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 peripheral
(=sarcoplasm) multiple nuclei cell membrane
(=sarcolemma)
myofibrils
cross-striations
(alternating light
and dark bands)
Characteristic
banding pattern
due to the
arrangement of
the myofilaments
MYOFIBRIL
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 A single
the myofibril do not SARCOMERE myofibril
extend the entire length
of the MYOFIBRILS but
are arranged in repeating
Thin actin Thick myosin
units called SARCOMERES filament with
Z LINE filament Z LINE
cross bridges
SARCOMERE
Myofilaments
Obvious
Z-Line
‘dark’
A Band
Is this ‘light’
muscle
relaxed or I Band
contracted?
mitochondria
Actin and myosin are protein molecules that form the myofilaments
Each thin filament is a strand THIN ACTIN FILAMENT
of actin molecules twisted into
a helix Each actin molecule has a
myosin-binding site where myosin
heads can attach
Each thick filament consists of THICK MYOSIN FILAMENT
about 200 molecules of the protein
myosin
Each myosin molecule is a long
fibrous protein with a ‘head’ at one end. Myosin
molecule
Myosin
Myosin ‘heads’ attach to the head
actin molecules and form cross-bridges
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 The edges of the A
bands are very dark
(I bands) appear I BAND A BAND I BAND as thick and thin
where there are filaments are present
thin filaments together
only
H
Across the middle
of each I-band is z ZONE
z The centre of the
A band contains
a dark line called
the Z line: The section thick filaments only
SARCOMERE and is slightly
of myofibril between
these Z lines is the lighter (H Zone)
SARCOMERE
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 The sarcolemma
within the muscle is the outer
cell limiting membrane
of the muscle cell
Myofibrils extend
along the entire Transverse T-tubules
length of the muscle cell extend inwards
across the muscle
A network of smooth
cell from the
endoplasmic reticulum
sarcolemma
(sarcoplasmic reticulum)
surrounds each myofibril
Cisternae or sacs Mitochondria extend
of the sarcoplasmic in rows throughout
reticulum where calcium the sarcoplasm
ions are stored providing the energy
for muscle contraction
Following the arrival The inward spread
of a nerve impulse of the impulse
depolarises the
at the neuromuscular sarcoplasmic
junction, a wave of reticulum
depolarisation spreads
throughout the
sarcolemma
and inwards via the The inward spread
transverse tubules of depolarisation
triggers the release
of calcium ions
from the sarcoplasmic
Calcium ion
concentrations reticulum
in the vicinity
of the thin filaments
increase
Z-Line
H Band Sarcolemma
A Band
I Band
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/0008-
swf_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
WHOLE
MUSCLE
MAGNIFY
Muscle consists of bundles MAGNIFY
of very long cells called Muscle fibres are very long,
muscle fibres cylindrical cells containing many
Each muscle fibre nuclei (multinucleate). The nuclei
contains numerous are located at the periphery of the
myofibrils cell just underneath the cell
membrane
MAGNIFY Muscle cells show cross-striations
composed of alternating light and
MYOFIBRIL dark bands
Each myofibril is composed of a
Z Z regular arrangement of myofilaments
SARCOMERE known as the thick and thin filaments
THIN ACTIN FILAMENT
THICK MYOSIN FILAMENT