What is the RVLM's part in the hierarchy?
-RVLM is part of the autonomic centre in the brain stem reticular formation, driving excitation of the preganglionin neurons, then post
- then constricts the blood vessel
What do we talk about today?
-baroreceptor reflex today, another level of control, this one above the RVLM
Is blood pressure, heart rate stable over time?
-rats' blood pressure, over a month -if you take average activity the blood pressure in an individual doesn't change very much -there are swings on a small scale but not on average -in a SHAM rat, minor fluctuations, when it is sleeping it is lower -morning surge of blood pressure when you are about to wake up -heart rate fluctuates a lot -sinoaortic denervation= lost control in the blood pressure and heart rate
How does sympathetic nervous acitvity affect the blood pressure?
-SNA= sympathetic nerve activity
-TRP= total peripheral resistance
-SV= stroke volume
-HR= heart rate
-P/sNA parasympathetic nervous system
-CO= cardiac output
-TRP+CO together give you arterial pressure
What is this?
-recording from a conscious sheep
-EEG (heart rate)
-CSNA (sympathetic innervation)
-give substance that causes dilation, drop in blood pressure, so there is an increase in sympathetic activity and higher heart rate
-this is the barorecpetor reflex -brings the blood pressure within narrow limit
-need to keep the blood pressure in a narrow limit so can perfuse the brain and to not damage the tissues
-if don't have perfusion to the brain, you faint and the brain perfuses again
What is the anatomy of the baroreceptors?
-the sensors are on the way to the head -the baroreceptors -the aortic arch (2), sense the stretch of these vessels, send axons to brain in the vagus nerve -common corrotid goes to the brain, then separates into internal corrotid is the main to the brain (just before it separates = the corrotid sinus and there the corrotid sinus body is -head up to the brain and glossopharyngeal nerve -subclavian (vessel) goes to the spinal cord -blood flow to the head
How are baroreceptors activated?
-in response to stretch APs are formed and go to the brain -cell body is in the no dose ganglion (under chin) -then have a central arm (same neuron has two neurons, peripheral sensing one and then one to the brain) -sensory afferent
What happens to baroreceptors when blood pressure increases?
-blood pressure increase= excites the baroreceptors -at resting in about midrange
Where do the baroreceptors project to?
-the nucleus of the solitary tract (NTS)
-collection of vagal afferent fibres
-the first port of call for most visceral afferent information (thorax and abdomen)
What happens when blood pressure decreases?
-in response to decrease in blood pressure= decrease of baroreceptor afferent neuron= increase in sympathetic activity= increase in heart rate
How does the sympathetic activity gets increased in response to decrease in afferent baroreceptor neurons?
-want decrease in parasympathetic activity and increase in sympathetic
-baroreceptor synapse to the STN, from there sympathetic and parasympathetic
-all those go onto the SA node (sinoatrial)
What are we doing?
-blood pressure low, want to increase the RVLM activity of neurons that drive the sympatehtic neurons and decrease the parasympathetic
How does the parasympatehtic system responds to decrease in blood pressure?
-parasympathetic is simple, we know that barorecpetor afferents are glutemrgic so excitatory, so excite the next neurons in the chain -they have a high safety margin (high probability that enough neurotransmitter to cause AP) -the neuron in NTS faithfully responds -direct projection from NTS to parasympathetic bit that is also glutergic -if blood pressure then low, then decrease the baroreceptor afferents, which decrease input onto NTS, which decreases input to parasympatehtic bit that doesn't fire, same as inhibiting it! -as have tonic activity can decrease drive same as the inhibition
How do the sympathetic system responds to the low blood pressure?
-decrease in baroreceptor afferent -less glutamate onto NTS -coming out of the NTS is a neuron that is excitated to caudal ventro lateral medulla (CVLM) which contains neurons that are short projecting neurons (6mm) onto CVLM neuron that excite sympatehtic neurons and the CVLM neurons are GABAergic, that means if you excite a CVLM neuron it decreases of the sympatehtic activity -so now have change of sign in the circuit -blood pressure decreases, less activity of the STN, so less active in CVLM so less inhibition of the sympathetic (RVLM) so have more activity of sympathetic (RVLM)
What happens when too high a blood pressure?
-if blood pressure is too high, want to decrease blood pressure so want to vasodilate, want to less sympathetic activity and more parasympathetic activity -want bradychardia -the activity of the afferent increases, (the baroreceptors) so the NTS to parasympathetic increases so more parasympathetic -NTS to CVLM is increased, so the inhibition of RVLM is stronger so decrease of sympathetic
• The baroreceptor reflex is an example of a negative feedback loop. – It serves to ensure constancy of brain perfusion • The sensors are bipolar nerve cells – peripheral terminals in the large arteries to the head – So main ganglia in the neck – Central terminals in the brainstem (nucleus of the solitary tract) • The effector is the autonomic nervous system – At rest activity is mid‐range meaning that both increases and decreases can occur • (heating and cooling!) • The processor is the simple circuitry of the brainstem – The inhibitory interneurons of the CVLM allow an excitatory input to differentially regulate two outputs. -• Sensory afferent neurons (baroreceptors) provide information to the brain regarding blood pressure. • When blood pressure changes the brain enacts a coordinated response via the autonomic nervous system to restore homeostasis. • This involves a relatively simple relay circuit in the brainstem. • Yet, in hypertension, blood pressure increases as does sympathetic activity and heart rate.
What are the classes of neurons of RVLM?
-within the RVLM have two classes of neurons (both make glutamate) -one class also makes catecholamines (adrenaline, dopamine, noradrenaline) -provide input to the sympathetic system