The atrioventricular bundle (A.V
bundle) originates from the A.V node. These cross the fibrous ring
that separates the atria and ventricles. They then divide into
branches left/right at the upper end of the ventricular septum.
These branches then break up in the ventricular myocardium into fine
fibres called purkinje fibres. All these combined convey electrical
impulses from the A.V node to the apex of the myocardium. This is
where the wave of contraction begins. It then sweeps up and outwards
pumping the blood into the pulmonary artery and the aorta.
PARASYMPATHETIC AND
SYMPATHETIC NERVES
Not only is the intrinsic impulses a
huge part in the heart beating, the heart is influenced by autonomic
nerves. These originate in the cardiovascular centre in the medulla
oblongate. This is reached through the autonomic nervous system.
There are two types of nerves present here, the parasympathetic (P.N)
and sympathetic (S.N) nerves. There actions are antagonistic to one
another. The P.N main supply is to the A.V and S.S nodes, and the
atria muscle. The stimulation from P.N reduces the rate of impulses,
and lessens the rate and force of a heart beat. The S.N works
opposite to this. These work with the S.A and A.V nodes and the
myocardium of atria and ventricles. It increases the rate and force
of a heart beat. Chemicals can give the same effect as a S.N by
increasing the heart beat. These chemicals include; hormone
adrenaline, and noradrenaline. Other hormones including thyroxine
increase the heart beat by their metabolic effect. Drugs, dissolved
gases and electrolytes in the blood can have either effect.
Another occasion when the heart beat
may increase is when the S.N is activated in times of stress, or
experiencing fear or anxiety.
