Regulation of Cardiac Activity: The cardiac cycle is a series of heart contractions that pressurise distinct chambers of the heart, forcing blood to flow in one direction. The autonomic (involuntary) nervous system has two branches influencing heart rate. The sympathetic nervous system (SNS) produces hormones (catecholamines – epinephrine and norepinephrine) to accelerate the heart rate. The parasympathetic nervous system (PNS) produces the hormone acetylcholine to reduce the heart rate.

The hormones adrenaline and noradrenaline, which are released by the renal medulla, are involved in the hormonal regulation of heart activity. Adrenaline (epinephrine) and noradrenaline (norepinephrine) are two hormones produced by the renal medulla that have a similar basic action on the body. By connecting to the body’s alpha and beta receptors, epinephrine and norepinephrine play a critical part in a fight-or-flight situation.

Regulation of Cardiac Activity: Overview

The normal activity of the human heart is automatically regulated by the nodal tissues- sinoatrial node (SA node) and atrioventricular node (AV node), hence the heart is called myogenic. 

Many physiological functions are controlled by the autonomic nervous system (ANS). It increases the heart’s contraction force and heart rate. It also regulates blood vessel resistance at the periphery. The sympathetic and parasympathetic divisions of the autonomic nervous system work together to keep the body in balance. The cardiac cycle promotes blood circulation throughout the body. Two basic systems control the cardiac cycle.

1. Intrinsic regulation of cardiac activity  
2. Extrinsic regulation of cardiac activity 

Intrinsic regulation of heart rate involves SA-node, AV-node, a bundle of His, Purkinje fibres. SA-node generates an impulse by itself and promotes the cardiac cycle.

Extrinsic regulation is separated into two categories.

Sympathetic and parasympathetic nerves control the heartbeat in a process known as neural regulation. Hormones like catecholamine and acetylcholine regulate the heart rate through hormonal or chemical regulation. The heartbeat is also regulated by other factors such as environmental stress and medicines.

Nervous Regulation

The sympathetic nervous system causes the heart to beat faster. It constricts arteries, causing blood pressure to rise. The heart rate is slowed by the parasympathetic nervous system. It works by widening arteries and lowering blood pressure. The vagus (X) cranial nerve carries these parasympathetic nerve fibres. Stress, caffeine, and excitement can temporarily speed up your heart rate, whilst meditating or taking calm; deep breaths can assist in bringing it down.

Baroreceptor Mechanism

1. Baroreceptors, also known as pressure receptors, are nerve endings found in the arteries’ walls. The number of baroreceptors in the body is quite high.
a. The carotid sinus is the area just above the carotid bifurcation where the internal carotid arteries meet.
b. The aortic arch’s inner wall.

2. The brain’s medullary centres are in charge of the autonomic nervous system’s total output, and they use information from baroreceptors to organise a response:
a. The parasympathetic route is initiated to reduce heart rate when an increase in arterial pressure is noticed. This, together with increased vascular vasodilation, helps to lower arterial pressure.
b. The sympathetic pathway is engaged when a reduction in arterial pressure is recognised, causing the heart rate and contractility to increase. The arterial pressure rises as a result of this, as well as increased vasoconstriction of arteries.

3. Baroreceptors are sensitive to changes in artery pressure and respond quickly. The baroreceptors are stretched when blood pressure rises, causing them to relay signals to the central nervous system. “Feedback” signals are then delivered back to the circulation via the autonomic nervous system, lowering arterial pressure to a more normal level.

4. The impulses travel from each carotid sinus to the glossopharyngeal nerve and then to the tractus solitarius in the medulla oblongata of the brain through Hering’s nerve, which is very tiny. Signals from the aortic arch are sent to the same location of the medulla oblongata via the vagus nerves.

Chemoreceptor Mechanism

1. Carotid and aortic bodies are the names for these receptors.
2. At the bifurcation of the common carotid artery (at the start of the occipital artery), the carotid body is present, and aortic bodies are present at the arch of the aorta.
3. The sinus nerve and aortic nerve, respectively, will carry impulses from these receptors’ afferent nerves.
4. They react to changes in biological fluids, such as a drop in pO₂, an increase in pCO₂, and a rise in hydrogen ion concentration.
5. Afferent impulses from chemoreceptors are transported by sinus and aortic nerves when they are triggered by any of the above conditions.
6. A rise in heart rate will be the eventual effect.

Bainbridge Reflex

1. Stretch receptors can be found in the walls of big veins. Low pressure or volume receptors are the terms used to describe them.
2. These receptors are stimulated when the great veins are dilated.
3. The vagus nerve will carry afferent signals from these receptors.
4. The activity of the cardioinhibitory centre is inhibited by afferent signals, resulting in an increase in heart rate.
5. Afferent impulses along the vagus will also excite neurons in the brainstem, causing sympathetic nerve activity to rise. This causes greater sympathetic activity in the heart and an increase in heart rate.

Chemical Regulation

1. Norepinephrine is the chemical secreted at the end of the vasoconstrictor nerves (noradrenaline).
2. The medulla of the adrenal endocrine glands also secretes norepinephrine (=nor- adrenaline).
3. The main functions of epinephrine and norepinephrine are to enhance metabolism and prepare organisms to deal with certain conditions brought on by physical stress, such as a drop in blood pressure or blood sugar, rage, anxiety, fear, and so on.
4. Under normal circumstances, norepinephrine regulates blood pressure. It causes a restriction of almost all of the body’s blood arteries.
5. The medulla of the adrenal endocrine glands also secretes another hormone called epinephrine (adrenaline). This hormone travels through the bloodstream to every area of the body, where it acts directly on cells. Epinephrine influences cardiac activity more than norepinephrine.
6. It also induces a modest constriction of muscle blood vessels, compared to the considerably stronger constriction caused by norepinephrine.
7. As a result, adrenal medullary hormones raise the heart rate.

Summary

Impulse generation in the autonomic nervous system (ANS) can potentially be used to control cardiac output and rhythm. This ANS activity is controlled by the brain’s medulla oblongata. The autonomic nervous system (ANS) is made up of two opposing nerves: sympathetic and parasympathetic nerves. Sympathetic nerve stimulation causes stronger ventricular and atrial contractions, which enhances cardiac output. It also raises your heart rate. In contrast to sympathetic stimulation, parasympathetic stimulation causes the atria and ventricles to contract less.

As a result, cardiac output and heart rate are reduced. Aside from autonomic modulation, certain chemicals can have an impact on heart activity regulation. Hormones such as epinephrine, norepinephrine, thyroxine, and others are among these substances. They have the ability to stimulate contractions and heart rate. Other compounds that have an effect on the heart are ions. These aren’t the only things that influence heart activity regulation. It can even be influenced by a person’s gender.

FAQs on Regulation of Cardiac Activity

Q.1. What are the regulations of cardiac activity?
Ans: The cardiac cycle is a natural behaviour of the human heart that is controlled by the nodal tissues—the sinoatrial node (SA node) and the atrioventricular node (AV node)—automatically (AV node). The cardiac output increases or decreases in response to changes in the cardiac cycle.

Q.2. Which hormones regulate cardiac activity?
Ans: To speed up the heart rate, the sympathetic nervous system (SNS) produces hormones (catecholamines – epinephrine and norepinephrine). The parasympathetic nervous system (PNS) produces the hormone acetylcholine to reduce the heart rate.

Q.3. Is the heartbeat regulated by the cardiac cycle?
Ans: The term “cardiac cycle” refers to the relaxation and contraction of the heart as it works to pump blood throughout the body. The frequency of the cardiac cycle is referred to as heart rate. It is a controlled variable that is one of the four vital signs.

Q.4. Why is the cardiac cycle important?
Ans: The heart’s primary function is to circulate blood across the body in a cycle known as the cardiac cycle. It is defined as the electrical signals that cause the heart muscles to contract and relax that coordinate the filling and emptying of blood.

Q.5. How can the heart be regulated extrinsically?
Ans: Neuronal, humoral, reflex, and chemical regulatory mechanisms are examples of extrinsic cardiovascular regulation. To maintain cardiac output, blood flow distribution, and arterial blood pressure, these extrinsic regulators regulate heart rate, myocardial contractility, and vascular smooth muscle.

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