Nerve Supply in the Heart
Parasympathetic supply is by the vagus nerve and Sympathetic supply is by Cardiac plexus.
Heart beat: The spontaneous, rhythmic contraction and relaxation of the heart to pump out and receive blood to and from the body is called Heartbeat. Depending upon the nature of control of the heart beat, hearts are of 2 types-
Neurogenic: Impulses originate from nervous system. E.g. Hearts of some annelids and most arthropods.
Myogenic (autorhythmic): Impulses originate from the heart itself. E.g. Hearts of molluscs and vertebrates.
The cardiac muscles have less actin and myosin. So functionally, they become similar to neurons.
Conduction system of the heart
Cardiac centre is present in the medulla oblongata along with vasomotor center and respiratory center.
Transmission of impulse in Heart [KU 2012]
1. Sinuatrial node
Sinuatrial node generates impulse (made up of muscle cells): SA node lies near the opening of the superior venacava in the right auricle. It is also called pacemaker of heart [BPKIHS 2012] (or node of keith and flack, heart of heart, brain of heart or pulsation centre)
The rhythmic impulses produced are called Sinus rhythmia or sinus rhythm.
In frog, S.A. node is present in sinus venosus.
2. Impulse is transmitted to:
a. The wall of auricle and causes it contraction
b. Atrioventricular(AV) node:
- Lies near the junction of interauricular and interventricular septum close to the opening of coronary sinus
- Also called reserve pacemaker, node of Twara and Aschoff
- Impulse generated by AV node is called nodal rhythm.
- In frog, A.V. node is absent
3. Bundle of His or AV Bundle:
It arises from AV node and descends in the interventricular septum, bifurcates into two branches innervating the wall of right and left ventricle respectively.
The impulses produced by bundle of His are non-rhythmic.
4. Purkinje fibers (in the wall of the ventricles)
Modified muscle fibers which act as sympathetic nerve fibers.
They arise from branches of bundle of His and provide impulse to myocardium of ventricles.
The impulses produced by these fibers are also non-rhythmic.
5. Wall of ventricles (myocardium) -
Finally, the impulse is transmitted to the wall of ventricles and they contract.
The rate of impulse generation is highest in SA node but the velocity of conduction is lowest while rate of impulse generation is lowest in Purkinje fibers but the velocity of conduction is highest.
Ectopic pacemaker: If any cardiac muscle other than the conducting tissue (nodes) generates impulse, then extra beats are heard and such muscles are called Ectopic pacemakers
Heart rate:
Heart beat per minute or number of cardiac cycles minute.
Frog: 64/min,
Rabbit: 200/min
Human: Adult: 60-100/min, New born baby: 120/min [IOM]
Females have higher heart rate than males.
Normal regular heart rate Rhythmia
Abnormal irregular heart rate -Arrhythmia
Decrease in heart rate Bradycardia
Increase in heart rate > Tachycardia
Regulation of heart beat:
The centre controlling the heart rate (cardiac centre) is present in medulla oblongata of brain. This centre is under the influence of hypothalamus which is the controller of autonomic activities.
Nervous control of heart
Brain has two sets of nerve fibres to control heart: Sympathetic and parasympathetic (vagus)
Increase in blood CO2 causes the sympathetic nerve fibres to stimulate SA node by producing sympathin (adrenaline + noradrenaline). This compound induces impulse generation by inducing entry of Ca2 into cardiac muscles. So, heart beat and force of contraction increase (Tachycardia).
When there is increase in blood O2, the parasympathetic or vagal (10th cranial) nerve inhibits the S.A. node by producing acetylcholine which increases contraction time and heart beat is decreased (Bradycardia). After action, acetyl choline is destroyed by enzyme acetylcholine esterase (AchE).
Hormonal control of heart
Hormones from adrenal medulla- adrenaline and nor adrenaline accelerate the heart beat
Nor adrenaline accelerates heart rate under normal conditions and the adrenaline at the time of emergency.
Pounding:
Very fast heart beat during some conditions like anger and love is called Pounding
Thyroxin hormone also increases the heart beat by increasing energy production.
Factors affecting heart rate:
Heart rate increases in following conditions:
Increased inspiration, muscular exercise, low oxygen tension, injection of adrenaline, thyroxin, sympathin
Increase in basal metabolic rate (BMR).
Decreased size of the body
Decrease in pH
Increase in temperature.
Increase in Na+ ions in blood or in cardiac muscles
Increase in Ca²+ ions in blood (but if it is injected in cardiac muscles, heart stops in contracted phase which called Systolic Arrest) while injection of K+ ions in heart muscles stop impulse generation and heart stops in diastolic or relaxation phase.
Force of contraction of heart is reduced by H+ ions
Heart rate decreases during sleep and increased expiration and injection of acetylcholine.
Athletes normally have low heart rate (Bradycardia). [IOM]
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