4. Muscle contraction

Question 1

What are the three types of muscle?

Answer 1

Skeletal
Smooth
Cardiac

Question 2

What is meant by the term contraction?

Answer 2

The ability of a muscle to actively shorten and develop tension

Question 3

What are the structures that make up the muscle?

Answer 3

Muscle 
Fasciculus
Fibre/cell 
Myofibril 
Sarcomere 
Filaments

Question 4

Describe the actin filaments in the relaxed state?

Answer 4

The ends of actin filaments from Z discs barely overlap

Question 5

Describe the actin filaments in the contracted state?

Answer 5

Actin filaments pulled inward, overlap to maximum extent

Question 6

What are the features of the actin filaments?

Answer 6

Double stranded F-actin molecules comprising of polymerised G-actin molecules
2 strands of tropomyosin molecules
Troponin complex on each tropomyosin for contraction (binds to Ca2+)

Question 7

How many myosin and actin filaments on each myofibril?

Answer 7

1500 myosin

3000 actin

Question 8

Describe the first three steps of the sliding-filament mechanism

Answer 8

1. Low Ca2+ conditions, tropomyosin blocks access to the myosin binding site of the actin
2. In high Ca2+ conditions, Ca2+ binds to troponin
3. The positions of troponin and topomyosin are altered on actin + myosin can now access its binding site on actin

complete the full theory

Question 9

Describe what happens when the sarcomere length decreases

Answer 9

Unstretched muscle

Question 10

Describe what happens when the sarcomere is at optimal resting length

Answer 10

Muscle can develop maximal tension

Question 11

Describe what happens to increased sarcomere length?

Answer 11

Actin is pulled to end of myosin with no overlap. No tension

Question 12

Describe excitation-contraction coupling in skeletal muscles

Answer 12

Action potential travels along a motor nerve to the neuromuscular junction

Nerve secretes small amount of acetylcholine

ACh opens channels on muscle membrane

Na+ flows into the cell – initiates an action potential

AP travels along muscle fibre membrane

Question 13

Describe the T-tubule endoplasmic reticulum system?

Answer 13

Muscle membrane depolarized, AP flows rapidly through centre of fibre via the T-tubules. Sarcoplasmic Reticulum releases Ca2+

Rise in intracellular Ca2+ enables Ca2+-binding to troponin, tropomyosin moves and cross-bridge cycling occurs

Ca2+ actively pumped back into SR store. Relaxation

Question 14

What causes the release of Calcium ions?

Answer 14

Action potential from Sarcoplasmic reticulum

Question 15

What causes re-uptake of calcium ions?

Answer 15

Calcium pump

Question 16

What is the force of muscle contraction regulated by?

Answer 16

Increasing the number of motor units contracting simultaneously

Increasing the stimulation frequency

Question 17

What is meant by tetany?

Answer 17

A continuous contraction occurring at high stimulation frequency

Question 18

Describe concentric contraction

Answer 18

Isotonic- constant tension- change in length

Concentric - Muscle shortens

Question 19

Describe eccentric contraction

Answer 19

Isotonic and eccentric (muscle lengthens)

Question 20

Describe isometric contraction

Answer 20

Isometric- constant length, produces tension

Muscle length stays constant

Question 21

What are the two classifications of skeletal muscle fibres?

Answer 21

Slow twitch type I and fast twitch type II

Question 22

Give an example of slow twitch type I

Answer 22

Postural muscles of the back

Question 23

What are the features of the slow twitch type I?

Answer 23

Slow, sustained contractions 
Oxidative metabolism 
Many mitochondria 
Red-myoglobin 
Resist fatigue

Question 24

What are the two fast twitch, type II?

Answer 24

Type IIA and Type IIB

Question 25

Give an example of Type IIA

Answer 25

Soleus in calf

Question 26

Give an example of type IIB

Answer 26

Extraocular muscle

Question 27

Describe the features of Type IIA fast twitch

Answer 27

``` Fast contractions Oxidative metabolism Many mitochondria Red-pink myoglobin Intermediate fatigue resistance ```

Question 28

Describe type IIB fast twitch features

Answer 28

``` Rapid contractions Anaerobic glycolysis Few mitochondria White - little myoglobin High glycogen content ```

Question 29

Where is smooth muscle found?

Answer 29

Found in the gastrointestinal tract, uninary tract and walls of blood vessels

Question 30

What are the features of smooth muscle?

Answer 30

Single nuclei | No striations

Question 31

Describe the structure of smooth muscle

Answer 31

Actin filaments radiating from dense bodies

Question 32

What innervates smooth muscle?

Answer 32

Automatic nerve fibres

Question 33

Describe smooth muscle excitation

Answer 33

Typical smooth muscle action potential elicited by an external stimulus

Question 34

Describe smooth muscle spontaneous activity

Answer 34

Repetitive spike potentials, occur spontaneously in the smooth muscle of the wall

Question 35

Describe the smooth muscle excitation- contraction coupling

Answer 35

Has spontaneous electrical and mechanical activity Activity can be phasic or tonic Activity is modified by circulating hormones and neurotransmitters Contraction velocity is less than skeletal muscle Smooth muscle can maintain prolonged contractions at less energy cost of the other muscle types

Question 36

What are the features of skeletal muscles?

Answer 36

Striations Troponin Cross-bridge cycling Neurogenic stimulation only excites in skeletal muscle E-C coupling- depolarisation Ca2+ release from SR

Question 37

What are features cardiac muscle?

Answer 37

``` Striations troponin Cross-bridge cycling Spontaneous activity Neurogenic stimulation- modultaes cardiac muscle contraction E-C coupling- depolarisation Ca2+ ```

Question 38

What are the features of smooth muscle?

Answer 38

Calmodulin Spontaneous activity Neurogenic stimulation- modulates smooth muscle contraction E-C coupling - smooth muscle -depolarisation -Ca2+ - influx and CICR second messenger system