Occurs When the Muscle Does Not Get a Chance to Relax Completely and Contracts Again

SKELETAL Muscle: WHOLE Musculus PHYSIOLOGY

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MOTOR UNITS

Image fatigued past BYU-I student Nate Shoemaker Spring 2016

The motor neurons that innervate skeletal muscle fibers are chosen blastoff motor neurons. Equally the alpha motor neuron enters a muscle, it divides into several branches, each innervating a muscle fiber (note this in the image above). 1 blastoff motor neuron along with all of the musculus fibers it innervates is a motor unit . The size of the motor unit correlates with the function of the musculus. In muscles involved with fine, coordinated command, the motor units are very small with three-5 muscle fibers per motor neuron. Muscles that control heart movement and muscles in our hands take relatively small motor units. On the other hand in muscles involved with more than powerful but less coordinated deportment, like the muscles of the legs and back, the motor units are big with 1000s of muscle fibers per motor neuron.

Musculus TWITCH

Championship: File:1012 Muscle Twitch Myogram.jpg; Author: OpenStax College; Site:https://commons.wikimedia.org/wiki/File:1012_Muscle_Twitch_Myogram.jpg; License: This file is licensed under the Creative Commons Attribution three.0 Unported license.

When an action potential travels downwards the motor neuron, it will result in a contraction of all of the muscle fibers associated with that motor neuron. The contraction generated by a single action potential is called a musculus twitch. A unmarried musculus twitch has iii components. The latent menstruation, or lag phase, the contraction phase, and the relaxation phase. The latent period is a short delay (1-2 msec) from the time when the action potential reaches the musculus until tension can be observed in the muscle. This is the time required for calcium to diffuse out of the SR, bind to troponin, the motion of tropomyosin off of the active sites, germination of cross bridges, and taking up any slack that may be in the musculus. The contraction phase is when the muscle is generating tension and is associated with cycling of the cantankerous bridges, and the relaxation phase is the time for the muscle to return to its normal length. The length of the twitch varies betwixt unlike musculus types and could be as short as 10 ms (milliseconds) or as long as 100 ms (more on this later).

If a muscle twitch is just a single quick contraction followed immediately past relaxation, how do we explicate the smooth continued movement of our muscles when they contract and move bones through a large range of move? The answer lies in the ordering of the firing of the motor units. If all of the motor units fired simultaneously the entire musculus would quickly contract and relax, producing a very hasty motion. Instead, when a musculus contracts, motor units burn down asynchronously, that is, one contracts then a fraction of a 2d after another contracts before the beginning has time to relax and so some other fires and so on. So, instead of a quick, jerky movement the whole muscle contraction is very smooth and controlled. Even when a muscle is at rest, there is random firing of motor units. This random firing is responsible for what is known as musculus tone. So, a musculus is never "completely" relaxed, fifty-fifty when comatose. However, if the neuron to a muscle is cutting, there will exist no "muscle tone" and this is called flaccid paralysis. There are several benefits of musculus tone: First it takes upward the "slack" in the muscle so that when information technology is asked to contract, it tin immediately brainstorm to generate tension and move the limb. If you lot have ever towed a automobile you know what happens if you don't take the slack out of the tow rope earlier starting to pull. The 2nd thing muscle tone does is deter musculus cloudburst.

TYPES OF MUSCLE Wrinkle

Muscle contractions are described based on two variables: force (tension) and length (shortening). When the tension in a muscle increases without a corresponding change in length, the wrinkle is chosen an isometric contraction (iso = same, metric=length). Isometric contractions are important in maintaining posture or stabilizing a articulation. On the other mitt, if the muscle length changes while muscle tension remains relatively constant, and then the contraction is chosen an isotonic contraction (tonic = tension). Furthermore, isotonic contractions tin be classified based on how the length changes. If the muscle generates tension and the entire muscle shortens than it is a concentric contraction. An example would exist curling a weight from your waist to your shoulder; the bicep muscle used for this movement would undergo a concentric contraction. In contrast, when lowering the weight from the shoulder to the waist the bicep would too be generating force but the muscle would be lengthening, this is an eccentric wrinkle. Eccentric contractions work to decelerate the motility at the joint. Additionally, eccentric contractions can generate more force than concentric contractions. Call back about the large box you take downward form the top shelf of your cupboard. Yous tin can lower it nether total command using eccentric contractions but when you try to render it to the shelf using concentric contractions you lot cannot generate enough strength to lift it back up. Force training, involving both concentric and eccentric contractions, appears to increment muscle strength more than just concentric contractions alone. Still, eccentric contractions crusade more harm (violent) to the musculus resulting in greater muscle soreness. If yous take e'er run downhill in a long race and and so experienced the soreness in your quadriceps muscles the adjacent day, you lot know what nosotros are talking about.

Muscle size is determined past the number and size of the myofibrils, which in turn is determined by the corporeality of myofilament proteins. Thus, resistance training will induce a cascade of events that result in the production of more than proteins. Often this is initiated by small, micro-tears in and around the the muscle fibers. If the tearing occurs at the myofibril level the muscle will reply past increasing the amount of proteins, thus strengthening and enlarging the muscle, a phenomenon called hypertrophy. This tearing is thought to account for the musculus soreness we experience after a workout. As mentioned to a higher place, the repair of these minor tears results in enlargement of the muscle fibers but information technology besides results in an increase in the amount of connective tissue in the muscle. When a person "bulks upward" from weight training, a significant percent of the increase in size of the muscle is due to increases in the amount of connective tissue. It should be pointed out that endurance preparation does not result in a significant increment in muscle size simply increases its ability to produce ATP aerobically.

FACTORS THAT INFLUENCE THE FORCE OF MUSCLE Wrinkle

Apparently our muscles are capable of generating differing levels of force during whole muscle wrinkle. Some actions crave much more than force generation than others; think of picking up a pencil compared to picking up a bucket of water. The question becomes, how tin can unlike levels of strength be generated?

Multiple-motor unit summation or recruitment: It was mentioned before that all of the motor units in a muscle usually don't burn down at the aforementioned time. One way to increment the amount of force generated is to increase the number of motor units that are firing at a given fourth dimension. We say that more than motor units are beingness recruited. The greater the load nosotros are trying to movement the more motor units that are activated. However, fifty-fifty when generating the maximum force possible, nosotros are just able to use nearly 1/3 of our full motor units at i fourth dimension. Unremarkably they will fire asynchronously in an effort to generate maximum forcefulness and forbid the muscles from becoming drawn. Every bit fibers begin to fatigue they are replaced by others in club to maintain the force. There are times, still, when nether farthermost circumstances we are able to recruit even more motor units. Y'all have heard stories of mothers lifting cars off of their children, this may not be totally fiction. Watch the following clip to run into how astonishing the human trunk can exist. Muscle recruitment. (Video Transcription Available)

Title: 1013_Summation_Tetanus.jpg; Author: OpenStax; Site: http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@6.6:67/Anatomy-&-Physiology; License: This piece of work is licensed by Rice Academy under a Creative Eatables Attribution License License ( 3.0).

Wave summation: Think that a muscle twitch can last upwards to 100 ms and that an action potential lasts merely 1-2 ms. Also, with the muscle twitch, at that place is not refractory period so it tin be re-stimulated at any time. If you were to stimulate a unmarried motor unit with progressively college frequencies of activeness potentials you lot would observe a gradual increase in the force generated by that musculus. This miracle is chosen wave summation. Eventually the frequency of action potentials would exist so high that there would be no time for the muscle to relax between the successive stimuli and it would remain totally contracted, a status called tetanus. Essentially, with the high frequency of action potentials there isn't fourth dimension to remove calcium from the cytosol. Maximal force, so, is generated with maximum recruitment and an action potential frequency sufficient to event in tetanus.

Title: 1011_Muscle_Length_and_Tension.jpg; Author: OpenStax; Site: http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@6.6:67/Anatomy-&-Physiology; License: This work is licensed by Rice University under a Creative Commons Attribution License License ( three.0).

Initial Sarcomere Length: It has been demonstrated experimentally that the starting length of the sarcomere influences the amount of force the muscle tin can generate. This observation has to do with the overlap of the thick and thin filaments. If the starting sarcomere length is very short, the thick filaments volition already be pushing up confronting the Z-disc and in that location is no possibility for further sarcomere shortening, and the muscle volition exist unable to generate equally much forcefulness. On the other hand, if the muscle is stretched to the bespeak where myosin heads tin can no longer contact the actin, then once more, less forcefulness volition be generated. Maximum forcefulness is generated when the muscle is stretched to the signal that allows every myosin head to contact the actin and the sarcomere has the maximum altitude to shorten. In other words, the thick filaments are at the very ends of the thin filaments. These data were generated experimentally using frog muscles that were dissected out and stretched betwixt two rods. Intact muscles in our bodies are not ordinarily stretched very far beyond their optimal length due to the arrangement of muscle attachments and joints.

Notwithstanding, you can do a petty experiment that will aid yous see how force is lost when a muscle is in a very short or a very stretched position. This experiment will use the muscles that help y'all pinch the pad of your pollex to the pads of your fingers. These muscles are near maximal stretch when y'all extend your arm and also extend your wrist. As your wrist is cocked back into maximal extension, effort to pinch your pollex to your fingers. Run into how weak information technology feels? At present, gradually flex your wrist back to a directly or neutral position. You lot should feel your pinch get stronger. At present, flex your elbow and your wrist. With your wrist in maximal flexion, the muscles y'all use to compression with are most their most shortened position. Try pinching again. It should feel weak. Just, once again, as you extend your wrist back to neutral you should feel your pinch go stronger.

ENERGY SOURCE FOR MUSCLE CONTRACTION

The ultimate source of energy for muscle contraction is ATP. Recall that each cycle of a myosin caput requires an ATP molecule. Multiply that by all of the myosin heads in a muscle and the number of cycles each head completes each twitch and you can beginning to see how much ATP is needed for musculus function. Information technology is estimated that we fire approximately our entire body weight in ATP each solar day so it becomes apparent that we need to constantly furnish this important energy source. For muscle contraction, there are 4 ways that our muscles get the ATP required for contraction.

1. Cytosolic ATP: This ATP represents the "floating" pool of ATP, or that which is present and available in the cytoplasm. This ATP requires no oxygen (anaerobic) to make it (because information technology is already there) and is immediately available simply it is short lived. Information technology provides enough energy for a few seconds of maximal activity in the musculus-non the all-time source for long term contraction. Still, for the muscles of the eyes that are constantly contracting chop-chop just for short periods of time, this is a smashing source.
2. Creatine Phosphate: One time the cytosolic stores of ATP are depleted, the cell calls upon another rapid energy source, Creatine Phosphate. Creatine phosphate is a high energy chemical compound that can rapidly transfer its phosphate to a molecule of ADP to speedily replenish ATP without the apply of oxygen. This transfer requires the enzyme creatine kinase, an enzyme that is located on the M-line of the sarcomere. Creatine phosphate can replenish the ATP puddle several times, enough to extend muscle wrinkle up to near 10 seconds. Creatine Phosphate is the near widely used supplement by weight lifters. Although some benefits have been demonstrated, most are very pocket-size and limited to highly selective activities.
3. Glycolysis: Glycolysis, as the proper name implies, is the breakdown of glucose. The main source of glucose for this process is from glycogen that is stored in the musculus. Glycolysis tin office in the absence of oxygen and as such, is the major source of ATP product during anaerobic activity. This serial of chemical reactions will be a major focus in the next unit. Although glycolysis is very quick and can supply energy for intensive muscular activity, it can merely be sustained for about a minute before the muscles begin to fatigue.
4. Aerobic or Oxidative Respiration: The mechanisms listed above can supply ATP for maybe a little over a infinitesimal before fatigue sets in. Obviously, we appoint in muscle activity that lasts much longer than a minute (things similar walking or jogging or riding a bike). These activities require a constant supply of ATP. When continuous supplies of ATP are required, the cells use metabolic mechanisms housed in the mitochondria that utilize oxygen. We usually refer to these processes as aerobic metabolism or oxidative metabolism. Using these aerobic processes, the mitochondria can supply sufficient ATP to ability the muscle cells for hours. The down side of aerobic metabolism is that it is slower than anaerobic mechanisms and is not fast enough for intense action. However, for moderate levels of activeness, information technology works great. Although glucose tin also be utilized in aerobic metabolism, the nutrient of selection is fatty acids. As described below, boring-twitch and fast-twitch oxidative fibers are capable of utilizing aerobic metabolism

FATIGUE

When we call up of skeletal muscles getting tired, we frequently use the word fatigue, however, the physiological causes of fatigue vary considerably. At the simplest level, fatigue is used to describe a status in which the muscle is no longer able to contract optimally. To make give-and-take easier, we will carve up fatigue into two broad categories: Fundamental fatigue and peripheral fatigue. Central fatigue describes the uncomfortable feelings that come from being tired, it is often called "psychological fatigue." It has been suggested that central fatigue arises from factors released by the muscle during exercise that signal the brain to "feel" tired. Psychological fatigue precedes peripheral fatigue and occurs well before the muscle fiber can no longer contract. One of the outcomes of training is to acquire how to overcome psychological fatigue. Equally we railroad train we learn that those feelings are non so bad and that we tin can continue to perform even when it feels uncomfortable. For this reason, aristocracy athletes hire trainers that push them and force them to move past the psychological fatigue.

Peripheral fatigue can occur anywhere betwixt the neuromuscular junction and the contractile elements of the muscle. It can be divided into two subcategories, low frequency (marathon running) and high frequency (circuit training) fatigue. Loftier frequency fatigue results from impaired membrane excitability as a result of imbalances of ions. Potential causes are inadequate performance of the Na⁺/K⁺ pump, subsequent inactivation of Na⁺ channels and damage of Ca^two+ channels. Muscles can recover apace, usually within 30 minutes or less, following high frequency fatigue. Low frequency fatigue is correlated with dumb Caⁱⁱ⁺ release, probably due to excitation coupling contraction problems. It is much more difficult to recover from low frequency fatigue, taking from 24 hours to 72 hours.

In add-on, there are many other potential fatigue contributors, these include: accumulation of inorganic phosphates, hydrogen ion accumulation and subsequent pH change, glycogen depletion, and imbalances in Grand⁺. Delight note that factors that are not on the listing are ATP and lactic acid, both of which do not contribute to fatigue. The reality is we still don't know exactly what causes fatigue and much research is currently devoted to this topic.

SKELETAL Muscle Fiber TYPES

Classically, skeletal muscle fibers tin be categorized according to their speed of contraction and their resistance to fatigue. These classifications are in the process of beingness revised, but the basic types include:

1. Slow twitch oxidative (type I) muscle fibers,
2. Fast-twitch oxidative-glycolytic (Type IIA) muscle fibers, and
3. Fast-twitch glycolytic (Blazon IIX) fibers.

Fast-twitch (type II) fibers develop tension two to three times faster than tiresome-twitch (type I) fibers. How fast a fiber tin contract is related to how long it takes for completion of the cross-bridge bicycle. This variability is due to different varieties of myosin molecules and how rapidly they tin hydrolyze ATP. Recall that it is the myosin head that splits ATP. Fast-twitch fibers accept a more rapid ATPase (splitting of ATP into ADP + P_i) ability. Fast-twitch fibers as well pump Ca^two+ ions back into the sarcoplasmic reticulum very quickly, so these cells have much faster twitches than the slower variety. Thus, fast-twitch fibers can consummate multiple contractions much more quickly than slow-twitch fibers. For a consummate list of how muscle fibers differ in their ability to resist fatigue see the table below:

	Boring Twitch Oxidative (Type I)	Fast-twitch Oxidative (Type IIA)	Fast-Twitch Glycolytic (Type IIX)
Myosin ATPase activity	slow	fast	fast
Size (diameter)	small	medium	large
Duration of contraction	long	short	short
SERCA pump activeness	slow	fast	fast
Fatigue	resistant	resistant	easily fatigued
Energy utilization	aerobic/oxidative	both	anerobic/glycolytic
capillary density	high	medium	depression
mitochondria	high numbers	medium numbers	low numbers
Color	reddish (contain myoglobin)	red (comprise myoglobin)	white (no myoglobin)

In human skeletal muscles, the ratio of the diverse fiber types differs from muscle to muscle. For instance the gastrocnemius muscle of the calf contains about one-half wearisome and half fast blazon fibers, while the deeper calf musculus, the soleus, is predominantly slow twitch. On the other hand the centre muscles are predominantly fast twitch. As a result, the gastrocnemius muscle is used in sprinting while the soleus muscle is important for standing. In addition, women seem to have a higher ratio of boring twitch to fast twitch compared to men. The "preferred" fiber type for sprinting athletes is the fast-twitch glycolytic, which is very fast, however, nearly humans have a very low percentage of these fibers, < ane%. Muscle biopsies of one world class sprinter revealed 72% fast twitch fibers and amazingly 20% were type IIX. The Holy Grail of muscle research is to determine how to change skeletal muscle fibers from i blazon to another. It appears that muscle fiber types are determined embryologically by the type of neuron that innervates the muscle fiber. The default muscle appears to be slow, blazon I fibers. If a muscle is innervated by a minor neuron that muscle fiber will remain tedious, whereas large mylenated fibers induce the fast isoforms. In addition, the frequency of firing rates of the neuron likewise alters the musculus fiber blazon. Inquiry suggests that humans have subtypes of fibers, making upwardly well-nigh <5% of the muscle, that are dually innervated and allow for switching between tiresome and fast to occur. Generally, information technology would appear that genetics determine the type of innervation that occurs and subsequent muscle fiber types and that grooming may be able to slightly alter the ratios due to the dually innervated muscles. However, since <5% have dual innervation, genetics is going to play a much greater role in your fiber types than your training.

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