lecture 31 - skeletal muscle 2

Question 1

motor unit =

Answer 1

motor neuron and all the muscle fibres it innervates

Question 2

steps leading up to the stage where ACh is released into neuromuscular junction

Answer 2

• AP arrives at axon terminal
• Ca2+ influx
• vesicles containing ACh fuse with membrane

Question 3

Activation of ACh receptors

Answer 3

• binding of ACh to the receptors on the muscle end-plate
• opening of ligand-gated cation channels
• predominantly Na+ moves into the muscle cell, depolarising it.

Question 4

why are the effects of ACh only short-lived? (less than 1ms)

Answer 4

acetylcholinesterase rapidly breaks it down.

Question 5

conditions for muscle AP to be triggered

Answer 5

if enough ligand (ACh) gated channels are opened, the end-plate potential reaches threshold. Voltage-gated Na+ channels open and AP is triggered.

Question 6

How does AP reach all the necessary fibres?

Answer 6

AP propagates along the sarcolemma into the t-tubule system

Question 7

end-plate potential does not always result in an action potential TRUE/FALSE

Answer 7

FALSE. The end-plate potential will ALWAYS result in an action potential, in skeletal muscle

Question 8

RMP in skeletal muscle

Answer 8

-90mV (very low permeability to Na+)

Question 9

what happens as the AP propagates through the t-tubule?

Answer 9

because they come into close contact with the sarcoplasmic reticulum, the voltage-gated Ca2+ channels in the SR open and Ca2+ floods into the cytosol

Question 10

when does contraction end?

Answer 10

when Ca2+ is actively pumped back into the SR via Ca2+ATPase pumps and Ca2+ levels fall

Question 11

when calcium levels fall, what happens in the sarcomere?

Answer 11

Troponin moves back into position, holding tropomyosin over the myosin-binding site on actin.

Question 12

creatine phosphate

Answer 12

• used as an ATP store for very quick bursts of activity (less than 15s)
• Donates P to ADP = ATP
• anaerobic process - mainly in type 2 muscles

Question 13

Anaerobic glycolysis

Answer 13

• good for short, high workloads (mainly type 2 muscles)
• fast but inefficient
• build up of lactic acid (H+ limits duration to 30-40s)
• 2 ATP net

Question 14

Aerobic metabolism

Answer 14

• important in postrural muscles and endurance exercise (Type 1 muscles)
• efficient but slow
• requires oxygen (good blood supply)
• max 300W
• 36ATP

Question 15

Type I muscle

Answer 15

• endurance
• slow but efficient aerobic cells
• darker red due to more mitochondria and myoglobin
• needs a good blood supply

Question 16

Type II muscle

Answer 16

• develop large forces
• fatigue rapidly - fast glycolytic
• larger than type I (think about weightlifters)
• pale appearance
• not many mitochondria
• don’t need so much blood

Question 17

regulation of force in muscles depends on…

Answer 17

1. the number of motor units recruited 2. rate of stimulation of individual motor units

Question 18

single stimulus leads to…

Answer 18

a single twitch (contraction + relaxation)

Question 19

time for AP vs. time for twitch

Answer 19

twitch is always longer (100ms) than an AP (2ms)

Question 20

low stimulation frequency leads to…

Answer 20

unfused tetanus (temporal summation)

Question 21

temporal summation

Answer 21

If another stimulus is applied, before the muscle relaxes completely, then more tension results

Question 22

complete tetanus

Answer 22

the result of high stimulation frequency, where the tensions summate. there is NO RELAXATION between stimuli, until maximum tension is reached

Question 23

the bacteria Clostridium tetani causes

Answer 23

all muscles of the body reach tetani at the same time, until you can’t breathe.

Question 24

recruitment

Answer 24

• another way of increasing muscle tension is to recruit more motor units. The more you recruit the higher the tension
• motoneurons are recruited in order of size i.e. smaller first, then medium, then large

Question 25

moving from a low intensity (casual jog) to a higher intensity (hill sprint), type _ muscles are recruited before type _. At ____ intensities the forces of the multiple motor units are _______ to create an overall higher force.

Answer 25

moving from a low intensity (casual jog) to a higher intensity (hill sprint), type _1_ muscles are recruited before type _2_. At _high_ intensities the forces of the multiple motor units are _summating_ to create an overall higher force.

Question 26

X =

Y =

Answer 26

X = type I muscle fibre (smaller diameter, darker)

Y = type II (larger diameter, pale)

Question 27

label the following

Answer 27

1 = sarcoplasmic reticulum

2 = t-tubules

3 = sarcolemma

Question 28

tetani can occur because…

Answer 28

AP time is shorter than contraction time