contact. Only within the optimal brackets of sarcomere lengths can the maximum number of cross-bridges be formed thereby generating a greater tension. When the muscle fibers are stimulated within a small range of resting lengths they produce a forceful contraction. The sarcomere lengths of the masseter and temporalis muscle range between 75 to 130 percent of the optimal length. . Muscle fiber orientation is considered in functionality of the jaw.

For a given muscle mass, fibers oriented at a particular angle to the muscle force-generating axis tend to be shorter, in contrast to longer fibers that are aligned parallel. In this regard the fiber layout of the prominent mastication muscles in tree-gouging, show lengthened muscle fibers, greater proportions of fiber length to muscle bulk and more excursion powers compared with non-gouging tamarins. Q2How is the bite force required of these 3 species in their feeding strategies illustrated in the anatomy of the bone, muscles, and joints that make up the jaw?

What difference does size make?A2The anatomy of the bone, muscles, and joints that make up the jaw greatly determine the feeding strategy of animal species. The marmosets have long fibers that increase feeding function by producing greater power and large jaw gapes. These species show smaller muscle PCSAs; an indication of lower force production relative to cotton-top tamarins species. The results suggest that idealizing by jaw posture may be a better method of checking for fiber length variations when sarcomere length normalization is not feasible.

The length of the mandible, height of the ramus, the angle of the jaw and the arrangement of masticatory muscles, prevents the extreme compression or tension. For instance, a wide gape stretches the temporalis and masseter muscles, but the bone and ligament tissue of masticatory apparatus halt this muscle movement before the muscle fibers tear.During the normal animal movements, muscle fibers have a wide range of lengths; the tension generated changes with the initial size of the muscle fibers.

A tree gouging activity causes cyclic muscles contraction and relaxation where the muscle fibers are extended to almost "ideal" length before they are made to contract. The masseter and temporalis muscles contract over a wide range of resting lengths and are assisted by contractions from other muscles involved in mastication. These Mastication muscles are long fibered in the common marmosets species are suited for wide jaw gapes production. In contrast, cotton-top tamarins have shorter fibers and larger Physiologic Cross Sectional Areas and are thus better suited for generating larger muscle forces.

Pygmy marmosets species are smaller in size than their counterpart cotton-top tamarins and common marmosets. However, their muscle fiber lengths are comparable to cotton-top tamarins at a quarter of their body weight, an indication of long fibers best suited to large excursions.Q3What additional information did you find to help resolve this case? For example, you located a database of blood test values and learned that the result reported in the case was abnormal in some way.A3“Research article on the morphology of the chewing apparatus that creates wide jaw gapes among the tree-gouging common marmosets”.

The Journal of Experimental Biology. http://jeb.biologists.org/content/212/24/4040.fullQ4What does each of these additional items contribute to the solution of the case? For example, the item contains analyzes the use of specific tests, experiments, or analyses for this sort of study.A4Evolutionary adaptation and plasticity of muscle architecture,Natural selection may have favored the evolution of comparatively longer fibers to improve gouging ability. This evolutionary adaptive hypothesis is arguably challenged by the observation that skeletal muscle is a highly plastic tissue (Botticelli and Reggiani, 2006).

In fact, several thousand sarcomeres can be serially added to an existing fiber in a matter of days with muscle stretch, or stretch combined with electrical stimulation (Williams et al., 1986).This evolutionary adaptive hypothesis is further challenged by our essential ignorance of the genetic basis offiber architecture in skeletal muscle.The heritability of muscle fiber architecture has crucial implications for the evolution of behavioral performance in humans and other primates. Preliminary research using an inbred mouse model (Taylor et al., 2008) demonstrates significant genetic variation in both masseter fiber lengths and PCSA.

Furthermore, this heritable variation in fiber length may contribute to variation in maximum jaw opening in these mice. While further studies are needed to solidify this initial finding, these preliminary results provide the necessary foundation for hypotheses of evolutionary adaptation in jaw muscle architecture of marmosets and other primates.Q5What other findings or information in this case are consistent with the information you located? A5It is most likely that other animals that use wide jaw gapes during eating behaviors may have same musculoskeletal configurations to aid force production at these prominent excursions.

Q6What findings or information are inconclusive or in conflict with the information in 2? A6The musculoskeletal variations between C. jacchus and S. oedipus are connected to evolutionary adaptations and/or phenotypic plasticity related to the mechanical demands of tree gouging.Q7If your proposed resolution of the case is correct, what other observations might we expect to find in this case?A7Observe that tree-gouging species must open their mouths wider; and that tree-gouging species must generate more speed and cover more in closing the jaws to be able to gouge holes in the bark, compared to the actions needed for the other foods that all 3 species share: insects and fruits.

Q8Give your solution to this case and, on a scale of 1–5; rate how confident you are in your solution.A8The main outcome of the case is that the features that affect jaw function are: type of lever; mechanical advantage (based on relative lengths of effort and resistance arms); length of sarcomeres (which determine maximum tension); thickness of sarcomeres (shows increase in number of myofibrils which produces more tension; and overall size of the muscle (length and thickness) which relates to the cumulative force of all the sarcomeres.

Rate of 4 in confident to this solutionQ9In considering your confidence level from question 8, what other information you would like to have to resolve this case. A9Information on other animal species whose masticatory behaviors may have musculoskeletal configurations similar to those mentioned in the study so as to facilitate greater comparison and contrast through experimentation.Environmental factors such as climate, physiological conditions that go in hand with adaptation to environment.Work cited1.

Burkholder TJ, Fingado B, Baron S, Lieber RL. “Relationship between musclefiber types and sizes and muscle architectural properties in the mouse hindlimb.” J Morph 220(1994): 1–14.2. Cavagna GA, Heglund NC,Taylor CR. “Mechanical work in terrestrial locomotion: two basic mechanisms for minimizing energy expenditure”. Am J Physiol 233(1977): R243–R261.3. Lucas, P. Dental Functional Morphology: How Teeth Work. Cambridge: Cambridge University Press, 2004.4. Kier, William M. "Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats".

Zoological Journal of the Linnean Society 83(1985): 307–3245. Weijs WA, Hillen B. “Relationships between masticatory muscle cross-section and skull shape.” Journal of Dental Research 63(1984): 1154-1157.