Conduction system anatomy Flashcards
2 types of tissue
o Nodal tissue: spontaneous automaticity → act as PM
o Conduction tissue: cells organized in bundles, separated from working myocardium by sheath of connective tissue
3 main components and circuit of impulse
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
What are gap jcts + fct
component of intercalated discs → electrical coupling of myocytes
o cell-to-cell propagation of impulses
o Small # in nodes → slow impulse velocity
Location of SA node
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
Blood supply SA node
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
how to identify SA node histo
staining connexin 43, which is absent in SA node (vs atrial cell)
SA node complex anatomy
Compact SA node
Exit pathways
Transitional cells
Compact SA node cells
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:
SA node exit pathways
mostly determined by autonomic nervous system
Superior: near CrVC → ↑∑
Inferior: lower RA floor → ↑p∑
Features of SA node
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
Innervation of SA node
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
Internodal tracts
- 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
Interatrial bundles
Bachmann’s bundle
Inferior inter atrial fascicle
Epicardial portion of CS
Bachmann’s bundle: role, location, cells
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
Inferior inter-atrial fascicle: location, role
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
Epicardial portion of CS: role
inter atrial connection btw lower R and L atrium
Parts of AV jct
Atrionodal bundles and proximal AV bundle
Compact AV node
Distal AV bundle
Atrionodal bundles and proximal AV bundle
- Controversial; superior, medial, lateral → converge into proximal AV bundle
o Distal continuation of internodal tracts
Compact node: shape, regions, cells
- 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
Site of anisotropic conduction
determined by myocyte orientation
AV node blood supply
o 2 arterial branches from Cx LCA + terminal septal artery → also supply His bundle + prox part of BB
o Venous drainage: Thebesian veins
AV node innervation
o p∑: predominantly influenced by L vagal nerve
o ∑: sympathetic nerve
Particularities of AV nodal conduction
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
Max reduction in conduction velocity in AV node
compact node
Normal slowing of conduction → allows mechanical AV synchrony + optimize ventricular filling
Delay approx. 90ms
Direction of AV nodal conduction
Anterograde → atrio-ventricular
Retrograde → ventriculo-atrial
* Concentric: along AV conduction axis
* Eccentric: accessory pathway
Pathways of AV node conduction
preferential in anterior atrial septum → longitudinal dissociation
Fast: antero superior RA region
Slow
Distal AV bundle
- 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)
Distal AV bundle segments
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
Distal AV bundle blood supply
artery from RCA or Cx LCA
Role of Intraventricular conduction system
Allows ventricular depol/contraction in synchronized fashion
RBBB anatomy and location
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
LBBB anatomy and location
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
Purkinje network anatomy and location
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
Feature of Purkinje cells
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
