Factors affecting
Simple muscle twitch
By:
Dr. Shaheenkousar
Objectives:
By the end of this section, you should :
1- Enumerate factors affecting simple muscle twitch (SMT).
2- Describe the mechanism by which each factors change SMT.
3- Define types, sites and causes of fatigue.
4- Define staircase, clonus, tetanus and how each of them is converted to one
another.
5- Differentiate between the types of skeletal muscle fibers.
6- Describe the effect of loading on SMT.
Simple Muscle twitch
Effect of Temperature on SMT
Mild warming
Mild cooling
Effect on SMT:
Effect on SMT:
*  amplitude (stronger contraction).
*  amplitude (stronger contraction).
* shortening of the duration of all phases * prolongation of the duration of all phases
(faster contraction).
(faster contraction).
Mechanism:
Mechanism:
1- acceleration of the metabolic reactions The opposite to warming
needed to provide energy for muscle
contraction.
2-  the viscosity of the muscle and
facilitates the sliding of actin over myosin.
Significance:
During
muscular
exercise
the
muscle
temperature which makes contraction
stronger and more rapid.
Heating a skeletal muscle above 45ºC leads to its
stiffening due to denaturation of the muscle proteins.
This is called “heat rigor”.
Effect of Fatigue on SMT
Definition:
It is  in the mechanical work of the muscle following rapid
repeated stimulation.
Effect on SMT:
1-  Amplitude (weaker contraction).
2- Prolongation of the duration of all phases (slow contraction)
especially the relaxation phase.
In extreme fatigue, relaxation becomes incomplete (contracture).
Contracture:
- It is a state of sustained weak muscle contraction (incomplete
relaxation) due to extreme fatigue.
- It is due to depletion of ATP which is important for muscle
relaxation (Ca2+ reuptake).
Sites & types of fatigue:
1- Transmission fatigue: includes:
a- Central fatigue:
* It is failure of transmission of motor impulse inside CNS.
* Due to depletion of the central neurotransmitter.
b- Neuromuscular fatigue:
* It is failure of transmission of nerve impulse to the muscle at
the MEP.
* Due to depletion of acetylcholine.
2- Contraction fatigue:
* It is failure of cross-bridges cycling inside the muscle.
* due to depletion of energy sources.
To differentiate between neuromuscular & contraction fatigue:
Neuromuscular fatigue appears during indirect stimulation of the nerve &
disappear by direct stimulation of the muscle WHILE contraction fatigue is
present in both.
In isolated muscles (outside the body), fatigue occurs rapidly due to:
Failure of the excitation-contraction coupling mechanism:
1- Failure of neuromuscular transmission due to depletion of
acetylcholine.
2- failure of cross-bridge cycling (necessary for contraction) &
failure of active transport of Ca2+ ions into the sarcoplasmic
reticulum (necessary for relaxation) due to: - Decreased energy
stores inside the muscle (ATP, Cr~P, and glycogen).
- Decreased O2 supply.
- Accumulation of metabolites e.g. CO2 and lactic acid.
- Decreased pH of the muscle cells.
- Electrolyte disturbance.
In intact muscles (inside the body), muscle fatigue is delayed because:
- The circulatory system: supplies the muscle with O2 and nutrients
and removes the metabolites from the muscle.
- The CNS: regulates the muscle contraction. so that not all the
muscle fibers are contracted at the same time, but , there is
alternation between contracted and relaxed muscle fibers.
- The endocrine system (Hormones):
(e.g. adrenaline, noradrenaline, glucocorticoids, thyroxin and
insulin) regulate the metabolic rate, glycogen stores, blood pressure
and excitability of the nervous system.
Type of the muscle
Skeletal muscles contain mainly two types of muscle fibers.
Slow (red) muscle fibers (type I) Fast (pale) muscle fibers (type II)
adapted
for
long
slow
contractions which maintain the
body posture (support the body
against gravity)
e.g. back muscles, soleus muscle
etc.
 S.M.T:
- duration: about 100 m.sec.
- long latent period.
- slow contraction.
- slow relaxation.
 adapted for fine and rapid
movements.
e.g. extra ocular muscles.
 S.M.T:
- duration: less than 10 m.sec.
- short latent period.
- rapid contraction.
- rapid relaxation.
Speed of contraction:
Slow (red) muscle fibers (type I)
Fast (pale) muscle fibers (type II)
 These muscle fibers:
- smaller in size.
- contain less sarcoplasmic
reticulum and glycogen granules.
- contain the respiratory pigment
myoglobin which facilitates the
uptake
of O2 from the blood stream (red
fibers).
- contain much mitochondria
(aerobic oxidation)
- surrounded by numerous blood
capillaries (red fibers).
 innervated by small (less rapidly
conducting) motor nerves.
 These muscle fibers:
- larger in size.
- contain much more sarcoplasmic
reticulum and glycogen granules.
- the myoglobin is absent. (pale
fibers).
- few blood capillaries. (pale
fibers).
- few mitochondria.
 innervated by large (rapidly
conducting) motor nerves.
Slow (red) muscle fibers (type I)
Fast (pale) muscle fibers (type II)
 do not show rapid fatigue
because:
- They are slowly contracting i.e.
use ATP at a slow rate.
- These muscles are richly supplied
with blood which is able to supply
them with O2 and nutrients
(glucose and free fatty acids).
- These muscles are adapted to use
aerobic oxidation (contain much
mitochondria, myoglobin and blood
capillaries) which provides much
energy (38 ATP molecules for each
molecule of glucose).
 adapted to use anaerobic
glycolysis (absent myoglobin, few
mitochondria,
few
blood
capillaries and more sarcoplasmic
reticulum and glycogen granules).
So, these muscles are able to
produce ATP rapidly and at high
rates but quickly fatigued once
their glycogen stores are depleted.
N.B.: A third type of muscle fibers (Type IIa = intermediate type; fast
oxidative) are present. These fibers share the characteristics with each
of the other two types. They have high ATPase activity like the fast
(anaerobic) fibers and high oxidative capacity like the slow (aerobic)
fibers. They contract more rapidly than the slow fibers and can
maintain contraction for a longer period of time than the fast fibers.
(rare in humans).
Treppe
Effects of stimulation of skeletal muscle by several successive stimuli
 The response of the muscle to several successive stimuli depends
on the frequency of stimulation.
 If the frequency of stimulation is low ------> Separate twitches
- The first few contractions gradually increase in strength. This
condition is known as the “staircase phenomenon”.
- staircase is separate twitches produced by stimulation of the muscle
by several successive stimuli of low frequency, so that the next
stimulus falls immediately after the relaxation phase of the preceding
twitch, & the first few twitches show gradual  in strength.
- The cause of this phenomenon is that the second stimulus finds the
muscle in a better physiological condition (see before). The third and
fourth stimuli find the muscle in a more and more better conditions
producing increasing contraction up to a certain limit where there is
no further increase in contractions.
– Incomplete tetanus
• Muscle fibers partially
relax between contraction
• There is time for Ca 2+ to
be recycled through the
SR between action
potentials
 If the frequency of stimulation is mild ------> Clonus (incomplete
tetanus) is produced.
Clonus is: rapid repeated contractions produced by stimulation of
the muscle by several successive stimuli of mild frequency, so that the
next stimulus falls during the relaxation phase of the preceding
twitch.
– Complete tetanus
• No relaxation between
contractions
• Action potentials come sp
close together that Ca 2+
does not get resequestered in the SR
 If the frequency of stimulation is high -------> complete tetanus is
produced.
 Tetanus is: continuous contraction produced by stimulation of the
muscle by several successive stimuli of high frequency, so that the
next stimulus falls during the contraction phase of the preceding
twitch.
 Tetanus results from fusion of successive contractions caused by
persistent release of Ca2+ ions which lead to continuous contractions.
The tension developed during a complete tetanus is about 4 times
that developed by a simple muscle twitch.
 Tetanus is the commonest type of muscle contraction which occurs
in the human body.
The frequency of stimulation needed to produce tetanus is around
20-60 times per second for most skeletal muscles.
The minimal frequency needed to produce complete tetanus is
determined by the duration of the simple muscle twitch. The
longer the duration of the twitch, the lower the frequency of
stimuli needed. So, the frequency depends on:
1. Type of the muscle: Slow (red) muscle needs a less frequency of
stimulation to produce tetanus.
2. All factors that lengthen the contraction phase lower the
minimal frequency needed to produce tetanus e.g.
cooling, fatigue, decrease O2 supply, decrease blood supply and
decrease Ca2+ ions.
3. Anticholinestrases (e.g. prostigmine) prolong the contraction
phase by preventing hydrolysis of acetylcholine. Stimulation of
the motor nerve leads to accumulation of acetylcholine at the
motor end plate producing tetanus.