Muscle Contraction

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 relaxation, a reduction in measured tension, when the contraction is analyzed by electromyography.

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

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 (force of contraction) 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.

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.

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.

twitch contraction

The RESPONSE of a muscle to a SINGLE brief threshold stimulus; a QUICK contraction followed by relaxation

muscle contraction

The normal 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

Three myograms:

(1) a muscle twitch (2) multiple motor unit recruitment (spatial summation. (3) wave (temporal) summation

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.

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 of a muscle twitch, etc.

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."

tetanus

A state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli (a form of wave = temporal summation).

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

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

Describe 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. 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.

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

muscle twitch a single stimulus - to one or to more than one motor unit multiple motor unit recruitment multiple stimuli - to than one motor unit, with a possible increase in stimulus strength or stimulus frequency to increase contraction force wave (temporal) summation multiple stimuli of identical strength - to one or to more than one motor unit with little increase in contraction force