S25
Reflex Control of the Circulation during Exercise Loring B. Rowell Department of Physiology and Biophysics and of Medicine, University of Washington School of Medicine, Seattle, WA 98195, U. S. A. Seattle, WA98195, U.S.
Abstract Rowell, Reflex LoringB. Rowe!!, Reflex Control Control of of the the CircuCirculation during Exercise. Tnt J Sports Med, Vol 13, Suppi Suppl 1, pp S25 —S27, 1992. —S27,1992.
cisc. Activation is in direct proportion to the number of motor cise. units required to maintain a given force of contraction. Central command appears to increase heart rate (HR), cardiac output, and also blood pressure (BP) immediately at the onset of exer-
cise by rapid vagal withdrawal, but command signals have little direct influence on sympathetic nervous activity (SNA)
higher blood pressure (BP). A new hypothesis is that central command works by resetting the baroreflex to a higher BP and withdraws vagal activity to raise heart rate, cardiac output and BP at the onset of exercise. The key to the hypothesis is that the rise in cardiac output at exercise onset must be fast enough to raise BP to its new reset level immediately,
otherwise a BP error occurs that must be corrected by baroreflex and SNA. Key words
The key question is: what causes the increase in
SNA and why does it begin at such low rates of exercise (shown in Fig. 1)? Do the reflexes initiated by exercise correct mismatches between blood flow and metabolism, called blood
"flow errors", or mismatches between cardiac output and vascular conductance, called "BP errors"? B. Muscle chemoreflexes
These reflexes are elicited from chemosensitive afferent fibers in the muscle whenever muscle blood flow (MBF) falls below the critical level needed to maintain adequate oxygen transport to the muscle (7). Release of lactic acid from the muscle appears to signal the onset of the reflex (7)
(additional references in 4). Figure 1 shows that SNA increases well before lactic acid is released into the circulation
(although SNA and lactate release are closely associated during isometric contractions — this does not hold during dynamic namic exercise) (4). In voluntarily exercising dogs dogs the the muscle muscle chemoreflex has a distinct threshold that is not reached reached until until
MBF is artificially reduced (partial occlusion) to a point at
Baroreflex, arterial blood pressure, exercise ci
100
I. What Signals Control the Circulation during Exercise?____________________
One general hypothesis is that centrally generated motor command signals set the basic patterns of effector activity, and this activity is modulated by arterial baroreflexes, muscle chemoreflexes, chemoreflexes, and and muscle muscle mechanoreflexes mechanoreflexesas as appropriate error signals develop (2).
A. Central command Central command is the term for motor command signals originating from subthalamic neurons involved
in locomotion. These signals signals activate activate in in parallel parallel both both carcar diovascular and skeletal muscle motor systems during exermt. J. Sports Med.13(1992)525—S27 l3(1992)S25 —S27 IntJ.SportsMed. GeorgThieme Verlag Stuttgart New York
120 20
60
HEORT HEART RATE (beats miS1) mind)
Fig. 1 Summary of human sympathetic responses to dynamic exercise (4). Total SNA (arrow) and muscle SNA (MSNA), plasma NE, and plasma renin activity (PRA) rise when HR approaches 100 beats min - when vagal withdrawal is complete. SNA causes decreased renal and splanchnic blood flow (RBF and SBF). Dashed line (HLa) shows rise in lactate at much higher HR.
Downloaded by: East Carolina University. Copyrighted material.
(see ref. 6).
Current theory is that circulatory control in exercise is governed by central command which sets basic patterns of effector activity that is modulated by arterial baroreflexes and chemo- and mechanoreflexes from active muscle. Because central command acts on vagal activity rather than sympathetic nerve activity (SNA), and because muscle chemoreflexes are not normally active during mild to moderate dynamic exercise, current theory cannot explain why SNA to virtually all organs, including active muscle, increases even during mild exercise. Are arterial baroreflexes involved? Baroreflex sensitivity is maintained during exercise, and most importantly, the reflex is reset to
S26 mt. .1. J. Sports Sports Med. Med. 13 (1992)
/
___( i
Loring B. Rowe!! Rowell
Skelelo IMaids Muscle SiIsIaI
-Feedback Contro e
Motor System
___
Central Cardiovascular Feed-Forward Control System [Command
a
Operating Paint
-
Aulonomic Pathways
Arterial Arleriat
Heort and L!L!Isuce Vosculature(
Borareceplors Respiratory Respiolory System
illustration of of an an hypothesis hypothesis that that central centralcommand, command,aafeed-forward feed-forwardsignal, signal,could couldexert exertitsitseffects effectson onthe theheart heartand andvascula vascula Fig. 2 Schematic illustration ture during exercise by resetting or changing the operating point of the arterial baroreflex. The baroreflex then becomes the primary controller of autonomic pathways (sympathetic nervous system). (Modified from Houk [1]).
a large margin for flow error and must therefore not be tonically active. Although the reflex appears much easier to elicit in humans than in dogs (human muscles have relatively low oxidative capacity), there is no evidence of tonic activity suggesting it causes the rise in SNA (5). In contrast, the muscle chemoreflex could explain part of the increase in SNA during severe exercise; when MBF becomes marginally adequate; muscle oxygen extraction is nearly complete; and lactic acid is produced within the muscle. This is not yet established.
C. Arterial baroreflex Despite earlier disagreement, it is now clear that the arterial baroreflex operates with a sensitivity during exercise that is about the same as that during rest. It is also established that the arterial baroreflex is reset to a higher BP; the rise is in proportion to the intensity of exercise (references in 4). This means that, in theory, cardiovascular control during exercise could be explained by continuous corrections of BP error signals. That is, small disparities between the prevailing BP and that BP sought by the central nervous system as a new
"set point" or operating point could explain all the cardiovascular responses to exercise. This hypothesis has two I) so problems: 1) so far far we we have have no no way way of of knowing knowing what what BP BP is is at at the baroreflex operating point, and 2) until we know what resets the baroreflex, the nature of the "exercise stimulus" or "exercise reflex" is still undefined. The next section puts forward an hypothesis.
The second part of the hypothesis is that the speed with which cardiac output rises at the onset of exercise determines whether there is a BP error signal, i. e., a discre-
pancy between the actual BP and the baroreflex operating point. For example when exercise is mild (requiring an HR of
Reflex Control of the Circulation during Exercise Loring B. Rowell Department of Physiology and Biophysics and of Medicine, University of Washington School of Medicine, Seattle, WA 98195, U. S. A. Seattle, WA98195, U.S.
Abstract Rowell, Reflex LoringB. Rowe!!, Reflex Control Control of of the the CircuCirculation during Exercise. Tnt J Sports Med, Vol 13, Suppi Suppl 1, pp S25 —S27, 1992. —S27,1992.
cisc. Activation is in direct proportion to the number of motor cise. units required to maintain a given force of contraction. Central command appears to increase heart rate (HR), cardiac output, and also blood pressure (BP) immediately at the onset of exer-
cise by rapid vagal withdrawal, but command signals have little direct influence on sympathetic nervous activity (SNA)
higher blood pressure (BP). A new hypothesis is that central command works by resetting the baroreflex to a higher BP and withdraws vagal activity to raise heart rate, cardiac output and BP at the onset of exercise. The key to the hypothesis is that the rise in cardiac output at exercise onset must be fast enough to raise BP to its new reset level immediately,
otherwise a BP error occurs that must be corrected by baroreflex and SNA. Key words
The key question is: what causes the increase in
SNA and why does it begin at such low rates of exercise (shown in Fig. 1)? Do the reflexes initiated by exercise correct mismatches between blood flow and metabolism, called blood
"flow errors", or mismatches between cardiac output and vascular conductance, called "BP errors"? B. Muscle chemoreflexes
These reflexes are elicited from chemosensitive afferent fibers in the muscle whenever muscle blood flow (MBF) falls below the critical level needed to maintain adequate oxygen transport to the muscle (7). Release of lactic acid from the muscle appears to signal the onset of the reflex (7)
(additional references in 4). Figure 1 shows that SNA increases well before lactic acid is released into the circulation
(although SNA and lactate release are closely associated during isometric contractions — this does not hold during dynamic namic exercise) (4). In voluntarily exercising dogs dogs the the muscle muscle chemoreflex has a distinct threshold that is not reached reached until until
MBF is artificially reduced (partial occlusion) to a point at
Baroreflex, arterial blood pressure, exercise ci
100
I. What Signals Control the Circulation during Exercise?____________________
One general hypothesis is that centrally generated motor command signals set the basic patterns of effector activity, and this activity is modulated by arterial baroreflexes, muscle chemoreflexes, chemoreflexes, and and muscle muscle mechanoreflexes mechanoreflexesas as appropriate error signals develop (2).
A. Central command Central command is the term for motor command signals originating from subthalamic neurons involved
in locomotion. These signals signals activate activate in in parallel parallel both both carcar diovascular and skeletal muscle motor systems during exermt. J. Sports Med.13(1992)525—S27 l3(1992)S25 —S27 IntJ.SportsMed. GeorgThieme Verlag Stuttgart New York
120 20
60
HEORT HEART RATE (beats miS1) mind)
Fig. 1 Summary of human sympathetic responses to dynamic exercise (4). Total SNA (arrow) and muscle SNA (MSNA), plasma NE, and plasma renin activity (PRA) rise when HR approaches 100 beats min - when vagal withdrawal is complete. SNA causes decreased renal and splanchnic blood flow (RBF and SBF). Dashed line (HLa) shows rise in lactate at much higher HR.
Downloaded by: East Carolina University. Copyrighted material.
(see ref. 6).
Current theory is that circulatory control in exercise is governed by central command which sets basic patterns of effector activity that is modulated by arterial baroreflexes and chemo- and mechanoreflexes from active muscle. Because central command acts on vagal activity rather than sympathetic nerve activity (SNA), and because muscle chemoreflexes are not normally active during mild to moderate dynamic exercise, current theory cannot explain why SNA to virtually all organs, including active muscle, increases even during mild exercise. Are arterial baroreflexes involved? Baroreflex sensitivity is maintained during exercise, and most importantly, the reflex is reset to
S26 mt. .1. J. Sports Sports Med. Med. 13 (1992)
/
___( i
Loring B. Rowe!! Rowell
Skelelo IMaids Muscle SiIsIaI
-Feedback Contro e
Motor System
___
Central Cardiovascular Feed-Forward Control System [Command
a
Operating Paint
-
Aulonomic Pathways
Arterial Arleriat
Heort and L!L!Isuce Vosculature(
Borareceplors Respiratory Respiolory System
illustration of of an an hypothesis hypothesis that that central centralcommand, command,aafeed-forward feed-forwardsignal, signal,could couldexert exertitsitseffects effectson onthe theheart heartand andvascula vascula Fig. 2 Schematic illustration ture during exercise by resetting or changing the operating point of the arterial baroreflex. The baroreflex then becomes the primary controller of autonomic pathways (sympathetic nervous system). (Modified from Houk [1]).
a large margin for flow error and must therefore not be tonically active. Although the reflex appears much easier to elicit in humans than in dogs (human muscles have relatively low oxidative capacity), there is no evidence of tonic activity suggesting it causes the rise in SNA (5). In contrast, the muscle chemoreflex could explain part of the increase in SNA during severe exercise; when MBF becomes marginally adequate; muscle oxygen extraction is nearly complete; and lactic acid is produced within the muscle. This is not yet established.
C. Arterial baroreflex Despite earlier disagreement, it is now clear that the arterial baroreflex operates with a sensitivity during exercise that is about the same as that during rest. It is also established that the arterial baroreflex is reset to a higher BP; the rise is in proportion to the intensity of exercise (references in 4). This means that, in theory, cardiovascular control during exercise could be explained by continuous corrections of BP error signals. That is, small disparities between the prevailing BP and that BP sought by the central nervous system as a new
"set point" or operating point could explain all the cardiovascular responses to exercise. This hypothesis has two I) so problems: 1) so far far we we have have no no way way of of knowing knowing what what BP BP is is at at the baroreflex operating point, and 2) until we know what resets the baroreflex, the nature of the "exercise stimulus" or "exercise reflex" is still undefined. The next section puts forward an hypothesis.
The second part of the hypothesis is that the speed with which cardiac output rises at the onset of exercise determines whether there is a BP error signal, i. e., a discre-
pancy between the actual BP and the baroreflex operating point. For example when exercise is mild (requiring an HR of
