Introduction to the Nervous System and Cardiac Function
Now that we have an idea of how the bioelectrical energy is created in the heart and how the muscle contracts, let’s turn our attention to how the brain and the heart communicate to control the hemodynamic status of the body. This communication uses chemical messengers and receptors to transport the information back and forth between the various organs. Understanding how these chemical messengers and receptors work, and how we can manipulate them in treatment, is critical to our study of arrhythmias.
Anatomically, the nervous system is made up of the central nervous system (brain and spinal cord) and various types of peripheral nerves. The two major types of peripheral nerves we will be discussing are the afferent nerves (Latin: adferre, to bear), which carry sensory impulses from all parts of the body to the brain and efferent nerves (Latin: efferens, to bring out), which carry messages from the brain to the muscles and all other organs of the body.
Functionally, the nervous system is divided into two primary components: the central nervous system and the peripheral nervous system. The peripheral nervous system is further subdivided into several subdivisions, which we will discuss further later in this chapter (Figure 2-7).
Figure 2-7 The human nervous system is composed of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS receives its information about the outside world, and responds to that information, via the peripheral nervous system. The sensory nerves are responsible for sensing the environment. They transmit their data to the CNS (see green arrows) via the afferent portion of the PNS (cranial nerves and spinal nerves). The CNS then sends information back to the body via the efferent portion of the PNS (see blue arrows) to respond to the environment. That response could either effect muscle movement or it can invoke the autonomic nervous system (ANS). The ANS controls the “fight or flight” response and all of our internal organs.
The CNS consists of the brain and spinal cord and functions as the control center for all other nervous system function. One could easily think of the CNS like a CPU (central processing unit) in a home computer. The CPU in a computer carries out all calculations, coordinating all incoming and outgoing information through various cables and connections. The CNS receives input from various receptors throughout the body, interprets the stimulus received via these sensory neurons, makes decisions, and directs actions to be carried out. These messages are then sent back out to the body via motor neurons or effectors, which carry out the desired action in various muscles and glands throughout the body. Computers work in a very similar manner: The CPU interprets data from the keyboard, mouse, disk drives, and so forth, and makes decisions and produces output actions such as printing. The whole system is connected by cables that function like the efferent and afferent nerves, sending and receiving information back and forth between the various components.
Peripheral Nervous System (PNS)
The PNS consists of all nervous tissue outside of the brain and spinal cord and is subdivided into two divisions, the somatic and autonomic nervous system. Of these two, we will spend the majority of our discussion on the autonomic nervous system, because this area has the greatest direct effect on cardiac function.
Autonomic Nervous System
The autonomic nervous system sends sensory impulses from internal structures (such as the blood vessels, the heart, and organs of the chest, abdomen, and pelvis) through afferent autonomic nerves to the brain. The responses to these stimuli are carried back to the organ systems by efferent autonomic nerves, which cause appropriate responses from the heart, vascular system, and other organs of the body to change the way they are functioning or behaving. We are rarely aware that this exchange of information is occurring, as these messages do not reach our consciousness, but cause a reflexive or automatic response.
The autonomic nervous system is considered to be “automatic” or involuntary because we cannot control or dictate functions under its control to happen. The autonomic nervous system is divided into two further subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These two systems have opposing effects and are constantly in a tug-of-war over control of the body (Figure 2-8). The sympathetic system causes a “speeding up” of the system while the parasympathetic “slows down” the system.
Figure 2-8 The sympathetic and parasympathetic nervous systems are in a constant tug-of-war, competing for final control over the body’s response to stimuli.
The heart, like most of our major organs and glands, receives both sympathetic and parasympathetic stimuli. The sympathetic stimulus increases the heart rate and contractility of the muscle, while the parasympathetic impulses decrease the rate. The constant tug-of-war between these two different sets of stimuli determines the final heart rate and contractility status.
Sympathetic Nervous System and the Heart. Also known as the “fight or flight” response, the sympathetic nervous system is dominant during periods of stress or activity. Control of the heart rate and force of contraction in response to stress is primarily under the control of the sympathetic nervous system. Sympathetic nerve fibers stimulate all parts of the atria and ventricles. Stimulation of the sympathetic nervous system causes the release of various chemical messengers, including epinephrine and norepinephrine (Figure 2-9).
Figure 2-9 Two nerve endings are ending in their neuromuscular junctions (area where the nerve ending and the muscles meet). The close-up shows some vesicles filled with chemical messengers or neurotransmitters, which are released from the nerve ending to the gap between the cells. The neurotransmitters cross the gap and attach to the receptors on the surface of the myocardial cell in order to affect it.
Epinephrine is the sympathetic nervous system’s primary chemical messenger that activates a specific type of receptor in the heart known as the beta-1 (ß1) adrenergic receptors. The results of epinephrine’s effect on the heart are an increased heart rate (positive chronotropic effect), increased conduction velocity (positive dromotropic effect), and finally an increased force of contraction in the ventricular muscle itself (positive inotropic effect). Epinephrine causes a shortening of phase 4 of the action potential, essentially speeding up the pacemaking action of the SA node and all of the other pacemaker cells as well.
Parasympathetic Nervous System and the Heart. Parasympathetic nerve fibers also stimulate the atria, ventricles and, especially, the SA and AV nodes. The main efferent pathway from the CNS to the heart occurs via the vagus nerve (cranial nerve 10). The primary neurotransmitter, or chemical messenger, of the parasympathetic nervous system is acetylcholine. Acetylcholine influences the system by slowing the rate of depolarization, essentially making cardiac cells less excitable.
In the SA node, the slowing qualities of acetylcholine decrease the rate of firing for the pacemaking cells, which in turn reduces the heart rate. Another effect of acetylcholine is to decrease the conduction velocity through the AV node, which can create a temporal separation between the contraction of the atria and ventricles. Sometimes the effect on the AV node can actually cause a complete block, essentially shutting down all communication between the atria and the ventricles.
In closing, this discussion was intended as a brief introduction into the autonomic nervous system and its control over the heart. As you can see, the effect of the autonomic nervous system on arrhythmia generation and management strategies is extensive. However, for our purposes, this brief discussion should suffice. Further discussion of the various chemical messengers and receptors is more appropriate for a textbook on pharmacology or an advanced text on arrhythmias.
Additional Information
Sympathetic Nervous System
Adrenergic: Relating to nerve fibers that release norepinephrine or epinephrine.
Sympathomimetic: Effects resembling those caused by stimulation of the sympathetic nervous system, such as the effects seen following the injection of epinephrine into a patient.
Sympatholytic: Interfering with or inhibiting the effect of the impulses from the sympathetic nervous system.
An easy way to remember these two terms:
When you mimic something, you imitate it (sympathomimetic).
When you lyse it, you dissolve it (sympatholytic).
Additional Information
Parasympathetic Nervous System
Parasympathomimetic: Effects resembling those caused by stimulation of the parasympathetic nervous system.
Parasympatholytic: Interfering with or inhibiting impulses from the parasympathetic nervous system.
Cholinergic: Liberating acetylcholine or an agent that produces the effects of acetylcholine.
