1. Muscle: classification based on histological criteria.Smooth, cardiac,
2. Categories of muscles: based upon their functional role in an organism,
muscles may either be visceral or somatic, skeletal or
Individual muscle cells are myofibers. These are arranged into specialized
Smooth muscle: uninucleate, fusiform cells without striations. Serve a
visceral function and are innervated by the ANS.
Cardiac muscle: syncytium of cells containing actin and myosin myofilaments.
Branching fibers which communicate via intercalated disks. Intrinsic contractility.
Striated muscle: multinucleate myofibers containing myofibrils which show
periodic cross striations. These contain myofilaments of actin and myosin
arranged in functional units known as sarcomeres. Innervated by somatomotor
fibers and primarily serve somatic functions. Also known as skeletal muscle.
nonskeletal, or, voluntary and involuntary.
3. Skeletal Muscles: consist of muscular and tendinous elements. Tendons
attach muscle to bone and are extensions of the
Somatic muscle: orients and moves animal within its environment. Body wall,
appendages, and tail. Primarily striated. Derive from embryonic myotomes.
Include (1) muscles of body wall, (2) hypobranchial and tongue muscles,
(3) extrinsic eye muscles, and (4) appendicular muscles.
Visceral muscle: maintain homeostasis. Usually smooth muscles innervated
by ANS. Include (1) branchiomeric, (2) cardiac, (3) intrinsic eye, and
(4) erector muscles as well as smooth muscles of tubes, vessels, and hollow
Skeletal muscles: attach and move skeletal elements. Complexity of tetrapod
skeletal muscle correlates with life on land. Also known as voluntary.
Nonskeletal muscle: all other muscles, ie. smooth. These are involuntary.
muscle fascia (epimysium) which wraps individual
4. Origins, insertions, and actions: denote the structural and functional
relationship of muscles. Origin: site of attachment
Muscle fascicles are enclosed by perimysium. Individual myofibers
are also enclosed by connective tissue, the endomysium.
Red fibers: high content of myoglobin and a rich blood supply. Contract
slowly and resist fatigue. Allow muscles to function for extended periods.
White fibers: lack myoglobin, contract rapidly and fatigue quickly.
that remains fixed and does not move.
5. Names and homologies of skeletal muscles:
Insertion: attachment site that is displaced when muscle contracts.
Action: describes the functional movement of the muscle.
Names: can denote the direction taken by fibers (oblique, rectus), location
or position (supraspinatus), number of subdivisions (quadriceps), shape
(deltoid), origin or insertion (sternocleidomastoid), action (levator scapulae),
size (pectoralis major), or for other criteria.
Homologies: just because muscles in two different species have the
same name does not mean they are homologous. Muscles sometimes alter shape,
size, and site(s) of attachment during evolution. Homologous muscles
have the same embryonic origins (not necessairly adult characteristics).
1. Skeletal muscles of trunk and tail, hypobranchial, tongue, and
extrinsic eye muscles. Characteristically, these muscles exhibit metamerism.
2. Metamerism is due to origin of these muscles in the segmental
mesodermal somites. Somite mesenchyme moves lateral and then ventrally
to fuse in the midventral line as the linea alba. Myosepta may seperate
muscle of one body segment from another.
3. Fishes: axial muscles consist of myomeres seperated by myosepta.
Except in the agnathans, these are divided into dorsal (epaxial) and ventral
(hypaxial) masses by the horizontal skeletogenous septum (fiberous connective
tissue sheet between vertebral column and lateral body wall).
Metamerism of hypaxial muscles is interrupted by pelvic and
pectoral girdles, and by the gills. Dorsal to gills are the epibranchials
which extend to the skull. Beneath gills, the hypobranchials extend to
the lower jaw.
4. Tetrapods: except for the urodele amphibians, tetrapods have lost
most axial metamerism due to locomotion on land. Amniotes develop epaxial
muscles arranged as straplike or pinnate bundles above the transverse processes.
Similar modifications below the transverse process allowed for much greater
flexibility of the vertebral column.
Hypaxial myomeres were also replaced by broad muscular sheets
whose fibers ran in different directions (except the intercostals).
Epaxials of the trunk: extend from base of skull into tail.
Collectively called the dorsalis trunci in amphibians. In amniotes, they
form 4 groups: intervertebrals, longissimus, spinales, and iliocostales.
Hypaxial muscles of the trunk: divided into 4 groups (subvertebrals,
oblique, transverse, and rectus abdominus).
Intervertebrals: deepest epaxials (retain metamerism) and extend between
successive zygopophyses (interarticulares), transverse processes (intertransversarii),
neural arches (interarcuales), or neural spines (interspinales). Maintain
Longissimus and Spinales: lateral and medial above transverse
processes. Bundles named according to location (capitus: head;
cervicus: neck; dorsi: trunk). Longissimus is the longest functional epaxial
mass. Spinales connect neural spines or transverse processes with others
Iliocostales: lateral to longissimus. Originate on ilium as thin sheet
and insert on ribs.
5. The mammalian diaphram: formed by a contribution of mesenchyme
from several somites which form a dome-shaped muscle separating the
thorax from the abdominal cavity.
Subvertebrals: longitudinal flexors beneath transverse processes from atlas
to pelvis. Prominant in lumbar region as the quadratus lumborum and the
Oblique and Transverse Muscles: include the external and internal
obliques of the abdomen, the transverse muscles of the abdomen, the external
and internal intercostals of the thorax, and the transverse muscles of
the thorax. A portion of the internal oblique forms the cremaster muscle
in male mammals.
Rectus Muscles: flex the trunk and support the abdomen.
6. Muscles of the tail: continuation of epiaxial and hypaxial
muscles of the trunk which also contribute to sphincter muscles of
cloaca and anus.
7. Hypobranchial and tongue muscles: originate from mesenchyme of
postbranchial somites which moves forward in floor of pharynx beneath
the branchial arches (anterior extensions of hypaxial muscles).
8. Extrinsic eye muscles: allow voluntary movement of the eye in
all directions. Include superior and inferior rectus, lateral and
medial rectus, and superior and inferior obliques. Muscles of the eyelid
also belong to this group.