Muscles II: Microscopic Anatomy and Contraction
October 29 -30 2015
Muscle Structure
Muscle
Fascicle (bundle of fibers)
Muscle Fiber (single cell)
Myofibril (organelle)
Sarcomere (unit of contraction)
Microscopic Anatomy of Skeletal Muscle
• Large, cylindrical, multinucleate cells• Contain many mitochondria; nearly filled with
myofibrils• Some organelles have unique vocabulary:
– Sarcolemma: cell membrane– Sarcoplasm: cytoplasm– Sarcoplasmic reticulum: modified ER, surrounds each myofibril; store Ca2+
• Each myofibril can be divided into contractile units called sarcomeres.
• Sarcomeres consist of overlapping protein filaments of actin and myosin.
• Regular arrangement of dark and light bands. Dark bands occur where myosin is present.
Microscopic Anatomy of Skeletal Muscle
• The M line is where the myosin attaches
• Z discs (a membrane) mark the edge of each sarcomere; serve as attachment site for actin
Microscopic Anatomy of Skeletal Muscle
Use the picture to come up with a definition of the following:
I band
A band
H zone
Microscopic Anatomy of Skeletal Muscle
Use the picture to come up with a definition of the following:
I band – area without myosin fibers; aka light band
A band – area with myosin fibers; aka dark band
H zone – area without actin fibers
Microscopic Anatomy of Skeletal Muscle
First match the words …
actin cell
myofibril group of cells
sarcomere cell membrane
fascicle protein
muscle fiber organelle
sarcolemma contractile unit Then, write a paragraph that uses all the words in both
columns above and explains that structure of the muscle.
Turn & Talk
Contraction Overview• Globular heads of
myosin filaments attach to actin filaments.
• Myosin pulls actin filaments : “Sliding filament theory”
• Causes sarcomere to shorten, particularly the light bands
Contraction Overview• Globular heads of
myosin filaments attach to actin filaments.
• Myosin pulls actin filaments : “Sliding filament theory”
• Causes sarcomere to shorten, particularly the light bands
light dark light
light dark light
Contraction Overview
Which shows contracted muscle fibers?How can you tell?
Contraction Overview
Relaxed muscle Contracted muscle (large light bands) (small light bands)
Contraction Overview
What are these?
See animation!
Mitochondria
Contraction Details1. A motor neuron stimulates the muscle cell by
releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.
Note:A motor unit is a single motor neuron and all the muscle fibers it activates
Contraction Details1. A motor neuron stimulates the muscle cell by
releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.
2. ACh causes an electric current called an action potential to move through the muscle cell.
Contraction Details1. A motor neuron stimulates the muscle cell by
releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.
2. ACh causes an electric current called an action potential to move through the muscle cell.
3. The action potential causes the release of Ca2+ from the sarcoplasmic reticulum.
Contraction Details4. Ca2+ exposes myosin-binding sites on actin
filaments.
5. Myosin heads (& ADP) attach to actin binding sites, forming cross-bridges.
Muscle relaxed. Ca2+ present. No Ca2+ present. Cross-bridge formed.
myosin
actin
ADP + P
myosin head
Contraction Details6. Myosin heads release ADP, move the actin filament
in “power stroke”
Power stroke,ADP + P released
myosin
actin
Contraction Details6. Myosin heads release ADP, move the actin filament
in “power stroke”
7. ATP binds to myosin head. The crosslink between actin and myosin breaks.
8. ATP becomes ADP + P, readying the myosin head to reattach to actin.
Power stroke, ATP binds,ADP + P released cross-links break
myosin
actin
Contraction Details• If Ca2+ is still present, cycle will repeat, with myosin
heads reattaching and contracting the muscle even more.
• Once the action potential is over, the Ca2+ is reabsorbed into the sarcoplasmic reticulum. Without Ca2+, myosin cannot attach to actin.
Watch me!
Contraction DetailsNOTE:
ATP is required to breakcross-links, not to form them.
Explains rigor mortis
Why then do muscles need
ATP?
To reset head so it can contract further -- contraction is a series of sliding motions.
Turn & TalkDescribe the role of each of the following in muscle
contraction
Scholar with more siblings….• ACh• Ca2+
Scholar with less siblings … • ATP• Action potential
Exit Ticket1. In comparing electron micrographs of a
relaxed skeletal muscle fiber and a fully contracted muscle fiber, which would be seen only in the relaxed fiber?
a) Z discsb) Triadsc) I bandsd) A bandse) H zones
Exit Ticket2. Which word describes the unit of contraction of a muscle?
a) Myofibrilb) Sarcomerec) A bandd) H band
Exit Ticket3. Which of the following correctly lists the order of structure of the muscle from
largest to smallest?
a) fascicle, myofibril, sarcomere, muscle fiberb) myofibril, fascicle, sarcomere, muscle fiberc) fascicle, muscle fiber, myofibril, sarcomere d) muscle fiber, fascicle, myofibril, sarcomere
Exit Ticket4. Which of these stores calcium ion?
a) Sarcoplasmic reticulumb) Sarcomerec) Sarcolemmad) mitochondria
Exit Ticket5. Which of these best describe the
process of muscle contraction?
a) The actin filaments shortenb) The myosin filaments shortenc) The light bands shortend) The dark bands shorten
Exit Ticket6. Which of these best describe the process of muscle contraction?
a) Myosin heads attach to actin filaments that are exposed by the presence of ATP
b) Myosin heads attach to actin filaments that are exposed by the presence of Ca2+
c) Actin heads attach to myosin filaments that are exposed by the presence of ATP
d) Actin heads attach to myosin filaments that are exposed by the presence of Ca2+