Normal heart rate is about 60-90 beats per minute. On an average, the rate at which the heart beats is about 75 per minute. It depends on the balanced activity between the sympathetic and parasympathetic nerve influence that are acting on it.
Heart rate can be increased because of either an increased activity of sympathetic nerve fibers or a decreased activity of parasympathetic nerve fibers and vice versa for a decrease in heart rate.
In a newborn infant, the heart rate is about 120 beats per minute. The rate at which the heart beats is proportionate to the metabolic rate of the body. In canary birds, it can be as much as 1000 beats per minute.
An increase in heart rate is known as tachycardia and a decrease is known as bradycardia.
Innervations to the Heart (Fig. 3.27):
i. The efferent nerve supply to the heart is from both sympathetic and parasympathetic nerves.
ii. The parasympathetic nerve supplying the heart comes along the vagus whereas sympathetic is from the lateral horn cells of T1-T5 segments of spinal cord. The sympathetic fibers reach heart as superior, middle and inferior cardiac nerves.
iii. Vagus nerve takes origin from the cardioinhibitory center present in the reticular formation of the brainstem. The preganglionic fibers synapse in the ganglion cells present in the walls of the atria. From these the short postganglionic fibers supply almost all parts of heart except the apex.
The neurotransmitter liberated both at the pre- and postganglionic regions will be acetylcholine. The receptors through which acetylcholine acts at the preganglionic region are termed as nicotinic receptor and at the postganglionic region are muscarinic receptor. The right vagus predominantly supplies the SA node whereas the left vagus predominantly supplies the AV node.
iv. Even under normal resting conditions, there is some amount of constant activity of the vagus on heart. This is termed as vagal tone. Because of this, the normal heart rate is maintained around 75 beats per minute. If there is bilateral vagotomy (cutting of vagi on either side), even at rest the heart rate may increase to about 140-180 beats per minute.
v. The sympathetic fibers take origin from the lateral horn cells of the upper five thoracic segments. The preganglionic fibers that have emerged out of the spinal cord ascend up and synapse in the superior, middle and inferior cervical ganglia. From these ganglia, the postganglionic fibers take origin and supply the heart.
The neurotransmitter liberated by the preganglionic fibers is acetylcholine and the postganglionic fibers release noradrenaline. The influences of parasympathetic and sympathetic nerve stimulation on various activities of the heart have been indicated in Table 3.7.
Phases of Respiration and Heart Rate:
Sinus arrhythmia:
During inspiration, the heart rate is increased and during expiration, it is decreased.
Mechanism:
a. During inspiration, there will be irradiation of the impulses from the inspiratory center to the cardioinhibitory center which is present nearby in the reticular formation of the brainstem. These impulses from the respiratory center will inhibit the activity of the cardioinhibitory center and this in turn decreases the activity of vagus nerve and hence vagal tone. Consequently, heart rate gets increased.
b. During inspiration as air enters the alveoli, the stretch receptors present in the walls of the alveoli get stimulated. The impulses are carried to the brainstem through afferent vagal fibers. These afferent impulses not only inhibit the inspiratory center but also the cardioinhibitory center. Hence vagal tone is decreased and heart rate increases.
Regulation of Heart Rate:
Baroreceptor Mechanism:
i. There are specialized receptors namely the baroreceptors in the walls of carotid sinus and arch of aorta. The carotid sinus is located at the beginning of the internal carotid artery.
ii. The baroreceptors are stretch receptors present in the walls of the above blood vessels. Whenever there is an increase in blood pressure, the receptors get stimulated. They respond better when the blood flow in the above vessels is pulsatile.
iii. The afferent impulses from the carotid sinus are carried by sinus nerve a branch of glossopharyngeal nerve and from arch of aorta by aortic nerve a branch of vagus.
iv. The afferent impulses will stimulate the cardio inhibitory center present in the brainstem. This will increase the number of efferent impulses along the vagus to heart. The end result will be a decrease in heart rate (Fig. 3.28). The vagal tone depends on the impulses coming from the baroreceptors. When the baroreceptors are denervated vagal tone is lost completely.
v. There is an inverse relationship between blood pressure and heart rate. Heart rate is inversely proportional to blood pressure and this is termed as Marey’s law. Accordingly, when blood pressure increases the heart rate is decreased. In certain conditions, like muscular exercise, anxiety, etc., there is increase of both blood pressure and heart rate (exception to Marey’s law).
Nerves:
From baroreceptors (BR) to CIC—IX and X cranial nerves.
Chemoreceptor Mechanism:
i. These receptors are called as carotid and aortic bodies.
ii. The carotid body is present at the bifurcation of the common carotid artery (at the commencement of occipital artery) and aortic bodies are present at the arch of aorta.
iii. The afferent nerve that carries impulses from these receptors will be sinus nerve and aortic nerve respectively.
iv. They respond for chemical changes in blood namely, decrease in pO2, increase of pCO2 and increase in hydrogen ion concentration.
v. When chemoreceptor get stimulated by any of the above factors, the afferent impulses from these receptors are carried by sinus and aortic nerves.
vi. The end result will be an increase in heart rate.
Bainbridge Reflex (Fig. 3.29):
i. In the walls of great veins, there are stretch receptors present. They are termed as low pressure or volume receptors.
ii. Distension of the great veins leads to stimulation of these receptors.
iii. Afferent impulses from these receptors will be carried by vagus nerve.
iv. The afferent impulses inhibit the activity of cardioinhibitory center and thereby leading to increase in heart rate.
v. The afferent impulses along the vagus will also stimulate the neurons present in the brainstem which can increase the activity of sympathetic nerves. This leads to increased sympathetic activity on heart and heart rate increases.
The other factors which can influence the heart rate are (Table 3.8):
1. Pain receptor stimulation will have differential effect. When pain is from superficial parts of body (cutaneous pain), it brings about an increase in heart rate and if pain is visceral in origin, it leads to decrease in heart rate.
2. Joint receptors that are present in and around the joint will get stimulated during muscular exercise and increase the heart rate during exercise.
3. Increased intracranial tension: When there is an increased intracranial tension (e.g. in improper drainage of CSF), this will bring about a reflex bradycardia.
4. Oculocardiac reflex: When pressure is applied on the eyeball, it will bring about a decrease in heart rate.
5. Increase of body temperature will bring about an increase of heart rate.
6. Effect of adrenaline and noradrenaline: In an intact heart, adrenaline increases the heart rate while noradrenaline decreases the same. The decrease of heart rate by noradrenaline is brought about by reflex mechanism acting through baroreceptors since noradrenaline brings about an increase of mean arterial blood pressure.
7. Impulses coming from higher parts of the CNS, like limbic system, hypothalamus will also influence the heart rate, e.g. anger, fear, worry excitement, etc.