Chapter 18 Nervous System Control of Circulation

• The nervous system has global functions for nervous control of circulation.
• Global functions include redistributing blood flow, increasing/decreasing heart activity and rapid control of arterial pressure.
• Nervous system controls circulation through the autonomic nervous system

Autonomic Nervous System

• Sympathetic nervous system is most important for regulating the circulation.
• Parasympathetic nervous system contributes to regulation of heart function.

Sympathetic Nervous System

• Sympathetic vasomotor nerve fibers leave the spinal cord through the thoracic and lumbar spinal nerves.
• Fibers pass into a sympathetic chain on each side of the vertebral column
• Pass by two routes to the circulation:
  • Specific sympathetic nerves innervate internal viscera and the heart.
  • Peripheral portions of the spinal nerves distribute to the vasculature of peripheral areas.

Sympathetic Innervation of Blood Vessels

• Sympathetic nerve fibers distribute to blood vessels, most vessels except capillaries are innervated.
• Precapillary sphincters and metarterioles get innervated in some tissues like mesenteric blood vessels.
• Innervation of small arteries/arterioles allows sympathetic stimulation to increase resistance to blood flow.
• Innervation of large vessels makes it possible for sympathetic stimulation to decrease the volume of these vessels. -This decrease pushes blood into the heart and regulates heart pumping.

Sympathetic Stimulation

• Sympathetic fibers go directly to the heart
• Sympathetic stimulation increases heart activity, increases heart rate and volume of pumping

Parasympathetic Stimulation

• Plays a minor role in regulating vascular function in most tissues.
• Important circulatory effect is to control heart rate through vagus nerves to the heart.
• Parasympathetic stimulation causes a marked decrease in heart rate with a slight decrease in heart muscle contractility.

Sympathetic Vasoconstrictor System and Central Control

• The sympathetic nerves carry large numbers of vasoconstrictor nerve fibers.
• The vasoconstrictor fibers distribute to all segments of the circulation, but more to some tissues more than others such as kidneys, intestines, spleen, and skin.
• Vasomotor center is located bilaterally in the reticular substance of the medulla and lower third of the pons.
• Vasomotor center transmits parasympathetic impulses through the vagus nerves to the heart.
• Sympathetic impulses transmitted through the spinal cord and peripheral sympathetic nerves to arteries, arterioles, and veins of the body.

Vasomotor Center Organization

• Vasoconstrictor area located bilaterally in the anterolateral portions of the upper medulla distributes fibers to all levels of the spinal cord and excite preganglionic vasoconstrictor neurons of the sympathetic system.
• Vasodilator area is located bilaterally in the anterolateral portions of the lower half of the medulla, and fibers project to the vasoconstrictor area, inhibiting its activity and causing vasodilation.
• Sensory area is located bilaterally in the nucleus tractus solitarius in the posterolateral portions of the medulla and lower pons. -This area of neurons receives sensory nerve signals from the circulatory system through the vagus and glossopharyngeal nerves.
• Output signals from this sensory area then help control activities of the vasoconstrictor and vasodilator areas of the vasomotor center. This provides reflex control of circulatory functions such as the baroreceptor reflex for controlling arterial pressure.
• Under normal conditions, signals transmit continuously to the sympathetic vasoconstrictor nerve fibers causing slow firing called sympathetic vasoconstrictor tone and maintains a partial state of constriction in the blood vessels called vasomotor tone.
• Spinal anesthetic blocks transmission of sympathetic nerve impulses from the spinal cord, lowering arterial pressure.
• Norepinephrine is injected to constrict vessels and increase arterial pressure.
• The lateral portions of the vasomotor center transmit excitatory impulses through sympathetic nerve fibers to the heart increases the heart rate and contractility.
• Medial portion sends signals to the dorsal motor nuclei of the vagus nerves which then transmit parasympathetic impulses to the heart to decrease heart rate and heart contractility when a decrease in heart pumping is needed.

Vasomotor Center Influences

• Higher brain centers like the pons, mesencephalon, and diencephalon reticular substance can excite or inhibit the vasomotor center.
• Hypothalamus plays a role in controlling the vasoconstrictor system through excitatory or inhibitory effects on the vasomotor center.
• Cerebral cortex parts can excite or inhibit the vasomotor center.
• The substance secreted at endings of vasoconstrictor nerves is norepinephrine, which acts on alpha-adrenergic receptors to cause vasoconstriction.
• Sympathetic impulses transmitted to the adrenal medullae, secrete epinephrine and norepinephrine into circulating blood where they act on blood vessels and usually cause vasoconstriction
• Sympathetic nerves carry sympathetic vasodilator fibers to skeletal muscles
• Pathway for CNS control of the vasodilator system; the principal area of the brain controlling this system is the anterior hypothalamus

Sympathetic Vasodilator System

• Does not appear to play a major role in circulation control in humans.
• Sympathetic system might cause initial vasodilation in skeletal muscles to allow increased blood flow at onset of exercise.
• Vasodilatory reaction occurs in people experiencing emotional disturbances that invoke fainting.
• Muscle vasodilator system is activated and vagal cardioinhibitory center transmits strong signals to the heart
• Arterial pressure falls rapidly causing loss of consciousness called vasovagal syncope

Role of the Nervous System in Controlling Arterial Pressure

• One of nervous control of circulation functions is to cause rapid increases in arterial pressure through vasoconstrictor and cardioaccelerator functions of the sympathetic nervous system that are stimulated together. This reciprocally inhibits parasympathetic vagal inhibitory signals to the heart.
  • Most arterioles are constricted
  • Veins are constricted
  • Heart is directly stimulated by the autonomic nervous system.

Nervous Control

• Important characteristic of response rapidity, beginning within seconds and increases by 2x within 5-10 seconds.
• Inhibition of nervous cardiovascular stimulation can decrease arterial pressure to half-normal within 10-40 seconds.
• Rise in pressure occurs during muscle exercise because of increased blood flow due to local vasodilation of muscle vasculature.
• Exercise causes incresed arterial pressue, brain motor areas activate increasing reticular systems stimulation of vasoconstriction and cardio-acceleratory areas of vasomotor center
• Alarm reaction provides elevated arterial pressure can supply blood to the muscles needed to escape dangers.

Reflex Mechanisms for Normal Arterial Pressure

• Subconscious nervous control mechanisms that maintains arterial pressure near normal through negative feedback reflex mechanisms
• Baroreceptor reflex is initiated by stretch receptors (baroreceptors/pressoreceptors) in the walls of systemic arteries; arterial pressure rises, the baroreceptors stretch and transmit signals into the CNS and sends feedback through the autonomic nervous system to circulation to reduce arterial pressure toward/at the normal level

Baroreceptor location and function

• Found in the walls of arteries, spray-type nerve endings are stimulated when stretched.
• Extremely abundant in:
  • The wall of each internal carotid artery, slightly above the carotid bifurcation, in the carotid sinus
  • The wall of the aortic arch
• Signals from the carotid baroreceptors transmits through small Hering’s nerves to the glossopharyngeal nerves in the high neck, and then to the nucleus tractus solitarius in the medullary area of the brain stem.
• Signals from the aortic baroreceptors are transmitted through the vagus nerves to the nucleus tractus solitarius of the medulla
• Carotid sinus baroreceptors are not stimulated at pressures between 0 - 60 mm Hg, operate progressively more rapidly at 180 mm Hg.
• Aortic baroreceptors similar except they operate at arterial pressure levels about 30 mm Hg higher.
• In the normal operating range of arterial pressure, a slight change in pressure causes a response to readjust arterial pressure back to normal; most effective in the pressure where it is most needed
• Baroreceptors respond rapidly to arterial pressure changes; rate of impulse increases during each systole and decreases during diastole.
• Baroreceptors respond more to rapidly changing pressure more than to stationary pressure, therefore it may be twice as much for rising pressure than stationary pressure

Baroreceptor Signals

• After signals enter the nucleus tractus solitarius of the medulla, secondary signals inhibit the vasoconstrictor center of the medulla and excite the vagal parasympathetic center. The net effects include:
• Vasodilation of the veins and arterioles
• Decreased heart rate and strength of heart contraction
• Excitation of the baroreceptors by high pressure in arteries, causes arterial pressure to decrease due to decrease in peripheral resistance and cardiac output, conversely, low pressure causes pressure to rise back toward normal.
• Removal of occlusion allows pressure in the carotid sinuses to rise, in which the carotid sinus reflex drops the aortic pressure almost immediately, but then returns to normal in another minute
• Baroreceptors attenuate blood pressure changes during changes in body posture to maintain relatively constant arterial pressure

Pressure buffer system

• The baroreceptor system opposes increases or decreases in arterial pressure
• Baroreceptors reduce moment to moment variation in arterial pressure to about ⅓ what would occur without the system
• Baroreceptors tend to reset in 1–2 days to pressure level
• May attenuate their potency as a control system for longer changes

Arterial regulation

• May mediate decreases in nerves that promote greater excretion of sodium and water
• Chronic electrical stimulation of carotid sinus nerve reduces sympathetic nervous system activity & arterial pressure
• Resetting might be due to resetting of sinus nerve mechanoreceptors
• Chemoreceptor reflex operates to maintain arterial pressure, but uses chemoreceptors instead of stretch receptors
• Chemoreceptors sensitive to low oxygen or elevated carbon dioxide and hydrogen ion levels, located in chemoreceptor organs
• Transmit signals from the chemoreceptors to excite the vasomotor center and elevates pressure
• Arterial pressure falls below 80 mm Hg this reflex becomes important to prevent decreases in pressure
• Chemoreceptors also contribute to pressures in severe conditions that have breathing cessation.

Atrial and pulmonary artery reflexes

• Low-pressure, Stretch receptors in walls that is similar to baroreceptor functions
• Plays a role in minimizing artery pressure changes
• Increase pressure and elicits reflexes equal to the baroreceptor function
• Stretch of atria causes reduction in sympathetic nerve activity and dilation.

Hypothalamus signals

• Decreased afferent arteriolar resistance, increases pressure, decreases filtration and anti-diuretic hormone.
• Volume reflex returns blood closer to normal blood by filtration and hormone.
• Volume drives the heart output and the volume reflex is a mechanism for volume control.
• A Bainbridge reflex triggers when the atrial pressure is increased raising the heart rate
• Bainbridge is overridden to cause volume balance
• Signals back through vagal nerves increases stretch and heart contractions
• Reflex prevents the veining from damming the pulmonary and atrial circulation

Decreased blood flow (CNS Ischemic response)

• Most nervous control of blood is reflexes by chemo/ pressure receptors;
• Vasomotor center responds to ischemia by exciting neurons which raises BP as high as heart can pump
• Failure of blood to carry carbon dioxide causes the Ischemic response which raises BP
• High as 250 mm/Hg for 10 min.
• The CNS Ischemic response elevates pressure and sometimes the renal production of urine stops
• Pressure will maintain BP, but functions as an emergency system.
• Cushing reaction is a special type of ischemic response triggered by pressure that compresses the brain
• Increased cerebrospinal fluid causes ischemia and initiates pressure response. The Cushing reaction protects centers of lose nutrients.

Abdominal Compression Reflex

• Used when baroreceptor or chemoreceptor functions are triggered
• Compress venous and shift blood to the heart
• Abdominal compression reflex causes output and sympathetic impulses
• Paralyzed muscles are hypotension from lack of compression
• Skeletal muscle compression is an element of cardiac regulation
• Anticipation tightens muscles
• This compression translocates blood from the peripheral vessels into the heart and lungs and, therefore, increases cardiac output
• It is an essential component in cardiac output

Arterial Waves

• With each respiratory rate, artery will rise and fall normally, but deeper changes change factors also.
• Many pressure signals overspill cycles
  • Thoracic becomes negative from expansion
  • Which diminishes output
    • Thoracic vessels and the heart are triggered causing rates

Baro and Chemo Reflex oscillation

• Oscillations with delays are reflex oscillations of nerves or the pressure.
• High pressure excite bars
• Chemo plays wave when the pressure is low
• Ischemic comes from responses where fluid is overreactive until it resets
• Waves follow basic mechanical system like an auto for planes causing over steering instead of straight.