Lecture nerual regulation of circulation
- 1. Nervous Regulation of Circulation Dr Shamshad Majamaah University Lecture 48
- 2. Objectives: 1. Describe the physiological anatomy of autonomic supply to the heart & blood vessels. 2. Discuss the nervous control of circulatory functions esp. heart rate. 3. Describe the role of vasomotor center in regulation of circulatory functions. 4. Describe the various factors affecting activity of vasomotor center.
- 3. Control of Heart & Vessels ANS Innervation Cardiac Peripheral Sympathetic Vasoconstrictors Epicardial Parasympathetic Vasodilators. Endocardial Preganglionic Postganglionic
- 4. 1:Sympathetic N cells lie in the intermediolateral horn of spinal cord T1-T5 segments. 2:Myelinated preganglionic fibers pass into the sympathetic trunk to superior , middle & inferior cardiac ganglia. 3:Synapse then unmylienated post- ganglionic fibers pass via the superior, middle & inferior cardiac sympathetic fibers. Supply: Nodal tissues: SAN, AVN & muscles of atria & ventricles. T1 –T5 segments 1 2 3
- 5. Brief Sympathetic NS Ionotropic Chronotropic Dromotropic Bathmotropic Target NT substances Vessels with smooth muscles Norepinephrine ATP Neuropeptide Y γ 1NPYVC Receptors Purnergic P2 ATP VC Adrenergic α1VC α2VC/VD β1 & β2VD Vascular Heart +Ve
- 6. 1. Cell bodies of Vagus nerves lies in medulla oblongata in Nucleus ambiguous 2. Myelinated preganglionic fibers travel in vagi & synapse with ganglionic cells located near the SAN , AVN & in atria. 3. Unmylienated Postganglionic fibers arise & are distributed to SAN , AVN & atrial muscles. Vagal tone At rest : Vagal tone predominates over sympathetic tone in heart . 1 2 3 Ventricles have NO Vagal Fibers
- 7. Peripheral vascular innervation I:Vasoconstriction By Sympathetic NS : Sympathetic tone Cell bodies from intermediate lateral horn cells of T1 – L2 spinal segments. Tonic effect on all vessels. Except: Cerebral vessels , true capillaries & coronary vessels Dense innervation :Arterioles, metarterioles ,pre-capillary sphincters & post-capillary venules.
- 8. Actions : 1:Vasocontriction:Through β2 adrenergic receptors . *On resistance vessels: Peripheral resistance Regulate BP. *Capacitance vessels(veins): Regulate blood Volume. *Arterioles Vasoconstriction regulated VR & thus control BP. 2:Vasodilation: On skeletal muscles, erectile tissue of reproductive system & facial blood vessels .
- 9. II:Vasodilation 1: Reduction of sympathetic constrictors tone. 2:Specific activation of vasodilators through A:Sympathetic cholinergic Nn :Ex: Skeletal muscles, heart, lungs liver, kidneys & uterus .Exercise /child birth increases blood flow. B:Parasympathetic vasodilator nerves: Nervi erigentes Ex: Nn : erectile tissue of reproductive system, salivary gland. 3:Dorsal root vasodilation : Axon reflex: Local neural mechanisms: :
- 10. Brief Parasympathetic NS Ionotropic Chronotropic Dromotropic Bathmotropic Target NT substances Arteries & Veins Ach Receptors Muscuranic M3VD Few no. Vascular Heart -Ve Receptors M2 Ventricles have NO Vagal Fibers
- 12. Medullary control the heart Cardiovascular center are group of neurons in CNS that regulate HR, SV and vascular tone . Medullary regulation of CVs is brought by two different centers located in the medulla: 1: Medullary cv center: Vasomotor center / vasomotor area • Pressor and Depressor areas 2:Cardiac vagal center Regulated by signals from baroreceptors.
- 14. Vasoconstrictor center/vasopressor area Location: Bilaterally in the antrolateral portion of the upper medulla. Neurons: Glutaminergic neurons excitatory effect on spinal sympathetic neurons. Under normal conditions, transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body, slow firing of these fibers at a rate of about ½-2 impulses/ second. lead to sympathetic vasoconstrictor tone. Significance: Impulses normally maintain a partial state of contraction in the blood vessels, called vasomotor tone.
- 15. Pressor effect on CVS Increase HR(Heart Rate) Increase BP (Blood pressure) Vasoconstriction leads to decrease blood stored in venous reservoir and increase venous return (VR).
- 16. Depressor area Location: Near medial & caudal parts of 4th ventricle in medulla. Action: Stimulation leads to decrease in sympathetic activity by inhibiting the tonically discharging impulses of the Pressor area neurons Effect: Decrease HR,SV BP (due to vasodilation) Venodilatation increases storage of blood in venous reservoirs & decrease VR.
- 17. Cardiovagal Centre Inhibitory pathways in the form of vagal fibers from the medulla to converge on Sympathetic preganglionic neurons of the spinal cord to decrease sympathetic activity. On Heart: Decrease the HR & force of contraction.
- 18. Vagal fibers arise from 3 different nuclei located in the medulla. A:Dorsal motor nucleus of the Vagus (Cardio- inhibitory Centre). B: Nucleus of tractus solitarius (NTS) Receives afferents from most of the baroreceptors & its fibers are projected to the nucleus ambiguous & dorsal motor nucleus of Vagus. C: Nucleus ambiguous : lies lateral to the medullary reticular neurons . Receives afferents via NTS and in turn sends impulses to the heart. Also called as cardiac vagal center . Neurons are not tonically active.
- 19. Factors affecting the VMC 1: Neural factors: A: Higher centers: Cerebral cortex, Hypothalamus ,Respiratory center.. 2:Afferent impulses: Circulatory system Baroreceptors of Arterial & Atria & Chemoreceptors.. 3: Other parts of the body: Cutaneous pain, Visceral pain Proprioceptors, Loven’s reflex, Cold Pressor test. 2: Chemical factors :CO2.O2,H+ ions.
- 20. Nervous Regulation of HR 1:Impulses from higher centers: Cerebral cortex :Motor cortex ,Ant temporal lobe, frontal cortex ,amygdala hippocampus act through hypothalamus. Action: Excitatory/ Inhibitory Direct /indirect action the hypothalamus (emotions) Conditioned reflex in to response to visual, smell, hearing stimuli or even thoughts.
- 21. 3:Respiratory center: HR ↑ with inspiration & ↓ with expiration . Respiratory sinus arrhythmia". Mechanism: i: Central mechanism: Active inspiratory center activation causing stimulation of the CAC. II: Peripheral mechanism: Lung inflation stimulates the stretch receptors in the alveoli -> afferent impulses via the pulmonary vagi --> -- CIC & ++ CAC. * Harrison's reflex: During expiration, HR ↓ due to decreased activity of inspiratory center, elastic recoil of the lungs and decreased venous return.
- 22. 2: Hypothalamus Anterior hypothalamic nuclei :Control parasympathetic activities -> ↓ HR stimulate CIC. During sleep & severe emotion. Posterior hypothalamic nuclei: control sympathetic activities -> ↑ HR stimulate CAC. Mild emotion, stress, muscular exercise or anxiety. Voluntarily: ↑ /↓ HR : meditation/yoga. Emotions : Mild : fear, anger, sexual excitation etc. -> ↑ HR. Severe: Sudden shocking news -> ↓ HR.
- 23. Baroreceptors: Mechanoreceptors/Stretch receptors Sensitive to stretch of vessels by dynamic blood flow Increased baroreceptors discharge inhibit tonic discharge of pressor area of VMC & excite CVC. Range of activation is 60 to 200 mmHg 60 mmHg is the threshold of baroreceptor reflex. 1:Arterial baroreceptors Innervation: Vagus nerve Except carotid sinus: By carotid sinus N branch of IX N.
- 24. Afferents : To NTS secretes NT Glutamate. Efferents: Excitatory glutaminigergic fibers terminates into VMC pressor area thus stimulate GABA secreting inhibitory neurons NA & Dorsal motor nucleus of vagus thus activate the neurons. Buffer nerves : IX & X N branches Sino aortic nerves regulate BP 2:Atrial receptors/Nonmylinated atrial afferents Innervated by nonmylinated C fibers of X N. Discharge is irregular & slow .Present all over atria & interseputm of atria Function: atrial pressure impulses vasodilation mainly in renal vascular circuit.
- 25. pO2 <50 mmHg ++Chemoreceptors impulses along IX a& X N chemosenstive area of medullastimulating respiratory center Ventilation drive &Parasympathetic --HR Sensitive to chemical changes in blood :Co2 ,O2 and pH. Carotid bodies: At bifurcation of CC A:N:Carotid sinus N IX N Aortic bodies : around arch of aorta N: Aortic N of X N Afferent: to NTS of medulla Operate best 40 -100 mmHg. When stimulated they stimulate VMC & respiratory neurons in medulla. 3:chemoreceptors
- 26. 2:Afferent impulses from the body other than the CV system: Stimulation of the central end of any nerve containing afferent fibers e.g. radial, sciatic or splanchnic ↑ / ↓ HR according to the nature of the stimulus. a- Impulses from skeletal muscles (Alams Smirk reflex) Contraction of voluntary muscles produces reflex tachycardia. Mechanism: Muscle contraction --> + + Proprioceptors in the muscle & joints --> Afferent somatic sensory nerves --> + + CAC & - - CIC --> Increase HR. Significance: Helps to ↑ blood supply to the active muscle.
- 27. 1) Cutaneous pain: Mild pain---> ++ VMC ,severe pain--> -- VMC 2) Severe visceral pain afferent fibers from the trigger zones -- VMC Trigger zone reflexes : Carotid sinus, Eye balls, Larynx, Epigastrium & Testes are sensitive. A blow on these areas produces reflex severe bradycardia /fainting / cardiac arrest/. 3) Afferent impulses from the Proprioceptors of skeletal muscles during exercise ++ VMC. Impulses from pain receptors:
- 28. 4) Loven reflex: Stimulation of the afferent nerve of an organ produces local vasodilatation in the active organ. But generalized vasoconstriction due to stimulation of the VMC. Leads to HT & localized vasodilation and increased blood flow to the organ. 5) Cold pressor test: If a normal person drips his hand in a cold water (4oC), ABP rises by 10-20 mmHg, due to stimulation of the VMC.
- 29. McDowall reflex: Decreased venous return, i.e., decreased right atrial pr as in hemorrhage leads to reflex increase of HR & of vasoconstrictor tone of arterioles and venules. Significance: This helps to maintain the normal ABP to allow sufficient supply of blood flow to the vital organs like brain, heart, & kidneys.
- 30. Marey's law HR is inversely proportional to the ABP provided that other factors that affect the HR remain constant. i.e., ↑ ABP ↓ HR Stimulus: ↑ ABP. Receptors: Arterial baroreceptors. Carotid sinus & aortic arch. Afferent: sinus nerve of IX & aortic nerve of X. Called "depressor nerves" as they resist rise in ABP. Center: + + CIC & - - CAC. Efferent: X N. Response: - - SAN ↓ HR. Significance:↓ ed HR antagonizes the elevation of ABP & helps to restore it to its normal level.
- 31. Carotid sinus syndrome Condition in which the carotid sinus is abnormally hypersensitive. Manifestations: A slight pressure on the carotid sinus (e.g. during shaving or by tight collar) stimulates the baroreceptors in the carotid sinus reflex ↑ vagal tone to the heart & ↓ sympathetic vasoconstrictor tone to the blood vessels bradycardia (↓ HR) and hypotension ↓ ABP cerebral ischemia & momentarily loss of consciousness (fainting).
- 32. Treatment: Denervation of the hypersensitive carotid sinus cures the condition. Clinical significance: Carotid sinus reflex could be used to terminate an attack of paroxysmal atrial tachycardia (PAT) but not of ventricular origin, as the vagus does not supply the ventricles.