Electrical Properties of the Heart Muscle Flashcards
Describe the general structure of cardiac muscle fibres
Similar to skeletal muscles, composed of long thin myofibrils and contract as sarcomere shortens
Cardiac muscle fibres are connected via gap junctions
What are intercalated discs
Sites of thickening of sarcolemma where cells join together
i think these are like gap junctions that allow transfer of things like ions between fibres, specific to myocardial cells
What does it mean when myocytes form electrical/functional syncytium
Where cells contract in a synchronous fashion - this is important for the pumping action of the heart.
Which ion has the greatest influence on magnitude of the resting potential and why
K+
There is a substantial K+ gradient and the membrane is relatively permeable to K+
Are the equilibrium potentials of K+ and Na+ relatively more or less negative than the resting potential
K+ is more negative
Na+ is a lot more positive
What is an equilibrium potential of an ion
the equilibrium potential is the membrane potential where the net flow through any open channels is 0.
Describe the phases of a cardiac muscle’s action potential
- Na+ floods in, down gradient, membrane potential rises to 20-30mV and Na+ channels are inactivated
- K+ permeability increases and K+ leaves at a higher rate down gradient
- Ca2+ channels simultaneously open and Ca2+ flows in causing a plateau of the membrane potential
- K+ efflux exceeds Ca2+. Ca2+ channels are inactivated.
- Membrane potential falls to K+ equilibrium
What is the absolute refractory period
When sodium channels close at the peak of an action potential and remain closed during the plateau phase. Muscle stimulation cannot happen in this period.
What is the Relative Refractory Period (RRP)
Between -50mV and complete repolarisation a further action potential can be generated but this requires a greater than normal stimulation
What is the importance of the long plateau phase in the cardiac action potential
The plateau outlasts the mechanical activity, so individual contractions cannot fuse into a maintained titanic contraction as in skeletal muscle.
This is important as the heart has to beat rhythmically.
What is pacemaker tissue in the heart
Denervated cardiac muscle continues to contract rhythmically
What does it mean that pacemaker tissues have automaticity
They have the ability to initiate their own beat. Cells of this tissue can spontaneously depolarise
Give examples of pacemaker tissues in the heart
sinoatrial (fastest)
atrioventricular nodes
Bundle of His
Purkinje fibres
Describe the shape of pacemaker tissue action potentials
Upstroke is more gradual, no depolarisation plateau
Describe the process of an action potential of in pacemaker tissue
- Depolarisation happens at threshold potential. Causing Na+ channels to open and followed by Ca2+ influx
- Depolarisation due to K+ efflux
- Cells have an unstable resting potential and gradually depolarise from -60 to -40mV due to slow continuous influx of Na+ and decreased efflux of K+
What is the role of the Sinoatrial Node (SAN)
Serves as the primary pacemaker of the heart and determines the rate at which the whole heart beats
What is the role of the Sinoatrial Node (SAN)
Serves as the primary pacemaker of the heart and determines the rate at which the whole heart beats
(has the fastest rate of drift of the resting potential??)
Where is the SAN
Right atrium
Where does the wave of electrical activity from the SAN spread
- Across the right and then the left atrium (via the bachmanns bundle)
- Enters the Atrioventricular (AV) node
What is the role of the AV node
Acts as a conductor of impulses from the atria to the ventricles
Impulse conducts very slowly through the AV node producing the delay that allows the ventricles to fill with blood after the atria have contracted.
After the AV node where does the electrical impulse go
From the AV node through the Bundle of His (right and left branches).
These then subdivide into a complex network of conducting fibres, Purkinje system
Allows almost simultaneous contraction of the right and left ventricles
What is an ECG
An electrocardiogram is a graphic made by an electrocardiograph that records the electrical voltage in the heart in the form of a continuous strip graph
What is the P wave formed by on an ECG
The conduction of the electrical impulse through the atria, this is the first bump on the ECG
What forms the PR interval on the ECG
The delay in the AV node forms much of the PR interval along with part of atrial repolarisation
What forms the QRS complex on the ECG
The spread of electrical activity through the ventricular myocardium via the bundle of His and purkinje fibres.
Seen as the main spike in the ECG
What forms the T wave on the ECG
The repolarisation of the ventricles
What is the normal range for the PR interval
120-200ms
What is the normal range for the QRS complex
80-120ms
What does a long QRS complex indicate
Conduction abnormalities
What does a shorter and longer PR interval indicate
Shorter - leaky annulus fibrosis (Wolf-parkinson-white syndrome)
Longer - AV conduction block
What is the consequence of an AV block
A pacemaker site distal to the block will become the new pacemaker for the heart
These secondary pacemaker sites generally have a slower intrinsic rate.
Therefore an AV conduction block = Ventricular bradycardia
