Muscle Physiology 2.Mechanical Properties of Striated Muscle (2024) PDF

Mechanical Properties of Striated Muscle PROF.DR.MİTAT KOZ Motor Unit: The Nerve-Muscle Functional Unit Each muscle has one motor nerve. Each motor neuron innervates several muscle fibers. Each branches of the axon terminals connect with a single muscle fiber. A motor neuron and all th...

Mechanical Properties of Striated Muscle PROF.DR.MİTAT KOZ Motor Unit: The Nerve-Muscle Functional Unit Each muscle has one motor nerve. Each motor neuron innervates several muscle fibers. Each branches of the axon terminals connect with a single muscle fiber. A motor neuron and all the muscle fibers it supplies is called a Motor Unit  The number of muscle fibers per motor unit can vary from a few to several hundred Muscles that control fine movements (fingers, eyes) have small motor units-fewer muscle fibers Large weight-bearing muscles (thighs, hips) have large motor units-more muscle fibers Stimulation of a single motor unit causes weak contraction of the entire muscle but, Stimulation of more than one motor unit leads to stronger contraction. TYPES OF SKELETAL MUSCLE CELL- FIBERS Skeletal muscle fibers do not all have the same mechanical and metabolic characteristics. Different types of fibers can be identified on the basis of (1) their maximal velocities of shortening or contracting fast or slow (2) the major pathway used to form ATP Oxidative(aerobic) or glycolytic(anaerobic). On the other hand, all the fibers in one motor neuron are same characteristics or same types. Three types of skeletal muscle fibres have been identified Type I: Slow Twitch;Slow-oxidative fibers contain low myosin-ATPase activity, but high oxidative capacity. Type II: Fast Twitch Type IIa: Fast Twitch; Fast-oxidative fibers contain high myosin-ATPase activity and high oxidative capacity. Type IIb: Fast Twitch;Fast-glycolytic fibers contain high myosin-ATPase activity with high glycolytic capacity. Type IIb As described earlier, whole muscles are made up of many muscle fibers organized into motor units. All the muscle fibers in a single motor unit are of the same fiber type. Most muscles are composed of all three motor unit types interspersed with each other.  Electromyography (EMG) The electrical activity of the skeletal muscle motor units can be measured with a recorder called 'electromyography' (EMG). These measurements can be made via disc electrodes outside the muscle or hypodermic needle electrodes. Various traces can be observed on the oscilloscope during resting or contraction of the muscle. EMG used in clinics for the diagnosis muscle and nervous disesase it is determined whether there is a defect in motor neuron, axon and muscle fibers. MECHANICS OF SINGLE-FIBER CONTRACTION; TENSION AND LOAD Tension ? The force exerted on an object by a contracting muscle is known as muscle tension, Load ? and the force exerted on the muscle by an object (usually its weight) is the load. Muscle tension and load are opposing forces. In order for muscle fibers to shorten, and thereby move a load, muscle tension must be greater than the opposing load. Muscle Twitch The mechanical response of a single muscle fiber to a single action potential is known as a muscle twitch. The fiber contract quickly and then relax. Single muscle fiber twitch develops in accordance with all or none law. Muscle Twitch can be divided 3 phase Latent Period is the first few ms after stimulation when excitation-contraction is occurring-there is no contraction Period of Contraction – cross bridges are active and the muscle shortens if the tension is great enough to overcome the load Period of Relaxation – Ca2+ is pumped back into SR and muscle tension decreases to baseline level Muscle Twitch; Latent Period Following the action potential, there is an interval of a few milliseconds known as the latent period before the tension in the muscle fiber begins to increase. During this latent period, the processes associated with excitation– contraction coupling are occurring. Muscle Twitch; Contraction Time The time interval from the beginning of tension development to the peak tension is the contraction time. Muscle Twitch Duration is not same in all muscles Not all skeletal muscle fibers have the same twitch contraction time. It can vary between 10- 100 msec. Twitch contraction of some muscles (extraocular) are rapid and brief, others (gastrocnemius, soleus) are slower and longer Contraction types Isometric Isotonic Eccentric Isometric contraction When a muscle develops tension but does not shorten (or lengthen), the contraction is said to be isometric (constant length). Such contractions occur when the load is greater than the tension developed by the muscle. They also called static contractions Isotonic Contraction A contraction in which the muscle shortens, while the tension on the muscle remains constant, is said to be isotonic (constant tension). Isotonic Contraction This illustrates a concentric isotonic contraction Shortening contractions are also referred to as concentric or dynamic contractions. In isotonic contractions, the amount of shortening (distance in mm) is measured Lengthening-Eccentric Contraction A third type of contraction is a lengthening contraction (eccentric contraction). This occurs when an unsupported load on a muscle is greater than the tension being generated by the cross-bridges. In this situation, the load pulls the muscle to a longer length in spite of the opposing force being produced by the cross-bridges. lengthening contraction (eccentric contraction) In an eccentric contraction, a muscle is extended (while active) by an external force. Examples for eccentric contraction; Activities such as descending stairs or landing from a jump utilize this type of contraction. Such contractions can be potentially dangerous because the muscle can experience forces that are higher than it could develop on its own contraction and tearing (strain) injuries can result. Muscle strain How are the graded muscle contractions developed? Single muscle fiber twitch cotracts in accordance with all or none law. Graded muscle contractions are; the variations in the degree or strength of muscle contraction in response to demand and required for proper control of skeletal movement Factors affecting development of muscle tension are Frequency of the stimulus Number of motor units recruited Degree of muscle length-stretch Frequency of the stimulus The duration of the action potential in a skeletal muscle fiber is short (about 5 msec) compared to the duration of a contraction. Action potential is electrical Twitch is mechanical This means that the absolute refractory period of the muscle is also short and the muscle fibre can be reactivated long before the muscle relaxes.  The contraction following the first stimulus, S1, lasts 150 msec. The second stimulus, S2, applied to the muscle fiber 200 msec after S1, when the fiber has completely relaxed, causes a second identical contraction. If a stimulus is applied before a fiber has completely relaxed, it induces a contractile response greater than that produced in a single contraction (S3 and S4). If the interval between stimuli is reduced further(frequency is increased), the resulting peak tension is even greater (S5 and S6).. The increase in muscle tension from successive stimulation is called as temporal summation. If stimuli are given more repeatedly and rapidly, the result is a smoot sustained contraction called a tetanus. A maintained contraction in response to repetitive stimulation is known as a tetanic contraction. At low stimulation frequencies, the tension may oscillate as the muscle fiber partially relaxes between stimuli, producing an incomplete (unfused) tetanus. A complete (fused tetanus), with no oscillations, is produced at higher stimulation frequencies  Because it involves events that occur close together in time, a tetanus is a form of temporal (wave) summation. What is the physiological bases of tetanic contractions ? When the second stimulus closely follows the first (even before force has begun to decline), the intracellular calcium concentration is still high, and the effect of the additional calcium ions is to increase the force and the duration of the contraction because a larger amount of calcium is present in muscle. Answ: Increases intracellular Ca ion concantrations. Graded Muscle Responses Factors affecting development of muscle tension: Frequency of the stimulus Number of motor units (fibers) recruited Degree of muscle stretch  The number of muscle fibers per motor unit can vary from a few to several hundred Muscles that control fine movements (fingers, eyes) have small motor units-fewer muscle fibers Large weight-bearing muscles (thighs, hips) have large motor units-more muscle fibers Stimulation of a single motor unit causes weak contraction of the entire muscle but, Stimulation of more than one motor unit leads to stronger contraction.  When a muscle is composed of small motor units, the total tension produced by the muscle can be increased in small steps by activating additional motor units. If the motor units are large, large increases in tension will occur as each additional motor unit is activated. Thus, finer control of muscle tension is possible in muscles with small motor units. motor unit recruitment. The process of increasing the number of motor units that are active in a muscle at any given time is called motor unit recruitment. The greater the number of active motor neurons, the more motor units recruited, and the greater the muscle tension. Motor neuron size Motor neuron size plays an important role in the recruitment of motor units. The size of a motor neuron refers to the diameter of the nerve cell body, which is usually correlated with the diameter of its axon. The smallest neurons will be recruited first—that is, will begin to generate action potentials first. Since the smal motor neurons innervate the large motor units, these motor units are recruited first.  In conclusion, the neural control of whole-muscle tension involves (1) the recruitment of motor units (to vary the number of active fibers). Motor unit recruitment (2) the frequency of action potentials in individual motor units (to vary the tension generated by the fibers in that unit) temporal (wave) summation Which is the primary cause of varying tension in an entire muscle?? Summation or motor unit recruitment Recall that the tension of a single fiber increases only three to five fold when going from a twitch to a maximal tetanic contraction. temporal (wave) summation can increase contraction only three to five fold. Motor unit recruitment is the primary cause of varying tension in the entire muscle. Recruitment is controlled by the central commands from the motor centers in the brain to the various motor neurons. Graded Muscle Responses Factors affecting development of muscle tension: Frequency of the stimulus Number of motor units recruited Degree of muscle stretch Degree of muscle stretch There is a muscle length and tension relationship. If the fiber is stretched and than it is released, its length will return to an resting length, much like releasing a stretched rubber band. This is called passive tension- there is no contraction. Active tension is produced by muscle contreaction. » If the muscle starts to contract in a stretched position, it can produce more tension. Total tension = Active tension + Passive tansion Degree of muscle stretch Titin is a non contractile protein which is attached to the Z line at one end and the thick filaments at the other,. The springlike characteristics of the protein titin is responsible for most of the passive elastic properties of relaxed muscles. Passive tension Optimal length ? If you stretch a muscle fiber to various lengths and stimulate it at each length, the magnitude of the active tension will vary with length. The length at which the fiber develops the greatest active tension is termed the optimal length(ol). OL for maximum force generetaion, For maximum force generetaion, OL of muscle fibre is approximately 80-120 % of it’s relaxed length. Load-Velocity Relation There is an inverse relation between load and shortening velocity (contraction speed). Light objects can be moved faster than heavy objects. That is, the velocity at which a muscle fiber shortens decreases with increasing loads. The shortening velocity is maximal when there is no load. Muscle Fatigue Muscle fatigue – the muscle is physiologically not able to contract Occurs when oxygen is limited and ATP production fails to keep pace with ATP use Lactic acid accumulation and ionic imbalances may also contribute to muscle fatigue When ATP is no available, cramps (continuous contraction) may result because cross bridges are unable to detach Heat Production During Muscle Activity Only 40% of the energy released in muscle activity is useful as work The remaining 60% is given off as heat Heat is dissipated by radiation of heat from the skin and sweating ADDITIONAL EXAMPLES of MUSCLE CONTRACTION  HYPERTROPHY AND ATROPHY IN SKELETAL MUSCLES If the skeletal muscles are given a resistance exercise of at least 70-80% of their maximal strength, even for a few minutes a day, the muscles will improve their structure and function within 6-10 weeks. This changes is called hypertrophy. In hypertrophy, actin and myosin filaments of the muscle increase. Muscle fiber diameter increases. In contrast, skeletal muscles undergoes the loss of structure, function and strength during prolonged inactivity. This is called atrophy.  ATROPHY OF DENERVATION If the neurons of the skeletal muscle are cut, the nerve-muscle junction becomes non-functional and the amount of contractile protein of muscle decreases, and the denervated muscle fibers become smaller. This is called denervation atrophy. Atrophy develops more rapidly in denervation. Protein breakdown, which starts in muscle fibers within a few weeks, reaches its maximum level in a few months. If reinnervation is not provided up to 1 year after denervation, contractile proteins in muscles are destroyed; replaces fat and connective tissue. This situation is irreversible. RIGOR MORTIS A few hours after death, the skeletal muscles partially contract, fixing the joints. This condition, called rigor mortis, may continue for seventy-two hours or more. It results from an increase in membrane permeability to calcium ions, which promotes cross-bridge attachment, and a decrease in availability of ATP in the muscle fibers, which prevents cross-bridge release from actin. Thus, the actin and myosin filaments of the muscle fibers remain linked until the muscles begin to decompose. MUSCLE CRAMPS Involuntary contraction of skeletal muscles produces muscle cramps. During cramping, nerve action potentials fire at abnormally high rates, a much greater rate than occurs during maximal voluntary contraction. The specific cause of this high activity is uncertain but it is probably related to electrolyte imbalances in the extracellular fluid surrounding both the muscle and nerve fibers. These imbalances may arise from over-exercise or persistent dehydration, and they can directly induce action potentials in motor neurons and muscle fibers. Hypocalcemic (Low extracellular Ca) Tetany Hypocalcemic tetany is the involuntary tetanic contraction of skeletal muscles that occurs when the extracellular Ca concentration decreases to about 40% of its normal value. Low extracellular Ca (hypocalcemia) increases the opening of Na channels in excitable membranes, leading to membrane depolarization and the spontaneous firing of action potentials. This causes the increased muscle contractions, which are similar to muscle cramping. Muscular Dystrophy Muscular dystrophy is a common genetic disease, affecting an estimated one in every 3500 males (but many fewer females). It is associated with the progressive degeneration of skeletal and cardiac muscle fibers, Weakening of muscles leads ultimately to death from respiratory or cardiac failure. Muscular dystrophy is caused by the absence or defect of one or more proteins in striated muscle. Duchenne muscular dystrophy It was named by French neurologist Guillaume- Benjamin-Amand Duchenne Duchenne muscular dystrophy is a sex- linked recessive disorder caused by a mutation in a gene on the X chromosome that codes for the protein dystrophin. Dystrophin was the first protein discovered to be related to a muscular dystrophy, Dystrophin is an extremely large protein that normally forms a link between the contractile filament actin and proteins embedded in the sarcolemma. In its absence, fibers subjected to repeated structural deformation during contraction are susceptible to membrane rupture and cell death. Duchenne muscular dystrophy Muscles of the hip girdle and trunk are the first to weaken, requiring patients to use their arms to “climb up” the legs in order to go from lying to standing.  Myasthenia Gravis Myasthenia gravis is a collection of neuromuscular disorders characterized by muscle fatigue and weakness. Myasthenia gravis affects more often in women than men. The most common cause is the destruction of nicotinic ACh-receptor proteins of the motor end plate, mediated by antibodies of a person’s own immune system The release of ACh from the axon terminals is normal, but the magnitude of In this disease, which is thought the end-plate potential is markedly reduced because of the decreased to play a role in hereditary availability of receptors. factors, autoimmune antibodies develop in the body, destroying nicotinic ACh receptors.  Myasthenia Gravis The difficulty of contraction in the extraocular eye muscles and other skeletal muscles of the body gradually increases, Difficulties in the eye movement and the patient may be lost due to respiratory (diaphragm muscle) failure.  Contracture Contracture is the collection of the free end of the muscle towards the origo. It can be a complication of stroke or amputation Due to the shrinkage of skeletal muscle proteins, this leads to shortening of the muscle's length and loss of myofibrils and myofilaments over time. It is difficult to return. Therefore, in paralysis and amputations, it is necessary to keep the muscles in long positions and prevent their shortening. Metabolic Myopathies (McArdle's Syndrome) They develop due to mutations of enzymes related with metabolism of carbohydrate, protein and lipits. There are impairments the production of endproducts of these metabolic pathways. CO2, H2O and ATP Glycogen is deposited in the muscles due to deficiencies glycogenolysis. As e result, muscle pain, muscle demolition, muscle stifness and execise intolerance develops due to accumulated toxic metabolites