Muscular Physiology Newsletter

Rigor Mortis

Rigor mortis is the process that your body goes through after you have died. It is the latin phrase for “stiffness of death”(1). It is where your skeletal muscles stiffen because the stimulation of muscle cells stop. And the muscle fibers might have been in mid-action at the time of death. And this may be when the cross bridges are still intact and your body needs ATP to release the cross bridges but because the ATP is used up when you die, so the cross bridges become “stuck” in a position (1). This leaves the muscles in a dead body stiff because there was no more ATP to “turn off” the contraction.

4 Factors that Influence the Strength of Muscle Contractions

The muscle is capable of many different things. That helps us do things such as lifting weights, to shaking hands, and writing. Sometimes we need a lot of strength and other times we don’t need a lot. Four factors that affect the strength of muscle contractions are the number of cross bridges that can make contact, the length of the fibers, and the frequency of the stimulation; summation and recruitment (8.) The length influences the contraction because at a certain point it is the strongest. And the length affects the cross bridges contact, if its too long the cross bridges can’t reach, making contraction weak and if the muscle is too short, a lot of cross bridges will be able to reach but it would get crowded and also make contraction weak. (8.) The frequency of the stimulation impulses also affect the contraction.

Phases of a Twitch Contraction

There are 3 phases of a twitch contraction called the latent period,

contraction phase, and relaxation phase. During the latent period the impulse

travels through the sarcolemma and T tubules into the SR where it triggers the

release of calcium ions (1). This calcium binding to troponin is what causes the

contraction to start, which is the contraction phase. After a few milliseconds the

contraction ceases and the relaxation phase begins.

How the Treppe effects relates to Athletes

The Treppe is a gradual, steplike increase in the strength of contraction. This

relates to the warm up of athletes because they use the principle of the staircase

phenomenon when they warm up. A muscle contracts more forcefully after it has

contracted a few times then when it first contracts (1). This allows the warm up of

the athletes to be more beneficial because it helps them get stronger and work their

muscles more.

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One of the interesting things for Skeletal muscles is the a muscle organ is composed of bundles of contractile muscle fibers held together by connective tissue. Closer magnification of a fiber shows another fiber, myofibrils, in the sarcoplasm, note sarcoplasm reticulum and t tubules forming a three part structure called a triad. A unique feature of the skeletal muscle cell is the t tubules, which are extensions of the plasma membrane, or sarcolemma, and the sarcoplasmic reticulum (SR), which forms networks of tubular canals and sacs. A triad is a triplet of adjacent tubules: a terminal sac of the sr, a t tubule, and another terminal sac of the sr. (1)

First of there is four different type of Myofilaments which are myosin, actin, tropomyosin, and troponin. The thin filaments are made of a combination of three proteins. The way they work is that the myosin head is chemically attracted to the actin molecules of the nearby thin filaments, so they angle the filaments. When they bridge the gap between adjacent myofilaments, the myosin heads are usually called cross bridges. (1)

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Sliding Filament Theory

The sliding filament theory is a muscle contraction. It allows for the shortening of

muscle fibers because the myosin heads attach themselves to the thin filaments

and pull with a lot of force that moves the filament closer together, past them. This

shortens the entire myrofibril and muscle fiber. (1)

Extremely large protein complexes involved in the Ca2+-regulatory system of the excitation-contraction-relaxation cycle have been identified in skeletal muscle, i.e. clusters of the Ca2+-binding protein calsequestrin, apparent tetramers of Ca2+-ATPase pump units and complexes between the transverse-tubular alpha1-dihydropyridine receptor and ryanodine receptor Ca2+-release channel tetramers of the sarcoplasmic reticulum. While receptor interactions appear to be crucial for signal transduction during excitation-contraction coupling, avoidance of passive disintegration of junctional complexes and stabilization of receptor interactions may be mediated by disulfide-bonded clusters of triadin. Oligomerization of Ca2+-release, Ca2+-sequestration and Ca2+-uptake complexes appear to be an intrinsic property of these muscle membrane proteins. During chronic low-frequency stimulation, the expression of triad receptors is decreased while conditioning has only a marginal effect on Ca2+-binding proteins. In contrast, muscle stimulation induces a switch from the fast-twitch Ca2+-ATPase to its slow-twitch/cardiac isoform. These

alterations in Ca2+-handling might reflect early functional adaptations to electrical stimulation. Studying Ca2+-homeostasis in transformed muscles is important regarding the evaluation of new clinical applications such as dynamic cardiomyoplasty. Studies of Ca2+-handling in skeletal muscle fibers have not only increased our understanding of muscle regulation, but have given important insights into the molecular pathogenesis of malignant hyperthermia, hypokalemic

periodic paralysis and Brody disease. (1)

When you work out you are essentially tearing your

muscle (actin) and when your body repairs your muscle

it makes it bigger and stronger (more actin). During

excersise your muscles use up oxygen and tries to

Ca++ in

excitation,

contraction,

and

relaxation of

a muscle

cell

Work a Muscle Until you

"Feel the

Burn"

replenish it through anaerobic respiration which results in the formation of lactic

acid, which is what causes the burning sensation. (1)

Most mature extrafusal skeletal muscle fibers in mammals are innervated by only a single α motor neuron. Since there are more muscle fibers by far than motor neurons, individual motor axons branch within muscles to synapse on many different fibers that are typically distributed over a relatively wide area within the muscle, presumably to ensure that the contractile force of the motor unit is spread evenly (Figure 16.4). In addition, this arrangement reduces the chance that damage to one or a few α motor neurons will significantly alter a muscle's action. Because an action potential generated by a motor neuron normally brings to threshold all of the muscle fibers it contacts, a single α motor neuron and its associated muscle fibers together constitute the smallest unit of force that can be activated to produce movement. Sherrington was the first to recognize this fundamental relationship between an α motor neuron and the muscle fibers it innervates, for which he coined the term motor unit. (1)

motor

unit

The cardiac muscle fiber does not taper like a

skeletal muscle fiber. Cardiac fibers to form a

continuous, electrically coupled mass called a SYNCYTIUM “unit of combined cells”.

The Cardiac muscle forms a continuous, contractile band around the heart

chambers. It conducts a single impulse across a virtually continuous sarcolemma.

These features are necessary for an efficient, coordinated pumping action. (1)

Skeletal Muscles provide movement, heat production,

and posture. Skeletal muscle contractions allow for the

movement of the body as a whole. Skeletal muscle cells

are very active and numerous which allows for a huge

Unit of Combined Cells

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Muscles

Provide

…

amount of body heat because each cell goes through the process of catabolism to

make heat. The muscle contractions also allow for people to stand, sit, and maintain

posture throughout the day doing various activities. (1)

Our body muscles are capable of many things. One

of those things is the ability to feel things or excitability

which coincides with the nervous system because the

muscle cells can respond to nerve signals. Contractility

is the muscle being able to contract or shorten, While

extensibility is the muscles being able to extend or

stretch.(1) These two things relate to agonist and

antagonist because they are both opposites and are the

motions of what the agonist and antagonists do.

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Muscles

Provide

…

Excitability,

Contractility, and

Extensibility