Skeletal Muscle Physiology

Front 1

Characteristics of sub-threshold potentials

Back 1

Proportional to stimulus strength, not propagated, decremental with distance, summation

Front 2

Characteristics of action potentials

Back 2

Independent of stimulus strength, propagated unchanged in magnitude, summation not possible

Front 3

Factors that affect conduction velocity of the action potential

Back 3

Cell diameter and amount of myelination are directly proportional to conduction velocity

Front 4

Absolute refractory period

Back 4

No stimulus can depolarize the cell

Front 5

Relative refractory period

Back 5

A large stimulus can depolarize the cell

Front 6

Neuromuscular transmission

Back 6

Action potential travels down axon and opens pre-synaptic Ca channels --> calcium influx --> release Ach vesicles --> Ach diffuses and attaches to nicotinic ion channels --> ↑gNa --> end-plate depolarization (local) spreads to areas with voltage-gated Na channels --> depolarization of muscle fiber

Front 7

Excitatory postsynaptic potentials

Back 7

Transient subtreshold depolarizations due to ↑gNa --> summation reaches axon hillock at the junction of cell body and axon --> voltage-gated Na channels depolarize the axon

Front 8

Inhibitory postsynaptic potentials

Back 8

↑gCl or ↑gK hyperpolarize the cell and lower threshold for depolarization

Front 9

Electrical synapse

Back 9

Action potential transmitted from one cell to the next via gap junctions, without synaptic delay and in both directions. Cardiac muscle, smooth muscle.

Front 10

Sarcomere A band

Back 10

Contains overlapping actin and myosin. Does not shorten during contraction.

Front 11

Sarcomere H zone

Back 11

Contains thick myosin filaments. Shortens during contraction.

Front 12

Sarcomere I band

Back 12

Contains thin actin filaments. Shortens during contraction.

Front 13

Sarcomere Z line

Back 13

Within the I band.

Front 14

Sarcomere M line

Back 14

Within the H zone.

Front 15

Actin

Back 15

Structural protein of the thin filaments, contains attachment sites for myosin cross-bridges.

Front 16

Myosin

Back 16

Structural protein of the thick filaments, contains cross-bridges that attach to actin. Has ATPase activity to terminate actin-myosin cross-bridges. ATP decreases actin-myosin affinity.

Front 17

Tropomyosin

Back 17

Part of thin filaments. Covers the actin attachment sites for the myosin cross-bridges

Front 18

Troponin

Back 18

Part of thin filaments, binds calcium, which moves tropomyosin out of the way exposing actin binding sites for cross-bridges.

Front 19

What happens if calcium is removed to the sarcoplasmic reticulum?

Back 19

Muscle goes back to resting state. Removal of calcium requires ATP.

Front 20

Rigor mortis

Back 20

Depletion of ATP - cycling stops with myosin attached to actin - (muscle contracted).

Front 21

Muscle contraction steps

Back 21

Action potential travels down T-tubules --> activates dihydropiridine voltage sensors --> foot processes are pulled aways from ryanodine calcium release channels of sarcoplasmic reticulum --> calcium is released --> calcium attaches to troponin --> tropomyosin moves exposing actin binding sites for myosin cross-bridges --> myosin binds actin --> myosin ATPase breaks down cross bridges producing active tension and shortening --> contraction terminated by active pumping of Ca into the sarcoplasmic reticulum.

Front 22

Myosin ATPase

Back 22

Hydrolizes ATP to supply energy for active tension and shortening. ATP decreases myosin-actin affinity

Front 23

Sarcoplasmic calcium-dependent ATPase

Back 23

Supplies energy to terminate contraction and pump Ca back into sarcoplasmic reticulum.

Front 24

Source of calcium for skeletal muscle contraction

Back 24

Sarcoplasmic reticulum. No extracellular calcium is involved because it doesn’t have voltage-gated Ca channels.

Front 25

Source of calcium for heart and smooth muscle contraction

Back 25

Sarcoplasmic reticulum and extracellular. Cardiac and smooth muscle have voltage-gated calcium channels.

Front 26

Tetanus

Back 26

Multiple action potentials increase release of calcium thus increasing contraction. Muscle cells have a short refractory period.

Front 27

Preload

Back 27

Stretch prior to contraction. ↑ preload --> ↑ prestretch of the sarcomere --> ↑ passive tension

Front 28

Afterload

Back 28

The load the muscle is working against. ↑ afterload --> ↑ cross-bridge cycling --> ↑ active tension

Front 29

What is the best measure of preload?

Back 29

Sarcomere length

Front 30

Preload-length tension curve

Back 30

It’s a function of the legth of the relaxed muscle. A positive parabola.

Front 31

Isomertric contraction

Back 31

Active tension is produced but length stays the same. Afterload is greater than active tension, load not moved.

Front 32

How is active tension produced?

Back 32

Calcium binds troponin --> tropomysion exposes actin sites --> myosin cross-bridges bond to actin --> myosin ATPase generates energy to break cross-bridge link --> cycle repeats --> active tension. The more cross-bridges that cycle, the greater the active tension.

Front 33

Total tension

Back 33

Passive (preload) tension + active (afterload) tension

Front 34

Active tension curve

Back 34

It's a function of the number of cross-bridges capable of cross-linking with actin. Negative parabola.

Front 35

What is L0?

Back 35

The optimum length to produce maximum active tension. Beyond L0, muscle is overstretched; below L0, it's understretched.

Front 36

Isotonic contraction

Back 36

Muscle contracts and shortens to move the load. Occurs when total tension equals the load.

Front 37

Most energy demanding phase of cardiac cycle

Back 37

Isovolumetric contraction. Active tension is generated. Equivalent to isometric contraction of skeletal muscle.

Front 38

Relationship between load, muscle force and muscle velocity

Back 38

↑ ATPase activity --> ↑ velocity; ↑ muscle mass --> ↑ force generated; ↑ afterload --> ↓ velocity

Front 39

Regulation of skeletal muscle force and work

Back 39

↑ frequency of action potentials, ↑ recruitment, ↑ preload and ↑ afterload --> ↑ force and work

Front 40

Regulation of cardiac and smooth muscle force and work

Back 40

Factors that regulate force and work are preload, afterload and contractility (which is altered by hormones). No summation nor recruitment.

Front 41

Characteristics of white muscle

Back 41

Large mass, high ATPase activity (fast muscle), anaerobic glycolysis, low myoglobin

Front 42

Characteristics of red muscle

Back 42

Small mass, low ATPase activity (slower muscle), aerobic metabolism (mitochondria), high myoglobin.

Front 43

Characteristics of skeletal muscle

Back 43

Actin and myosin form sarcomeres, sarcolema lacks junctional complexes, each fiber innervated, troponin binds calcium, high ATPase activity, triadic contacts by T-tubules at A-I junctions, no calcium channels on membrane

Front 44

Characteristics of cardiac muscle

Back 44

Actin and myosin form sarcomeres, gap junctions, electrical syncytium, troponin binds calcium, intermediate ATPase activity, dyadic contacts by T-tubules near Z-lines, voltage-gate calcium channels.

Front 45

Characteristics of smooth muscle

Back 45

Actin and myosin not organized in sarcomeres, gap junctions, electrical syncytium, calmodulin binds calcium, low ATPase activity, lacks T-tubules, voltage-gated calcium channels.