Muscle (Exam 1)

Contractile elements of muscle

Contractile elements:

• Actin:

  • thin filament

  • binding site for myosin head

  • has troponin + tropomyosin attached to it

• Myosin:

  • thick filament

  • head region acts like an enzymatic binding site for ATP hydrolysis

• Myofibril = bundles of myofilaments

Non-contractile elements of muscle

• sarcolemma = cell membrane

• perimysium = CT that binds muscle fibers into fascicles

• muscle fascicle = bundles of muscle fibers

(myofibrils + fibers + non-contractile CT = arranged in parallel → producing force)

Structure of the sarcomere

• Spans Z-disc → Z-disc

• arranged in series along the length of the muscle fiber (allowing for transmit force)

• Titin:

  • structural protein that helps anchor Z-disc to thick filament

  • NOT contractile (aka active)

7 steps of Cross-bridging cycle

1) Uncoupling

• sacromere @ rest,

• ATP bound to myosin head and troponin + tropomyosin block actin binding site

2) Hydrolysis of ATP

3) Recharging

• Ca2+ attaches to troponin and tropomyosin releases from actin binding site

4) Coupling

• myosin head attaches to actin → cross bridge form

5) Release of Phosphate

• helps w/ making energy for powerstroke (sliding of filaments of each other)

6) Contraction

• powerstroke occurs

7) Release of ADP

Types of muscle contractions

Concentric:

• shortening

• small Z-disc to Z-disc distance

Eccentric:

• Cross bridging still occurs

• lengthening

• large Z-disc to Z-disc distance

Isometric:

• Cross bridging still occurs

• Active (most force from it)

• NO change in Z-disc to Z-disc distance

Determinants of strength/active force production of muscle

• increased # of muscle fibers in a motor unit → increased active force production

• increased size in a motor unit → increased active force production

• increased # of motor units firing → increased active force production

• increased freq of motor units firing → increased active force production

• Slow muscle contraction → increased active force

• Optimal sarcomere length → increased active force produced

Comprises a motor unit and how are motor units organized to facilitate different functions (i.e. size, speed)?

Motor unit:

• alpha motor neuron + all the muscle fibers it innervates

Principle of motor units:

• Recruit first: Small MU have small axon, few fibers and are Type 1 —> fine motor

• Recruit later: large MU have large axon, many fibers and are Type 2 —> forceful contractions

What are the muscle fiber types and can you predict the role (Endurance vs. Force Production) based on the fiber type?

Types of Muscle fibers

Type 1 (slow oxidative):

• slow contraction

• slow rate of fatigue

Type 2A (fast oxidative glycolytic)

• fast contraction

• intermediate rate of fatigue

Type 2X (fast oxidative)

• fast contraction

• fast rate of fatigue

For endurance → want to use Type 1

For force production (strength improvement) → want to use Type 2

What are examples of muscles with each fiber type?

Vastus (superficial) → mostly type 2B and a little bit of type 2X

Gastroc (superficial) → mostly type 2B and a little bit of type 2X

Vastus (deep) → has all muscle fiber

Gastroc (deep) → has all muscle fibers

Soleus → only type 1

Active, Passive, and Total length tension curves

Active tension is greatest @ mid-position of muscle length

• want the optimal actin-myosin overlap

Passive tension occurs only when the muscle is being stretched

Total tension is greatest when the muscle is in a lengthened position

• b/c it is non-contractile that is helping produce force

***too shortened —> slack***

Can you describe the force/velocity relationship?

• can move heavier load slowly

• can NOT move heavier load when moving rapidly

• Eccentric >> isometric >> Concentric

***Eccentric = external torque wins meaning highest force cuz you go against gravity**

***Concentric = muscle force has winning torque***

***Isometric: Internal torque = External torque***

Muscle architecture

Fusiform/strap:

• Long + parallel fibers→ good for lengthening + shortening (producing ROM)

• PCSA = ACSA

Pennate:

• more muscle fibers → good for force production

• shorter muscle fibers → less drastic length change

• PCSA is larger than ACSA

Anatomical Cross-Sectional area vs Physiological Cross-Sectional area

ACSA: measure around muscle

PCSA: measure around muscle @ 90 dg/ perpendicular to the muscle fibers

• determines force production

Active Insufficiency vs Passive Insufficiency

Active Insufficiency:

• inability for a biarticulate muscle to make enough torque to maximally shorten across all of the joints it crosses

  • Ex: hamstring fully extends hip but can NOT fully flex the knee → No full shortening of hamstring

• avoid for strength training b/c to be stronger there needs to be a lot of force production and this will produce less force

Passive Insufficiency

• inability for a biarticulate muscle to make enough torque to maximally stretch across all of the joints it crosses

  • Ex: Rectus femoris NOT lengthening enough to allow maximal knee flexion