1. Muscle Physiology

2. Striated muscle tissue x 400
Muscle histology
Striated muscle tissue x 400
Notice the fine ‘stripes’ or striations along each muscle cell.

3. same view – different stain

4. Higher magnification showing dark and light bands on 5 cells/fibres.

5. The Muscle Cell
Skeletal muscle cells (fibres) are long, cylindrical and multinucleate
cytoplasm
(=sarcoplasm)
peripheral
multiple nuclei
cell membrane
(=sarcolemma)
myofibrils
cross-striations
(alternating light
and dark bands)
Characteristic
banding pattern
due to the
arrangement of
the myofilaments
MYOFIBRIL

6. T.E.M. showing ‘light’ ‘I’ bands made of Actin
And broader ‘dark’ ‘A’ bands made of Myosin
‘I’ Band
‘A’ Band
MYOSIN
ACTIN

7. The Muscle Cell Z LINE A single myofibril Myofilaments
myofilaments inside
the myofibril do not
extend the entire length
of the MYOFIBRILS but
are arranged in repeating
units called SARCOMERES
SARCOMERE
Thin actin
filament
Thick myosin
filament with
cross bridges
Z LINE
Myofilaments
SARCOMERE

8. Obvious
Z-Line
‘dark’
A Band
Is this muscle relaxed or contracted?
‘light’
I Band
mitochondria

9. The Contractile Proteins Myosin and Actin
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
molecule
Myosin
head
Myosin ‘heads’ attach to the
actin molecules and form cross-bridges

10. The Banding Pattern of Skeletal Muscle
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
The edges of the A
bands are very dark
as thick and thin
filaments are present
together
A BAND
I BAND z H
ZONE
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
The centre of the
A band contains
thick filaments only
and is slightly
lighter (H Zone)
SARCOMERE

11. I BAND A BAND H ZONE No overlap Z line
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
H ZONE
No overlap

13. A Detailed View Of A Skeletal Muscle Fibre
Sarcoplasm contained
within the muscle
cell
The sarcolemma
is the outer
limiting membrane
of the muscle cell
Myofibrils extend
along the entire
length of the muscle cell
Transverse T-tubules
extend inwards
across the muscle
cell from the
sarcolemma
A network of smooth
endoplasmic reticulum
(sarcoplasmic reticulum)
surrounds each myofibril
Mitochondria extend
in rows throughout
the sarcoplasm
providing the energy
for muscle contraction
Cisternae or sacs
of the sarcoplasmic
reticulum where calcium
ions are stored

16. depolarisation spreads
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
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
Calcium ion
concentrations
in the vicinity
of the thin filaments
increase

18. Z-Line
H Band
Sarcolemma
A Band
I Band

19. Motor end plate

21. The Sliding Filament Theory

22. 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

23. 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

24. 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

26. THE SLIDING FILAMENT MECHANISM IS TRIGGERED INTO ACTION AND MUSCLE Z
THE SLIDING FILAMENT MECHANISM
IS TRIGGERED INTO ACTION AND MUSCLE
CONTRACTION TAKES PLACE

27. THE SLIDING FILAMENT MECHANISM IS TRIGGERED INTO ACTION AND MUSCLE
Notice that it is the H-Band which shortens Z
Sarcomere Shortens
THE SLIDING FILAMENT MECHANISM
IS TRIGGERED INTO ACTION AND MUSCLE
CONTRACTION TAKES PLACE

28. Both the below are very good simulations of the slilding filament muscle contraction
Good animation of pairs of muscle contracting:
An excellent animation on structure and function of muscle:

29. Summary: Structure Of Skeletal Muscle
WHOLE
MUSCLE
Muscle consists of bundles
of very long cells called
muscle fibres
MAGNIFY
MAGNIFY
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
Each muscle fibre
contains numerous
myofibrils
Muscle cells show cross-striations
composed of alternating light and
dark bands
MAGNIFY
MYOFIBRIL
Each myofibril is composed of a
regular arrangement of myofilaments
known as the thick and thin filaments Z
SARCOMERE
THIN ACTIN FILAMENT
THICK MYOSIN FILAMENT