Chapter 13

neurons that are embedded in the heart that responsible for the heart’s conduction system

the ability of the heart to generate its own rhythm

autorhythmicity

autorhythmicity cells provide a starting point for

spreading excitation through the heart

pacemakers are what type of cells?

autorhythmic cells

neurons in our hearts that spontaneously depolarize their membranes → generating an action potential

• coordinates and provides rhythm to heartbeat

pacemaker cells

rapidly conduct action potentials started by pacemaker cells to heart muscle/myocardium

conduction fibers

the conduction velocity of the conduction fibers is

4 meter per sec

autorhythmic neurons located on the upper right side of heart are

sinoatrial node

two sets of conducting fibers located in the medial region of the heart

atrioventricular node

have axons that go up and down through the interventricular septum

atrioventricular node

coming from the AV node, a bundle of axons is called

bundle of His aka atrioventricular bundle

conduction fibers of the myocardium

internodal pathways, bundle of His, purkinje fibers

the bundle of His separates into the left and right

av branches

Separates from the bundle of His into two branches

• this one goes around towards the left ventricle at the apex of the heart

left bundle branch

Separates from the bundle of His into two branches

• this one goes towards the right ventricular

right bundle branch

The smaller branches coming off of the left and right bundle branches are called the

purkinje fibers

the autorhythmic cells are found all around the different parts of the

• nodes that have autorhythmic cells

  • SA node

  • AV node

  • Bundle of His

  • Purkinje fibers

conduction system

the node that does action potentials at rest the fastest is the

SA node

the cells within the conduction system of the heart are linked by

• cardiac cells are linked by

gap junctions aka electrical synapses

no neurotransmitters needed between

cardiac muscle cells

fastest cells =

• set the rate for rest of heart/ control other cells

pacemakers

interatrial pathway

internodal pathway

both _____ contract at the same time

atria

conduction through the AV node is

slower

there is a delay with the AV node because it makes sure that both

atria contract before the ventricles

the conduction system of the heart

• AP initiated in SA node, signals spread through atrial muscle via interatrial pathways

• signal travels to AV node via internodal pathway—AV node delay

• atrioventricular bundle (bundle of His)

• splits into left and right bundle branches

• purkinje fibers

Heartbeat is generated

within itself

Nodal cells are connected to the

muscle cells through connexions and gap junctions

autorhythmic cells have

pacemaker potentials

Pacemaker potentials

bring the membrane potential past AP threshold -50mV

Pacemaker potentials are

graded depolarizations caused by closing K channels and opening two types of voltage gated channels

• channels that are permeable to Na and K

• as the membrane potential becomes hyperpolarized these channels open up

• and once the open up they depolarize the cell

• close at around -60 and -55mV

  • “initial subthreshold of cardiac neurons”

funny channels

• channel opens up at -60mV allowing Ca through

• depolarizing AKA late subthreshold depolarization

• after funny channels close

• depolarizes potential until -50 mV (AP threshold)

  • brings membrane to AP threshold

T type calcium channels

Ca concentration is more; drives calcium

outside the cell; into the cell

• once AP threshold is reached another Ca channel opens

• allow Ca in faster → causing faster depolarization

• generates calcium mediated AP

• open longer to depolarize membrane

• cells have delayed rectifier potassium channels (lazy)

L type calcium channels

after the delayed rectifier potassium channel opens up in L type calcium channels the membrane potential

repolarizes

After about -50 delayed rectifier potassium channels begin to; then _________ open

close; funny channels

action potentials of the cardio are from: ________ such as in the

autorhythmic neurons; SA node

• excitable cells

• driven by nodal neuronal AP

• transferred to and between cardiac muscle fibers through gap junctions

cardiac muscle fiber cells

what are the phases from 0-1

depolarization, brief repolarization, plateau, repolarization, rest

in the cardiac muscle cells there are

rest periods

In the cardiac muscle there are

rest phases for the muscle to relax

Five phases of electrical activity

• depolarization of mV

• very fast increased permeability to Na+

  • voltage gated Na+ channels open at a threshold

• inactivation gate opens after short time

Phase 0

cardiac muscle AP are

longer

A inactivation gate opens up after a short while during phase 0 and causes the depolarization to

stop and creates the peak

Five phases of electrical activity

• brief repolarization

• inactivation of Na+ channels

• leak channels that generate a resting mV start to repolarize

Phase 1

Five phases of electrical activity

• slight plateau

• increased permeability to Ca2+ (L type channels)

• increasing permeability to inward rectifier K+ channels

Phase 2

what do inward rectifier K + channels do

open when cells is very depolarized and allow K to leave cell

Ca++ entering the cell while K+ leaves the cell causes a

balance with little to no change in mV

what phase extends the duration of the cardiac AP to keep cardiac muscle to keep contracted

phase 2

Five phases of electrical activity

• L type Ca++ are starting to close

• increased permeability to K +

• delayed rectifier K channels open up

phase 3

Five phases of electrical activity

• resting membrane potential

• both K+ channels are open

phase 4

cardiac contractile cell AP have a

long duration of 250-300 msec

cardiac muscle fiber cells have a

straited appearance

Cardiac muscle fiber cells have properties that are similar to

skeletal muscles

cardiac muscle fiber cells have:

sarcomeres, t tubules, sarcoplasmic reticulum, troponin, tropomyosin cross bridge

properties similar to smooth muscles in the cardiac muscles are

gap junctions, extracellular Ca plays an important role in depolarization

1

current spreads through gap junctions to contractile cell

2

action potentials travel along plasma membrane and T tubules

As ap goes by on membrane or sarcoplasmic reticulum

there are voltage calcium channels that open up

calcium voltage gated channels in the cardiac muscle are

L type ca channels

calcium voltage gated channels in the cardiac muscle are

attached to sarcoplasmic reticulum

AP triggers the release of Ca++ from the voltage gated channels, binding to _________ and pulls the ___________, exposing the binding sites on the actin

troponin; tropomyosin

After the binding sites are exposed with the help of Ca ________ binds to the sites on actin and this causes the cross bridge cycling

myosin

The cross bridge used for the actin uses ____, which allows the myosin heads to bind to actin

ATP

After the myosin heads does its power stroke the _____ falls off, disconnecting the myosin and actin

ADP

After Ca++ randomly unbinds from troponin it:

diffuses around and is bumped back into sarcoplasmic reticulum by an active transport pump that uses ATP

The pumps that pump Ca++ back into the sarcoplasmic reticulum can also be found on the ________ so it can be pumped out of the cell through _________

plasma membrane; ATP

Secondary transport in the cardiac muscle

calcium sodium cotransporter

The calcium sodium cotransporter uses the power of

sodium rushing in to pump calcium out