- Skeletal muscle fibers are multinucleated cells containing myofibrils, which are packed with thick and thin filaments that create striations when overlapped. - The neuromuscular junction is the connection between a motor neuron and muscle fiber, located at the motor end plate on the muscle fiber membrane. - A motor unit includes a motor neuron and all the muscle fibers it innervates; when activated, the entire motor unit contracts in an all-or-none response. Fine motor control is achieved by recruiting smaller motor units. - Muscle contraction occurs via the sliding filament theory, in which myosin cross-bridges attach to and pull on actin filaments, short

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Neuromuscular
Junction &
Muscle
Contraction

2.  Muscle fibers are muscle cells
Ensheathed by thin connective tissue layer called
endomysium
 Plasma membrane is called sarcolemma
 Muscle fibers are similar to other cells except are multinucleate
and striated
Skeletal Muscle Structure continued
12-6

3. Most distinctive feature of skeletal muscle is its
striations
Skeletal Muscle Structure continued
12-7

4. Neuromuscular Junction
12-8

5. Neuromuscular Junction (NMJ)
 Includes the single synaptic ending of the motor neuron
innervating each muscle fiber and underlying specializations of
sarcolemma
12-9

6. Place on sarcolemma where NMJ occurs is the motor
end plate
Neuromuscular Junction (NMJ) continued
12-10

7. Motor Unit
12-11

8.  Each motor
neuron branches
to innervate a
variable # of
muscle fibers
 A motor unit
includes each
motor neuron
and all fibers it
innervates
Motor Unit
12-12

9. When a motor neuron is activated, all muscle fibers in
its motor unit contract
Number of muscle fibers in motor unit varies according
to degree of fine control capability of the muscle
Innervation ratio is # motor neurons : muscle fibers
Vary from 1:100 to 1:2000
Fine control occurs when motor units are small, i.e.
1 motor neuron innervates small # of fibers
Motor Unit continued
12-13

10. Since individual motor units fire "all-or-none," how do
skeletal muscles perform smooth movements?
Recruitment is used:
Brain estimates number of motor units required
and stimulates them to contract
It keeps recruiting more units until desired
movement is accomplished in smooth fashion
More and larger motor units are activated to
produce greater strength
Motor Unit continued
12-14

11. Mechanisms of Contraction
12-15

12. Structure of Muscle Fiber
 Each fiber is packed with myofibrils
Myofibrils are 1µ in diameter and extend length of fiber
Packed with myofilaments
 Myofilaments are composed of thick and thin filaments
that give rise to bands which underlie striations
12-16

13.  A band is dark, contains
thick filaments (mostly
myosin)
Light area at center of
A band is H band
= area where actin
and myosin don’t
overlap
 I band is light, contains
thin filaments (mostly
actin)
At center of I band
is Z line/disc where
actins attach
Structure of Myofibril
12-17

14. 12-18

15.  Are contractile units of skeletal muscle consisting of
components between 2 Z discs
 M lines are structural proteins that anchor myosin during
contraction
 Titin is elastic protein attaching myosin to Z disc that contributes
to elastic recoil of muscle
Sarcomeres
12-19

16. How Fiber Contracts
12-20

17. Sliding Filament Theory of Contraction
Muscle contracts because myofibrils get shorter
Occurs because thin filaments slide over and
between thick filaments towards center
Shortening distance from Z disc to Z disc
12-21

18. Sliding Filament Theory of Contraction continued
 During contraction:
 A bands (containing actin)
move closer together, do
not shorten
 I bands shorten because
they define distance
between A bands of
successive sarcomeres
 H bands (containing
myosin) shorten
12-22

19. 12-23

20. Cross Bridges
 Are formed by heads of myosin molecules that extend toward
and interact with actin
 Sliding of filaments is produced by actions of cross bridges
Each myosin head contains an ATP-binding site which
functions as an ATPase
12-24

21. Cross Bridges continued
 Myosin can’t bind to actin unless it is “cocked” by ATP
After binding, myosin undergoes conformational change
(power stroke) which exerts force on actin
After power stroke myosin detaches
12-25

22. 12-26

24. Control of Contraction
 Control of cross bridge attachment to actin is via troponin-
tropomyosin system
Serves as a switch for muscle contraction and relaxation
The filament tropomyosin lies in grove between double row
of G-actins (that make up actin thin filament)
Troponin complex is attached to tropomyosin at intervals of
every 7 actins
12-27

25. Control of Contraction continued
 In relaxed muscle,
tropomyosin blocks
binding sites on actin so
crossbridges can’t occur
This occurs when Ca++
levels are low
 Contraction can occur
only when binding sites
are exposed
12-28

26. Role of Ca++
in Muscle Contraction
When Ca++
levels rise, Ca++
binds to troponin causing
conformational change which moves tropomyosin and
exposes binding sites
Allowing crossbridges and contraction to occur
Crossbridge cycles stop when Ca++
levels decrease
12-29

27. Role of Ca++
in Muscle Contraction
 Ca++
levels decrease
because it is
continually pumped
back into the
sarcoplasmic
reticulum (SR - a
calcium reservoir in
muscle)
 Most Ca++
in SR is in
terminal cisternae
 Running along
terminal cisternae are
T tubules
12-30

28. Excitation-Contraction Coupling
 Skeletal muscle
sarcolemma is
excitable
Conducts Action
Potentials
 Release of ACh at
NMJ causes large
depolarizing end-plate
potentials and APs in
muscle
 APs race over
sarcolemma and
down into muscle via
T tubules
12-31

30. Excitation-Contraction Coupling continued
 T tubules are extensions
of sarcolemma
 Ca++
channels in SR are
mechanically linked to
channels in T tubules
 APs in T tubules cause
release of Ca++
from
cisternae via V-gated
and Ca++
release
channels
Called
electromechanical
release
channels are 10X
larger than V-gated
channels 12-32

31. Excitation-Contraction Coupling continued
12-33

32. Cardiac Muscle (Myocardium)
 Contractile apparatus similar to skeletal
 Striated like skeletal but involuntary like smooth
 Branched; adjacent myocardial cells joined by intercalated disks
(gap junctions)
Allow Action Potentials to spread throughout cardiac muscle
12-77