Muscle structure

Question 1

Three types of muscle tissue:

Answer 1

Skeletal, cardiac and smooth (visceral)

Question 2

Properties of muscle tissue:

Answer 2

• Excitability (irritability) - responds to stimuli

-Contractility - can shorten

• Extensibility - can lengthen

-Elasticity - returns to original length after stretching

Question 3

characteristics of skeletal muscle:

Answer 3

• Attach to bones
• Cells are multinucleate
  (many nuclei in each cell)
• Has visible striations under microscope
• Under voluntary control

Question 4

functions of skeletal muscle tissue

Answer 4

▪ Movement

▪ Maintaining posture

▪ Stabilizing joints

▪ Heat production

Question 5

Gross anatomy of a skeletal muscle:
connective tissue coverings

Answer 5

1. Endomysium – surrounds each individual muscle cell or fiber

Question 6

2. Perimysium – surrounds groups of fibers, bundling them into fascicles

Answer 6

fascicle – a bundle of muscle fibers

Question 7

3. Epimysium – surrounds entire muscle

Answer 7

• All three connective tissue coverings connect with each other and with tendons.

Question 8

Attachments:

Answer 8

Tendons: cords of dense irregular c.t. that are continuous with the three connective tissue layers surronding muscle.
- attach muscle to bone
- fuse with the periosteum of bone

Question 9

nerve & blood supply

Answer 9

Vascular – each muscle is served by at least one artery and one or more veins

Nerves – each skeletal muscle fiber is attached to a nerve cell axon at a neuromuscular junction

Both embedded in c.t. coverings

Question 10

Microscopic anatomy of skeletal muscle fiber-
structure

Answer 10

▪ Muscle fibers (= muscle cells) - are made up of many myofibrils.
→ Myofibrils are made up of sarcomeres aligned end-to-end.
→ Within the sarcomeres are many myofilaments with a specific

Question 11

Two types of myofilaments:

Answer 11

1-Thick myofilaments:
Mostly comprised of a protein called myosin.

Question 12

second type of myofilaments

Answer 12

2. Thin myofilaments – mostly comprised of a protein called actin but also contains two regulatory proteins called troponin and tropomyosin

Question 13

Histology:

Answer 13

A band – dark band due to presence of both actin & myosin

I band – light band due to presence of actin only

Z disc – located in center of I band

H zone – lighter region in center of A band and containing only myosin

Question 14

Histoloy:

Answer 14

Sarcomere – functional unit that extends from one Z disc to the next

Question 15

Histology:

Answer 15

The striations are due to the arrangement of thin and thick myofilaments.

Question 16

Proteins involved in contraction:

Answer 16

1. Myosin – composed of a rod-like tail and two heads
   ▪ the heads have ATP binding sites and contain the ATPase enzyme.
   ▪ comprise thick myofilaments

Question 17

Proteins involved in contraction: 2

Answer 17

2. Actin – spherical proteins linked in a chain
   ▪ have active sites for myosin heads to bind
   ▪ comprise thin myofilaments

Question 18

Proteins involved in contraction: 3

Answer 18

3. Troponin & Tropomyosin – two proteins that form the TT complex;
   ▪ coil around actin in thin myofilaments
   ▪ configuration is controlled by calcium

Question 19

protein involved contraction: 3 pt2

Answer 19

3. Troponin & Tropomyosin (cont.)
   ▪ troponin – binds & releases calcium; also bound to tropomyosin
   ▪ tropomyosin – strand-like; blocks active sites in relaxed muscle

Question 20

special cellular structures:

Question 21

1- sarcolemma

Answer 21

muscle cell’s plasma membrane

Question 22

2- sarcoplasma

Answer 22

muscles cell’s cytoplasm
- stores large amounts of:
glycogen & myoglobin ( red pigment - stores oxygen)

Question 23

3- Transverse tubules (T tubules):

Answer 23

tubular continuations of the sarcolemma that run through the cell like tunnels

Question 24

4 - sarcoplasmic reticulum (SR)

Answer 24

muscle cell’s smooth ER
- terminal cisternae- expanded portions that run along side.
- stores calcium

Question 25

5 Triads

Answer 25

two terminal cisternae and a T tubule clustered together.

Question 26

Skeletal muscle fiber at rest:

Answer 26

▪ Thick and thin filaments overlap only slightly.

▪ Ca2+ in sarcoplasm is low.
(Ca2+ is stored in the SR.)

• ATP, the cell’s energy source, is bound to myosin heads.

▪ Tropomyosin occupies the binding sites on actin.
(Therefore, no cross bridges are formed)

Question 27

Sliding filament model overview - contraction of skeletal muscle

Answer 27

▪ Contraction of a muscle occurs by the sliding of thin filaments past thick filaments due to power strokes of the myosin cross bridges.

▪ Adjacent A bands are pulled closer together as the I bands between them shorten (the
H zones also shorten.)

Question 28

sliding filament model (cont)

Answer 28

▪ As the sarcomeres and myofibrils shorten, the entire muscle shortens (contracts).

▪ The actin and myosin myofilaments themselves do not change length.

Question 29

Sliding Filaments Model-

Answer 29

when the myosin heads attach to actin, heads to pull the actin, the heads swivel to pull the actin, and shorten the sarcomere.

Question 30

initiatin of muscle contraction:

Answer 30

▪ Contraction is initiated by stimulation of the muscle by a motor neuron at the neuromuscular junction.

Question 31

Mechanics of muscle contraction: 1

Answer 31

1-Axon terminals of motor neuron release acetylcholine (ACh) into synaptic cleft at the neuromuscular junction.

Question 32

Mechanics of muscle contraction- 2

Answer 32

2- The ACh diffuses across the synaptic cleft and binds to ACh receptors on the muscle fiber. This triggers propagation of an action potential along the sarcolemma and down the T-tubules of muscle fiber.

Question 33

Mechanics of muscle contraction - 3

Answer 33

3. Action potential stimulates release of Ca2+ ions from the sarcoplasmic reticulum (SR) into the sarcoplasm.

Question 34

Mechanics of muscle contraction - 4

Answer 34

4. Ca2+ released from the SR binds to troponin, causing it to change shape. This conformation change causes tropomyosin to move away from actin’s active sites.

Question 35

Mechanics of muscle contraction - 5

Answer 35

5. With tropomyosin out of the way, the active sites are exposed, and myosin heads attach to actin forming cross bridges.

Question 36

Mechanics of muscle contraction - 6

Answer 36

6. ADP & Pi are given off by the myosin head as ATP is broken down to supply energy for the power stroke.

Question 37

Mechanics of muscle contraction - 7

Answer 37

Power stroke - the myosin molecule tilts and bends toward the center of the sarcomere, thus propelling actin toward the center. This is called the working stroke or power stroke.

Question 38

Mechanics of muscle contraction - 8

Answer 38

8. A new ATP binds to the myosin head, which causes it to detach from actin. Now, the cycle can repeat.

Question 39

Mechanics of muscle contraction - a few notes

Answer 39

▪ There are about 200 myosin per thick filament (with each myosin head contributing to the movement of the thin filament).

▪ Each myosin head attaches & detaches many times to maximize movement of the thin filament.

▪ This process of the thin filaments “sliding” over the thick ones is known as the sliding filament model.

Question 40

Mechanics of muscle contraction - a few notes pt2

Answer 40

▪ Myosin heads attach & detach asynchronously.*
◦ produces smooth, continuous movement.
◦ ensures that thin filament will not snap back to its relaxed position as it might if all myosin heads detached at the same time.

Question 41

Mechanics of muscle contraction - a few notes pt 3

Answer 41

▪ Shortening (contraction) of the sarcomere occurs as the myosin cross bridges pull the thin filaments toward center of the sarcomere.

→ provides the basis for the contraction of the myofibrils, muscle fibers & also the whole muscle.

Question 42

Muscle’s return to relaxed state

Answer 42

▪ When the nerve impulse stops, calcium from sarcoplasm is actively transported back into the sarcoplasmic reticulum.

▪ As a result, tropomyosin again blocks actin’s active sites, and contraction ends as cross bridge formation is prevented.

Question 43

Muscle’s return to relaxed state: pt 2

Answer 43

▪ When the nerve impulse stops, calcium from sarcoplasm is actively transported back into the sarcoplasmic reticulum.

▪ As a result, tropomyosin again blocks actin’s active sites, and contraction ends as cross bridge formation is prevented.

Question 44

Muscle’s return to relaxed state - pt3

Answer 44

▪ With no cross bridges linking the thick and thin filaments, thin filaments slide back to their resting or relaxed position.

Question 45

Final notes on muscle conytraction - ATP

Answer 45

▪ As ATP is usually readily available in skeletal muscle fibers, ATP is not the factor regulating muscle contraction.

▪ Instead, sarcoplasmic levels of calcium determine when a muscle can contract.

Question 46

ATP in muscle contraction

Answer 46

ATP is required:
•To maintain the resting membrane potential via the Na+/K+ ATPase pump so the cell can depolarize when stimulated.
•For active transport of calcium into the sarcoplasmic reticulum.
•To provide energy for the power stroke.
•To allow the myosin heads to detach.
oRigor mortis occurs when ATP runs out. The heads get locked in place causing stiffness.