Reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
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 in
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
In cardiovascular physiology, stroke volume (SV) is the volume of blood pumped from the left ventricle per beat. Stroke volume is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood i ...
(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
baroreceptors in the
carotid sinus sense this increase in blood pressure and relay the information to the
cardiovascular centres in the
medulla oblongata
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...
. In order to maintain
homeostasis, the cardiovascular centres activate the
parasympathetic nervous system
The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...
. 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 M
2 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, M
2 receptors reduce the contractile forces of the atrial cardiac muscle and reduce the conduction velocity of the
atrioventricular node (AV node). However, M
2 receptors have no effect on the contractile forces of the ventricular muscle.
Stimuli causing reflex bradycardia include:
*
Oculocardiac reflex The 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 ...
[
* Sympathetic response to intracranial hypertension][ Page 254]
* Systemically administered norepinephrine (α-adrenergic effects on systemic vasculature exceed the effects of β1-adrenergic effects on the heart)
References
{{Cardiovascular physiology
Cardiovascular physiology