Physiology Lecture 60
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Vasomotor
center
Areas of the brain that
play important roles in
the nervous regulation of
the circulation
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Vasomotor Center
– Located in Brain
• Bilaterally mainly in the reticular substance
of the Medulla and lower third of the Pons
– Autonomic Nervous System
• Sympathetic
• Parasympathetic (Vagus)
Tanveer Raza MD MS MBBS
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Nervous Regulation of Circulation
• Vasomotor center
– Vasoconstrictor area
– Vasodilator area
– Sensory area
• Receive sensory nerve signals through
CN X and CN IX
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Vasomotor Center
– Control of Heart Activity
• Sympathetic impulses
–Increase heart rate and contractility
• Parasympathetic impulses (Vagus Nerve)
–Decrease heart rate and contractility
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Vasomotor Center
– Control by Higher Nervous Centers
• Can excite or inhibit vasomotor center
• Hypothalamus plays a special role
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Sympathetic Nervous
System
– Vasomotor nerve
fibers
• Leave spinal cord
through all the
thoracic spinal
nerves and L1 and
L2 spinal nerves
Tanveer Raza MD MS MBBS
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Nervous Regulation of Circulation
• Sympathetic innervation of Blood Vessels
Tanveer Raza MD MS MBBS
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Nervous Regulation of Circulation
• Sympathetic Innervation
– Blood Vessels
• Innervates all the vessels except
capillaries, precapillary sphincters and
metarterioles in most tissues
– Innervation of small arteries and arterioles
increase resistance
– Innervation to the large veins increase venous
return
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Sympathetic Innervations
– Blood Vessels
• Vasoconstriction
– Distributed to essentially all segments of the
circulation
» More to kidneys, intestines, spleen, and
skin but less potent in skeletal muscle and
the brain
• Vasodilation
– Only a few fibers
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Sympathetic Innervations
– Vasoconstrictor tone
• Under normal condition a partial state of
contraction in the blood vessels, called
vasomotor tone is maintained by
sympathetic impulses
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Sympathetic Innervations
Effect of total spinal
anesthesia on the arterial
pressure, showing marked
decrease in pressure
resulting from loss of
"vasomotor tone."
Tanveer Raza MD MS MBBS
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Nervous Regulation of Circulation
• Sympathetic Innervation
– Secretes Norepinephrine (NE) at nerve
endings
– Adrenal Medullae
• Secrete both Epinephrine and NE
– Usually causes vasoconstriction
» Epinephrine in certain cases cause
vasodilation
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Sympathetic Innervation
– Vasodilation
• To skeletal muscles
– Carries constrictor fibers also
• Of little or no importance
• May cause anticipatory vasodilation in
skeletal muscles before exercise
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Nervous Regulation of Circulation
• Parasympathetic innervations
– Control of Heart function
• through CN X
• Marked decrease in heart rate and a slight
decrease in heart muscle contractility
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
VASOVAGAL SYNCOPE
Tanveer Raza MD MS MBBS
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Vasovagal Syncope
• Most common cause of Syncope (fainting)
• Mechanism
– Increased Parasympathetic activity
– Decreased Sympathetic activity
Tanveer Raza MD MS MBBS
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Vasovagal Syncope
• Mechanism
– Increased Parasympathetic activity
• Cardioinhibitory effect
– Negative Ionotropic (heart contraction) and
Chronotropic (heart rate) effect
» Decreased Cardiac Output
– Decreased Sympathetic activity
• Vasodilation
– Pooling of blood in the periphery, decreased
venous return
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Vasovagal Syncope
• Mechanism
– Decreased blood flow to brain
– Loss of Consciousness
• Person falls down, blood flow to brain
restored and patient wakes up
Tanveer Raza MD MS MBBS
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Vasovagal Syncope
• Cause
– Many triggers
• Prolong standing
• Stress
• Painful stimuli
– Venepuncture
• Extreme emotion
• etc
Tanveer Raza MD MS MBBS
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Nervous Regulation of Circulation
• Control of Arterial Pressure
– Rapid
– Sympathetic stimulation
• Vasoconstriction
– Increased cardiac Output
» Increase in Total Peripheral resistance
» Increased venous return
• Positive Chronotropic & Ionotropic action
– Parasympathetic inhibition (to heart)
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Reflex Mechanisms for Maintaining Normal
Arterial Pressure
– Negative feedback reflex mechanism
• Rise in arterial pressure
• Stretching of baroreceptors
• Receptors transmit signals into CNS
• "Feedback" signals (by ANS) to circulation
• Arterial pressure reduced to normal level
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Reflex Mechanisms for Maintaining Normal Arterial Pressure
– Negative feedback reflex mechanism
• Rise in arterial pressure
• Stretching of baroreceptors
• Receptors transmit signals into CNS
• "Feedback" signals (by ANS) to circulation
• Arterial pressure reduced to normal level
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Arterial pressure rise
– In normal operating
range of arterial
pressure, around 100
mm Hg, a slight change
in pressure causes a
strong change in the
baroreflex signal
Activation of the baroreceptors
at different levels of arterial
Tanveer Raza MD MS
MBBS
pressure
razajju2@yahoo.com
Baroreceptor Reflex
• Reflex Mechanisms for Maintaining Normal Arterial Pressure
– Negative feedback reflex mechanism
• Rise in arterial pressure
• Stretching of baroreceptors
• Receptors transmit signals into CNS
• "Feedback" signals (by ANS) to circulation
• Arterial pressure reduced to normal level
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Baroreceptors
– Stretch receptors
• Stimulated when stretched
– Spray-type nerve endings
– Location
• Carotid sinus
• Wall of the aortic arch
• Few in the wall of almost every large artery
of thoracic and neck regions
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Carotid Sinus
– Localized dilation of Internal Carotid Artery at
its origin
– Function
• Contains numerous baroreceptors
– Innervated by Nerve of Hering (CN IX branch)
– CN IX synapses in the Nucleus Tractus
Solitarius located in the medulla oblongata
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor
Reflex
• Carotid Sinus
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Carotid and Vertebral
angiography
Carotid Sinus
Tanveer Raza MD MS MBBS
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Baroreceptor Reflex
• Carotid body
– Chemoreceptors and supporting cells located near
the bifurcation of the carotid artery
• Innervated by branches of CN IX and CN X
– Function
• Detects changes in arterial blood
– O2 (mainly)
– Also
» CO2
» pH
» Temperature
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Baroreceptors at the
wall of aortic arch
– Innervated by branches
of CN X and travels to
the Nucleus Tractus
Solitarius located in the
medulla oblongata
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Reflex Mechanisms for Maintaining Normal Arterial Pressure
– Negative feedback reflex mechanism
• Rise in arterial pressure
• Stretching of baroreceptors
• Receptors transmit signals into CNS
• "Feedback" signals (by ANS) to circulation
• Arterial pressure reduced to normal level
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Receptors transmit signals in CNS
– The baroreceptor signals entered the Nucleus
of tractus solitarius of the medulla,
secondary signals inhibit the vasoconstrictor
center of the medulla and excite the vagal
parasympathetic center.
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Reflex Mechanisms for Maintaining Normal Arterial Pressure
– Negative feedback reflex mechanism
• Rise in arterial pressure
• Stretching of baroreceptors
• Receptors transmit signals into CNS
• "Feedback" signals (by ANS) to circulation
• Arterial pressure reduced to normal level
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• "Feedback" signals (by ANS) to
circulation
– High pressure in the arteries reflexly causes
the arterial pressure to decrease
• Decrease in peripheral resistance
• Decrease in cardiac output
– Low pressure has opposite effects
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Arterial baroreceptor system reduces the
minute by minute variation in arterial pressure
to about one third that which would occur were
the baroreceptor system not present
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Two-hour records of
arterial pressure in a
normal dog (above)
and in the same dog
(below) several
weeks after the
baroreceptors had
been denervated
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Long-Term Regulation
– Important in short term (seconds to minutes) control
– Long term regulation unimportant
• Baroreceptors rapidly reset to new pressure
• Total baroreceptor denervation has no lasting
effect on the average daily arterial pressure
• Recent studies in intact experimental animals suggest
baroreceptor may play a role in long term regulation
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Control of Arterial pressure by chemoreceptors
– Chemoreceptors
• Chemosensitive cells
• Respond to
– O2 , CO2 and H+
• Location
– In several small chemoreceptor organs
» two Carotid bodies
» One to three aortic bodies adjacent to aorta
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Baroreceptor Reflex
• Control of Arterial pressure by chemoreceptors
– Impulse pass along with the baroreceptor
fibers, hrough Hering's nerves (CN IX) and
the CN X into the vasomotor center
– When arterial pressure falls below critical
level, chemoreceptors become stimulated
because diminished blood flow causes
decreased O2 as well as excess CO2 and H+
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Bainbridge Reflex
• Atrial Reflex Control of Heart Rate
– Increased atrial pressure increases in heart
rate
• Direct effect of the increased atrial volume
to stretch the SA node
• Bainbridge reflex
– Helps prevent damming of blood in the veins,
atria, and pulmonary circulation
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Bainbridge Reflex
• Atrial Reflex Control of Heart Rate
– Increased atrial pressure increases in heart rate
• Bainbridge reflex
– Nervous reflex
– Stretch receptors of atria elicit the reflex
– Afferent signals through CN X to the medulla
– Efferent signals through CN X and sympathetic
nerves
» Increase heart rate and strength of
contraction
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Renal Control
Increases in cardiac output,
urinary output, and arterial
pressure caused by
increased blood volume in
dogs whose nervous
pressure control
mechanisms had been
blocked
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Renal Control
• Renal-body fluid systems play a
dominant role in long-term BP regulation
– Excess ECF
– Blood volume and
arterial pressure rise
– Kidneys excrete extra
ECF
– Pressure returns back
to normal
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Renal Control
• Pressure diuresis
– Increased urinary output in response to
increased pressure
• In humans being, at an arterial pressure of
– 50 mmHg urine output is essentially zero
– 100 mm Hg urine output is normal
– 200 mm Hg urine output is 6-8 times normal.
– Pressure natriuresis
• Increased arterial pressure also increases
sodium output
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Renal Control
• Increased Fluid Volume Can Elevate
Arterial Pressure
– Increases Cardiac Output
– Increases Total Peripheral Resistance
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Renal Control
• Salt (NaCl) intake elevate the arterial
pressure
– Increased Salt in ECF
– Increased osmolality of ECF
• Increased thirst
• Increased ADH secreton
– Increased ECF
– Raised Pressure
Tanveer Raza MD MS MBBS
razajju2@yahoo.com
Thank You