by Gerald Epling
Take a look at the signal that drives a normal human heart. I’ve made a recording and you can see it in figure 1, just down the page. Notice that the signal changes over time. Here is what’s happening. When we inhale, the signal becomes smaller. As we exhale, the signal driving the heart becomes stronger and taller. The strength of each heartbeat changes as we breath. In Figure 1, you can see the heart-driving signal grow taller on the second beat, and then shorter as time moves from the beginning, on the left, to four seconds later, on the right. Notice the first upward or positive-going spike is a little shorter than the second positive-going spike. The first two spikes occurred as the person was exhaling. The next several spikes were recorded as the person breathed in fresh air. Less oxygen in the blood makes the heart work harder. More oxygen makes the job of the heart easier. This is why the heart control signal gets taller when we breath out and shorter as we inhale.
Figure 1. Signals that control the heartbeat.
The brain needs the oxygen, nourishment, and hormones that come along with normal blood flow. Adrenaline is a hormone that circulates in the blood stimulating heart activity. Adrenaline is released from the adrenal glands, which sit atop the kidneys. Adrenaline is very similar to the brain stimulating neurotransmitter epinephrine. Epinephrine is produced in the brain when norepinephrine is released in various parts of the brain. Norepinehrine is rapidly changed to epinephrine when released between brain cells.
From the Brain to the Heart
The brain controls the activity of the heart by passing signals through six identifiable nerve bundles that connect the brain to the heart. Five of these nerves are part of the sympathetic system and stimulate action in the heart to increase heart rate, increase the force of heart contraction, and balance the subtle rhythms of heart activation. The sixth nerve conducts signals that can decrease heart rate, reduce the force of heart contraction and otherwise mediate the stimulating effect of the other five sympathetic nerves.
Signals that travel in the nerves of the sympathetic system and contact the heart lead to the release of norepinephrine. Once released, norepinephrine is routinely modified to become epinephrine. Epinephrine is also called adrenaline. Signal receptors that are excited by norepinephrine and epinephrine are also indirectly stimulated by nicotine. Is it any wonder that people get hooked on tobacco?
In contrast with the sympathetic system, is the parasympathetic system. The vagus nerve is part of the parasympathetic system that carries signals from the base of the brain to the heart. This mediating system signals a reduction in heart rate, heart force, and conduction velocity. What would life be like if there was only stimulation and no modulation of heart activity?
The release of norepinephrine can be inhibited by signals that connect the parasympathetic vagus nerve to a sympathetic nerves. The hormone that the vagus nerve releases is acetylcholine (ACh). As a counterbalance, the release of ACh can be limited by the release of epinephrine on vagus nerve endings. This system allows for fine control of the heart activity without pouring increasing levels of competing neurotransmitters onto the receptor sites of the heart.
From the Heart to the Brain
Signals also travel from the heart back to the brain via the vagus nerve. Some of the signals from the heart stimulate a portion of the brain known as the amygdala. The amygdala supports memory formation and rapid, memory-based responses to whatever is near or crosses our mind.
The release of adrenaline into the bloodstream causes our pulse to quicken and memories to form. Adrenaline that stimulates the heart is also sensed by the vagus nerve and reported to the brain (Miyashita & Williams, 2006). When portions of the vagus nerve that send messages to the brain are stimulated, there is an increase in the release of norepinephrine in the amygdala (Hassert, Miyashita & Williams, 2004). When norepinephrine is released in the amygdala, memories are formed which last longer than memories that are not punctuated with excitement.
REFERENCES
Hassert, D. L., Miyashita, T., Williams, C. L.(2004). The Effects of Peripheral Vagal Nerve Stimulation at a Memory-Modulating Intensity on Norepinephrine Output in the Basolateral Amygdala. Behavioral Neuroscience 118: 79-88.
Miyashita T. & Williams C.L. (2006) Epinephrine administration increases neural impulses propagated along the vagus nerve: Role of peripheral beta-adrenergic receptors. Neurobiol Learn Mem. 85:116-24.