For skeletal muscles to contract, based on excitation–contraction coupling, . the cell membrane against time, the action potential begins with depolarization. Below are two different but similar descriptions of muscle contraction that (4) At the motor end plate, the action potential causes the release of packets or. between force and EMG (Lippold, ), however this relationship is not The term muscle strength refers to the ability of a muscle to generate tension. The contraction of muscle fibers occurs when action potentials are.
If the starting sarcomere length is very short, the thick filaments will already be pushing up against the Z-disc and there is no possibility for further sarcomere shortening, and the muscle will be unable to generate as much force.
On the other hand, if the muscle is stretched to the point where myosin heads can no longer contact the actin, then again, less force will be generated. Maximum force is generated when the muscle is stretched to the point that allows every myosin head to contact the actin and the sarcomere has the maximum distance 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 between two rods.
Intact muscles in our bodies are not normally stretched very far beyond their optimal length due to the arrangement of muscle attachments and joints. However, you can do a little experiment that will help you 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 you pinch the pad of your thumb to the pads of your fingers. These muscles are near maximal stretch when you extend your arm and also extend your wrist. As your wrist is cocked back into maximal extension, try to pinch your thumb to your fingers.
See how weak it feels? Now, gradually flex your wrist back to a straight or neutral position. You should feel your pinch get stronger. Now, flex your elbow and your wrist.
With your wrist in maximal flexion, the muscles you use to pinch with are near their most shortened position. It should feel weak. But, again, as you extend your wrist back to neutral you should feel your pinch get stronger. Recall that each cycle of a myosin head 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 start to see how much ATP is needed for muscle function.
It is estimated that we burn approximately our entire body weight in ATP each day so it becomes apparent that we need to constantly replenish this important energy source.
For muscle contraction, there are four ways that our muscles get the ATP required for contraction. This ATP requires no oxygen anaerobic to make it because it is already there and is immediately available but it is short lived.
It provides enough energy for a few seconds of maximal activity in the muscle-not the best source for long term contraction. Nevertheless, for the muscles of the eyes that are constantly contracting quickly but for short periods of time, this is a great source.
Once the cytosolic stores of ATP are depleted, the cell calls upon another rapid energy source, Creatine Phosphate. Creatine phosphate is a high energy compound that can rapidly transfer its phosphate to a molecule of ADP to quickly replenish ATP without the use 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 pool several times, enough to extend muscle contraction up to about 10 seconds. Creatine Phosphate is the most widely used supplement by weight lifters.
Although some benefits have been demonstrated, most are very small and limited to highly selective activities. Glycolysis, as the name implies, is the breakdown of glucose. The primary source of glucose for this process is from glycogen that is stored in the muscle. Glycolysis can function in the absence of oxygen and as such, is the major source of ATP production during anaerobic activity. This series 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 only be sustained for about a minute before the muscles begin to fatigue. Aerobic or Oxidative Respiration: The mechanisms listed above can supply ATP for maybe a little over a minute before fatigue sets in. Obviously, we engage in muscle activity that lasts much longer than a minute things like walking or jogging or riding a bicycle. These activities require a constant supply of ATP. When continuous supplies of ATP are required, the cells employ metabolic mechanisms housed in the mitochondria that utilize oxygen.
We normally refer to these processes as aerobic metabolism or oxidative metabolism. Using these aerobic processes, the mitochondria can supply sufficient ATP to power 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 activity.
However, for moderate levels of activity, it works great. Although glucose can also be utilized in aerobic metabolism, the nutrient of choice is fatty acids. As described below, slow-twitch and fast-twitch oxidative fibers are capable of utilizing aerobic metabolism FATIGUE When we think of skeletal muscles getting tired, we often use the word fatigue, however, the physiological causes of fatigue vary considerably.
At the simplest level, fatigue is used to describe a condition in which the muscle is no longer able to contract optimally. To make discussion easier, we will divide fatigue into two broad categories: Central fatigue and peripheral fatigue.
Central fatigue describes the uncomfortable feelings that come from being tired, it is often called "psychological fatigue. Psychological fatigue precedes peripheral fatigue and occurs well before the muscle fiber can no longer contract. One of the outcomes of training is to learn how to overcome psychological fatigue.
As we train we learn that those feelings are not so bad and that we can continue to perform even when it feels uncomfortable. For this reason, elite athletes hire trainers that push them and force them to move past the psychological fatigue. Peripheral fatigue can occur anywhere between 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. High frequency fatigue results from impaired membrane excitability as a result of imbalances of ions.
Muscles can recover quickly, usually within 30 minutes or less, following high frequency fatigue. It is much more difficult to recover from low frequency fatigue, taking from 24 hours to 72 hours. In addition, there are many other potential fatigue contributors, these include: Please note that factors that are not on the list 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. These classifications are in the process of being revised, but the basic types include: Fast-twitch type II fibers develop tension two to three times faster than slow-twitch type I fibers.
How fast a fiber can contract is related to how long it takes for completion of the cross-bridge cycle. This variability is due to different varieties of myosin molecules and how quickly they can hydrolyze ATP. Recall that it is the myosin head that splits ATP. Thus, fast-twitch fibers can complete multiple contractions much more rapidly than slow-twitch fibers. For a complete list of how muscle fibers differ in their ability to resist fatigue see the table below: Cardiac muscle is also striated in appearance, but it differs significantly from other striated muscle in both its structure and its behavior.
Still other muscles, called smooth muscleslack the characteristic cross-striations, but contain the same contractile proteins.
- MUSCLE CONTRACTION
- MUSCLE TWITCH
- Summary of Events in Muscle Contraction and Relaxation
The smooth muscles are important as linings of the gastrointestinal tract that churn and propel food through the tract, as linings of blood vessels that control their diameters and thus flow through them, as valves that control the passage of gases and fluids in the body, and as controllers at many other places in the body. Of the three types of muscle, skeletal and cardiac muscle have been studied most thoroughly.
It is presumed that the mechanism of contraction is the same for both types and only the details of initiating and controlling the contraction differ. Not all striated muscle, however, behaves in the same way. For example, skeletal muscles of vertebrates all appear to initiate contractions with sodium spikes, whereas striated muscles of some invertebrates initiate contractions with calcium spikes.
We will confine our discussion primarily to vertebrate skeletal muscle, pointing out the distinctive features of structure and function of cardiac and smooth muscle. Two arrangements of muscle fibers within a muscle. Tendons are lines radiating from rectangles muscle fibers at each end. Tendons are vertical lines extending from the two sides of the parallelogram.
SKELETAL MUSCLE: WHOLE MUSCLE PHYSIOLOGY
Double headed arrows f indicate direction of force exerted by individual muscle fibers; single-headed arrows F indicate direction of force exerted by whole muscle. Mechansims of muscle contraction and its energetics. Skeletal muscles are composed of masses of fibers, each an individual cell. There are several types of muscles, each with different arrangements of fibers, but these can be divided into two major classes: Figure shows these two classes.
In the parallel arrangement Aeach muscle fiber, or a small group of fibers, is attached to its own tendon, the tendons converging on a common point 1. The muscle fibers are side-by-side, i. The pennate muscle fibers B attach to a common tendon, so that the direction of shortening of the individual fibers double-headed arrow, f is different from the direction of shortening of the whole muscle single-headed arrow, F.
As a result, the pennate muscle cannot shorten as much as the parallel muscle. Pennate muscles are located in positions requiring small but powerful movements; parallel muscles are located in positions requiring longer movements with less power or faster movements.
Muscles, fibrils and filaments To understand how a muscle works it is necessary to understand the fine-structure of muscle cells because it is the internal parts of the cells that do the work. The relevant internal structures are the myofibrils, the myofilaments and the sarcoplasmic reticulum.
Muscles are composed of muscle fibers; fibers are composed in part of myofibrils; and myofibrils are composed of myofilaments. Skeletal muscles have a characteristic striated appearance because the myofibrils are characteristically striated and because the myofibrils are more or less in register the same stripes are lined up.
The myofibrils are striated because the myofilaments are not homogeneously distributed within them, but rather occur in regular, repeating arrays. Levels of organization within a skeletal muscle, including counterclockwise from top left whole muscle and fascicles, bundles of muscle fibers, myofibrils, thin and thick filaments, and myosin and actin molecules. Warwick R, Williams PL [ed]: Note the striated appearance of all three. Each muscle fiber contains about myofibrils that are 1 m in diameter and run the length of the fiber.
Myofibrils have no membrane, being simply surrounded with cytoplasm.
Chapter 14 - Muscle Contraction
The cross-striations of the myofibrils are serially repeating units called sarcomeres. A sarcomere can be from 1. Each sarcomere contains an anisotropic doubly refractive, therefore dark in phase microscopy band bounded by two isotropic singly refractive, therefore light bands. The anisotropic band is called the A band ; the isotropic band is called the I band. Actually, each sarcomere contains two half-I bands one at each end because a single I band straddles the Z line and therefore is part of two adjacent sarcomeres.
In the center of the A band, there is a lighter region known as the H zone or H band. During contraction the A band does not change length 2though the sarcomere shortens, the distance between Z lines lessens, and the I and H bands narrow. Any theory of muscle contraction must account for these observations. The myofibrils, as shown in Figureare composed of proteinaceous structures called myofilaments. One filament is thick, about 11 nm in diameter and 1. These filaments are referred to as the thick filaments and thin filamentsrespectively.
Thick filaments are made up of several hundred myosin molecules, proteins of a molecular weight of about , and some other minor proteins whose function is unknown. The myosin molecule has a tail region that is rodlike, and head region, with two globular subunits projecting out at approximately right angles with the filament.
The structure has been likened to two golf clubs with their shafts twisted together. Drawings of a myosin molecule, and its position within the thick filaments are shown in Figure The myosin molecules of thick filaments are arranged in a sheaf with heads oriented toward each end and tails toward the center. Each subsequent myosin molecule attaches 14 nm further toward the end of the filament, and its head is rotated 60 around the filament from its predecessor.
Thus, the thick filament is studded with projections except at its center, which contains only myosin tails. The thick filaments are coincident with the A band of the sarcomere. Each thin filament contains three protein molecules: A single thin filament is composed of to actin molecules and 40 to 60 troponin and tropomyosin molecules.
Actin is a small, nearly spherical molecule that is arranged in the filament into two helical strands, as shown in Figurewith about 13 actin molecules per complete turn of the helix.
Troponin and tropomyosin are sometimes called regulator proteins because of their central role in regulating muscle contraction. Tropomyosin is a filamentous protein that is thought to form two strands that lie in the grooves formed between the actin strands. Troponin, a globular protein, binds to tropomyosin at only one site and therefore is thought to sit astride the tropomyosin molecule strand at regular intervals approximating 40 nm.
Figure shows the relationships between the three proteins as they are currently thought to exist. The thin filaments attach to the Z disc, a flat protein structure.
Thin filaments may be connected end-to-end in the H band by slender threadlike processes. Organization of the sarcomeres.
Pattern of cross-striation in skeletal muscle with bands labeled. Arrangement of thick and thin filaments that accounts for the pattern of cross-striations. Hexagonal arrays of thick and thin filaments in cross sections through the sarcomere in the A band, H band and I band.
The relatively high anisotropicity of the A band results from the presence of both thin and thick filaments shown in longitudinal section in the upper part and cross-section in the lower part of Fig.
The I band is only slightly anisotropic because it contains only thin filaments. The H band is not optically as dense as the rest of the A band because it does not contain any thin filaments when the muscle is at rest.
As can be seen in the cross section of Figurethe thin filaments are organized into regular hexagonal arrays within the myofibrils, with a thick filament at the center of each array in the A band.
Three thick filaments are equidistant from each thin filament, whereas six thin filaments are equidistant from each thick filament as shown in the left panel.
A cross section through the I band shows only the thin filament array; a section through the H band shows only the thick filament array plus the slender, thread-like processes interconnection thin filaments. Proposed structure of thin filament with relative positions of actin, troponin and tropomyosin indicated. Quart Rev Biophys 2: Because they are staggered around the thin filament at 60 intervals, each projects in the direction of a thin filament, and each thin filament has projections toward it from three thick filaments.
AK LECTURES - Action Potential vs. Muscle Contraction Graphs
These projections have been termed either cross-bridges or cross-projectionsdepending upon whether the heads are thought to contact and bind thin filaments or not. As we shall see, there are two schools of thought, in fact, two different mechanisms proposed to account for the generation of the mechanical force of contraction.
Three-dimensional reconstruction of skeletal muscle illustrating organizatin of myofibrils, sarcoplasmic reticulum and T tubules. Gray's Anatomy, 35th British ed, Philadelphia, W. Saunders, T tubules and sarcoplasmic reticulum Muscle cells have a unique membrane structure, called the transverse tubule or simply the T tubule.
The T tubule is an invagination of the muscle membrane, much like the invagination produced in a balloon by pushing a finger into its side without puncturing it, but the T tubules are long and tortuous. The T tubule system forms a ring around every myofibril either at the Z line, in which case there is one per sarcomere, or at the A-I-band junction, in which case there are two per sarcomere.Neuromuscular Junction Process in Skeletal Muscle
These perifibrillar rings are inter-connected, forming a kind of honeycomb arrangement, as shown in Figure The position of the T tubule with respect to the sarcomere is somewhat species specific; frog skeletal muscle has only one tubule per sarcomere, whereas human skeletal muscle has two.
It should be noted that in human cardiac muscle there is only one tubule per sarcomere as shown in Figure The inside of the T tubule is continuous with the extracellular space and presumably contains a fluid like extracellular fluid, but, because the tubular space is small and not well stirred, it is likely that ionic movements across the tubule membrane produce significant changes in ionic concentration, at least on a short-term basis.
Fortuitous sections through triads, one at right angles to the other.