Anatomy & Embryology of Skeletal Muscle

Question 1

Is skeletal muscle striated or non-striated?

Answer 1

Striated

Question 2

Is skeletal muscle single or multi-nucleated?

Answer 2

Multi-nucleated

Question 3

Is skeletal muscle branched or unbranched?

Answer 3

Unbranched

Question 4

What are myonuclei?

Answer 4

The pleural term for the numerous nuclei in each individual skeletal muscle cell.

Question 5

Where are the nuclei located in skeletal muscle cells?

Answer 5

At the peripheries.

Question 6

What does syncytial mean?

Answer 6

A cytoplasm controlled by multiple nuclei.

Question 7

What is a myofibril and where are they found?

Answer 7

They are fibres consisting of multiple adjacent sarcomeres (the contractile units). Myofibirils are found in great volumes in muscle cells.

Question 8

What are the X3 layers of fascia which surround various sections of the muscles/muscle fibres?

Answer 8

Endo-, peri- and epimysium .

Question 9

Where do the endo/peri/epimysium fuse to?

What is the name of this junction?

Answer 9

They fuse to tendons at the myotendinous junctions

‘myo’ = myocyte =muscle cell

Question 10

Explain the components of a muscle starting from myofibrils.

Include the fascia containing each layer.

Answer 10

Multiple myofibrils = myocyte 
Myocyte = wrapped in endomysium
Multiple myocytes = form a fascicle = wrapped in perimysium
Multiple fascicles (whole muscle) = wrapped in endomysium

Question 11

What is the sarcolemma?

Answer 11

The name given to the cell membrane of skeletal muscle.

Question 12

What is isotonic contraction?

Answer 12

A muscle action in which the muscle fibres change length (either shorten or lengthen)

Question 13

What is isometric contraction?

Answer 13

A muscle action in which the muscle fibres do NOT change length.

Question 14

What are the X2 type of isotonic muscle action?

What happens to the muscle fibres in each of these?

Answer 14

Isotonic concentric (muscle fibres shorten)

Isotonic eccentric (muscle fibres lengthen)

Question 15

What is a synergist muscle?

Answer 15

Complements the prime mover (the agonist muscle doing the main body of the work)

Question 16

How do skeletal muscle cells form?

What is this process called?

Answer 16

Single myoblast cells proliferate and fuse into a myotube (muscle fibre).

This is called myogenesis.

Question 17

What are satellite cells and where are they found?

Answer 17

They are found on the surface of skeletal muscle cells and are resident stem cells, which can be activated to form further satellite cells or myoblasts to regenerate further muscle fibres.

Question 18

What gives skeletal muscle cells their striated appearance?

Answer 18

The presence of contractile sarcomere units

Question 19

What X2 filament proteins form the sarcomeres?

Which is thick and which is thin in terms of their banding length?

Answer 19

Actin (thin)

Myosin (thick)

Question 20

Where are the actin and myosin located in terms of the sarcomere? Which is central and which is ‘at the edges’?

Answer 20

Myosin = central

Actin = edges

Question 21

Which lines separate sarcomeres?

Answer 21

Z lines

Question 22

What band is the thick myosin band?

Answer 22

The A band

Question 23

What band is the thin actin band?

Answer 23

The I band

Question 24

What is the thin central band where no actin overlaps myosin?

Answer 24

The H band

Question 25

Which lines and bands move during contraction and which do not?

Answer 25

The Z lines move closer together and the H and I bands narrows The A band never changes

Question 26

What are the X2 muscle fibre types?

Answer 26

Type 1 slow Type 2 fast

Question 27

Which muscle fibre type is further subdivided and what is it subdivided into?

Answer 27

Type 2 fast = Fast type 2a = Fast type 2b = Fast type 2x

Question 28

How are skeletal muscles innervated in terms of neuron:muscle fibre ration?

Answer 28

It is a 1:1 ratio, each myfibre is innervated individually by a single motor neuron. Each motor neuron can innervated muscle muscle fibres however.

Question 29

What is the name of the numerous sarcolemma invaginations into skeletal muscle cells, which are open to interstitial fluid?

Answer 29

T-tubules

Question 30

What is the function of the T-tubules?

Answer 30

They ensure the action potential spreads evenly and quickly throughout the muscle fibre.

Question 31

What is the sarcoplasmic reticulum?

Answer 31

A fluid filled system of membranous sacs which encircle each myofibril.

Question 32

What are the names of the sacs of the sarcoplasmic reticulum where it meets the T-tubules?

Answer 32

Terminal cisterna

Question 33

Explain the relationship between the T-tubules and the terminal cisterna.

Answer 33

Each T-tubule has a terminal cistern either side in a triad organisation. They are actually called TRIADS.

Question 34

What is the role of the sarcoplasmic reticulum?

Answer 34

Stores calcium

Question 35

The sarcoplasmic reticulum components (thick and thin bands) are not hovering in air. Which molecules attach each of these and where do they extend from/to?

Answer 35

Actin is held by NEBULIN, which extends from the Z-line for the length of the actin Myosin is held by TITIN which extends from the Z-line to the M-line

Question 36

What is the name of the location on each actin molecule where the myosin heads bind?

Answer 36

Myosin binding site

Question 37

What are troponin and tropomyosin? What do they each do? What is the name of the complex they form together?

Answer 37

They are regulatory proteins. Tropomyosin = blocks myosin binding sites Troponin = the complex which holds tropomyosin in place Together they form the Troponin-tropomyosin complex

Question 38

What does calcium bind to in order to initiate muscle contraction? What happens when it binds?

Answer 38

Troponin C When calcium binds, the Troponin-tropomyosin complex moves, exposing the myosin binding sites on the actin filaments with the myosin heads.

Question 39

In terms of initiating muscle contraction, what happens when neurotransmitter (released in the synapse where the nerve meets the muscle fibre) comes into contact with the muscle fibre? How does this eventually lead to muscle contraction?

Answer 39

It stimulates voltage-gates Na channels to open and allow Na into the cell. This depolarises the cell with the depolarisation spreading all over the cell due to the T-tubules. Calcium is then released from the sarcoplasmic reticulum into the cytoplasm (due to voltage induced calcium release in skeletal muscle and CICR in cardiac muscle) which then binds to Troponin C of the Troponin-tropomyosin complex allowing muscle contraction to be initiated.

Question 40

What causes relaxation to occur in terms of calcium?

Answer 40

The ATPase pump SERCA pumps calcium back into the SR and a Na/Ca exchanger pumps it out of the cell, decreasing the cytoplasmic calcium concentration and causing calcium to unbidden from troponin C.

Question 41

Explain the X4 stages of sarcomere contraction.

Answer 41

1) ATP binds to myosin which releases it from actin 2) Myosin hydrolysis ATP —-> ADP + Pi which rotates it’s head and binds it weakly to actin 3) THE POWER STROKE! The Pi is released as the head swivels and moves the actin closer towards the m-line of the sarcomere. 4) At the end of the power stroke the ADP is released and the myosin stays bound to the actin in a RIGOR state

Question 42

Do skeletal muscle cells contain gap junctions?

Answer 42

No, they are each individually innervated by nerve motor axon. Cardiac and smooth muscle are the only types to have gap junctions between cells.