5 Baroreceptor reflex
Learning objective 5: Describe the Baroreceptor reflex in response to high or low blood pressure.
An autonomic reflex regulating blood pressure
The peripheral somatic system has reflexes such as the familiar tendon-jerk reflex involving a short and involuntary arc through the spinal cord leading to motor output. Similarly the ANS has short and involuntary reflex arcs from peripheral receptors through the CNS and out again through ANS ganglia to ANS effectors (Fig. 4). We now describe the Baroreceptor Reflex, an autonomic reflex that regulates blood pressure. Like all reflexes it has three parts:
- (i) afferent input from sensory receptors,
- (ii) a processing center in the spinal cord or brain, and
- (iii) efferent output, which in the case of the baroreceptor reflex is mediated by efferent activity in sympathetic and parasympathetic nerve fibers and adjusts blood pressure ( 5). (ASIDE: Baro- and bary- mean pressure or weight–as in barometer and bariatric surgery.). Here’s how it works:
- Sensory receptor afferent input. Baroreceptors are stretch-sensitive mechanoreceptors in the walls of the aortic arch and carotid sinus that monitor blood pressure by responding to changes in the tension of the arterial wall. Stretch activates them.
- Central processing Center. Afferent input carried by spike activity (action potentials) in baro- receptor axons increases with elevated blood pressure and is sent to the medulla of the brainstem for integration.
- Efferent outflow. Increased input from receptor afferents excites the parasympathetic NS and inhibits the sympathetic NS, which lowers blood pressure.
The baroreceptor reflex stabilizes blood pressure by adjusting the activity of the sympathetic NS and the parasympathetic NS. For example, a drop in blood pressure reduces tension in the walls of the aortic arch and carotid sinus, decreasing excitation of the stretch sensitive baroreceptors that results in a corrective increase in segmental sympathetic outflow and a decrease in parasympathetic outflow (in the Vagus nerve) as illustrated in Fig. 5.
![](https://uw.pressbooks.pub/app/uploads/sites/545/2022/05/baroreceptor-826x1024.png)