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The muscular system
The muscular system is responsible for the movement of the human body
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Each of
these muscles is a discrete organ constructed of skeletal muscle tissue,
blood vessels, tendons, and nerves
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In these organs, muscles
serve to move substances throughout the body

Muscle Types
There are three types of muscle tissue: Visceral, cardiac, and skeletal
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Visceral Muscle
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The weakest of all muscle
tissues, visceral muscle makes organs contract to move substances
through the organ
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The term “smooth muscle” is
often used to describe visceral muscle because it has a very smooth,
uniform appearance when viewed under a microscope
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2
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Found only in the heart, cardiac muscle is responsible for
pumping blood throughout the body
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While hormones and
signals from thebrain adjust the rate of contraction, cardiac muscle
stimulates itself to contract
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Because of its self-stimulation, cardiac muscle is considered to
be autorhythmic or intrinsically controlled
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The
arrangement of protein fibers inside of the cells causes these light and
dark bands
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The cells of cardiac muscle are branched X or Y shaped cells tightly
connected together by special junctions called intercalated disks
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The branched structure and intercalated disks allow the muscle cells
to resist high blood pressures and the strain of pumping blood throughout
a lifetime
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3
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Skeletal muscle is the only voluntary muscle tissue in the
human body—it is controlled consciously
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g
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The function of skeletal muscle is to contract to move
parts of the body closer to the bone that the muscle is attached to
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Skeletal muscle cells form when many smaller progenitor cells lump
themselves together to form long, straight, multinucleated fibers
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Skeletal muscle derives its name from the fact that these muscles always
connect to the skeleton in at least one place
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Tendons are tough bands of dense regular connective tissue whose strong
collagen fibers firmly attach muscles to bones
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Muscles move by shortening their length, pulling on tendons, and moving
bones closer to each other
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The place on the stationary bone that is
connected via tendons to the muscle is called the origin
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The belly of the muscle is the fleshy part of the muscle in
between the tendons that does the actual contraction
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Location
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The rectus abdominis and transverse abdominis, for example, are found in
theabdominal region
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Other muscles use a hybrid of these two, like the
brachioradialis, which is named after a region (brachial) and a bone
(radius)
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Some muscles are named based upon their
connection to a stationary bone (origin) and a moving bone (insertion)
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Examples of this type of muscle
include the sternocleidomastoid (connecting thesternum and clavicle to
the mastoid process of the skull) and the occipitofrontalis (connecting
the occipital bone to the frontal bone)
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Some muscles connect to more than one bone or to
more than one place on a bone, and therefore have more than one origin
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A muscle with three origins is
a triceps muscle
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Shape, Size, and Direction
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For
example, the deltoids have a delta or triangular shape
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The rhomboid major is a
rhombus or diamond shape
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The gluteal
region contains three muscles differentiated by size—the gluteus maximus
(large), gluteus medius (medium), and gluteus minimus (smallest)
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In the abdominal region, there are several sets of wide, flat
muscles
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Function
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Most of the muscles of the forearms are named based on
their function because they are located in the same region and have
similar shapes and sizes
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Thesupinator is a muscle that supinates
the wrist by rolling it over to face palm up
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Groups Action in Skeletal Muscle
Skeletal muscles rarely work by themselves to achieve movements in the
body
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The
muscle that produces any particular movement of the body is known as an
agonist or prime mover
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For
example, the biceps brachii muscle flexes the arm at the elbow
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When the triceps is extending the arm, the biceps would be
considered the antagonist
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Synergists are muscles that help
to stabilize a movement and reduce extraneous movements
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Because skeletal muscles move the insertion closer to the
immobile origin, fixator muscles assist in movement by holding the origin
stable
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Skeletal Muscle Histology
Skeletal muscle fibers differ dramatically from other tissues of the body
due to their highly specialized functions
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The sarcolemma is the cell membrane of muscle fibers
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Connected to the sarcolemma are transverse tubules (T-tubules) that help
carry these electrochemical signals into the middle of the muscle fiber
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Mitochondria, the “power

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houses” of the cell, are abundant in muscle cells to break down sugars
and provide energy in the form of ATP to active muscles
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Myofibrils are made up of many proteins fibers
arranged into repeating subunits called sarcomeres
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(See Macronutrients for more information
about the roles of sugars and proteins
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Thick filaments
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Myosin is the protein that causes muscles to contract
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Thin filaments are made of three proteins:

1
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Actin forms a helical structure that makes up the bulk of the thin
filament mass
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2
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Tropomyosin is a long protein fiber that wraps around actin
and covers the myosin binding sites on actin
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Troponin
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Function of Muscle Tissue
The main function of the muscular system is movement
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Related to the function of movement is the muscular system’s second
function: the maintenance of posture and body position
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The muscles responsible for the body’s posture have
the greatest endurance of all muscles in the body—they hold up the body
throughout the day without becoming tired
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The cardiac and visceral muscles are primarily
responsible for transporting substances like blood or food from one part of
the body to another
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As a
result of the high metabolic rate of contracting muscle, our muscular
system produces a great deal of waste heat
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When we
exert ourselves more than normal, the extra muscle contractions lead to a
rise in body temperature and eventually to sweating
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The muscle acts as the effort force; the joint acts as the

fulcrum; the bone that the muscle moves acts as the lever; and the object
being moved acts as the load
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A third class lever is a system in which the
fulcrum is at the end of the lever and the effort is between the fulcrum
and the load at the other end of the lever
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The tradeoff for this increase in distance is that the force required to move
the load must be greater than the mass of the load
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A very slight
change in the length of the biceps causes a much larger movement of the
forearm and hand, but the force applied by the biceps must be higher
than the load moved by the muscle
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Each motor
neuron controls several muscle cells in a group known as a motor unit
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The size of motor units varies throughout the body, depending on the
function of a muscle
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Muscles
that need a lot of strength to perform their function—like leg or arm
muscles—have many muscle cells in each motor unit
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This explains why
the same muscles that are used to pick up a pencil are also used to pick
up a bowling ball
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Motor neurons contact muscle cells at a point called the Neuromuscular
Junction (NMJ)
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The motor end plate contains many ion channels that open in
response to neurotransmitters and allow positive ions to enter the muscle
fiber
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When the positive ions reach the sarcoplasmic reticulum, Ca2+ ions are
released and allowed to flow into the myofibrils
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Tropomyosin is moved away from
myosin binding sites on actin molecules, allowing actin and myosin to bind
together
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Myosin proteins act like oars
on a boat, pulling the thin filaments closer to the center of a sarcomere
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Myofibrils of muscle fibers are made of many sarcomeres in a
row, so that when all of the sarcomeres contract, the muscle cells
shortens with a great force relative to its size
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When a motor neuron stops the release of the
neurotransmitter, the process of contraction reverses itself
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Sarcomeres return to their elongated resting state once the force of
myosin pulling on actin has stopped
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A single nerve impulse of a motor
neuron will cause a motor unit to contract briefly before relaxing
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If the motor neuron
provides several signals within a short period of time, the strength and
duration of the muscle contraction increases
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If the motor neuron provides many nerve
impulses in rapid succession, the muscle may enter the state of tetanus,
or complete and lasting contraction
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Not all muscle contractions produce movement
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When people tense their bodies due to
stress, they are performing an isometric contraction
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A contraction that does produce movement is an isotonic contraction
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Muscle tone is a natural condition in which a skeletal muscle stays
partially contracted at all times
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All muscles
maintain some amount of muscle tone at all times, unless the muscle has
been disconnected from the central nervous system due to nerve damage
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1
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They are
very resistant to fatigue because they use aerobic respiration to produce
energy from sugar
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Near the spine and neck regions, very high
concentrations of Type I fibers hold the body up throughout the day
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Type II fibers are broken down into two subgroups: Type II A and Type II
B
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Type II A fibers are found throughout the body,
but especially in the legs where they work to support your body
throughout a long day of walking and standing
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Type II B fibers are also much lighter in color than
Type I and Type II A due to their lack of myoglobin, an oxygen-storing
pigment
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Muscle Metabolism and Fatigue
Muscles get their energy from different sources depending on the situation
that the muscle is working in
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Aerobic
respiration requires oxygen to produce about 36-38 ATP molecules from a
molecule of glucose
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When we use muscles to produce a high level of force,
they become so tightly contracted that oxygen carrying blood cannot
enter the muscle
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Anaerobic
respiration is much less efficient than aerobic respiration—only 2 ATP are
produced for each molecule of glucose
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To keep muscles working for a longer period of time, muscle fibers contain
several important energy molecules
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The oxygen from myoglobin allows muscles to
continue aerobic respiration in the absence of oxygen
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Muscles use
energy in the form of ATP, converting ATP to ADP to release its energy
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Finally, muscle
fibers contain energy-storing glycogen, a large macromolecule made of
many linked glucoses
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When muscles run out of energy during either aerobic or anaerobic
respiration, the muscle quickly tires and loses its ability to contract
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A fatigued muscle contains very
little or no oxygen, glucose or ATP, but instead has many waste products
from respiration, like lactic acid and ADP
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Oxygen debt (or recovery
oxygen uptake) is the name for the extra oxygen that the body must take
in to restore the muscle cells to their resting state

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