Muscle histology
Striated
muscle
tissue
x 400
Notice the fine ‘stripes’ or striations along each
muscle cell.
same view – different stain
Higher magnification showing dark
and light bands on 5 cells/fibres.
Skeletal muscle cells (fibres) are long, cylindrical and multinucleate
cytoplasm
(=sarcoplasm)
peripheral
multiple nuclei
cell membrane
(=sarcolemma)
myofibrils
MYOFIBRIL
cross-striations
(alternating light
and dark bands)
Characteristic
banding pattern
due to the
arrangement of
the myofilaments
T.E.M. showing ‘light’ ‘I’ bands made of Actin
And broader ‘dark’ ‘A’ bands made of Myosin
‘I’ Band
‘A’ Band
MYOSIN
ACTIN
myofilaments inside
the myofibril do not
extend the entire length
of the MYOFIBRILS but
are arranged in repeating
units called SARCOMERES
Z LINE
SARCOMERE
Thin actin
filament
Thick myosin
filament with
cross bridges
SARCOMERE
A single
myofibril
Z LINE
Myofilaments
Obvious
Z-Line
‘dark’
A Band
Is this
muscle
relaxed or
contracted?
‘light’
I Band
mitochondria
Actin and myosin are protein molecules that form the myofilaments
Each thin filament is a strand
of actin molecules twisted into
a helix
THIN ACTIN FILAMENT
Each actin molecule has a
myosin-binding site where myosin
heads can attach
Each thick filament consists of
about 200 molecules of the protein
myosin
THICK MYOSIN FILAMENT
Each myosin molecule is a long
fibrous protein with a ‘head’ at one end.
Myosin ‘heads’ attach to the
actin molecules and form cross-bridges
Myosin
molecule
Myosin
head
Under a light microscope the striated nature of skeletal muscle can be observed
This is seen as a regular alternation of light and dark bands
This banding pattern is due to the arrangement of the thick and thin filaments
within the myofibrils
Dark bands (A bands) appear where thick myosin filaments are located
Light bands
(I bands) appear
where there are
thin filaments
only
I BAND
A BAND
I BAND
z
H
z
Across the middle
of each I-band is
a dark line called
the Z line: The section
of myofibril between
these Z lines is the
SARCOMERE
ZONE
SARCOMERE
The edges of the A
bands are very dark
as thick and thin
filaments are present
together
The centre of the
A band contains
thick filaments only
and is slightly
lighter (H Zone)
This electron micrograph of a longitudinal
section of skeletal muscle shows the
myofibrils and Z-lines of the sarcomeres
(magnification X75 000)
Z line
I BAND
A BAND
Z line
H ZONE
No overlap
Sarcoplasm contained
within the muscle
cell
Myofibrils extend
along the entire
length of the muscle cell
A network of smooth
endoplasmic reticulum
(sarcoplasmic reticulum)
surrounds each myofibril
Cisternae or sacs
of the sarcoplasmic
reticulum where calcium
ions are stored
The sarcolemma
is the outer
limiting membrane
of the muscle cell
Transverse T-tubules
extend inwards
across the muscle
cell from the
sarcolemma
Mitochondria extend
in rows throughout
the sarcoplasm
providing the energy
for muscle contraction
Following the arrival
of a nerve impulse
at the neuromuscular
junction, a wave of
depolarisation spreads
throughout the
sarcolemma
and inwards via the
transverse tubules
Calcium ion
concentrations
in the vicinity
of the thin filaments
increase
The inward spread
of the impulse
depolarises the
sarcoplasmic
reticulum
The inward spread
of depolarisation
triggers the release
of calcium ions
from the sarcoplasmic
reticulum
Z-Line
H Band
A Band
I Band
Sarcolemma
Motor end plate
The Sliding Filament Theory
Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to move
away from the myosin binding sites
Calcium
Ion
TROPONIN
Binding
sites
Myosin
head
Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to move
away from the myosin binding sites
The myosin binding sites on the actin molecules are EXPOSED
Myosin heads make contact with the actin filaments
Binding
sites
Myosin
head
Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to move
away from the myosin binding sites
The myosin binding sites on the actin molecules are EXPOSED
Myosin heads make contact with the actin filaments
Binding
sites
Myosin
head
Z
Z
THE SLIDING FILAMENT MECHANISM
IS TRIGGERED INTO ACTION AND MUSCLE
CONTRACTION TAKES PLACE
Notice that it is the H-Band which shortens
Z
Z
Sarcomere Shortens
THE SLIDING FILAMENT MECHANISM
IS TRIGGERED INTO ACTION AND MUSCLE
CONTRACTION TAKES PLACE
Both the below are very good simulations of the slilding filament muscle
contraction
http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
http://msjensen.education.umn.edu/1135/Links/Animations/Flash/0008swf_sarcomere_shor.swf
Good animation of pairs of muscle contracting:
http://biology.clc.uc.edu/courses/bio105/muscles.htm
An excellent animation on structure and function of muscle:
http://entochem.tamu.edu/MuscleStrucContractswf/index.html
MAGNIFY
Muscle consists of bundles
of very long cells called
muscle fibres
Each muscle fibre
contains numerous
myofibrils
MAGNIFY
MAGNIFY
MYOFIBRIL
Z
Z
SARCOMERE
WHOLE
MUSCLE
Muscle fibres are very long,
cylindrical cells containing many
nuclei (multinucleate). The nuclei
are located at the periphery of the
cell just underneath the cell
membrane
Muscle cells show cross-striations
composed of alternating light and
dark bands
Each myofibril is composed of a
regular arrangement of myofilaments
known as the thick and thin filaments
THIN ACTIN FILAMENT
THICK MYOSIN FILAMENT