Frog Muscle: Temporal Summation

Stimulation, response, and key results

Voltage was set to maximal level (where all fibers are recruited) and multiple shocks were delivered at while increasing the frequency. Eliciting temporal summation, where the firing rate of each active unit increases

What stimulus is used? Why?

The stimulator is set to maximal voltage (V), so all muscle fibers are firing, and any change in muscle tension is not due to increased recruitment. So, by firing at the maximal voltage at a higher frequency (Hz), it causes each individual muscle fiber to develop more tension

So as the muscle receives a higher frequency (Hz) of stimulation it goes through various stages of?

• why?

goes through various stages of temporal summation because the time interval is too short and the muscle fiber is not able to reset externally and internally, causing an increased response (muscle tension)

What is temporal summation?

• how do we know

increased response (tension) due to increased frequency (Hz) of active units (muscle fibers) firing

• using the maximal stimulus means only temporal summation was observed

What is the experiment?

increasing the frequency (Hz) of maximal voltage and measuring the tension (g)

• applying greater stimuli per second

• Hz= # of stimulus/second

What are the three stages of temporal summation?

Treppe, wave summation, and tetanus

Treppe

• frequency

• relaxation

• summation

• cause of summation

2-6 Hz

complete, returns to baseline; looks like a muscle twitch, but know it’s not because 1) because maximal voltage is used, and 2) amplitude increases

temporal summation, leading to an increase in force

1) Not enough time is given to the Ca ATPase to requester all of the Ca2+ from the cytosol. So when the next contraction comes in, more Ca2+ is released in addition to what was left over from the previous contraction.

Wave summation

• frequency

• relaxation

• summation

• cause of summation

6-10 Hz

Incomplete, it does not return to the baseline

temporal summation, leading to an increase in force

increase in the response bu the muscle does not relax (return to baseline), because

1) increased availability of Ca2+

• Because not enough adequate time is available to the requester Ca2+, Ca2+ accumulates. Causing more crossbridging, therefore a greater contraction strength

2) prestretch of series elastic elements (SEE)

• Because the contraction does not go back to the baseline, the muscle is partially stretched, and less energy goes into prestretching SEE, so more energy goes into the actual contraction, resulting in a larger contraction

Tetanus

• frequency

• relaxation

• summation

• cause of summation

10-18 Hz

none

temporal summation, leading to an increase in force

Stimuli are so frequent, reaching the maximum strength of contraction, and the muscle has no time to relax because..

1) greater increase of Ca2+, and CA ATPase has no chance to requester because contractions are coming constantly

2) prestretched SEE allows more energy to go into the contraction, resulting in a larger contraction

3) Specific to the frog gastrocnemius muscle is the recruitment of tonic muscles, leading to an increase in contraction strength due to spatial summation

Ca2+ importance in the three stages of temporal summation?

more Ca2+ available means the more cross bridges formed, creating greater tension

Why does fatigue occur?

muscle running out of ATP and metabolites accumulating