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Lecture 10

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ZOO 2090
Fred Laberge

Lecture 10 October 16, 2013 ZOO*2090 Muscles Objectives - Classify muscles according to commonly used criteria - Highlight the function of major muscle groups in vertebrates What is a muscle? - collection of modified cells that can generate force along the axis of their fibers  contractile - force can be used to produce movement or restrain movement - in order to do that, muscle cells are electrically excitable  responsive to nervous stimulation - muscles can also generate heat through shivering - some muscles are modified to specifically generate heat (eg. brown fat) or electric chocks (eg. electric eel) or electric fields (eg. electrosensory fishes) upon stimulation Classification - by location (somatic and visceral) - by method of nervous control (voluntary or involuntary) - by microscopic appearance (smooth, cardiac or skeletal) o skeletal muscle  associated with skeleton o cardiac  muscle of heart wall o smooth  muscle of blood vessels and many visceral organs Embryonic Origin of Muscles - all muscles originate from mesoderm - differentiation into segmented somites (epimere), hypomere and mesenchyme Lecture 10 October 16, 2013 ZOO*2090 - somites are divided further into dermatome (skin), myotome (muscle) and sclerotome (vertebral column) o somites  anterior mesoderm clusters do not become fully segmented - fate of mesoderm divisions o 1) myotome divisions of somites  most postcranial and some cranial muscles o 2) somitomeres  cranial muscles o 3) hypomere  muscles of gut and heart o 4) mesenchyme  some limb and smooth muscles Actions of muscles - usually two groups of muscles working opposite to each other - flexors/extensors – bend/straighten a limb or a part of a limb - abductors/adductors – draw a limb away or toward the midline of the body - pronators/supinators – turn palm or sole down/up - levators/depressors – raise/lift or lower - constrictors/dilators – close/open orfices - protracts/retractors Major Muscle Groups 1) Postcranial muscles a. trunk (axial) musculature: body wall, divided into epaxial and hypaxial divisions b. appendicular musculature: fin and limbs 2) Cranial muscles a. branchiomeric muscles: jaw and pharynx muscles derived from somitomeres (innervated by cranial nerves) b. hypobranchial muscles: jaw and pharynx muscles derived from cervical somites (innervated by spinal nerves) c. extrinsic eye muscles 3) Muscles of gut and heart 1a. Trunk Muscles in fishes - myotomes from distinct myomeres (segments) separated by connective tissue (myosepta)  propulsive locomotion - dorsal (epaxial) and ventral (hypaxial) portions separated by the horizontal septum Lecture 10 October 16, 2013 ZOO*2090 - ventral portions of first few myotomes moved forward to form the hypobranchial muscles  open jaws and pull gills down and backward 1a. Trunk Muscles in tetrapods - decreased role in locomotion allows more specialized roles o flexion of vertebral column o movement of rib cage o weight support o movement of the head Epaxial musculature of salamanders is one segmented muscle mass, the dorsalis trunci Hypaxial musculature differentiated into 3 bands of lateral muscle and a ventral muscle band Epaxial musculature of amniotes differentiated further and the septa disappeared; three longitudinal bundles attach to vertebrae and contribute to complex trunk and head movement 1) transversospinalis: short fibers 2) longissimus dorsi: longer fibers, all the way to neck 3) iliocostalis: most lateral, some contact ribs In birds and turtles epacial muscles are reduced and the transversospinalis and longissimus dorsi become many small specialized muscles for complex head movement Hypaxial musculature of lizards attaches to rib cage (red
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