Heart Physiology

Front 1

What stimulates cardiac myocytes to contract?

Back 1

Membrane depolarisation

Front 2

Arrangement of cells of the heart

Back 2

Arranged as a branching functional syncitium in which electrical depolarisation passes from one cell to the next via gap junctions
Certain cardiac muscle cells contain few myofibrils and are specialised conduction fibres which allow a wave of depolarisation to spread throughout the heart in a rapid, co-ordinated manner

Front 3

How are cardiac electric impulses generated?

Back 3

Intrinsically - not dependent on external nervous input, though cardiac function can be modulated by the activity of the autonomic nervous system

Front 4

SA node

Back 4

Area of specialised cardiac tissue on the posterior wall of the right atrium - the normal pacemaker region of the heart (where electrical depolarisation is initiated)

Front 5

Why is the SAN the pacemaker?

Back 5

It exhibits the highest frequency of spontaneous activity and overrides other potential pacemaker regions (ectopic pacemakers)

Front 6

SAN node cells

Back 6

Contain few myofibrils and are not specialised for contraction - they spontaneously depolarise in a rhythmic manner, producing action potentials that are conducted to the surrounding atrial tissue

Front 7

Ion movement in SAN

Back 7

Following an action potential, the resting membrane potential of an SA node cell is -55 to -60mV
Slow inward leak of Na+ ions ('funny' current) depolarises SA node cells until an action potential is fired at about -40mV
Upstroke of the action potential in the SA node cell is a result of Ca2+ ion influx through voltage-gated Ca2+ channels

Front 8

How is heart rate determined?

Back 8

Frequency of SAN depolarisation's determines the frequency of cardiac contraction

Front 9

What prevents the excitation spreading prematurely to the ventricles?

Back 9

Atria are almost completely insulated from the ventricles by the annulus fibrosus and electrical impulses can only pass between them via the AV node

Front 10

AV node

Back 10

Specialised area of conducting tissue within the atrial septum which slows conduction of the electrical impulse, allowing the atria to contract before the ventricles

Front 11

Path of impulse after AVN

Back 11

Bundle of His transmits depolarisation across the annulus fibrosis and along the interventricular septum - bundle of His divides into left and right bundle branches which pass down the left and right sides of the interventricular septum and transmit impulses initially to the endocardial regions of the left and right ventricles

Front 12

Purkinje fibres

Back 12

Left and right bundle branch fibres transmit impulses to Purkinje fibres - large-diameter cells which conduct electrical impulses very rapidly.

Front 13

Why do large-diameter fibres propagate electrical impulses more quickly than small-diameter?

Back 13

Because their internal resistance is lower

Front 14

How can all parts of the ventricles contract simultaneously?

Back 14

From the endocardium, contractile cells transmit impulses to each other

Front 15

Sympathetic effect on conducting system of the heart

Back 15

Positive chronotropic effect - tachycardia:
Sympathetic nerve activity to the SAN increases the magnitude of funny current, decreasing time taken for depolarisation to occur and so increasing heart rate
Also, decrease time taken for conduction through the AVN

Front 16

Cause of sympathetic effect?

Back 16

Catecholamines bind to β1 adrenoreceptors, leading to an increase in intracellular cAMP - increases the open-state probability of channels which conduct the funny current

Front 17

Parasympathetic (vagal) effect on heart rate

Back 17

Slows heart rate - bradycardia
Slows rate of depolarisation of the pacemaker potential in the SA node cells
Pacemaker cells are hyper polarised, increasing time required to reach action potential threshold

Front 18

Cause of parasympathetic effect?

Back 18

Acetylcholine binding to muscarinic (M2) receptors: Reduced pacemaker potential is the result of reduced intracellular cAMP concentration
Hyperpolarisation is result of K+ channel activation

Front 19

What is an ECG?

Back 19

Record of the electrical events associated with depolarisation and repolarisation of the myocardium - measured as changes in the surface potential of the skin

Front 20

Purpose of ECG

Back 20

Allows abnormalities in the electrical activity of the myocardium to be detected and diagnosed

Front 21

Principle of ECG

Back 21

As atria and then ventricles undergo sequential depolarisation followed by repolarisation, the extracellular myocardial compartment can be treated as two moving dipoles of opposite charge
Each dipole is an aggregation of the depolarised (-ve) and hyperpolarised (+ve) regions of the heart - charge flows between these dipoles and the potential arising from these currents is what the ECG detects as tiny potential differences (1mV) at the skin

Front 22

Features of the ECG

Back 22

P wave - arises from depolarisation of the atria, lasts around 0.08sec
PR interval - period from the start of the P wave to start of the QRS complex (in reality, it is the PQ interval), large proportion is flat, representing the time taken for conduction through the AV node, lasts around 0.2 sec
QRS complex - record of ventricular depolarisation, lasts around 0.1 sec (short length demonstrates almost synchronous depolarisation of the ventricular myocardium)
ST segment - corresponds to the plateau phase of the ventricular AP. Just like most of the PR interval, represents heart in an isoelectric state
T wave - record of ventricular repolarisation

Front 23

ECG leads

Back 23

3 electrodes - one on each arm and one on the left leg
Form Einthoven's triangle
ECG trace generated by resolving the electrical vector arising from the electrical dipole onto one of the three leads connecting the above three electrodes