Neuronal Control of Blood Pressure

Question 1

What are the two systems that regulate blood pressure?

Answer 1

Fast-acting neuronal system

Slower-acting hormonal system

Question 2

What does the neuronal system do? What would happen if it did not function?

Answer 2

It provides moment to moment regulation, for example when you go from a lying down to a standing posture it automatically regulates blood flow to the legs
Without this regulation blood would pool in the legs, less would go to your brain and you would become dizzy or even faint. This condition is called orthostatic hypotension

Question 3

When is it vital that the neuronal system maintains blood pressure?

Answer 3

After haemorrhage

Question 4

What sort of time scale does the hormonal system work at? What is the name of this system?

Answer 4

The hormonal system works on a slower time scale of minutes or hours. This is the renin-angiotensin-aldosterone system

Question 5

What type of homeostatic process is the neuronal control of blood pressure? What is a key feature of this process?

Answer 5

Negative feedback

All negative feedback systems have sensors to monitor the controlled variable

Question 6

Where are the sensors to control blood pressure found?

Answer 6

In the carotid sinus in the internal carotid artery just above the bifurcation of the carotid arteries
As well as the carotid sinus there are also pressure sensors in the aortic sinus, at the base of the aortic valve

Question 7

Why do we have baroreceptors at two locations?

Answer 7

The aortic sensors detect the blood pressure at the start of the aorta and the carotid sensors detect the blood pressure in the internal carotid. It is possible that the brainstem vasomotor centre uses these two pressures to compute flow in the internal carotids and thus blood flow in the anterior circulation to the brain. Brain blood flow is autoregulated and does not change over a wide range of systemic blood pressures, but the mechanism of this autoregulation is still unclear

Question 8

How is the artery wall different in the sinus? How does this trigger action potentials in the sensory nerve fibres?

Answer 8

The artery wall is more compliant in the sinus. An increase in arterial pressure selectively stretches the sinus wall and thus also the sensory nerve fibres embedded in the wall. The stretch opens mechanically sensitive sodium and calcium channels in the membrane and triggers action potentials in the sensory nerve fibres.

Question 9

How does the negative feedback process work in the barreceptor system?

Answer 9

The baroreceptor (blood pressure) system is a negative feedback system. In all such systems a receptor (e.g. a thermostat) senses a variable (temperature). A control centre compares the actual variable value with a desired reference value, and if there is a difference, it activates some effector system (e.g. a heater or air conditioner) to drive the variable back to the reference level.

Question 10

What is located next to the carotid sinus? What is found here?

Answer 10

Next to the carotid sinus is the carotid body, which is where carotid chemoreceptors sensitive to the oxygen level in the blood are found. Sensory nerve fibres from the carotid body also travel in the IX or X cranial nerves

Question 11

What is the difference between what is sensed by the carotid sinus and carotid body?

Answer 11

Carotid sinus senses blood pressure
Carotid body senses hypoxia
DON’T CONFUSE THE TWO!

Question 12

What is the glossopharyngeal (IX cranial) nerve?

Answer 12

A mixed cranial nerve, primarily sensory (containing afferents from tongue, pharynx, larynx and the carotid sinus) but with a secretomotor (parasympathetic) output to parotid gland and motor output to a single muscle (stylopharyngeus)

Question 13

What is the vagus (X cranial) nerve?

Answer 13

A large mixed (motor and sensory) cranial nerve which contains motor output and (visceral) sensory afferent nerve fibres from lungs and gut, pharynx & larynx.

Question 14

Where might carotid sinus afferents travel in?

Answer 14

The glossophageal (IX cranial) or vagus (X cranial) nerves, or both.

Question 15

Where do the afferent fibres from the sinus nerve enter the brainstem and where do they terminate?

Answer 15

They enter the brainstem in the vagus or glossopharyngeal nerve.
They terminate in the nucleus of the solitary tract (NTS) in the medulla oblongata, (often referred to simply as ‘medulla’), the lowest part of the brainstem. The caudal end of the medulla merges with the rostral end of the spinal cord.

Question 16

What is the nucleus of the solitary tract (NTS)? What is its function?

Answer 16

A column of cells running rostro-caudally within the lower medulla. Other parts of NTS receive afferents from taste receptors in the tongue and throat.
The NTS can be regarded as an integrating centre for visceral afferents from mouth, throat and neck.

Question 17

What does the NTS connect to and where? What does it compute from these connections?

Answer 17

The nucleus of the solitary tract (NTS) connects to the vasomotor centre in the rostral medulla and the nucleus ambiguus in the nearby lateral medulla.
The NTS computes whether the information from the sinus nerve matches the blood pressure ‘set point’ and if not activates a corrective output either via the vasomotor centre or the nucleus ambiguus.

Question 18

What does the NTS do if blood pressure is too low or too high?

Answer 18

If blood pressure is too low the NTS activates the vasomotor centre which stimulates sympathetic outflow to the heart via the reticulospinal tract.
If blood pressure is too high the NTS activates the nucleus ambiguus which stimulates parasympathetic outflow to the heart via the vagus nerve

Question 19

What does the vasomotor centre activate? What does this cause the release of?

Answer 19

The vasomotor centre activates reticulospinal tract* axons which synapse on sympathetic pre-ganglionic neurons in the intermediate part (IML) of the lumbar and thoracic ventral horn. These activate post-ganglionic sympathetic fibres which release noradrenaline on arteriolar blood vessels
*The lateral reticulospinal tract is a group of axons in the dorsolateral spinal cord that arises from cells in the reticular formation of the medulla of the brainstem. The vasomotor axons are a (small) part of the lateral reticulospinal tract

Question 20

What happens if the input from baroreceptors (i.e. rate of action potential firing) is too low? What does this result in?

Answer 20

The vasomotor centre activates the sympathetic nervous system. The sympathetic outflow results in a constriction of peripheral arterioles that raises total peripheral resistance (TPR). A rise in total peripheral resistance increases blood pressure (assuming cardiac output, the outflow in litres/min from the heart, stays constant)

Question 21

What is a second result of increased sympathetic outflow?

Answer 21

The sympathetic outflow also results in an increase in heart rate that raises cardiac output. A rise in cardiac output also increases blood pressure and has synergistic effect to the rise in peripheral resistance

Question 22

What is a third result of increased sympathetic outflow?

Answer 22

Finally the sympathetic outflow also results in constriction of veins that raises venous return and preload and thus raised stroke volume and cardiac output.

Question 23

What happens if baroreceptor input (sinus action potentials) is too high?

Answer 23

The vasomotor centre is inhibited. This leads to reduced sympathetic outflow, relaxation of arterioles, lowered heart rate and reduction of TPR. This lowers blood pressure back to the desired level.

Question 24

What happens at the same time that the sympathetic nervous system is inhibited? What are the effects of this?

Answer 24

The parasympathetic nervous system is activated via the nucleus ambiguus. This stimulates the vagus nerve. The vagus acts at the sinoatrial node of the heart to slow down the heart and thus reduce cardiac output (CO). A reduced CO also reduces BP. (Note that the parasympathetic nervous system does not affect peripheral vascular resistance)

Question 25

What is the baroreceptor reflex?

Answer 25

This is simply the observation that stimulation of baroreceptors reduces blood pressure.
As we have seen in earlier slides this is mediated by a decrease in sympathetic outflow from the vasomotor centre and/or an increase in vagal outflow from the nucleus ambiguus (cardioinhibitory centre) to the heart

Question 26

What is a carotid massage and how is it carried out? What are the positive effects of doing so? Why can it be dangerous to carry out?

Answer 26

A carotid massage, often called a carotid sinus massage or CSM, is a medical manoeuvre that can be used to reduce blood pressure or slow down a dangerously rapid heartbeat or to diagnose certain heart rhythm disturbances.
Massaging the sinus in this way increases the rate of firing in the sinus nerve and increases vagal output.
To perform a carotid massage, you’ll need to massage the area at the base of the patient’s neck, where the carotid artery enters the head.
An incorrectly performed CSM can cause serious health repercussions, especially in elderly patients and should only be used by experienced health professionals (see URL below)
https://www.wikihow.com/Perform-a-Carotid-Massage

Question 27

What is hypertension? What is essential/type 1 hypertension?

Answer 27

Hypertension, where the blood pressure is consistently above 140/90, is one of the most common pathologies known. Hypertension is much more common that hypotension, where the pressure is below 90/60. Hypotension is only regarded as significant if symptoms (eg dizzines, fainting) are present. Essential or type 1 hypertension is where there is no clear pathology associated with the condition.

Question 28

How does hypertension affect the feedback control system? What causes this?

Answer 28

People with hypertension generally still have a controlled blood pressure; thus it must be the case that the feedback control system is still working, but either the reference (set) point generated in the vasomotor centre has increased or the feedback signal from the carotid sinus has decreased.
One possibility is that the feedback signal has decreased because the receptors in the carotid sinus have come less sensitive to stretch

Question 29

What happens if the carotid sinus has become less sensitive to stretch?

Answer 29

If this has happened then a normal blood pressure will generate a lower than normal rate of firing of sinus afferents, this will be computed by the vasomotor centre as due to a low blood pressure. The system will ‘correct’ this by increasing sympathetic output, raising the blood pressure, until the input from the baroreceptors is restored to the ‘correct’ level.
The result is a controlled but higher than normal blood pressure.

Question 30

How do the walls of the sinus compare to other arteries?

Answer 30

The walls of the sinus are normally more elastic than those in other arteries to enable them to detect small changed in arterial pressure

Question 31

What can damage to the internal elastic lamina on the sinus lead to?

Answer 31

Replacement of elastic fibres in the lamina by collagen fibres, thus increasing the stiffness of the lamina and making it less elastic. Thus it will require a greater pressure to stretch them the normal amount. Thus a normal blood pressure will give a reduced stimulus to the baroreceptor afferent nerves.
This will result in increased sympathetic drive and decreased parasympathetic drive, resulting in a higher blood pressure.
The blood pressure will be actively regulated but at a higher than normal higher pressure.

Question 32

What does hypertension frequently coexist with?

Answer 32

“Diabetes and hypertension frequently coexist, leading to additive increases in the risk of life‐threatening cardiovascular events. Hypertension is a common comorbid condition in patients with type 1 or type 2 diabetes when compared with the general population and occurs in 75% of patients with the more prevalent form of diabetes, type 2” *.
*Diabetes and Hypertension: Epidemiology of the Relationship and Pathophysiology of Factors Associated With These Comorbid Conditions

Question 33

What is the strange paradox between hypertension and smoking?

Answer 33

We would expect smokers to have hypertension as it damages endothelial cells. However smokers have a lower average blood pressure than non-smokers. (We have no explanation of this).
“The findings revealed that the adjusted blood pressure were lower in current smokers versus nonsmokers and former smokers. No significant dose-dependent effect of current smoking on blood pressure indices except PP was observed. Smoking cessation was significantly associated with an increased risk of hypertension. However, current smoking was not a risk factor of hypertension”**.
**The association between smoking and blood pressure in men: a cross-sectional study

Question 34

What is the relationship between stress and hypertension? What personality type is particularly at risk?

Answer 34

Stress appears to contribute to hypertension.
Stress increases the output of the sympathetic nervous system as it prepares the body for ‘fight or flight”.
This increases blood pressure, but normally only temporarily.
However chronic stress may permanently increase blood pressure, especially in the type A personality.

Question 35

When was type A behaviour first described as a potential risk factor for hypertension? What was the theory for its cause? How does this compare to type B?

Answer 35

Type A behaviour was first described as a potential risk factor for hypertension and heart disease in the 1950s.
It was at first thought that chronic activation of the sympathetic nervous system was the cause of stress-induced hypertension, but recent research has indicated that chronic release of cortisol may be the crucial factor
People with a Type B personality in contrast generally live at a lower stress level and enjoy achievement but do not become stressed when goals are not achieved.
When faced with competition, they do not mind losing and either enjoy the game or back down.
They may be creative and enjoy exploring ideas and concepts.
They are often reflective, thinking about the outer and inner worlds.

Question 36

What is under increasing emphasis as a treatment for hypertension?

Answer 36

Although there are many drugs which can be used to reduce blood pressure, there is nowadays increasing emphasis on ‘lifestyle’ changes to reduce blood pressure.
Perhaps the most important of these is increased levels of exercise.
Exercise has a very beneficial effect on the cardiovascular system, and on its own even without medication can often reduce blood pressure to normal levels.