Introduction: From the SAN to the AVN and finally Purkinje fibres, all these structures play a vital part in ensuring the heart beats regularly and continually.
During times of stress our heartbeat increases, during times of relaxation, our heartbeat decreases.
This is caused by other chemicals in our bodies called hormones, released to either slow or quicken the SAN firing process.
The sinoatrial node (SAN) is located between the right atrium and the superior vena cava.
The SAN is self-contained and does not require stimulation from the nervous system; although it is innervated by sympathetic and parasympathetic nerves, which regulate the heart beat.
The SAN is the heart's electrical pacemaker.
It is responsible for the intrinsic rhythmic activity of the heart.
The SAN is composed of closely and densely packed pacemaker cells.
The Na+ channels of the pacemaker cells are more open than other cardiac cells and the resting potential is less negative than the other cells.
The action potential of pacemaker cells is due to voltage gated Ca2+ channels not the usual Na+ and K+ ions.
Following a resting potential, Na+ ions permeate the cells more readily than K+ ions, making the inside more negative causing Ca2+ channels to open, the membrane potential rises causing an action potential to generate and start the chain reaction.
Cardiac muscle cells are in electrical contact with each other via gap junctions, enabling action potentials to spread rapidly, causing the cells to contract in unison.
As the action potential is generated in the SAN, the signals pass quickly through the electrically joined cardiac muscles of the atria, like a lit fuse, and they contract in unison, causing the right and left atria to contract.
The electrical wave continues to the junction of the atria and ventricles and stimulates the atrioventricular node (AVN).
Before the AVN fires, however, there is a delay of 0.
1 seconds to allow the atria to finish contracting.
When the AVN does fire, the electrical impulse travels through the atrioventricular bundle or bundle of His, which are specialist larger Purkinje fibres emanating from the AVN.
The His divide into finer Purkinje fibres, through which the current flows, finally across conducting fibres, ending at the apex of the myocardium of the ventricles at the base of the heart.
Here the ventricular contraction begins, sweeping upwards and outwards through the ventricles, pumping blood into the pulmonary artery and aorta.
The process then repeats itself, up to 76 beats/processes per minute for a normal healthy adult at rest.
Although the electrical activity within the SAN is not transmitted from the sympathetic or parasympathetic nerves, they do play a part in regulation of the heart's beat.
The sympathetic nerves originate in the vasomotor centre of the brain, situated in the medulla.
They travel via the spinal cord to the sympathetic ganglion and finally to the SAN, AVN and atria/ventricle myocardium.
The parasympathetic nerves travel from the cardio inhibitory centre of the brain, in the medulla, via the 10th cranial nerve (vagus nerve), to the parasympathetic ganglion and finally to the SAN, AVN and atrial muscle.
The sympathetic and parasympathetic nerves have opposite effects on the heart's beat.
The sympathetic nerves use the neurotransmitters adrenalin and noradrenalin to increase the force and rate of the heartbeat.
These are the "fight, flight and fright" hormones.
When we are frightened, threatened or facing other stressful situations, the release of these neurotransmitters causes the heart to beat faster.
If we engage in exercise, they are released to ensure the heart beats faster to provide blood more readily to muscles.
Parasympathetic nerves use the neurotransmitter acetylcholine to decrease the force and rate of the heartbeat, for example, when we sleep, relax, the heart does not have to beat so quickly.
Therefore the SAN is continually monitored and kept in check by these sets of nerves.
This process is called cholinergic vagal input or vagal restraint.
Conclusion So it starts with an electrical impulse originating in the SAN, which spreads through the heart across conductive cardiac cells, causing cells to contract in unison, starting at the atria, following, via the AVN, through to the ventricles (allowing for a delay of 0.
1 seconds for the atria to complete their contraction cycle).
The pulses travel through the Purkinje fibres to finish at the bottom of the heart, causing the ventricles to contract upwards and empty the heart of blood.
However, if left to its own devices the heart would beat quite quickly under all circumstances; therefore it is kept in check by the body releasing hormones.
Hormones are released to quicken the heart beat and conversely other hormones are released to slow the heart beat, depending on the needs of the body's metabolism.