Practical- 5
Factors affecting simple muscle twitch
At the end of session student should be able to:
a. Define fatigue and describe its effect on SMT.
b. List the sites and types of fatigue and explain the delayed onset of fatigue inside the body.
c. Define contracture and mention the possible mechanism.
d. Describe the effect of several successive stimulation of the muscle.
e. Discuss the mechanism of tetanus and list the factors that determine frequency of stimuli
needed to produce.
f. Discuss the effect of initial length of the muscle on SMT.
Dr. Shaheenkousar Haroon Rashid
Simple Muscle twitch
Factors affecting simple muscle twitch are
1. Temperature:
Effect of mild warming on SMT:
 amplitude (stronger contraction). Shortening of the duration of all phases (faster contraction).
Mechanism:
Increased metabolic reactions needed to provide energy for muscle contraction.
Decreased 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 cooling warming on SMT:
Decreased amplitude and prolongation of the duration of all phases (faster contraction).
Mechanism:
Decreased metabolic reactions needed to provide energy for muscle contraction.
Increased viscosity of the muscle and facilitates the sliding of actin over myosin.
2. Effect of Fatigue on SMT
It is  in the mechanical work of the muscle following rapid repeated stimulation.
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:
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.
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.
3. Effects of stimulation of skeletal muscle by several successive stimuli
a. 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,
this is called beneficial effect where there is no further increase in contractions.
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.
– 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 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.
4. effect of initial length of the muscle on SMT
Starling law muscle contraction: The force of contraction is directly proportional to
initial length of muscle fiber within physiological limits.
The initial length of muscle fiber can be increased by applying suitable pre-load to stretch the
muscle before contraction.
Mechanism: the stretching of the skeletal muscle allowing a maximal overlapping of the myosin
and actin filaments, with optimal stretching allowing each myosin head to have access to a
binding site on actin. Also, stretching the muscle increases the Ca2+ sensitivity of the myosinactin interaction.
Over-stretching the muscle reduces the access of myosin heads to actin.