Heart Electrical Conduction

Action potentials In pacemaker cells

1. Na+ enters the cell - these channels are slightly leaky This raises the membrane potential slowly over time K+ channel is closed 2. T-type Ca2+ opens - calcium enters the cells Causes an abrupt depolarization when the threshold value is reached 3. Once threshold attained, L-type Ca2+ channels carry the depolarization 4. Repolarization occurs K+ channel opens - K+ can flow out of the cell F-type Na+ channel closes T-type Ca2+ channel closes This causes the cell's membrane potential to decrease

Steps for Contracting heart cells

1. Rapid depolarization Na+ channel open - Na+ enters the cell K+ channel closes Causes the inside of the cell to become more positive 2. Plateau/Maintenance phase Ca2+ (L-type-long) channel opens Ca2+ can enter the cell to help maintain the depolarization in the cell 3. Repolarization K+ channels open - K+ leaves the cell Na+ and Ca2+ channels close Achieve a positive membrane potential

Steps of Electrical Conduction

1. The sinoatrial node sends an electrical impulse down to the atrioventricular node 2. Once the signal is spread, it goes to all of the cardiac muscle cells in the wall of the atria and depolarizes them - atrial excitation About one millisecond after they are depolarized, the cells contract Both atria contract in unison 3. The impulse goes through the AV node to the bundle of His The atria relax and the cells repolarize

Steps of electrical conduction

4. Then, it travels down the interventricular septum to both sides of the heart This excites the ventricles - ventricular excitation When this is complete, all of the muscle cells in the ventricles will be depolarized and they will contract in unison Both ventricles contract at the same time and send the blood out of the heart 5. After the contraction of the ventricles is complete, the ventricles relax and the cell repolarize

Pacemaker Potential

Action Potentials in Pacemaker Cells Membrane potential spontaneously drifts toward threshold Four ion channels: F-type Na+ (funny) T-type Ca2+ (transient) L-type Ca2+ K+ (repolarize)

Cardiac muscle

Action potential and contraction have almost the same (prolonged) duration No tetanus, ensures completion of cycle; maintenance of heart beat Provides time for ventricular filling

Diastole

Do not get the ventricles completely filled without the atrium contracting and pulling the blood into the ventricles This pushes the blood into the ventricles As the atria contract, you get in as much blood into the ventricles as possible

Electrocardiogram Basics

Electrical currents in the myocardium can be detected by electrodes on the limbs By convention, depolarization moving toward a (+) electrode results in an upward spike One approach: three limb leads: Einthoven's triangle Depolarizations moving toward left leg will cause positive deflections in leads II and III

The Cardiac Cycle

Heart is a double pump with isolated left and right sides, but working together in the cardiac cycle During systole The atria contract together followed by the ventricles contracting together Ejection of blood from the heart Diastole follows, a rest phase, when the chambers relax and fill Refills the heart with blood

What if the SA node fails?

Other cells have pacemaker potentials and serve as a backup Nervous system input NOT required SA node: 100 bpm AV node: 40-60 bpm Bundle: 20-40 bpm In healthy heart, these slower pacemakers are "overdrive suppressed"

Ventricular filling

Pulls the blood back into the right atrium

Ejection fraction (EF)

SV/EDV This measures how well the heart is pumping blood to the body The normal value is about 50 percent This value will be higher in individuals who are athletic training If this is less than 40 percent, there will be pathological consequences Not getting enough blood into the body Congestive Heart Failure starts to occur

Systole

Tension develops in the ventricles as depolarization starts The tension causes the pressure to increase in the ventricles Valves between the atrium and ventricle closes In the second stage, the ventricles contract and the aortic and pulmonary valves open This allows blood to empty out of the heart

Sinoatrial Node

The SA node is the heart's pacemaker - it initiates the electrical impulse Electrical impulse is delivered to the apex of the heart and then travels upward, "squeezing" blood from the ventricle

Cardiac output

The amount of blood flow in the body The amount of blood the heart pumps through the circulatory system in a minute SV (mL/beat) x Heart rate (HR; beats/min) = CO (mL/min; L/min)

Systole

The atria contract together followed by the ventricles contracting together Ejection of blood from the heart

Automaticity

The capacity of pacemaker cells for spontaneous, rhythmic, self-excitation Inherent rate: 100 depolarizations per minute (or bpm) for SA node cells

Heart Electrical Conduction System

The conducting system starts at the sinoatrial node Located in the right atrium Responsible for keeping the heart beating at the correct pace Contraction of the heart is coordinated by electrical impulses traveling through conducting cells Purkinje fibers lie just under endocardial surface Depolarize adjacent myocardial cells

Diastole

a rest phase, when the chambers relax and fill Refills the heart with blood All the valves of closed first Then, it moves when the pressure in the ventricle drops enough that blood can start rushing into the atrium and ventricles This occurs because the muscle cells start to repolarize and relax Pulls the blood back into the right atrium Called ventricular filling

P wave

atrial depolarization

Heart rate

defined as number of beats per minutes (bpm) The heart contracts (or beats) ~ 72 times/min (at rest varies from 60-80 beats/min) Each heartbeat lasts ~ 0.85 sec SA node cell: 100 bpm so why isn't resting HR faster? Other stimuli impact it as well This is set by pacemaker cells

Tachycardia

fast heartbeat (>100 bpm)

Arrhythmia

irregular heartbeat

Tetanus

maintained contraction in response to a repetitive stimuli

Q-T interval

measure of the length of contraction Used as an important measure of pathology!

Heart valves

prevent backflow When the ventricle is relaxed, there is higher pressure in the aorta, so the valve closes When the ventricle contracts, it increases the pressure and opens the valve Without the valves, you would have a very leaky heart When they work properly, they open and close when they should

Skeletal Muscle

short action potential (green), can fire another while muscle is still contracting (red) Potential for tetanus (maintained contraction in response to a repetitive stimuli)

Bradycardia

slow heartbeat (<60 bpm)

End diastolic volume (EDV)

the max volume of blood the heart can hold This will increase during exercise

End Systolic Volume (ESV)

the residual volume in the heart that cannot be pushed out

Stroke volume (SV)

the volume of blood that is pushed out of the heart after a contraction SV = EDV - ESV

QRS complex

ventricular depolarization corresponds to initial depolarization of ventricular myocardium (pacemaker cells)

T wave

ventricular repolarization The action potential and contraction last until the end of this wave