Lecture 31: Muscle Lecture 2

Question 1

length tension relationship

Answer 1

• isometric contraction
• maximum active force (tension developed) is dependent on the degree of actin and myosin overlap

Question 2

active tension

Answer 2

• maximum force between 2.0-2.2um
• 2.2< active forces decline as extent of overlap between filaments reduces, reducing number of cross bridges
• <2.0um filaments collide + interfere with each other, reducing force developed.

Question 3

passive force

Answer 3

• total tension = active dependent on the sarcomere length and passive force
• muscle stretched the connective elements resists the stretch = passive force

Question 4

what is a motor unit?

Answer 4

a motor unit consists of a motor neuron and all the muscle fibers it innervates

Question 5

what is step 1 of ecc

Answer 5

1. ach released into neuromuscular junction:
   ca2+ channels open at axon terminal + ca2+ enters the axon terminal

• this triggers the vesicles containing ACh to fuse with the terminal membrane, releasing ACh into the neuromuscular junction (synaptic cleft)

Question 6

what is step 2 of ecc

Answer 6

2. activation of ach receptors
   - binding of ACh to the receptors on the muscle end plate causes opening of the ligand (ACh) gated ion channels
   - opening of these channels allows movement of predominantly na+ into the muscle cell making it less negative (end plte potential)

**effects of ACH are short lasting as the enzyme acetylcholine esterase rapidly breaks down ACh

Question 7

what is step 3 of ecc

Answer 7

a muscle action potential is triggered
- end plate potential is reached when sufficient ligand gated channels are opened.
- voltage gated na+ channels open + action potential is triggered
- the action potential is then propagated along the sarcolemma into the t tubule system

Question 8

step 4 of ecc

Answer 8

calcium released from the sr
action potential is conducted down the t-tubules that are close to the sarcoplasmic reticulum
- voltage gated ca2+ channels in the sarcoplasmic reticulum opening
- ca2+ is then released into the cytosol

Question 9

step 5 of ecc

Answer 9

ca2+ binds with troponin
- ca2+ concentrations reach a critical threshold
- myosin binding sites on actin filament are exposed, allowing the cross-bridge cycle to occur

Question 10

what are the stages of the cross bridge cycle?

Answer 10

6. cross-bridge formation
7. power stroke
8. detachment
9. energization of myosin head

Question 11

what is step 10 of ecc

Answer 11

contraction ends when ca2+ levels fall
- calcium is actively pumped back into the sarcoplasmic reticulum via ca2+ atpase pumps
tropomyosin moves back covering the myosin binding site.

Question 12

what are the sources of atp for muscle metabolism?

Answer 12

creatine phosphate
anaerobic glucolysis
aerobic metabolism

Question 13

creatine phosphate

Answer 13

15seconds
anaerobic (no oxygen required)
1atp per cycle

Question 14

anaerobic glycolysis

Answer 14

no oxygen
fast, but inefficient (2atp)
short intense exercise
10-30seconds
build up of metabolites (H+ limits the duration to max 120 seconds)