Skeletal Muscle Physiology 1 (Week 8)

Question 1

Our skeletal muscles are composed of bundles of ___________

Answer 1

fascicles

Question 2

Each fascicle is composed of linearly aligned ______________

Answer 2

muscle fibers (myofibres)

Question 3

a single, multi-nucleated, elongated cell, composed of many sarcomeres that are arranged linearly

Answer 3

muscle fibre/myofiber

Question 4

What the cell membrane of a muscle fibre called?

Answer 4

sarcolemma

Question 5

Sarcomeres are composed of _______________

Answer 5

myofibrils

Note: myofibrils are organelles

Question 6

the connective tissue sheath that surrounds the whole muscle, that extends from the tendons

Answer 6

epimysium

Question 7

the sheath surrounding each fascicle

Answer 7

perimysium

Question 8

the sheath surrounding each individual muscle fibre

Answer 8

endomysium

Note: below the endomysium is the sarcolemma (cell membrane of a muscle cell)

Question 9

Each muscle fibre contains how many myofibrils?

Answer 9

1000-2000

Question 10

Each myofibril is composed of many myofilaments. What is a myofilament composed of?

Answer 10

• contractile proteins (e.g., actin, myosin)
• regulatory proteins (e.g., tropomyosin, troponin)
• accessory proteins (e.g., actinin, dystrophin, titin, nebulin)

Question 11

A sarcomere runs from __________________

Answer 11

Z disc to Z disc

Question 12

the contractile unit of skeletal muscle

Answer 12

sarcomere

Question 13

Thin filaments are made of __________

Answer 13

actin

Question 14

Thick filaments are made of _____________

Answer 14

myosin

Question 15

What gives the striated appearance of skeletal muscle?

Answer 15

the overlapping of thick and thin filaments

Question 16

the center of the sarcomere

Answer 16

M-line

Question 17

True or False: thin filament (actin) attaches to the Z discs and thick filament (myosin) attaches to the M-line

Answer 17

True

Question 18

The A band (darker band) is composed of ___________

Answer 18

actin and myosin (they overlap)

Note: However, within the A band, there is a portion that is only myosin = H zone

Question 19

The I band (lighter band) is composed of ____________

Answer 19

actin only

Question 20

centre of I bands (light bands)

Answer 20

Z disc

Question 21

What part of the sarcomere shortens during contraction?

Answer 21

I band

Question 22

monomer of actin

Answer 22

G actin

Question 23

polymer of actin

Answer 23

F actin

Question 24

Thin filaments are composed of two strands of _____________ wound together

Answer 24

F actin

Question 25

Each G actin (monomer of actin) has a binding site for ____________

Answer 25

myosin

Question 26

Each unit of ___________ is composed of a head and tail region

Answer 26

myosin

Question 27

Head region of myosin forms _____________ that interact with adjacent actin filaments

Answer 27

cross-bridges

Question 28

What are the three important chemical features at the myosin head region?

Answer 28

1) ATP activity 2) actin-binding region 3) ATP binding region Note: ATP is required to change the conformation of the hinge

Question 29

___________ and __________ are regulatory proteins that associates with actin

Answer 29

tropomyosin and troponin

Question 30

When a skeletal muscle is in a ______________ (relaxed/contracted) state, tropomyosin molecules cover the myosin-binding site on G-actin monomers

Answer 30

relaxed Note: this prevents cross-bridge formation between actin and myosin

Question 31

Troponin forms a complex of 3 subunits. What are they?

Answer 31

1) Troponin C 2) Troponin T 3) Troponin I

Question 32

Troponin C binds to ?

Answer 32

calcium

Question 33

Troponin T binds to?

Answer 33

tropomyosin

Question 34

Troponin I binds to?

Answer 34

actin

Question 35

True or False: When calcium binds to troponin C, the troponin complex undergoes a conformational change and troponin T "pulls" tropomyosin and troponin I off of the myosin-binding site of G actin subunits. This allows cross-bridge formation to occur.

Answer 35

True

Question 36

invaginations within the sarcolemma that allows the action potential to be carried deep into the muscle fibre

Answer 36

transverse tubules (T tubules) Note: T tubules are continuous with the extracellular fluid

Question 37

a specialized endoplasmic reticulum containing high concentrations of calcium

Answer 37

sarcoplasmic reticulum

Question 38

___________________ are specialized regions of the sarcoplasmic reticulum that associated with the T-tubules

Answer 38

terminal cisternae

Question 39

The junction between the T-tubules and the terminal cisternae is called what?

Answer 39

muscle triad (think of T tubule "sandwiched" between terminal cisternae on either side of it)

Question 40

True or False: The concentration of calcium in the ECF (extracellular fluid) and the sarcoplasmic reticulum (SR) is generally low

Answer 40

False The calcium concentration is high in both the ECF and SR (compared to the cytosol). This means that if you open up a calcium channel, it will go right into the cytosol.

Question 41

a motor nerve axon contacts each muscle fiber near the middle of the fiber, forming a synapse called what the ________________

Answer 41

neuromuscular junction

Question 42

the region of the sarcolemma in closest contact with the presynaptic nerve terminal is called the _____________

Answer 42

motor end plate

Question 43

What neurotransmitter does the motor nerve synapse release onto the muscle fibre's nicotinic receptors?

Answer 43

acetylcholine (Ach)

Question 44

When Ach binds to the nicotinic receptors on the sarcolemma, this activates voltage-gated sodium channels which triggers an ______________, which propagates along the sarcolemma

Answer 44

action potential Note: a skeletal muscle action potential looks very similar to a neuronal action potential but is slightly longer in duration skeletal muscle AP = 5-10 msec neuronal AP = 1-2 msec

Question 45

The action potential travels into the T-tubule, bringing depolarization deep into the muscle fiber, signalling/exciting _____________ release from the sarcoplasmic reticulum

Answer 45

calcium

Question 46

When the action potential propagates along the T-tubules, it activates _____________ channels in the sarcolemma

Answer 46

L-type Ca2+ Note: activation of these channels also triggers activation of Ryanodine receptors on the terminal cisternae... let's out a ton of calcium out of the sarcoplasmic reticulum (SR)

Question 47

After calcium flows down its concentration gradient (out of the SR and into the cytosol), it then binds to ______________, to expose the binding sites for myosin

Answer 47

troponin C

Question 48

True or False: Most of the calcium that activates skeletal muscle comes from the ECF

Answer 48

False Most comes from the sarcoplasmic reticulum (SR)

Question 49

What is the average number of alpha motor neurons that innervate a typical muscle?

Answer 49

~100

Question 50

The number of muscle fibers that a motor neuron reaches out to is referred to as what?

Answer 50

a motor unit

Question 51

the result of a single action potential from a motor neuron that barely causes any force development in the muscle fibers that it innervates

Answer 51

a twitch

Question 52

How do you get smooth, forceful movements?

Answer 52

occur when the alpha-motor neuron sends MANY action potentials to the motor unit, at a relatively high frequency summation of the APs prevent calcium concentrations from dropping (calcium keeps getting released), creating a peak of force

Question 53

True or False: fused tetanus/maximal force generation in skeletal muscle requires a lot of ATP

Answer 53

True

Question 54

True or False: Individual "twitch times" vary depending on the role of the muscle

Answer 54

True Example: Your soleus, meant to keep you standing, is a slow muscle. Therefore, one action potential results in a very long twitch (~200msec). Whereas your gastrocnemius helps you more when you're running to help you push off the ground. This has a faster twitch time (~70msec), because it generates more force. Ocular muscle twitch times are very fast (~25msec)