Baroreflex pathway IX Medulla NTS X PA DA
Baroreflex pathway: IX Medulla NTS X PA DA CS Spinal cord Aa Heart Adrenal Medulla Blood vessels
NTS Medulla Dorsal motor nucleus Nucleus Ambiguus Vagus nerve Heart
• RESPONSES OF BARORECEPTORS: WHEN BLOOD PRESSURE INCREASES – BP Carotid sinus & Aa arch distension Sino-Aortic n. discharge (stimulation) end in NTS (Glutamate - excitatory NTR) Projection of NTS (Glutamate NTR) to: 1. Depressor area (CVLM & IVLM) – GABA (inhibitory NTR) secreting neurons stimulated 2. Vagal motor neurons (in NA & DMN)
Contd: These effects (1 & 2) cause: + Vagus Inhibition of VMC Inhibition of Sympathetic outflow Bradycardia HR Contractility Venodilation Vasodilation Cardiac Output BP RETURNS TO NORMAL PR
WHEN BLOOD PRESSURE DECREASES – BP Less stretch (activation) of baroreceptors Sino-Aortic n. discharge Less stimulation of NTS 1. Disinhibition (activation) of VMC (DA inhibited; Pressor area activated) 2. Cardiac Vagal centre inhibited
These effects (1 & 2) cause: - Vagus HR HR Stimulation of Sympathetic outflow Vasoconstriction Contractility Venoconstriction Cardiac Output PR BP RETURNS TO NORMAL
Baroreceptor firing rate Pressure range for responses: 200 150 100 50 Mean arterial pressure (mm Hg)
Types of responses (Receptors sensitive to): 1. Change in pulse pressure (PP) 2. Change in Mean arterial pressure(MAP) Arterial Pressure (AP) (mm. Hg) 1. Effect of change in PP (MAP constant)Sinus nerve traffic 120 100 80 70 0. 3 Time (s) 0. 8 0. 0 0. 3 Time (s) 0. 8
Different levels of MAP (mm. Hg) 2. Effect of change in MAP (PP constant)120 70 50 70 100 140 200 0. 0 Time (s) 0. 3 0. 8
• BARORECEPTOR RESETTING: Baroreceptors are rapidly adapting receptors • CAROTID CLAMPING & BILATERAL VAGOTOMY: • LYING TO STANDING:
Typical intra-arterial and finger arterial pressure registrations during the Valsalva manoeuvre (left) and standing-up (right). Ben P. M. Imholz et al. Cardiovasc Res 1998; 38: 605 -616 Copyright © 1998, European Society of Cardiology
CARDIAC BARORECEPTORS: 1. Atrial stretch receptors & 2. Ventricular receptors 1. ATRIAL STRETCH RECEPTORS: Types – Role of atrial stretch receptors – a. As low pressure receptors b. Atrial reflex control of blood volume (Volume reflex) c. Atrial reflex control of heart rate(Bainbridge reflex)
a. As low pressure receptors: Increased venous return Increased atrial filling Stimulation of type – B receptors Sympathetic inhibition Vasodilation, BP, but HR
b. Atrial reflex control of blood volume(Volume reflex): Volume overload Atrial stretch Signals to Hypothalamus Renal afferent arteriolar relaxation Diuresis Decreased ADH Rise in Glomerular capillary pressure Release of ANP Decreased water reabsorption from tubules Increased fluid filtration in tubules
c. Atrial reflex control of heart rate(Bainbridge reflex): Infusion of saline or blood (with initial slow HR) Increased venous filling of atria Tachycardia producing atrial receptors stimulated Tachycardia (afferent & efferent via X)
• VENTRICULAR RECEPTORS: • PULMONARY BARORECEPTORS:
CHEMORECEPTORS • Location: • Receptors sensitive to: • Innervation of Chemoreceptors: (Afferent pathway) • Pressure range for responses: • Mechanism:
Aortic body x Medulla Respiratory centre Cardioinhibitory center VMC Aortic arch IX Carotid body Vasoconstriction CCA HR Hyperventilation Catecholamines (Adrenal medulla) Tachycardia Mild tachycardia; Vasoconstriction
CNS ISCHAEMIC RESPONSE (Mechanism) Gross hypotension or Intra-cranial pressure Decreased blood flow to VMC Hypoxia & hypercapnia at VMC Strong stimulation of VMC Intense vasoconstriction Increased pressure in Carotid sinus Activation of Baroreflex Reflex bradycardia
Capillary Fluid shift mechanism: Blood pressure Hydrostatic pressure in capillaries Shift of fluid from extravascular compartment into intravascular compartment circulating blood volume Blood pressure returns to normal
Stress relaxation: Abrupt increase of Blood pressure Distension of blood vessels Acute stretch of vessel wall Smooth muscles of vessels relax Vascular tone decreases Blood pressure decreases
RENIN-ANGIOTENSIN SYSTEM: Arterial pressure Angiotensin I Renin Angiotensinogen Converting enzyme Source: lungs & Kidneys Angiotensin II Blood pressure returns to normal
SOMATOSYMPATHETIC REFLEX: Sensory cortex Collateral to medulla Somatic afferents Nociceptor; Proprioceptor Vasoconstriction BP Sympathetic outflow Ascending pathway VMC
II. LONG TERM BLOOD PRESSURE REGULATION Renin - Angiotensin - Aldosterone mechanism: Arterial pressure Angiotensin I Renin Angiotensinogen Converting enzyme Source: lungs & Kidneys Angiotensin II: 1. Vasoconstriction 2. Aldosterone release 3. Stimulation of thirst Angiotensin II
Angiotensin II Contd: Release of Aldosterone Vasoconstriction Subfornical region of brain Na+ reabsorption (Kidneys) Thirst Na+ & H 2 O retention Blood volume Water intake BLOOD PRESSURE RETURNS TO NORMAL
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