The Muscular System
ANS 215
Anatomy & Physiology
of Domesticated
Animals
Primary Functions
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Movement
Blood
 Ingesta/excreta
 Gametes/conceptus
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Support
 Generation of body heat
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Calssifications
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Smooth
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No striations
Individual cells are spindle shaped with
centrally located nuclei
Regulated by the autonomic nervous system
Located in visceral structures
Aggregates of myofilaments are composed of
actin and myosin
Filaments are not arranged in order
Smooth Muscle
Classifications
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Cardiac
Found only in the heart
 Regulated by the autonomic nervous
system
 Shows striations characterized by
alternating white and dark bands
 Composed of elongated, branching cells
with irregular contours at junctions
with other cells
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Classifications
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Cardiac
Boundary area where cells meet is
called intercalated disk
 This specialized structure facilitates
transmission of nerve impulses
 Each cell has a nucleus (sometimes 2)
that is centrally located
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Cardiac Muscle
Classifications
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Skeletal
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Long bundles of cells – striated
Multinucleated with nuclei at periphery of cells
Innervated by spinal or cranial nerves
Three types
• red or dark (highest myoglobin and mitochondria
content)
• white or pale (lowest myoglobin and mitochondria
content)
• intermediate (characteristics between white and red
fibers)
Classifications
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Skeletal
Skeletal muscle comprises majority of
body muscle mass
 Skeletal muscle fiber can run the length
of the muscle with which it is a part
 Skeletal muscle fibers do not branch or
anastomose
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Classifications
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Skeletal
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Often described according to type of
movement performed
• Flexors – if they are located on the side of the limb
towards which the joint decreases the joint angle
• Extensors – if they are located on the side of the
limb towards which the joint increases the joint
angle
• Adductors – if they pull a limb toward the median
plane
• Abductors – if they pull a limb away from the
median plane
• Sphincters – are arranged circularly to constrict
body openings
Skeletal Muscle
Arrangement
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Function of muscles is to contract or
shorten and thereby move an object
Primary consideration for accomplishing
this goal is arrangement
Examples include:
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Sheets
Sheets rolled into tubes
Bundles
Rings
Cones
Discrete cells or clusters of cells
Arrangement
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The effects of skeletal muscles (apart
from sphincters) are noted some distance
from their location
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This means the contraction must be
transmitted
One end must be anchored and the other
attached directly to a tendon or moveable
part
Accordingly, anatomic description of a skeletal
muscle sometimes refers to its origin and
insertion
Arrangement
Skeletal Muscle Harnessing
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Harness for skeletal muscle fibers is
composed of connective tissue elements
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Epimysium
Perimysium
Endomysium
Continuous from the individual muscle
fibers to the connective tissue structure
to which it attaches and on which it
exerts its contraction
Skeletal Muscle Harnessing
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Connective tissue elements of
skeletal muscle are:
Muscle fibers comprise a muscle bundle
which are attached by their cell
covering (sarcolema) to connective
tissue division, endomysium
 The endomysium is continuous with the
connective tissue that envelopes
muscle bundles, the perimysium
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Skeletal Muscle Harnessing
The perimysium is continuous with the
connective tissue that envelops the
muscle (collection of bundles), the
epimysium
 Then epimysium is continuous with the
tendon or aponeurosis, which can travel
some distance from its attachment
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Microstructure of Skeletal
Muscles
Depending on diameter of the
muscle fiber, there might be several
hundred to several thousand
myofibrils within one muscle
 Each myofibril has striations or
banding
 Myofibrils are further broken into
repetitive units (sarcomeres)
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Microstructure of Skeletal
Muscles
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Sarcomeres contain the protein
myofilaments actin and myosin
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Give rise to striations by their arrangement
Sarcomeres of a myofibril are in
alignment with the sarcomeres of all the
other myofibrils of the muscle fiber
The Z line is located at each end of a
sarcomere and is common to both
sarcomeres that it separates
Microstructure of Skeletal
Muscles
Actin filaments project from the Z
line into the sarcomeres that it
separates
 Actin of two sarcomeres common to
the same Z line comprise an I band
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Microstructure of Skeletal
Muscles
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The myosin filaments are centrally
located within a sarcomere, coupled with
the overlap of actin filaments provide for
the dark banding (A band)
There is a regular 2:1 ratio of actin to
myosin
Sarcotubular System
Skeletal muscle fibers contain a
network of tubules known as the
sarcotubular system
 Tubules are located within the
muscle fiber, but outside the
myofibrils
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Sarcotubular System
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Two tubule sets
Arranged transversely (right angles) to
myofibrils
 Open to the outside of fibers
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• contain extracellular fluid
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Found at boundary of A and I bands
• each sarcomere has two
Neuromuscular Junction
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Each skeletal muscle fiber is innervated by a
motor neuron
Axon terminal end bulb forms synapse with
muscle fiber
Release of Acetylcholine (Ach) causes
depolarization
One motor neuron innervates many muscle
fibers
 Motor unit
Motor unit ration of 1:150 means one nerve fiber
supplies 150 muscle fibers
Smaller ration increases precision
Scanning Electron
Micrograph of
Neuromuscular Junction
Skeletal Muscle Contraction
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Depolarization of muscle fibers
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Acetylcholine released into synapse
• Ca++ facilitates release
• Parturient paresis occurs if not enough Ca++
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Opens ligand-coated Na+ channels
Causes a propagated action potential
• sarcolema
• T-tubule
• sarcoplasmic reticulum
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Cholinesterase destroys Ach
Skeletal Muscle Contraction
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Depolarization of muscle fibers
Depolarization of sarcoplasmic
reticulum causes Ca++ release
 Ca++ diffuses to myofibrils
 Ca++ initiates contraction
 Ca++ pumped back into sarcoplasmic
reticulum
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• active transport
• Ca++ ATPase Pump