Muscles

COMPARATIVE ANATOMY | LECTURE 5

I. MUSCLE TISSUE 3. Smooth Muscle Tissue
 Function:  Cells are:
 shorten when stimulated  Fusiform
 stabilize or operate skeleton  Uninucleate
 final determinants of posture,  Have myofibrils
locomotion, and orientation of the  Lack cross striations
body in the environment  Autonomic nervous system
 physiological functions

How Do Muscles Contract?

 when the thin actin and thick myosin
filaments slide past each other.

Kinds of Muscle Tissue

1. Skeletal Muscle Tissue

 Cell: long, cylindrical, multinucleate muscle
fibers Summary of Muscle Types
 Striations caused by MYOFIBRILS
 Each myofibril consist of SARCOMERES
 Each sarcomere composed of
MYOFILAMENTS
 Myofilaments are: ACTIN & MYOSIN

2. Cardiac Muscle Tissue

 Myocardium Categories Of Muscle Tissue
 Similar to the skeletal muscle fibers Somatic Visceral
 Syncytium with intercalated disc
 Excitation is autonomic (Na, K, Ca)
Voluntary vs. Involuntary Muscles
 Useful classification to human subjects
 Not very applicable to lower animals
 Voluntary – under conscious control
 Involuntary – unconscious contraction

Red vs White Muscle Fibers

 A histochemical variation
 Striated muscle
 White – fast twitch, fatigueable, lack
myoglobin
 Red – slow twitch,
“unfatigueable”,
Homologies
richer blood supply,
 shark: hypaxial musculature
with myoglobin
vs
 necturus:rectus abdominis, external oblique,
internal oblique, transversus abdominis
vs
 cat: rectus abdominis, external oblique,
internal oblique, transversus abdominis
 More
reliable criteria
for determining
homologies are:
Names of Skeletal Muscles are based on:
 embryonic
 direction of fibers (e.g., oblique)
origin
 location or position (e.g., superficial)
 nerve supply
 number of divisions (e.g., triceps)
 shape (e.g., deltoid)
 origin and/or insertion (e.g., iliocostalis)
 action (e.g., levator scapulae)
 size (e.g., major)
 or some combination of these

How it Functions?

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 SYNERGISTIC ACTIONS
 ELEVATOR/DEPRESSOR
 CONSTRICTOR
 FLEXION
 PRONATION
 ABDUCTOR

 Axial musculature of an aquatic

salamander, Necturus maculosus. The layers
of lateral hypaxial musculature are exposed
from superficial to deep in the cranial to
caudal direction. The number of external
oblique layers varies between one and two in
this species (the figured specimen exhibits
two). Abbreviations: oes, M. obliquus
externus superficialis; oep, M. obliquus
II. AXIAL MUSCLES externus profundus; oi, M. obliquus internus;
 Include the skeletal muscles of the trunk & tail ta, M. transversus abdominis
 extend forward beneath the pharynx as
hypobranchial muscles & muscles of the Trunk & Tail Muscles of Fish:
tongue are present in orbits as extrinsic
eyeball muscles
 are metameric (most evident in fish and
aquatic amphibians where the axial muscles
are used in locomotion; in other tetrapods,
metamerism is obscured due to presence of
paired appendages responsible for
locomotion on land)
 are segmental because of their embryonic
origin; arise from segmental mesodermal
somites Trunk & Tail Muscles of Tetrapods
 Tetrapods, like fish, have epaxial & hypaxial
masses, & these retain some evidence of
metamerism even in the highest tetrapods.
 Modifications:
1. epaxials are elongated bundles that
extend through many body segments
& that are located below the
expanded appendicular muscles
required to operate the limbs

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 2. hypaxials of the abdomen have no
myosepta & form broad sheets of
muscle
3. hypaxials are oriented
into oblique, rectus, &
transverse bundles

Epaxials of Tetrapods
 lie along vertebral column dorsal to
transverse processes & lateral to neural
arches
 extend from base of the skull to tip of the tail
 Urodeles & some lizards - epaxials are
obviously metameric & are referred to
as the dorsalis trunci
 Higher tetrapods - superficial epaxial
bundles form long muscles that
extend over many body segments;
deep bundles are still segmented
 Longest bundles:
1. longissimus group
 lies on transverse
processes of vertebrae;
includes the longest Epaxials of Tetrapods
epaxial bundles  Shortest bundles - intervertebrals
 subdivisions include:  remain segmented
o longissimus dorsi
 connect processes (spinous,
o longissimus cervicis
transverse, & zygapophyses) of
o longissimus capitis
adjacent vertebrae
2. iliocostalis group
 lateral to longissimus &
Function of Epaxials of Tetrapods
spinalis
 arises on ilium & inserts on 1. short epaxials perform same function as in
dorsal ends of ribs or uncinate fish (side-to-side movements of vertebral
processes column)
3. spinalis group 2. short & long bundles arch & support the
 lies close to neural arches vertebral column
 connects spinous processes or 3. most anterior bundles = attach to & move the
transverse processes with skull
those several vertebrae
anteriorly

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Hypaxials of Tetrapods HYPAXIALS: Rectus muscles
1. Muscles of lateral body wall:  weakly developed in most fish; 'stronger' in
 oblique (external & internal), tetrapods
transverse, & rectus muscles  support ventral body wall & aid in arching the
2. Muscles that form longitudinal bands in roof back
of body cavity (subvertebral muscles)  in mammals - rectus abdominis (typically
extends from the anterior end of the sternum
to the pelvic girdle)

Hypaxials: Oblique & Transverse Muscles

 Early amphibians & reptiles
 ribs developed in myosepta along
entire length of the trunk Function of Hypaxials of Tetrapods
 urodeles still have myosepta the 1. Aquatic urodeles = used chiefly for swimming
length of the trunk, but ribs no longer 2. Terrestrial urodeles = assist in locomotion
form in all of them 3. Other tetrapods = reduced in volume
 Modern amniotes compared to fish (because of shift in mode of
 myosepta & ribs are restricted to the locomotion); now support contents of
thorax (so abdominal muscles are abdomen, assist in respiration
not obviously segmented) (especially intercostal muscles), & assist
 hypaxials form 3 layers: external epaxials in bending vertebral column (rectus
oblique, internal oblique, & transverse muscles)
(in the thorax region: external &
internal intercostals, which play an Hypobranchial & Tongue Muscles:
important role in respiration, &  Fish
transverse muscle)  hypobranchials
extend forward from
pectoral girdle & insert
on mandible, hyoid, &
gill cartilages
 hypobranchials
strengthen floor of
pharynx & assist
branchiomeric muscles
in elevating floor of
mouth, lowering jaw, &
extending gill pouches

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  Tetrapods
 hypobranchials stabilize & move
hyoid apparatus & larynx
 the tongue of amniotes is a 'sac'
anchored to hyoid skeleton & filled
with hypobranchial muscle

Extrinsic Appendicular Musculature

 Dorsal group of the forelimbs, e.g., trapezius
and latissimus dorsi, arise on:
1. fascia of trunk in lower tetrapods
III. APPENDICULAR MUSCLES
2. skull, vertebral column, & ribs to a
 move fins or limbs
point well behind the scapula in higher
 Extrinsic - originate on axial skeleton or
tetrapods & converge on the girdle &
fascia or trunk & insert on girdles or limbs
limb
 Intrinsic - originate on girdle or proximal
 Ventral group, e.g., pectoralis, arises on
skeletal elements of appendage & insert on
sternum & coracoid, & converge on limb
more distal elements

 Fish - appendicular muscles serve mostly as

stabilizers; intrinsic muscles are limited in
number & undifferentiated

 Tetrapods - appendicular muscles are much  RESULT:

more complicated than in fish pectoral girdle
 greater leverage required for locomotion & limb are
on land joined to trunk
 jointed appendages (as opposed to fins) by extrinsic
require complex muscles appendicular
muscles as
illustrated in
this diagram:

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Extrinsic Appendicular Muscles  Mammals - similar to reptiles but more
1. most develop from hypaxial blastemas in the diverse
body wall
2. referred to as secondary appendicular
muscles because it was not their original
function to operate appendages
3. chief extrinsic muscles of forelimbs of
tetrapods include: scapular deltoid,
latissimus dorsi, rhomboideus, serratus
ventralis, & pectorals

IV. BRANCHIOMERIC MUSCLES

1. associated with the pharyngeal arches
2. series of skeletal & smooth muscles
3. adductors, constrictors, & levators operate
jaws plus successive gill arches

Intrinsic Appendicular Muscles

1. form from blastemas within the limb bud
2. called primary appendicular muscles

Appendicular Muscles
 Amphibians - much
more complex than
in fish
 Reptiles - more
numerous & diverse
than in amphibians;
better support of body & increased mobility of
distal segments of the limbs
 Birds - intrinsic musculature is reduced;
pectoralis (downstroke muscle) &
supracoracoideus (upstroke muscle) are
enlarged

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Muscles of the Mandibular Arch
 Squalus & other fish - operate the jaws
(adductor mandibulae & intermandibularis)
 Tetrapods
 muscles of 1st arch still operate jaws
 adductors of mandible:
o masseter & temporalis
o pterygoid
o digastric

 Tetrapods- muscles further reduced; primary

muscles include:
 stylopharyngeus (Arch III) - used for
swallowing
Muscles of the Hyoid Arch  intrinsic muscles of the larynx or
 move hyoid arch 'voicebox' (remaining arches)
 aid in hearing (stapedial muscle)  cucullaris - gives rise to trapezius,
 assist in moving lower jaw (e.g., digastric) cleidomastoid, & sternocleidomastoid
muscles of amniotes

Muscles of 3rd & Successive Arches

 Squalus - constrictors above & below gill
chambers plus levators (including the
cucullaris) that compress & expand the gill
pouches
 Bony fish - muscles reduced; operculum
plays important role in respiration
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 VI. ELECTRIC ORGANS
 consist of a number of electric discs (up to
20,000) piled in either vertical or horizontal
columns
 each disc (electroplax) is a large coin-shaped
cell
 evolved several times in a variety of fish (good
example of convergent evolution)

V. Integumentary Muscles
 Extrinsic integumentary muscles (e.g.,
platysma)
 originate (usually) on the skeleton &
insert on the underside of the dermis
 striated
 move skin of amniotes
 Intrinsic integumentary muscles (arrector pili
muscles)
 entirely within the dermis
 found in birds & mammals
 mostly smooth muscles  The electric organ contains electrically
excitable cells called 'electrocytes', which
receive simultaneous command signals from
the brain to 'fire'. At the moment of 'firing', the
electrocytes are asymmetrically polarized
acting as serially connected batteries.

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 The simultaneous firing of electrocytes results
in the electric organ discharges (EODs)
which are emitted in the surrounding water.
In strongly electric fishes, such as the electric
eel, electric catfish, and electric rays, the
electric organ is huge containing numerous
electrocytes. Therefore, their discharge
voltage can reach as high as 600 volts. In
weakly electric fishes, which use electricity for
navigation and communication, the
discharge voltage is small -- often less than a
volt.
 There are two types of EODs, pulse type and
wave type. All strongly electric fishes and
some weakly electric fishes are pulse-type
 Electric fishes are divided into the three main
electric fishes. They discharge short electrical
categories.
pulses intermittently. Some weakly electric
 Strongly electric fish
fishes are wave type. They produce wave-like
 electric eel
continuous A.C. electricity.
 electric catfish
 electric rays
 Weakly electric fish
 knife fishes
 elephant nose
 Fishes that can only sense electricity
 All electric fishes mentioned so far not only  sharks
produce electricity but sense it with a very  rays
sensitive sensory organ called  skate
'electroreceptors' which are embedded in the  catfish
skin.  paddle fish
 Electroreceptors are used to detect a slight  Platypus (though not a fish, they are
change of electric field cause by nearby electroreceptive.)
objects. Electric fishes can thus electrically
'see' objects in an environment where vision
is useless (at night, or in murky water). This
process is called 'active electrolocation'
because the source of electricity that they use
for electrolocation is their own electric organ.

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