The autonomic nervous system (ANS), along with the endocrine system, coordinates the regulation and integration of bodily functions. The endocrine system sends signals to target tissues by varying the levels of blood-borne hormones. In contrast, the nervous system exerts its influence by the rapid transmission of electrical impulses over nerve fibers that terminate at effector cells, which specifically respond to the release of neuromediator substances.
Drugs that produce their primary therapeutic effect by mimicking or altering the functions of the autonomic nervous system are called autonomic drugs and are discussed in the following four chapters. These autonomic agents act either by stimulating portions of the autonomic nervous system or by blocking the action of the autonomic nerves. This chapter outlines the fundamental physiology of the ANS and describes the role of neurotransmitters in the communication between extracellular events and chemical changes within the cell.
Introduction to the Nervous System
The nervous system is divided into two anatomical divisions: the central nervous system (CNS), which is composed of the brain and spinal cord, and the peripheral nervous system, which includes neurons located outside the brain and spinal cord—that is, any nerves that enter or leave the CNS (Figure 3.1). The peripheral nervous system is subdivided into the efferent division, the neurons of which carry signals away from the brain and spinal cord to the peripheral tissues, and the afferent division, the neurons of which bring information from the periphery to the CNS. Afferent neurons provide sensory input to modulate the function of the efferent division through reflex arcs, or neural pathways that mediate a reflex action.
Figure 3.1. Organization of the nervous system.
A. Functional divisions within the nervous system
The efferent portion of the peripheral nervous system is further divided into two major functional subdivisions, the somatic and the autonomic systems (see Figure 3.1). The somatic efferent neurons are involved in the voluntary control of functions such as contraction of the skeletal muscles essential for locomotion. The autonomic system, conversely, regulates the everyday requirements of vital bodily functions without the conscious participation of the mind.
Because of the involuntary nature of the autonomic nervous system as well as its functions, it is also known as the visceral, vegetative, or involuntary nervous system. It is composed of efferent neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature, and the exocrine glands, thereby controlling digestion, cardiac output, blood flow, and glandular secretions.
B. Anatomy of the ANS
1. Efferent neurons
The ANS carries nerve impulses from the CNS to the effector organs by way of two types of efferent neurons (Figure 3.2). The first nerve cell is called a preganglionic neuron, and its cell body is located within the CNS. Preganglionic neurons emerge from the brainstem or spinal cord and make a synaptic connection in ganglia (an aggregation of nerve cell bodies located in the peripheral nervous system). These ganglia function as relay stations between a preganglionic neuron and a second nerve cell, the postganglionic neuron. The latter neuron has a cell body originating in the ganglion. It is generally nonmyelinated and terminates on effector organs, such as smooth muscles of the viscera, cardiac muscle, and the exocrine glands.
Figure 3.2. Efferent neurons of the autonomic nervous system.
2. Afferent neurons
The afferent neurons (fibers) of the ANS are important in the reflex regulation of this system (for example, by sensing pressure in the carotid sinus and aortic arch) and in signaling the CNS to influence the efferent branch of the system to respond.
3. Sympathetic neurons
The efferent ANS is divided into the sympathetic and the parasympathetic nervous systems as well as the enteric nervous system (see Figure 3.1). Anatomically, the sympathetic and the parasympathetic neurons originate in the CNS and emerge from two different spinal cord regions. The preganglionic neurons of the sympathetic system come from thoracic and lumbar regions (T1 to L2) of the spinal cord, and they synapse in two cordlike chains of ganglia that run close to and in parallel on each side of the spinal cord. The preganglionic neurons are short in comparison to the postganglionic ones. Axons of the postganglionic neuron extend from these ganglia to the tissues that they innervate and regulate (see Chapter 6).
The sympathetic nervous system is also called the thoracolumbar division because of its origins. In most cases, the preganglionic nerve endings of the sympathetic nervous system are highly branched, enabling one preganglionic neuron to interact with many postganglionic neurons. This arrangement enables this division to activate numerous effector organs at the same time. [Note: The adrenal medulla, like the sympathetic ganglia, receives preganglionic fibers from the sympathetic system. Lacking axons, the adrenal medulla, in response to stimulation by the ganglionic neurotransmitter acetylcholine, influences other organs by secreting the hormone epinephrine, also known as adrenaline, and lesser amounts of norepinephrine, into the blood.]
4. Parasympathetic neurons
The parasympathetic preganglionic fibers arise from cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus) as well as from the sacral region (S2 to S4) of the spinal cord and synapse in ganglia near or on the effector organs. [The vagus nerve accounts for 90% of preganglionic parasympathetic fibers in the body. Postganglionic neurons from this nerve innervate most of the organs in the thoracic and abdominal cavity.]
Due to the origin of the parasympathetic nervous system, it is also called the craniosacral division. Thus, in contrast to the sympathetic system, the preganglionic fibers are long, and the postganglionic ones are short, with the ganglia close to or within the organ innervated. In most instances there is a one-to-one connection between the preganglionic and postganglionic neurons, enabling the discrete response of this division.
5. Enteric neurons
The enteric nervous system is the third division of the ANS. It is a collection of nerve fibers that innervate the gastrointestinal (GI) tract, pancreas, and gallbladder, and it constitutes the “brain of the gut.” This system functions independently of the CNS and controls the motility, exocrine and endocrine secretions, and microcirculation of the GI tract. It is modulated by both the sympathetic and parasympathetic nervous systems.
C. Functions of the sympathetic nervous system
Although continually active to some degree (for example, in maintaining the tone of vascular beds), the sympathetic division has the property of adjusting in response to stressful situations, such as trauma, fear, hypoglycemia, cold, and exercise (Figure 3.3).
Figure 3.3. Actions of sympathetic and parasympathetic nervous systems on effector organs.
1. Effects of stimulation of the sympathetic division
The effect of sympathetic output is to increase heart rate and blood pressure, to mobilize energy stores of the body, and to increase blood flow to skeletal muscles and the heart while diverting flow from the skin and internal organs. Sympathetic stimulation results in dilation of the pupils and the bronchioles (see Figure 3.3). It also affects GI motility and the function of the bladder and sexual organs.
2. Fight or flight response
The changes experienced by the body during emergencies have been referred to as the “fight or flight” response (Figure 3.4). These reactions are triggered both by direct sympathetic activation of the effector organs and by stimulation of the adrenal medulla to release epinephrine and lesser amounts of norepinephrine. Hormones released by the adrenal medulla directly enter the bloodstream and promote responses in effector organs that contain adrenergic receptors (see Figure 6.6).
The sympathetic nervous system tends to function as a unit and often discharges as a complete system, for example, during severe exercise or in reactions to fear (see Figure 3.4). This system, with its diffuse distribution of postganglionic fibers, is involved in a wide array of physiologic activities. Although it is not essential for survival, it is nevertheless an important system that prepares the body to handle uncertain situations and unexpected stimuli.
Figure 3.4. Sympathetic and parasympathetic actions are elicited by different stimuli.
D. Functions of the parasympathetic nervous system
The parasympathetic division is involved with maintaining homeostasis within the body. To accomplish this, it maintains essential bodily functions, such as digestive processes and elimination of wastes. The parasympathetic division is required for life. It usually acts to oppose or balance the actions of the sympathetic division and is generally dominant over the sympathetic system in “rest and digest” situations. The parasympathetic system is not a functional entity as such and it never discharges as a complete system. If it did, it would produce massive, undesirable, and unpleasant symptoms, such as involuntary urination and defecation. Instead, discrete parasympathetic fibers are activated separately and the system functions to affect specific organs, such as the stomach or eye.
E. Role of the CNS in the control of autonomic functions
Although the ANS is a motor system, it does require sensory input from peripheral structures to provide information on the state of affairs in the body. This feedback is provided by streams of afferent impulses, originating in the viscera and other autonomically innervated structures that travel to integrating centers in the CNS, such as the hypothalamus, medulla oblongata, and spinal cord. These centers respond to the stimuli by sending out efferent reflex impulses via the ANS (Figure 3.5).
Figure 3.5. Baroreceptor reflex arc responds to a decrease in blood pressure.
1. Reflex arcs
Most of the afferent impulses are translated into reflex responses without involving consciousness. For example, a fall in blood pressure causes pressure-sensitive neurons (baroreceptors in the heart, vena cava, aortic arch, and carotid sinuses) to send fewer impulses to cardiovascular centers in the brain. This prompts a reflex response of increased sympathetic output to the heart and vasculature and decreased parasympathetic output to the heart, which results in a compensatory rise in blood pressure and tachycardia (see Figure 3.5). [Note: In each case, the reflex arcs of the ANS comprise a sensory (or afferent) arm, and a motor (or efferent, or effector) arm.]
2. Emotions and the ANS
Stimuli that evoke strong feelings, such as rage, fear, and pleasure, can modify the activities of the ANS.
F. Innervation by the ANS
1. Dual innervation
Most organs in the body are innervated by both divisions of the ANS. Thus, vagal parasympathetic innervation slows the heart rate, and sympathetic innervation increases the heart rate. Despite this dual innervation, one system usually predominates in controlling the activity of a given organ. For example, in the heart, the vagus nerve is the predominant factor for controlling rate. This type of antagonism is considered to be dynamic and is fine-tuned at any given time to control homeostatic organ functions. The activity of a system represents integration of influence of both divisions.
2. Organs receiving only sympathetic innervation
Although most tissues receive dual innervation, some effector organs, such as the adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system. The control of blood pressure is also mainly a sympathetic activity, with essentially no participation by the parasympathetic system.
G. Somatic nervous system
The efferent somatic nervous system differs from the autonomic system in that a single myelinated motor neuron, originating in the CNS, travels directly to skeletal muscle without the mediation of ganglia. As noted earlier, the somatic nervous system is under voluntary control, whereas the autonomic system is involuntary. Responses in the somatic division are generally faster than those in the ANS.
H. Summary of differences between sympathetic, parasympathetic, and motor nerves
Major differences in the anatomical arrangement of neurons lead to variations of the functions in each division (Figure 3.6). The sympathetic nervous system is widely distributed, innervating practically all effector systems in the body. In contrast, the parasympathetic division’s distribution is more limited. The sympathetic preganglionic fibers have a much broader influence than the parasympathetic fibers and synapse with a larger number of postganglionic fibers. This type of organization permits a diffuse discharge of the sympathetic nervous system. The parasympathetic division is more circumscribed, with mostly one-to-one interactions, and the ganglia are also close to, or within, organs they innervate.
This limits the amount of branching that can be done by this division. [A notable exception to this arrangement is found in the myenteric plexus, where one preganglionic neuron has been shown to interact with 8000 or more postganglionic fibers.] The anatomical arrangement of the parasympathetic system results in the distinct functions of this division. The somatic nervous system innervates skeletal muscles. One somatic motor neuron axon is highly branched, and each branch innervates a single muscle fiber. Thus, one somatic motor neuron may innervate 100 muscle fibers. This arrangement leads to the formation of a motor unit. The lack of ganglia and the myelination of the motor nerves enable a fast response by this (somatic nervous) system.
Figure 3.6. Characteristics of the sympathetic and parasympathetic nervous systems.