HUBS 192 - LECTURE 9

The Excitable Heart

what type of cells are the majority of heart cells?

• contractile cells

how much of heart cells do electrical cells make

• 1%

what are the key differences between contractile cells and electrical cells?

• not a lot of actin and myosin - all pushed to the periphery

• no striations

• dont contract Don't contract.

what is the purpose of pushing the actin and myosin to the sides?

• keep the centre space open to allow electricity to efficiently move through

how do action potentials propagate through the heart?

• action potentials propagate along the surface of electrical and contractile cells.

• the signal begins in the conduction cells - moves through them rapidly because theyre empty

• passes the signals onto neighbouring contractile cells - which start contracting

where does the signal begin

• SA node

how does the signal pass onto neighbouring contractile cells from conduction cells?

intercalated disks

explain which type of intrercalated disks connect neighbouring cells and allow signals to pass through

• gap junctions

• pores with low resistance to current

• allow current to flow between adjacent cells

where will you find gap junctions

• along conduction pathway

• between conduction and contractile cells

• between contractile cells (pass on calcium as well to allow physical propagation)

what allows the heart to work as a functional syncytium?

• gap junctions and spreading impulse

what is a conduction pathway

series of structures through which signal travels around the heart to spark a contraction

where does electrical signal intitiate

• sinoatrial node

what allows the heart to beat completely alone

• sinoatrial node

does the sinoatrial node require initiation from the brain

no

what structure is also known as the pacemaker and why?

• sinoatrial node

• it sets the pace for the contraction

how many signals does the sinoatrial node fire everytime it initates a contraction

• send signals in 3 directions

wheres the first place the sinoatrial node sends a signal

• directly to the right atrium - trying to get it to depolarise and contract

wheres the next place the sinoatrial node sends a signal

• across the interatrial bundle to the left atrium

• arrives almost simultaneously to the next signal

what is the interatrial bundle

• collection of conduction cells that connects the SA node tot he left atrium

• winds itself through arteries and veins and valves

do the atria contract together

yes

• the signals arrive pretty much at the same time

where is the third place the sinoatrial node signals

• internodal bundle

what is the internodal bundle

• made of conduction cells which moves the signal between two nodes

describe the path of the 3rd signal of the sinoatrial node

• the first point is the internodal bundle, which connects to the atrioventral node

• the atrioventral node then holds the signal to allow the atria to contract and start relaxing before the signal travels any further

• the av node then sends the signal to the ventricle through the av bundle which runs down the septal wall

• the av bundle branches out into the left and right av branch which run to the bottom of the heart and then back up purkinje fibers

• purkinje fibers transport the signal to the walls of the ventricles to cause a contraction

what are purkinje fibers

• finger like projections that work into the walls of the ventricles

what does quiescent mean

• no electrical signal in that area

what does depolarisation mean

• electrical signal present

• contraction occuring

what does repolarisation mean

• electrical signal leaving

• area is relaxing

describe the polarisation of areas as signal travels through the heart

• electrical signal begins

• depolarisation at atria occurs

• complete depolarisation and contraction of atria - as well as signal as reached AV node

• atria begin repolarising and electrical signal travels down av bundle depolarising areas - purkinje fibers - as this is happening atria are in quienscence

• signal reaches ventricle walls and they depolarise

• from the base of the ventricle - repolarisation occurs

• then ventricles fully repolarise

what is an electrocardiogram

• measures chagnes in voltage of the heart over a period of time

what is the ECG unable to measure

• not able to measure how much the voltage has changed - increase/decrease, simply that it has changed

due to the inability of the ECG to quantify the voltage change - how is repolarisation and depolarisation shown

• upwards peaks as simply the change is measured

what are the main parts of the ECG

• P wave

• QRS complex

• T wave

describe what happens during the first peak of the ECG

• depolarisation of the atrium is read as the P wave

• it is small compared to other waves due to the atrium being small

describe what happens inbetween the first and second peak of the ECG

• eventually the atrium is fully depolarised but since the ECG is unable to detect this change - it goes back to neutral

describe what happens during the second peak of the ECG

• signal moves to the ventricles and they begin depolarising and simultaneously the atria are repolarising

• this is shown by the QRS complex

• the ventricles are very large and they depolarise fast so theres a very skinny sharp tall electrical peak assosciated with this

• atrial repolarisation is also a change and it exists within the QRS complex

describe what happens between the 2nd and 3rd peaks of the ECG

• the ventricles fully depolarise but this change isnt detected and so we return back to neutral

what happens during the third peak of the ECG

• ventricular repolarisation occurs and this is shown as a T wave

• it is shorter but broader than teh QRS complex bc it is slower (i think)

• then we get quescence state