Lab 6 Sample Solutions

Lab 6 Sample Solutions 1. Compare the demonstration microscope slides of skeletal, smooth, and cardiac muscle. Make a quick sketch of each of these three types of muscle). Label the nuclei, as well as any other features that will help you to distinguish between these three muscle types. Refer to your own sketches of skeletal, smooth, and cardiac muscle. 2. Examine the dissections showing the extrinsic eye muscles of the dogfish ( Squalus ). Complete Table 5.1 in your lab manual. Muscle Origin Insertion Predicted Action Superior Oblique anterior medial corner of orbit dorsal surface of eyeball look up and forward Inferior Oblique anterior medial corner of orbit ventral surface of eyeball look down and forward Superior Rectus posterior medial corner of orbit dorsal surface of eyeball look up and back Lateral Rectus posterior medial corner of orbit posterior-lateral surface of eyeball look back Medial Rectus posterior medial corner of orbit medial surface of eyeball, towards the anterior look forward Inferior Rectus posterior medial corner of orbit ventral surface of eyeball look down and back 3. Complete the Tables 5.2, 5.3 and 5.4 of your lab manual. Identify each muscle in your animals and predict its action based on its origin and insertion. Table 5.2: Branchiomeric Muscles Muscle Animal Origin Insertion Action adductor mandibulae Squalus palatoquadrate cartilage Meckel’s cartilage (lower jaw) elevates (adducts) lower jaw (closes mouth) masseter and temporalis Rattus masseter: zygomatic arch temporalis: temporal fenestrae dentary bone (lower jaw) elevates (adducts) lower jaw (closes mouth) Columba orbit and temporal fenestrae dentary bone (lower jaw) elevates (adducts) lower jaw (closes mouth) intermandibularis Squalus Meckel’s cartilage (lower jaw) central raphe elevates floor of mouth 1

mylohyoideus Columba dentary bone (lower jaw) mid-ventral (central) raphe elevates/tenses the floor of the mouth digastric Rattus dentary bone (lower jaw) hyoid bone depresses lower jaw (opens mouth) and/or elevates hyoid cucullaris Squalus fascia of epaxial muscle scapular cartilage elevates scapular cartilage and gill arches trapezius Rattus nuchal crest of skull and neural spines of vertebrae scapula and clavicle draws scapula dorsally (and anterodorsally) sternomastoideus sternum nuchal crest of skull turns (rotates) head Table 5.3: Hypobranchial Muscles Muscle Animal Origin Insertion Action coracoarcuals Squalus coracoid bar hyoid arch helps to depress the lower jaw (opens mouth) sternohyoid Rattus sternum hyoid bone draws hyoid posteriorly Table 5.4: Appendicular Muscles Muscle Animal Origin Insertion Action pectoral abductor Squalus Scapular process Fin pterygiophores elevates (abducts) pectoral fin latissimus dorsi Rattus thoracic and lumbar vertebrae shaft of humerus pulls forelimb caudo-dorsally Columba thoracic vertebrae humerus (posterior surface) abducts and retracts wing triceps brachii Rattus scapula and shaft of humerus ulna, on the olecranon process (“the elbow”) extends forearm Columba humerus ulna, on the olecranon process (“the elbow”) extends forearm pectoral adductor Squalus coracoid bar pectoral fin pterygiophores adducts (depresses) pectoral fin 2

pectoralis b) In which of the three animals is this muscle the largest (relative to overall body size)? Explain why this muscle is so large. Pigeon. In all three animals, this muscle adducts the forelimbs. In the pigeon, adducting the forelimbs (wings) produces the powerful down-stroke that generates lift for flight. The pectoralis must be very large and strong to produce the amount of force required to generate lift. c) Where would you locate the antagonists to the muscles that adduct the forelimb/fin in each of: the shark, rat, and pigeon? Are these muscles all homologous to each other? Explain. In the shark, the antagonist to the pectoral adductor is on the opposite (dorsal) side of the pectoral fin; it is called the pectoral abductor. In the rat, the antagonists to the pectoralis muscles are on the opposite/dorsal side of the forelimb, e.g. the latissimus dorsi muscles. In the pigeon, the antagonist to the pectoralis is located on the same side of the forelimb as the pectoralis! It is called the supracoracoideus. The supracoracoideus lies parallel and dorsal to the pectoralis muscle, and also originates on the sternum, and inserts onto the dorsal side of the humerus via a tendon that travels through a hole in the pectoral girdle. No, these muscles are not all homologous to each other: The pectoral abductor of the shark is homologous to the latissimus dorsi of the rat, however, neither of these muscles are homologous to the supracoracoideus of the pigeon. Instead, the supracoracoideus of the pigeon is homologous to the pectoral adductor of the shark and the pectoralis of the rat. (note the location of the supracoracoideus!) 6. Consider the temporalis muscle in Rattus . a) Identify an antagonist of the temporalis muscle in the rat. Explain your choice. The temporalis muscle elevates (adducts) the lower jaw to close the mouth. The digastric muscle is an antagonist of the temporalis muscle because it has the opposite function: it depresses the lower jaw to open the mouth. (The digastric is actually one of several muscles involved in depressing the lower jaw. Other muscles include the lateral pterygoid, mylohyoid, and geniohyoid, but these muscles are located too deep to view in this dissection.) b) Does the temporalis muscle have an agonist? Explain. Yes. Both the temporalis and the masseter elevate the lower jaw to close the mouth. They both move the same body part (the lower jaw) in the same direction, therefore they are agonists. c) In vertebrates, what might be the benefit of having two muscles that both act to move the lower jaw in the same direction? Although both of these muscles elevate the lower jaw, they insert onto different locations along the dentary bone and at different angles, such that they exhibit 4

different mechanical advantages when closing the mouth. The temporalis can produce a more powerful bite when the jaws are far apart (as they would be, for example, when carnivores bite large prey), while the masseter can produce larger forces when the jaws are closer together (as they would be, for example, when herbivores use the molars for grinding). Also, in many herbivores, the masseter muscles are also positioned such that they can produce lateral movements of the lower jaw for chewing (=prolonged mastication or grinding). Colour the muscle diagrams in your lab manual with 4 colours: yellow for axial, green for hypobranchial , red for branchiomeric , and blue for appendicular . (Note that these colours do not represent embryonic germ layers.) Note that you are not responsible for knowing all of the muscles in the following diagrams, only the muscles listed in the Important Terminology for Lab 5. Lateral view of musculature in Squalus acanthias Ventral view of musculature in Squalus acanthias 5

7