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FM1020 > P: Contraction & electrical activity > Flashcards

P: Contraction & electrical activity Flashcards

1
Q

Contraction of cardiac muscle

A
  • Requires external stimulation by somatic motor nerves
  • Specialized noncontractile myocardial cells, autorhythmic or pacemaker cells are responsible for triggering contraction of cardiac muscle
  • Cell membranes spontaneously depolarize + degenerate Aps which spread into surrounding contractile myocardial cells.
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2
Q

Sinoatrial node

A
  • Autorhythmic cells are concentrated in sinoatrial node located in the right atrium near opening of superior vena cava
  • Spontaneous depolarizations generated here pass into surrounding myocardial cells and generate contraction:
    1. Atrial myocardial cells
    2. Pause (fibrous layer)
    3. Ventricular myocardial cells
    4. Atrial syncytium + ventricular synsytium
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3
Q

Atrial conduction

A
  • Impulses spread through atrial fibres at rate of 1m/sec
  • Bachmann’s bundle conduct impulse from right into left atrium
  • Impulses then spread to atrioventricular node
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4
Q

Atrioventricular conduction

A
  • AV node is located on base of right atrium near interatrial septum
  • Conduction velocity slows to 0.05m/s
  • Results in a delay between atrial and ventricular contraction
  • Allows optimal ventricular filling during atrial contraction
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5
Q

Ventricular conduction

A
  • AV node conducts to bundle of His, then to the bundle branches
  • These subdivide into Purkinje fibres which conduct impulses into both ventricles.
  • Conduction in purkinje fibres is 1-4m/s due to their large cell size.
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6
Q

Auto-rhythmic cells

A
  • SA node in right atrium: APs generated spread over entire cardiac tissue
  • 2 AV nodes
  • 3 pacemakers in Purkinje fibres. Aps generated by SA node will inhibit autorhythmic activity of these cells.
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7
Q

Types of action potential in cardiac tissue

A
  1. Fast response: atrial + ventricular myocytes
  2. Slow response: autorhythmic cells in SA and AV node.
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8
Q

Action potential in myocardial contractile cells

A
  1. Arrival of AP at contractile myocardial cell opens Na+ channels
  2. Voltage-gated Ca2+ channels open slowly
  3. At +20mV, Na+ channels close and K+ channels open. Repolarization begins.
  4. Slow inward diffusion of Ca2+ then balances outward diffusion of K+ plateau phase.
  5. Ca2+ channels close and K+ channels complete repolarization.
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9
Q

Role of calcium during cardiac AP

A

Inward diffusion of extracellular Ca2+ during depolarization also opens Ca2+ channels on SR
Extracellular Ca2+ is used to initiate contraction in myocardial cells instead of intracellular stores.
Increase in intracellular [Ca2+] triggers contraction in an identical mechanism to skeletal muscle. During repolarization, Ca2+ is transported out of the cell and relaxation occurs.

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10
Q

Myocardial cell contraction

A

Length of AP in myocardial cell (250ms) is much longer than an AP in a skeletal muscle cell (20msec) due to a plateau phase.
Myocardial cells are refractory during almost their entire contraction.
Summation cannot occur –> tetany is prevented.

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11
Q

Type of Ca2+ channels in cardiac muscle

A

Predominantly L-type Ca2+ channels

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12
Q

Ca2+ channel antagonists

A

Verapamil + diltiazem decrease duration of action potential + contractility of myocardial cells.

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13
Q

Slow response action potential

A
  • Generated spontaneously in pacemaker cells
  • Resting memory potential during phase 4 is less negative and is unstable
  • Depolarization ( phase 0) is not as large or rapid
  • Early repolarization (phase 1) is not apparent and plateau phase (phase 2) is less prolonged and not as flat.
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14
Q

Myocardial autorhythmic cells

A
  • Membrane potential is unstable due to slow Na+ and Ca2+ channels which open at -60mV
  • Slow drift In Vm from -60 to -50mV
  • At -50mV (threshold) fast Ca2+ and Na+ channels open, spontaneous membrane depolarization occurs
  • K+ channels open and membrane repolarization occurs.
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15
Q

Refractory period

A
  • Na+ channels (fast response) and Ca2+ channels (slow response) inactivate when cell is depolarized so no further AP can be generated and there is an effective refractory period
  • These channels revert to a closed state as cell is repolarizing (phase 3) and are fully closed in phase 4: cell is now fully excitable
  • Pacemaker cells have a prolonged refractory period and also have post-repolarization refractoriness.
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16
Q

P wave

A

atrial depolarization

17
Q

QRS wave

A

ventricular depolarization

18
Q

T wave

A

ventricular repolarization

19
Q

P-Q (P-R) interval

A

0.16 sec. Indicative of delay in conduction of impulse into ventricles

20
Q

Q-T interval

A

0.35 seconds. Duration of ventricular contraction

21
Q

R-R interval

A

Duration between 2 consecutive R waves (0.83 sec)

22
Q

Principles of ECGs

A
  • Depolarization wave enters ventricles via septum + spreads towards apex
  • For almost entire ventricular depolarization electrical current flows from depolarized area to polarized area in large circuitous routes
  • Heart electrical currents have vector properties
  • Normally orientation of cardiac vector during QRS is +60 degrees from horizontal plane.
23
Q

How to set up ECGs

A
  • Measured using bipolar limb leads
  • One positive and one negative recording electrode placed at each side of heart on limbs
  • Reference electrode placed on left leg
  • Connected via wires to ECG forming a complete circuit.
24
Q

Einthoven’s triangle

A
  • If direction of cardiac vector is towards +ve electrode: upward deflection on ECG
  • If direction of cardiac vector is parallel to direction of lead: maximum deflection
  • Magnitude of QRS is largest in lead II if cardiac vector is parallel to lead II
25
Q

Unipolar leads

A
  • Single recording electrode placed on body
  • 3 augmented unipolar limb leads in same locations as bipolar leads: left arm, right arm and left leg
  • Chest leads: 6 electrodes placed on surface of chest (V1-V6)
  • 12-lead ECG
  • Bipolar + unipolar leads are recorded simultaneously
26
Q

Normal variations in mean electrical axis

A

Short/stocky people < 60 degrees
Long/thin people > 60 degrees.

27
Q

Is an ECG wave generated when the ventricles are fully depolarised

A

No as all myocardial cells are depolarised and there’s no flow of current

28
Q

What conduct electrical activity to skin surface in ECG

A

Tissue fluids

FM1020 (21 decks)

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