Muscle Tissue Physiology - Skeletal Muscle Length-Tension Relationship and Twitch PDF

MUSCLE TISSUE PHYSIOLOGY - SKELETAL MUSCLE LENGH-TENSION RELATIONSHIP AND TWITCH Dr Mungai Ruth LEARNING OUTCOMES 1. Describe the relationship between motor unit activation and force generation 2. Compare and contrast isotonic and isometric contraction. 3. Compare and contrast c...

MUSCLE TISSUE PHYSIOLOGY - SKELETAL MUSCLE LENGH-TENSION RELATIONSHIP AND TWITCH Dr Mungai Ruth LEARNING OUTCOMES 1. Describe the relationship between motor unit activation and force generation 2. Compare and contrast isotonic and isometric contraction. 3. Compare and contrast concentric and eccentric contraction. 4. Interpret a graph of the length-tension relationship and describe the anatomical basis for that relationship. 5. Describe the three phases of a muscle twitch 6. Describe wave summation, tetanus, treppe,tone MOTOR UNIT Each muscle fiber is innervated by only one motor neuron but a single motor neuron can innervate multiple muscle fibers. The contraction of skeletal muscle fibers is triggered by signaling MOTOR UNIT ACTIVATION AFFECTS FORCE GENERATION The size of a motor unit dictates its function. Gastrocnemius 2,000 muscle fibers per motor neuron. Large motor units Extraocular muscles Less than 10 muscle fibers per motor neuron. A small motor unit Ratio of muscle fibres to motor neurons affects the precision of movement: If the ratio is low the action is specific ( more precise ) If the ration is high the action is less specific ( less precise ) precise movements muscle consist of a large number of motor units and few muscle fibers in each motor unit e.g. Hand and eye muscles. MOTOR UNIT ACTIVATION AFFECTS FORCE GENERATION During contraction, the force exerted by a muscle depends on: 1. how many motor units are recruited 2. how frequently each of the active motor neurons fires action potentials. This orderly recruitment of motor units is called the size principle because the motor units are recruited in order of motor neuron axon size. Small motor neurons are more excitable, conduct action potentials more slowly, and excite fewer fibers that tend to be slow twitch (type I). Large motor neurons are less excitable, MOTOR UNIT ACTIVATION AFFECTS FORCE GENERATION To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system thus uses recruitment as a mechanism to efficiently utilize a skeletal muscle where additional motor units are added to produce more tension. This process allows a muscle such as the biceps to pick up a feather with minimal MOTOR UNIT ACTIVATION AFFECTS FORCE GENERATION To move an object(a load) the muscle fibers of a skeletal muscle must shorten. The force generated by a contracting muscle is called muscle tension and is produced internally within the sarcomeres, Muscle tension is transmitted to bone as the contractile component tightens the series-elastic component. All muscle contractions shorten muscles and move bones. For a muscle to shorten during contraction, the tension developed in the muscle must exceed the MOTOR UNIT ACTIVATION AFFECTS FORCE GENERATION Muscle contraction can only pull and not push bone, two different antagonistic muscles or muscle groups are situated to pull on opposite sides of the joint. For example, the biceps can pull the joint in one direction (flexion) and the triceps can pull the joint in the other direction (extension). Muscle tension can be generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions isometric contractions ISOTONIC Isotonic contraction occurs when the CONTRACTIONS force of the muscle contraction is greater than the load, and the tension on the muscle remains constant during the contraction. When the muscle contracts, it shortens and moves the load. A muscle contracts and the joint angle it controls increases and decreases while the muscle either shortens or lengthens. Concentric contraction: The muscle fibre shortens when force is greater than load. Think of holding a weight with your elbow straight then bending it to 90 degrees. Your bicep has just performed a concentric contraction Eccentric contraction: The muscle is lengthen ISOTONIC Eccentric strengthening is an effective way CONTRACTIONS to rehabilitate a weak muscle following surgery. For instance, many people cannot perform a straight leg raise following knee surgery because the quadriceps has basically shut down. We work instead on eccentrically lowering the leg with control, and before you know it, the person can begin lifting the leg up! Another use of eccentric contractions in physical therapy is during the treatment of tendinopathies. Slow, controlled heel drops or decline board squats are great for treating Achilles tendon and patellar tendon symptoms, respectively. The benefits of eccentric exercises are due to increased collagen synthesis and tendon remodeling found during force fluctuations. The drawback of heavy eccentric training is it is more likely to produce muscle soreness, both during and after exercises. ISOMETRIC CONTRACTION CONTRACTIONS Isometric contraction occurs when the load is greater than the force of the muscle contraction; the muscle creates tension when it contracts, but the overall length of the muscle does not change. Isometric contractions are important for maintaining posture (such as keeping the legs stiff while standing) and for supporting objects in a fixed position (such as holding a beverage between sips). Groups examples of isometric and isotonic contractions ISOTONIC AND ISOMETRIC CONTRACTION LENGTH-TENSION RELATIONSHIP When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension(force) of the muscle contraction. The cross-bridges can only form where thin and thick filaments overlap; thus, the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship. LENGTH-TENSION RELATIONSHIP The ideal length of a sarcomere to produce maximal tension occurs at 80 percent to 120 percent of its resting length. This length maximizes the overlap of actin-binding sites and myosin heads. If a sarcomere is stretched past the ideal length (beyond 120 percent), thick and thin filaments do not fully overlap, which results in less tension produced. If the muscle is stretched to the point where the thick and thin filaments do not overlap at all, no cross-bridges can be formed, and no tension is generated. This amount of stretching does not usually occur as accessory proteins and connective tissue oppose extreme stretching. If a sarcomere is shortened beyond 80 percent, the zone of overlap is reduced with the thin filaments jutting beyond the last of the myosin heads. Eventually, there is nowhere else for the thin MUSCLE TWITCH Muscle twitch, is a brief and transient contraction of a muscle fiber. For a twitch to occur, the stimulus must surpass a threshold, Action potentials are all-or-nothing responses, meaning that they either occur, or they don't. For instance, a smaller stimulus won't result in a smaller action potential. A single action potential from a motor neuron will produce a single twitch A twitch can last anywhere from a few milliseconds to 100 milliseconds, depending on the muscle fiber type. Fast twitch muscles such as those which move the eyeball have twitches which reach maximum contraction in 3 to 5 ms (milliseconds) have small motor units for precise control. slow twitch muscles like the postural muscles (e.g. back muscles, soleus) have twitches which reach maximum tension in 40 ms or so. intermediate twitch lengths of 10 to 20 ms most muscles which exhibit most of our body movements have it. MUSCLE TWITCH a single twitch can be measured by a myogram and it has has 3 phases Latent period a brief delay between the arrival of the action potential and the observable tension in the muscle. During this period, several processes occur, including the release of calcium ions from the sarcoplasmic reticulum, the binding of calcium to troponin, and the formation of cross-bridges between actin and myosin filaments Contraction phase characterized by the generation of tension in the muscle as the cross-bridges cycle and interact.occurs as the muscle generates increasing levels of tension; the Ca++ ions in the sarcoplasm have bound to troponin, tropomyosin has shifted away from actin-binding sites, cross-bridges have formed, and sarcomeres are actively shortening. Relaxation phase, when tension decreases as Ca++ ions are pumped out of the sarcoplasm back into the sarcoplasmic reticulum, the muscle returns to its initial length after contraction resting state. SUMMATION A single twitch does not produce ‘useful’ activity in a living body Instead we use graded contractions/ graded muscle response by varying the rate at which a motor neuron fires action potentials Muscle fibers within a motor unit can produce graded contractions by undergoing summation the process by which multiple twitches or muscle contractions combine to produce a more forceful contraction. There are two types of summation: Temporal /wave summation: This occurs when successive stimuli are applied to a muscle fiber before it has completely relaxed from the previous twitch. The second stimulus triggers a stronger contraction because the muscle fiber has not fully returned to its resting state. TETANUS Refers to a state of sustained muscle contraction or a prolonged series of contractions that occur when the motor nerve signals are delivered at a high frequency. Tetanus can be categorized into two types: Incomplete tetanus:Muscle fibers are stimulated at a high frequency but still have some relaxation time between contractions. This results in a sustained, yet oscillatory, contraction. Complete tetanus: If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous with no relaxation time between contractions.The concentration of Ca++ ions in the sarcoplasm allows virtually all of the sarcomeres to form cross-bridges and shorten, so that a contraction can continue uninterrupted (until the muscle fatigues and can no longer produce tension). Tetanic contractions are important for activities that require sustained muscle force, such as maintaining posture or gripping objects tightly. TREPPE When a skeletal muscle has been dormant for an extended period and then activated to contract, with all other things being equal, the initial contractions generate about one-half the force of later contractions. The muscle tension increases in a graded manner that to some looks like a set of stairs. This tension increase is called treppe, a condition where muscle contractions become more efficient. It’s also known as the “staircase effect” (Figure 5). It is believed that treppe results from a higher concentration of Ca++ in the sarcoplasm resulting from the steady stream of signals from the motor neuron. It can only be maintained with adequate ATP. https://www.youtube.com/watch?v=Jg8uz8hAUmM TONE Skeletal muscles are rarely completely relaxed, or flaccid. Even if a muscle is not producing movement, it is contracted a small amount to maintain its contractile proteins Even relaxed muscles are constantly being stimulated to produce muscle tone, the minimal graded contraction possible, continuous random firing /partia contraction of motor units in the muscle at rest that keeps them in a state of readiness. This muscle tone helps take up the “slack” in the muscle, allowing for immediate tension generation when the muscle is called upon to contract. It also helps prevent muscle atrophy, as the continuous activation and stimulation of muscle fibers help maintain their integrity and functionality. Muscle tone is accomplished by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, most likely in a cyclical manner. The absence of the low-level contractions that lead to muscle tone is referred to as hypotonia or atrophy, and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and display functional impairments, such as weak reflexes. Flaccid paralysis can occur when the connection between a motor neuron and muscle fibers is disrupted. In this condition, there is a loss of muscle tone, resulting in a limp and weakened muscle state. Conversely, excessive muscle tone is referred to as hypertonia, accompanied by hyperreflexia (excessive reflex responses), often the result of damage to upper motor neurons in the CNS. Hypertonia can present with muscle rigidity (as seen in Parkinson’s disease) or spasticity, a phasic change in muscle tone, where a limb will “snap” back from passive stretching (as seen in some strokes). NEUROMUSCULAR ABNORMALITIES Muscle Spasm A disturbance to the normal flow of information in the CNS caused by diseases, infections, toxins, and injuries can lead to disturbances ranging from spasms to paralysis. Results from violent and painful involuntary muscle contraction usually caused by muscle overstretching, joint wrenching, and tendon or ligament tearing. When this happens, the injured area floods sensory impulses to the spinal cord and it responds by eliciting intense muscle contraction. Pain from muscle spasms is due to lactic acid accumulation that occurs when blood flow is cut off during contractions. Sensory impulses continue to flood and a vicious cycle of contraction develops. Spasticity Muscle spasticity occurs when damaged neurons are within the CNS rather than the peripheral areas. The site of damage makes this abnormality permanent. There is an interruption in the balance of excitatory and inhibitory influences within the CNS which can lead to hypertonia (excessive muscle stimulation) and consequent contractures and structural changes. There is no loss of coordinated muscle activity. MUSCLE RELAXANTS WATCH:https://youtu.be/ 6noV8AHcM6E?t=406  Watch:https://mediaspace.illinois.edu/media/t/ 1_knslni9m