STUDY GUIDE: Skeletal Muscle – microscopic anatomy
Sarcolemma – Cell membrane of a
muscle cell
Myofibril – Long ribbon like organelles
that fill cytoplasm
Light (I) Band – Lighter colored band that
contain only the actin (thin) filaments and
is reason for striated appearance
Dark (A) Band - Darker colored bands
that contain the Myosin (thick) filaments
Z Disc – Zig zag midline
interruption in light (I) band
H Zone – Lighter central
zone in dark (A) band –
Actin and myosin filaments
do not overlap here, so it
appears somewhat lighter
M Line – Line through the
center of the H zone –
Contains tiny protein rods
that hold thick filaments
together – Thin filaments do
not overlap here
Sarcomere – Tiny
contractile units that make
up myofibrils
Myofilaments – Filaments
that compose the
myofibrils
Thick (myosin) filament –
Filaments composed of
the protein myosin that
extend the entire length of
the dark (A) bands
Thin (actin) filaments Filaments composed of
the protein actin that are
found in the light bands
and overlap into the dark
bands
STUDY GUIDE: Skeletal Muscle – microscopic anatomy
Cross Bridges – Studs on myosin
filaments that link actin and
myosin strands during muscle
contraction
Sarcoplasmic reticulum – Specialized
endoplasmic reticulum found only in
muscle cells that stores and releases Ca+
for use during muscle contraction
Key Concept Questions
1) What are sarcomeres composed of and how are they arranged within a myofibril?
Composed of Myofilaments
Arrangement: Aligned end to end along the length of myofibrils
2) Write a brief paragraph that explains the arrangement of the following in a muscle fiber: Sarcolemma, Myofibrils,
Nuclei, Dark (A) Bands, and Light(I) Bands.
A muscle fiber is composed of myofibrils. A bunch of myofibrils and several nuclei are surrounded by the
sarcolemma (Cell membrane). Each myofibril is composed of alternating bands: The Dark (A) bands and the Light (I)
bands.
STUDY GUIDE: Skeletal Muscle – microscopic anatomy
3) Label the following in the diagram below: Sarcolemma, Myofibrils, Nuclei, Dark (A) Bands, and Light(I) Bands.
4) Explain how the thick (myosin) filaments and the thin (actin) filaments are arranged within a myofibril.
The thick (myosin) filaments are found in the dark bands. The thin (actin) filaments are attached to the z disc and run
through the light bands and overlap the thick bands until halfway through the dark band. The H zone is where the
thin filaments DO NOT overlap the thick filaments. The M line runs through the middle of the H zone and serves to
hold the thick filaments in place.
5) Label the following parts in the diagram below: thin (actin) filament, thick (myosin) filament, Z disc (2 times), H
zone, I band, A band, and M line.
6) While looking at the diagram above, explain what accounts for the striated appearance of skeletal muscle.
The alternating I and A bands account for the striation. The presence of thick (myosin) filaments makes the A bands
appear darker than the I bands, which have no thick filaments, just the thin filaments.
7) How does the M line contribute to the structure of a myofibril?
The M line is the line that runs through the middle of the H zone and is composed of tiny protein rods that hold the
thick (Myosin) filaments together which holds them in place.
8) What role do the myosin heads on the thick filaments play during muscle contraction?
The myosin heads link the actin and myosin filaments together during contraction. This allows the contraction to be
held by preventing the filaments from slipping back to their resting position.
9) Why is the sarcoplasmic reticulum essential for muscle contraction?
The sarcoplasmic reticulum serves to store and release Ca+, which serve as the trigger for muscle contractions.