Physiology Chapter 12: Muscle contractions and response

Phases of a twitch

1. Latent period 2. Contraction 3. Relaxation

Factors affecting muscle force

1. The force generated by individual muscle fibers: Depends on the number of active crossbridges formed within the muscle fiber. The more crossbridges, the greater the force. 2. The number of muscle fibers that contract at the same time: Muscles contract together in *motor units*. The more muscles that contract at the same time, the more crossbridges are active, and the greater is the force.

Motor unit

A motor neuron and all the muscle fibers it innervates are collectively called a motor unit. • An action potential in a motor neuron triggers contraction of all the muscle cells that are connected to that neuron. • Large motor units control more muscle cells, and therefore more force.

Eccentric contraction

An isotonic contraction that causes muscles to elongate in response to a greater opposing force. • ex: lowering a dumbbell back down causes an eccentric contraction

Concentric contraction

An isotonic contraction that causes muscles to shorten, thereby generating force • Ex: lifting a dumbbell causes a concentric contraction in your bicep

Fiber length

Another factor that affects the formation of crossbridges and force generated by muscle fibers. • There is an ideal length for sarcomeres such that myosin and actin participate at the highest level. • If the fibers length differs from the ideal, it loses some of its ability to generate force. • This phenomenon is illustrated on the *length-tension curve* • The optimal length is between 100% to 120% of the sarcomeres resting length

Frequency of stimulation

Crossbridges and force are affected by the amount of calcium released from the SR and the frequency of stimulation • As stimulation occurs more frequently, there is less time for the cytosolic calcium to return to the SR after the twitch, leading to a greater contractile force due to *summation* which occurs when additional action potentials arrive before the twitches are completes. • Although a single muscle twitch is an all-or-nothing event, when twitches occur back to back, the effects of the two twitches add up as cytosolic calcium concentration increases. • The second contraction will generate less tension that the first contraction. • Two twitches have a summed effect called a treppe

Recruitment of motor units based on muscle fiber type

During exercise, individual motor units are recruited based on the size principle. Order of recruitment: 1. Slow oxidative fibers, located in the smaller motor units. 2. Fast oxidative-glycolytic fibers, located in intermediate motor units 3. Fast glycolytic fibers, located in the large motor units.

Adaption to exercise: endurance training

Endurance exercises enhance the oxidative capacity of fibers and greatly reduce their chances of becoming fatigued. • The muscles of marathon runners will have more numerous, larger mitochondria than the average person, but will have a decreased glycolytic capacity as those fibers have been converted to fast oxidative fibers. • Fibers will be smaller in diameter. • Over time Type IIx fibers are converted to Type IIa fibers.

Differences in Speed of contraction: fast-twitch fibers and slow-twitch fibers

Fast and slow twitch fibers generally differ in the myosin that is present. • The heads of myosin contain ATPase which dephosphorylates ATP. • Myosin ATPase enzymes have different catalytic rates as some catalyze reactions more quickly than others. • In fast twitch cells, fast-type myosin ATPase is found, and in slow twich cells, slow-type myosin ATPase is found.

Differences in the primary mode of ATP production: Glycolytic fibers and oxidative fibers

Glycolytic fibers have a very high concentration of cytosolic glycolytic enzymes, so they are bale to generate ATP rapidly though glycolysis. • They are less capable of generating ATP through oxidative phosphorylation because they do not contain many mitochondria. Oxidative fibers have numerous mitochondria and can generate ATP easily though oxidative phosphorylation, although this is countered by a low glycolytic capacity. In a stain micrograph, the deeper color stained cells are usually oxidative fibers.

Adaption to exercise: Intensity training

High intensity exercise over a short duration increases the glycolytic capcity of fibers but lower their oxidative capacity. • These muscles have fewer capillaries and mitochondria but an increased concentration of glycolytic enzymes • They also increase in diameter over time, which explains why weight lifters are more bulky. • Over time, Type IIa fibers and converted to Type IIx fibers.

Maximum tetanic tension

If the stimulus intensity is increased past a complete tetanus, tetanic tension increases, but only up to a point; further increases in frequency beyond this point yield no further increase in force. • Under these conditions, the muscle is generating all the force it can, which is referred to as maximum tetanic tension.

Isometric contractions

In an *isometric contraction*, the muscle remains the same length. • All-or nothing event • For example, if you try to life an object that you are physically unable to life, your muscle contracts but does not change in length. • Because the load is too heavy, shortening does not occur even though the force is still generated • Thus, when a load exceeds the amount of force the muscle can generate, the muscle cannot move it and therefore contracts isometrically.

Isotonic contractions

In an *isotonic contraction*, the tension in the muscle remains constant but the muscle changes in length. • Only possible when the muscle's maximum force of contraction is greater than the total load on the muscle. • Curve for the isotonic twitch shoes a distinct plateau, indicating that the force remains constant for a period of time. • NOT an all or nothing event. It depends on the size of the load that is placed on the muscle. • Two types of isotonic contraction: concentric and eccentric

Sarcomeres in isometric contractions

In an isometric contraction, the *contractile components* (the sarcomeres in a myocyte) still shorten even though the entire muscle does not. • This is made possible by *series elastic components* in the myocytes, which don't contraction (and do not produce tension), but are slightly elastic and therefore transfer tension to the ends of the muscle cell. • The tendons attached to the muscle constitute the majority of the series elastic components. • when the sarcomeres shorten in an isometric contraction, the series elastic elements stretch out so that the entire muscle does not shorten.

Two kinds of twitched

Isometric and Isotonic • Key difference between the two is that the muscle is allowed to shorten in isotonic contractions, but not in isometric.

Muscle spindles (stretch sensors)

Muscle spindles contain two to twelve modified muscle fiber that are arranged to run parallel to the extrafusal fibers (the skeletal muscle cells). • They are located underneath the extrafusal fibers, so they are referred to as *intrafusal fibers* • The combination of the intrafusal fibers with their sensory axons is a muscle spindle. • The sensory axons associated with the muscle spindle are Type Ia afferent and Type II afferent. • The centers of muscle spindles do not have striations, but their outer tips do. These striations are sarcomeres, which are contractile . • This is why the ends of this type of intrafusal fiber have moderate tension generating abilities. • Stretch receptors are innervated by gamma motor neurons and 1A afferent nerve fibers. The stretching of muscle spindles results in the activation of alpha motor neurons, which innervate the extrafusal fibers and cause a muscle contraction • Muscle spindles are responsible for the stretch reflex, or the myotatic reflex. Muscle spindles go slack when muscles shorten. • This would usually makes them ineffective as stretch receptors, however, gamma motor neurons fire at the same time as alpha motor neurons, causing the muscle spindles to contract and "reset" so that they can appropriately detect changes in the length of the muscle regardless of the extent to which the extrafusal fibers are contracted.

Myonuclear domain

Refers to the volume of cytosol associated with each nucleus in a skeletal muscle cell. • As the cell get larger, the myonuclear domain gets larger as well. • Therefore, the skeletal muscle cell has to take on new nuclei in order to keep the size of each myonuclear domain roughly constant. • It does so by recruiting a satellite cell to insert itself into the plasma membrane and become a new nucleus.

Research on contractions

Researchers can analyze the contraction of a single muscle fiber in the laboratory by removing a single muscle cell and anchoring it to an experimental device. • An electrical stimulator can then be used to "zap" the muscle cell and cause a contraction, and the tension produced by the contraction can be measured with a *force transducer*. • The change in the lengths of the muscle fiber could be measured with a *length transducer.

Load velocity curve

Shows the relationship between the velocity of muscle shortening and the size of the load. • As you might expect, the velocity of shortening is higher when the load is smaller. • Makes sense as you can lift smaller weight faster. • As size of load increases, shortening velocity decreases.

Types of skeletal muscle fibers

Skeletal muscle fibers can be divided into three types based on how they manufacture energy (Glycolytic versus oxidative) and how quickly they contract (slow or fast). • Slow-oxidative (Type 1) muscle fibers • Fast oxidative-glycolytic (Type IIa) muscle fibers • Fast glycolytic (Type IIb/IIx) muscle fibers

Motor coordination: actions at joints

Skeletal muscles are attached to bones by tendons, which are continuation of a muscle tissue sheath that transfers muscle force to nones and causes movement in joints. • Flexion: What causes the movement, agonist, is the bicep, the triceps is the antagonist • Extension: What causes the movement, antagonist, is the bicep, the triceps is the agonist • There are *sensory receptors* and associated *afferent axons* within the muscles that communicated movements to the CNS • Muscle spindles: detects lengths • Tendon organs detect tension, feedback of how much force is being produced.

Protective sensorys

Skeletal muscles maintain protective sensors called *muscle spindles* and *golgi tendon organs (GTO)* which measure length and muscle tension, respectively. • This is important because it provides information to the central nervous system and helps it prevent the muscles from over stretching and tearing

Phases of a twitch: Contraction

Starts at the end of the latent period and ends when the muscular tension peaks • This stage begins with the crossbridge cycling and the rising levels of cytosolic calcium • During this phase, cytosolic calcium levels are increasing as the release of these ions exceed their reuptake

Creatine Phosphate System

The ATP that powers muscle cell contraction is produced by substrate-level phosphorylation and oxidative phosphorylation. • When muscles need ATP produced at a high rate at contraction, they rely on an immediately available store of high-energy phosphate that is present in the form of *creatine phosphate*, a compound that donates its phosphate to ADP (which is always present) to form ATP Creatine phosphate + ADP =* creatine + ATP *Catalyzed by creatine kinase • When muscle activity begins, ATP levels fall and ADP levels rise, driving the creatine reaction. • Creatine phosphate can be reestablished after exercise is finished

Factors affecting force generated by individual muscle fibers

The force generated in a muscle depends on two factors: (1) the force generated in the individual muscle fibers and (2) the number of muscle fibers contracting. • These depend on the active number of crossbridges. More cross bridges, mean more force.

Phases of a twitch: Relaxation

The longest of the three phases • This is the time between the peak tension and the end of the contraction, when tension returns to zero. • During this time, cytosolic calcium is returned to the sarcoplasmic reticulum and the number of cross-bridges declines, relaxing the muscle. • Longest because calcium is moving against its concentration gradient when it is actively pumped back into the SR from the cytosol.

Muscle twitch

The mechanical response of an individual muscle cell, group of cells, or an entire muscle to a single action potential • Like an action potential, a twitch is an all-or-nothing event, producing the same amount of contraction each time it is stimulated because the action potential produces the same amount of depolarization, and the same amount of calcium is released from the SR. • It is also a reproducible event. • Not all muscle twitches are the same in terms of time and amplitude, affected by both the diameter of the muscle cell as well as the type of myosin ATPase at the head of the myosin. • Fast myosin ATPas hydrolyzes ATP faster than slow myosin ATPase, and therefore generates cross bridges and tension more quickly than slow myosin ATPase

Skeletal muscle metabolism

The mechanism for producing ATP in a myocyte will vary depending on the type and the intensity of work • This is based on how muscle cells generate ATP & how muscle cell metabolism changes with exercise

Phases of a twitch: Latent period

The millisecond time delay between the action potential and the initiation of the contraction, when the cell first begins to generate force. • This includes the time it takes for the impulse to travels down the nerve, ACh to be released, and all of the reactions that muscle occur before the filaments begin to slide. • Thus, events of the excitation-contraction coupling must occur before crossbridge cycling, and force generation, can begin. • No mechanical response occurs during the latent period.

Fiber diameters

The most important variable in determining the capacity of a muscle fiber to generate force. • Muscles with larger diameters have more sarcomeres in parallel, so they have more myosin and actin. • This means that they have a greater capacity to form crossbridges, so they can generate more force.

Fast glycolytic (Type IIb/IIx) muscle fibers

These fibers contract quickly and primarily produce ATP anaerobically, though glycolysis. • These fibers shorten quickly, but they have very low fatigues resistance. • They are typically white due to their low myoglobin content (myoglobin is not necessary as they do not need oxygen) • Used in body builders

Fast oxidative-glycolytic (Type IIa) muscle fibers

These muscle fibers contract quickly and use both anaerobic and aerobic metabolic processes for ATP production. • Their myoglobin content, fatigue resistance, pinkish color, and fiber diameter lie between the slow oxidative and fast glycolytic muscle fibers. • They are primarily used for walking or sprinting.

Slow oxidative (Type 1) muscle fibers

These muscle fibers contract slowly and primary produce ATP aerobically. • Although these fibers shorten slowly, they have high fatigue resistance, as they can continue to contract for a long time without fatigues. • Because they rely on aerobic respiration, they have a high content of *myoglobin*, the red pigment that carries oxygen, giving these muscle fibers a dark red color. • Type 1 muscle fibers also have very small diameters. (This is why endurance runners are very skinny, as they have more Type 1 muscle fibers)

Regulation of the force generated by whole muscles

To control the amount of force generated overall by a muscle, the number of active motor units can be increased or decreased. • An increase in the number of active motor units is known as *recruitment*. We ALWAYS recruit the smaller motor units first. • *Size principle* is a general rule that larger motor units and controlled by larger somatic motor neurons, and vice versa. Large stimulus is necessary to depolarize large somas.

Golgi tendon organs (tension sensors)

Unlike muscle spingles, they do not run parallel to skeletal muscles. • Instead, GTOs are arranged in series, and are innervated by the types 1B afferent nerve fibers, but not by any motor neurons. • This is because there are no contractile elements associated with GTOs The job of the GTOs is the sense the tension being placed on a tendon and respond accordingly to prevent injury. • GTO stimulation leads to alpha motor neuron inhibition and muscle relaxation (opposite of stretch receptors) • GTOs also detect high and low levels of contraction as well as passive stretch of muscles.

Metabolism changes with intensity

When a muscle exercised at light to moderate intensity, more ATP is supplied by oxidative phosphorylation. • Initially, muscles use their glycogen stores to supply glucose for ATP. • As exercise continues, muscles shift to use glucose and fatty acids delivered to them by the blood. • After about 30 minutes, glucose utilization decreases, and fatty acids become the dominant energy source. Creatine phosphate utilized quickly (30s - 1min) Glycolysis kicks in and peaks 1-2 min Oxidative phosphorylation is last energy source (steady state at about 3-4 min) Glycolysis produces a whole lot during intense exercise and thus it does not last that long. In heavy exercise, oxidative phosphorylation becomes less important for ATP, and substrate-level phosphorylation becomes more important. • This uses lactic acid

Summation

When a muscle is stimulated repetitively such that additional action potentials arrive before twitches can complete, the twitches become surperimposed on one another, yielding a force greater than that of a single twitch • This is called *summation*

Muscle growth

when muscles hypertrophy (get bigger, though growth or exercise) they do so by adding additional myofibrils rather than adding new cells. • Damaged muscles also add new myofibrils • *myosatellite cells* (also called satellite cells) help with repair and growth • *Myonuclear domain* assists in muscle growth as well

Tetanus

• At higher frequencies of stimulation, summation reaches a peak in contraction called a *tetanus* will result. • Incomplete and complete tetanus

Isotonic shortening

• Shortening does not begin until enough crossbridges have formed to match the load, which is why there is a latent period. • It is only when the force becomes equal to the load does the muscle begin to shorten. • Once enough tension has been produced to match the force of the load, a constant tension is maintained and the muscle shortens.

Incomplete versus complete tetanus

• When two twitches are sequential, the total muscle tension rises, but there is a subsequent decrease in muscle tension. This is called an *incomplete tetanus* • When twitches are sent in rapid succession, the overall muscle tension is maintained at a high level, known as a *complete tetanus*. Here, you cannot distinguish between individual twitches.

Treppe

•Twitches occurring back to back have a summed effect called a treppe • NOT summation, as the force comes all the way down to zero as Ca has enough time to go back into SR. • The treppe occurs at a frequency where independent twitches follow one another closely such that the peak tension rises in a stepwise fashion with each twitch, until eventually reaching a constant level. • The cause of this additive effect is unknown and the tension will continues to rise with each twitch until it reaches a plateau.