Reflex bradycardia
Encyclopedia
Reflex bradycardia is a bradycardia
(decrease in heart rate
) in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases blood pressure
. In the presence of high mean arterial pressure
, the baroreceptor reflex produces a reflex bradycardia as a method of decreasing blood pressure by decreasing cardiac output
.
Blood Pressure
(BP) is determined by cardiac output (CO) and total peripheral resistance
(TPR), as represented by the formula BP = CO x TPR. Cardiac output (CO) is affected by two factors, the heart rate (HR) and the stroke volume
(SV), the volume of blood pumped from one ventricle
of the heart with each beat (CO = HR x SV, therefore BP = HR x SV x TPR). In reflex bradycardia, blood pressure is reduced by decreasing cardiac output (CO) via a decrease in heart rate (HR).
An increase in blood pressure can be caused by increased cardiac output, increased total peripheral resistance, or both.
The baroreceptor
s in the carotid sinus
sense this increase in blood pressure and relay the information to the cardiovascular centre
s in the brainstem. In order to maintain homeostasis
, the cardiovascular centres activate the parasympathetic nervous system
. Via the vagus nerve
, the parasympathetic nervous system stimulates neurons that release the neurotransmitter acetylcholine
(ACh) at synapses with cardiac muscle cells. Acetylcholine then binds to M2 muscarinic receptors
, causing the decrease in heart rate that is referred to as reflex bradycardia.
The M2 muscarinic receptors decrease the heart rate by inhibiting depolarization
of the sinoatrial node
via Gi protein
-coupled receptors
and through modulation of muscarinic potassium channels. Additionally, M2 receptors reduce the contractile forces of the atrial cardiac muscle and reduce the conduction velocity of the atrioventricular node
(AV node). However, M2 receptors have no effect on the contractile forces of the ventricular muscle.
Stimuli causing reflex bradycardia include:
Bradycardia
Bradycardia , in the context of adult medicine, is the resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. It may cause cardiac arrest in some patients, because those with bradycardia may not be pumping enough oxygen to their heart...
(decrease in heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
) in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
. In the presence of high mean arterial pressure
Mean arterial pressure
The mean arterial pressure is a term used in medicine to describe an average blood pressure in an individual. It is defined as the average arterial pressure during a single cardiac cycle.-Calculation:...
, the baroreceptor reflex produces a reflex bradycardia as a method of decreasing blood pressure by decreasing cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
.
Blood Pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
(BP) is determined by cardiac output (CO) and total peripheral resistance
Total peripheral resistance
Vasculature throughout the entire body can be thought of as two separate circuits - one is the systemic circulation, while the other is the pulmonary circulation. Total peripheral resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation...
(TPR), as represented by the formula BP = CO x TPR. Cardiac output (CO) is affected by two factors, the heart rate (HR) and the stroke volume
Stroke volume
In cardiovascular physiology, stroke volume is the volume of blood pumped from one ventricle of the heart with each beat. SV is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood in the ventricle at the end of a beat from the volume...
(SV), the volume of blood pumped from one ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
of the heart with each beat (CO = HR x SV, therefore BP = HR x SV x TPR). In reflex bradycardia, blood pressure is reduced by decreasing cardiac output (CO) via a decrease in heart rate (HR).
An increase in blood pressure can be caused by increased cardiac output, increased total peripheral resistance, or both.
The baroreceptor
Baroreceptor
Baroreceptors are sensors located in the blood vessels of several mammals. They are a type of mechanoreceptor that detects the pressure of blood flowing through them, and can send messages to the central nervous system to increase or decrease total peripheral resistance and cardiac output...
s in the carotid sinus
Carotid sinus
In human anatomy, the carotid sinus is a localized dilation of the internal carotid artery at its origin, the common carotid artery.-Functions:...
sense this increase in blood pressure and relay the information to the cardiovascular centre
Cardiovascular centre
The cardiovascular centre is a part of the human brain responsible for the regulation of the rate at which the heart beats. It is found in the medulla. Normally, the heart beats without nervous control, but in some situations , the cardiovascular centre is responsible for altering the rate at...
s in the brainstem. In order to maintain homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
, the cardiovascular centres activate the parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...
. Via the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
, the parasympathetic nervous system stimulates neurons that release the neurotransmitter acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
(ACh) at synapses with cardiac muscle cells. Acetylcholine then binds to M2 muscarinic receptors
Muscarinic acetylcholine receptor M2
The muscarinic acetylcholine receptor M2, also known as the cholinergic receptor, muscarinic 2, is a muscarinic acetylcholine receptor.-Heart:...
, causing the decrease in heart rate that is referred to as reflex bradycardia.
The M2 muscarinic receptors decrease the heart rate by inhibiting depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
of the sinoatrial node
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...
via Gi protein
Gi alpha subunit
Gi alpha subunit is a heterotrimeric G protein subunit that inhibits the production of cAMP from ATP.- Receptors :The following G protein-coupled receptors couple to the Gi subunit:* Acetylcholine M2 & M4 receptors...
-coupled receptors
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
and through modulation of muscarinic potassium channels. Additionally, M2 receptors reduce the contractile forces of the atrial cardiac muscle and reduce the conduction velocity of the atrioventricular node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...
(AV node). However, M2 receptors have no effect on the contractile forces of the ventricular muscle.
Stimuli causing reflex bradycardia include:
- Oculocardiac reflexOculocardiac reflexThe oculocardiac reflex, also known as Aschner phenomenon, Aschner reflex, or Aschner-Dagnini reflex, is a decrease in pulse rate associated with traction applied to extraocular muscles and/or compression of the eyeball. The reflex is mediated by nerve connections between the trigeminal cranial...
- sympatheticSympathetic nervous systemThe sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
response to intracranial hypertension