Skeletal muscles cause the skeleton to move at joints They are attached to skeleton by tendons. Tendons transmit muscle force to the bone. Tendons are made of collagen fibres & are very strong & stiff

1. Muscles
13.8 Muscles are effectors which
enable movement to be carried out

2. Muscle
 Is responsible for almost all the
movements in animals
 3 types
Cardiac muscle
Smoot h muscle
I nvolunt ary
cont rolled by
aut onomic
nervous syst em
Skelet al muscle
(aka st riped or
st riat ed muscle)
volunt ary
cont rolled by
somat ic nervous
system

3. Muscles & the Skeleton
 Skeletal muscles cause the skeleton to
move at joints
 They are attached to skeleton by
tendons.
 Tendons transmit muscle force to the
bone.
 Tendons are made of collagen fibres &
are very strong & stiff

4. Antagonistic Muscle Action
 Muscles are either contracted or
relaxed
 When contracted the muscle exerts a
pulling force, causing it to shorten
 Since muscles can only pull (not push),
they work in pairs called antagonistic
muscles
 The muscle that bends the joint is
called the flexor muscle
 The muscle that straightens the joint is

5. Elbow Joint
 The best known example of antagonistic
muscles are the bicep & triceps muscles
E lb o w jo in t fle x e d
F le x o r m u s c le s c o n tra c te d
E x te n s o r m u s c le s re la x e d
E lb o w jo in t e x te n d e d
E x te n s o r m u s c le s c o n tra c te d
F le x o r m u s c le s re la x e d
b ic e p s
tric e p s
S e c tio n th r o u g h a r m
F le x o r
m u s c le s
E x te n s o r
m u s c le s
H u m e ru s
B o n e

6. Muscle Structure
 A single muscle e.g.
biceps contains approx
1000 muscle fibres.
 These fibres run the
whole length of the
muscle
 Muscle fibres are joined
together at the tendons
Bicep Muscle

7. Muscle Structure
 Each muscle fibre is actually a
single muscle cell
 This cell is approx 100 m in
diameter & a few cm long
 These giant cells have many
nuclei
 Their cytoplasm is packed full
of myofibrils
 These are bundles of protein
filaments that cause
contraction
 Sarcoplasm (muscle
cytoplasm) also contains
mitochondria to provide energy
for contraction
n u c le i s trip e s m y o fib rils

8. •Sarcomere = the basic contractile unit

11. Muscle Structure
 The E.M shows that each myofibril is made up of
repeating dark & light bands
 In the middle of the dark band is the M-line
 In the middle of the light band is the Z-line
 The repeating unit from one Z-line to the next is called
the sarcomere
d a r k
b a n d s
lig h t
b a n d s
M
lin e
Z
lin e
1 s a r c o m e r e
1myofibril

12. Muscle Structure
 A very high resolution E.M reveals that each myofibril
is made up of parallel filaments.
 There are 2 kinds of filament called thick & thin
filaments.
 These 2 filaments are linked at intervals called cross
bridges, which actually stick out from the thick
filaments
T h ic k
f ila m e n t
T h in
f ila m e n t
C r o s s
b r id g e s

13. The Thick Filament (Myosin)
 Consists of the protein
called myosin.
 A myosin molecule is
shaped a bit like a golf
club, but with 2 heads.
 The heads stick out to
form the cross bridge
 Many of these myosin
molecules stick
together to form a
thick filament
o n e m y o s in
m o le c u le
m y o s in h e a d s
( c r o s s b r id g e s )
m y o s in ta ils

14. Thin Filament (Actin)
 The thin filament consists of a protein called
actin.
 The thin filament also contains tropomyosin.
 This protein is involved in the control of
muscle contraction
a c tin m o n o m e rs tro p o m y o s in

15. •Sarcomere = the basic contractile unit

16. The Sarcomere
T h ic k f ila m e n ts
( m y o s in )
T h in fila m e n t s
( a c tin )
M
lin e
Z
lin e
Z
lin e
p r o t e in s in
t h e Z lin e
ju s t
th in
fila m e n t
o v e r la p z o n e
- b o th
th ic k & th in
fila m e n ts
ju s t
th ic k
f ila m e n t
m y o s in
b a r e z o n e
- n o
c r o s s b r id g e s
p r o te in s
in th e M lin e

18. I Band = actin
filaments

20. Anatomy of a Sarcomere
 The thick filaments produce the dark A band.
 The thin filaments extend in each direction
from the Z line.
 Where they do not overlap the thick filaments,
they create the light I band.
 The H zone is that portion of the A band where
the thick and thin filaments do not overlap.
 The entire array of thick and thin filaments
between the Z lines is called a sarcomere

21. Sarcomere shortens when
muscle contracts
 Shortening of the
sarcomeres in a
myofibril produces
the shortening of
the myofibril
 And, in turn, of the
muscle fibre of
which it is a part

22. Mechanism of muscle contraction
 The above micrographs show that the
sarcomere gets shorter when the muscle
contracts
 The light (I) bands become shorter
 The dark bands (A) bands stay the same length
R e la x e d
m u s c le
C o n t r a c t e d
m u s c le
r e la x e d s a r c o m e r e
c o n t r a c te d s a r c o m e r e

23. The Sliding Filament Theory
 So, when the muscle contracts,
sarcomeres become smaller
 However the filaments do not change in
length.
 Instead they slide past each other
(overlap)
 So actin filaments slide between myosin
filaments
 and the zone of overlap is larger

24. Repetition of the cycle
 One ATP molecule is split by each cross
bridge in each cycle.
 This takes only a few milliseconds
 During a contraction 1000’s of cross bridges
in each sarcomere go through this cycle.
 However the cross bridges are all out of
synch, so there are always many cross
bridges attached at any one time to maintain
force. http://199.17.138.73/berg/ANIMTNS/SlidFila.htm