Lecture 32

Question 1

T-tubules in skeletal muscle

Answer 1

A-I band intersection

Question 2

T-tubules in cardiac muscles

Answer 2

in ventricles at Z-lines

Question 3

Intercalated discs:

Answer 3

Desmosomes prevent cells from separating contraction.

• Contain gap junctions that allow the action potentials to be carried from one cell to the next
• Allows for the coordinated contraction of all the myocytes

Question 4

Ventricular myocyte action potential

Answer 4

• Action potential is long lasting > 100 ms long
• Has plateau phase due to presence of a large sustained Ca2+ current (ICaL)
• Membrane potential depolarised throughout most of the twitch

Question 5

The three major stages of an action potential in a cardiac muscle

Answer 5

1. Rapid depolarisation due to fast voltage-gated Na+ channel
2. Plataeu phase due to slow voltage gated Ca2+ channel (L-type Ca2+ channel)
3. Repolarisation due to closing of Ca2+ channels and opening of K+ (outward) channels

Question 6

LTCC:

Answer 6

L-type voltage gated calcium cahnnel (ICaL)

Question 7

RyR

Answer 7

Ryanodine receptor (Calcium channel in sarcoplasmic reticulum)

Question 8

NCX

Answer 8

Sodium/Calcium exchanger

Question 9

NKA

Answer 9

Sodium/Potassium ATPase

Question 10

Cardiac muscle: Excitation contraction coupling

Answer 10

1. Depolarization opens Na+ channels in the sarcolemma.
2. Ca2+ influx by a Na+/Ca2+ exchanger
3. Ca2+ influx triggers opening of Ca2+ sensitive channels in the SR (RyRa), which liberates bursts of Ca2+
4. Ca2+ influx triggers. The raised intracellular Ca2+ concentration allows Ca2+ to bind to troponin, which then switches on the contractile machinery.

Question 11

For relaxation to occur ___

Answer 11

Ca2+ must decline, allowing Ca2+ to dissociate from troponin. This requires Ca2+ transport out of the cytosol by four pathways:

• SR Ca2+ ATPase
• Sarcolemmal Na+/Ca2+ exchange
• Sarcolemmal Ca2+ ATPase
• Mitochondrial Ca2+ uniport

Question 12

Ca2+ can exit the cytosol by four pathways:

Answer 12

• SR Ca2+ ATPase
• Sarcolemmal Na+/Ca2+ exchange
• Sarcolemmal Ca2+ ATPase
• Mitochondrial Ca2+ uniport

Question 13

Trigger for Ca2+ release from SR in skeletal muscle

Answer 13

-Na+ influx

Question 14

Trigger for Ca2+ release from SR in cardiac muscle

Answer 14

-Ca2+ influx

Question 15

Heart rate is set by:

Answer 15

The pacemaker cells in the sinoatrial node. The rate can then be modified, especially via the autonomic nerves releasing neurotransmitters

Question 16

Stroke volume is increased by:

Answer 16

Increased rate of firing

• Increasing stretch of ventricles
• Certain neurotransmitters

Question 17

Pacemaker cells

Answer 17

• Unstable resting membrane potential

- Depolarization due to relatively slow Ca2+ current (not fast Na+)

Question 18

Autonomic innervation of the heart

Answer 18

The vagus nerve decreases heart rate. Release ACh.

Question 19

Sympathetic cardiac nerves

Answer 19

increase heart rate and force of contraction (release noradrenaline).

Question 20

Increasing heart rate also increases:

Answer 20

contractile force

Question 21

Starlings law of the heart:

Answer 21

-As the resting ventricular volume is increased the force of the contraction is increased.

Question 22

Noradrenaline acting on B receptors and via second messengers acts on:

Answer 22

L-type channels resulting in more calcium entering the cell.

• Ca2+ pump in SR so SR increases its Ca2+ stores.
• Net result: bigger/shorter concentration

Question 23

Noradrenaline released by sympathetic nerves leads to increased cytosol calcium

Answer 23

Due to increased HR shortening time for extrusion