Exam 3 Review:  Chapter 09:  Electromyography

electromyography - A test which measures the voltage changes of skeletal muscle contraction in response to nerve stimulation; the x axis represents time and the y axis represents the muscle's internal voltage potential; this test is used clinically to evaluate muscle weakness and to determine if the weakness is related to the muscles themselves or a problem with the nerves that supply the muscles.

electromyogram (EMG)  = myogram - A graphic record of the electrical activity of a skeletal muscle contraction as recorded by an electromyograph; it demonstates the latent period, contraction period, and the relaxation period.

muscle contraction - The macroscopic increase in tension and, perhaps, shortening in length, of a muscle or muscle cell as a result of the action of actin and myosin proteins within sarcomeres at the molecular level, the sliding filament mechanism.

twitch contraction - The response of a muscle to a single brief threshold stimulus; a quick contraction followed by relaxation.

latent period - The brief initial time interval observed in a myogram between the initial stimulation of a muscle fiber and the beginning of its actual contraction.

contraction period - The time interval (usually 10-100 msec) observed in a myogram after the latent period following the initial stimulation of a muscle fiber when the sliding filament mechanism is active at the molecular level, actin-myosin cross-bridges are being formed, and tension develops, rising to a peak; if the tension is sufficient to overcome the resistance of a load, the muscle fiber will shorten.

relaxation period - The time interval (usually 10-100 msec) observed in a myogram after the contraction period and following development of peak tension, when the sliding filament mechanism is inactived at the molecular level, actin-myosin cross-bridges are broken and tension is reduced, until the resting state is restored or another contraction is initiated.

refractory period (muscle) - The time interval, after a muscle fiber has been stimulated and a contraction has been achieved, which has to pass before the muscle cell can be stimulated to contract again.

wave summation = temporal summation - The physiological phenomenon in which the tension developed in a muscle fiber during a muscle twitch will be greater in a second twitch compared to a first twitch because the second stimulus was applied before the muscle had relaxed completely after the first twitch, despite the fact that both stimuli were identical in strength; i.e., because the second twitch began while some tension remained, the final tension is summed; the strength of contraction increases when there is an increase in the frequency with which a muscle is stimulated; with rapid repeated stimulation (so rapid that the muscle does not completely relax between successive stimulations), a muscle fiber is re-stimulated while there is still some contractile activity, and as a result, there is a 'summation' of the contractile force; in addition, with rapid stimulation there is too little time between successive stimulations to remove all the calcium from the sarcoplasm, so, with several stimulations in rapid succession, calcium levels in the sarcoplasm increase; more calcium ions means more active cross-bridges form and, therefore, a stronger contraction develops until the contraction is truly maximal for that muscle cell.

tetanus - Physiology. a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli (a form of wave = temporal summation).  [2.  an acute, often fatal disease characterized by spasmodic contraction of voluntary muscles, especially those of the neck and jaw, and caused by the toxin of the bacillus Clostridium tetani, which typically infects the body through a deep wound.  a.k.a. lockjaw].

incomplete tetanus - In a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli  (a form of wave = temporal summation), when there is still evidence of partial relaxation, a reduction in measured tension, when the contraction is analyzed by electromyography.

fused tetanus = complete tetanus - In a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli  (a form of wave = temporal summation), when there is no evidence of partial relaxation, a reduction in measured tension, when the contraction is analyzed by electromyography.

treppe = staircase effect - A phenomenon in muscle tissue in which a number of stimuli of the same strength are applied to the muscle after a period of rest in the relaxed state  (a form of wave = temporal summation), the first few contractions of the series show a successive increase in amplitude (strength); the increase in efficiency is due to local temperature increase, increased enzyme efficiency, increased calcium ion availability and some increase in structural elasticity; nickname "the warm-up effect."

recruitment = motor unit summation = multiple motor unit recruitment - At the level of an entire muscle, the main means of increasing tension production, by stimulating a greater proportion of the individual motor units to contract simultaneously.

Describe: (or you may sketch and label)

11. The cellular events that are occurring during each period of the muscle twitch.

1.  Latent Period:  motor nerve impulse arrives at neuro-muscular junction; acetyl choline (ACh) neurotransmitter is released into the neuro-muscular junction's synaptic cleft; ACh binds non-covalently, reversibly, to chemically-gated Na+ channels of the motor end plate; chemically-gated Na+ channels open; Na+ ions flow into the sarcoplasm, depolarizing the sarcolemma of the motor end plate; depolarization of the sarcolemma spreads by the successive destabilizing, and therefore, opening of nearby voltage-gated Na+ channels; the wave of depolarization spreads along the T-tubule system, again by opening of nearby voltage-gated Na+ channels; Na+ ions flow into the sarcoplasm near the T-tubules destabilizing nearby voltage-gated Ca+2 channels in the sarcoplasmic reticulum; Ca+2 ions flow into the general sarcoplasm; Ca+2 ions then diffuse into the myofibrils, and, acting as second messengers, initiate contraction of the sarcomeres.  This long series of molecular processes result in a time delay between the arrival of the stimulus (nerve impulse) and the start of contraction. 2.  Period of Contraction:  Ca+2 ions diffuse into the myofibrils, and, acting as second messengers, initiate contraction of the sarcomeres; myosin head bind to acti n fibers, and initiate the power stroke.  The power stroke is followed successively be release and reattachment of the myosin heads to actin, and continued sliding of the actins over the myosin fibers.  This is the "ratchet effect." 3.  Period of Relaxation:  The stimulus = nerve impulse ends.  In the absence of ACh, the chemically-gated Na+ channels of the motor end plate close, the action of the Na+ K+ ATPase pumps repolarizes the motor end plate; repolarization spreads across the sarcolemma and through the T-tubule system as voltage-gated Na+ channels continue to close; repolarization of  the T-tubule system affects the nearby sarcoplasmic reticulum; voltage-gated Ca+2 channels in the sarcoplasmic reticulum close; Ca+2 ions are removed from the general sarcoplasm by Ca+2 ATPase pumps.  In the absence of the second messenger, Ca+2 ions, the myosin heads take up a final ATP molecule and release the actin fibers.  Contraction ends.

12. The following types of muscle contraction and how they occur:

a. wave (temporal) summation - a series of contractions which sum (build-up contraction force = tension) because a series of stimuli are applied to the muscle one after another.  See (2) above.  Stimulus strength did not vary; only the time between incoming stimuli.         b. incomplete (unfused) tetanus - a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli, when there is still evidence of partial relaxation, a reduction in measured tension, following each successive stimulus.  See (3) above.  Stimulus strength did not vary; only the time between incoming stimuli.         c. complete (fused) tetanus - a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli, when there is no evidence of partial relaxation, a reduction in measured tension, following each successive stimulus.  See (4) above.  Stimulus strength did not vary; only the time between incoming stimuli.         d. staircase (treppe) effect - a phenomenon in muscle tissue in which a number of stimuli of the same strength are applied to the muscle after a period of rest in the relaxed state, the first few contractions of the series show a successive increase in amplitude (strength); the increase in efficiency is due to local temperature increase, increased enzyme efficiency, increased calcium ion availablility and some increase in structural elasticity; nickname "the warm-up effect."  See the figure below.

e. recruitment (multiple motor unit summation) - MMUR is a process observed at the level of an entire muscle; it is the main means of increasing tension production, by stimulating a greater proportion of the individual motor units to contract simultaneously.    See the MMUR-BioPac figure below where successive peaks/plateaus represent a person attempting to increase grip strength from 25% to 50% to 75% to 100% of their maximum grip strength.

Sketch and Label:

6. A myogram of a muscle "twitch," indicating the names of all time periods and the scales of the x and y axes.

7. Three myograms:

(1) a muscle twitch:  See Figure in question 6 above.

(2) multiple motor unit recruitment (spatial summation):  See MMUR-BioPac figure immediately above "Sketch and Label" header.

and (3) wave (temporal) summation:  See the figure below.

Describe the differences in the stimuli necessary to cause these three types of muscle response and how the responses differ from each other in terms of the strength and/or number of muscle fibers stimulated.