The action of muscles in the
body
Learning outcomes – A grade questions
(n) describe, with the aid of diagrams,
photographs and electron micrographs, the
histology and ultrastructure of skeletal
muscle;
(o) describe the sliding filament theory of
muscle contraction, to include the
importance of the power stroke and the role
of ATP and calcium ions.
Key words/phrases
Skeletal muscle
Sliding filament theory
Power stroke
Calcium ions
Ultrastructure
Success criteria
Success
criteria
Success
criteria
Success
criteria
• Make flash cards of key words
• Label diagrams of key words
• Label electron micrographs, identifying key structures
• Complete past paper questions to assess and test
knowledge.
• Identify the proteins found in a sarcomere using
diagrams and electron micrographs
Types of muscles in the body
• Three types of muscle
– Cardiac
– Skeletal
– Smooth
When talking about
muscle,
say exactly which type
you mean!
• All three types of muscle contain the proteins actin and myosin
forming myofilaments that allow contraction to occur.
Skeletal Muscle
• Stimuli from nerve impulses
causes skeletal muscle to
contract.
• Contraction uses ATP to
produce movement and also
releases heat to warm the
body.
• Specialised muscle fibres that
make up the structure of
muscle enable it to carry out
these important functions.
Skeletal Muscle
• Muscles fibres are surrounded
by a membrane called the
sarcolemma.
• The sarcolemma encases a
specialised cytoplasm called the
sarcoplasm.
• This membrane has special
finger-like projections called Ttubules that link up to the
sarcoplasmic reticulum.
• This creates a complete cellular
network in the muscle.
Skeletal Muscle
• The cytoplasm contains many mitochondria, plus stores of myoglobin
and glycogen.
• The cytoplasm also contains the proteins (filaments) actin and mysoin,
arranged in bundles of myofibrils.
• The sarcoplasmic reticulum contains calcium ions that are essential
for muscle contraction.
Learning outcomes – A grade questions
(n) describe, with the aid of diagrams,
photographs and electron micrographs, the
histology and ultrastructure of skeletal
muscle;
Task 1
(o) describe the sliding filament theory of
muscle contraction, to include the importance
of the power stroke and the role of ATP and
calcium ions.
• Make flash cards of the following
words with definitions on the
back.
–
–
–
–
–
–
–
Actin
Myosin
Sarcolemma
Sarcoplasm
T-tubules
Sarcoplasmic reticulum
Myofibrils
End
12 minutes
Mini Plenary
Actin and myosin form myofilaments
True
False
Mini Plenary
Sodium ions are essential for muscle
contraction
True
False
Mini Plenary
The sarcoplasm is a specialised membrane
that surrounds muscle fibres
True
False
Mini Plenary
Muscle membrane has special finger-like
projections called T-tubules that link up to
the sarcoplasmic reticulum.
True
False
The structure of a sarcomere
• Sarcomeres are made up of myofilaments, which are
bundles of myofibrils.
• The two types of myofilament are actin and myosin.
• Actin in the thin band, whereas myosin in thicker.
• Actin is actually made up of three different proteins
bound together:
– Actin
– Tropomyosin
– troponin
The structure of a sarcomere
• Myosin filament is made of bundles of
polypeptide molecules with a globular
head for binding to ATP and to the actin
filament.
Learning outcomes – A grade questions
(n) describe, with the aid of diagrams,
photographs and electron micrographs, the
histology and ultrastructure of skeletal
muscle;
(o) describe the sliding filament theory of
muscle contraction, to include the importance
of the power stroke and the role of ATP and
calcium ions.
Task 2
• Make flash cards of the following words
with definitions on the back.
–
–
–
–
Tropomyosin
Troponin
Sarcomeres
Myofilaments
• Test each other using all your
flashcards to ensure that you have a
good understanding of what these
words mean
End
15 minutes
Sliding filament theory
Sliding filament theory
• Contraction of
skeletal muscle is
initiated by
neurotransmitters
(acetylcholine) which
are released from
motor neurones.
• A series of strictly
controlled events
happens as a result…
Sliding filament theory
• Nerve impulses stimulate the neuromuscular junction (NMJ)– causes
the release of acetylcholine (Ach), which is an excitatory
neurotransmitter.
• ACh travels across the synapse of the NMJ and binds to receptor
sites on the motor end plate.
• Causes impulses to spread quickly through the nerve fibre to the T
tubule system and then to the sarcoplasmic reticulum.
Sliding filament theory
• Ca2+ ions bind to troponin which
causes tropomyosin to detach
from actin.
• This results in actin being
exposed so myosin heads can
now bind to it because binding
sites have been exposed.
• ADP is normally attached to
mysoin heads. The enzyme
mysoin kinase removes the ADP
so that myosin can attach to the
binding sites on actin.
http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
Sliding filament theory
• In the “power stroke”, the myosin
head changes position and “pulls”
the actin
• This results in the filaments sliding
past each other.
• If ATP is available, myosin heads
bind to it and hydrolyse it to release
energy.
• This causes myosin heads to detach
from the binding sites on the actin
and return to their original position.
• This cycle will repeat as long as
there is a supply of calcium ions and
ATP
Sliding filament theory
• As the filaments slide past each other, the attachment
breaks and new attachments form further along the
molecule. This can happen 100 times per second.
• The contractions continue to shorten the fibre until
nervous stimulation stops and the calcium ions return to
the sarcoplasmic reticulum
• If the concentration of Ca ions falls, actin filaments
reattach to troponin and tropomyosin. The muscle fibre
then returns to its original shape
Learning outcomes – A grade questions
(n) describe, with the aid of diagrams,
photographs and electron micrographs, the
histology and ultrastructure of skeletal
muscle;
(o) describe the sliding filament theory of
muscle contraction, to include the importance
of the power stroke and the role of ATP and
calcium ions.
Task 3
• In your own words, write out a
flow diagram to describe the
stages of the sliding filament
theory.
• Do this in pairs.
• Feedback your flow chart to the
rest of the group. Try to
describe it in detail, but make
sure you understand the detail
you are giving!
End
15 minutes
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
Courtesy of Dr. H. Huddart – Department of
Biological Sciences, The University of Lancaster
Z LINE
Z LINE
SARCOMERE
x 40,000
A BAND
I BAND
I BAND
The arrangement of the myofilaments can also be seen in cross
sections of the myofibrils
Thin Filament
SARCOMERE
Thick
Filament
Longitudinal
Section
Cross
Sections
Myosin and Actin
(A band - very dark)
Myosin Only
(A band - dark)
Actin Only
I band - light
Cross sections of myofibrils show the HEXAGONAL organisation
of actin and myosin filaments
The ends of
each thick
filament are
surrounded
by six thin
filaments
3-D
SIX ACTIN
FILAMENTS
MYOSIN
ELECTRON MICROGRAPH OF A CROSS SECTION OF SKELETAL
MUSCLE SHOWING THE ARRANGEMENT OF THICK AND THIN
FILAMENTS
THICK FILAMENT
HEXAGONAL ARRAY OF THIN FILAMENTS