Week 9 - More Nervous System

Skeletal muscle - contraction

Crossbridge cycling - consequences:

• Movement = only about 10 micrometers

  • Thick (myosin) pulls thin (actin) toward M line

  • Shortening of:

    • Sarcomere

    • I band

    • H zone

    • NOT A band

Skeletal muscle - relaxation

Must stop crossbridge cycling by stopping…

• Motor neuron action potentials

• ACh

• Myofiber action potentials

• Ca²⁺ exiting sarcoplasmic reticulum

  • Return Ca²⁺ to sarcoplasmic reticulum

    • From cytosol

    • Via ATPase pump/transporter

    • Into sacroplasmic reticulum

Skeletal muscle - twitch

Twitch = the pattern of tension (T) produced by a single action potential

• T = force generated from crossbridge cycling

• AP = 1-2 milliseconds (over before any T exists)

• Delay before any T

  • Latent period (& release of Ca²⁺)

  • Get crossbridge cycling started

• Build to peak

• Delay before there’s no T

  • Remove all Ca²⁺

Skeletal muscle - ATP

• Needs (lots)

  • Crossbridge cycling

    • Unbinding myosin from actin

    • Energizing crossbridge for next cycle

  • Relaxation → ATP pumps Ca²⁺ back into SR to relax the muscle

Skeletal muscle - ATP - sources

• 1. Creatine Phosphate

• 2. Glycolysis

• 3. Kreb’s Cycle

• 4. Oxidative Phosphorylation

1. Creatine Phosphate

• 1 ATP

• Initial source

• Anaerobic (can do aerobic)

• Replenished during relaxation

• Problems: osmolarity, kidney damage

2. Glycolysis

• 2 ATPs

• Breaks down glucose

  • Blood

  • Oxygen

• Anaerobic (can do aerobic)

  • Intense (need lots of ATP now)

    • “Sprinting to library as fast as possible”

  • Prolonged

    • “Walking to Cleveland without stopping”

  • Lactate created mainly during intense activity: cleared during rest

    • Producing energy anaerobically: break down glucose (glycolysis), create pyruvate, which turns into lactate to keep producing energy when O₂ is limited

3. Kreb’s Cycle

• 2 ATPs

• Aerobic only

• Sources:

  • Glycolysis

  • Fatty acids

  • Amino acids

4. Oxidative Phosphorylation

• 28-34 ATPs

• Aerobic only

• Sources:

  • Kreb’s Cycle

  • Glycolysis

Skeletal muscle - whole muscles - contraction

• Contraction:

  • Generation of tension

  • Opposed by load (L)

    • Bucket = load, but we’re also the load!

  • Shortening ONLY if tension > load

Types of contraction

• Isometric (tension < load)

  • Independent of load; contraction but no change in muscle length

    • Independent = the muscle will still try to contract and generate tension regardless of how heavy the load is

• Eccentric (tension < load)

  • Dependent of load; contraction & lengthening of muscle as load wins

    • Dependent = how much the muscle lengthens depends on how heavy the load is

• Isotonic (tension > load)

  • No change in tension (in order to conserve ATP/energy); muscle shortens as tension wins

Isotonic contraction - keys

• Isometric must come 1st (to build tension)

• As load increases…

  • Increased latency (b/c we must generate a bigger tension to overcome the load, which takes time → “latent period”)

  • Decreased velocity

  • Decreased response (b/c the initial tension isn’t enough anymore to overcome the increased load)

Summed tension

• Tension is summed → builds the tension

  • This maintained contraction = tetanus

    • Unfused tetanus: partial relaxation (shown in image)

    • Fused tetanus: no relaxation

      • Done when we need to do it fast → like in dangerous situations or in American Ninja Warriors

Difference between myofibers

1. Speed of contraction

2. Main ATP source

1. Speed of contraction

• Slow vs. fast twitch

  • Fast: gets to tension quickly, but uses up ATP quickly

  • Slow: uses up ATP slowly, but gets to tension slowly

• ATPase speed in crossbridge cycling

2. Main ATP source

• Glycolytic (anaerobic)

  • Few mitochondria

  • Lots of glycogen

• Oxidative (aerobic)

  • Many mitochondria

  • Highly vascularized (more vessels)

  • Lots of myoglobin → allows us to store O₂ b/c O₂ can’t be stored in myofibers

Skeletal muscle - whole muscles - myofiber types

1. Slow-oxidative

2. Fast-oxidative (aka fast-oxidative-glycolytic)

3. Fast-glycolytic

Myofiber types - characteristics chart

Most muscles are a mix of all 3 types

• Genetic basis → based on what the muscle has always done

• Slight modification is possible (you can improve how a muscle fiber functions, but you can’t completely change its type)

  • Change in structure (ex. adding/subtracting mitochondria)

  • Change in function (ex. focusing more on glycolysis than on using mitochondria)

Source of muscle type modification

• Disuse & some diseases = atrophy

• Use, especially exercise = hypertrophy

  • Aerobic = increased vascularization, increased number of mitochondria, decreased fatigue (shouldn’t be intense or prolonged)

  • Anaerobic = increased myofiber diameter (by adding more sarcomeres), increased glycolytic enzymes (b/c more intense & prolonged)

Extra Credit

• Working hard to relax

• “Clean-up principle” → easy to make a mess, but harder to clean it up

• Creatine gives body builders bigger/swollen muscles

• Anaerobic - intense = sprinting to library as fast as possible

• Anaerobic - prolonged = walking to Cleveland without stopping

• Physics

• She-hulk

• $25

• Increased latency: Olympics → pause before weight lift to build tension against load

• Bad tetanus lasts a decade

• American Ninja Warrior

• Speed skating

• Usane Bolt

• Sprinter vs. Marathoner