S13) Muscle

Question 1

Define the following terms:

• Myalgia
• Myasthenia

Answer 1

• Myalgia: muscle pain

- Myasthenia: muscle weakness

Question 2

Define the following terms:

• Myocardium
• Myopathy
• Myoclonus

Answer 2

• Myocardium: muscular component of the heart

- Myopathy: disease of the muscles

- Myoclonus: sudden spasm of the muscles

Question 3

Define the following terms:

• Sarcolemma
• Sarcoplasm
• Sarcoplasmic reticulum

Answer 3

• Sarcolemma: outer membrane of a muscle cell

- Sarcoplasm: cytoplasm of a muscle cell

- Sarcoplasmic reticulum: smooth endoplasmic reticulum of a muscle cell

Question 4

What are the three histological forms of muscle?

Answer 4

• Skeletal muscle (striated)
• Cardiac muscle (striated)
• Smooth muscle (non-striated)

Question 5

Describe the morphology, connections, control and power of skeletal muscle

Answer 5

• Morphology: long parallel cylinders, multiple nuclei, striations
• Connections: fascicle bundles, tendons
• Control: somatic, voluntary
• Power: rapid, forceful

Question 6

Describe the morphology, connections, control and power of cardiac muscle

Answer 6

• Morphology: short branched cylinders, central nucleus, striations
• Connections: junctions
• Control: intrinsic rhythm, involuntary autonomic
• Power: lifelong variable rhythm

Question 7

Describe the morphology, connections, control and power of smooth muscle

Answer 7

• Morphology: spindle-shaped, tapering ends, central nucelus
• Connections: connective tissue, gap junctions
• Control: involuntary, autonomic
• Power: slow, sustained or rhythmic

Question 8

What is myoglobin and what does it do?

Answer 8

Myoglobin is a red protein containing haem, which functions as an oxygen-storing molecule, providing oxygen to the working muscles

Question 9

Which type of muscle contains myoglobin?

Answer 9

It is present in skeletal and cardiac muscle only

Question 10

Explain the relationship between haemoglobin and myoglobin

Answer 10

• Haemoglobin gives up oxygen to myoglobin, especially when pH is lowered
• Active muscles produce CO₂ (or lactic acid - anaerobic respiration) both of which result in the more acidic conditions that promote this transfer

Question 11

What is a muscle fibre?

Answer 11

A muscle fibre is a striated muscle cell

Question 12

Describe skeletal muscle structure

Answer 12

Question 13

What is muscle atrophy?

Answer 13

Muscle atrophy is a decrease in the mass of the muscle due to a reduction in the number of cells and/or size of cells

Destruction > replacement

Question 14

What is muscle hypertrophy?

Answer 14

Muscle hypertrophy involves an increase in size of skeletal muscle through a growth in size of its component cells

Replacement > destruction

Question 15

What are the causes of atrophy?

Answer 15

• Muscle inactivity
• Malnutrition
• Cancer
• Neurogenic

Question 16

What are the changes that accompany hypertrophy?

Answer 16

• More contractile proteins (increase in fibre diameter)
• Metabolic increases: enzyme activity for glycolysis, mitochondria, stored glycogen, blood flow

Question 17

Outline the arrangement of skeletal muscle

Answer 17

• Skeletal muscle is composed of fascicles
• Fasciscles are composed of muscle fibres (cells)
• Muscle fibres are composed of myofibrils
• Myofibrils are composed of myofilaments (actin& myosin)

Question 18

Explain and describe the use of Troponin in enzyme assays

Answer 18

• Troponin is used as a marker for cardiac ischaemia as it is released from ischaemic cardiac muscle within an hour
• The smallest changes in troponin levels in the blood are indicative of cardiac muscle damage

Question 19

Describe the structure of a myosin molecule

Answer 19

• A myosin molecule has a rod-like structure with two protruding ‘heads’
• Each thick filament contains many myosin molecules, whose heads protrude at opposite ends

Question 20

Describe the structure of a thin actin filament

Answer 20

• The actin filament forms a helix around which tropomyosin molecules coil, to reinforce it
• A troponin complex is attached to each tropomyosin molecule

Question 21

What is creatine kinase and what is it used for?

Answer 21

• CK is an important enzyme in metabolically active tissues like muscle
• Used to measure and diagnose heart attacks as the enzyme increase is proportional to infarct size (superseded by troponin assay)

Question 22

CK is an enzyme that is also released into the blood by damaged skeletal muscle and brain.

A rise in plasma CK can result from which events/conditions?

Answer 22

• Intramuscular injection
• Vigorous physical exercise
• A fall
• Rhabdomyolysis (severe muscle breakdown)

Question 23

Identify the steps in the contraction mechanism

Answer 23

• Stage 1: Attachment
• Stage 2: Release
• Stage 3: Bending
• Stage 4: Force Generation
• Stage 5: Reattachment

Question 24

Outline the 5 stages in the contraction mechanism

Answer 24

⇒ Attachment – myosin head tightly binds to actin molecule

⇒ Release – ATP binds to myosin head causing it to uncouple from the actin filament

⇒ Bending – ATP hydrolysis causes uncoupled myosin head to bend and advance a short distance

⇒ Force Generation – myosin head binds weakly to actin filament, releasing P_i which strengthens binding, (power stroke → myosin head returns to former position)

⇒ Reattachment – ATP binds to the myosin head causing detachment from actin.

Question 25

What is the role of ionic calcium in the contraction mechanism?

Answer 25

- **Ionic calcium** bind to TnC of troponin which moves tropomyosin away from actin's binding sites - This displacement allows myosin heads to bind actin and contraction begins

Question 26

What is a neuromuscular junction?

Answer 26

A **neuromuscular junction** is a chemical synapse formed by the contact between a motor neuron and a muscle fibre

Question 27

What happens at a neuromuscular junction?

Answer 27

A nerve impulse causes the release of ACh which binds to receptors on the sarcolemma to intiate an action potential propagated along the muscle

Question 28

Outline the 9 steps involved in the contraction of skeletal muscle

Answer 28

⇒ Nerve impulse arrives at **neuromuscular junction** ⇒ **Ach released** into synaptic cleft ⇒ **Local depolarization** of sarcolemma ⇒ Voltage-gated Na⁺ channels open (**Na⁺ enters** cell) ⇒ **General depolarization** spreads over sarcolemma (& into T tubules) ⇒ Activation of voltage gated **Ca²⁺ channels** ⇒ **Ca²⁺ rapidly released i**nto sarcoplasm ⇒ Ca²⁺ binds to **troponin** ⇒ Initation of **contraction cycle**

Question 29

Differentiate between hyperplasia and hypertrophy

Answer 29

- **Hypertrophy**: enlargement of their individual cells - **Hyperplasia:** multiplication of their cells

Question 30

Why is there is a close association of T tubules and diads the sarcoplasmic reticulum?

Answer 30

It permits the release of ionic calcium into the sarcoplasm and allows for subsequent muscle contraction

Question 31

What are natriuretic peptides?

Answer 31

**Natriuretic peptides** are peptide hormones that are synthesized by the heart, brain and other organs: - Atrial natriuretic peptide (ANP) - Brain-type natriuretic peptide (BNP)

Question 32

Why are natriuretic peptides released?

Answer 32

Release stimulated by **atrial and ventricular distension** (response to heart failure)

Question 33

What do natriuretic peptides do?

Answer 33

They **reduce arterial pressure** by decreasing blood volume and systemic vascular resistance (counter-regulatory system for RAAS)

Question 34

What is ANP?

Answer 34

**ANP** is a peptide that is synthesized, stored, and released by atrial myocytes in response to atrial distension

Question 35

What is BNP?

Answer 35

**BNP** is a peptide synthesised largely by the ventricles (and brain)

Question 36

What are Purkinje fibres?

Answer 36

**Purkinje fibres** are specialised myocardial cells which carry impulses to ventricular muscle from the AVN

Question 37

Identify 3 unique cellular features of purkinje fibres

Answer 37

- Abundant glycogen - Sparse myofilaments - Extensive gap junction sites

Question 38

What do Purkinje fibres do?

Answer 38

Purkinje fibres **conduct action potentials rapidly,** enabling the ventricles to contract in a synchronous manner

Question 39

Where can smooth muscle be found?

Answer 39

Smooth muscle often forms the contractile walls of **passageways or cavities** *e.g. of vascular structures, respiratory tract and genitourinary system*

Question 40

What are indicators of muscle injury/necrosis?

Answer 40

- Creatine kinase (all muscle) - Myoglobinuria (skeletal muscle) - Troponin I (cardiac)

Question 41

Identify some diseases caused by smooth muscle dysfunction

Answer 41

- Asthma - Irritable Bowel Syndrome - Primary hypertension - Atherosclerosis

Question 42

Modified smooth muscle cells can occur as myoepithelial cells or myofibroblast cells. What are myoepithelial cells?

Answer 42

**Myoepithelial cells** are stellate cells which form a basketwork around the secretory units of some exocrine glands *e.g sweat, salivary and mammary glands*

Question 43

Modified smooth muscle cells can occur as myoepithelial cells or myofibroblast cells. What are myofibroblast cells and what do they do?

Answer 43

**Myofibroblasts** are modified fibroblasts which produce collagenous matrix and contract at sites of wound healing

Question 44

Compare and contrast the limited nature of repair possible in the different forms of mature muscle

Answer 44

- **Skeletal muscle** **cells** cannot divide but regenerate through **satellite** cells, (hyperplasia follows muscle injury) **- Cardiac muscle** **cells** are incapable of regeneration (scar tissue forms after damage) **- Smooth muscle cells** retain their mitotic activity and can form new smooth muscle cells