Lecture 4: Excitation Coupling

Question 1

What is the cardiac skeleton?

Answer 1

a fibrous plate with five holes
one fibrous ring for 
- left AV valve 
- right AV valve 
- aortic semi-lunar valve
-pulmonic semi lunar valve
-atrio-ventricular bundle

Question 2

what are the three functions of the cardiac skeleton

Answer 2

1. insulation for electrical activity ‘electrophysiological discontinuity’
2. attachment of atrial and ventricular myocardium
3. strong yet flexible support for valvular openings

Question 3

the cardiac skeleton is reinforced with cartilage or bone, in which species is there bone and how many pieces?

Answer 3

ox - 2 bone
sheep - 1 bone
horse, pig, dog, cat = cartilage

Question 4

what are and describe the 3 layers of the heart tissue from outside to inside:

Answer 4

x3 layers

1. epicardium - covers exterior: has outter flattened epithelium of mesothelium cells, supported by CT w/ elastic fibres and adipose tissue
2. myocardium - contain cardiac myocytes, pacemaker cells and cardiac skeleton
3. endocardium - endothelium in contact with the lumen on the heart, has two sub endothelial layers.

Question 5

what is a sneaky way to tell endocardium apart from epicardium?

Answer 5

adipose tissue will be found UNDER the epicardium

Question 6

why are cardiac myocytes abundant in mitochondria?

Answer 6

cardiac mycoses are dependent on aerobic respiration, no rest periods, no oxygen debt possible.

Question 7

list 6 histological features of cardiac myocytes.

Answer 7

1. structurally have intercalated discs contain desmosomes (structural attachment) and gap junctions (electrical attachment)
2. cell membrane invaginations - t tubules
3. sarcoplasmic reticulum
4. branched fibres
5. central nucleus
6. purkinje fibres - specialised conducting cells, pale staining = glycogen filled

Question 8

the rise in Ca seen in the action potential, couples the Ap to contraction.
describe how Ca enters the cell and rises in intracellular [Ca]

Answer 8

• AP conducted across mycoyte memb, Ca enter cell via Ltype VO channels (20%)
• increase in intracellular Ca, stimulating the release ofCa from the ryanodine channels on the SR

Question 9

describe the generation of tension within cardiac muscle

Answer 9

tension generated through cross bridging mechanism.

Ca binds to troponin, alters the tropomyosin position, moves actin aside exposing myosin binding site not the actin filament –> contraction

Question 10

how does Ca release from the SR contribute to the plateau phase of the action potential?

Answer 10

Ca release from the SR contributes to the plateau phase, keeping the intracellular calcium levels positive, contributing to the refactatory period

Question 11

how is relaxation achieved?

Answer 11

CaATPase active pumps return calcium to the SR from the extracellular fluid
Ca exchanged extracellularly for Na
intracellular ca levels drop and the muscle relaxes
–> need relaxation for filling of heart

Question 12

what does the generation of tension depend on

Answer 12

how many cross bridges are formed

continues until all troponin C are bound to Ca

Question 13

what are the two kinds of contraction in cardiac muscle?

Answer 13

isometric

isotonic

Question 14

what is an isometric contraction? give an example

Answer 14

the ability of the muscle to generate force at a fixed length
–> a measure of the muscles maximum ability to develop tension
eg/ cardiac contraction before valves open

Question 15

what is isotonic contraction? give an example

Answer 15

activating an unrestrained muscle causing it to shorten without force development.
eg/ when the valves open and cardiac muscle push blood out

Question 16

what is the effect of cell length on cardiac contraction?

Answer 16

cell length determines the overlap of thick and thin filaments
length of the relaxed cell is determined by amount of blood in ventricle

Question 17

what is the consequence of the length of the cell being too close or too far?

Answer 17

too close = will not let contraction, no room for sliding

too far = can’t contract, cards myopathy, heart is floppy and can’t power stroke

Question 18

what is contractility and how can it be adjusted?

Answer 18

contractility is the amount of tension that can be developed at any given stretch of cardiac muscle
contractility can be adjusted physiologically and pathologically (with ionotropes)
contractility increases with increasing calcium

Question 19

what is the relationship describing length tension?

Answer 19

the frank starling relationship
X axis = EDV
Y axis = Stroke vol