The sympathetic components increase heart rate by releasing the neural hormone catecholamines – epinephrine and norepinephrine. The combination of the neural and chemical components regulates heart rate and other heart functions
Myogenic heart when separated out from the body keep on beating on their own but when in the body the rate of heart beat is modified through various stimuli such as nervous and chemical stimuli.
The main structure that regulates the heart beat is medulla oblongata.
It has 3 types of reflexes
Activator area causing activation
Inhibition area causing inhibition.
Respiratory reflex centres
Vasoconstrictor reflex centres.
The activator or inhibitor area
In the activator and inhibitor area there are interneurons. The excitatory or inhibitory stimuli go to the corresponding areas from where motor stimuli causing excitation or inhibition are sent to the heart, through motor neurons or effector neurons.
The motor neurons either bring about the change directly or through pacemakers.
The cardiac reflex centres respond to various signals from cerebral hemispheres.
In medulla oblongata there are also chemo receptors sensitive to blood O2, blood CO2 and pH which directly affect the cardiac reflex centre.
In the invertebrates with open circulatory system the heart is under control of cardiac ganglion which is governed by excitatory and inhibitory neurons from CNS.
There are various chemical substances affecting heart rate. As mentioned earlier dissolved O2 in blood, CO2 in blood and pH of blood affect the chemo receptors in medulla oblongata affect heartbeat. They also have direct effect on heartbeat.
Low O2 stimulates Heart beat
Very low O2 inhibits Heart beat
And prolonged low O2 causes heart failure.
High CO2 causes inhibition of heartbeat.
Low blood CO2 causes excitation.
Optimum pH should be 7.4 and acidic pH causes relaxation of heart and reduces strength of contraction.
Transmitter substances and hormones
Mainly Acetylcholine is secreted by parasympathetic pathway and causes inhibition heart.
NE/E – Noradrenaline/Adrenaline are secreted by sympathetic pathways and cause excitation of heart rate. Adrenaline secreted by Adrenal makes the heart beat stronger and faster.
Thyroxine secreted by thyroid increases the rate of heart beat.
Inorganic ions — in which K, Na and Ca are very important.
If K is in excess it causes relaxation of heart and very high ‘K’ stops the heart in relaxed condition or flaccid condition. Effect is called as K-inhibition.
When vagus nerve is stimulated it secretes high ‘K’ on heart muscles, causing inhibition.
If Vagus nerve is stimulated for long time then the heart, which is inhibited, again starts functioning. This phenomenon is known as vagus escape.
Excess ‘Na’ causes relaxation of heart muscles but this is due to antagonistic effect or action with calcium. When ‘Na’ excess it competes with ‘Ca’ so ‘Ca’ availability reduces and ‘Ca’ essential for contraction.
‘Ca’ is important mainly for contraction of heart but its excess causes sudden contraction of heart but it stops in contracted state known as ‘Ca’ rigour.
Ringer’s solution is having balance of Na, K, Ca. The solution has composition similar to blood. It is used for heart perfusion
Digitalis – increases strength of heart muscles and accelerates heart rate. Used as drug for low B.P. and very high concentration can stop heart.
Atropin – is a stimulatory drug which accelerates the heart rate by suppressing Acetylcholine and binding with ‘K’ ions, hence used as antidote against toxic effect many poisonous substances.