Answer
Heartbeat is
myogenic,that is the beatings of the heart are started by cardiac muscles and
not by external stimulation.The excitation that starts the heart beat
originates from the sinoatrial
node(SAN).Sinoatrial node(SAN) is a patch of specialized cardiac muscle tissues
located in the wall of the right atrium
near the entry of the vena cava.SAN is responsible for starting the heartbeat
and the control of instrinsic rate of heart beat.The popential difference
across the plasma membrane of SAN is -90mV.SAN cells have a permanent high
permeability towards natrium ions.Natrium ions diffuse into SAN tissue cells
continuously and depolarize these cells.This depolarization will generate wavw
of excitation that are transmitted out of SAN to produce contraction of the
heart.The wave of excitation is conducted from SAN to the walls of both
atria.This causes both atria to contract simultaneously;atrial systole
occurs.Atrioventricular node (AVN) is a specialized myocardial tissue that is
similar to SAN.AVN is located in the wall between the atrium and the
ventricle.AVN is connected to the Fine branches from the bundle of His are
called Purkinje fibers. Purkinje fibres permeates the muscles of the entire
ventricular wall.As the atrium begins to contract,the wave of excitation from
SAN reaches AVN.AVN then transmits this excitation via the bundle of His to the
apex of the ventricle.From this apex,the wave of excitation are then
transmitted to the ventricular muscles via the Purkinje fibers.Such mode of
transmission causes both ventricles to contract from the apex,forcing the blood
out of the heart into the pulmonary artery and aorta.Contracting cardiac
muscles cannot be stimulated by any other stimulation until it relaxes
again.This period where the muscles are insensitive is called absolute
refractory period.Cardiac muscles have a longer absolute refractory period
compared to other muscles.Because of his nature,the heart can contract quickly
and strongly without experiencing gatigue or oxygen debts.Ventricular systole
is followedby ventricular diastole.
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