Cardiac & Smooth Muscle Pt. 2 Flashcards
What causes the rising phase of the action potential during SA node depolarisation?
Calcium influx opposed to sodium
How does the resting membrane potential of the SA node differ?
It does not have a stable resting membrane potential
How is the pacemaker potential of the SA node shown?
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
When does an action potential occur?
At the threshold value of -40 mV
During SA node depolarisation, when and why does the depolarising phase occur?
It occurs from -40 - 0 mV
This is due to the opening of Ca2+ channels and rapid calcium influx
During SA node depolarisation, when and why does the repolarising phase occur?
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
Why is the resting membrane potential of the SA node not stable?
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
What is the Frank-Starling law of the heart?
The resting length of cardiac muscle is set below its optimal level
Why is having sub-optimal overlap in cardiac muscle beneficial?
It gives cardiac muscle the ability to adjust the force of contraction
What happens to the myosin-actin overlap when cardiac muscle cells are stretched?
It increases the degree of overlap
This increases the force generated when the cells contract
Where is smooth muscle found?
It lines the walls of most hollow organs
This includes vascular, gastrointestinal, respiratory, urinary and reproductive systems
Why is smooth muscle an important therapeutic target?
It regulates variables such as blood flow, gastrointestinal motility and ventilation of the lungs
What happens to smooth muscle during septic shock?
Inadequate tone
Infection releases inflammatory mediators that cause dilation of systemic blood vessels
This leads to severe hypotension
How does the structure of smooth muscle allow for peristalsis?
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
What is the diameter and length of a smooth muscle cell?
2 - 10 micrometers diameter
40-600 micrometers length
Why is smooth muscle not striated?
Cells have more actin than myosin
The thick and thin filaments are not well organised
Do smooth muscle cells have T-tubules?
No but they have caveolae
Small indentations in the sarcolemma that may act like t-tubules
What does a smooth muscle cell look like?
It is not branched
It is an individual spindle-shaped cell with a central nucleus
Why does smooth muscle rely on external sources of calcium?
The sarcoplasmic reticulum is poorly developed
They have a poor internal calcium store
What two components of skeletal muscle does smooth muscle lack?
troponin and sarcomeres
In smooth muscle, where is actin and myosin anchored?
What is the motion of contraction and why?
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
Is smooth muscle autorhythmic?
Some of it but not all of it
What does smooth muscle contraction depend on and how is it regulated?
It depends on an increase in cytosolic Ca2+
It is regulated by myosin molecules of the thick filament, rather than the thin filament
What happens in smooth muscle contraction after an increase in cytosolic Ca2+?
Ca2+ binds to calmodulin
Calmodulin interacts with myosin kinase to phosphorylate myosin
Why must myosin be phosphorylated in smooth muscle contraction?
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
What happens in smooth muscle contraction when cytoplasmic Ca2+ falls?
Ca2+-calmodulin complex dissociates
This inactivates myosin kinase
How are the cross-bridges dephosphorylated?
After cytosolic Ca2+ falls, myosin phosphatase will dephosphorylate the cross-bridges
How can smooth muscle maintain force over long periods of time?
What is the benefit to this method?
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
Why is cross-bridge cycling much slower in smooth muscle?
- Smooth muscle myosin has a slow ATPase rate
It takes a long time for each cross-bridge to detach from the actin filament
- rate of Ca2+ removal from the cytoplasm is slow
this prolongs the duration of contraction
How does nervous input differ in skeletal and smooth muscle?
Skeletal muscle cells receive input from only one nerve
Smooth muscle cells may receive inputs from more than one nerve
What is meant by single-unit smooth muscle?
It contains gap junctions allowing it to act as a single unit
Where is single-unit smooth muscle found?
Gastrointestinal, respiratory, urinary and reproductive tracts
In the walls of small arteries
How is single-unit smooth muscle regulated?
How does electrical activity arise?
It is regulated by the autonomic nervous system
Electrical activity may arise spontaneously due to the presence of pacemaker cells
What is meant by the myogenic response in single-unit smooth muscle?
It will contract in response to a stretch
What is meant by multi-unit smooth muscle?
It lacks gap junctions so each cell is innervated individually
It allows for fine control
Where is multi-unit smooth muscle found?
It allows for fine control so is found in ciliary muscle of the eye which controls pupil size
Piloerector muscles of hair follicles
How is multi-unit smooth muscle innervated?
How is it different from single-unit?
It is innervated by the autonomic nervous system
There is no inherent response to stretch
It is not spontaneously active
What are varicosities?
Autonomic nerves make multiple contacts with the cell
Each contact is a varicosity
Why do autonomic nerves have no specialised post-junctional membrane?
Receptors are spread across the membrane
What is dilated cardiomyopathy?
The heart enlarges as the ventricles become dilated
Function is impaired due to the enlarged ventricle
How does dilated cardiomyopathy affect the lungs?
Muscles become weak and inefficient leading to a build up of fluid in the lungs
This is caused by left heart failure
Causes breathlessness
What does right heart failure lead to?
Fluid build up in tissues and organs
What is the prognosis for dilated cardiomyopathy?
Poor
50-60% of patients will survive more than 2 years
What are the symptoms of dilated cardiomyopathy?
Shortness of breath, swelling of the ankles, tiredness, palpitations and chest pain
what can cause dilated cardiomyopathy?
viral infection, autoimmune disease, excessive alcohol consumption or exposure to toxic compounds, pregnancy or familial disease
What is hypertrophic cardiomyopathy?
A disease of the myocardium where a portion of the myocardium thickens with no obvious cause
What are the symptoms of hypertrophic cardiomyopathy?
Which age group does it usually affect?
Usually asymptomatic
Symptoms can include shortness of breath, chest pain, palpitation, light-headedness and blackouts
Usually affects young people who are physically active
What causes thickening of the tissue in hypertrophic cardiomyopathy?
Myocardial disarray
The normal alignment of muscle cells is absent
What causes hypertrophic cardiomyopathy?
genetic mutation of important proteins needed for contraction of the heart
What is leiomyoma (fibroids)?
benign growth of the female reproductive tract
it is a smooth muscle disorder
How do fibroids usually present themselves?
Usually multiple growths with a diameter of 5mm or greater
When are fibroids more prevalent and what are the symptoms?
More prevalent when the female is approaching the menopause
Cause heavy uterine bleeding and/or pain
What is the cause of leiomyoma?
How many women experience symptoms?
Cause is unknown but is more common in the Afro-Caribbean race
20-50% of women are symptomatic
