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Muscular system
Purpose of muscles

Stabilize joints
 Work
with tendons to reinforce and stabilize joints that
have poorly fitting articulating surfaces

Posture
 Skeletal muscles
 Fibers shorten at different times

Movement
 Moves bones by pulling on them
 Primary –muscle that provides the most movement
 Synergist- muscles that contract and assist the prime mover
 Antagonist-resist a prime mover’s action and cause
movement in the opposite direction

Heat production
 Maintain homeostasis
 By product of muscle activity
Muscle introduction

Three types of muscles
 skeletal
 Smooth
 cardiac

A skeletal muscle is an organ of the
muscular system
 Composed
of skeletal muscle tissue, nervous
tissue, blood, and connective tissue
Smooth muscle
Elongated with tapering ends
 Lack striations
 Sarcoplasmic reticulum in not well
developed
 Two major types of smooth muscle

 Multiunit
smooth muscle
 Visceral smooth muscle
Multiunit smooth muscle
Muscle fibers are separate rather than
organized into sheets
 Found in the irises of the eyes and the
walls of the blood vessels
 Contracts only in response to stimulation
by motor nerve impulses and certain
hormones

Visceral smooth muscle

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Composed of sheets of spindle shaped cells in close
contact with one another
More common type of smooth muscle
Found in walls of hollow organs, such as the stomach,
intestines, urinary bladder, and uterus
Fibers can stimulate each other

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Display rhythmicity- a pattern of repeated contractions

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When one fiber is stimulated, the impulse moving over its
surface may excite adjacent fibers, which in turn stimulate others
This is due to self-exciting fibers that deliver spontaneous
impulses periodically into surrounding muscle tissue
These two features are largely responsible for
peristalsis- the wave like motion that occurs in certain
tubular organs such as the intestines
Smooth muscle contractions

Two neurotransmitters affect smooth muscle
 Acetylcholine
 Norepinephrine



Can also be affected by hormones
Slower to contract and relax than skeletal
muscle
Can maintain forceful contraction longer
Cardiac muscle


Found only in the heart
Opposing ends are connected by intercalated
disks
 Elaborate
junctions between cell membranes
 Help to join cells and to transmit the force of
contraction from cell to cell



Allow muscle impulses to pass freely so that they travel
rapidly from cell to cell
Responds in an all or none manner
Self-exciting and rhythmic
Skeletal muscle
Voluntary
 Structure:

 Long
and cylindrical
 Multinucleated
 Striations

Contract rapidly but tire easily
Structure of skeletal muscle

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Endomysium- connective tissue sheath that
encloses each muscle fiber
Fascicle- bundle of fibers
Perimysium- connective tissue that wraps a
fascicle
Epimysium- connective tissue that covers the
entire muscle
Tendon- strong cordlike structure formed from
epimysia that attaches muscles to bones
Aponeuroses- sheet-like structure that connects
muscles indirectly to bones, cartilage, or
connective coverings
Microscopic anatomy of skeletal
muscle

Alternating light (I) bands and dark (A)
bands
The
light I band has a midline
interruption which is a darker area called
the z disc
The dark band A has a lighter central
area called the H zone
The M line is the center of the H zone
 Contains
tiny protein rods that hold thick
filaments together
Microscopic anatomy cont.

Two types of myofilaments
 Thick filaments (myosin filaments)
 Made mostly of myosin
 Extend the entire length of the A band
 Cross bridges- projections off the ends of the myofilaments
that link the thick and thin filaments during contractions
 Thin filaments (actin filaments)
 Composed of actin
 Anchored to the Z disc
 Makes up the I band
Microscopic anatomy cont.

When a contraction occurs actin
containing filaments slide toward each
other into the center of the sarcomere the
H zones disappear because myosin and
actin filaments are completely overlapped
Sliding filament theory

that the head of a myosin cross bridge can
attach to an actin binding site and bend
slightly
 Myosin
molecule is composed of two twisted
protein strands with globular parts called crossbridges projecting outward along their lengths
This pulls the actin filament with it
 Then the head can release, straighten, and
combine with another binding site farther
down the actin filament and pull again

Skeletal muscle activity

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Irritability- the ability to receive and respond to
stimulus
Contractibility- the ability to shorten when
adequate stimulus is received
Must be stimulated by nerve impulses to
contract
One motor neuron may stimulate a few muscle
cells to hundreds
One neuron and all the skeletal muscle cells it
stimulates are a motor unit
Nerve stimulus and action potential
The extension of the neuron (axon)
reaches the muscle it branches into a
number of axon terminals
 Each forms junctions with the sarcolemma
of a different muscle cell

 Junctions

are neuromuscular junctions
The gap between nerve endings and
muscle cell membrane is the synaptic cleft
Nerve stimulus and action potential

Nerve impulse reaches axon terminals neurotransmitter
is released

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Acetylcholine (Ach) stimulates skeletal muscles
Ach diffuses across the synaptic cleft and attaches to
receptors that are part of the sarcolemma
Sarcolemma becomes temporarily more permeable to
Na+ and they rush into the cell while K diffuse out of the
cell
The electrical conditions of the membrane are reversed
and opens more channels to allow Na+ entry only
This “upset” generates an electrical current called an
action potential
Leads to contraction of the muscle cell
Oxygen debt
Oxygen is used to synthesize ATP
 When oxygen is low muscles produce lactic
acid anaerobic respiration
 As lactic acid accumulates, an oxygen debt is
produced
 Oxygen debt equals the amount of oxygen
required to convert lactic acid into glucose,
plus the amount muscle cells require to restore
ATP and creatine phosphate to their original
concentrations
 The conversion is slow and may take several
hours to pay back

Muscle fatigue
Fatigue- the ability to contract after
strenuous exercise
 Interruption in the muscle’s blood supply
or lack of acetylcholine in motor nerve
fibers
 Usually from accumulation of lactic acid

 Lowers
pH causing muscles not to respond to
stimulation
Muscle cramp
Painful condition in which a muscle
undergoes a sustained involuntary
contraction
 Thought to occur when changes in
extracellular fluid surrounding the muscle
fibers and their motor neurons somehow
trigger uncontrolled stimulation of the
muscle

Muscle responses
Threshold stimulus- the minimal strength
of stimulation to cause a contraction
 All-or-none response- a skeletal muscle
exposed to stimulus of threshold strength
or above responds to its fullest extent

 Does
not contract partially
 Increasing the strength of the stimulus does
not affect the fiber’s degree of contraction
Recording muscle contractions

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Myogram- recording of muscle that is stimulated electrically
Twitch- a single contraction that only last a fraction of a
second
 Muscle is exposed to a single stimulus of sufficient
strength to activate motor units
Latent period- the delay between the time the stimulus is
applied and the time the muscle responds
 Followed
by period of contraction- when the
muscle pulls at its attachments
 Period of relaxation- when it returns to former
length
Summation
A muscle fiber exposed to a series of
stimuli of increasing frequency reaches a
point when it is unable to completely relax
before the next stimulus in the series
arrives
 Tetanic contraction- When the resulting
forceful sustained contraction lacks even
partial relaxation

Muscle tone
Certain amounts of sustained contraction
occur in the fibers
 Is a response to nerve impulses that
originate repeatedly from the spinal cord
and stimulate a few muscle fibers
 Important in maintaining posture

 If
suddenly lost the body collapses
Skeletal muscle actions
Origin- the immovable end of the muscle
 Insertion- the movable end of the muscle

 When
a muscle contracts the insertion is
pulled towards the origin
 Some muscles have more than one origin or
insertion
Naming skeletal muscles

Direction of the muscle
 Named
for reference to some imaginary line
usually the midline
Rectus- fibers run straight
 Oblique- at an slant

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Relative size of the muscle
 Ex.
Maximus- largest
 Minimus- smallest
 Longus- liong
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Location of the muscles
 Named

with bone associated with
Temporalis and frontalis overlie the frontal and temporal
bones
Naming skeletal muscles

Number of origins
 Biceps-

two origins, triceps-three etc.
Location of muscle’s origin and insertion
 Named
for attachment sights
 Ex. Sternocleidomastoid- origins at sternum and
clavicle and inserts on mastoid process

Shape of the muscle
 Have


distinctive shape
Deltoid is roughly triangular
Action of the muscle
 Named

for their actions
Flexor, extensor, and adductor