M3: Heart - Electrical Properties

Question 1

what are auto-rhythmic cells and why do we have them in the heart?

Answer 1

• pacemaker cells
• can spontaneously depolarize/cause an AP in regular time intervals
• need them to set heart’s electrical rhythm

Question 2

special features of cardiac conducting system cells

Answer 2

AUTO-RYTHMIC
- spontaneous depolarization
DIFFERING # OF GAP JUNCTIONS
- control speed of electrical signal
DIFFERENT ION CHANNELS ON SACROLEMMA
- change what ions move in/out during AP
DIAMETER
- changes signal speed

Question 3

what is a node?

Answer 3

a lump/mass of specialized cells

Question 4

SA node

Answer 4

Sinoatrial node (pacemaker)
- right atrium
- fires most rapidly: dictates pace of AP firing for rest of heart

Question 5

AV node

Answer 5

Atrioventricular node
- slows signal down to allow atria to fully contract
- smaller diameter fibres and fewer gap junctions

Question 6

AV bundle

Answer 6

Atrioventricular bundle (bundle of His)
- passes through small hole into interventricular septum
- allows signal to move from atria to ventricles

Question 7

electrical signal pathway in heart

Answer 7

Sinoatrial (SA) node
Atrioventricular (AV) node
Atrioventricular (AV) bundle (bundle of His)
in interventricular septum
R and L bundle branches
- diamater and gap junctions increase
after apex
Purkinje fibres

Question 8

Purkinje fibres

Answer 8

• extensions of bundle branches that curve along outside of ventricels
• conducts signal to ventricular muscle cells
• large diameter
• many gap junctions
• few myofibrils
• signal travels VERY fast

Question 9

steps in contractile cardiac muscle cell action potential

Answer 9

1. -90mV, RAPID DEPOLARIZATION
   due to Na+ inflow when voltage gated FAST Na+ channels open
2. +20mV, PLATEAU,
   Na+ channels close, plateau maintained by Ca2+ inflow
3. REPOLARIZATION
   Ca2+ chanels close
   voltage K+ channels open

sodium-potassium pumps re-establish concentration gradient

Question 10

resting membrane potential in cardiac cell

Answer 10

-90mV

Question 11

how does calcium contribute to cardiac muscle contraction

Answer 11

• cross-bridge cycling
• calcium induced calcium released (CICR), influx of Ca+ binds to SR, causing more Ca+ to be released

Question 12

why is the refractory period longer than the contraction period in cardiac muscle APs?

Answer 12

• allows heart to fully contract before starting another contraction
• prevents sustained contraction (tetanus)

Question 13

differences between skeletal and cardiac muscle physiology

Answer 13

Action Potentials
C: conducted cell to cell via gap junctions
S: conduction along length of single fibre

Rate of AP propagation
C: slow because of gap junctions and small diameter of fibres
S: faster due to larger fibres

SR calcium release
C: calcium induced calcium release - Ca+ into sarcoplasm stimulated release of more Ca+ from SR
S: Ca2+ release channels on SR open when voltage changes in T tubules

Question 14

characteristics of autorhythmic AP

Answer 14

• no stable resting membrane potential (low end -60mV)
• PACEMAKER POTENTIAL: specialized sodium leak channels causes resting potential to move towards threshold

Question 15

autorhythmic action potential phases

Answer 15

1. PACEMAKER POTENTIAL
   - Na+ in via leak channels causes resting potential to move towards threshold
   - inside of cell more +. Triggers some Ca2+ v-gated channels to open
   - K+ channels closing
2. DEPOLARIZATION
   - more Ca2+ channels open
   - K+ channels closed
3. REPOLARIZATION
   - Ca2+ channels close
   - K+ channels open

Question 16

difference between nodes in length of time it takes to reach threshold

Answer 16

SA node is the fastest
Farther away from SA node, the longer it takes to reach threshold

Question 17

What is an electrocadiagram?

Answer 17

• measures movement of electrical signal in the heart
• ## electrodes placed on skin to detect signals in heart

Question 18

Explain waves and intervals in ECG

Answer 18

P: ATRIAL DEPOLARIZATION
- onset of atrial contraction
QRS complex: VENTRICULAR DEPOLARIZATION
- onset of ventricular contraction
- atria depolarizes, but signal masked on diagram
T: VENTRICULAR REPOLARIZATION
- ventricular relaxation

PQ interval
- start of atrial excitation to start of ventricular excitation
QT interval
- start of ventricular depolarization to end of ventricular repolarization

Question 19

two types of cardiac muscle cells that cause heart contraction

Answer 19

autorythmic cells
- create and transmit AP
contractile cells
- make up bulk of heart, responsible for movement of blood