Muscles: Look & Function
Structure of Striated Muscles
• Muscles are made of
cells which are
elongated and called
muscle fibers
• Muscles include:
muscle fibers,
connective tissues,
blood vessels, and
nerves
• Muscle cells have multiple nuclei just inside the
plasma membrane (which is called the
sarcolemma)
• The sarcolemma has multiple extensions that
penetrate the interior of the cell (called T
tubules)
• The cytoplasm of a muscle cell called
sarcoplasm; it contains:
– lots of glycosomes which store glycogen
– proteins called myoglobin
• Sarcoplasmic reticulum (internal membrane
system like smooth ER) surrounds myofibrils
and function to store and release Calcium ions
(Ca2+) into the sarcoplasm to trigger a muscle
contraction
• Myofibrils are rod-shaped bodies that run the
length of the muscle cell; myofibrils are packed
closely to one another and many mitochondria
are between each one
• Myofibrils contain individual units called
sarcomeres which allow the movement due to
myofilaments, myosin and actin
View animation:
YouTube
Muscle Contraction
By VenitaVance
• Each sarcomere within a
myofibril contains the
following from one side
to the other:
–
–
–
–
–
–
–
Z line
light section
dark section
intermediate section
dark section
light section
Z line
I band
Z line
H zone
M line
A band
I band
Z line
• Z lines – the ends of the sarcomere
• A bands – extend the entire length of the
myosin filaments (includes actin and
myosin)
• H band – in the middle of the A band;
contains only myosin, no actin
• M line – supporting protein in the middle of
the thick myosin filaments
• I band – only thin actin filaments, no
myosin
Actin vs. Myosin Filaments in Sarcomere
Actin
Myosin
Thin filaments (8 nm
diameter)
Contains myosin binding
sites
Individual molecules
form helical structures
Thick filaments (16 nm
diameter)
Contains myosin heads that
have actin binding sites
Individual molecules form a
common tube-like region with
outward protruding heads
Heads are referred to as
cross-bridges and contain ATP
binding sites and ATPase
enzymes
Includes two regulatory
proteins, tropomyosin
and troponin
View Animation:
YouTube
How a Muscle Contraction is Signaled
By Dagger Biology
How Muscles Contract:
Sliding Filament Theory
Basic idea states that myosin filaments contain
“hooks” that attach to the actin filaments and
this causes them to slide over each other.
Myosin then repeats the process further down
the actin to move actin closer again.
Because of this:
A band is always the same size whether muscle
is contracted or relaxed
How Muscles Contract:
1. Motor neuron carries action potential to
muscle and neurotransmitter acetylcholine is
released into synaptic gap
2. Acetylcholine binds to protein receptors which
allow Na+ to move into muscle cell
3. Muscle action potential moves along
membrane through T tubules
4. T tubules release Ca2+ ions into sarcoplasm
5. Ca2+ ions bind to troponin (on actin filaments)
which exposes myosin binding sites
6. Myosin heads (which have ATPase) splits ATP
(made by nearby mitochondria) and release
energy (this causes head to change position)
7. Myosin heads bind to the myosin binding sites
on actin after the movement of tropomyosin
8. The myosin-actin cross bridges rotate towards
the center of the sarcomere (this pulls the actin
in)
9. ATP rebinds to the myosin head resulting in
the detachment of myosin from actin
• If no more action potentials, Ca2+ levels drop and
muscle relaxes
• If more action potentials, the process repeats
Homework:
Clearly draw and label
a sarcomere in the
following stages:
a) Fully relaxed
b) Partially contracted
c) Fully contracted