Excitable tissue, Muscle Lecture: Two

Question 1

What is type of contraction is the length tension relationship relevant to?

Answer 1

• Isometric contraction i.e ACTIVE FORCE

Question 2

What does the length tension relationship describe?

Answer 2

At the level of the sarcomere the maximum active force (tension developed) is dependent on the degree of actin and myosin overlap (over stretch and under-stretch diminish the tension)

Question 3

Describe the length of the sarcomeres and force developed:

Answer 3

• Maximal force between 2.0 – 2.2 μm
• A lengths >2.2 μm active forces decline as the extent of overlap between filaments reduces, reducing the number of cross bridges
• At lengths <2.0 μm filaments collide and interfere with each other reducing force developed

Question 4

What is total tension the product of?

Answer 4

Active and passive force

•Total tension is the sum of the active tension dependent on the sarcomere length (length-tension relationship) and the passive tension (elastic property and connective tissue resisting force).

Question 5

What is the motor unit?

Answer 5

Consists of motor neuron and all the muscle fibres it innervates

Question 6

What are the steps of excitation contraction coupling? 10 steps

(steps 6-9 are the cross bridge cycle)

Answer 6

1. ACh released into neuromuscular junction
2. Activation of Ach receptors
3. A muscle action potential is triggered
4. Ca is released from the SR
5. Ca binds with troponin
   [Cross bridge cycle occurs]
6. Cross bridge formation
7. Power stroke
8. Detachment
9. Energisation of the myosin head
10. Contractions end when Ca levels fall

Question 7

Describe step one of the excitation contraction coupling; Ach released into the neuromuscular junction

Answer 7

• An action potential travels down the motor neuron
• At the axon terminal Ca2+ channels open, and 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 8

Describe step two of the excitation contraction coupling; Activation of the Ach receptors

Answer 8

• The 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 plate potential)

Question 9

What breaks down ACh in the Neuromuscular junction (NMJ)?

Answer 9

(the effects of ACh are short lasting as the enzyme acetylcholine esterase rapidly breaks down ACh)

Question 10

Describe step three of the excitation contraction coupling; A muscle action potential is triggered

Answer 10

• If sufficient ligand gated channels are opened the end plate potential reaches threshold
• Voltage gated Na+ channels open and an action potential is triggered
• The action potential is then propagated along the sarcolemma into the T tubule system

Question 11

Describe the action potential in skeletal muscle: (five steps)

Answer 11

Question 12

Roughly how long is an AP in skeletal muscle?

Answer 12

3msec

Question 13

Describe step four of the excitation contraction coupling; Ca is released from the SR

Answer 13

• The action potential is conducted down the t-tubules coming in close contact with the sarcoplasmic reticulum.
• This results in voltage gated Ca2+ channels in the sarcoplasmic reticulum opening.
• Ca2+ is then released into the cytosol

Question 14

Describe step five of the excitation contraction coupling; Ca binds with troponin

Answer 14

When Ca2+ concentrations reach a critical threshold the myosin binding sites on the actin filament are exposed allowing the cross-bridge cycle to occur

Question 15

Describe step ten of the excitation contraction coupling; Contractions end when Ca levels fall

Answer 15

• Calcium is actively pumped back into the sarcoplasmic reticulum via Ca2+-ATPase pumps
• Tropomyosin moves back covering the myosin binding site

Question 16

What are the three sources of ATP for muscle metabolism?

Answer 16

1. Creatine phosphate
2. Anaerobic glycolysis
3. Aerobic metabolism

Question 17

Write some notes on creatine phosphate as a muscle energy source:

Answer 17

• ATP Energy source = creatine phosphate
• Creatine phosphate + ADP = creatine + ATP (1 ATP per cycle)
• Creatine phosphate acts as ATP “store”, but quickly spent (<15s)
• Anaerobic (no oxygen required)

Question 18

Write some notes on anaerobic glycolysis:

Answer 18

• ATP Energy source = glucose
• Aerobic = no oxygen required
• Fast but inefficient (2 ATP per glucose)
• Good for short intense exercise
• Dominant system from about 10-30s of maximal effort
• Build up of metabolites, e.g. H+ limits duration to max 120s

Question 19

Write some notes on aerobic metabolism

Answer 19

• Energy source: Glucose, pyruvic acid, fatty acids, amino acids
• Efficient (32 ATP per glucose), but comparatively slow. Max 300 W.
• Requires oxygen, therefore good blood supply
• Important for postural muscles and endurance exercise

Question 20

What are the types of muscle fibres in humans to be aware of?

Answer 20

Type one: slow oxidative
Type two B: fast glycolytic

Question 21

Whats the main differences between type one and type 2B muscle fibres:

Answer 21

Type one has high mitochondria and slow ATPase rate, type two has low mitochondria and fast ATPase rate.

Question 22

Describe the type one slow twitch motor unit:

Answer 22

Units with neurons innervating the slow efficient aerobic cells (maintaining posture, walking)

Question 23

Describe the type two fast twitch motor unit:

Answer 23

Units with the neurons innervating the large fibres that fatigue rapidly but develop large forces (jumping, weight lifting)

Question 24

What is the regulation of force dependent on? what is it not dependent on?

Answer 24

• Rate of stimulation of individual motor units
• The number of motor units recruited

NOT dependent on the magnitude of the stimulus because it is an all or nothing response providing threshold is met.

Question 25

Describe the influence of rate of stimulation on force:

Answer 25

If a stimulus is applied before the muscle fully relaxes then more tension results (than the subsequent contraction tension) this is TEMPORAL SUMMATION and can result in an unfused tetanus. (i.e reaches max tension but has time to relax a little between stimuli)

At higher rates of stimulation there is no time to relax between stimuli and this results in a fused tetanus

Question 26

In what order are motor units recruited?

Answer 26

As more units are recruited tension increases

Usually the most fatigue resistant (small) motor units are recruited first