CLASS 10 PCA + LECTURE NOTES

what is the main energy source for muscles and how long does it last?

ATP

only stores limited reserves of ATP – can sustain 5-6 seconds of activity.

three main mechanisms of replenishing ATP

Phosphorylation by phosphocreatine

Anaerobic glycolysis and lactate formation

Oxidative phosphorylation

phosphocreatine

high energy molecule that acts as a rapid, anaerobic energy reserve in vertebrate tissues

how and when does phosphocreatine work

it is the first level of energy backup after the 5-6 seconds of ATP is used

• muscles convert phosphocreatine into ATP

• allows for 15 more seconds of sustained energy

when is glucose the main energy source

after atp and after phosphocreatine

glucose metabolic biochem path

glucose → glycolysis → pyruvate → anaerobic or aerobic pathways

aerobic pathway

oxidative pathway!

slow ATP but more of it

• Provides enough energy for hours

• Resource-intense

anaerobic pathway

glycolytic pathway!

Makes ATP more quickly, but less of it

• Provides enough energy for 30-40 secs

• Resource-light

energy source summary

skeletal muscle fiber classification

speed of contraction and main mechanism for ATP

features of fast contracting fibers

• myosin isoform that hydrolyzes ATP more quickly

• SERCA isoform that pumps Ca2+ more efficiently

• Fast-contracting fibers crossbridge and relax more quickly.

slow oxidative

low-intensity, sustained activity

fast glycolytic

high-intensity, short burst

fast oxidative-glycolytic

intermediate levels

fatigue

Inability to contract even when the muscle is receiving stimulation

→ low on ATP

slow oxidative features

Myosin ATPase + SERCA activity: slow

Primary pathway for ATP synthesis: oxidative phosphorylation

Diameter: small

Tendency to fatigue: low

Capillary density: high

Myoglobin content: high

Color: dark red

fast oxidative-glycolytic

Myosin ATPase + SERCA activity: fast

Primary pathway for ATP synthesis: Oxidative phosphorylation + anaerobic glycolysis

Diameter: medium

Tendency to fatigue: medium

Capillary density: medium

Myoglobin content: medium

Color: red

fast glycolytic

Myosin ATPase + SERCA activity: fast

Primary pathway for ATP synthesis: anaerobic glycolysis

Diameter: large

Tendency to fatigue: high

Capillary density: low

Myoglobin content: low

Color: pale

real world examples

Running a marathon: Primarily slow oxidative (aerobic)

Running a sprint: Primarily fast glycolytic (anaerobic)

how does endurance cause muscle fibers to adapt

• Increase the number of mitochondria

• Increased myoglobin

• Increased capillary density

• Increased reliance on oxidative pathways

motor unit

a single lower motor nueron and all the muscle fibers it innervates

features of motor units

can innervate up to hundreds of muscle fibers.

When one lower motor neuron fires, all the muscle fibers in a motor unit contract together

Each muscle fiber is only innervated by one motor neuron.

load

weight

tension

muscle force

isometric contraction BEFORE lifting

same length = no movement

tension < load

concentric contraction

muscle shortens

tension > load

eccentric contraction

muscle lengthens

tension < load

factors affecting muscle tension

1) # of motor units recruited

2) frequency of stimulation of each motor unity

3) resting sarcomere length before contraction

4) size of muscle fibers in motor units

1) Number of motor units recruited

more motor units → more tension (vice versa)

2) frequency of stimulation of each motor unit

• APs are far apart in time , we observe two distinct twitches

• APs are closer, the second contraction begins before the first relaxation finishes

refractory period ( for motor units)

he refractory period of the muscle action potential is shorter than time it takes for muscle to fully relax

unfused tetanus

If the frequency is higher still, the muscle reaches a steady state and maintains constant tension

→ partial contraction/relaxation

fused tetanus

if the frequency is super high

→ no relaxation and rapid fatigue

muscle cramps/ spasms

sudden, involuntary contractions of a muscle due to hyperexcitability (excess firing) of lower motor neurons

3) resting sarcomere length

4) size of muscle fibers

The larger the muscle fibers within a motor unit, the more tension they produce

strength training

working out induces hypertrophy (cell enlargement) of muscle fibers

motor unit recruitment

the process of activating additional motor units to increase muscle tension (force).