Muscle Physiology

Muscle

a complex tissue that is comprised of several levels of organization

What are muscles composed of?

fibers

what are fibers made up of?

myofibrils

What are myofibrils composed of?

sarcomeres

what are sarcomeres made up of?

myofilaments

What are the two types of alternating myofilaments in each sarcomeres?

thick myosin filaments and thin actin filaments

what are actin and myosin known as?

contractile proteins

what are the functional units of skeletal muscle?

sarcomere

how are the thick and thin filaments arranged in a sarcomere?

alternating pattern

what are the thick filaments made up of?

the contractile protein, myosin

What are the thin filaments made up of?

contractile protein, actin and the regulatory proteins troponin and tropomyosin.

Where are the thick filaments arranged?

They reach from one side of the A band to the next side of the A band.

What are the myosin heads?

tiny knobs that project from the thick filaments towards the binding sites on the thin filaments to form cross bridges.

What are cross bridges?

they join the thin and thick filaments together during contraction.

thin filament arrangement?

they are attached at one end to the x line and the other end of the filament does not reach all the way across the A band.

why is the middle area of the A band light?

thick filaments are only present

What is the light middle area of the A band called?

H zone

I band

the area of the sarcomere that is comprised of only thin filaments

what happens to the thin filaments during contraction?

the thin filaments are pulled closer together, causing the filaments to slide over each other, shortening the sarcomere.

What is muscle contraction?

the result of a series of events stimulated by a motor neuron

what do motor neurons stimulate?

several muscle fibers when it generates action potentials.

motor unit

the neuron and the muscle cells it stimulates

neuromuscular junction?

the nerve endings and and muscle cells do not touch each other.

How does action potential work/act?

1. Action potentials in the motor neuron result in the release of acetylcholine (ach) onto the motor end plate of the muscle fiber.

2. This opens the chemically-gated sodium channels

3. Sodium moves across the membrane and generates a graded potential. If the graded potential brings the membrane to threshold, the muscle fiber generates its own action potential with the opening of the voltage-gated sodium channels.

What is the second part of muscle contraction?

1. as the nerve impulse reaches the muscle cell, the cell membrane also becomes permeable to calcium that is sorted in the sarcoplasmic reticulum.

2. When calcium moves into the cell, the calcium reacts with the troponin and tropomyosin complex.

3. This reaction uncovers the binding sites on the actin filaments, allowing the cross-bridges to attach.

4. The cross-bridges pull on the actin filaments, sliding them closer together, changing the fibers’ position and not length.

how do the thin filaments and thick filaments work together?

1. the thin filaments have binding sites for the myosin knobs

2. In a relaxed cell, these sites are covered up with tropomyosin protein molecules

3. The tropomyosin molecules are bound to another protein called troponin.

What is the sliding filament theory?

The cross-bridges pull on the actin filaments, sliding them closer together, changing the fibers’ position and not length.

What is the last part of muscle contraction?

Contraction is followed by relaxation during which re-uptake of calcium ions into the SR occurs and filaments return to their original position.

what does the cross bridge cycle require?

ATP

how does ATP work during this process?

• ATP will hydrolyze to release energy for the actin-myosin binding to occur.

• ATP also binds to myosin during the cross-bridge cycle, breaking the actin-myosin bond, which then allows another such bond to form.

Rigor mortis

• After death ATP declines

• The binding of actin-myosin can still occur. However, any movement of the filaments or breaking of the bonds cannot

• Filaments are bound to each other, and results in a rigid muscle.

• This rigidity is temporary and diminishes as the process of decomposition begins to breakdown the contractile proteins.

Twitch

The contraction resulting from a single action potential manifest itself as a weak twitch that is usually not observed.

Contraction strength increases as…

More fibers are stimulated or recruited and as the frequency of stimulation increases.

Tetanus

If stimuli are delivered so quickly that no relaxation occurs, a strong sustained contraction known as tetanus occurs.

if a second action potential occurs before relaxation from a previous action is completed?

the second contractile response builds on the first and produces greater contraction strength

smooth muscle

• composed of sheets of cells, each cell with one nucleus

• contraction is involuntary

• This tissue is found in the walls of blood vessels, the digestive tract and the reproductive system

cardiac muscle

• found only in the heart

• composed of uninucleate cells bound together at dense plaques called intercalated disks

• This tissue appears to be striated or striped owing to the parallel arrangements of the bundles of contractile fibers

• involuntary control

What does ATP do?

1. pumps Ca2+

2. energizes myosin (power stroke)

3. cross-bridge detachment between myosin and actin

skeletal muscle

• Under voluntary control

• Attached to skeleton

• Multi-nucleated fibers

• Prominent striations are visible in the light microscope, due to the extreme regularity of arrangement of the contractile proteins.

what is one way the nervous system can control the amount of muscle tension?

Control the number of motor axons firing which controls the number of active motor units and therefore controls the number of contracting muscle fibers.

how is recruitment stimulated?

Stimulating the muscle with brief electrical shocks of varying intensities

what is the threshold level to activate any nerve fibers ?

around three

As the stimulus increases beyond threshold…

More motor units (nerve fibers+ the muscle fibers they control) are recruited.

stimulus intensity and muscle tension relartionship?

• Muscle tension increases as more nerve fibers are excited

• it reaches a maximum contraction beyond which more stimulation does not increase the muscle force.

length-tension relationship of a muscle fiber

• muscle fibers generate the most force when sarcomeres are at their optimal resting length (2.0-2.25 um)

• forces decreases when sarcomeres are too short( overlapping too much) or too long (not overlapping enough)

stimulus frequency and muscle response

1. low stimulus freqeuncy- seperate twitch- twitch

2. moderate/normal stimulus frequency- increasing, summating twitches- incomplete tetanus

3. very high- smooth sustained contraction- complete tetanus

an action potential in a skeletal muscle fiber travels…

from the synaptic region(end plate), down the transverse tubules to release calcium ions from the sarcoplasmic reticulum into the cytoplasm,

how is the tension complied to induce movement.

the breaking and making of bonds

An isotonic contraction of a skeletal muscle is a contraction in which:

the length of the muscle changes

Action potentials in a motor axon may create a threshold stimulus to a target muscle fiber, resulting in:

a single action potential in the muscle fibers it contacts and a single contraction.

An action potential in skeletal muscle fibers increases sarcoplasmic calcium levels because:

it travels down the T-tubules, stimulating voltage-gated calcium channels in the sarcoplasmic reticulum membrane

During incomplete tetanus, the amount of tension increases above that seen with a single twitch because:

there is accumulation of calcium in the sarcoplasm.

During incomplete tetanus, the muscle

contracted so that the single contractions created more tension than seen during single twitches.

During complete tetanus, the muscle:

produced a prolonged (fused) contraction, and single contractions could not be seen.

what zones get shorter during contraction?

I and H zone get shorter

what zone stays the same

the A zone