Question

You’re lying on your couch watching The Walking Dead when you hear loud pounding at your front door. As you leap to your feet, preparing to run like hell for the back door, gravity pulls the blood out of your brain sending it towards your feet, causing you to see stars and nearly black out. Baroreceptors in your carotid artery detect the drop in blood pressure and signal the medulla oblongata via the glossopharyngeal nerve. The medulla stimulates sympathetic nerves which increase your heart rate, and your blood pressure begins to rise, just in time for you to remember that you ordered a pizza.

1.) List the components of a feedback loop (stimulus, receptor, control center, effector and response) and then identify which items in this scenario are associated with each component.

2.) Is this a positive or a negative feedback loop? Explain your answer.

Answer 1

Baroreceptors are the receptors, which give response to change in blood pressure. Baroreceptors are also called pressoreceptors. .

Location - Depending upon the situation, baroreceptors are divided into two types:

1. Carotid baroreceptors, situated in carotid sinus, which is present in the wall of internal carotid artery near the bifurcation of common carotid artery.

2.Aortic baroreceptors, situated in the wall of arch of aorta.

Nerve Supply -

Carotid baroreceptors are supplied by Hering nerve, which is the branch of glossopharyngeal (IX cranial) nerve. Aortic baroreceptors are supplied by aortic nerve, which is a branch of vagus (X cranial) nerve

Nerve fibers from the baroreceptors reach the nucleus of tractus solitarius, which is situated adjacent to vasomotor center in medulla oblongata.

Role of baroreceptors when blood pressure increases

When arterial blood pressure rises rapidly, baro receptors are activated and send stimulatory impulses to nucleus of tractus solitarius through glassopharyngeal and vagus nerves. Now, the nucleus of tractus solitarius acts on both vasoconstrictor area and vasodilator areas of vasomotor center. It inhibits the vasoconstrictor area and excites the vasodilator area. Inhibition of vasoconstrictor area reduces vasomotor tone. Reduction in vasomotor tone causes vasodilatation, resulting in decreased peripheral resistance. Simultaneous excitation of vasodilator center increases vagal tone.

This decreases the rate and force of contraction of heart, leading to reduction in cardiac output. These two factors, i.e. decreased peripheral resistance and reduced cardiac output bring the arterial blood pressure back to normal level.

Role of baroreceptors when blood pressure decreases

The fall in arterial blood pressure or the occlusion of common carotid arteries decreases the pressure in carotid sinus. This causes inactivation of baroreceptors. Now, there is no inhibition of vasoconstrictor center or excitation of vasodilator center. Therefore, the blood pressure rises. Information regarding blood pressure within the range of 50 to 200 mm Hg (mean arterial pressure) reaches the vasomotor center through the carotid baro receptors. Information about the blood pressure range of 100 to 200 mm Hg goes through aortic baroreceptors. Both carotid and aortic baroreceptors are stimulated by the rising pressure than the steady pressure and their response depends upon the rate of increase in the blood pressure. Since the baroreceptor mechanism acts against the rise in arterial blood pressure, it is called pressure buffer mechanism or system and the nerves from baroreceptors are called the buffer nerves.

Ans 2 - Barroreceptor mechanism is a negative feedback loop mechansim.