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What are the elements of neural reflexes regulating MAP?
- Afferent neural pathway
- Coordinating center
- Efferent neural pathway
What type of system is made up of the neural reflexes regulating MAP?
Series of negative feedback loops
What are the primary sensors of the neural reflex system?
Baroreceptors - detect mechanical stretch
What are the secondary sensors of the neural reflex system?
Chemoreceptors - detect changes in blood PCO2, O2, and pH
What are the control centers for the neural reflex system?
Mostly the medulla, also sites in cerebral cortex and hypothalamus
What are the effectors of the neural reflex system?
- Pacemaker and muscle cells in the heart
- Vascular smooth muscle cells in arteries and veins
- Adrenal medulla
What is the maintenance function of the vasoconstrictor area of the VMC?
Vascular tone (vasomotor tone)
Where is the vasodilator area of the VMC located? What does it do?
Anterior hypothalamus - inhibits vasoconstrictor activity
Where is the part of the VMC responsible for controlling HR and contractility?
- Lateral portion - increases HR and contractility through sympathetic stimulation -> NE acting on 1 receptors
- Medial portion - decreases HR and contractility through parasympathetic stimulation -> ACh acting on M2 receptors
Where is the sensory area of the VMC?
In the tractus solitarius in posterior lateral portions of medulla and lower pons
Where does the tractus solitarius receive signals from?
Vagus and glossopharyngeal nerves
Describe the pathway of vasovagal syncope.
- Stress -> cerebral cortex ->hypothalamus -> Both AVP (ADP) release and signals medulla
- Medulla ->Both decreased sympathetic output (decreases TPVR) and increased vagal output (decreases VR and CO)
- End result is decreased arterial pressure -> decreased cerebral blood flow -> loss of consciousness
By what 3 mechanisms can the nervous system rapidly increase arterial pressure (seconds)
- 1) Constricting almost all arterioles in the body -> increases TPVR
- 2) Constricting large vessels -> increases VR and CO
- 3) Directly increasing CO by increasing HR and contractility
Is the baroreceptor reflex short-term or long-term?
What is assumed to be the primary insult when baroreceptors exert control of BP?
An increased MAP
Where are baroreceptors located?
- Carotid bifurcation (Carotid sinuses)
- Aortic arch
What are baroreceptors?
Spray-type nerve endings
Along what nerves do baroreceptors transmit signals from the carotid sinus?
Hering's nerve to glosopharyngeal nerves and then to the nucleus tractus solitarius (NTS) of the medulla
Along what nerves do baroreceptors transmit signals from the aortic arch?
Vagus nerve into the nucleus tractus solitarius (NTS)
What does stretching of baroreceptors cause?
Vasodilation and bradycardia at either location
At what pressure range are baroreceptors effective?
What do baroreceptors do below 60mmHg?
They're ineffective (they are not sensing stretch)
At what pressure are baroreceptors the most sensitive?
What are secondary signals of baroreceptors?
- 1) Inhibition of vasoconstrictor center in medulla
- 2) Excitation of vagal parasympathetic center
- The result is vasodilation of veins and arterioels, decreased HR, decreased contractility
What receptors maintain relatively constant pressure despite changes in position?
Describe the importance of baroreceptors in long-term BP control.
Unimportant because they adapt over time
What is the neurotransmitter for the vasoconstrictor nerves?
What stimulates alpha adrenergic receptors?
Norepinepherine and epinepherine
How can epinepherine cause vasodilation?
What are chemoreceptors sensitive to?
- O2 deficiency
- CO2 excess
- H+ ion excess (low pH)
Where are chemoreceptors located?
- Carotid bodies near the carotid bifurcation
- Aortic arch
What does activation of chemoreceptors result in?
Excitation of the vasomotor center -> increased sympathetic activity
At what pressure are chemoreceptors stimulated?
Describe the CNS ischemic response
In the presence of reduced blood flow, CO2 buildup stimulates the VMC thereby increasing arterial pressure.
What is one of the most powerful activators of the sympathetic vasoconstrictor system?
CNS ischemic response
At what pressure is the CNS ischemic response activated?
At what pressure is the CNS ischemic response have greatest activation?
What is the mechanism of the Cushing response?
- Increased ICP (CSF) to the point of vascular stasis. The brain and arteries become compressed to the point of cutting off blood flow (supply)
- The increase in ICP will trigger CNS ischemic response to increase arterial pressure ABOVE the ICP to permit blood flow.
What is the function of low pressure receptors in the atria and pulmonary arteries?
Minimize arterial pressure changes in response to changes in blood volume
How do atria and pulmonary artery receptors work?
- Increased blood volume activates the receptors and lowers arterial BP. Activation of the receptors enhances Na+ and water EXCRETION.
- 1) Decreasing the rate of ADH secretion
- 2) Increasing the GFR
- 3) Decreasing Na+ reabsorption
What prevents the damming of blood in veins, atria, and pulmonary circulation in response to increased volume?
How does the Bainbridge reflex work?
- Increased atrial pressure increases HR
- Stretch of atria sends signals to VMC via vagal AFFERENTs to increase HR and contractility -> DECREASED vagal tone
What is the significance of respiratory waves on arterial wave form?
- LARGER fluctuations with breathing are good indicators of HYPOvolemia
- Normal rise and fall of 4-6mmHg
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