6.4 Nerves and Neurotransmitters in the Heart

Physician's Notebooks 6 - http://physiciansnotebook.blogspot.com - See Homepage 

4. Nerves & Neurotransmitters in the Heart, Update 04 Septr 2021

The Autonomic Nerves are either sympathetic or parasympathetic. They are "autonomic" because free of conscious control.

The Sympathetic Nerves transmit stimulatory signals to heart and blood tests and vessels. The nerve cells (neurons) in spinal cord and brain that control the sympathetic nerves normally send low-level signal, which is increased or decreased under varied influences (Reflex responses to Blood Pressure and Heart Rate, and a general positive response to stress; Cf. Professor Hans Selye, Hungarian-Canadian endocrinologist who first popularized the stress response and was on the Nobel Committee’s short list 17 times but never won its physio. & med. prize.) The overall sympathetic response has been termed fight or flight or “stress hormone response.” Each nerve fiber ends in a micro space - the end synapse - between nerve ending and heart muscle fibers. The sympathetic-nerve signal, a.k.a. adrenergic signal, causes liberation of epinephrine (adrenaline) or norepineprine from its nerve endings.

   Epinephrine gives beta-adrenergic effect: Increases heartbeat rate and force, and relaxes small size arteries widening them and increasing oxygen-giving blood to heart muscle. This may be useful and life-saving in primitive situation of fight or flight, and the strengthening of heartbeat is useful in a failing heart, but it can be harmful to an already injured heart because it increases heart work and oxygen demand, and can produce heart injury if continued at unrelieved high level.

   Norepinephrine affects alpha-adrenergic receptors to raise the arterial blood pressure (BP). Since chronic stress increases sympathetic nerve output and norepinephrine, the stress is an important cause of chronic high blood pressure and other bad effects classed under degenerative heart and brain disease.

   The sympathetic nerves have a closely related hormone-producing counterpart atop each kidney; it is the adrenal medulla gland. The cells of the adrenal medulla are modified sympathetic nerve end-organ that respond to stress by releasing the β adrenergic hormone epinephrine (Cf. the famous beta-blocker meds oppose this.). This is a general body effect because epinephrine of the adrenal glands is released into the blood directly in response to body stress and goes to every cell in body, while the other sympathetic nerves being controlled in the brain and being passed in nerve fibers, affect only the organs and tissues innervated by the nerves. So, stimulating the adrenal glands is like an explosion of hormone in the body fluids, while stimulating the other sympathetic nerves is more like pressing a button to send signal to one particular organ.

   Reviewing the difference in the effects of norepinephrine and epinephrine, the epinephrine dilates small arteries and, alone, will drop the Blood Pressure while norepinephrine constricts large and medium-size arteries and raises the BP.  In the actual fight or flight stress condition, the combination results in a balance of norepinephrine and epinephrine, moderately elevating the BP, making the heartbeat more forceful and fast, and dilating coronary arteries to assure more oxygen for the increased work demand.

   The beta-blocker medications (Inderal, metaprolol, et al.) block the β-adrenergic affect and their most important effect is slowing the heartbeat, giving heart muscle more time to recover from each contraction and lowering its oxygen demand. But some beta-blocker (propranolol) has a minor bad effect of causing small bronchus constriction in lungs, increasing airways resistance and worsening, or bringing on asthma attacks, and another bad effect is its moderate decrease in heart contraction force that may worsen heart failure. (Overall, beta-blocker is a very useful pill to prolong good heart function in old age; I have been using it for 10+ years without problem.)
Angiotensin-converting-enzyme inhibitors (ACE-I) are a class of medication used primarily for the treatment of high blood pressure and heart failure. They work by causing relaxation of blood vessels as well as a decrease in blood volume, which leads to good effect of lower blood pressure and healthy slow resting  decreased oxygen demand from the heart. The ACE-I is often used with a beta blocker as I have used ACE-I for the last 10+ years, to keep my BP healthy normal and resting heart healthy slow..

   Certain recreational street drugs have a pro-beta adrenergic stimulation effect and can provoke coronary spasm heart attack and cerebral blood vessel brain hemorrhage leading to brain stroke. These are cocaine, amphetamines, methylphenidate (Ritalin) and the phenylpropanolamine in many cold and cough medicines.

The Parasympathetic Nerves have an opposite effect to the Sympathetic (The para effect is referred to as rest and digest.) Their neurotransmitter is acetylcholine, which slows the heartbeat and decreases heart muscle power. It also lowers the blood pressure. Overall effect of parasympathetic nerve – slowing of heart and lowering of BP – should cause heart muscle to require and to receive less oxygen and nutrients. With healthy coronary arteries, the reduced oxygen requirement from the slowing heart and lowered BP should protect against the low oxygen supply as seen in coronary artery disease; but the weaker contraction of heart with parasympathetic dominance may not be good where a heart is failing for other reason.
   The parasympathetic system is less important to the Heart than the sympathetic system because not involved as much in reflex feedback. It has no equivalent to the sympathetic’s adrenal medulla epinephrine general body stress effect. The tenth Cranial Nerve (CN Roman X is famous for its name, “Vagus nerve”, which has given a name to the parasympathetic effects), i.e., HR slowing and BP decrease; vagolytic is the name for the anti-parasympathetic effect of the plant-derived atropine.

The opioid drugs (morphine et al.) have parasympathetic effect (narrowed pinpoint eye pupils, mild slowing breathing and bronchi narrowing).

As mentioned, the parasympathetic system has an anti-drug, atropine, from the herbal Belladonna. It blocks acetylcholine and speeds heart. It is a key treatment of heart stoppage (Cardiac arrest) against sudden death. And it blocks the bronchial-constricting effect of acetylcholine so may be part of treatment of asthma attack. Atropine makes for the wide-eyed look by blocking the eye-pupil-constricting effect of parasympathetic nerve, so having drops of atropine in the eye dilates the pupil, good for the eye doctor to have a look in, but dangerous for persons with glaucoma. Atropine is also the treatment to reverse the poisonous effects of many parasympathetic blocking poison gases like the infamous Sarin gas.

End of Chapter. To read next now, click 6.5 Tests of Heart