BIOL 273 - Unit 3.4 +3.5

Question 1

What is the tension exerted by a muscle during a single twitch is influenced by (2)

Answer 1

1. Muscle type (fast twitch can generate more force)
2. Sarcomere length (at rest)

Question 2

Describe the overlaps at rest of the thick and thin filaments

Answer 2

1. Too little overlap - few crossbridges/little force
2. Too much overlap - actin filaments start to interfere with eachother/ less force generated
3. Way too much overlap - thick filaments collide with Z disks/ force rapidly decreases

Question 3

How to increase single tension

Answer 3

increase the rate of action potentials before cell can fully release
- summate tension

Question 4

What is the term of the state of a musce when it reches maximum force of contraction

Answer 4

Tetanus; 2 types that are dependant on rate of stimulation

Question 5

What is Incomplete (unfused) tetanus

Answer 5

slow stimulation rate - fibre relaxes slightly between stimuli
- won’t fully relax thus it will summate and lead to tetanus
- doesn’t stay at maximum tension

Question 6

What is complete (fused) tetanus

Answer 6

fast stimulation rate - fibre does not have time to relax
- stays at maximum tension

Question 7

What is the motor unit

Answer 7

the basic unit of contraction in an intact skeletal muscle
- many are found in a muscle
- all muscle fibres of a motor unit are of the same type

Question 8

What are the two components of the motor unit

Answer 8

1. A group of muscle fibres
2. A somatic motor neuron that controls them all

Question 9

How can the contraction of the muscle vary (2)

Answer 9

1. Changing the type of motor unit that is activated
2. Changing the number of motor units that are active

Question 10

describe the threshold for stimulation of slow oxidative fibres and fast glycolytic fibres

Answer 10

slow: have a low threshold for stimulation
fast: have a high threshold for stimulation

Question 11

Describe isotonic movement

Answer 11

• creates force and moves a load
• the load is usually constant, and the muscle length changes

Question 12

Describe isometric movement

Answer 12

• creates force without movement
• muscle length is constant (contracts)
• the load is usually greater than the force that can be applied

Question 13

Where is smooth muscle found in the body?

Answer 13

• walls of hollow organs and tubes - not attached to bones of skeleton
• ex. bladder, sphinchter, intestine, walls of blood vessels

Question 14

Two ways smooth muscle cells can be arranged

Answer 14

1. Single unit - cells connected via gap junctions
2. Multi unit - no connections

Question 15

Single unit characteristics

Answer 15

• not necessary to elctricaly stimulate each individual fibre because there are gap junctions
• found on walls of internal organs ex blood vessels

Question 16

Multi-unit characteristics

Answer 16

• each individual muscle fibre is separatley innervated
• eg. iris of eye, parts of reproductive organs

Question 17

What kind neuron innervates smooth muscle

Answer 17

autonomic neurons (sympathetic and parasympathetic)

Question 18

differentiate the contraction between smooth and skeletal muscle

Answer 18

• skeletal muscle changes length when contracts
• smooth muscle changes shape, not just length

Question 19

Differentiate the speed of contraction between smooth and skeletal muscle

Answer 19

• smooth muscle develops tension musch slower than skeletal muscle
• however that contraction can be sustained for a long period of time without fatiguing

Question 20

Differentiate between smooth and skeletal muscle on a cellular level

Answer 20

• fibres much smaller in smooth muscle than skeletal fibre
• No striations on smooth muscle / no sarcomeres
• actin and myosin arranged in long bundles around periphery of the cell
• actin anchored by dense bodies and not Z lines
• smooth muscles have caveolae instead of T-tubules in sarcolemma for cell signalling (to hold calcium)

Question 21

how does smooth muscle starts the path to begin to contract, with the absence of t tubules?

Answer 21

“calcium dependent calcium release “
- influx of calcium that causes the release of more calcium in the SR

Question 22

Differentiate between skeletal and smooth muscles on a molecular level

Answer 22

• less myosin per unit actin in smooth muscle
• Actin and myosin filaments are longer and overlap more in smooth muscle
• develop tension slower in smooth muscle because of slow Myosin ATPase activity
• myosin heads are located along all parts of myosin in smooth
• no troponin in smooth

Question 23

What is the major difference between contraction of smooth muscle and skeletal muscles

Answer 23

“Role of Phosphorylation”

Question 24

Signal to initiate contraction is increase in cytosolic calcium, how does calcium enter ECF? (3)

Answer 24

1. Voltage gated channels - due to depolarizing
2. Stretch activated channels - due to membrane stretch
3. Chemically gated channels - due to hormones

Question 25

What does calcium entry in the ECF in smooth muscle result in

Answer 25

release of additional calcium from the SR

Question 26

Since there is no troponin for calcium to bind to in smooth muscle, what does it bind to instead? What results in this?

Answer 26

calmodulin in the cytosol
- leads to the activation of the enzyme “myosin light chain kinase (MLCK)”
- which will phosphorylate myosin heads

Question 27

What is the role of MLCK (myosin light chain kinase)

Answer 27

activates myosin by phosphorylating the light chain of the myosin molecule using the inorganic phosphate from ATP
- ATPase activity depends on phosphorylation of myosin - which can then break apart ATP to initiate crossbridge cycle

Question 28

How is the crossbridge cycle/contraction initiated in smooth muscles

Answer 28

When phosphorylated myosin interacts with actin
- MLCK uses inorganic phosphate from ATP to activate myosin (turn it on)
- but more ATP is needed to go through crossbridge cycling contraction to occur

Question 29

What structure of smooth muscle and skeletal muscle determines the initiation of the crossbridge cycle

Answer 29

Smooth muscle: myosin is regulated via phosphorylation of myosin

Skeletal muscle: actin is regulated via troponin/tropomyosin interaction with actin

Question 30

Describe relaxation in smooth muscle ; what molecules are used to remove calcium

Answer 30

Calcium ions is removed from the cytosol as it is pumped out of the cell or into the SR

Ca2+ - Na+ antiport (calcium out and sodium in)
Ca2+ ATPase

Decrease levels of calcium in cytosol causes calcium to unbind from calmodulin (inactivates MLCK)

Myosin light chains are dephosphorylated by protein myosin phophatase (MLCP)

Question 31

Does dephosphorylation of myosin relax the smooth muscle

Answer 31

• does not automatically relax the muscle, smooth muscle enters latch state
• tension is maintained but with minimal ATP consumption