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:
  1. 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.
  2. 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.
  3. 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.

The baroreceptor reflex behaves as a negative feedback loop with gain. A. Arterial blood pressure is sensed by baroreceptors, a type of stretch receptor neuron, in the carotid sinus near the base of the brain. After integration in the medulla this information pro- vides negative feedback control of the cardiovascular system. The sympathetic component of the circuit includes outputs that stimulate the heart’s pumping capacity (cardiac output) by increasing heart rate and the strength of contractions. In addition, sympathetic stimulation causes arteries to contract, which raises the hydraulic resistance to blood flow. Together the effects of increased cardiac output and increased vascular resistance raise mean arterial blood pressure. Importantly, inhibitory projections from the caudal to the rostral ventral lateral medulla create negative feedback so that an increase. in blood pressure inhibits sympathetic activity, whereas a decrease raises sympathetic activity. Although omitted for simplicity, parasympathetic neurons in the cardiac ganglion also contribute to the reflex by creating an inhibitory cardiac input that is functionally antagonistic to the sympathetic pathway (see Figure 49–9B). During baroreceptor reflexes parasympathetic activity within the heart is therefore increased by hyper- tension and reduced by hypotension. B. The neurons mediating the baroreceptor reflex behave as a negative feedback loop with gain. By amplifying the activity that provides the signals for cardiovascular control, neurons in this circuit can accurately control blood pressure. In a healthy individual with reflex gain of 8, systemic blood pressure can be maintained within 10% of its set point.

 

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