Muscular physiology

Muscular Physiology

Thursday, January 17, 13

Skeletal Muscles Uniquely Provide The Body With:

•Heat Production

•Movement

•Posture Maintenance


Heat Production• Because skeletal muscles release such a massive

amount of heat, even while doing no work, the muscles are a great influence on body temperature

• When body temperature falls below the temperature set by the hypothalamus of the brain, temperature sensors send a message to the hypothalamus to instruct the skeletal muscle to contract (explained on the next slide), therefore releasing more heat, thus bringing the body temperature back up to homeostasis temperature


Movement

• An impulse, which creates a temporary imbalance, is conducted along the sarcolemma and inward along the T tubules. The impulse in the tubule triggers the release of a flood of calcium ions from the near-by sacs of the SR. In the sarcoplasm, the fluid in the sarcolemma, which contains the sarcomere, the calcium ions combine with troponin molecules in the thin filaments of the myofibrils. The troponin normally holds tropomyosin strands in a position that blocks the chemically active sites of actin molecules. After active sites are exposed, myosin heads become energized and bind to actin, which move the actin forward and backwards, therefore contracting the muscle.


Posture

• Because skeletal muscles are the muscles responsible for movement for the body, the muscles are unique in that they establish and maintain posture for the human body by creating the upper and lower limits, as well as the resting location, of most joints and appendages.


Similarities with other Systems

• The concept of excitability is also shared with the nervous system, which is the case because the nervous system, in most cases, is the system responsible for controlling muscle movement.

• Contractility and extensibility is related to agonist and antagonist in that the contractility of a muscle refers to it’s specific ability to contact, or cause movement, which is connected to agonist, the main muscle required for a specific movement. Antagonist refers to extensibility because muscles cannot extend on their own, but rather, they require muscles which cause the exact opposite movement, therefore extending the muscle itself.


• the term for the chains of actin and myosin that pack muscle fiber. These are the force generating structures. When looking at the longitudinal section through an electron microscope, a myofilament shows several distinct bands, each of which has been given a special letter. The lightest (least electron dense) band is known as the I-band and consists mostly of actin. The wide, dark band, known as the A band, is composed primarily of myosin. In the center of the I-band is an electron dense line, known as the Z-line. In the middle of the A band is another dense line known as the M line. In cross section, under very high magnification, both A and I bands can be seen to be hexagonal networks. These networks are apparently ordered and fixed at the M- and Z-lines. In the region where the A and I bands overlap (sometimes known as the H band) the two hexagonal networks intermesh so that each myosin filament is surrounded by six actin filaments. These networks appear to be anchored to (and through) the cell membrane in two ways. At the ends of fibrils, special structures anchor the terminal actin filaments to the membrane. There also appear to be connections between the Z and M lines and the cell membrane. In conclusion, the structure of the myofilament have gapes in them and which allow for them to overlap and make a muscle shorter, they can also stretch out to make a muscle longer.

Myofilament


Sliding Filament Theory

• The sliding filament theory allows for the shortening of the muscle fiber because the Z-lines contract which make the A-band shorten. There are myosin in the middle of the Z-lines and on each of the myosin there are little proteins that connect to them called actin that grab a hold to the fibers in the muscle and that allows for the muscle to shorten. The actin will continue to grab the muscle fibers until there is eventually no more room on the myosin.


Movement Terms

• Excitation: A neural synapse induces an action potential in a muscle cell (fiber) that, in turn, results in calcium ions to be released into the cytosol from the sarcoplasmic reticulum when calcium channels open.

• Contraction: Calcium ions are an intracellular signaling molecule for muscle contraction. Calcium binds to troponin-C to initiate contraction; this will continue until excitation ceases.

• Relaxation: removal of intracellular calcium allows the muscle to relax.


Marathon Runner

Type I muscle fibers are classified as “slow-twitch”, meaning they develop force slowly and relax at a similar pace with a longer “twitch” in between. Why? Because..

- It doesn’t utilize much energy from ATP.- It is more aerobic than anaerobic.- & it can function for longer periods of time with repetitive contractions, because of readily supply of oxygen.


Sprinter• Type II fibers are the “fast-twitch” explosive muscle fibers

associated with power and explosiveness in athletes.

“Inefficient and Fatigable” - characterizes high anaerobic power but very low aerobic power which equates to shorter duration, but more powerful contractions of the muscle as a whole.The main difference between type 2a and 2b muscle fibers comes from their “capacity for aerobic-oxidative energy supply.” Type 2a muscle fibers have a more efficient meanings of aerobic metabolism due to a higher number of surrounding capillaries that circulate blood throughout the muscle which results in a higher resistance to fatigue.


Units of Combined Cells

• Cardiac muscles are the muscles of the heart. Cardiac muscle cells make up the myocardium portion of the heart wall. They have an overlapping arrangement of light and dark striations. It measures about 10-15 micrometers in diameter and 50-100 meters in length.


Rigor Mortis

• A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. It is caused by the skeletal muscles partially contracting. The muscles are unable to relax, so the joints become fixed in place.


Motor Unit Activation

• A motor unit consists of an alpha-motor neuron and the muscle fibers it innervates. Depending on the size of the motor unit, the alpha-motor neuron connects to between 10 and 1,000 muscle fibers and sends a signal to trigger simultaneous contraction of all the fibers in that motor unit. This synchronized contraction allows the muscle to make coordinated movements. For movements requiring little force, such as picking up a pencil, you recruit small motor units in your arm muscles. For high-force movements, such as picking up a brick, you will recruit large motor units in addition to the small ones to execute the movement.


Stimulation Frequency

• A single stimulus of the muscle fiber from the nervous system will produce a small amount of muscle force, followed by a muscle relaxation as the fiber returns to baseline. However, if the nervous system delivers several stimuli before the fibers can fully relax, the muscle fibers produce more force than they would in response to a single stimulus. Continual nervous system stimulation of the motor unit and muscle fibers yields the maximal force possible of the given muscle fibers, increasing the strength of the muscle contraction.


Muscle Fiber Length

• Your muscles have thin and thick filaments, which are organized into contractile units called sarcomeres. Within each sarcomere, thick filament proteins slide and bind to proteins in the thin filament during muscle contraction. The sarcomeres have an optimal length at which the number of possible binding sites between the filaments is maximized. If your muscle fibers are shorter or longer than this optimal length, they do not have as much force-producing potential because there are fewer available binding sites between the filaments. For example, when your elbow is fully bent, the length of your biceps muscle fibers is shorter and less capable of producing force than when your elbow is extended.


Contraction Speed

• Muscle contraction speed determines the force-producing capacity of your muscle. For a concentric muscle contraction in which the muscle fibers shorten, your muscles' force-producing capacity decreases at faster contraction speeds. Conversely, an eccentric contraction in which the muscle lengthens produces greater force at faster contraction speeds, and force-production capacity is always greater in an eccentric contraction compared to a concentric contraction. For example, you may be capable of lowering a heavy barbell to your chest during a chest press, but cannot lift it off your chest. This is because the chest and shoulder muscles eccentrically contract to lower the weight and contract concentrically to lift the weight.


Contraction Phases

• Latent Phase: Is the interval from the stimulus application until the muscle begins to contract (shorten). Note that there is no traced activity during this phase, but there are some electrical and chemical changes taking place during this phase.

• Contraction Phase: This phase is when the muscle fibers shorten, the tracings will show during this phase (a) peak(s).

• Relaxation Phase: This phase is represented by the downward curve in your tracings, this is when the muscle is going back to its original state of relaxation and the muscle will once again lengthen.


Molecular Events

1. During contraction, the myosin molecule forms a chemical bond with an actin molecule on the thin filament (gripping the rope). This chemical bond is the crossbridge. For clarity, only one cross-bridge is shown in the figure above (focusing on one arm).2. Initially, the crossbridge is extended (your arm extending) with adenosine diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin.3. As soon as the crossbridge is formed, the myosin head bends (your arm shortening), thereby creating force and sliding the actin filament past the myosin (pulling the rope). This process is called the power stroke. During the power stroke, myosin releases the ADP and Pi.4. Once ADP and Pi are released, a molecule of adenosine triphosphate (ATP) binds to the myosin. When the ATP binds, the myosin releases the actin molecule (letting go of the rope).5. When the actin is released, the ATP molecule gets split into ADP and Pi by the myosin. The energy from the ATP resets the myosin head to its original position (re-extending your arm).


Warm up by Treppe

• The concept or phenomenon of "Treppe" occurs when a muscle contracts more forcefully after it has contracted a few times than when it first contracts. This is due to the fact that active muscles require decreasing degrees of succeeding stimuli to elicit maximal contractions. Returning to our example of the second set of squats feeling easier than the first, during the first set there was insufficient warm-up, and the second set felt easier because the first set actually served as a warm-up. The phenomenon in which the contraction strength of a muscle increases, due to increased Ca2+ availability and enzyme efficiency during the warm-up.


Works Cited

• All information derived from source #1


Date post:	03-Jun-2015
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Muscular physiology

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