L10 Skeletal Muscle

Question 1

Skeletal muscle

Answer 1

Storage and voluntary

Innervation: somatic

Movement 
Metabolic regulation: think diabetes 
Heat production: think shivering 
Adaptable to diff loading conditions 
Many clinical conditions involved 
Changes in muscle have important implications for physical function

Question 2

Muscle fibers

Answer 2

Large, multinucleated

Composed of bundles of myofibrils

Question 3

Myofibrils

Answer 3

Consists of a bundle of parallel myofilaments

Question 4

Myofilaments

Answer 4

Organized into contractile units called sarcomeres (smallest functional unit of skeletal muscle)

Question 5

Two types of myofilaments

Answer 5

Myosin and action

Question 6

Myofibrils banding patterns

Answer 6

Dark band- myosin

Light band - actin

Question 7

A band

Answer 7

Thick filaments (myosin)

Thin filaments also present in parts of A band

Question 8

I band

Answer 8

Thin filaments plus Z line

Question 9

H band

Answer 9

Centered around M line

Contains myosin plus M line proteins

Question 10

Z line

Answer 10

Anchors thin filaments

Defines the sarcomeres

Question 11

M line

Answer 11

Supports and organize the myosin filaments

Composed of cytoskeletal proteins

Question 12

Each fiber innervated by

Answer 12

One neuron at one location

Question 13

One neuron can innervate

Answer 13

Many fibers ( motor unit)

Motor unit can be different sizes

Question 14

Excitation-contraction coupling

Answer 14

1. Action potential at NMJ
2. Ca release
3. Myosin and action interaction
4. Muscle contraction (cross-bridge cycling)
5. Ca removal
6. Relaxation

Question 15

1. Action ptoential at NMJ

Answer 15

Action potential from motor neuron results in release of ACh

Opening of ACh receptors results in EPP

EPP depolarization the motor end-plate and intimates action potentials in the muscle sarcolemma

Question 16

2. Calcium release

Answer 16

Action potential travels through the entire sarcolemma and into the transverse (T) tubules (toward where Ca stores- sarcoplasmic reticulum)

T-tubules make contact with terminal cisternae of SR forming triads (SR, T, SR)

Muscles can contract

Question 17

Steps in Ca release from SR

Answer 17

T-tubules contain slow activating voltage gated Ca sensors called dihydropyridine (DHP) receptors in skeletal muscle (not acting as receptor! )

SR membrane contains Ca release channels called ryanodine receptors (RYR)(channels open when receive signal from DHP)

Mechanical connection btw DHP AND RYR

Question 18

Thick (myosin) filaments

Answer 18

Composed of two proteins twisted together forming a tail with two head (cross-bridge)
Two pairs of light chains (MLC, head)
One pair of heavy chains (MHC, tail)

Has ATPase activity

Question 19

Thin (actin) filaments

Complex of three proteins

Answer 19

Actin- attachment site for myosin cross bridges (Monomers make helix)

Tropomyosin- reg protein that sterically inhibits binding of myosin to actin (rod shape)

Troponin - regularity protein composed of 3 subunit (globular molecules)

Question 20

Subunits of troponin

Answer 20

TnC : TROPHONIN CALCIUM
TnI : troponin inhibitory
TnT: troponin tropomyosin

Question 21

Role of Ca in regulation of myosin binding to actin

3. Myosin and actin interaction

Answer 21

Ca binding to the troponin complex allows for physical repositioning of tropomyosin filament

Which exposes the myosin binding site of the action molecules

Question 22

Sliding filament hypothesis

Answer 22

Contraction occurs by sliding of the thin filament past the thick filaments with actual lengths of the filaments remaining unchanged

Series of cyclic reactions between the myosin head and the actin filament makes filaments slide: cross bridges

Predict that contractile force produced by the muscle fiber is proportional to the number of cross-bridges formed

Question 23

What makes filament slide?

Answer 23

Series of cyclic reactions between the myosin head and the actin filament : cross bridges

Predict that contractile force produced by the muscle fiber is proportional to the number of cross-bridges formed

Question 24

In skeletal muscle where does 100% of the Ca come from for the purpose of contractility l?

Answer 24

Sarcoplasmic reticulum

None comes from extracellular space!

Question 25

Step 4: muscle contraction (cross bridge cycling)

Answer 25

At rest-filaments are not interacting

Myosin binds to action making a cross bridge (myosin head is extended)

Head/cross bridge bends, pulling myofilament inward creating a power stroke

Cross bridge detached at end of power stroke and returns to original conformation

Cross bridge binds to a more distal action molecule and cycle repeats

Question 26

Muscle contraction/cross bridge cycling requires

Answer 26

ATP on myosin head

ATP hydrolyzed by ATPase, ADP and Pi remain attached to myosin as stored energy

No Ca- no excitation, muscle fibers remain at rest, no cross bridge

Ca- excitation, removes inhibitory influence from actin and the two bind (cross bridge)

Phosphate removal allows power stroke, ADP released immediately after bending

FRESH ATP binds head and detachment is possible (if not rigor mortis)

ATP hydrolyzed and head goes back to extended position , restart cycle

Question 27

Amount of force exerted by muscle is directly related to

Answer 27

The amount of cross bridges formed

Question 28

Capacity of sarcomere to generate cross bridges is directly proportional to

Answer 28

The alignment btw action and myosin filaments

Needs to be at maximum capacity for interaction

Question 29

Steps 5 and 6:

Calcium removal and relaxation

Answer 29

Ca removed from the ICF back into the SR by sarcoplasmic reticulum Ca ATPase (SERCA)

Pump that moves Ca against conc gradient from ICF to SR