Cardiac & Smooth Muscle Pt. 2

Question 1

What causes the rising phase of the action potential during SA node depolarisation?

Answer 1

Calcium influx opposed to sodium

Question 2

How does the resting membrane potential of the SA node differ?

Answer 2

It does not have a stable resting membrane potential

Question 3

How is the pacemaker potential of the SA node shown?

Answer 3

The pacemaker potential is a slow depolarisation

There is a gradual depolarisation from -60 mV

This is due to the slow influx of calcium and reduced K+ permeability

Question 4

When does an action potential occur?

Answer 4

At the threshold value of -40 mV

Question 5

During SA node depolarisation, when and why does the depolarising phase occur?

Answer 5

It occurs from -40 - 0 mV

This is due to the opening of Ca2+ channels and rapid calcium influx

Question 6

During SA node depolarisation, when and why does the repolarising phase occur?

Answer 6

This occurs when the Ca2+ channels close and the K+ channels open

K+ leaves the cell

The channels close at -60mV allowing the pacemaker potential to start over again

Question 7

Why is the resting membrane potential of the SA node not stable?

Answer 7

This is due to the slow influx of calcium

At the threshold, Ca2+ channels will open and rapid depolarisation will occur leading to a cardiac action potential

Question 8

What is the Frank-Starling law of the heart?

Answer 8

The resting length of cardiac muscle is set below its optimal level

Question 9

Why is having sub-optimal overlap in cardiac muscle beneficial?

Answer 9

It gives cardiac muscle the ability to adjust the force of contraction

Question 10

What happens to the myosin-actin overlap when cardiac muscle cells are stretched?

Answer 10

It increases the degree of overlap

This increases the force generated when the cells contract

Question 11

Where is smooth muscle found?

Answer 11

It lines the walls of most hollow organs

This includes vascular, gastrointestinal, respiratory, urinary and reproductive systems

Question 12

Why is smooth muscle an important therapeutic target?

Answer 12

It regulates variables such as blood flow, gastrointestinal motility and ventilation of the lungs

Question 13

What happens to smooth muscle during septic shock?

Answer 13

Inadequate tone

Infection releases inflammatory mediators that cause dilation of systemic blood vessels

This leads to severe hypotension

Question 14

How does the structure of smooth muscle allow for peristalsis?

Answer 14

It has an inner circular layer and outer longitudinal layer

Alternating contraction and relaxation of the 2 layers mixes substances in the lumen of hollow organs

Question 15

What is the diameter and length of a smooth muscle cell?

Answer 15

2 - 10 micrometers diameter

40-600 micrometers length

Question 16

Why is smooth muscle not striated?

Answer 16

Cells have more actin than myosin

The thick and thin filaments are not well organised

Question 17

Do smooth muscle cells have T-tubules?

Answer 17

No but they have caveolae

Small indentations in the sarcolemma that may act like t-tubules

Question 18

What does a smooth muscle cell look like?

Answer 18

It is not branched

It is an individual spindle-shaped cell with a central nucleus

Question 19

Why does smooth muscle rely on external sources of calcium?

Answer 19

The sarcoplasmic reticulum is poorly developed

They have a poor internal calcium store

Question 20

What two components of skeletal muscle does smooth muscle lack?

Answer 20

troponin and sarcomeres

Question 21

In smooth muscle, where is actin and myosin anchored?

What is the motion of contraction and why?

Answer 21

Anchored to dense bodies

Actin and myosin filaments appear to spiral down the long axis of the cell

The cell contracts in a corkscrew like way

Question 22

Is smooth muscle autorhythmic?

Answer 22

Some of it but not all of it

Question 23

What does smooth muscle contraction depend on and how is it regulated?

Answer 23

It depends on an increase in cytosolic Ca2+

It is regulated by myosin molecules of the thick filament, rather than the thin filament

Question 24

What happens in smooth muscle contraction after an increase in cytosolic Ca2+?

Answer 24

Ca2+ binds to calmodulin

Calmodulin interacts with myosin kinase to phosphorylate myosin

Question 25

Why must myosin be phosphorylated in smooth muscle contraction?

Answer 25

Once it is phosphorylated, the myosin can attach to the actin filament and form a cross-bridge It can generate tension in a similar way to skeletal muscle

Question 26

What happens in smooth muscle contraction when cytoplasmic Ca2+ falls?

Answer 26

Ca2+-calmodulin complex dissociates This inactivates myosin kinase

Question 27

How are the cross-bridges dephosphorylated?

Answer 27

After cytosolic Ca2+ falls, myosin phosphatase will dephosphorylate the cross-bridges

Question 28

How can smooth muscle maintain force over long periods of time? What is the benefit to this method?

Answer 28

Cross-bridge cycling occurs more slowly in smooth muscle Contraction occurs more slowly The duration of contraction in response to a stimulus is long Reduced ATP consumption

Question 29

Why is cross-bridge cycling much slower in smooth muscle?

Answer 29

1. Smooth muscle myosin has a slow ATPase rate It takes a long time for each cross-bridge to detach from the actin filament 2. rate of Ca2+ removal from the cytoplasm is slow this prolongs the duration of contraction

Question 30

How does nervous input differ in skeletal and smooth muscle?

Answer 30

Skeletal muscle cells receive input from only one nerve Smooth muscle cells may receive inputs from more than one nerve

Question 31

What is meant by single-unit smooth muscle?

Answer 31

It contains gap junctions allowing it to act as a single unit

Question 32

Where is single-unit smooth muscle found?

Answer 32

Gastrointestinal, respiratory, urinary and reproductive tracts In the walls of small arteries

Question 33

How is single-unit smooth muscle regulated? How does electrical activity arise?

Answer 33

It is regulated by the autonomic nervous system Electrical activity may arise spontaneously due to the presence of pacemaker cells

Question 34

What is meant by the myogenic response in single-unit smooth muscle?

Answer 34

It will contract in response to a stretch

Question 35

What is meant by multi-unit smooth muscle?

Answer 35

It lacks gap junctions so each cell is innervated individually It allows for fine control

Question 36

Where is multi-unit smooth muscle found?

Answer 36

It allows for fine control so is found in ciliary muscle of the eye which controls pupil size Piloerector muscles of hair follicles

Question 37

How is multi-unit smooth muscle innervated? How is it different from single-unit?

Answer 37

It is innervated by the autonomic nervous system There is no inherent response to stretch It is not spontaneously active

Question 38

What are varicosities?

Answer 38

Autonomic nerves make multiple contacts with the cell Each contact is a varicosity

Question 39

Why do autonomic nerves have no specialised post-junctional membrane?

Answer 39

Receptors are spread across the membrane

Question 40

What is dilated cardiomyopathy?

Answer 40

The heart enlarges as the ventricles become dilated Function is impaired due to the enlarged ventricle

Question 41

How does dilated cardiomyopathy affect the lungs?

Answer 41

Muscles become weak and inefficient leading to a build up of fluid in the lungs This is caused by left heart failure Causes breathlessness

Question 42

What does right heart failure lead to?

Answer 42

Fluid build up in tissues and organs

Question 43

What is the prognosis for dilated cardiomyopathy?

Answer 43

Poor 50-60% of patients will survive more than 2 years

Question 44

What are the symptoms of dilated cardiomyopathy?

Answer 44

Shortness of breath, swelling of the ankles, tiredness, palpitations and chest pain

Question 45

what can cause dilated cardiomyopathy?

Answer 45

viral infection, autoimmune disease, excessive alcohol consumption or exposure to toxic compounds, pregnancy or familial disease

Question 46

What is hypertrophic cardiomyopathy?

Answer 46

A disease of the myocardium where a portion of the myocardium thickens with no obvious cause

Question 47

What are the symptoms of hypertrophic cardiomyopathy? Which age group does it usually affect?

Answer 47

Usually asymptomatic Symptoms can include shortness of breath, chest pain, palpitation, light-headedness and blackouts Usually affects young people who are physically active

Question 48

What causes thickening of the tissue in hypertrophic cardiomyopathy?

Answer 48

Myocardial disarray The normal alignment of muscle cells is absent

Question 49

What causes hypertrophic cardiomyopathy?

Answer 49

genetic mutation of important proteins needed for contraction of the heart

Question 50

What is leiomyoma (fibroids)?

Answer 50

benign growth of the female reproductive tract it is a smooth muscle disorder

Question 51

How do fibroids usually present themselves?

Answer 51

Usually multiple growths with a diameter of 5mm or greater

Question 52

When are fibroids more prevalent and what are the symptoms?

Answer 52

More prevalent when the female is approaching the menopause Cause heavy uterine bleeding and/or pain

Question 53

What is the cause of leiomyoma? How many women experience symptoms?

Answer 53

Cause is unknown but is more common in the Afro-Caribbean race 20-50% of women are symptomatic