Sensory Receptors And Reflexes Notes

SENSORY RECEPTORS AND REFLEXES

Mean arterial pressure

- calculated by cardiac output x peripheral resistance

-It is important for the body to maintain a high constant blood pressure within a narrow range (120/80mmHg) as it ensures optimum organ perfusion and efficient glomerular filtration.

-If the pressure is too low the patient is in shock whereas the pressure is too high the patient is hypertensive and if pressure is at either of these two extremes local blood flow can no longer be regulated by myogenic and metabolic autoregulation.

4 key factors are responsible for setting blood pressure, cardiac output, total peripheral resistance, compliance and blood volume

-CO increases BP due to Bernouilli’s equation-faster moving fluid has a greater pressure

-TPR- energy needed to circulate the peripheral tissue, resistance is calculated by Pouiselle’s equation

-Compliance is the change in volume of a vessel for a given change in pressure- can’t be directly modified but plaque build up on vessel walls increases with age leading to atherosclerosis

-Short term regulation of blood pressure, which takes seconds to minutes occurs through neural pathways and targets the heart, vessels and the adrenal medulla.

- long term regulation targets mostly the kidneys which control the extracellular fluid. In patients whose biological mechanisms that regulate blood pressure have failed require drugs are required to maintain a constant blood pressure.

Neural reflexes: arterial baroreceptor

-The activity of baroreceptor reflexes was shown by an expirement carried out by Heyman. When adrenaline was injected into dogs there was a subsequent rise in blood pressure but due to the neural reflex this was followed by a decrease in heart rate. To show that is was neuronal activity rather than hormones in the blood stream that resulted in bradycardia Heyman cross perfused 2 dogs where one dog had nerves connected but its blood supply to its head coming from the 2^nd dog. When the 1^st dog was injected with adrenaline there was a decrease in heart rate showing that it was neuronal activity.

-Baroreceptors are stretch receptors that detect expansion in vascular walls- baroreceptors are sprayed sensory fibre endings and are connected to afferent axon- they are found in the adventitia of arteries at two main locations- carotid sinus and the aortic arch

-The carotid sinus- thin walled dilatation at the orgin of the internal carotid artery- afferent fibres from the carotid sinus baroreceptors form the carotid sinus nerve- this joins the glossopharyngeal nerve

- The second high pressure baroreceptor is found in the tranverse arch of the aorta – their fibres form the depressor nerve and then asend in the vagus

-Both of the aortic arch and carotid sinus baroreceptors are made up of a mixture of A and C fibres. A fibres have a large diameter, are fast conducting, myelinated and have a low threshold. These fibres are active at normal blood pressure. Whereas C fibres which are more abundant, are myelinated, slow conducting, have a small diameter and high thresholds.

- Having a mixture of the two types of muscle fibres allows recruitment of different fibres according to the blood pressure. The terminals of these fibres express nonselective cation channels which are from the TRP family.

-rise in arterial pressure stretches the artery wall- deforms and excites the receptor terminals. Carotid sinus is thin, so is easily stretched and is sensitive to changes in blood pressure. Aortic arch- many elastic fibres and is very compliant and stretches during left ventricular ejection

-When there is an increase in transmural pressure difference the blood vessel is enlarged and this deforms TRPC1 channels which results in a depolarising inward current and forms the receptor potential. To show that these channels are stretch sensitive rather than pressure sensitive, the vessels was prevented from being stretched which resulted in the failure to open the TRPC1 channels even when there is an increase in transmural pressure.

-arterial baroreceptors respond to both the magnitude of pressure (static sensitivity) and rate of change (dynamic sensitivity) If carotid sinus is distended rapidly- baroreceptor fibres fires high frequency of action potentials (dynamic response to pressure change)- firing frequency declines (Adaptation)- stabalises at a lower frequency- this signals new pressure level (Static response)

- afferent nerves (glossopharyngeal/vagus nerves) transmit impulses to the medullary cardiovascular centre and synapse with neurons in the nucleus tractus solitarii by releasing glutamate which binds to AMPA receptors. This results in the stimulation of inhibitory interneurons which originate in the nucleus tractus solitarii and synapse with C1 neurons in the vasomotor area.

- During rest the C1 neurons of the medulla vasomotor centre exerts a tonic activity on the sympathetic vasoconstrictor nerves which results in basal vasoconstriction.

- The activity of C1 neurons was shown when clonidine was injected which bound to imidazole receptors and resulted in an inhibition of the C1 neurons and this led to a decrease in blood pressure.

- The inhibition of the C1 neurones due to stimulation of inhibitory interneurons results in a decrease in the amount of neurotransmitter released - This results in a decrease in basal activity of the sympathetic vasoconstriction which causes vasodilation and causes a reduction in a total peripheral resistance.

-The afferent nerve fibres in the nucleus tractus solitarii also innervate excitatory interneurons which synapse with the cardioinhibitory area which includes the nucleus ambiguous and dorsal motor nucleus of the vagus. Innervation of these neurons leads to a decrease in heart rate which ultimately causes a decrease in cardiac output.

-Therefore increase in activity of high pressure baroreceptors leads to a decrease in mean arterial pressure due to overall decrease in cardiac output and decrease in total...

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