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 are shown in Figure 2.
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.