Lecture 15 - Cardiac and Smooth Muscle Flashcards
Do cardiac muscles fire action potentials?
Yes
Describe the structure of cardiac muscle
Branching cells interconnected by intercalated disks
How do contractile cell in the myocardium get the signal to contract?
Autorhythmic cells initiate the contraction, the action potentials spread from neighbouring cells
How is the action potential of the cardiac muscle and the skeletal muscle different?
In cardiac muscle, the AP lasts much longer, as is the refractory period. This means that there is an absolute upper limit to the speed of heart beats
Describe the action potential in cardiac contractile cells
1. Depolarisation • Na+ in, fast 2. Initial repolarisation • K+ out, fast 3. Plateau • Ca2+ in, slow • Halt in K efflux 4. Repolarisation • K+ out, fast
Describe Excitation-Contraction coupling in cardiac muscle cells
- Action potential arrives from neighbour, flows down T tubules
- L type Ca2+ channels on plasma membrane open; Ca rushes into cell
- Calcium induced calcium release; RyR channels on the SR open; more Ca comes out.
- Calcium binds to troponin
- Tropomyosin moved off the myosin binding site, myosin binds to actin
- Crossbridge cycling
What is meant by calcium induced calcium release?
Once calcium flow in throu the L type Ca channel, the RuR channel opens and more calcium flows into the cytosol from the SR
What are the two channel types that are important for cardiac muscle contraction?
- L type Ca2+ channel on plasma membrane
* RyR channel on SR
Describe the conduction pathway in the heart
- Sinoatrial node
- Intermodal pathway
- Atrioventricular node
- Left and right branches of Bundle of His
- Purkinje fibres
Describe the action potential in cardiac autorhythmic cells
- Calcium in slowly
- Threshold reached –> calcium in fast
- K out fast
In cardiac muscle cells, most Ca comes from the …
Sarcoplasmic reticulum
More Ca from outside cell than in skeletal muscle, though.
Where is smooth muscle commonly found?
Wall of organs:
- stomach
- intestines
- kidneys
- blood vessels
- bladder
Describe the different layers of smooth muscle in organs such as the stomach
There are many layers, with the muscle arranged in different orientations.
When the different layers contract, the stomach restricts along the different planes
Describe the pathway leading to contraction in smooth muscle cells
- Muscle excited
- Calcium channel opens, calcium rushes into cytosol
- Calcium induced calcium release from SR
- Calcium in the cytosol binds to calmodulin, making a CaCaM complex
- This activates MLCK
- MLCK phosphorylates the myosin head
- Muscle contraction
What is the difference between single and multi unit smooth muscle cells?
Single unit smooth muscle cells are electrically linked by gap junctions.
These cells all contract together
Eg. In the small intestine
Multi unit smooth muscle cells must be excited individually
Eg. The eye muscle
What is different about the length of muscle contraction in smooth muscle?
In smooth muscle, the myosin heads remain bound in the rigor state. The muscle can remain contracted for hours
What is the structure of arteries?
Endothelium
Elastic tissue
Lots of smooth muscle
Fibrous tissue
What is the structure of arterioles?
Endothelium, smooth muscle
What is the structure of capillaries?
Endothelium
What is the structure of venules?
Endothelium
Fibrous tissue
What is the structure of veins?
Endothelium
Elastic fibres
Smooth muscle
Fibrous tissue
What allows electrical coupling of cardiac muscle cells?
Intercalated disks:
• gap junctions
How is calcium removed from the cytosol once the contraction needs to be inhibited?
Pumped back into the SR by Ca-ATPase pump
Compare the variability in force of contraction in heart and skeletal muscle
Skeletal: invariant
Cardiac: force production can be graded
Describe grading of cardiac muscle force production
Depends on Ca cytosolic concentration • more calcium enters the cytosol • more Ca bound to troponin • more active contractile fibres • increased force of contraction
Why is it important for cardiac contractile cells have a longer action potential?
Prevents tetanus
Heart cannot function if it is continually contracted, because it can’t fill with blood
What is the resting membrane potential of myocardial autorhythmic cells?
No resting membrane potential
Unstable membrane potential that fluctuates from -60 mV up to threshold
