lecture 32 - Cardiac muscle

Question 1

How does the cross-bridge cycle differ between smooth, skeletal, and cardiac muscle?

Answer 1

it doesn’t. It is exactly the same for all of them.

Question 2

length and diameter of cardiac muscle cells

Answer 2

ventricular: 100μm x 30μm atrial: 100μm x 10μm

Question 3

define myogenic and sate its relevance to cardiac muscle cells

Answer 3

myogenic means that control over muscle cells is involuntary. This is the case with cardiac muscle cells.

Question 4

T-tubules in cardiac muscle (vs. skeletal)

Answer 4

in cardiac muscle they are in the ventricles at the Z-lines, thus, there is only 1 T-tubule per sarcomere (as opposed to 2 in skeletal). N.B. atrial muscle cells have no T-tubules.

Question 5

is there a sarcoplasmic reticulum in the cardiac muscle or nah?

Answer 5

sarcoplasmic reticulum is present, however, it is not as extensive or as important as in the skeletal muscle.

Question 6

intercalated discs (contain? functions?)

Answer 6

• desmosomes prevent cells from separating during contraction - gap junctions allow APs to carry between cells - allows for the coordinated contraction of all myocytes

Question 7

action potential in the ventricular myocytes

Answer 7

• long lasting (greater than 100ms vs. 1ms in skeletal) - plateau (due to Ca2+) - very little chance of tetani

Question 8

sinoatrial node

Answer 8

found at the top of the right atrial wall. Initiates the AP.

Question 9

purkinje fibres

Answer 9

carry AP, from the atroventricular node, around the ventricles .

Question 10

rapid depolarisation (ventricular myocytes) due to…

Answer 10

fast voltage-gated Na+ channels, cause the membrane potential to reverse from -90mV to +30mV

Question 11

Plateau phase (ventricular myocytes) due to…

Answer 11

long acting, voltage-gated L-type Ca2+ channels open and there is a large sustained Ca2+ current, as the calcium moves in from the extracellular fluid

Question 12

repolarisation (ventricular myocyte) due to…

Answer 12

closing of Ca+ channel and opening of K+ channels (basic AP tekkers)

Question 13

how does the interaction between the SR and the T-tubules differ in cardiac vs. skeletal?

Answer 13

in cardiac muscle there is a separation between the SR and the T-tubules

Question 14

how is the influx of Ca2+ into the myocytes balanced?

Answer 14

Na+/Ca2+ exchanger

Question 15

DHPR =

Answer 15

Dihydrogen pyridine - same exact thing as L-type Ca2+ channel

Question 16

influx of Ca2+ into the sarcolemma/T-tubules, triggers?

Answer 16

CALCIUM INDUCED CALCIUM RELEASE Ca2+ sensitive channels in the SR (RyRa) open, liberating BURSTS (not constant) of Ca2+. Muscle contraction can then occur as it does in the skeletal muscle.

Question 17

as the action potential starts to repolarise, what happens to the Ca2+ ions?

Answer 17

Pumped into the SR through CaATPase (SERCA) and extruded from the cell via Na/Ca exchanger

Question 18

how is the contraction size graded?

Answer 18

by changing the concentration of Ca2+ i.e the presence of actin binding-sites

Question 19

can the heart increase the force in a contraction by recruiting more fibres?

Answer 19

NO!!! “all or nothing” when the heart beats, every muscle fibre is involved

Question 20

Cardiac output (CO) =

Answer 20

Stroke volume (SV) x Heart rate (HR)

Question 21

Stroke volume reflects…

Answer 21

tension developed by the cardiac muscle fibres in one contraction

Question 22

3 ways to increase stroke volume

Answer 22

1. increased stretch of ventricles (length) 2. increased rate of firing (HR) 3. certain hormones

Question 23

the pacemaker cells are…

Answer 23

Sinoatrial node and atrioventrical node

Question 24

pacemaker RMP

Answer 24

unstable resting membrane potential, spontaneously reaches threshold

Question 25

pacemaker potential

Answer 25

threshold is reached (spontaneously) when there is a slow depolarisation, due to ‘funny’ channels letting Na+ leak into the cells

Question 26

pacemaker depolarisation

Answer 26

when the threshold is reached, the Ca2+ T-type (transient) channels open and the rapid influx of Ca2+ produces the rising plane of the action potential

Question 27

pacemaker repolarisation

Answer 27

inactivation of Ca2+ channels and opening of K+ channels. Takes a while due to the L-type Ca2+ channels

Question 28

3 ways to increase HR (by altering the SA depolarisation)

Answer 28

1. decreasing the threshold potential 2. make the maximum distolic potential more positive 3. increasing the rate of spontaneous depolarisation

Question 29

parasympathetic vagus nerve

Answer 29

• decreases heart rate - releases ACh - mostly affects the HR

Question 30

sympathetic cardiac nerve

Answer 30

• increases heart rate - releases noradrenaline - affects both HR and SV

Question 31

Noraepinephrin vs. noradrenaline

Answer 31

same thing mayyyyyyynnne

Question 32

bmp without any neurotransmitters intervening

Answer 32

~100bpm

Question 33

noradrenaline

Answer 33

• increases the rate of spontaneous depolarisation, which increases the heart rate
• increases the SR uptake of Ca2+ (more can be released)

Question 34

ACh

Answer 34

decreases rate of spontaneous depolarisation and decreases membrane potential

Question 35

Starling’s Law of The Heart

Answer 35

“As the resting ventricular volume is increased, the force of contraction is increased”

Question 36

Na+/Ca2+ exchanger

Answer 36

voltage sensitive - stimulated by the repolarisation phase of the action potential.

Question 37

sarcomere length in resting cardiac muscle is ____ and develop maximum force when stretched to ____

Answer 37

sarcomere length in resting cardiac muscle is _1.8μm_ and develop maximum force when stretched to _2.1μm_

Question 38

Which numbers indicate:

a) where L-type Ca2+ channels open then close
b) where Na+ channels open and close
c) where K+ is pumped out

Answer 38

a) open at 1, close at 3
b) open at 0, close at 1
c) starts pumping out at 2, until a new AP initiates

Question 39

which region of the curve is due to the I_f (funny current)

Answer 39

1

Question 40

what is represented by each curve mayne?

Answer 40

1 = active tension in skeletal muscle

2 = active tension in cardiac muscle

3 = passive tension in cardiac muscle

4 = passive tension in skeletal muscle

Question 41

Cardiac muscle is electrically __________ whereas skeletal muscle is electrically _________

Answer 41

Cardiac muscle is electrically _coupled_ whereas skeletal muscle is electrically _isolated_

Question 42

increasing heart rate…

Answer 42

increases contractive force because there is less time for Ca2+ to be pumped out of the cell. Termed “automacity”

Question 43

increased inotropy (presence of Noradrenaline) will…

Answer 43

increase active tension at any given length