Set up: 2A. Recruitment requires increasing stimulus amplitude . • •
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Below threshold stimulus, there is no response from the muscle. As stimu stimulus lus amplit amplitude ude incre increas ases, es, only only the muscle muscle bers bers with with the the lowest lowest threshold initially respond and contract. As the stimulus amplitude continues to increase, an increasing number of motor motor units units will will be recr recruit uited ed so that that the the streng strength th of the contra contracti ction, on, or tension, increases as well. Once all motor units in a muscle have been recruited, an increase in the stimulus amplitude will not result in any increase in tension. The contraction of a whole muscle can produce produce strong, weak or intermediate contractions depending depending upon how many motor units are stimulated. The strength strength of the twitch increases increases with the increase increase in the magnitude magnitude of the graded (or shoc shock. k. Ther Theref efor ore, e, the the str strengt ength h of the the twit twitch ch is said said to be graded incrementally increased)
we will demonstr demonstrate ate the respons response e to increasin increasing g stimulus stimulus intensity intensity progr progressi essively vely higher voltage!. Remember that motor units within a whole muscle have variable thresholds. The slow twitch motor units have the lowest threshold and will be the rst to respond as the voltage of the stimulus is increased. "ntermediate "ntermediate motor units are recruited ne#t. The fast twitch motor units have the highest thresholds and are the last to be recruited. As more motor units are recruited with increased stimulus voltage, the tension produced by each twitch increases.
Defnition: motor unit - is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals. terminals.[1] Groups of motor units often work together to coordinate the contractions of a single muscle muscle;; •
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Stimulus current $ induces %ow of electric chargeions!, chargeions!, generated because there is di&erence in charge potential! Recruitment is increasing increasing the number of motor units responding to a single stimulus, which results in the increase in tension in whole muscle. hreshold stimulus ! weakest stimulus that could generate an action potential "aximal stimulus # where an increase in stimulus intensity will no longer result in any increase evoked potential $% # large temporary alteration in "% "%&& when depolariing G% reaches certain threshold potential G% # greater intensity o stimulus& greater change in "%
2B. Supramaximal stimulus – any stimulus ao!e maximal stimulus Summation is the increase in tension that results when a muscle ber is unable to rela# between twitches. 'ummation requires increasing stimulus "requency .
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At low fre(uency, the muscle ber will rela# before the ne#t stimulus impulse occurs. As the stimulus fre(uency increases and interval between the stimuli decreases, the muscle ber cannot fully rela# before the ne#t stimulus occurs
we will demonstrate how a muscle responds to increases in stimulus fre(uency the number of stimuli per second!. The stimulus voltage will remain constant throughout this e#periment. As fre(uency increases, summation and tetanus will begin to occur. 'ummation can be measured as the increase in the muscle response along the )*a#is. Tetanus will be observed as the loss of rela#ation between twitches Remember what causes contraction in a muscle ber. An action potential causes calcium ions to be released from the sarcoplasmic reticulum into the sarcoplasm. This allows myosin to form cross bridges with actin laments of the sarcomere and generate the movement associated with the contraction. The more cross bridges formed, the stronger the contraction. Rela#ation is brought about by the removal of the +a from the sarcoplasm. "f a second stimulus is delivered to a ber before all of the +a are removed the ber has not rela#ed!, more +a are released and the ber starts a second contraction without completely rela#ing from the rst.
#$y supramaximal stimulus% the response to any suprathreshold stimulus is ma#imal and therefore cannot be increased by increasing the intensity of the stimulus. -owever, when stimulating a whole muscle the response is graded muscle force increases with increased intensity of the stimulus. This is due to the increased number of activated bres. To obtain reproducible results, researchers usually use suprama#imal stimuli, which will induce contraction of all the bres at each stimulation. 'uprama#imal stimulation is stimulation having intensity voltage or current! signicantly above that re(uired to activate all the muscle bres. /enition0 Re"ractory period * period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation (hy is the magnitude of contraction more when the ) nd stimulus falls in the relaxation period of 1st one*
B&'&*A+ &&*,: The amount of tension is increasing due to an increase in the amount of +a1,as the +a1 released by previous stimulation is not mopped o& and +a1 continues to release due to subse(uent stimuli. The increase magnitude of contraction is actually due to summation of the responses contraction! rather than the summation of stimuli (ith rapid stimulation +so rapid that a muscle does not completely relax between successive stimulations,& a muscle fiber is re!stimulated while there is still some contractile activity.
$s a result& there is a 'summation' of the contractile force. -n addition& with rapid stimulation there isn't enough time between successive stimulations to remove all the calcium from the sarcoplasm.
o& with several stimulations in rapid succession& calcium levels in the sarcoplasm increase. "ore calcium means more active cross!bridges and& therefore& a stronger contraction
2*. This loss of rela#ation between stimuli is called tetanus. "f stimulated at progressively higher fre(uency, the amount of rela#ation that occurs in between each 2twitch3 is progressively reduced, until a steady state of tension tetanus, or tetany! is generated 4e say a muscle is in incomplete tetanus if the muscle ber is able to partially rela# between stimulus impulses see 5ig. 6!. A muscle ber is in complete tetanus if there is no rela#ation at all between stimulus impulses. Tetanus $ state of sustained contraction due to rapidly repeated stimulation * *
Rate of stimulation increase higher fre(uency!, tetanus ensues Rapid and repeated stimulation leads to stimuli to fall during the contraction phase so the contractile processes are repeatedly activated * no time to rela#
"nconplete tetanus7clonus7unfused $ when stimulated at lower tetani8ing fre(uency
'ummation vs "ncomplete tetanus -f a second stimulus arrives before the relaxation phase has ended& a second& more powerful contraction occurs. he addition of one twitch to another in this way constitutes thesummation of twitches & or wave summation -f the stimulation continues and the muscle is ne ver allowed to relax completely& tension will rise to a peak +/igure 10!1b ,. $ muscle producing peak tension during rapid cycles of contraction and relaxation is in incomplete tetanus .
