Chapter 8
Muscular System
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Recap:
-The three types of muscle in the
body are skeletal, smooth, and
cardiac muscle.
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Structure of a Skeletal Muscle
Each muscle is an organ comprised of
-skeletal muscle tissue
-connective tissues
-nervous tissue
-blood
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Connective Tissue Coverings
Fascia: Layers of dense connective tissue
that surround and separate each
muscle.
Fascia extends beyond the ends of the
muscle and gives rise to
tendons which are fused to bones
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Epimysium: The layer of connective tissue
around each whole muscle
Perimysium : The layer of connective tissue
surrounding individual bundles
(fascicles) within each muscle
Endomysium: The layer of connective tissue
around each muscle cell (fiber)
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Skeletal Muscle Fibers
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-Each muscle fiber is a single cell with…
-Sarcolemma: cell membrane
-Sarcoplasm : cytoplasm containing many
mitochondria and nuclei
-Myofibrils: organelles that are separated into
compartments called sarcomeres
-Sarcomeres: structures that have thick and thin
protein filaments
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The Sarcomere:
-Thick filaments of myofibrils are made up of
the protein myosin.
-Thin filaments of myofibrils are made up of
the protein actin.
-The organization of these filaments produces
striations.
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A sarcomere extends from Z line to Z line.
-I bands (light bands) made up of actin filaments
are anchored to Z lines.
-A bands (dark bands) are made up of overlapping
thick and thin filaments.
-In the center of A bands is an H zone, consisting
of myosin filaments only.
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 Skeletal Muscle Contraction
Muscle contraction involves several
components that result in
-the shortening of sarcomeres
-the pulling of the muscle against its
attachments.
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-Sliding filament theory: the myosin
attaches to the binding site on the actin
filament, pulls and releases it
*After release, it will move on to the next actin
binding site
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Stimulus for Contraction
1. The motor neuron releases
acetylcholine (signalled from brain)
*Signals muscle to contract
2. Calcium is released
3. Myosin is able to bind to actin and the
fiber contracts (sarcomeres shorten)
How does this stop?
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Energy Sources for Contraction
1. Energy for contraction comes from ATP.
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Muscle Fatigue
-Muscle Fatigue: A muscle loses its ability to
contract during strenuous exercise
-Usually arises from the accumulation of lactic
acid (lowered pH) in the muscle.
-As a result, no muscle response to stimulation
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Muscle Cramp
Muscle cramp: sustained involuntary
contraction
*Changes in fluid around cell
*Uncontrolled stimulation of muscle
(dehydration…)
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 THRESHOLD STIMULUS
 Muscles will only respond
(contract) if the stimulus
strength reaches a certain
point
 Minimum strength required
to make a muscle fiber
contract is called the
THRESHOLD STIMULUS
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 ALL – OR – NONE RESPONSE
 Muscles can’t “partially”
contract
 They will either contract
fully or not at all
 Increasing the stimulus
intensity does not increase the
force of a single muscle’s
response
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 Twitch: period of contraction and relaxation of a single
muscle fiber
 Movements result from multiple muscle fibers
contracting together
 Fast Twitch: weight lifting, sprints,
 Slow Twitch: low intensity, long distance run,
swimming
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Skeletal muscle contracts
involuntarily
Can be caused by:
*Muscle fatigue/strain
*Stress/anxiety
*Medication
*Caffeine
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 MUSCLE TONE –Even at “rest” a
muscle is contracting
 Important in
maintaining posture
 When a person collapses,
muscle tone has been lost
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 ISOTONIC - Muscle changes
length and produces force;
causes motion
○ Ex. Lifting weights
 ISOMETRIC - Muscle doesn’t
move but creates force
 Ex. Yoga
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 Smooth Muscles
Smooth muscle cells:
-elongated with tapered ends
-lack striations
tw0 relaxed smooth muscle cells
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Multiunit
 Blood vessels
 Iris of the eye
 Individual fibers
Visceral
 Walls of hollow organs
 Sheets
 Self-stimulating and
rhythmic
 Peristalsis in tubes
and hollow organs
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Smooth Muscle Contraction
-Slower to contract and relax than skeletal muscle
-Can contract longer using the same amount of ATP
(sustainable contractions)
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 Cardiac Muscle
-
has structures that supply extra calcium, and
can thus contract for longer periods
-
intercalated disks: join cells and transmit
impulses throughout the heart
-
self-exciting and rhythmic
-
the whole heart contracts as a unit
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