Mechanical Properties Of Striated Muscle PDF

MECHANICAL PROPERTIES OF STRIATED MUSCLE -3-  FACTORS AFFECTING MUSCLE TENSION There are three factors which affect muscle tension in a whole muscle: 1. Number of motor units recruited  2. Frequency of stimulation  3. Degree of muscle stretch  FACTORS AFFE...

MECHANICAL PROPERTIES OF STRIATED MUSCLE -3-  FACTORS AFFECTING MUSCLE TENSION There are three factors which affect muscle tension in a whole muscle: 1. Number of motor units recruited  2. Frequency of stimulation  3. Degree of muscle stretch  FACTORS AFFECTING MUSCLE TENSION Although single muscle cells respond to stimulation in an all or non fashion, a whole muscle shows variations in the development of tension (force of contraction) What is the ALL or NON fashion? Principle in physiology In any single nerve or muscle fiber the response to a stimulus above threshold level is maximal and independent of the intensity of the stimulus.  HOW MUSCLE RESPOND A SINGLE STIMULUS? Muscle Twitch is a muscle contraction in response to a single stimulus of adequate strength.  HOW MUSCLE RESPOND A SINGLE STIMULUS? Muscle Twitch is a muscle contraction in response to a single stimulus of adequate strength.  THE THREE PHASES OF MUSCLE TWITCH A complete muscle twitch is divided into three phases; 2. Contraction period Myosin cross bridge cycling causes sarcomeres to shorten 1. Latent period 3. Relaxation period The sarcolemma Calcium ions are and the T tubules actively transported depolarize back into the terminal cisternae Calcium ions are released into the Cross bridge cycling cytosol decreases and end Cross bridges Muscle return to its begin to cycle but original length there is no visible shortening of the muscle  WHAT IS MUSCLE TWITCH? A twitch is the force produced from the contraction event due to one action potential.  TEMPORAL SUMMATION OF TWO STIMULI * Muscle stimulated with same intensity, * Second stimulus applied before complete relaxation, * Second contraction added to first contraction. Temporal summation (Wave Summation): is an event which occurs when a second stimulus of the same intensity is applied to a muscle before the completion of the relaxation period of the first stimulus. This results in increased muscle tension.  WHY THE SECOND PICK IS HIGHER THAN THE FIRST CONTRACTION? In temporal summation, the second peak is higher than the first because, the additional influx of calcium ion promotes a second contraction, which is added to the first contraction. Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction.  EFFECT OF TIME INTERVAL ON SECOND CONTRACTION If you wait until relaxation is complete from the first stimulus, then give a second stimulus to the same muscle, temporal summation will not occur. 50 ms Attention for the height of the curve! 70 ms 90 ms  REPEATED STIMULATION If we stimulate muscle repeatedly, and the interval between stimuli gradually decreased (frequency gradually increased) What is going to happen? TREPPED; The frequency of stimulation was so slow here that relaxation was complete between contractions. (Curve goes down to the baseline after each contraction.) The strength of contraction did increase because muscle contraction causes heat to build up in the muscles and muscles then work better when they are warmer. Enzymes can work faster and more efficiently when a muscle is "warmed up". TREPPED; The frequency of stimulation was so slow here that relaxation was complete between contractions. (Curve goes down to the baseline after each contraction.) TEMPORAL SUMMATION The frequency of stimulation is increased to the point where relaxation cannot totally occur. The result is a continual increase in tension which may result from increased availability of intracellular calcium. INCOMPLETE TETANUS The frequency of stimulation is increased to the point where the muscle exhibits even shorter contraction-relaxation cycles, but there is still some degree of relaxation after each contraction.  COMPLETE TETANUS When the frequency of stimulation becomes fast enough, the contractions fuse into a smooth, continuous, total contraction with no apparent relaxation. This state is due to a continual depositing of calcium ions in the cytosol. As a result, the binding sites on actin continually stay exposed. At a slightly higher frequency, the strength of contraction reaches its maximum, and thus any additional increase in frequency beyond that point has no further effect in increasing contractile force.. FATIGUE Tetanus cannot continue forever. With continued rapid stimulation, there is a build-up of acidic compounds which affect protein functioning, a relative but not total lack of ATP, and ionic imbalances resulting from membrane activities. This causes muscle fatigue and the gradual inability of the muscle to respond to stimulation.  MUSCLE FATIGUE “Fatigued muscle produces less force and has a reduced velocity of shortening.” Muscle fatigue is defined as the inability to maintain a desired power output and a decline in both force and velocity of shortening. A decline in maximal force production with fatigue results from a reduction in the number of active cross-bridges as well as the force produced per cross-bridge. Other characteristics of fatigued skeletal muscle are lower rates of both force production and relaxation, owing to impaired release and reuptake of Ca2+ from the sarcoplasmic reticulum (SR). WHAT HAPPENS WHEN MUSCLE FATIGUE OCCURED? As a result, fast movements become difficult or impossible. Muscle fatigue is reversible with rest, which contrasts with muscle damage or weakness. FATIGUE CAN RESULT FROM ATP DEPLETION, LACTIC ACID ACCUMULATION, AND GLYCOGEN DEPLETION. Therefore, fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. ATP DEPLETION Muscle fibers require ATP for ① contraction, ② relaxation, and the ③ activity of the membrane pumps that maintain ionic homeostasis. Therefore, the cells must maintain [ATP]i to avoid fatigue. Intense stimulation of muscle fibers requires high rates of ATP utilization. As fatigue develops, [ATP]i can fall from 5 mM to less than 2 mM, particularly at sites of cross-bridge interaction. Accumulation of Pi in the myoplasm also results muscle fatigue! Phosphate concentrations; at rest around 2 mM, in working muscle 40 mM *Inhibition of Ca++ release from the SR *Decrease in the sensitivity of contraction to Ca++ *Alteration in actin-myosin binding LACTIC ACID ACCUMULATION Intense activity also activates glycolysis again, resulting in a high rate of lactic acid production and thus reducing pHi to as low as 6.2 This fall in pHi inhibits myosin ATPase activity and thereby reduces the velocity of shortening. The fall in pHi also inhibits cross-bridge interaction and the binding of Ca2+ to troponin (Inhibition of excitation- contraction coupling by H+; a decreased pH has been demonstrated to reduce the affinity of troponin for Ca2+.). *Fast-twitch fibers more sensitive than slow-twitch fibers. Accumulation of lactic acid results muscle fatigue! When lactic acid ↑ 15 to 26 mM, Myoplasmic pH ↓ ≈7 to ≈6.2 *Inhibits actin-myosin interactions *Sensitivity of the actin-myosin interaction to Ca++ ↓ *Inhibits myosin ATPase activity *Ca++ binding to troponin C is altered  FACTORS AFFECTING MUSCLE TENSION There are three factors which affect muscle tension in a whole muscle: 1. Number of motor units recruited  2. Frequency of stimulation  3. Degree of muscle stretch  3. Degree of muscle stretch “ LENGTH-TENSION RELATIONSHIP” Moderately stretched muscle Maximum tension is developed when there is an optimum overlap of thin and thick filaments so that all cross bridges can participate in the contraction.  Unstretched Muscle Over stretched muscle The overlapping thin filaments from The thin filaments are pulled almost to opposite ends of the sarcomere, interfere the ends of the thick filaments where and conflict with each other. little tension can be developed. This restricts productive cross bridge building and results less tension development. The unstretched muscle produces a relatively weak contraction.  At the optimum sarcomerelength (~ 𝟐. 𝟐 𝛍m), the fiber generate optimum force in thattwitch. Musclelength is expressed is the percent of optimal length (Lo), that is, the length at which active isometric tension is maximal.  When a muscle fiber length is 60 percent of Lo, the fiber develops no tension when stimulated. As length is increased from this point, the isometric tension at each length is increased up to a maximum at Lo. Further lengthening leads to a drop in tension. At lengths of 175 percent Lo or beyond, the fiber develops no tension when stimulated.  When all the skeletal muscles in the body are relaxed, the lengths of most fibers are near Lo and thus near the optimal lengths for force generation. The length of a relaxed fiber can be altered by the load on the muscle or the contraction of other muscles that stretch the relaxed fibers, but the extent to which the relaxed length can be changed is limited by the muscle’s attachments to bones. It rarely exceeds a 30 percent change from Lo and is often much less.  TYPES OF MUSCLE CONTRACTION ISOKINETIC ISOTONIC (we measure tension across the muscle) ISOMETRIC (meaning same length) CONCENTRIC ECCENTRIC Concentric contractions Eccentric contractions Isometric contractions occur when are those which cause are the opposite of there is no change in the length of the muscle to shorten concentric and occur the contracting muscle. as it contracts. when the muscle lengthens as it An example is when you grip An example is bending contracts. something, such as a tennis racket. the elbow from straight There is no movement in the joints to fully flexed, causing a of the hand, but the muscles are concentric contraction contracting to provide a force of the Biceps Brachii sufficient enough to keep a steady muscle. hold on the racket. In isotonic contraction, muscle tension stays the same, the length of the muscle changes as muscle contracts and pulls on the bone.  1. ISOTONIC (meaning same tension) CONCENTRIC CONTRACTION CONTRACTION = WHILE SHORTENING ECCENTRIC CONTRACTION WHILE CONTRACTION = LENGTHENING http://www.youtube.com/watch?v=T3OiOJ6-x34 1.Isotonic contractions are characterized by a consistent muscle tension as the contraction proceeds and a resulting movement of body parts. An arm curl with a 5 kg hand weight involves isotonic contractions throughout your arm. 2. Isometric contractions are characterized by a consistent muscle length throughout the contraction with no visible movement of body parts. An example of an isometric contraction is when you hold a hand weight at arm’s length in front of you; your arm is not moving, but you feel tension in your arm muscles ISOMETRIC CONTRACTION  The springlike characteristics of the protein titin, which is attached to the Z line at one end and the thick filaments at the other, is responsible for most of the passive elastic properties of relaxed muscles. With increased stretch, the passive tension in a relaxed fiber increases, not from active cross-bridge movements but from elongation of the titin filaments. ISOKINETIC contractions are similar to isotonic in that the muscle changes length during the contraction, where they differ is that isokinetic contractions produce movements of a constant speed.  An isokinetic muscle contraction happens when the muscle contracts and shortens at a constant and consistent rate of speed.  Muscle Hypertrophy Increase in total mass of a muscle – Enlargement of the individual muscle fibers – Increase the amount of force generated but has no effect on the maximal velocity of shortening – The enzyme systems that provide energy also increase – Load increase the hypertrophy – Testosterone When a muscle remains unused for many weeks, the rate of degradation of the contractile proteins is more rapid than the rate of replacement.  Hyperplasia of Muscle Fibers Increase in the number of muscle fibers Skeletal muscles have a limited ability to form new fiber the mechanism is linear splitting of previously enlarged fibers.  Muscle Atrophy Decrease in total mass of a muscle – When a muscle remains unused for many weeks – Space flight unloads muscles – Inhibition of protein synthesis – Stimulation of protein degradation When a muscle loses its nerve supply, it no longer receives the contractile signals that are required to maintain normal muscle size. Therefore, atrophy begins almost immediately. After about 2 months, degenerative changes also begin to appear in the muscle fibers. If the nerve supply to the muscle grows back rapidly, full return of function can occur in as little as 3 months, but from that time onward, the capability of functional return becomes less and less, with no further return of function after 1 to 2 years. Thank You