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Lecture 6

PHYL2001 Lecture Notes - Lecture 6: Atrioventricular Node, Cardiac Muscle, Skeletal Muscle


Department
Physiology
Course Code
PHYL2001
Professor
Tony Bakker
Lecture
6

This preview shows pages 1-2. to view the full 6 pages of the document.
Contractions are signalled electrically
Cells are normally negatively
charged
Cells are electrically charged
This is how you generate activity in
muscles and in nerves
Electrical signals are used as they
over long distances
Nerves and muscles use changes in
charge to send electrical signals called
action potentials (go from negatively to
positively charged)
Cardiac muscle cells use actions
potentials to co
-
ordinate contraction
across the heart and activate contraction
Contractions:
Striations can be seen (cardiac and skeletal muscle)
Gap junctions are the gaps in the cell membrane between
one cell and the next, allowing small particles eg. Ions to flow
from one charge to its neighbour
Within the atria and ventricles myocardial cells are connected by
gap junctions
Gap junctions allow the cardiac action potential to propagate from
cell to cell through a low resistance pathway
Atrioventricular node: where
the atria and ventricles connect
Function is to rapidly
conduct electrical signals
from the AV node
Want to start contraction
down at the apex in order
to pump blood OUT of the
heart
APEX
Electrical conductivity in the heart:
BASE
13/3/2019
Sunday, 10 March 2019 10:38 AM
Lecture Page 1

Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

Electrical activity can pass from cell to cell in the atria and ventricles
AV node is the only place atria and ventricles connect
The atria and ventricles are electrically isolated by the hearts fibrous skeleton the
Annulus fibrosus
The heart has specialised electrically active cells in addition to contractile myocardium
Conduction of electrical signals
These cells for the Sinoatrial (SA) node, Atrioventricular (AV) node, Bundle of His and Purkinje
Fibres
Pacemaker is located in the SA node
Electrical activity normally originates in the SA node.
The AV node forms the only site of electrical connection between the atria and ventricles
Electrical conductivity in the heart:
Propagation of the Cardiac AP:
Timing of after an action potential starts in the SA node, how long it takes to
arrive in different locations in the heart (in secs)
Action potential (AP) starts at SA node
AP conducted through atrial muscle
The AP is delayed at the AV node before entering the Bundle of His
Conduction through the Bundle of His and Purkinje fibres is extremely
rapid
The ventricles depolarise (shows electrical change) from endo to
epicardium and form apex to base
Atria already finished contracting before the ventricles
start to contract
If both contracting at the same time, atria cannot push
blood into a contracting ventricle
This is called atrial
-
ventricular delay
Reaches the AV node at around 0.03 seconds, but does not come out
of the AV node until around 0.16 seconds
Ventricle contracts from the apex towards the base
Able to beat by themselves
Some heart cells (SA, AV node and Purkinje network) show automaticity, the ability to generate a heart beat
These cells have an intrinsic rhythmicity which generates a pacemaker potential
The heart does not require nerve or hormonal input to beat
In heart transplant patients the nerves are severed but the heart beats on
Autorhythmicity:
The sinoatrial node has the fastest pacemaker potential (~90
-
100 beats/min) and is the normal
pacemaker
The atrioventricular node is the next fastest (~40
-
60 beats/min) followed by cells in the bundle of
his (15
-
30)
The fastest pacemaker normally drives the heart and suppresses other pacemaker (overdrive
suppression)
A beat generated outside the normal pacemaker is an ectopic beat
The site that generates an ectopic beat is known as an ectopic focus (foci pl.) or ectopic
pacemaker
Cardiac Pacemakers:
***under normal conditions, the autonomic nerves are slowing the heart
-
suppression of natural heart
rate
Agents that alter heart rate are chronotropic
Adrenaline and noradrenaline act on beta
-
adrenergic receptors on the heart
Positive chronotropic agents increase heart rate
Negative chronotropic agents slow the heart
Neural control of Heart Rate:
Lecture Page 2
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