Conduction system anatomy

Question 1

2 types of tissue

Answer 1

o Nodal tissue: spontaneous automaticity → act as PM
o Conduction tissue: cells organized in bundles, separated from working myocardium by sheath of connective tissue

Question 2

3 main components and circuit of impulse

Answer 2

SA node, AV jct, interventricular conduction system
o Electrical impulse originate in SA node → atrial myocardium/interatrial bundles (Bachmann’s, inferior fascicle) → internodal tracts (anterior, middle, posterior)
 Internodal tracts: controversial, accepted agreement that there is preferential pathways defined by myofibers orientation/ridges/ valvular annuli/venous ostia
o Atrionodal bundles → proximal AV bundle → compact AV node (floor of RA) → His bundle (cross fribrous skeleton) → interventricular specialized conduction system → Purkinje network → working myocardium
 L branch → antero-superior fascicle + postero inferior fascicle
 R branch

Question 3

What are gap jcts + fct

Answer 3

component of intercalated discs → electrical coupling of myocytes
o cell-to-cell propagation of impulses
o Small # in nodes → slow impulse velocity

Question 4

Location of SA node

Answer 4

below epicardial surface, at jct of CrVC & RA
o Near upper portion of crista/sulcus terminalis
 Extend btw cavae → jct of embryonic sinus venosus and pectinae muscle of RAAur
 Composed of fibrous tissue → important in SA node fct
o Can extend from CrVC to near CS
 Large size → wandering PM
* Reflection of atrial depol → depend on which exit pathway is used
 Normal: from middle/cranial regions

Question 5

Blood supply SA node

Answer 5

o Sinus node artery: // to sulcus/crista terminalis
 Terminal branch of RCA in 90%, LCA in 10%
 2 branches: surround central SA node
 Insulate SA node from atrium
 2/3 of blood supply to SA node
* 1/3 from collateral vessels
 Venous drainage: Thebesian veins → open into RA

Question 6

how to identify SA node histo

Answer 6

staining connexin 43, which is absent in SA node (vs atrial cell)

Question 7

SA node complex anatomy

Answer 7

Compact SA node
Exit pathways
Transitional cells

Question 8

Compact SA node cells

Answer 8

2 types of specialized atrial myocytes
 P cells → PM or typical nodal cells
* Center of SA node → 45-50% of cells
* Small, empty cells: few myofilaments, mitochondria, SR, low # gap jct
* 3 morphologies
o Type 1: ovoid cells w scattered myofibrils
o Type 2 (spindle cell): elongated shape w numerous myofibrils
o Type 3 (spinder shaped cell): central body w >3 extensions
 T cells → transitional
* Periphery of P cells → transition zone btw compact node and working atrial myocardium
* Intermediate morphology:

Question 9

SA node exit pathways

Answer 9

mostly determined by autonomic nervous system
 Superior: near CrVC → ↑∑
 Inferior: lower RA floor → ↑p∑

Question 10

Features of SA node

Answer 10

o Layer of atrial myocardium separates SA node from endocardium
o Size: medium dog → length 15-20mm, width 5-7mm, thick 200microm
 Cats → length 7mm, width 2mm, thick 300-500microm

Question 11

Innervation of SA node

Answer 11

modulated by autonomic tone
o Vagal tone: predominant at rest
 Dicrete vagal efferents + local/intrinsic network of autonomic nerves in epicardial fat pads
o ∑ innervation: L & R subclavian loops from stellate ganglia
 Mainly R symp fibers

Question 12

Internodal tracts

Answer 12

• Controversial; histopathologic evidence in dogs → Racker’s description
  o Anterior: from anterior aspect of SA node → anterior margin of CrVC → cross Bachmann’s bundle → anterior part of IAS → superior atrionodal bundle
  o Middle: // to posterior internodal tract → contour anteriorly fossa ovalis → medial atrionodal bundle
  o Posterior: // crista terminalis → posterior part of IAS → CS ostium → lateral atrionodal bundle

Question 13

Interatrial bundles

Answer 13

Bachmann’s bundle
Inferior inter atrial fascicle
Epicardial portion of CS

Question 14

Bachmann’s bundle: role, location, cells

Answer 14

responsible for LA activation
o From region of SA node on R → L auricle
o Discrete subepicardial bundle of myocytes in interatrial groove
 Characteristics of Purkinje fibers
* Conduct impulse at higher velocities
* More resistant to hyperkalemia

Question 15

Inferior inter-atrial fascicle: location, role

Answer 15

o Connect R and LA along path of CS → distal portion at level of ligament of Marshall
 Ligament of Marshall: remnant of L CrVC btwn upper and lower L PVs

Question 16

Epicardial portion of CS: role

Answer 16

inter atrial connection btw lower R and L atrium

Question 17

Parts of AV jct

Answer 17

Atrionodal bundles and proximal AV bundle
Compact AV node
Distal AV bundle

Question 18

Atrionodal bundles and proximal AV bundle

Answer 18

• Controversial; superior, medial, lateral → converge into proximal AV bundle
  o Distal continuation of internodal tracts

Question 19

Compact node: shape, regions, cells

Answer 19

• Elongated shape, concave surface is ventral (facing MV annulus)
• 3 regions
  o Atrionodal: large cells, similar morphology to Purkinje cells
   Separated by transitional  w elongated shape
   Mixed w P cells, adipocytes, atrial myocytes, collagen, nerve fibers
  o Nodal: transitional cells closely connected, no connective tissue = COMPACT NODE
  o Nodo-Hisian: P and transitional cells connected w Purkinje cells at AV distal bundle

Question 20

Site of anisotropic conduction

Answer 20

determined by myocyte orientation

Question 21

AV node blood supply

Answer 21

o 2 arterial branches from Cx LCA + terminal septal artery → also supply His bundle + prox part of BB
o Venous drainage: Thebesian veins

Question 22

AV node innervation

Answer 22

o p∑: predominantly influenced by L vagal nerve
o ∑: sympathetic nerve

Question 23

Particularities of AV nodal conduction

Answer 23

o Decremental conduction: progressive delay of impulse propagation w ↑HR
 Partial recovery from previous AP → affect slope of de pol of next AP
 Cumulative effect can lead to complete impulse block
 Important filter role
o Concealed conduction: alteration of ECG by previous hidden event (ex. PR prolongation from partial refractoriness of AV node)
 Antegrade conduction of APC, Atach, Afib, Aflutter
 Retrograde conduction of VPC

Question 24

Max reduction in conduction velocity in AV node

Answer 24

compact node
 Normal slowing of conduction → allows mechanical AV synchrony + optimize ventricular filling
 Delay approx. 90ms

Question 25

Direction of AV nodal conduction

Answer 25

 Anterograde → atrio-ventricular  Retrograde → ventriculo-atrial * Concentric: along AV conduction axis * Eccentric: accessory pathway

Question 26

Pathways of AV node conduction

Answer 26

preferential in anterior atrial septum → longitudinal dissociation  Fast: antero superior RA region  Slow

Question 27

Distal AV bundle

Answer 27

* Distal prolongation of compact node → only connection btw atrial/ventricular conduction system o Electrically insulated by fibrous skeleton  Central fibrous body → triangle of fibrous tissue btw MV, TV and AoV rings  Crossed posteriorly by penetrating portion of His Bundle (level of NC Ao cusp)

Question 28

Distal AV bundle segments

Answer 28

Start when cell loose reticulated distribution to form // fascicles → end at 1st branch o Non penetrating portion o Penetrating portion o Branching portion: from postero-inf fascicle branching → RBB

Question 29

Distal AV bundle blood supply

Answer 29

artery from RCA or Cx LCA

Question 30

Role of Intraventricular conduction system

Answer 30

Allows ventricular depol/contraction in synchronized fashion

Question 31

RBBB anatomy and location

Answer 31

o Direct continuation w His bundle o Travels down IVS (subendocardium) → anterior pap muscle → divide to intra cavitary false tendons  Anterior, medial, posterior → RVFW  Subendocardial Purkinje network

Question 32

LBBB anatomy and location

Answer 32

divides below AoV leaflets (subendocardium) o Truncular portion: flatt, ribbon shaped o Postero-inferior fascicle: → base of postero-medial pap muscle  Almost perpendicular to truncular portion o Antero-superior fascicle → base of antero-lat pap muscle

Question 33

Purkinje network anatomy and location

Answer 33

o Connect terminal portion of conduction system → endocardial surface of ventricles  Penetration 1/3 of muscle mass → continuous with cardiac muscle fibers  L side: subendocardial network denser around pap muscles, ↓ developed at base * Intracavitary fibers = false tendons  R side: directly connected to RBB

Question 34

Feature of Purkinje cells

Answer 34

o Rapid propagation of electrical impulse → ↑# of intercalated discs o Longer AP duration  Greater positive values, short plateau phase  Slower repol and longer refractory period o Absent in OT regions