ACTION POTENTIAL CHARACTERISTICS OF SPECIALIZED CELLS d The action potential curve shown here is demonstrated by which of the following?
SA node Atrial muscle Purkinje fibers Ventricular muscle Learning Objectives Action potential in SA and AV nodes and the effects of sympathetic and parasympathetic nervous system on them Conduction pathways in heart and the velocity of conduction Automaticity of various heart cells the slow response, occurs in the sinoatrial (SA) node, which is the natural pacemaker region of the heart, and in the atrioventricular (AV) node, which is the specialized tissue that conducts the cardiac impulse from the atria to the ventricles Slow response- SA and AV nodes
The resting potential of slow fibers is less negative Lack the early repolarization phase (phase 1). The slope of the upstroke (phase 0), the amplitude of the action potential, and the overshoot are lesser. RRP in the slow fibers extends well into phase 4 after the fibers have fully repolarized. The action potential amplitude and the steepness of the upstroke are important determinants of propagation velocity along the myocardial fibers. In slow-response cardiac tissue, the action potential is propagated more slowly and conduction is more likely to be blocked than in fast-response cardiac tissue. Slow conduction and a tendency toward conduction block increase the likelihood of some rhythm disturbances Slow response- SA and AV nodes
Slow fibers lack functioning fast channels. depolarization is slower and the action potential travels more slowly across the surface of the cell. Unstable membrane potential during phase 4;, (there is a slow, gradual depolarization toward threshold.) Once the threshold is reached, an action potential is generated. Characteristics of SA Nodal Cells
depolarization is achieved mainly by influx of Ca++ through L-type Ca++ channels instead of influx of Na+ through fast Na+ channels. Repolarization is accomplished in these fibers by inactivation of the Ca++ channels and by the increased K+ conductance through the iK1 and iK channels SA Nodal (Pacemaker) Action Potential The if funny current The funny current is an inward, depolarizing current through a sodium channel that is unlike every other sodium channel. This channel is voltage-dependent, but it opens when the cell repolarizes and closes when the cell depolarizes. As phase 3 ends, the funny channel opens and causes sodium influx that reverses the membrane potential and causes the cell to depolarize toward threshold. Phase 0 Phase 0 is mainly a slow channel (L-type) or calcium spike rather than a fast channel or sodium spike. Phase 3 As is the case with other action potentials, phase 3 is due to a rapid potassium efflux (gK increases via iK channels). Pacemakers Pacemaker ability is conferred on cells by HCN(funny channel). When open, HCN channels cause Vm to slide gradually toward the threshold for AP for-mation. The cyclic adenosine monophosphate (cAMP) dependence of this channel also provides the ANS with a way of regulating the rate of phase 4 depolarization, which, in turn, regulates HR. HCN = hyperpolarization activated, cyclic nucleotidedependent Na channel. SA node is the hearts primary pace-maker
SA node is the hearts primary pace-maker. It has an intrinsic rate of 100 beats/min, but HR is usu-ally lower because the PSNS reduces HR when the prevailing need for cardiac output (CO) is low (see Figure 17.8B). Should the SA node be damaged and fall silent, then the AV node takes over as pacemaker. The AV node is normally subservient to the SA node because its intrinsic rate is 40 beats/min. It takes 1.5 s for AV nodal phase 4 to reach Vth (see Figure 17.8C), but the new wave of excitation originating in the SA node typically ar-rives well before this time (overdrive suppression). Purkinje cells are tertiary pacemakers. Their intrinsic rate is very low (20 beats/min), partly because Vm is about 25 mV more negative in Purkinje cells than nodal cells, and, thus, it takes much longer for Vm to reach and cross threshold from this more negative level (see Figure 17.8D) Characteristics of AV nodal cells
The AV nodal action potential is similar to the SA nodal action potential but differs in that the AV node has a slower rate of phase 4 depolarization. Because the SA node reaches threshold sooner, it is the pacemaker of the heart. AV nodal conduction velocity depends on the rate of rise and the height of phase 0 (inward Ca2+current). REGULATION Regulation: Because HR is a primary determinant of CO, the SA node is heavily regulated by the ANS. The SNS increases HR by releasing norepinephrine onto beta 1-adrenergic receptors on nodal cells. These are G proteincoupled receptors (GP-CRs) that increase adenylyl cyclase (AC) activity and intracel-lular cAMPconcentration. cAMP binds to and increases HCN open probability and accelerates the rate of phase 4 depolariza-tion ( Figure 17.9). HR thus increases (positive chronotropy). PSNS terminalsrelease acetylcholine (ACh) onto nodal cells. ACh binds to muscarinic type-2 receptors, which are also GP-CRs that depress AC activity and decrease cAMP formation. The rate of phase 4 depolarization slows, and HR decreases (nega-tive chronotropy). Effect of sympathetics
The slope of the pacemaker potential increases, threshold is reached sooner, and the intrinsic firing rate increases. Activation of -adrenergic receptors causes increased funny current (sodium) andincreased calcium current ( gCa). Effect of sympathetics
Sympathetics increase the conductance to Ca2+ and increase the rate of rise and height of phase 0. This increases the conduction velocity (decreases the PR interval). normal Sympathetic Effects on SA Nodal Cells Effect of Parasympathetic stimulation: RMP becomes more negative, decreased phase 4 slope(opening of Ach sensitive K channels) Effect of parasympathetics
Parasympathetics hyperpolarize the cell via increasing potassium conductance. Thus, it takes longer to reach threshold, and the intrinsic firing rate decreases. There is also a decrease in the slope of the pacemaker potential caused by reduction of the sodium funny current and decreased gCa. Effect of parasympathetics
Parasympathetics increase K+ conductance. The persistent increase in the outward K+ current counteracts the inward Ca2+ current and decreases the rate of rise and the height of phase 0. This slows the conduction velocity (increases the PR interval). Adenosine increases K+ conductance, slows conduction velocity through the AV node, and is used to stop supraventricular tachycardias (SVT). Parasympathetic Effects on SA Nodal Cells Regulation of Heart rate
Sympathetic & parasympathetic (vagus) nerves control the heart beat rate. A normal heart beat is maintained by slow continuous discharge from sympathetic nerves The vagal fibers are distributed mainly to atria than ventricles Without neuronal influences, SA node will drive heart at rate of its spontaneous activity Normally Symp & Parasymp activity influence HR (chronotropic effect) Mechanisms that affect HR: chronotropic effect Positive increases; negative decreases Autonomic innervation of SA node is main controller of HR Symp & Parasymp nerve fibers modify rate of spontaneous depolarization Regulation of Heart rate
Strong sympathetic stimulation can increase the heart rate from normal 70 beats / min. upto beats / min. Strong vagal stimulation bring down the rate to beats/min & also can decrease strength of heart muscle contraction by 20-30% . 14-8 of changes in pressure in isolated carotid sinuses on neural activity in cardiac vagal and sympathetic efferent nerve fibers OTHER MECHANISM Local strech,(resp) Temp K Ca Thyroxine Limbic sys
reflexes Conduction system of the heart: SA node is the pacemaker?? conduction of cardiac impulse Conduction Pathways and Velocity of Conduction
SA node atrial muscle AV node (delay) Purkinje fibers ventricular muscle Velocity Fastest conducting fiber = Purkinje cell Slowest conducting fiber = AV node Automaticity SA nodal cells: Highest intrinsic rate, primary pacemaker of the heart (100120/min) AV nodal cell: Second highest intrinsic rate, secondary pacemaker of the heart (4060/min) Purkinje cells: Slower intrinsic rate (3040/min) If Purkinje cells fail to establish a rhythm, then ventricular cells may spontaneously depolarize and produce a ventricular beat. However, the frequency is very low and unreliable. Which of the following has the fastest conducting fibers in the heart?
SA node Atrial muscle AV node Purkinje fibers Ventricular muscle Which of the following conditions at the A-V node will cause a decrease in heart rate?
A) Increased sodium permeability B) Decreased acetylcholine levels C) Increased norepinephrine levels D) Increased potassium permeability E) Increased calcium permeability Which of the following best explains how sympathetic stimulation affects the heart?
A) Permeability of the S-A node to sodium decreases B) Permeability of the A-V node to sodium decreases C) Permeability of the S-A node to potassium increases D) There is an increased rate of upward drift of the resting membrane potential of the S-A node E) Permeability of the cardiac muscle to calcium decreases ORIGIN & SPREAD OF CARDIAC IMPULSE