Session 4

Question 0

Define action potential

Answer 0

Characteristic disturbance of potential difference between the inside and outside of the cell (whose form varies between ventricular cells, pacemaker cells and conducting).

Question 1

What is the function of pacemaker cells?

Answer 1

They generate an electrical event at regular intervals (cardiac action potential)

Question 2

How does a cardiac action potential differ to an action potential in skeletal muscle?

Answer 2

An increased influx of sodium (Na+)
This is the main factor driving the pacemaker potential towards threshold. Cardiac cells possess unique Na+ ion channels (‘funny channels’) that open in response to hyperpolarisation.
A decreased efflux of potassium (K+)
On top of the effect of sodium, potassium channels in the heart begin to close during hyperpolarisation (of the action potential). This limits the amount of potassium that can leave the cell, pushing the membrane potential up further.
Calcium flow is much more important
Lasts longer
Last

Question 3

How long is an average action potential in the heart when resting?

Answer 3

280 ms

Question 4

Why is the cardiac action potential the length that it is?

Answer 4

Length ensures that once the AP has begun it’s long enough for the cell to be depolarised when the last cell in the myocardium begins its AP.

Question 5

Why does the [Ca2+]i increase during the plateau phase?

Answer 5

Mostly due to calcium induced calcium released from SR/ER, partly because of Ca2+ from Ca2+ channels

Question 6

What does the force of contraction depend on?

Answer 6

Rate of entry of Ca2+ (to the cytoplasm)

Rate of removal of Ca2+

Question 7

Why does the membrane potential depolarise slowly during the pacemaker potential?

Answer 7

Due to channels that are permeable to Na+ ions. They are different to Na+ channels of action potential and are activated with more hyperpolarisation.

Question 8

What is the role of purkinje fibres?

Answer 8

Conduction of excitation through ventricular myocardium. Have long action potential’s.

Question 9

How does smooth muscle in the arteries and veins initiated?

Answer 9

By phosphorylation of MLC by MLCK.

Question 10

Describe what happens during ventricular systole (graphically with ion channels)

Answer 10

Upstroke - Na+ influx due to opening of V-gated Na+ channels
Bendy bit, slight downstroke - transient outward K+ current
Plateau - opening of V-gated Ca2+ channels (L-type) channels
Downstroke - Ca2+ channels inactivate, V-gated K+ channels open (take longer to inactivate than Na+ channels)
Begins at -90mV and lasts about 400ms. Goes up to about about 25mV and plateau lasts about 300ms

Question 11

Describe what happens during SAN AP graphically (ion channels)

Answer 11

Slope up - pacemaker potential, funny current (influx of Na+)
Bendy bit - opening of V-gated Ca2+ channels
Downstroke - opening of V-gated K+ channels
Starts at -60mV, goes up to about 10mV. Total time is about 800ms slope up lasts about 400ms.

Question 12

What is a desmosome?

Answer 12

A mechanical joint

Question 13

What is the function of gap joints?

Answer 13

Permit movement of ions and electrically couple cells.

Question 14

Where does Ca2+ bind in myocytes?

What is the significance of this?

Answer 14

Binds to troponin C, causing a conformational change which shifts tropomyosin to reveal myosin binding site on actin filaments. (sliding filament model)

Question 15

Where does Ca2+ bind in smooth muscle?

What is the significance of this?

Answer 15

Calmodulin.
Activates MLCK which phosphorylates myosin light chain to permit interaction with actin. Relaxation as Ca2+ levels start to decline (MLCP).

Question 16

How can contraction be inhibited?

Answer 16

Phosphorylation by PKA inhibits MLCK which therefore inhibits contraction.

Question 17

What type of receptors are activated to initiate depolarisation?

Answer 17

Alpha-adrenoreceptors

Question 18

Define re-polarisation

Answer 18

Change back to a negative potential

Question 19

What type of system is the autonomic nervous system?

Answer 19

An efferent system. One nerve cell in the pathway is located entirely outside of the CNS.

Question 20

Where do pre-ganglionic fibres of the parasympathetic system leave the CNS?

Answer 20

From cranio-sacral outflow (S2-S4).

Question 21

Which is longer in the ANS, pre-ganglionic fibres or post-ganglionic fibres?

Answer 21

Pre-ganglionic fibres tend to be longer

Question 22

Where do the pre-ganglionic fibres of the sympathetic nervous system leave the CNS?

Answer 22

Thoraco-lumbar outflow (T1-L2/3).

Symp, short, speedy

Question 23

Describe the neurotransmitters at each junction

Answer 23

Pre-g. of both release ACh which acts on a nicotinic receptor
Post-g. of parasympathetic also release ACh which act on muscarinic receptors
Post-g. of sympathetic release noradrenaline which acts on adrenoreceptors (alpha and beta)

Question 24

Where are alpha adrenoreceptors found?

Where are beta adrenoreceptors found?

Answer 24

Alpha - vascular smooth muscle.
Beta - heart, smooth muscle of airways of the lungs, adipose tissue and some blood vessels particularly in skeletal muscle

Question 25

Describe the relationship between the sympathetic nervous system and the adrenal glands

Answer 25

Connected to the adrenal medulla. Pre-g. fibrers run to AM which is made of modified post-g. cells (chromaffin cells)

Question 26

What is the resting heart rate?

Answer 26

60 bpm. Parasympathetic dominates at rest

Question 27

What is the function of baroreceptors?

Where are they located?

Answer 27

Baroreceptors sense changes in blood pressure (sensitive to stretching and an increase in arterial pressure causes them to stretch)
Located in the carotid sinus and aortic arch.

Question 28

How do the parasympathetic and sympathetic nervous systems act on the heart?

Answer 28

P - ACh released which acts on M2 muscarinic receptors
S - NA released which acts on B1 adrenoreceptors
(NB: adrenaline from adrenal medulla acts on the heart)

Question 29

What are the names of the nerves providing innervation to the heart?

Answer 29

Vagus nerve (parasymp) - decreases the slope of the pacemaker potential, decreases the cAMP and increases K+ conductance. Accelerator nerve (symp) - increases the slope of the pacemaker potential and increases cAMP.

Question 30

What effect does noradrenaline have?

Answer 30

Increases levels of cAMP, phosphorylation of Ca2+ channels, therefore increasing Ca2+ uptake and sensitivity of contractile machinery to Ca2+ therefore increasing the force of contraction.

Question 31

What are the effects of beta adrenoreceptors in smooth muscle?

Answer 31

Cause vasodilation. Increased levels of cAMP causes K+ channels to open thus causing relaxation of the smooth muscle

Question 32

What effects do alpha1-adrenoreceptors have in smooth muscle?

Answer 32

Cause vasoconstriction. Increase in [Ca2+]in from stores and via influx of extracellular Ca2+ thus causing contraction of smooth muscle.