initiating heart contraction Flashcards
what is fibrillation
uncoordinated contraction of the atria and ventricles
what is myogenic muscle
muscle that can initiate its own contraction
what is Purkyne tissue
consists of specially adapted muscle fibres that conduct the wave of excitation from the AVN down the septum to the ventricles
what is the sinoatrial node
the hearts pacemaker. it is a small patch of tissue that sends out waves of electrical excitation at regular intervals in order to initiate contractions
what can cardiac muscle do
initiate muscle contraction
why can heart muscle initiate its own muscle contractions
it is myogenic and has its own intrinsic rhythm at around 60 beats per minute
can heart muscle still relax and contract even its not connected to the body
yes
which muscle contracts at a higher frequency
the atrial muscle
what is the benefit of myogenic contraction
it prevents the body wating resources maintaining the basic heart rate
what is the average resting heart rate of an adult
70bpm
which factors can affect heart rate
exercise, excitement, stress
what can cause inefficient pumping
if the contractions of the chambers aren’t synchronised
what is the term for unsynchronised contractions
fibrillation
what are myocytes
muscle cells
what do myocytes have
a slight electrical charge across their membrane - they are polarised
what happens when the electrical charge across the membrane is reversed
they are depolarised and this causes them to contract
where is depolarisation initiated
in the sinoatrial node
what is another term for the sinoatrial node
the pacemaker
where is the SAN located
near the point where the vena cava empties blood into the right atrium
what does the SAN do
initiate a wave of excitation at regular intervals (55-80x a minute in humans)
what is the SAN a patch of
cardiac muscle that controls the cardiac cycle
what are the names of the two nodes
the sinoatrial node and the atrioventricular node
how are heart muscle contractions coordinated
electrical impulses are sent from the SAN to the AVN
what do the cell membranes of the two nodes allow
a charge to be maintained across the membrane.
what happens at rest
the nodes in the heart become polarised
what happens at polarisation
a positive charge builds up on the inside of the node and a negative on the outside. this is caused by ions building up
when do the nodes become depolarised
when a contraction occurs
what happens at depolarisation
positively charged ions move out of the nodes stimulating a move in the electrical activity of the heart
what happens at atrial systole
- the sinoatrial node sends a wave of depolarisation throughout the atria
- this causes the walls of the atria to contract almost simultaneously
- blood is forced through the bicuspid and tricuspid valves into the ventricles
what happens at ventricular systole
- the AVN picks up the wave from the SAN and initiates its own wave of depolarisation down the Bundle of His then into the Purkyne tissues
- this causes the ventricular walls to contract
- this forces blood out of the semilunar valves through the aorta and the pulmonary artery
describe the electrical coordination of the heartbeat
- the sinoatrial node initiates the electrical impulses that set the heartbeat. a wave of depolarisation spreads from the SAN over the atria. the atria depolarise causing them to contract and push blood into the ventricles. an electrical signal reaches the AVN which waits for the atria to fully contract and for the ventricles to fill with blood. the electrical impulses travel down the Bundle of His and along the Purkyne fibres into the ventricles. the Purkyne fibres cause the ventricles to depolarise and contract therefore pumping blood through the arteries to the lungs and the rest of the body. the ventricles repolarise and the heart muscle cells return to their resting state waiting for the next cycle. the heart undergoes diastole and the atria fills with blood from the vena cava and pulmonary waiting for the next systole
why is there a brief delay when the impulse reaches the AVN
to ensure the atria have emptied completely and stopped contracting
what is the bundle of his
bundle of conducting tissue made up of Purkyne fibres
