Skeletal Muscle Design

Question 1

Parallel muscles

Answer 1

• long fibers arranged parallel to length of muscle
• produce greatest range of movement
• most common skeletal muscle formation

Question 2

Pennate muscles

Answer 2

• have shorten fibers arranged obliquely in a manner similar to a feather
• arrangement increases the cross sectional area of the muscle, thereby producing more tension
• Because the muscle cells pull at an angle, contracting pennate muscles do not move their tendons as far as parallel muscles do; shorter range

Question 3

Convergent muscles

Answer 3

• muscle fibers are spread over a broad area
• versatile, because the stimulation of only one portion of the muscle can change the direction of the pull
• when the entire muscle contracts, the muscle fibers do not pull as hard on the attachment site as would a parallel muscle of the same size

Question 4

circular muscles

Answer 4

• fibers concentrically arranged around an opening or recess

- when muscle contracts, the diameter of the opening decreases

Question 5

Henneman Size Principle (of motor units)

Answer 5

• motor neurons with the smallest cell bodies are the ones most easily brought to firing threshold and are thus the first recruited.
• The smaller the motor neuron the smaller the number of muscle fibers it innervates. Thus as more units are recruited, larger and more forceful units are recruited.
• The distribution of motor unit sizes is such that the percentage increase in force that a unit adds to the total muscle force is relatively constant– this enables a smooth grading of force as recruitment proceeds in to the motor unit pool
• recruit slow, then fast-fatigue resistant, then fast fatigable

Question 6

Increasing the amount of time myosin is spent in strong binding states do what to the amount of force a muscle cell can generate?

Answer 6

-Increases it

Question 7

What is the consequence of smooth muscle’s myosin being “slower” than a fast myosin in skeletal muscle?

Answer 7

Skeletal and smooth muscle myosin generate the same amount of unitary force, but smooth muscle holds the force longer
-expression of different contractile proteins results in muscles with diverse mechanical properties

Question 8

Muscle efficiency is dependent on

Answer 8

-sliding velocity and muscle fiber type

Question 9

When is muscle efficiency the greatest?

Answer 9

-At mid-range velocities

Question 10

When is power output maximal?

Answer 10

• at mid-range velocities
• thus existence of different fiber types (each composed of different protein isoforms) with differing force-velocity relationships allows the muscle to perform a variety of tasks with optimal efficiency

Question 11

Fenn Effect

Answer 11

Rate of heat liberation is higher in shortening muscle compared to one developing isometric tension

Question 12

Rate limiting step of isotonic (unloaded) contraction vs. isometric contraction

Answer 12

isotonic: Pi release; release ADP really fast so ATP can bind and new cycle can start and can shorten
isometric: ADP release; remains here for a while so it can remain in strong binding state and not shorten

Question 13

Muscle fiber type is predominantly determined by __________.

Answer 13

Innervation: can switch fiber type innervation and alter fiber type

Question 14

Muscles and bones are connected via ______.

Answer 14

tendons, which are primarily composed of parallel arrays of collagen fibers
-tendons transmit muscle to bone

Question 15

Lateral and radial forces are transmitted in skeletal muscles via _______.

Answer 15

connective tissues: “share force” and thus sarcomere shortening with neighbors

Question 16

_________ transmits the lateral force of muscle contraction to the extracellular matrix.

Answer 16

• Dystrophin
• similar to spectrin
• renders muscle fibers capable to withstand and transmit longitudinal and radial mechanical forces to the ECM

Question 17

During hypertrophy, myofibrils are added in _______. What does this result in?

Answer 17

• parallel

- greater force generation

Question 18

Growth and hypertrophy of skeletal muscle involves activation, proliferation, and fusion of satellite cells (muscle stem cells) into existing fibers or even forming new fibers (hyperplasia). What are the pro- and anti-growth factors in regulating this process?

Answer 18

Pro: Hepatic growth factor (Scatter factor)- cytokine; found bound within ECM of skeletal muscle, where upon injury, it is released and activated satellite cells via their c-met receptors
Pro: Insulin-like growth factors (IGF-1)-
Anti: myostatin: secreted TGF-B family member that functions as negative regulator of muscle growth. In adults, its expression is restricted almost exclusively to skeletal muscle