Muscle Anatomical Organization and Physiology 10/21 and 10/24
Levels of muscle organization: organ to myofiber
Levels of muscle structure: myofiber to actin/myosin
Sacromere bands: Why do they have different shades?
Isometric/Isotonic: which changes tension and which
changes the length of a muscle?
• Incomplete vs. Complete Tetany
• What is the rational for the strength/tension relationship?
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Potentially Confusing Things about the words that describe the organization
of muscle cells into an organ called a muscle (i.e. Biceps brachii):
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Organization from tiniest to largest (a whole muscle or organ)
Sarcomeres: many units of z-line to z-line.
Myofilament: consist of actin and myosin filaments.
Myofibrils: groups of myofilaments surrounded by a sarcoplasmic reticulum.
Sarcolemma: plasma membrane that surrounds several myofibrils, the sarcoplasm is the
fluid inside the “cell”.
Skeletal Myocyte= Skeletal Myofiber
Endomysium: connective tissue sheath that surrounds a single myofiber (or myocyte or
muscle cell).
Perimysium: connective tissue sheath that surrounds several endomysiums/myofibers.
Think of the strands of meat you can pull out of a well cooked pot-roast!
Muscle Fascicle is another term to describe a unit within a single muscle with perimysiums
and many endomysiums/myofibers inside.
Fascia: an additional sheath surrounding the whole muscle.
Remember that all muscles come in pairs: one is a primer mover or agonist and the other
being an antagonistic muscle
Consider the arm: Triceps brachii that extends your arm, the biceps brachii is required so
you can flex your arm back to its original condition.
Potentially Confusing Things about the words that describe the
organization in myocyte Tricky Words:
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Sarcolemma: is another word for plasma membrane that is specific to that
surrounding a muscle cell (also called a myocyte or myofiber)
Endomysium: This fibrous sheath that wraps around a single myofiber and
strengthens the sarcolemma.
Sarcoplasm: is another word for cytoplasm that is specific to that found inside a
myofiber.
Sarcomere: is the unit of contraction in a myofiber or space extending from one Z-line
to the next Z-line. A single myofiber would have hundreds clustered into groups
called myfibrils. (think: Z-line Actin+ Myosin + Actin + Z-line)
Myofibrils are bundles inside a single myofiber, with each bundle consisting of a
few hundred myofilaments.
Myofilaments are inside a larger myofibril and consists of many sarcomeres (z-line to
Z-line) that are end to end.
Sarcoplasmic Reticulum (SR): consists of a series of tubes running the length of the
myofiber, these hollow tubes store calcium so a muscle can relax between
contractions. When the muscle contracts Ca++ is released from the SR into the
sarcoplasm (cytoplasm).
Transverse tubules extend from the SR deep into the myofiber so Ca++ is release
evenly into the cell for contractile force generation.
How is a muscle structurally organized?
Remember: each muscle is an organ that makes up the muscular
system.
• Actin/Myosin• Muscle fibermultiple nuclei and very large
• Endomysium• Fascicle• Perimysium• Epimysium• Tendon vs. Ligament• The 600 different individual muscles are the
organs composing the skeletal system.
Muscle Structure From Smallest to Largest:
Actin/myosin>Filaments>Myofibrils>Myofiber>Endomysium>
Perimysium> Epimysium(Fascia)> Muscle attached to bone by
tendons
Can you identify these parts?
A’ is the belly of the muscle and each myofiber (D) is surrounded by a bit of areolar
connective tissue (endomysium), several endomysiums wrapped together by connective
tissue is the perimysium, several perimysiums together makes the epimysium (fasciae),
which connect to bone at tendons at either end.
What are the structural components in a
skeletal muscle cell (myofiber)?
Muscle Organization:
Fascicle(meat grain)(endomysium is areolar CT
around each MyofiberMyofibrilActin/Myosin
Myofiber: (Endomysium)SarcolemmaTtubulesSarcoplasmic reticulumSarcoplasm(fluid)
Actin/Myosin Organization:
“Sarcomere”Z disc to Z disc
Inside sarcomere: I-band (actin: light), A-band
(myosin: very dark) and H-band (just myosin: dark
middle part of A-band)
Other Organelles:
• Mitochondria:
• Nuclei and transcription/translation:
• Sarcoplasmic Reticulum and T-tubules:
Relative to a healthy membrane phospholipid bilayer, where should you
find sodium, potassium and calcium?
Calcium stored/transported in special structures: Sarcoplasmic reticulum/Ttubules
Review of structural organization
How does the orientation of actin and myosin heads at the start of
contraction effect potential force generation?
Review: Muscles/Myofibers/Sarcomeres have an ideal length that gives
them the best contractile performance for a given stimulation. This is called
the Length-Tension Relationship
The Total elapsed time for a single
myofiber twitch is extremely variable, only
7 to 100 msec. This is still pretty short!
• To Twitch or Not to Twitch, this is the
question for a myofiber.
• What is a motor endplate?
In coming action potential
• What is vesicular release?
Release of ACH into synapse
ACH opens Na channels!
• What is multiple motor unit summation?
• Threshold contraction for a myofiber vs.
maximal contraction for an entire muscle?
• The % of myofibers in the entire muscle are
ultimately recruited determines the strength
of contraction.
How do BOTH all-or-none and graded responses coexist in a muscle?
• Consider how a single myofiber contracts:
• How do several myofibers together contract:
• How do cells modify their force of contraction? AllOr-None and Graded?
• How does exercise and pre-conditioning affect the
potential maximal force generation?
What are the phases of a myofiber depolarization
and contraction?
Total elapsed time for a single twitch is short and still
extremely variable: 7 to 100 msec
• 1) Arrival of stimulus at
motor end plate
• 2) Latent period
• 3) Contraction phase
• 4) Relaxation phase
• How is muscle fiber
recruitment effected by:
nicotine, temp, or nerve
damage?
• Myasthinia gravis and
ACH-receptors?
• Cholinesterase inhibitors
and warfare
Why does muscle tension increase with repeated
contractions? What is muscle tetany?
What are the pre-failure limits?
• Force generation: myofiber vs. whole muscle
• All-or-none vs. Graded responses
• Multiple motor unit summation:
• Recovery duration and ATP replenishment
• Treppe and wave summation:
When myofibers in a muscle depolarize and the muscle
contracts, a change in muscle length OR a change in
muscle tension can occur (or both).
Why is this significant?
• # myofibers recruited determines strength!
• Isometric Contractions:
• Isotonic Contractions:
• Concentric contraction: shortens as it contacts
• Eccentric contraction: lengthens as it contracts
• How does a muscle warm-up help an athlete to
reach top performance?
How do we maintain adequate ATP supplies in
active muscle when there is so little ATP present
at any one time?
ADP Phosphorylation:
Glycolysis in cytosol:
Oxidative phosphorylation in mitochondria:
Fatty Acid Oxidation:
Phosphagens: Phospho(CREATINE)kinase
phosphocreatine+ADP--> ATP+creatine
• Myokinase: ADP + ADP  AMP + ATP
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• Sequential times of ATP supply depletion:
– When does ATP become available and from what
source?
• Consider Peak Performance Activities:
• How does carbohydrate loading work?
• How do dietary supplements like PCr work?
What sorts of delay occur in a cell with respect to the time needed to produce
ATP from PCr, glucose and fatty acids when you begin exercise, when you
sustain it for a minute and sustain it for 15 minutes? How does this describe
the runner surge 15 minutes into a workout?