We have learned, however, that communication between the heart and brain actually is a dynamic, ongoing, two-way dialogue, with each organ continuously influencing the other’s function. Research has shown that the heart communicates to the brain in four major ways: neurologically (through the transmission of nerve impulses), biochemically (via hormones and neurotransmitters), biophysically (through pressure waves) and energetically (through electromagnetic field interactions). Communication along all these conduits significantly affects the brain’s activity. Moreover, our research shows that messages the heart sends to the brain also can affect performance.
Some of the first researchers in the field of psychophysiology to examine the interactions 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.
In physiologist and researcher Walter Bradford Cannon’s view, when we are aroused, the mobilizing part of the nervous system (sympathetic) energizes us for fight or flight, which is indicated by an increase in heart rate, and in more quiescent moments, the calming part of the nervous system (parasympathetic) calms us down and slows the heart rate. Cannon believed the autonomic nervous system and all of the related physiological responses moved in concert with the brain’s response to any given stimulus or challenge.
Presumably, all of our inner systems are activated together when we are aroused and calm down together when we are at rest and the brain is in control of the entire process. Cannon also introduced the concept of homeostasis. Since then, the study of physiology has been based on the principle that all cells, tissues and organs strive to maintain a static or constant steady-state condition. However, with the introduction of signal-processing technologies that can acquire continuous data over time from physiological processes such as heart rate (HR), blood pressure (BP) and nerve activity, it has become abundantly apparent that biological processes vary in complex and nonlinear ways, even during so-called steady-state conditions. These observations have led to the understanding that healthy, optimal function is a result of continuous, dynamic, bidirectional interactions among multiple neural, hormonal and mechanical control systems at both local and central levels. In concert, these dynamic and interconnected physiological and psychological regulatory systems are never truly at rest and are certainly never static.
For example, we now know that the normal resting rhythm of the heart is highly variable rather than monotonously regular, which was the widespread notion for many years. This will be discussed further in the section on heart rate variability (HRV).
Figure 1.1 Innervation of the major organs by the autonomic nervous system (ANS). Parasympathetic fibers are primarily in the vagus nerves, but some that regulate subdiaphragmatic organs travel through the spinal cord. The sympathetic fibers also travel through the spinal cord. A number of health problems can arise in part because of improper function of the ANS. Emotions can affect activity in both branches of the ANS. For example, anger causes increased sympathetic activity while many relaxation techniques increase parasympathetic activity.
The Laceys noticed that the model proposed by Cannon only partially matched actual physiological behavior. As their research evolved, they found that the heart in particular seemed to have its own logic that frequently diverged from the direction of autonomic nervous system activity. The heart was behaving as though it had a mind of its own. Furthermore, the heart appeared to be sending meaningful messages to the brain that the brain not only understood, but also obeyed. Even more intriguing was that it looked as though these messages could affect a person’s perceptions, behavior and performance. The Laceys identified 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, French researchers stimulated the vagus nerve (which carries many of the signals from the heart to the brain) in cats and found that the brain’s electrical response was reduced to about half its normal rate. This suggested that the heart and nervous system were not simply following the brain’s directions, as Cannon had thought. Rather, the autonomic nervous system and the communication between the heart and brain were much more complex, and the heart seemed to have its own type of logic and acted independently of the signals sent from the brain.
While the Laceys research focused on activity occurring within a single cardiac cycle, they also were able to confirm that cardiovascular activity influences perception and cognitive performance, but there were still some inconsistencies in the results. These inconsistencies were resolved in Germany by Velden and Wölk, who later demonstrated that cognitive performance fluctuated at a rhythm around 10 hertz throughout the cardiac cycle. They showed that the modulation of cortical function resulted from ascending cardiovascular inputs on neurons in the thalamus, which globally synchronizes cortical activity. An important aspect of their work was the finding that it is the pattern and stability of the heart’s rhythm of the afferent (ascending) inputs, rather than the number of neural bursts within the cardiac cycle, that are important in modulating thalamic activity, which in turn has global effects on brain function. There has since been a growing body of research indicating that afferent information processed by the intrinsic cardiac nervous system (heart-brain) can influence activity in the frontocortical areas and motor cortex, affecting psychological factors such as attention level, motivation, perceptual sensitivity and emotional processing.
Neurocardiology: The Brain On the Heart
While the Laceys were conducting their research in psychophysiology, a small group of cardiologists joined forces with a group of neurophysiologists and neuroanatomists to explore areas of mutual interest. This represented the beginning of the new discipline now called neurocardiology. One of their early findings is that the heart has a complex neural network that is sufficiently extensive to be characterized as a brain on the heart . The heart-brain, as it is commonly called, or intrinsic cardiac nervous system, is an intricate network of complex ganglia, neurotransmitters, proteins and support cells, the same as those of the brain in the head. The heart-brain’s neural circuitry enables it to act independently of the cranial brain to learn, remember, make decisions and even feel and sense. Descending activity from the brain in the head via the sympathetic and parasympathetic branches of the ANS is integrated into the heart’s intrinsic nervous system along with signals arising from sensory neurons in the heart that detect pressure, heart rate, heart rhythm and hormones.
The anatomy and functions of the intrinsic cardiac nervous system and its connections with the brain have been explored extensively by neurocardiologists. In terms of heart-brain communication, it is generally well-known that the efferent (descending) pathways in the autonomic nervous system are involved in the regulation of the heart. However, it is less appreciated that the majority of fibers in the vagus nerves are afferent (ascending) in nature. Furthermore, more of these ascending neural pathways are related to the heart (and cardiovascular system) than to any other organ. This means the heart sends more information to the brain than the brain sends to the heart. More recent research shows that the neural interactions between the heart and brain are more complex than previously thought. In addition, the intrinsic cardiac nervous system has both short-term and long-term memory functions and can operate independently of central neuronal command.