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Neuromuscular Junction
Components of neuromuscular junction
• Motor neuron
• End plate region
• Presynaptic terminal ( mitochondria and synaptic vesicles
10,000 Ach per vesicle)
• Synaptic cleft (or gap) (Cholinesterase)
• Postsynaptic membrane ( neurotransmitter’s receptors)
Mechanism of neuromuscular transmission
• Action potential is conducted from the motor neuron to the muscle chemically through
the neuromuscular junction via a substance called neurotransmitter (acetylcholine, Ach)
• Events during transmission:
1. Synthesis - in presynaptic terminals by the enzyme choline acetyltransferese
2. Storage – 10,000 to 20,000 Ach molecules per vesicle
3. Release :
- Action potential arrives at terminal and causes depolarization and increases calcium
influx concentration in the terminal.
- Ca2+ in turn causes the vesicles to fuse with presynaptic membrane to empty its
content in the cleft.
- Ach diffuses across to the postsynaptic membrane where it activates its receptor .
- membrane conductance increases to Na + , results in depolarization called the end
plate potential( EPP), if the EPP exceeds threshold, action potential is produced and
muscle contracts.
Mechanism of neuromuscular transmission
4. Reuptake – Ach action in the cleft lasts only a short time because Ach is cleaved by the
action of cholinesterase , by products are reabsorbed and taken up by the presynaptic
terminal
End plate Potentials EPPs
• They are graded potentials with the amplitude depends upon amount of Ach
Acetylcholine Opens Na+ Channel
End plate potential and excitation
of the skeletal muscle fiber
• Ach activation of its receptors leads to local membrane potential at the end plate called EPP.
• The magnitude of this EPP usually about 50 – 75 millivolts. This is more than sufficient to
depolarize the muscle cell and to initiate an action potential.
• The action potential is all or none phenomenon, the depolarization must reach the
threshold(20 – 30 millivolts) in odder for an action potential to take place.
• While the EPP is graded potential depends on the strength of the stimulus and the amount
of Ach released.
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Fatigue of the neuromuscular
junction
• The impulse that reaches the neuromuscular junction causes three times as much end
plate potential as that required to stimulate the muscle fiber, this is called the safety
factor for transmission.
• But stimulation with a frequency more that 100 times/ second for several min. often
diminishes the number of Ach vesicles so much that impulses then fail to pass into
muscle fiber, this is called fatigue of the neuromuscular junction
Ach formation and release
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Drugs that affect the transmission at the neuromuscular junction
Ach release :
1. Ca2+
2. Mg+ and Mn+
3. Botulin toxin
Bind to the receptors
1. D – tubocurare ( curare) inhibits transmission
2. Carbachole
3. Methacholine
Ach like effect
4. Nicotine
Cholinesterase inhibitors:
1. Irreversible – nerve gas ( diisopropyl flluorophosphate) and insecticides
Respiratory muscles
2. Reversible - neostigmine and physiostigmine
Myasthenia gravis: autoimmune disease where
antibodies against the Ach receptors are produced.
Which consequences do you expect?
How do you think you can ameliorate the situation?
Spread of the action potential to the interior of the muscle fiber by way of a transverse
tubule system
• Muscle fiber is large … action
potential can’t spread deep
into the muscle fiber
• T tubules penetrate deep in the
muscle from one side to the
other.
• T tubule action potential
causes release of calcium ions
in the vicinity of all the
myofibrils.
• Calcium ions cause contraction
• And this is what's called
excitation-contraction coupling
Release of calcium ions from the sarcoplasmic reticulum (SR)
T tubules depolarization is sensed by a voltage sensors called the dihydropyridine
receptors DHPR.
DHPR is in direct contact with the calcium release channel, the ryanodine receptor
located on the SR.
Calsequestrin is a protein in the SR that augments SR calcium storage ( low affinity
high capacity.
Judith A. Heiny
SERCA Ca-ATPase pump ends the Ca2+ transient by pumping Ca2+
back into the SR
• To relax a muscle, Ca2+ must return back to the SR
• It is returned by the action of the smooth endoplasmic reticulum calcium ATPase (SERCA)
• SERCA links the hydrolysis of ATP with the pumping of 2 Ca2+ ions back into the lumen of SR
For your own knowledge
Myosin is a Molecular Motor
Myosin is a hexamer:
2 myosin heavy chains
4 myosin light chains
2 nm
Coiled coil of two a helices
C terminus
Myosin head: retains all of the motor functions of myosin,
i.e. the ability to produce movement and force.
Nucleotide
binding site
Myosin S1 fragment
crystal structure
NH2-terminal catalytic
(motor) domain
neck region/lever arm
Ruegg et al., (2002)
News Physiol Sci 17:213-218.
Characteristics of muscle contraction
• Single action potential (stimulation) causes single muscle contraction (Twitch)
• Twitch three phases : Latent, contraction and relaxation
+ Stimulator
Nerve
• Don’t confuse the action potential with the muscle twitch
The muscle twitch lasts much longer than the action potential
(The trigger for muscle contraction)
• A very short stimulus causes a single muscle contraction (twitch). Force rises then falls,
the falling time is longer than the rise time
Figure 12.16
Muscle force depends on the number of motor units that are activated
•
Gradual increase in stimulus strength produces stronger twitch , as progressively
increasing stimulus activates more motor neurons , which activates more motor units
which leads to more force.
(recruitment)
•
Motor unit is the motor neuron and all of its innervated muscle fibers
• The size principle : Motor units are recruited in order of their size, What do you think the
rational behind this phenomenon?
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Recall The Motor Unit:
motor neuron and the muscle fibers it innervates
Spinal
cord
• The smallest amount of
muscle that can be activated
voluntarily.
• Gradation of force in skeletal
muscle is coordinated largely
by the nervous system.
• Recruitment of motor units
is the most important means
of controlling muscle tension.
• Since all fibers in the motor
To increase force:
1. Recruit more M.U.s
2. Increase freq.
(force –frequency)
unit contract simultaneously,
pressures for gene expression
(e.g. frequency of stimulation,
load) are identical in all fibers
of a motor unit.
Muscle force can be increased by increasing the frequency of motor
neuron firing
• The action potential is much shorter than the muscle twitch
• Thus, the nerve can stimulate the muscle before the muscle has relaxed or even before
it reaches its peak tension
• The frequency must exceed 1/twitch time (period) in order for summation to take place
• At high frequency the force shows no waviness, this is called Tetanus
Molecular rational behind
frequency summation and
tantalization
• Single action potential causes single Ca2+
transient.
• Sequential SR release leads to summation of
myoplasmic calcium concentration.
• Force development depends on intracellular
Ca2+ concentration , so repetitive stimulation
causes repetitive Ca2+ transients and hence
more force.
Effect of consecutive stimuli: Treppe
• Treppe: gradual increase in
contraction intensity during
sequential stimulation
• Might be due to calcium ions
accumulating in the cytoplasm
with each stimulation
Figure 12.15
Isometric/isotonic contractions
• Isometric: muscle
contraction without
movement  no
muscle shortening
• Isotonic: muscle
contraction with
movement  muscle
shortens
Three Potential Actions During Muscle Contraction:
• shortening
Biceps muscle shortens
during contraction
(Isotonic: shortening against fixed
load, speed dependent on M·ATPase
activity and load)
• isometric
• lengthening
Biceps muscle lengthens
during contraction
Most likely to cause
muscle injury
Muscle force depends on the length of the muscle
•
•
•
•
Stretching a muscle produces a passive force
The active tension rises and then falls with the stretch of the muscle
Active tension = Total tension - passive tension
The relationship between active force and muscle length is the Length-tension curve
The sliding filament hypothesis predicts that force depends on
the overlap of thick and thin filaments
• At a sarcomere length of 3.65u there is no force because there is no overlap.
• At progressively shorter length the overlap increases and the force increases as well
Until at 2.2 sarcomere length, there is maximal overlap and maximal force.
This force does not decrease until the sarcomere shortens to less that 1.95.
• At shorter length the thin filaments begin to run into each other and the number of
cross bridges decrease .
• When the thick filaments butt up against the Z-disk the force falls precipitously.
Figure 12.18
The velocity of muscle contraction varies inversely with the afterload
• Concentric contraction – shortening of the muscle
• Eccentric contraction lengthening
Muscle power
• Power is the force times the velocity
• Muscle power peaks at about one-third of maximal force
Skeletal Muscle Tone
Even when muscles are at rest, a certain amount of tautness usually remains.
This is called muscle tone .
Because normal skeletal muscle fibers do not contract without an action potential to
stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve
impulses coming from the spinal cord
Fiber types and muscle energetics
• Various muscles with different Twitch time.
• They are all the same active force, they differ in their velocity of shortening
+ Stimulator
Nerve
Rate of shortening in a muscle fiber (sarcomere) depends directly on
the turnover rate of the cross-bridges
• Each cross-bridge cycle slides the thin filament about 10 nm past the thick filament
• Rapid cross-bridge cycling means that the thin filament slides the thick filament more
quickly.
• Thus, the velocity of shortening the muscle (each sarcomere), depends on the
turnover rate of the cross-bridges.
 The turnover rate of the cross-bridges depends on the ATPase activity of
myosin, which depends on the myosin isoforms
 Myosin isoforms are encoded by separate genes, in the adult there are two basic varieties:
- Slow myosin – slow fibers
- Fast myosin - fast fibers
 Myosin isoforms stain differently in histological sections.
 Myosin staining is one basis for fiber classification.
Brook classification of muscle fiber :
depending on myosin staining
• Type I ( Slow) and Type II ( Fast) fibers
Type IIb
Type IIa
Type I
Fiber types
characterized using
ATPase
histochemistry
Note: single muscle contains all
isoforms with different ratios
 Muscles can be classified based on their metabolic properties (Peter and coworker):
1. Slow oxidative (SO)
2. Fast glycolytic (FG)
3. Fast oxidative-glycolytic (FOG)
In general:
Red fibers contains
-
A lot of mitochondria
A lot of myoglobin
Have large oxidative capacity
They are slower and fatigue resistant
• Burke classified muscle fiber based of their mechanical properties into:
1. Slow (S)
2. Fast fatigue resistant (FR)
3. Fast intermediate (FI)
4. Fast fatigable (FF)
• Whole muscles in the body are mixtures of muscle fiber types
• Single muscle can be predominantly one type or another.
• The ratio of a given muscle fiber types in a specific muscle vary between individuals
• Muscle fiber types differ also in the isoforms of many different proteins for example:
Fast twitch fiber contains SERCA1a & TnC2 while slow twitch fiber contains SERCA2a &
TnC1
Muscle fiber types also differ in the relative amount oF organelles:
1. mitochondria
2. SR volume
3. SR calcium pump
4. myoglobin
ATP hydrolysis is the source of energy for mechanical work
(Cross-bridges formation)
How? Myosin ATPase
H2O
ATP  ADP + Pi + Energy
57 KJ
ATP is also needed for other reactions in muscle :
1. Calcium reuptake into SR
2. Sodium-Potassium ATPase to maintain the ionic composition of the two side of the
cell membrane
3. Other functions of the cell such as protein expression
 Note that during heavy activity, cross-bridges formation is the main drain on ATP
stores in muscle cell.
 Rate and amount of ATP consumption varies with the intensity and duration of the
exercise
Metabolism regenerates ATP in different time scales and capacities
1. Cytoplasmic ATP (5 mM) can support full contraction for about 1-2 second at most.
2. Creatine phosphate(CP) regenerates ATP fastest to its normal cytoplasmic concentration
CP + ADP = ATP + Creatine
This source of energy supports maximal muscle contraction for another 5 to 8 seconds
3. Glycolysis rapid but low capacity supply of ATP for fast twitch fibers
(Glycogen or blood) 1 glucose…….2 ATP
Metabolism regenerates ATP in different time scales and capacities
4. Oxidative phosphorylation: slower but high capacity source of ATP:
Electron transport chain
1 glucose……30 ATP
Fuel sources
1. Carbohydrates: stored as Glycogen which mobilized by glycogenolysis
rapid muscle activity utilizes Glycogenolysis, resorted during rest.
Glycolysis the source of glucose either from glycogen or blood
Glut4 …..Effect of exercise
Glucose converted into ATP, pyruvate and NADH and it doesn’t require oxygen
(anaerobic metabolism)
Mitochondria generates NAD+ in order for glycolysis to continue
Or Lactic dehydrogenase converts pyruvate into lactic acid and generates NAD+
during rapid bursts of glycolysis
During intense exercise there will be:
1. short rest periods between contractions.
2. More fast glycotic fibers are recruited over the oxidative fibers, causing more lactic
acid release
3. Increase sympathetic innervation leading to more glycogenolysis, meaning more
pyruvate and thereby more lactate
Thus, During intense Exercise there is more lactic acid production in the muscle even if
it is fully oxygenated.
Fuel sources
2. Fat
3. Protein
Fuel type vary with the type, intensity and duration of exercise
Muscle fatigue
• Muscle fatigue is a reduction in developed force
resulting from previous muscle activity
maximal force that can be generated from resting muscle,
any decrease of this maximal
force is called fatigue.
Maximal force can be sustained only very short time (only
once)
• Metabolic fatigue is a reduction in submaximal force
after prolonged repetitive stimulation
usually exercise is done at submaximal force from many
repetition, then we become tired and be unable to do this
submaximal force
Fatigue in maximum sustained contraction is not in the brain in humans
Pi and H+ in muscle interfere with force development by actomyosin ATPase
• Fast twitch muscle use PC and glycolysis for ATP generation, PC accumulates Pi and
Glycolysis accumulates lactic acid. The pH of and exercising muscle falls to pH 6.0
• Both Pi and pH reduce developed force at the level of cross-bridges formation which
is perceived as fatigue.
Fatigue at submaximal force can be postponed by glycogen supercompensations
(carbohydrate loading)
 Exercise increases glucose transporter (GLUT4) in the muscle sarcolemma
 Resistance training hypertrophies muscle, increases muscle fiber size not number
 Signals that control muscle mass:
1.
2.
3.
4.
5.
6.
Stretch
Hypoxia
Androgens
glucocorticoids
Ca 2+
Myostatin negative regulator
 Hypertrophy takes place through recruitment of satellite cells
Myostatin knock-out
Exercise and force velocity relation ship
Muscle Atrophy
– Lack of muscle
activity
• Reduces muscle
size, tone, and
power
Steroid Hormones
• Stimulate muscle growth and hypertrophy
Growth hormone
Testosterone
stimulate synthesis of contractile proteins &
enlargement of skeletal muscles
Thyroid hormones: elevate rate of energy consumption in resting & active skeletal
muscles

Epinephrine: stimulate muscle metabolism and increase the duration of stimulation and
force of contraction