2.07 - Neural Regulation Of Blood Pressure Flashcards
What three parameters control Mean Arterial Pressure (MAP)?
TPR, SV & HR
MAP = TPR • SV • HR = TPR • CO
How does the autonomic nervous system control the parameters of MAP?
HR: sympathetic (+), parasympathetic (-; vagal). At the level of the heart
SV: sympathetic (+), parasympathetic (-;vagal) at the level of the heart. Sympathetic (+) at level of TPR (afterload)
TPR: sympathetic (+) ONLY at level of vessels
What are the two main neural feedback controls on blood pressure?
Arterial Baroreceptor reflex
Cardio-pulmonary reflex
Describe the function of the arterial baroreceptor reflex
AB helps to stabilise BP control, with a fast response within a narrow range.
When BP is increased –> Increased parasympathetic and decreased sympathetic activity –> decreased HR, SV and TPR
Where are the receptors for the AB reflex located? And nerves do the afferent fibres use?
Pressure (mechanoreceptors) receptors in the aortic arch and carotid sinus
Carotid sinus afferents via IX (glossopharyngeal) and aortic arch afferents via X (vagal)
Which types of fibres are activated and over what range?
A-fibres are sensitive over the lower pressures. Increased firing rate as pressure increases but reach a saturation point at high pressures
C-fibres are activated at high pressures and continue to increase in activity as pressure increases
What effect does exercise have on the Arterial Baroreceptor reflex?
The set-point is reset so that work in a higher range can be undertaken without incurring a depressor response
Describe the efferent fibres of the AB reflex
Sympathetic output over the whole sympathetic chain (T1-L3)
Parasympathetic output via Vagus nerve and lumbosacral cord (S2-S4)
An increase in parasympathetic outflow decreased sympathetic activity and vice versa
Describe the central AB pathways in the brain
The NTS neurons send excitatory fibers (glutamatergic) to the caudal ventrolateral medulla (CVLM), activating the CVLM. The activated CVLM then sends inhibitory fibers (GABAergic) to the rostral ventrolateral medulla (RVLM), thus inhibiting the RVLM. The RVLM is the primary regulator of the sympathetic nervous system, sending excitatory fibers (glutamatergic) to the sympathetic preganglionic neurons located in the intermediolateral nucleus of the spinal cord.
Hence, when the baroreceptors are activated (by an increased blood pressure), the NTS activates the CVLM, which in turn inhibits the RVLM, thus decreasing the activity of thesympathetic branch of the autonomic nervous system, leading to a relative decrease in blood pressure. Likewise, low blood pressure activates baroreceptors less and causes an increase in sympathetic tone via “disinhibition” (less inhibition, hence activation) of the RVLM. Cardiovascular targets of the sympathetic nervous system includes both blood vessels and the heart.
Describe the Cardiopulmonary Reflexes
The CP relfexes are a collections of varios reflexes.
They are low pressure receptor reflexes
Describe the Veno-Atrial Stretch Receptors
They measure atrial blood volume i.e. central venous pressure/volume) in the low pressure part of circulation (central veins/atria)
As venous return increases, the pressure in the superior and inferior vena cava increase. This results in an increase in the pressure of the right atrium, which stimulates the atrial stretch receptors (low pressure receptor zones). These receptors in turn signal the medullary control centers to increase the heart rate (Tachycardia). Unusually, this tachycardia is mediated by increased sympathetic activity to the sinoatrial node (SAN) with no fall in parasympathetic activity.
Increasing the heart rate serves to decrease the pressure in the superior and inferior venae cavae by drawing more blood out of the right atrium. This results in a decrease in atrial pressure, which serves to bring in more blood from the vena cavae, resulting in a decrease in the venous pressure of the great veins. This continues until right atrial blood pressure returns to normal levels, upon which the heart rate decreases to its original level.
Also works to decrease blood volume by promoting renal vasodilation –> increased renal sodium excretion; decreased ADH release –> increased diuresis; increase release of atrial natiuretic peptide (ANP) –> increased diuresis
Describe the effect of atrial chemoreceptors
Sense PO2 and PCO2. Located in carotid and aortic bodies.
A drop in pressure below 80mmHg (asphyxia, shock etc.) results in: increased TPR as renal, splanchnic and muscle vascular beds are constricted. Splanchnic veins constrict –> decreased pooling –> increase MSFP –> increased SV –> increase CO. Tachycardia due to increased respiratory rate (lung stretch receptors inhibit parasympathetic activity
Under resting conditions which arm of the autonomic nervous system has the greatest effect on the heart?
Parasympathetic is more effective than sympathetic, although both are tonically active
Jack West, a 28 year old bike rider was involved in a car accident on the road to the snowfields. He suffered a broken femur and was immediately transferred to Cooma Base Hospital. On admission, his HR was 112BPM, and his BP 65/45torr. Which of the following statements best describes the state of the arterial baro- and cardiopulmonary reflex?
C
Describe the tonic inhibition of the CP reflexes
Cardiac de-afferentiation (transplant) reveals a tonic inhibition of HR and TPR only, of the CP reflexes.
