Muscle

Question 1

What are the types of muscle and how are they differentiated?

Answer 1

Skeletal (Striated, Voluntary)
Cardiac (Striated, Involuntary)
Smooth (Unstriated, Involuntary)

Question 2

What is myoglobin and what is it’s specific function?

Answer 2

Protein with a single haem group that binds to O2 (single-subunit of Haemoglobin)
Present in Striated muscle to transfer O2 to muscles (Higher affinity at lower pH - when there’s higher [CO2])

Question 3

What is the structure of skeletal muscle?

Answer 3

Muscle fibres surrounded by endomysium
Muscle fibres bundled into fascicles
Fascicles are surrounded by perimysium
Groups of fascicles bundled into skeletal muscle
Muscles are surrounded by epimysium

Question 4

What is the structure of muscle fibres?

Answer 4

Contain myofibrils of thin actin and thick myosin filaments

Myofibrils are divided into sarcomeres and surrounded by outer membrane sarcolemma

Question 5

Describe the sections of a sarcomere

Answer 5

A-band (dark): length of myosin
I-band (light): length of actin only
H-zone: length of myosin only
Z-disc: connection point for actin
M-line: middle point of sarcomere

Question 6

Describe the features of hypertrophy

Answer 6

Replacement > Destruction of muscle fibres
Increase in fibre diameter (more contractile proteins)
Increase in number of sarcomeres

Question 7

Describe the features of atrophy

Answer 7

Replacement < Destruction of muscle fibres
Decrease in fibre diameter (less contractile proteins)
Decrease in number of sarcomeres

Question 8

What causes hypertrophy/atrophy?

Answer 8

Changes in the rate of metabolism
Changes in the frequency of stretching (contraction/relaxation)
Changes in the amount of innervation

Question 9

Describe the features of actin filaments

Answer 9

Dimers of actin form a double helix
Coils of tropomyosin surround actin for reinforcement and blocking myosin
Troponin complex (TnI, TnT, TnC) attached to every tropomyosin controls attachment of actin to myosin

Question 10

Describe the features of myosin filaments

Answer 10

Many individual myosin molecules group to form a filament

Each myosin molecule contains 2 myosin heads in regions of potential overlap (none in the H-zone)

Question 11

Describe the stages of muscle contraction

Answer 11

1) Attachment of the myosin head to actin filaments:
Additional Ca2+ binds to TnC of the troponin complex
Displaces tropomyosin away from the actin filaments allowing binding of myosin
2) Working stroke:
Myosin pulls actin toward the m-line, releases ADP and Pi
3) Low-energy configuration:
ATP binds to myosin heads releasing actin
4) Preparation:
ATP is hydrolysed but not released, ‘cocking’ the myosin head
(This will repeat as long as there is Ca2+)

Question 12

What happens to the regions of a sarcomere during contraction?

Answer 12

H-zone shortens
I-band shortens
A-band is constant but moves closer to Z-discs

Question 13

What are markers for:

Cardiac muscle damage and skeletal muscle damage?

Answer 13

Cardiac: Increase in troponin levels
Skeletal: Increase in Creatine Kinase levels

Question 14

What is sarcolemma and what is it’s specific function?

Answer 14

Outer membrane of myofibres (formed from T-tubules and sarcoplasmic reticulum)
T-tubules which conduct the Action Potential
Sarcoplasmic reticulum contain Ca2+ which is released to bind to TnC
T-tubules form triad with adjacent terminal cisternae of sarcoplasmic reticulum

Question 15

After post-synaptic depolarisation by VGNaC’s how does Ca2+ get released?

Answer 15

Action potential travels down T-tubules causing conformational change of voltage-sensing proteins
Conformation change of proteins opens gated Ca2+ channels of adjacent terminal cisternae
Rapid efflux of Ca2+ into sarcoplasm

Question 16

What are muscle spindles?

Answer 16

Intrafusal muscle fibres surrounded by a capsule facilitating proprioception
3 neurones:
Gamma - keeps fibre taught
Type Ia sensory - rate of change of muscle length
Type II sensory - position of muscle

Question 17

What special structure do cardiac muscle cells contain?

Answer 17

Intercalated discs (allows contraction of cardiac muscle simultaneously):
Fascia adherens (connect sarcomeres via anchoring actin filaments)
Desmosomes (prevent separation of muscle fibres by binding actin filaments)
Gap junctions (allow passage of ions, continuing the AP and depolarisation between sarcomeres)

Question 18

What is the difference between hypertrophy and hyperplasia?

Answer 18

Hypertrophy: Increase in cell size
Hyperplasia: Increase in cell number

Question 19

What are Natriuretic peptides and what causes their release?

Answer 19

Natriuretic Peptides (reduce hypertension)
Distension from hypertension (from hyperbole is) causes their release

Question 20

What are the effects of ANP and BNP?

Answer 20

Decrease the level of Renin (causes increase in blood pressure)
ANP: Increases Glomerular Filtration Rate (kidneys)
BNP: Increases vasodilation

Question 21

What does a reduction in Renin cause?

Answer 21

Reduction of Renin, causes reduction of Angiotensin II (which activates aldosterone - mineralocorticoid)
Increases Diuresis (Excretion via urine) and natriuresis (Excretion of Na), which decreases blood volume as water also leaves, lowering blood pressure

Question 22

Describe the features of smooth muscle

Answer 22

Unbranched: fusiform
Unstriated: no sarcomeres/T-tubules
Involuntary: under autonomic nervous control (still use myosin/actin)

Question 23

How does each type of muscle cell regenerate?

Answer 23

Skeletal muscle (cannot divide) - 
Satellite cells differentiate into skeletal muscle cells  and can undergo Mitosis (hyperplasia) or fuse with existing muscle fibres (hypertrophy)
Cardiac muscle (cannot regenerate) -
Following damage, fibroblasts invade area, divide and lay down scar tissue
Smooth muscle (can regenerate) -
As cells can divide (hyperplasia) and grow (hypertrophy)