M2 L6 Flashcards
How do fibers generate different forces? (3 reasons)
Based on how many cross bridges are working in parallel.
- Also greater cross sectional area = greater force production.
- also force produced by myosin heavy chain (fast) is greater than slow MHC
What is the myosin type determined by?
The type of motor neuron that stimulates it
- Slow motor neurons → stimulate fibers to express slow myosin (Type I) → suited for endurance.
- Fast motor neurons → stimulate fibers to express fast myosin (Type IIa or IIb) → suited for speed and power.
What type of MHC isoform are in the same myofibril
each myofibril will only contain the same type of MHC isoform.
meaning all the myosin heavy chain (MHC) isoforms are the same—either all slow (Type I) or fast (Type IIa/IIb). This allow fibers to contract at the same time w same force.
- If different myosin types were mixed within the same myofibril, some parts would contract faster or slower than others, leading to inefficient and uncoordinated contractions.
Keeping the same MHC isoform ensures that all sarcomeres contract together with the same speed and force, making movement smooth and efficient.
What did the experiment with cross innervating show?
As time passes or endurance training occurs, the fibers can adapt and change myosin type.
ex:
* Endurance training shifts Type IIb → IIa → I (more fatigue-resistant).
* Strength training shifts Type I → IIa → IIb (more powerful).
Whats a motor unit
When one motor neuron can innervate and supply multiple different muscle fibers.
They express the same MHC and have similar metabolic characteristics.
What properties are similar in a motor unit
- MHC is-form
- activation start time
- metabolic processes (rely on the same energy system (oxidative for slow units, glycolytic for fast units))
- contractility
Motor unit type 1 characteristics:
* v max:
* metabolic process:
* fiber diameter:
* fatiguability:
* conc of Ca2+:
* # of fibers per motor unit:
* force production
- slow/low
- o2 phos
- small
- low
- Low (slow Ca²⁺ release/reuptake)
- few (less than 3 - for precise control and fine movements like finger moving)
- Low (steady, endurance-based)
Motor unit type 2 characteristics:
* v max:
* metabolic process:
* fiber diameter:
* fatiguability:
* conc of Ca2+:
* # of fibers per motor unit:
* force production
- fast/high
- glycolysis
- large ((stronger force, fatigues faster))
- high
- High (fast Ca²⁺ cycling for rapid contractions)
- many (100s to generate large force for quick powerful movement)
- High (explosive, short bursts)
Why does type 2 generate more force than type 1?
- Ca2+ is released quickly from SR (so more cross bridges and stronger force)
- larger cross sectional area (more cross bridges available)
- more cross bridges in parallel (more force)
- bc of fast MHC - its more efficient for chemical to mechanical energy conversion
When is type 1 used
* ex
- low forces for long periods of time
- ex: eye muscles
when is type 2 used
* ex
large amount of force for short period of time
* ex: quads or glutes
How are motor units distributed in a muscle?
All the fibers are distributed around the muscle so there is a distributed force produced/muscle contraction
- Type 2 are more present and are bigger with more muscle fibers.
- each motor unit connects to its respective motor neuron.
How do we modify force production (2 ways)
- activate different motor units
- increase frequency of activation of muscle fiber with inc ca2+
what is recruitment
activating more motor units to generate more force during muscle contraction.
what is tetanus
When a muscle is stimulated repeatedly without enough time to relax between stimuli, the calcium levels in the muscle remain high, leading to sustained contraction.
on a graph it looks like a high plateau where the muscle reaches max force from stimulus
What happens with recruitment
As you turn on more motor units, velocity will increase with the more working cross bridges.
Also once you get to c and d the larger type 2 motor units, you will have more muscle fibers and force
start with nerve A bc its less fatiguable and lasts the longest
Whats the process of turning them off
The first on will be the last to turn off.
explain the effects of increased force on velocity (more motor units)
More motor units recruited → more muscle fibers stimulated → more cross-bridge cycling @ more rapid speed
Differences between slow and fast twitch muscles?
Slow twitch:
* contains more type 1
* more vasculature
* red in color
* more mitochondria
* small
Fast twitch:
* more type 2
* whiter in color:
* large
* less vasculature
* some mitochondria
list all ways to change force production in skeletal muscle
- # of working CB in parallel
- modify via recruitment
- change MHC isoform expression
- change frequency stimulation
- change cross sectional area
- change sarcomere length (like is it at optimal or no)
