Sympathetic Effect on Heart Rate


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Summary Statement:  The sympathetic nervous system tends to accelerate 
the heart rate in response to fear.

First, let us examine the sympathetic nervous system in some detail.  The sympathetic 
nervous system consists primarily of two cords, beginning at the base of the brain and 
proceeding down both sides of the spinal column.  These are made up of nerve fibers and 
ganglia of nerve cell bodies.  The cord between the ganglia is a cable of nerve fibers, 
closely associated with the spinal cord.  Sympathetic nerves extend to all the vital 
organs, including the liver and pancreas, as well as INNERVATING THE HEART, stomach, 
intestines, blood vessels, the iris of the eye, sweat glands and the bladder. 

Let us next trace the path of the cardiac sympathetic fibers to the heart.  The cardiac 
sympathetic fibers originate in the INTERMEDIOLATERAL columns of the upper five or six 
thoracic and lower one or two cerviccal segments of the spinal cord.  The postganglionic 
cardiac sympathetic fibers approach the base of the heart along the adventitial surface 
of the great vessels, and upon reaching this base, these fibers are distributed to the 
various chambers as an extensive epicardial plexus.  They then penetrate the myocardium, 
usually along the coronary vessels.  

Two factors are responsible for the MORE GRADUAL ONSET of the heart rate RESPONSE to 
SYMPATHETIC STIMULATION -- the use of a secondary messenger system and the slower 
rate of release of the sympathetic neurotransmitters (norepinephrine).  

Some side facts.  First, a bilateral assymetry of effects from the left and right 
sympathetic fibers on the contractility of the heart, and therefore on the heart rate, 
probably exists in humans.  Second, the effects of sympathetic stimulation decay very 
gradually after the cessation of stimulation, in contrast of the abrupt termination of 
the response after vagal activity.  This is because most of the norepinephrine released 
during sympathetic activity is slowly taken up again by the nerve terminals, while much 
of the remainder is carried away by the bloodstream.  


Sample Problems:

1)  T/F: Sympathetic nerves are able to exert beat-by-beat control of the heartrate.
2)  What happens when the cardiac sympathetic and vagus nerves are both stimulated 
    intensely and at the same time?  



1)  False.  Heart rate responds more gradually to sympathetic activity than to 
    parasympathetic stimulation.  Enough acetylcholine can be released during 
    a brief period of intense vagal activity to stop the heartbeat entirely, but
    enough norepinephrine is released to change cardiac behavior by only a small bit.  
2)  The vagus, or parasympathetic, effects on heart rate will predominate.  Because 
    parasympathetic neurotransmitter (ACh) acts must faster than norepinephrine, very 
    soon after the onset of stimulation, ACh will have markedly suppressed the release
    of NE from the sympathetic nerves.  Therefore, the response to simultaneous 
    stimulation is often equivalent to the response to vagal stimulation alone.  


For more information on this topic, please refer to Berne & Levy , pp. 86-87, and for more depth, the entire Chp. 4 :)

Also, check out the following links that may be helpful:


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This page was written by Ethan Y. Chen, a student in this course. Email me!

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