PY 105 Chapter Notes - Chapter 11: Voltage-Dependent Calcium Channel, Vagal Tone, Sympathetic Nervous System
Rhythmic Excitation of the Heart
• The initiation of each action potential that starts each cardiac cycle occurs automatically from
within the heart itself, in a special region of the right atrium called the sinoatrial (SA) node
• The cells of the SA node act as the pacemaker of the heart
• The SA node exhibits automaticity and its action potential is commonly divided into 3 separate
phases: phase 0, phase 3 and phase 4
o The SA node has an unstable resting potential - this is phase 4 and is caused by special
sodium leak channels that are responsible for its rhythmic, automatic excitation
o This inward sodium leak brings the cell potential to the threshold for voltage-gated calcium
channels; when they open they cause phase 0
o Cause by the influx of Ca+2
o This Ca2+ drives the membrane potential of the SA nodal cells toward the positive Ca2+
equilibrium potential
o Phase 3 is repolarization
• Caused by the closure of the Ca2+ channels and opening of the K+ channels, leading to
an outward flow of K+ from the cell
• The SA node cells transmit their action potential through intercalated discs to the rest of the
conduction cells in the heart, repolarize, then start the process over again, repeated once per
heartbeat for the life of the individual
• Phase 4: slow inward Na+ current through leak channels
• Phase 0: inward Ca current through voltage gated channels
• Phase 3: outward K+ current through voltage gated channels
• Phases of action potential in cardiac muscle cells:
o Phase 0 (depolarization): upstroke of the action potential and is caused by the transient
increase in Na+ conductance
o Phase 1 (initial repolarization): Na_ channels inactivate and K+ channels open
• Efflux of K+ and a slight drop in cell potential
• Opening of voltage gated Ca2+ channels
o Phase 2 (plateau phase): influx of Ca ions balance the K+ efflux from phase one leading to
equilibrium in cell potential
o Phase 3 (repolarization): Ca channels close and K channels continue to allow K to leave
o Phase 4 (resting membrane potential): inward and outward current are equal
• Each heartbeat begins as an action potential in the sinoatrial (SA) node then spreads throughout
the atria, causing them to contract and fill the ventricles with blood
• The pathway connects the SA node to the atrioventricular (AV) node
• Since this pathway connects the two nodes, it is referred to as the internodal tract
Regulation of the Heart by the Autonomic Nervous System
• Regulates the rate of contraction
• The vagus nerve (cranial nerve) contains preganglionic axons which synapse in ganglia near the SA
node
• The postganglionic neurons innervate the SA node, releasing acetylcholine (Ach)
• The Ach inhibits depolarization by binding to receptors on the cells of the SA node
• The constant level of inhibition provided by the vagus nerve is known as vagal tone
• The role of the parasympathetic system in controlling the heart is to modulate the rate by
inhibiting rapid automaticity
• The sympathetic system kicks in when increased cardiac output is needed during a "fight or flight"
• Sympathetic postganglionic neurons directly innervate the heart, releasing norepinephrine
• Epinephrine secreted by the adrenal medulla binds to receptors on cardiac muscle cells
find more resources at oneclass.com
find more resources at oneclass.com
