Muscle Physiology Lecture 4 PDF

Muscle Physiology Lecture :4 Dr. HUSSEIN GHANI Ph.D. Neurophysiology Relation between electrical and mechanical events of single muscle twitch. action potential and mechanical response (simple muscle twitch curve) plotted on the same time scale after being recorded separately. Fr...

Muscle Physiology Lecture :4 Dr. HUSSEIN GHANI Ph.D. Neurophysiology Relation between electrical and mechanical events of single muscle twitch. action potential and mechanical response (simple muscle twitch curve) plotted on the same time scale after being recorded separately. From this relationship graph, it can be concluded that: The twitch starts about 2 ms after the start of depolarization of the membrane but always before the repolarization is completed, The refractory period (absolute refractory period) is very short and lies in the first half of the latent period of the single muscle twitch. FIGURE 2.3-18 Electrical (A) and mechanical (B) responses of mammalian skeletal muscle fibre to a single maximum stimulus plotted on same time scale. Factors affecting contractile response The factors that can affect the contractile response (force of contraction) of a skeletal muscle are: Strength of stimulus, Frequency of stimulus, Load on the muscle (preload and after-load), Initial length of muscle Temperature. Strength of stimulus. A single muscle fibre obeys the all or none law, i.e. A subthreshold stimulus evokes no response, With threshold, maximal and supramaximal stimuli the contractile response remains constant. In the isolated nerve-muscle preparation. The graded response is obtained when stimuli of different intensities were applied through the nerve in a nerve-muscle preparation: Subthreshold stimuli do not evoke any response. Threshold stimulus, i.e. a stimulus just sufficient to elicit response and produces minimal contraction. Suprathreshold stimuli produce a graded response (i.e. the force of contraction goes on increasing with the increase in strength of stimulus till a maximum limit is reached). This is because, as the strength of stimuli is increased, more and more muscle fibres are recruited into activity. This phenomenon is called quantal or multifibre summation.  Maximal stimulus is that stimulus which produces the maximal response ( excites all the motor units). Supramaximal stimulus refers to the stimulus which exceeds the maximal value. Supramaximal stimuli do not increase the response beyond the maximal response. This is because, at the maximal stimulus, all the motor units are already contracting to their maximum extent and thus any further increase in the strength of stimuli (supramaximal stimulus) has no effect in increasing the force of contraction. Frequency of stimulus The effect of repeated stimuli on the contractile response of a skeletal muscle depends upon the number of stimuli (frequency). The effect of number of stimuli can be better understood by keeping the strength of stimuli constant and varying the time interval between the successive stimuli. Effect of two successive stimuli. Depending upon the length of interval between the two successive stimuli, the following types of effects are observed: No response, Summation, Superposition Beneficial effect. i. No response. When the second stimulus is applied during first half of the latent period, no response is obtained to the second stimulus as this period corresponds with the absolute refractory period (ARP) of the muscle. The muscle responds only to the first stimulus and the curve obtained is similar to a simple muscle twitch (Fig. A). ii. Summation. When the second stimulus is applied from second half of the latent period to the contraction phase, the effect of two stimuli is summed up and a single curve is achieved. This phenomenon is called complete summation. The graph obtained shows an increase in the force of contraction—an effect called summation of contractions or wave summation. This is due to the beneficial effect. the summation curve (Fig. B) so obtained is different from the simple muscle curve by having: A greater amplitude, A broader base. iii. Superposition. the second stimulus is applied during relaxation phase of the curve due to first stimulus, the relaxation phase is cut short and another contraction occurs. The second curve is superimposed over the first curve (Fig C). This is called phenomenon of superposition or incomplete summation of waves. The amplitude of the second curve is more than that of the first curve. This is also due to the beneficial effect,. iv. Two separate curves with beneficial effect. When the second stimulus is applied soon after the relaxation phase of the curve due to first stimulus, another complete curve is obtained. However, the force of second contraction is greater than that of the first contraction (Fig.D). The increase in the force of contraction of second curve is due to the beneficial effect. Beneficial effect and its causes. when second stimulus is applied at any stage after the first half of the latent period (i.e. at any time after the absolute refractory period), the force of second contraction is more than that of the first one. the contraction produced by first stimulus proves beneficial for the second one. This is called beneficial effect. FIGURE 2.3-20 Effect of two successive stimuli in an isolated gastrocnemius sciatic nerve preparation: A. no response (when second stimulus applied in absolute refractory period of the first response); B. summation effect; C. superposition (incomplete summation); and D. beneficial effect Causes. of beneficial effect are: Some of the calcium ions released from the terminal cisterns into the sarcoplasm during first contraction + those released by the second stimulus when the two stimuli are applied Viscosity of the muscle and thus the elastic inertia of the muscle is decreased to some extent by the first contraction, Increase in H ion concentration due to the first + contraction Increase in the temperature due to first contraction decreases the viscosity of the muscle and thus contributes to the beneficial effect. Effect of multiple stimuli. the contractile response of a skeletal muscle depends upon the total duration of twitch and the frequency of stimuli. the response obtained will depend upon whether the next stimulus falls: After the first twitch, or On relaxation phase of first twitch or On contraction phase or to second half of latent period of first twitch. Based on the above facts, following types of responses are observed due to multiple stimuli: Discrete responses, Incomplete tetanus Complete tetanus. i. Discrete responses. the next successive stimulus falls after completion of relaxation phase of the previous twitch, the contractions obtained, with brief intervals between them, are complete individual twitches (with contraction and relaxation phases). Such a response is called a discrete response. Further, each successive twitch has increased force of contraction (due to beneficial effect of previous twitch) till a maximal beneficial effect is achieved (Fig. A). This phenomenon is called the staircase effect or treppe (a German word for staircase). if the total duration of a twitch is 100 ms, then frequencies less than 10/s (i.e. each stimulus coming after every 100 ms) will produce discrete responses with a staircase effect. ii. Incomplete tetanus or clonus. the next successive stimulus falls on the relaxation phase of the previous twitch, the succeeding contraction obtained will be superposed over the previous twitch due to incomplete summation of waves. The record so obtained shows a progressive increase in amplitude up to a certain level beyond which there is no further increase. On stoppage of stimulation, the lever returns to normal (Fig. B). The series of jerky contractions of the muscle, with period of incomplete relaxation in between, is referred to as state of subtetanus or incomplete tetanus or clonus. Thus, when the total duration of a twitch is 100 ms, then frequencies between 10 and 20/s (i.e. each stimulus coming after every 50 ms but before 100 ms) will fall on the relaxation phase of previous twitch and produce incomplete tetanus. iii. Complete tetanus. the next successive stimulus falls in the second half of latent period to the contraction phase of the previous twitch (i.e. before relaxation begins), then due to complete summation effect, the muscle will remain in a state of sustained, smooth and forceful contraction called tetanus or tetanic contraction (Fig. C). The graph shows an increasing slope of the uninterrupted tracing, which exceeds the peaks of single twitches. When the stimulation is stopped, the muscle relaxes immediately.  if the stimulation is continued further, a plateau is maintained until the muscle begins to fatigue, after which it relaxes gradually.  During complete tetanus, the tension developed in the muscle is four times greater than that developed during the individual muscle twitch.  when the total duration of twitch is 100 ms, then frequencies more than 20/s (i.e. each stimulus coming before 50 ms) will fall on the contraction phase of previous twitch and produce complete tetanus.  The rate of stimulation at which there is complete fusion of individual contraction to produce tetanus is called the tetanizing or fusion frequency. Ionic Basis of Tetanus. The Ca2+ ions released in sarcoplasm during single twitch are removed quickly and relaxation occurs. When the muscle is stimulated in rapid succession, there occurs a progressive accumulation of Ca 2+ ions in the sarcoplasm. The longer stay of Ca2+ ions in the sarcoplasm increases the duration of active state (due to continuous recycling of myosin heads). This increases the amount of stretch on the series elastic component (SEC) and the tension developed rises to tetanic levels. Effect of temperature The contractile response is altered due to the effect of temperature. The effect of temperature noted on the amplitude of contraction, and its various periods is given below: At room temperature, a normal simple muscle is recorded as shown in Fig. A. At moderately high temperature (say 40°C), there occurs: Faster diffusion of Ca2+ ions from sarcoplasmic reticulum to sarcoplasm leading to: An increase in the muscle excitability, Acceleration of the chemical processes involved in muscle contraction and A decrease in muscle viscosity. Because of the above effects of temperature, the following changes are noted in an isotonic muscle twitch (Fig. B): Total duration of the twitch is decreased with shortening of all the periods, Speed of contraction increases Relaxation is also faster An increase in amplitude of muscle curve occurs due to increase in isotonic shortening of muscle. At low temperature (say 5–10°C), the reverse changes occur, i.e. A decrease in the muscle excitability, Slowing down of chemical processes involved in muscle contraction An increase in the muscle viscosity. Because of the above effects of cold, there occurs (fig C): A decrease in the force of contraction, Prolongation of all the periods. However, the effects of cold are reversible. So, if the muscle is gradually rewarmed, excitability is regained. At high temperature (above 50–60°C), there occurs coagulation of the muscle proteins leading to stiffness and shortening of the muscle fibres. This condition is called heat rigor. It is an irreversible phenomenon. Other types of rigors. Some other types of rigors are also described here because of the similar changes: Cold rigor. It occurs following exposure to severe cold. It is a reversible phenomenon. Calcium rigor. It occurs due to increased calcium content. It is also a reversible phenomenon. Rigor mortis