1-4 Twitch

Muscle Twitch in Frog Experiment

• Threshold is minimum voltage necessary to produce action potential
• a single brief stimulus at that voltage produces a quick cycle of contraction & relaxation called a twitch (lasting less than 1/10 second)
• A single twitch contraction is not strong enough to do any useful work

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Twitch contraction

• Phases of a twitch contraction
• latent period (2 msec delay)
• only internal tension is generated
• no visible contraction occurs since �only elastic components are being �stretched
• contraction phase
• external tension develops as muscle �shortens
• relaxation phase
• loss of tension & return �to resting length as calcium returns to SR

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Recruitment & Stimulus Intensity

• Stimulating the whole nerve with greater and greater voltage produces stronger contractions
• More motor units are being recruited
• called multiple motor unit summation
• lift a glass of milk versus a whole gallon of milk

Maximal

recruitment

Production of Variable Contraction Strengths�Twitch and Treppe Contractions

• Stimulating a muscle at variable frequencies
• low frequency (up to 10 stimuli/sec)
• each stimulus produces an identical twitch response
• moderate frequency (between 10-20 stimuli/sec)
• each twitch has time to recover but develops more tension than the one before (treppe or staircase phenomenon)
• calcium was not completely put back into SR
• heat of tissue increases myosin ATPase effeciency (warm-up exercises)

Production of Variable Contraction Strengths�Incomplete and Complete Tetanus

• Higher frequency stimulation (20-40 stimuli/second) generates gradually more strength of contraction
• each stimuli arrives before last one recovers
• temporal summation or wave summation
• incomplete tetanus = sustained fluttering contractions
• Maximum frequency stimulation (40-50 stimuli/second)
• muscle has no time to relax at all
• twitches fuse into smooth, prolonged contraction called complete tetanus
• rarely occurs in the body

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