ex phys exam 3

how much of the human body is skeletal muscle?

human body contains over 600 skeletal muscles

• 40-50% of total body weight

what are the functions of skeletal muscle?

force production for locomotion, breathing, and postural support

heat production during cold stress

skeletal muscle gross structure

epimysium (whole muslce), perimysium (bundle of muscle fibers- fascicle), endomysium (between fibers)

tendons connect muscle to bone

microstructure of skeletal muscle

muscle fiber (muscle cell)

• myofibrils

  • sarcomeres

    • myofilaments (actin and myosin)

      • actin = thin filament — troposin complex and tropomyosin

      • myosin = thick filament

arrangement of myosin and actin

resting: myosin and actin overlap with myosin heads pointing outward from M line

• actin extends inward from Z discs

• myosin is centered around M line

• topomyosin blocks the myosin binding sites on actin

during contraction:

• actin slides toward M lines

• myosin heads move through cross bridge interactions

sarcolemma

muscle fiber’s cell membrane

t-tubules

inward extensions of sarcolemma, carry electrical signal (AP) from the surface deep into muscle fiber

SR

specialized network of membrane bound tubules that surrounds the myofibrils

stores calcium ions and releases them when muscle fiber is stimulated

takes calcium back up when the muscle needs to relax

the sliding filament model

proposes that muscle shortens or lengthens because thick and thin filaments slide past each other without changing length

myosin crossbridge cycling

myosin continually attaches and detaches from actin filaments with energy from ATP hydrolysis

contractions in sliding filament model

1. Concentric contraction: fiber length shortens during force generation

2. Isometric contraction: fiber length remains unchanged during force generation

3. Eccentric contraction: fiber lengthens during force generation

cross bridge formation during muscle contraction

1. AP triggers Ca2+ release from SR → Ca2+ binds to troponin → tropomyosin shifts to expose myosin binding sites on actin

2. myosin head (carrying ADP and Pi) binds to actin, forming cross bridge

3. myosin head pivots and pulls actin toward the M line (releasing Pi and ADP) → generates force (POWER STROKE, causing filaments to slide)

4. new ATP binds to myosin, causing myosin head to detach from actin

5. myosin hydrolyzes ATP into ADP and Pi → energy released re energizes the head and returns it to its cocked position, ready to bind to actin again if binding sites remain exposed

**cycle repeats as long as calcium remains elevated and ATP is available

explain the sarcomere length tension curve

shows that the amount of force (tension) a muscle can generate is dependent upon the initial length of the sarcomere before contraction (which is determined by the degree of overlap between actin and myosin)

• if a muscle is over stretched or overly shortened before contracting, its ability to generate force is compromised

steps of muscle excitation

1. sac-like vesicles within axon terminal release Ach → diffuses across synaptic cleft + attaches to specialized Ach receptors on sarcolemma

2. MAP depolarizes transverse tubules at the sarcomere’s A-I junction

3. t-tubule system depol causes CA2+ release from SR lateral sacs

steps of muscle contraction

4. Ca2+ binds to troponin-tropomyosin in actin filaments

5. actin joins myosin ATPase to split ATP with energy release during muscle action → tension from energy release produces myosin cross bridge movement

6. muscle shortening occurs after ATP binds to the myosin cross bridge, which breaks the actin myosin bond and allows cross bridge dissociation from actin and sliding of thick and thin filaments

7. cross bridge activation continues when Ca2+ concentration remains high (From membrane depol) → this prevents troponin-tropomyosin action

8. when muscle stimulation ceases, Ca2+ moves back into the SR lateral sacs through active transport via ATP hydrolysis

9. Ca2+ removal restores troponin-tropomyosin inhibitory action → with ATP present, actin and myosin remain in the dissociated relaxed state

how do muscle fiber types differ

skeletal muscle contains two main types of fibers that differ in three characteristics:

• primary mechanism to produce ATP

• motor neuron innervation type

• myosin heavy chain type expressed

how do the proportions of each type of muscle fiber vary?

they vary from muscle to muscle and from person to person:

on average, how many slow twitch fibers to men, women, and children possess in their arm and leg muscles?

45-55%

what is the muscle biopsy technique?

removes cross sectional area of muscle to determine muscle fiber type

how does the muscle biopsy technique work?

1. Serial cross sections obtained by muscle biopsy- using acidity (pH), different muscle fiber types stain different colors

2. More basic solution shows more slow twitch; more acidic shows more fast twist

type I fibers (slow twitch)

SO (slow oxidative)

aerobic, highly fatigue resistant (endurance activities)

type IIa (fast twitch)

FOG (fast oxidative glycolytic)

fast contraction speed, utilizes both aerobic and anaerobic energy pathways

type IIx (fast twitch)

fast twitch intermediate fiber type bridging type IIa and type IIb

type IIb (fast twitch)

FG (fast glycolytic)

primarily anaerobic, generates high force rapidly, but fatigues very quickly

contraction time of fibers

type I- slow

type IIa- moderately fast

type IIx- fast

type IIb- very fast

motor neuron size of m fibers

type I- small

type IIa- medium

type IIx- large

type IIb- very large

fatigue resistance of m fiber types

type I- high

type IIa- fairly high

type IIx- intermediate

type IIb- low

activity of m fiber types

type I- aerobic

type IIa- long term anaerobic

type IIx- short term an

type IIb- short term an

useful duration of m fiber types

type I- hours

type IIa- <30 minutes

type IIx- <5 minutes

type IIb- <1 min

force production of m fiber types

type I- low

type IIa- medium

type IIx- high

type IIb- very high

mitochondrial density of m fiber types

type I- high

type IIa- high

type IIx- medium

type IIb- low

capillary density of m fiber types

type I- high

type IIa- intermediate

type IIx- low

type IIb- low

oxidative capacity of m fiber type

type I- high

type IIa- high

type IIx- intermediate

type IIb- low

glycolytic capacity of m fiber types

type I- low

type IIa- high

type IIx- high

type IIb- high

major storage fuel of muscle fiber types

type I- triglycerides

type IIa- PCr glycogen

type IIx- PCr glycogen

type IIb- PCr glycogen

myosin heavy chains of each muscle fiber type

type I- MYH7

type IIa- MYH2

type IIx- MYH1

type IIb- MYH4

primary subtypes of fast twitch fibers (type II)

type IIa- represents fast-oxidative glycolytic fibers (FOG)

type IIx- midway between a and b types in physiologic and metabolic characteristics

type IIb- possesses greatest anaerobic potential and most rapid shortening velocity; represents ‘true’ fast glycolytic fiber

key features of fast twitch fibers

high electrochemical transmission of APs

high myosin ATPase activity

rapid Ca2+ release and uptake by SR

high cross bridge turnover rate

key features of slow twitch fibers

low myosin ATPase activity

slow calcium handling ability and shortening speed

less glycolytic capacity than fast twitch fibers

large and numerous mitochondria

generate energy for ATP resynthesis through aerobic system of energy transfer

fiber type differences among athletic groups

fast twitch fibers distribute equally between type IIa and type IIx subdivisions

although no gender differences exist in fiber distribution, larger inter individual variation occurs

endurance athletes possess predominantly slow twitch fibers predominate for elite sprint athletes

force velocity relationship

explain this: force power relationship

1. Peak power happens at something less than maximum force and less than maximum velocity

   1. If force x velocity = power, then if im at max force, then velocity is zero, and power is zero (more force doesn't always mean more power)