To read this article with accompanying illustrations, as well as all the articles in the series, you can go to the mini-site: "Aging of Your Heart and Blood Vessels is Risky" by clicking here.

Introduction

Reality is far more incredible than fiction! The communication between the human brain and the heart is a striking example. This article will show you how the brain communicates with the heart through the nervous system. And, we will discuss how, with aging, some of the messages get weakened. Thus, the older heart no longer responds like the younger heart.

The Brain Talks to the Heart Differently Than to Other Muscles

To understand how brain to heart communication deteriorates with aging you first need to know how nervous stimulation from the brain to the heart, which is a muscular organ, is different than communication from the brain to other muscles in your body. Consider how most muscles work! For example, in order to function, the muscles in your arms and legs and elsewhere in your body, called skeletal muscles, require a connection from your brain to them by way of nerves.

Nerves are bundles of fibers interconnecting the central nervous system (the brain and spinal cord) with organs and other body parts. Nerves transmit either sensory stimuli (meaning those that results in sensations such as pain, heat, etc.) or motor impulses (those that result in movement of the muscles) from one part of the body to another. Now, consider what happens, if, for example, the area in your brain that provides stimuli to a leg muscle via nerve fibers is damaged by a stroke or other brain trauma, or if there is permanent damage to the interconnecting nerves anywhere along the route from the brain to the muscle. Essentially, all skeletal type muscle, which would normally be stimulated by way of this "destroyed" pathway, would atrophy (or waste away), as well as the nerve fibers themselves.

In contrast to skeletal muscle, heart muscle can continue to function even when nerve fibers from the brain to the heart are severed!
Does this sound like a scene from the X-Files, a Vincent Price Thriller or the plot for a new Stephen King Novel? Truth is stranger than fiction! The ability of the heart to beat after its nerves fibers are cut is one of these truths. The heart beats on in spite of severed nerves. The classic example of the ability of the heart to beat after its nerves have been severed is heart transplantation. Everyone knows that a heart can be removed from one person and transplanted into another. The process of removing the heart from an organ donor requires that all nerves connected to the donor's heart be severed. After being implanted into the recipient and stimulated, the heart will then beat without nerves. So, if the nerves connecting the brain to the heart are not essential for it to beat, why then, do they exist in the first place? The answer is that these nerves exist to function in fine-tuning the heart's action. They assist at determining how fast the heart beats and how hard the heart pumps. As one ages, brain-heart communication diminishes. As a result the older heart does not respond as it did at an earlier age.

To understand this aging effect let's first take a look at the part, or division, of the nervous system that functions in brain heart communication, and then discuss the role of the neurotransmitters, which are biochemical products of this division to see why brain-heart communication "withers with aging".

Jekyll and Hyde, or The Autonomic Nervous System

The nerves that link the brain to the heart are part of what is called the autonomic nervous system. The autonomic nervous system pathways connect the heart and other internal body organs to the brain. This system functions in an involuntary and reflexive manner. It directs activities of the body that do not require conscious control. You could think of it as allowing things to happen automatically. For example, in most cases, your intestines and your heart operate without you knowing it. You eat a hamburger without having to say, "O.K. stomach and intestines, start working now to digest this Big Mac". And when your favorite sports team makes the winning score in a crucial match, you don't need to tell your heart to beat faster, it just does.

The autonomic nervous system is made up of two divisions: sympathetic and parasympathetic. Autonomic nerve fibers originate from the brain and spinal cord and deliver impulses to your heart's pacemaker and other parts of the heart. They exert a substantial modulatory influence over how fast and how hard the heart pumps. These two divisions could be thought of as the Jekyll and Hyde of the Automonic Nervous System, because they have opposite actions on your heart. The sympathetic division signals both your heart's pacemaker to increase its firing rate and your heart's muscle cells to increase the strength of their contraction; and the parasympathetic division sends signals to slow down your heart rate. The sympathetic fibers, which increase the heart rate, are activated in times of stress or emergency situations, sometimes called "fight", or take "flight", situations.
The parasympathetic fibers slow the heart rate and allow us to "rest" and "digest".

Sympathetic or Parasympathetic Shifts in Nerve Traffic Related to Physical and Mental Stress

Previous articles in this series focused on cardiovascular reserve function, largely in the context of increased demands for blood flow to your body during the stress of exercise. Now we must look at what happens to the autonomic nervous system under similar conditions. During exercise stress (and mental stress as well), major shifts in nerve traffic occur within the sympathetic and parasympathetic autonomic nerves. In the basal state, meaning completely resting and lying down, parasympathetic input to your heart and blood vessels predominates over sympathetic regulation. But during graded degrees of stress, (sitting up, standing, walking, jogging), or performing during different gradations of exercise, impulses via the parasympathetic nerves wane and impulses via the sympathetic nerves increase. This shift in the type of autonomic nervous "tone" to your heart and blood vessels during stress occurs in a way that does not involve the thinking (cerebral input) or upper part of your brain (cerebrum). Rather, the shift occurs via a change in signals from the nerve body stations along your spinal cord and other nerve bodies within your lower brain. Hence, the graded shift in nerve traffic in response to graded stress occurs through an action called a reflex. You don't have to think about it to make it happen.

Another distinction between these two divisions is that they emerge from the central nervous system (brain or spinal column) from different points of origin. The sympathetic fibers arise from the middle portion of the spinal cord. The parasympathetic arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. Together, but in opposing fashion, these two divisions control the functions of the heart and circulatory system and other internal organs.

The Brain Also Talks to the Heart Using the Language of "Biochemistry"

The sympathetic nervous system, one of the two divisions of the autonomic nervous system, as previously noted, is sometimes referred to as the adrenergic nervous system (meaning having activity like that of adrenalin). This system has alpha and beta adrenergic components. Neurotransmitters, or signaling substances, called epinephrine and norepinephrine, activate the heart's beta adrenergic receptors. Autonomic nerve fibers as well as your adrenal gland release these neurotransmitters during exercise and other kinds of stress. These substances travel to the heart cells through the blood. They deliver the brain's message by binding with special receptors on the membranes of the heart cells, and set off a chain of molecular events within these cells that might end with a faster beating heart, stronger contractions, and faster relaxation between beats. Or, depending on what neurotransmitter is called upon, the autonomic nervous system can tell the heart to reverse all these effects and slow down.

We can define the essence of sympathetic nervous system influence on the heart and blood vessels by examining the results of the following study. The study compared a young person's cardiovascular performance during vigorous exercise (when full sympathetic nervous input occurs) with that measured during vigorous exercise in the presence of a drug that blocked (or substantially reduced) sympathetic signaling. The drug used is called a beta blocker, i.e. it blocks the beta adrenergic component of the autonomic system. You may have heard of this type of drug, as it is used in clinical medicine in the treatment of cardiovascular diseases (a future article will address this). Compared to the situation when the beta adrenergic stimulation was intact, in the presence of the beta-blocking drug the heart rate during vigorous exercise in the young volunteer did not increase as much, the heart size dilated, and the usual increase in ejection fraction was reduced. Does this pattern sound familiar to you? It should if you have been following this Series, "Aging of Your Heart and Blood Vessels is Risky".

The effects of the beta blocking drug on the young heart:

1. acute cardiac enlargement during exercise
2. diminished increase in heart rate and
3. reduced ejection fraction

are the characteristics of the exercise response of older persons compared to younger persons. This response was discussed in Article 3 "How Good a Pump is Your Older Heart?" In essence, by blocking the beta adrenergic system of the young volunteer the investigators of this study converted the cardiovascular performance profile of a young person into one of an older person! Thus, the essence of the beta adrenergic modulation of heart function during exercise is to make the heart beat faster and stronger and to keep its size small. A young heart normally responds in this way to vigorous exercise. However, aging, even in otherwise healthy persons, is accompanied by a reduction in the effectiveness of the beta adrenergic nerve influence on the heart. In other words, the beta adrenergic signaling which acts in young hearts to accommodate vigorous exercise tends to falter with age, even in normal healthy people. Why is this? The reduced beta adrenergic influence on the older heart during exercise could be attributed to a reduction in the production of the neurotransmitters (norepinephrine and epinephrine), resulting in reduced delivery of these signaling substances to "docking sites" on the heart and blood vessel cells. (Docking sites, called receptors, are areas on cells where specific substances, like biochemicals, can be accepted, absorbed or passed through.) Alternatively, the age-associated deficit could be due to a reduced response to these substances by the older heart and blood vessel cell's docking sites. Which is it? The answer is that it is a reduced response to the neurotransmitters by the docking sites on the heart and blood vessel's cells. Is this normal human aging? What do scientists know about this? Can anything be done to prevent or delay this age related process? We will discuss some of these issues in our next article.

Dr. Ed is a physician/scientist, who is internationally recognized for studies that range from humans to molecules on how the heart and blood vessels work in health and disease as the body ages.