Excitation of cardiac cells

Question 1

What is sinus rhythm?

Answer 1

Heart rate controlled by S.A. node, rest rate approx. 72 beats/min (wide variation)

Question 2

Describe normal excitation of the heart (steps)

Answer 2

• Action potential the activates atria
• Atrial A.P. activates A.V. node
• A.V. node (small cells, slow conduction velocity) introduces delay of 0.1 sec
• A.V. node activates Bundle of His/ Purkinje fibres
• Purkinje fibres activate ventricles

Question 3

What causes the delay in conduction?

Answer 3

Non-conducting connective tissue

Question 4

Why is cardiac muscle myogenic?

Answer 4

Generates its own potentials

Question 5

How are A.P. conducted from cell to cell?

Answer 5

Via intercalated discs which have gap (or nexus) junctions

-A.P. develop spontaneously at S.A. node

Question 6

Describe a desmosome

Answer 6

Two plasma membranes of adjacent cells with intercellular space in between
Plaque inside cell has intracellular filaments and intercellular filaments within cells (cadherins) between cells

Question 7

Describe gap junctions

Answer 7

Connexins between cells= channel for passage of small molecules and ions

Question 8

What are the changes in ion concentration during S.A. node action potential?

Answer 8

Pacemaker potential due to : increase in gCa, gNa and decrease in gK
Action potential upstroke due to increase in gCa
Repolarisation due to increase gK, decrease in gCa
Noradrenaline- increase gCa
Acetyl choline increases gK decrease gCa
(g= conductance)

Question 9

What are the differences in cardiac and skeletal muscle?

Answer 9

• Cardio= neurogenic (needs nervous impulse to initiate contraction), cardiac= myogenic
• Long cardiac action potential (skeletal= 50 msec, cardiac= 500 msec)
• A.P. controls duration of heart contraction, acts as only a trigger in skeletal
• Ion currents during A.P.= skeletal simple, cardiac complex
• Source of Ca for contraction
• Relaxation (Ca reduction)

Question 10

What currents are responsible for cardiac action potential?

Answer 10

Depolarisation- large gNa
Plateau- small gNa, increase gCa, decrease gK
Repolarisation- decrease gCa, increase gK

Question 11

Describe the cardiac sarcomere structure

Answer 11

Sarcolemma
Mitochondria
I band and A band
Sarcotubular network
Z line
Cisterna
Sarcoplasmic reticulum
T tubule

Question 12

What is the concentrations of calcium at rest and at contraction in skeletal and cardiac muscle?

Answer 12

Skeletal= 10^-7 M (rest), 10^-5 M (contraction)
Cardiac= 10^-7 M (rest), 10^-6/-5 M (contraction)

Question 13

Where is calcium sourced in cardiac contraction?

Answer 13

Ca released from sarcoplasmic reticulum but for heart cells Ca entry from outside is needed (Ca induced Ca release)

Question 14

Describe skeletal muscle excitation-contraction coupling

Answer 14

A.P.- sarcolemma- T tubule- Ca++ release channel
Troponin to 4 Ca++-Troponin
Ca++ pump requires energy from ATP to pump into sarcoplasmic reticulum

Question 15

How is the sarcoplasmic reticulum associated with the T tubule?

Answer 15

Covalent association between dihydropyridine receptor and ryanodine receptor subtype 1 (RyR1)

Question 16

What can point mutations cause?

Answer 16

• Malignant hyperthermia
• Halogenated or depolarising anaesthetics induce this
• Prior to discovery of dantrolene biggest killer under anaesthesia
• Central core disease= myopathy, hypotonia, loss of channel function

Question 17

Describe relaxation (Ca reduction)

Answer 17

Uptake of Ca by S.R. via an ATP driven Ca pump
Exit of Ca from cell:
-An ATP driven Ca pump (weak)
-Na-Ca exchange protein (energy from Na entry gradient)