SNAB Topic 7 spec point 10: control of heart rate

snab biology spec point 10 with notes

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  • Created on: 21-03-16 14:37
Preview of SNAB Topic 7 spec point 10: control of heart rate

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10. Understand that cardiac muscle is myogenic and describe the normal electrical activity of the heart,
including the roles of the sinoatrial node (SAN), the atrioventricular node (AVN) and the bundle of His, and how
the use of electrocardiograms (ECGs) can aid the diagnosis of cardiovascular disease (CVD) and other heart
conditions.
Cardiac muscle is myogenic it can contract without external nervous stimulation (from the nervous
system)
Small changes in the electrical charge of cardiac muscle cells causes contraction of the cardiac muscle
When these cells have a slight positive charge on the outside= they are polarised
When this is reversed= they are depolarised
A change in polarity spreads from cell to cell which causes the cells to contract
Depolarisation starts at the sinoatrial node (SAN) which is a small area of specialised muscle fibres
located in the wall of the right atrium, beneath the opening to the superior vena cava
The sinoatrial node is also known as the pacemaker
The SAN generates an electrical impulse> this spreads to the right and left atria= causing them to
contract at the same time
The impulse also travels to specialised cells called the atrioventricular node (AVN)
From here the impulse is conducted to the ventricles after a short delay (0.13s)
The delay ensures that the atria have finished contracting and that the ventricles have filled with blood
before they contract
After the delay the signal reaches the Purkyne fibres these are large specialised muscle fibres that
conduct impulses rapidly to the apex (tip) of the ventricles
There are right and left bundles of fibres which are collectively called the bundle of His
The Purkyne fibres continue around each ventricle and divide into smaller branches that penetrate the
ventricular muscle
These branches carry the impulse to the inner cells of the ventricles > and from here it spreads through
the entire ventricles
The first ventricles to be depolarised are at the apex of the heart so that contraction begins at this point
and travels upwards towards the atria
This produces a wave of contraction moving up the ventricles= pushing the blood into the aorta and the
pulmonary artery

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Measuring electrical activity
Electrocardiogram (ECG) can be used to measure the electrical activity of the heart by displaying a
graphic record of the electrical activity during the cardiac cycle
It is carried out by attaching electrodes to a person's chest and limbs to record electrical currents
produces during the cardiac cycle
When there is a change in polarisation of the cardiac muscle> a small electrical current can be detected
at the skin's surface
An ECG is usually carried out whilst a patient is at rest…read more

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