Lab 3 - Skeletal muscle contraction Flashcards
Describe how an action potential initiated at the NMJ can cause an increase in incracellular calcium. How is the intracellular calcium concentration returned to normal?
When the action potential invades the presynaptic terminal it depolarises it, there are voltage gated sodium channels on the axon that are responsible for the propagation of the action potential and the depolarisation of the action potential spreads into the nerve terminal and opens voltage gated calcium channels that are located throughout the terminal and many are at the same location as the vesicles to allow for tight coupling
Tropomyosin
Tropomyosin covers myosin binding sites on actin
Troponin
troponin binds calcium
calcium binding sites blocked, calcium binds to troponin complex which then moves tropomyosin out of the way, exposing myosin binding sites
Tetanus
When the frequency of muscle contraction is such that the macimal force is generated without any relaxation of the muscle
Twitch
The period of contraction and relaxation of a muscle after a single stimulation
The force transducer …
produces a voltage output in proportion to the force applied to it
EDL (extensor digitorum longus)
fast twitch fibres
Soleus
slow twitch fibres
In this experiment of EDL and soleus what is recorded
In this experiment the two channels are recorded. One shows the stimuli delivered, and the othe rshows the muscle response. These two channels are useful to show the time delay of the muscle response after the stimulus
Minimum contraction tension
Find the stimulus voltage when the contraction first occurs, and measure the tension developed
Maximum contraction tension
Find the stimulus voltage where the contraction is at its maximum, and measure the tension developed
Describe how the twitch amplitude changed when you increased the stimulus voltage.
Twitch amplitude increases with increasing stimulus voltage until maximum is reached where increase in the stimulus voltage will not increase the twitch amplitude anymore
Why did the twitch amplitude change when you increased the stimulus voltage? (and Q3 part 1 what is the mechanism for the increase).
Recruited more muscle fibers with increasing voltage
• Larger fibres recruited first (due to lower resistance with artificial stimulus)
In this experimental set up, explain the process for the observed twitch amplitude change. How would this be different in the body?
Recruit more motor units
(motor unit: alpha motor neuron + all fibres it innervates)
• Smaller motor units recruited first
Time to peak
from start of the response until maximum tension occurs
Latency
from the onset of the stimulus to the start of the response (time delay)
Fusion frequency
is the frequency where individual twitches can no longer be seen in response to stimuli (referred to as tetanus or a tetanic contraction). Individual twitches become fused at this particular frequency
at fusion frequency the response will be a jagged line and not a smooth one like textbooks
EDL has a _____ time to peak than soleus
faster
Explain why the latency and duration of the contraction differ for EDL and soleus.
• • •
Soleus has a higher proportion of slow twitch fibres
EDL has a higher proportion of fast twitch fibres
Compare the cellular characteristics of slow vs fast twitch fibres
Ryanodine receptors - slow twitch
low density = slower calcium release
Ryanodine receptors - fast-twich
high density = faster calcium release
Myosin type - slow twitch
slow ATP hydrolysis = slow cross bridge cycling
Myosin type - fast twitch
fast ATP hydrolysis = fast cross bridge cycling
SERCA pump - slow twitch
SERCA2a pumps Ca2+ back into SR slowly = slower drop in tension
SERCA pump - fast twitch
SERCA1a pumps Ca2+ back into SR rapidly = faster drop in tension
Energy production - slow twitch
oxidative = lots of ATP generated by requires oxygen and is slower
32-36 ATP
Energy production - fast twitch
glycolytic = fewer ATP generated quickly but creates lactic acid
2 ATP
Mitochondria - slow twitch fibres
many
Mitochondria - fast twitch fibres
few
Oxidative enzymes - slow twitch
many
Oxidative enzymes - fast twitch
few
Myoglobin - slow twitch
abundant
Myoglobin - fast twitch
sparse
Type I =
slow twitch
oxidative
Type IIA
fast twitch
oxidative-glycolytic
Type IIB
fast twitch
glycolytic
Slow twitch fibres
Fatigue resistant
Abundant in postural muscles
Utilised most during endurance activities
Fast twitch fibres
Fatigues easily
Abundant in muscles of power or fine movement
Utilised most during power and sprint activities
Fused tetanus soleus vs edl
Fused tetanus happens at a lower frequency for Soleus than for EDL.
There is a difference in how smooth the fused tetanus trace is, Soleus is smoother, EDL fluctuates.
EDL does not hold peak tension for long
(it fatigues quickly).
If you applied continuous stimulation for a period of 30 minutes to EDL and soleus muscle, what would you expect to occur to the tension recorded in each muscle?
EDL - fast twitch therefore quick fatigue
Soleus - slow twitch therefore slow fatigue
Why is the tetanic force greater than the twitch?
- Each AP causes Ca2+ release, many AP’s cause a lot of Ca2+to be released
- Ca2+ reuptake lower than release
- Sustained Ca2+ available in cytoplasm
- Because tissue does not relax, elastic recoil does not need to be overcome with each twitch
Active tension =
total tension - passive tension
Describe what occurred to the muscle twitch as you increased the length of
the muscle and explain why.
Increased then decreased
More cross-bridges = more tension
Results of active tension experiment support which theory of skeletal muscle contraction?
Sliding Filament Theory: Actin and myosin slide over one another
Myasthenia gravis
Autoimmune disease • Antibodies to nicotinic ACh receptors • AChR receptors blocked/destroyed • Decreased efficiency at NMJ • Muscle weakness (especially of frequently used muscles)
During clinical tests, Mrs CHB was given a repetitive closely timed stimulus (Repetitive Nerve Stimulation Test) on the ulnar nerve and the muscle action potential was recorded on flexor carpi ulnaris. Would the test show any abnormalities? If yes, how does it differ to normal conditions?
Yes - EMG decreased in amplitude with successive stimulation
What would be the effect of administering edrophonium chloride (acetylcholinesterase blocker) to Mrs CHB on her arm weakness and ptosis?
- Increase availability of ACh by blocking AChE
- Relieve symptoms of muscle weakness and ptosis
- Temporary
- Diagnostic
