Answers-2, BIO 3220, Muscular System


1. Explain the overall function of muscles.
Muscles function in locomotion, posture, protection, and heat production.

2. Classify muscle tissue according to microscopic appearances and nervous control.
Skeletal muscle appears striated, or striped, and cylindrical with the nuclei peripheral. Skeletal muscle cannot contract without stimulation from a motor neuron. Cardiac muscle appears striated, cylindrical and branched, with the nuclei central, with intercalated discs. Its movement is involuntary. Smooth muscle appears smooth, spindle-shaped with the nucleus central. Its movement is involuntary and is controlled by the autonomic nervous system.

3. Describe the parts of a muscle cell (fiber), including myofibril, myofilament, actin, myosin, and sarcomere.
Each muscle fiber contains an array of myofibrils that are stacked lengthwise and run the entire length of the fiber, mitochondria, an extensive endoplasmic reticulum, many nuclei. Each myofibril is made up of arrays of parallel filaments. The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin. The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins, troponin and tropomyosin. The myofibrils have distinct, repeating microanatomical units, termed sarcomeres, which represent the basic contractile units. The sarcomere is defined as the region of myofilament structures between two Z-lines. Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fiber of which it is a part.

4. Define motor neuron and motor unit.
Motor neuron – a neuron that stimulates muscle contraction
Motor unit – the motor neuron plus the number of myofibers it innervates

5. Define intercalated disc.
Intercalated disc are regions where adjacent cardiocytes interlock and where gap junctions permit electrical coupling between the cells.

6. Define and give an example of a somatic muscle.
Somatic muscles consist of all skeletal muscle except branchiomeric muscle. It is voluntary muscle and is found in the body wall and appendages.

7. Define and give an example of a visceral muscle.
Visceral muscle is found in various parts of the body such as the arteries, the bladder, the digestive tract as well as in many other organs. Visceral muscle is smooth muscle as it doesn’t have cross striations. Visceral muscle contracts slower than skeletal muscle but the contraction can be sustained over a longer period of time.

8. Compare and contrast red and white muscle fibers.
Red – more blood supply; aerobic metabolism; has myoglobin – red pigmented protein that stores oxygen; fatigue resistant; ex. fish swimming long distances, posture of tetrapods
White – not as much blood supply; anaerobic metabolism; fatiguable; fish spurts of swimming, tetrapod sprints

9. Describe the following gross anatomical features:
Origin – the relatively fixed attachment to skeleton
Insertion – the muscle attachment point that is relatively free to move
Fascia – a loose connective tissue that binds muscle to muscle and skin to muscle
Tendon – join muscle to bone; cord-like
Aponeurosis – a tough flat sheet of connective tissue serving to distribute the tension of a muscle; sheet-like tendon

10. Characterize the various muscle shapes.
Strap – fleshy, wide attachments at each end; ex. sternocleidomastoid
Teardrop – teardrop shaped muscle; ex. pectorals
Spindle – cigar-like shape with pinched ends; ex. extremities
Heads – multiple origins; ex. forearm of cat
Sheets – fiber strands lie in flat bundles; ex. abdominals
Fan – fan shaped muscle converging towards attachment point; ex. trapezius
Circular/sphincter – circular-shaped muscles; ex. sphincter muscles
Pinnate – breathing muscle unique to mammals; ex. diaphragm

11. Review the following muscle actions:
Flex – reduce angle between adjacent bones
Extend – increase the angle
Adduct – move parts inward toward midline
Abduct – move parts outward
Levator – raise parts
Depressor – lower parts
Protract – push part away from its base – forward movement
Retract – draw it back
Sphincters – constrict openings
Constrict – compress spaces
Dilator – oppose both of above
Rotator – turning
Supinator – rotators that turn palms of hands upward
Pronator – turn them downward
Tensor – pulling tight/taut

12. Contrast agonist, synergist, and antagonist.
Agonist is the primary mover. Synergist is muscle that supplements/helps movement. Antagonist is muscle that opposes movement.

13. Discuss the embryologic development of muscle. How is phylogeny of muscles studied?
The mesoderm gives rise to the epimere, mesomere, and hypomere. The epimere gives rise to the dermotome which becomes some smooth muscle, the sclerotome, and the myotome which becomes most muscles except for branchiomeric muscle. The hypomere gives rise to somatic muscles, muscles of the body wall, and splanchnic muscles, smooth muscle of the gut and cardiac muscle. Embryology and nerve supply are also used to study the phylogeny of muscles. Embryology helps to provide for major muscle categories. The nerve supply has been shown to be the most consistent indicator of homology.

14. Define metamerism, somite, and myoseptum.
Metamerism – The condition of having the body divided into metameres (somites), exhibited in most animals only in the early embryonic stages of development
Somite – segmental mass of mesoderm in the vertebrate embryo, occurring in pairs along the notochord and developing into muscles and vertebrae; also called metamere
Myoseptum – the septum or divider between adjacent myotomes which are segments of a somite in a vertebrate embryo that differentiates into skeletal muscle

15. Distinguish axial from appendicular muscles.
The axial musculature begins are myotomes separated by myosepta. The axial muscles are then further divided into two regions, muscles on the dorsal part of the body (epaxial muscles) and mucles on the muscles on the ventral part of the body (hypaxial muscles). These regions are separated by the lateral or horizontal septum. The axial musculature associated with the trunk can function either in locomotion or breathing. They include the muscles of the trunk and tail, hypobranchial, tongue, and extrinsic eye muscles. Appendicular muscle development originates from the somites as outgrowths of the somite myotome into the limb bud. Such outgrowths are called the myotomic buds to the appendages. As the limb buds grow, the appendicular musculature subdivides into the muscle mass that lies above the appendicular skeleton (dorsal muscles) and the mass that lies below the appendicular skeleton (ventral muscles).

16. Characterize epaxial muscles. Discuss their function and innervation. (See Muscle Handout.)
Epaxial muscle is the region of axial muscle on the dorsal part of the body. It functions to extend or straighten the spine (dorsoflex) and provides some lateral flexion. It is innervated by the dorsal rami of spinal nerves.

17. Characterize hypaxial muscles. Discuss their function and innervation.
Hypaxial muscle is the region of axial muscle on the ventral part of the body. It functions to bend the spine (ventroflex) and provides some lateral bending. It is innervated by the ventral rami of spinal nerves.

18. Characterize hypobranchial muscles. Discuss their function and innervation.
Hypobranchial muscle is located below the pharynx, from the pectoral girdle to the jaw. It functions in respiration and feeding. It is innervated by 12th cranial nerve and ventral rami of cervical nerves.

19. Characterize extrinsic eye muscles. Discuss their function and innervation.
Extrinsic eye muscles are formed from epaxial parts of 3 pairs of head myotomes. They function as a pattern of muscles that move the eye. They are innervated by the 3rd, 4th, and 6th cranial nerves.

20. Describe the axial muscles of agnathans, fish, and tetrapods, including a description of the following muscles: dorsalis trunci, intervertebrals, longissimus, spinales, iliocostalis, subvertebrals, abdominal obliques, transverses, intercostals, rectus abdominus, and diaphragm.
Agnathans – lateral septum lacking; segmentation present; axial skeleton virtually absent; no hypobranchial musculature; branchial muscles not prominent; extrinsic eye muscles develop from head myotomes (but hard to homologize with other vertebrates)
Jawed fishes – lateral septum divides epaxial and hypaxial portions; myomeres are more angled than in cyclostomes (dorsal ribs, if present, in this septum); hypobranchial muscles distinct; extend from pectoral girdle to visceral arches; longitudinal orientation; coracomandibularis responsible for opening of jaw; extrinsic eye muscles; 4 rectus muscles (anterior, posterior, superior, inferior); 2 oblique muscles (superior and inferior); 3rd cranial nerve – anterior, superior, inferior recti and inferior oblique (premandibular myotome); 4th cranial nerve – superior oblique (mandibular myotome); 6th cranial nerve – posterior rectus, derived from both mandibular and hyoidean myotomes
Tetrapods – several trends evident in evolution of the axial musculature of vertebrates: in fishes the axial muscles are the main propulsive muscles and the most massive of the body, in tetrapods the appendicular muscles enlarge and the axial musculature diminishes
Amphibians (axial musculature transitional between that of fishes and reptiles) – epaxial muscles (dorsalis trunci); conservative; hypaxial muscles; primitive on tail but on trunk more advanced; divided into three groups: Subvertebral group (below transverse processes; flexes the spine), Rectus abdominis muscle (ventral, between the girdles; ventroflxes body, supports viscera), Lateral group (3 sheetlike layers; support and compress body wall) – external oblique, internal oblique, transverses; pelvic girdle – stronger in amphibians than in fishes; since it has gained articulation with the spine it does not require musuclar support; pectoral girdle no longer articulates with the head and does not have an articulation with the spine, several muscles evolve to hold this girdle to trunk; hypobranchial muscles; generally like fishes, but fleshy tongue has important added derivatives
Reptiles and mammals – epaxial muscles, those in tail without myosepta, those in cervical region form in layers, those of trunk complex; higher tetrapods – superficial epaxial bundles form long muscles that extend over many body segments; deep bundles are still segmented; longest bundles: longissimus group lies on transverse processes of vertebrae; includes the longest epaxial bundles; subdivisions include: longissimus dorsi longissimus cervicis, longissimus capitis; iliocostalis group is lateral to longissimus & spinalis arises on ilium and inserts on dorsal ends of ribs or uncinate processes; spinalis group lies close to neural arches connects spinous processes or transverse processes with those several vertebrae anteriorly; shortest bundles – intervertebrals remain segmented connect processes (spinous, transverse, & zygapophyses) of adjacent vertebrae; hypaxial muscles; cervical and caudal regions similar to amphibians; on trunk the subvertebral group much reduced (except in lumbar area); lateral group has modifications on thorax; the transversus is excluded from thorax and external and internal obliques become the external and internal intercostal muscles (new function of ventilation of the lungs); muscles in shoulder region derived from lateral group of hypaxial muscles become more prominent: Serratus, Levator scapulae, Rhomboideus; Diaphragm found only in mammals is of hypaxial origin and is innervated by ventral rami of cervical nerves; Hypobranchial muscles are much the same as for amphibians, but altered by regression of gills and complicated by enlargement of larynx

21. Describe the hypobranchial and tongue muscles of tetrapods.
In amphibians, hypobranchial muscles are generally like fishes, but fleshy tongue has important added derivatives. In reptiles and mammals, hypobranchial and tongue muscles are much the same as for amphibians, but altered by regression of gills and complicated by enlargement of larynx. They function to stabilize the hyoid and larynx. The roots lingu- and glosso- mean tongue.

22. Describe the appendicular muscles of fish.
Appendicular muscles of fish are divided into dorsal mass of extensors (upward and forward), and ventral mass of flexors (downward and backward). These muscles have origins on the girdles (which are anchored within the axial musculature) and adjacent fascia.

23. Describe the dorsal and ventral muscle groups of the pectoral and pelvic limb as described on your muscle handout.
Pectoral limb –
Dorsal group – includes deltoid, latissimus dorsi, teres major, subcoracoscapularis, teres minor, triceps, forearm and hand extensors, cutaneous trunci = maximus
Ventral group – pectoralis, supracoracoideus, biceps brachii, brachialis, forearm, wrist, and digit flexors
Pelvic limb –
Dorsal group – iliofemoralis, iliofibularis, iliotibialis, femorotibialis, ambiens, puboischiofemoralis internus, ankle and foot extensors
Ventral group – puboischiotibialis, pubotibialis and flexor tibialis, adductor femoris, ankle and foot flexors

24. Characterize the branchiomeric muscles. What is their origin?
Branchiomeric muscles are associated with the pharyngeal arches, are a series of skeletal & smooth muscles, are adductors, constrictors, & levators operate jaws plus successive gill arches. They have a visceral origin.

25. Discuss the branchiomeric muscles of each pharyngeal arch per your muscle handout.
Muscles of the Mandibular Arch (first arch): in Squalus & other fish – operate the jaws (adductor mandibulae & intermandibularis); in tetrapods, muscles of 1st arch still operate jaws; adductors of mandible: masseter and temporalis, pterygoid, digastric
Muscles of the Hyoid Arch: move hyoid arch, aid in hearing (stapedial muscle), assist in moving lower jaw (e.g., digastric)
Muscles of 3rd & successive arches: in Squalus – constrictors above and below gill chambers plus levators (including the cucullaris) that compress and expand the gill pouches; in bony fish – muscles reduced; operculum plays important role in respiration; in tetrapods – muscles further reduced; primary muscles include: stylopharyngeus (Arch III) – used for swallowing, intrinsic muscles of the larynx or ‘voicebox’ (remaining arches), cucullaris – gives rise to trapezius, cleidomastoid, and sternocleidomastoid muscles of amniotes

26. Discuss the incidence, derivation, function and structure of electric organs.
Electric organs are found in more than 500 species of fish. They are primarily derived from muscle cells, but also from glands and nervous tissue. They function in communication, orientation, detection of prey, and offense or defense. Electric organs consist of large number of electric discs in vertical or horizontal columns. Each disc is a functional unit called an electroplax. Each electroplax is a modified multinucleate muscle fiber.