Skeletal Muscle Contraction (Lecture 4) Flashcards Preview

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What are the 4 characteristics of slow (red, dark) twitch fibers?

1) more myoglobin than fast twitch muscle fibers
2) more mitochondria than fast twitch muscle fibers
3) smaller concentration of ATPase than fast twitch muscle fibers
4) primarily use aerobic respiration


True or False:

After birth, the number of myofibers can be increased.

False -> after birth, the number of myofibers cannot be increased.


What can be increased within the myofiber that will increase the mass of it, thus, increasing the mass of the muscle?

The number of myofibrils.

**remember: the number of myofibers cannot be increased after birth, just the number of myofibrils within the myofiber.


What is lost muscle tissue replaced with?

Scar Tissue (fibrous connective tissue)


What type of myofiber is the soleus mostly composed of?

Dark Fibers (slow twitch; red fibers)


What type of myofiber is the gastrocnemius mainly composed of?

Light Fibers (white fibers; fast twitch)


True or False:

A single nerve cell (neuron) may innervate from a few to several hundred myofibers.



What makes up a motor unit?

A neuron and the myofibers it innervates constitute a motor unit.


Does all of the myofibers in a motor unit contract when the neuron of that motor unit fires?

YES, the all-or-none really refers to a motor unit.


Discuss the process of summation in a specific muscle as it builds up to tetany of that muscle.

electrical events occur faster than mechanical events --> an additional spike can occur before the previous calcium ions have been returned to the sarcoplasmic reticulum (SR) --> this increases the total amount of calcium ion in the cytosol and increases the rate of cycling between the myosin and actin cross-bridges --> this increases muscle tension --> each additional spike adds to the effects of the previous spikes --> if the frequency of spikes is fast enough, there is no time for relaxation between spikes --> muscle remains at maximal contraction (tetany is reached)


What happens to the muscle when tetany is reached?

The muscle remains at maximal contraction.


Fill in the blank:

______transmit forces from one place to another?

machines (consider bone-muscle systems as machines)


What two forces do machines involve?

1) force applied to the machine (in-force (Fi) or effort)

2) force derived from the machine (out-force (Fo) or resistance)


What is a lever?

A lever is a rigid body (bone) that rotates around a pivot (joint) or fulcrum.


What is the distance from the in-force (muscle attachment) to the fulcrum (joint)?

The in-lever arm


What is the distance from the out-force to the fulcrum (joint)?

The out-lever arm.


True or False:

A functional lever must have at least two moments.



How do you calculate Mi?
How do you calculate Mo?
What is the formula at equilibrium?

* Mi = FiLi

*Mo = FoLo

*equilibrium = FiLi = FoLo


What is a first-class lever and example of one?

*the fulcrum is in the middle
*the in-force and out-force move in opposite directions

*Example: raising chin using sternocleidomastoids or similar muscles (fulcrum = atlas/axis complex)
Also, a sea-saw or a pry bar.


What is a second-class lever system and an example?

*resistance (out-force) is in the middle
*fulcrum = ball of foot
*both in and out forces are on the same side of the fulcrum

*Example: raising the body on the ball of the foot


What is a third-class lever and an example?

*effort (in-force) is in the middle
*both in and out forces are on the same side of the fulcrum
*both forces move in same direction

*Example: lifting a weight in the palm of your hand


How are lever systems classified?

Lever systems are classified according to the position of the fulcrum in relation to the in-force and the out-force.


Where is ATP required for muscle contraction?

1) most is used for sliding filament mechanism
2) pumping calcium ions from sarcoplasm back into sarcoplasmic reticulum
3) pumping sodium and potassium ions through the sarcolemma to reestablish resting potential


What is the concentration of ATP in muscle fiber?

about 4 mmol (at least enough to maintain contraction for 1-2 seconds)


What are the 3 main things that provide the energy for rephosphorylation?

1) Phosphocreatine
2) Glycolysis
3) Oxidative Metabolism


What provides more than 95% of all energy needed for the long-term contraction of muscle?

oxidative metabolism


What does phosphocreatine do to provide energy for rephosphorylation?

*it releases energy rapidly
*it reconstitutes ATP
*ATP + phosphocreatine provides enough energy for 5-8 seconds of contraction


How does glycolysis provide energy for rephosphorlyation?

*by lactic acid build-up
*can sustain contraction for 1 minute


What is the definition of preload?

The load on a muscle in the relaxed state (before it contracts).


What is directly proportional to the preload in muscle?

The passive tension.

***The greater the preload, the greater the passive tension in the muscle.