One motor nerve innervates many muscle fibers. Acetylcholine plays a key role in transmitting excitation from the nerve to the muscle fiber. For contraction to occur, calcium must be released from the sarcoplasmic reticulum. The proportion of type I and type II muscle fibers determines the contractile properties of the muscle, and this proportion can change based on activity levels and other factors.

1. Skeletal Muscle II
SR2002
October 24, 2013
Dr. Arimantas Lionikas

2. Muscle II
Plan
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•
•
•
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Muscle Innervation
Neuromuscular transmission
Contractile properties
Electrical stimulation
Muscle fibre types
• Reading list:
1. Enoka R. Neuromechanics of human movement. 2002. Publishers:
Human Kinetics, p. 257-278
2. MacIntosh, B.R., Gardiner, P.F. McComas, A.J. Skeletal muscle, 2nd
edition. 2006. Publishers: Human Kinetics, p. 32-39, 126-150.
3. McArdle W.D. et al. Exercise Physiology: energy, nutrition, human
performance. 2001. Publisher: Lippincott Williams & Wilkins, p. p. 358-382.

3. Muscle Innervation
αMotor neuron

4. Muscle Innervation (cont)
• Microscopic analysis: Three
muscle fibres innervated by
branches of a motor nerve
• Diphtheria toxin damages
myelin sheaths of nerve fibres
(shown by arrows)
• One motor nerve
innervates a group
of muscle fibres
• Motor nerve divides
into terminals that
form
neuromuscular
junctions on muscle
fibres
• Neuromuscular
junctions are located
in the middle of the
fibres

5. Neuromuscular transmission
• Nerve fibres (axons)
conduct action potentials
(APs) at a fast rate (~4080 m/s)
• Terminals of motor nerves
form neuromuscular
junctions (also called
motor end plates)
• The key function of the
neuromuscular
junctions is transmission
of activation
from nerve to muscle fibres

6. Neuromuscular transmission
• Arrival of the AP at the nerve
terminal triggers release of
acetylcholine (Ach) into the
synaptic cleft (space between nerve
and muscle fibre)
• Ach is made from Ac-CoA from
mitochondria and choline in nerve
terminals
• Released Ach binds to receptors on
the muscle fibre and triggers influx of
Na+ followed by generation of action
potential (AP) in muscle fibres
• Ach acts for a short time since it is
degraded by Ach esterase enzyme
• Choline is taken up by nerve
terminals for re-synthesis of Ach

7. Synaptic vesicles of axon terminals
Synaptic vesicles
The active zone
The process leading to the release of Ach involves
docking of the synaptic vesicles to the active zone ,
fusion with the membrane and release of the
mediator.
Szule JA, Harlow ML, Jung JH, De-Miguel FF, et al. (2012)

8. Neuromuscular transmission
Clinical relevance
Before treatment
After treatment
• Myasthenia gravis due to low
levels of acetylcholine (Ach):
• Eyelids are dropped due to
muscle weakness
• Injection of a drug which blocks
Ach esterase helps to regain
muscle strength: eyelids open
• Myasthenia gravis
(autoimmune disease) can
be due to low levels of:
• (1) acetylcholine (Ach)
• (2) Ach receptor
• Drugs blocking
neuromuscular
transmission are muscle
relaxants used during
operations (ex.:
suxamethonium)
• Tubocurorine is found in
some plants. Indians used
it in hunting (poisoned
arrows)

9. Action potential (AP)
• After depolarization of postsynaptic
membranes, action potential (AP) is generated
• AP spreads to both ends of muscle fibres
Action potential (AP)

10. Contractile properties
Human muscles
Electrodes
Force
Dynamometer
strap
• Contractile properties
can be investigated in
humans muscles
• Electrical stimulation is
applied over surface
electrodes
• Torque in isometric
contraction is usually
recorded
• The set up is for
studies of the
quadriceps muscle

11. Muscle response to AP
Single twitch contractions
ES

12. Twitch and Tetanus

13. Contractile properties
Human muscles
• Muscle force depends on
the frequency of
stimulation
• Unfused contractions
are associated with
fluctuation in force during
a contraction
• Muscle fibres are
activated at 5-100 Hz
frequencies in voluntary
contractions
• Electrical stimulation
can be used for training of
muscles
Westerblad et al. 2002

14. Role of Calcium: Tetanic contractions
Isolated muscle fibre
• Continuous stimulation
at 100 Hz causes a
sustained increase in
intracellular Ca2+ - [Ca2+]I
and generates tetanic
contractions (smooth
contractions)
• [Ca2+]I matches force in
repeated contractions
• Muscle fibres were repeatedly
stimulated at 100 Hz for 0.5 s each
time
Westerblad et al. 2002

15. Applications of Electrical Stimulation
• Neuromuscular electrical stimulation (NMES) is used for
functional and therapeutic applications in subjects with spinal
cord injury or stroke.
A transcutaneous multichannel neuroprosthesis system allows persons with paraplegia unbraced ambulation for home and
short community distances. (Parastep I System User, courtesy of Sigmedic Inc., Fairborn, OH.)
SHEFFLER & CHAE 2007

16. Muscle fibre types
• Skeletal muscle is not a
homogeneous tissue.
• Muscle fibres that muscle
consists of can be
subdivided in two main
types: type I and type II
(the latter is further
divided into IIA & IIX).
• Type of myosin heavy
chains expressed in the
fibres is the primary
determinant of the fibre
type and their contractile
properties.
Eriksson et al 2005
50 µm
I
IIA
Mouse soleus. Acid preincubation pH 4.47

17. Properties of fibre types
Characteristic
Small*
Intermediate
Large
High
Intermediate
Low
High
Intermediate
Low
Myosin ATPase
Low
High
High
Low
High
High
Oxidative capacity
High
High/Medium
Low
Contraction
Slow
Fast
Fast
Relaxation
Slow
Fast
Fast
Fatigue resistance
Adaptation#
Fibre ∅
Glycolytic capacity
Contractility
Type IIX(B)
Mitochondrial vol
Histo- & Biochemistry
Type IIA
Capillaries/mm2
Morphology
Type I
High
Moderate/High
Low
Increase in fibre area
Low
Moderate
Moderate
* Not always; depends on the muscle. Size between type I and II is about the same in soleus
#
Adaptation to several weeks of strength training.

18. Fibre type distribution in various muscles
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•
81 ms
•
Proportion of fibre types differ among muscles.
Type I fibres dominate in postural muscles (e.g.
soleus) and muscles involved in fine
movements (e.g. adductor pollicis).
Type II fibres dominate muscles involved in
movements requiring quick and / or powerful
contractions (e.g. orbicularis oculi, biceps
brachii).
Muscle
% type I
120 ms
Orbicularis oculi
Tricep brachii
37
Quadriceps femoris
68 ms
15
52
Adductor pollicis
80
Soleus
80
Harridge, Bottinelli et al. 1996

19. Variation in fibre types among individuals
• Even though on average vastus lateralis muscle
consists of ~50 of type I fibres, there is a broad range of
variation between individuals (from ~20% to >80% of
type I fibres).
• Important consequences of variation of fibre type
composition:
– Prevalence of certain fibre type is favourable for some athletic
activities (e.g. type I – endurance events, type II – speed and
power events).
– Apparently increasing proportion of type II fibres is associated
with prevalence to weight gain and cardiovascular risk.

20. Factors affecting proportion of fibre types
• Genetic factors play important role in proportion of fibre types.
• Moderate changes may occur as adaptation to physical activity or
lack of it.
• Drastic change in degree of activity leads to marked shift in
proportion of fibre types.
Proportion of Fibre Types
Type II
Type I
Endurance Training
Immobilization
Spinal Cord Injury

21. Muscle II
Summary
• One motor nerve innervates many
muscle fibres
• Acetylcholine plays a key role in
transmission of excitation from nerve to
muscle
• For muscle contraction to occur
release of calcium from sarcoplasmic
reticulum is required
• Proportion of type I and type II fibres
are important determinants of
contractile properties.