Pacemaker potential

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Pacemaker rates

Slow, positive increase in voltage across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential.

- Pacemaker potential

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Sinoatrial node

Group of cells known as pacemaker cells, located in the wall of the right atrium of the heart.

Sinoatrial node shown at 1. The rest of the conduction system of the heart is shown in blue.
Figure 2: Low magnification stained image of the SA node (center-right on image) and its surrounding tissue. The SA node surrounds the sinoatrial nodal artery, seen as the open lumen. Cardiac muscle cells of the right atrium can be seen to the left of the node, and fat tissue to the right.
Figure 3: Sinoatrial node action potential waveform, outlining major ion currents involved (downward deflection indicates ions moving into the cell, upwards deflection indicates ions flowing out of the cell).
Schematic representation of the atrioventricular bundle

Instead, immediately after repolarization, the membrane potential of these cells begins to depolarise again automatically, a phenomenon known as the pacemaker potential.

Cardiac action potential

Brief change in voltage across the cell membrane of heart cells.

Drugs affecting the cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing efflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.
Different shapes of the cardiac action potential in various parts of the heart
Action potentials recorded from sheep atrial and ventricular cardiomyocytes with phases shown. Ion currents approximate to ventricular action potential.
Figure 2a: Ventricular action potential (left) and sinoatrial node action potential (right) waveforms. The main ionic currents responsible for the phases are below (upwards deflections represent ions flowing out of cell, downwards deflection represents inward current).
Figure 4: The electrical conduction system of the heart

In these cells, phase 4 is also known as the pacemaker potential.

Action potential

Action potential occurs when the membrane potential of a specific cell location rapidly rises and falls.

As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na+) and potassium- (K+) gated ion channels open and close as the membrane reaches its threshold potential. Na+ channels open at the beginning of the action potential, and Na+ moves into the axon, causing depolarization. Repolarization occurs when the K+ channels open and K+ moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.
Shape of a typical action potential. The membrane potential remains near a baseline level until at some point in time, it abruptly spikes upward and then rapidly falls.
Approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane. The membrane potential starts out at approximately −70 mV at time zero. A stimulus is applied at time = 1 ms, which raises the membrane potential above −55 mV (the threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and overshoots to −90 mV at time = 3 ms, and finally the resting potential of −70 mV is reestablished at time = 5 ms.
Action potential propagation along an axon
Ion movement during an action potential.
Key: a) Sodium (Na+) ion. b) Potassium (K+) ion. c) Sodium channel. d) Potassium channel. e) Sodium-potassium pump. In the stages of an action potential, the permeability of the membrane of the neuron changes. At the resting state (1), sodium and potassium ions have limited ability to pass through the membrane, and the neuron has a net negative charge inside. Once the action potential is triggered, the depolarization (2) of the neuron activates sodium channels, allowing sodium ions to pass through the cell membrane into the cell, resulting in a net positive charge in the neuron relative to the extracellular fluid. After the action potential peak is reached, the neuron begins repolarization (3), where the sodium channels close and potassium channels open, allowing potassium ions to cross the membrane into the extracellular fluid, returning the membrane potential to a negative value. Finally, there is a refractory period (4), during which the voltage-dependent ion channels are inactivated while the Na+ and K+ ions return to their resting state distributions across the membrane (1), and the neuron is ready to repeat the process for the next action potential.
When an action potential arrives at the end of the pre-synaptic axon (top), it causes the release of neurotransmitter molecules that open ion channels in the post-synaptic neuron (bottom). The combined excitatory and inhibitory postsynaptic potentials of such inputs can begin a new action potential in the post-synaptic neuron.
In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.
In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.
Comparison of the conduction velocities of myelinated and unmyelinated axons in the cat. The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter d (that is, v ∝ d), whereas the speed of unmyelinated neurons varies roughly as the square root (v ∝√d). The red and blue curves are fits of experimental data, whereas the dotted lines are their theoretical extrapolations.
Cable theory's simplified view of a neuronal fiber. The connected RC circuits correspond to adjacent segments of a passive neurite. The extracellular resistances re (the counterparts of the intracellular resistances ri) are not shown, since they are usually negligibly small; the extracellular medium may be assumed to have the same voltage everywhere.
Electrical synapses between excitable cells allow ions to pass directly from one cell to another, and are much faster than chemical synapses.
Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.
Giant axons of the longfin inshore squid (Doryteuthis pealeii) were crucial for scientists to understand the action potential.
As revealed by a patch clamp electrode, an ion channel has two states: open (high conductance) and closed (low conductance).
Tetrodotoxin is a lethal toxin found in pufferfish that inhibits the voltage-sensitive sodium channel, halting action potentials.
Image of two Purkinje cells (labeled as A) drawn by Santiago Ramón y Cajal in 1899. Large trees of dendrites feed into the soma, from which a single axon emerges and moves generally downwards with a few branch points. The smaller cells labeled B are granule cells.
Ribbon diagram of the sodium–potassium pump in its E2-Pi state. The estimated boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes.
Equivalent electrical circuit for the Hodgkin–Huxley model of the action potential. Im and Vm represent the current through, and the voltage across, a small patch of membrane, respectively. The Cm represents the capacitance of the membrane patch, whereas the four g's represent the conductances of four types of ions. The two conductances on the left, for potassium (K) and sodium (Na), are shown with arrows to indicate that they can vary with the applied voltage, corresponding to the voltage-sensitive ion channels. The two conductances on the right help determine the resting membrane potential.

The voltage traces of such cells are known as pacemaker potentials.

Sinoatrial block

Disorder in the normal rhythm of the heart, known as a heart block, that is initiated in the sinoatrial node.

Sinus rhythm (rate = 65/min) with Type II S-A (exit) block; (see laddergram). Note that the long cycles (191) are nearly identical to twice the short cycles (190).

It can be detected only during an electrophysiology study when a small wire is placed against the SA node from within the heart and the electrical impulses can be recorded as they leave the p-cells in the centre of the node [ see pacemaker potential ], followed by observing a delay in the onset of the p wave on the ECG.

Purkinje fibers

The Purkinje fibers (often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium.

Isolated heart conduction system showing Purkinje fibers
Purkinje fiber just beneath the endocardium.

The Purkinje fibers do not have any known role in setting heart rate unless the SA node is compromised (when they can act as pacemaker cells).

Otto Hutter

Austrian-born British physiologist who was Regius Professor of Physiology at the University of Glasgow.

Hutter speaking at his 90th birthday

They made the first recordings using microelectrodes of the pacemaker potential in heart muscle to study the cardiac pacemaker.

Graded potential

Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none.

Examples of graded potentials

They include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave potential, pacemaker potentials, and synaptic potentials, which scale with the magnitude of the stimulus.

Muscle contraction

Activation of tension-generating sites within muscle cells.

Types of muscle contractions
In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac
Organization of skeletal muscle
Structure of neuromuscular junction.
Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions
Cross-bridge cycle
Muscle length versus isometric force
Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.
Cardiac muscle
Key proteins involved in cardiac calcium cycling and excitation-contraction coupling
A simplified image showing earthworm movement via peristalsis
Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page.
Electrodes touch a frog, and the legs twitch into the upward position

Some types of smooth muscle cells are able to generate their own action potentials spontaneously, which usually occur following a pacemaker potential or a slow wave potential.