Re: how does the pulse rate work in a human body

Hello Khash!  Thank you for your question.

You asked:  "I want to know how and why the body and brain using the 
nervous system controls the heart rate."

The heart is basically one big muscle with one important job:  keep blood 
moving forward.  The brain needs blood, the rest of the body needs blood, 
and even the heart itself needs blood (there are blood vessels in the heart 
muscle that the heart pumps to).  Depending on what kind of situation your 
body is going through, be it eating or trying to get out of the way of a 
speeding car, your organs need a particular amount of blood in order to 
have oxygen for producing energy.  The brain therefore signals the heart 
accordingly.   

The autonomous nervous system of the body is responsible for maintaining 
balance in our body systems.  Two main divisions of this system, the 
sympathetic and the parasympathetic systems, both affect the heart.  Let's 
look at these briefly:

Parasympathetic Nervous System - (commonly known for inducing the "Rest and 
Digest" State).  These nerve fibers come from the very top and very bottom 
of the spine (collectively referred to as the craniosacral levels) and 
secrete acetylcholine.

Sympathetic Nervous System - (commonly known for inducing the "Fight or 
Flight" Response).  These nerve fibers come from the thoracic and lumbar 
regions of the spine and secrete norepinephrine.

These two systems work in cooperation throughout the day.  If I scarf a Big 
Mac, my parasympathetic system will kick in and innervate my stomach, 
bladder, and salivary glands to focus my body's energies on processing the 
food.  My heart rate will slow (since a fast heart rate during digestion is 
useless and would only waste energy that my digestive system could use).  
If, while I'm walking along eating my Big Mac, I accidentally step in front 
of a speeding car, my sympathetic system will suddenly kick in, and I'll 
attempt to run away.  OK, what does this have to do with the heart?  Well, 
the neat thing about the sympathetic nervous system is that it not only 
innervates the heart to increase the rate of signal conduction, but it also 
innervates a really cool organ, the Adrenal Medulla, which secretes 
hormones such as Epinephrine and Norepinephrine (epi. and norepi.) that do 
amazing things to the body.  That weird rush that I would get if I suddenly 
found myself in front of a speeding car is my sympathetic system 
activating.  The sensation is a combination of my blood vessels suddenly 
doing things that shifts the distribution of my blood around.  Here's what 
happens:

The sympathetic system directly affects the heart by increasing the rate of 
signal conduction.  This then leads to an increase in both the speed and 
strength of the heart beat (heart rate and contractility) to get more blood 
moving forward.  ***One note:  Your question dealt with heart rate 
specifically.  It seems logical that a heart beating faster would pump more 
blood forward, so that increasing the heart rate would be the best bet for 
helping the body.  Actually, a heart simply beating faster eats up more 
energy since it's using more oxygen to keep itself pumping frantically.  A 
more economical response by the brain is to signal the heart to increase 
the strength of each heart beat as well.  This is very important.  Now, a 
stronger heart beat can pump more blood forward per each beat, ultimately 
conserving energy.

The sympathetic system can also affect the heart by doing things to other 
body parts which then affect the heart.  Blood vessels in the skin and 
mucosa, for example, tighten up due to the effects of epi. and 
norepi.(you've seen this:  if I see a ghost in my basement, you'd see me 
turn pale --- that's because the vessels in my skin and lips suddenly 
constricted, so I lose my pinkish color and blanch as blood flow is 
reduced).  By doing this, less blood goes to these places, but more blood 
is saved for the heart, lungs, and brain.  Likewise, other blood vessels in 
the muscles and lungs widen (dilate) in order to contain more blood to 
transport more oxygen.  In total, your muscles, lungs, brain, and heart 
will be getting more oxygen from the blood by various vessel either 
constricting or dilating --- these organs are useful when you're trying 
frantically to scramble out of the way of that oncoming car...!

The heart rate is affected by numerous other factors.  Changes in blood 
pressures by changes in blood volume or by vessel dilation/constriction 
alter the stretch of certain arteries which ultimately leads to the brain 
speeding up or slowing down the heart rate as necessary.  Certain drugs and 
toxins can speed, slow, or even completely block signal conduction to those 
nerves in the heart, ultimately changing the heart rate.  One particular 
drug, Atropine, unbalances that sympathetic-parasympathetic balance by 
hiding the parasympathetic response so it ultimately looks like the body is 
only experiencing a sympathetic response.

Hope this helped to answer your question!  I kept my answer fairly general, 
but if you have further questions regarding the specific chemical processes 
involved with signal transduction in the brain and the heart, feel free to 
email me at monreal.1@osu.edu or submit another question to MadSci.

Sincerely, 
G. Monreal

For information available on the web, please consult the following 
excellent sites: http://paralia.com/athina/Heart/BeatRegulation.html http://sun.science.wayne.edu/~bio669/Cardio.html http://www.oucom.ohiou.edu/CVPhysiology/E010.htm