Tuesday, September 22, 2009

The Intelligent Heart

Some of the first modern psychophysiological researchers to examine the conversations between the heart and brain were John and Beatrice Lacey. During 20 years of research throughout the 1960s and ’70s, they observed that the heart communicates with the brain in ways that significantly affect how we perceive and react to the world.

A generation before the Laceys began their research, Walter Cannon had shown that changes in emotions are accompanied by predictable changes in heart rate, blood pressure, respiration and digestion. In Cannon’s view, when we are "aroused," the mobilizing part of the nervous system (sympathetic) energizes us for fight or flight, and in more quiescent moments, the calming part of the nervous system (parasympathetic) cools us down. In this view, it was assumed that the autonomic nervous system and all of the physiological responses moved in concert with the brain’s response to a given stimulus. Presumably, our inner systems tooled up together when we were aroused and simmered down together when we were at rest, and the brain was in control of the entire process.

The Laceys noticed that this simple model only partially matched actual physiological behavior. As their research evolved, they found that the heart seemed to have its own peculiar logic that frequently diverged from the direction of the autonomic nervous system. The heart appeared to be sending meaningful messages to the brain that it not only understood, but obeyed. Even more intriguing was that it looked as though these messages could affect a person’s behavior. Shortly after this, neurophysiologists discovered a neural pathway and mechanism whereby input from the heart to the brain could "inhibit" or "facilitate" the brain’s electrical activity. Then in 1974, the French researchers Gahery and Vigier, working with cats, stimulated the vagus nerve (which carries many of the signals from the heart to the brain) and found that the brain’s electrical response was reduced to about half its normal rate. In summary, evidence suggested that the heart and nervous system were not simply following the brain’s directions, as Cannon had thought.

Neurocardiology: The Brain in the Heart

While the Laceys were doing their research in psychophysiology, a small group of cardiovascular researchers joined with a similar group of neurophysiologists to explore areas of mutual interest. This represented the beginning of the new discipline of neurocardiology, which has since provided critically important insights into the nervous system within the heart and how the brain and heart communicate with each other via the nervous system.

After extensive research, one of the early pioneers in neurocardiology, Dr. J. Andrew Armour, introduced the concept of a functional "heart brain" in 1991. His work revealed that the heart has a complex intrinsic nervous system that is sufficiently sophisticated to qualify as a "little brain" in its own right. The heart’s brain is an intricate network of several types of neurons, neurotransmitters, proteins and support cells like those found in the brain proper. Its elaborate circuitry enables it to act independently of the cranial brain – to learn, remember, and even feel and sense. The recent book Neurocardiology, edited by Dr. Armour and Dr. Jeffrey Ardell, provides a comprehensive overview of the function of the heart’s intrinsic nervous system and the role of central and peripheral autonomic neurons in the regulation of cardiac function. The nervous system pathways between the heart and brain.

The heart’s nervous system contains around 40,000 neurons, called sensory neurites, which detect circulating hormones and neurochemicals and sense heart rate and pressure information. Hormonal, chemical, rate and pressure information is translated into neurological impulses by the heart’s nervous system and sent from the heart to the brain through several afferent (flowing to the brain) pathways. It is also through these nerve pathways that pain signals and other feeling sensations are sent to the brain. These afferent nerve pathways enter the brain in an area called the medulla, located in the brain stem. The signals have a regulatory role over many of the autonomic nervous system signals that flow out of the brain to the heart, blood vessels and other glands and organs. However, they also cascade up into the higher centers of the brain, where they may influence perception, decision making and other cognitive processes.

Dr. Armour describes the brain and nervous system as a distributed parallel processing system consisting of separate but interacting groups of neuronal processing centers distributed throughout the body. The heart has its own intrinsic nervous system that operates and processes information independently of the brain or nervous system. This is what allows a heart transplant to work: Normally, the heart communicates with the brain via nerve fibers running through the vagus nerve and the spinal column. In a heart transplant, these nerve connections do not reconnect for an extended period of time, if at all; however, the transplanted heart is able to function in its new host through the capacity of its intact, intrinsic nervous system.

The Heart Brain

The intrinsic cardiac nervous system, or heart brain, is made up of complex ganglia, containing afferent (receiving) local circuit (interneurons) and efferent (transmitting) sympathetic and parasympathetic neurons. Multifunctional sensory neurites, which are distributed throughout the heart, are sensitive to many types of sensory input originating from within the heart itself. The intrinsic cardiac ganglia integrate messages from the brain and other processing centers throughout the body with information received from the cardiac sensory neurites. Once information has been processed by the heart’s intrinsic neurons, the appropriate signals are sent to the sinoatrial and atrioventricular nodes as well as the muscles in the heart. Thus, under normal physiological conditions, the heart’s intrinsic nervous system plays an important role in much of the routine control of cardiac function, independent of the central nervous system. Dr. Armour and his colleagues have shown that the heart’s intrinsic nervous system is vital for the maintenance of cardiovascular stability and efficiency, and that without it, the heart cannot operate properly. Read More!

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