The Autonomic Nervous System
Much of the routine work of the body, particularly of the circulatory and respiratory apparatus, of the viscera, and of the smooth muscles, is regulated by the general visceral efferent component of the nervous system, called by Langley the autonomic nervous system.
This visceral efferent component, in contrast with the somatic efferent, always requires two successive neurons to conduct an impulse from the central nervous system to an effector organ. Further, general visceral reflexes, for which the autonomic nervous system serves as the efferent pathway, are not under voluntary control and tend to be more diffuse and not as delicately coordinated as somatic reflexes.
The afferent neurons which communicate with the autonomic nervous system are not essentially different from those involved in somatic reflexes. Although some are visceral sensory, with their dendritic endings in internal organs, others are somatic sensory, coming from external sense organs. It is chiefly through their peripheral connections that these two types of sensory neurons may be distinguished, for both enter the central nervous system by way of the dorsal nerve-roots and both have their cell bodies in the sensory ganglia of these roots.
This similarity in the afferent components of somatic and visceral reflex arcs is not extended to their remaining elements. As previously described, in simple somatic spinal reflex arcs the sensory neuron synapses with an intermediate neuron which is entirely within the central nervous system. The intermediate neuron synapses with an efferent, or motor, neuron which has its cell body in the ventral horn of the gray matter. The neurite of the motor neuron, running out through the ventral root of the spinal nerve, extends, without interruption, to the striated muscle fibers which it innervates. Thus the efferent pathway of a simple somatic reflex arc consists of but one neuron (Fig. 659).
In the case of visceral spinal reflexes involving the autonomic nervous system, the visceral motor neuron, with which the sensory cell synapses without intervention of an intermediate neuron, has its cell-body in the lateral column of the gray matter. Its neurite, which leaves the spinal cord through the ventral nerve-root, does not reach an effector but terminates instead in an outlying autonomic ganglion. Because this cell leads to a ganglion it is known as a preganglionic neuron. The preganglionic cell synapses with a second efferent neuron which has its cell-body within the ganglion and sends its neurite to an effector, either an involuntary muscle or a gland. This second unit in the efferent pathway is called a postganglionic neuron. As shown by Langley at the turn of the century, autonomic path-ways probably always consist of these two successive neurons.
Preganglionic neurites are myelinated and covered with neurolemma, as are most fibers, both sensory and motor, in cranial and spinal nerves. Postganglionic neurites, on the other hand, are usually non-myelinated, having only the thin neurolemmal sheath.
In mammals the nerves which include preganglionic fibers fall into three groups, namely: cranial, thoracico-lumbar, and sacral. These are known as autonomic outflows, or divisions of the autonomic nervous system.
The cranial division consists of five nerves, the oculomotor, facial, glossopharyngeal, vagus, and spinal accessory, as shown in Tables XI and XII under the heading “general visceral efferent.” The thoracico-lumbar division includes the thoracic and first few lumbar nerves. According to Sheehan the most posterior nerve belonging to this group in man is the second lumbar (L2), while in the monkey it is L3 or L4 and in both cat and dog L4. The sacral division is made up of the third and fourth sacral nerves in man, while it consists of sacrals one, two and three in monkey, cat, and dog.
In the cranial and sacral divisions, the autonomic ganglia, in which are located the postganglionic cell-bodies, are close to or embedded in the organs which they innervate (Fig. 660). Being near to the termination of the pathway from the central nervous system to the viscera, they are known as terminal ganglia.
In the thoracico-lumbar division, some of the ganglia are terminal while others are nearer to the nerve cord. Many, lying ventro-lateral to the vertebral column on either side, are connected longitudinally into a pair of ganglionic chains. They are known as chain, or vertebral, ganglia. In addition there are collateral ganglia lying on the dorsal aorta at the points where the three major arteries to the digestive tract arise (Fig. 661). Thus there are three types of ganglia in which postganglionic cell bodies associated with this outflow may be located.
One of the characteristic features of the thoracico-lumbar outflow is the chain of vertebral ganglia, also known as the sympathetic trunk (Fig. 660). Each segment, beginning with the first cervical, is represented in the chain, but the eight ganglia of the cervical region have coalesced into three, known as the anterior, middle, and posterior cervical ganglia. Although subject to some variation, the anterior probably corresponds to the ganglia of the first four neck segments, the middle to the fifth and sixth, and the posterior to the seventh and eighth. Sometimes the first thoracic ganglion fuses with the posterior cervical to form the stellate ganglion.
To the sympathetic trunk preganglionic fibers are carried by the nerves which make up the thoracico-lumbar outflows (T1-L2, inclusive, in man). Each of these fibers divides into a number of branches in the trunk (Fig. 662). Usually one or more branches terminate in the ganglion belonging to the nerve through which the fiber leaves the cord, but others run anteriorly and posteriorly in the trunk and occasionally some cross to the trunk of the other side. As there is no cervical outflow, the cervical ganglia are supplied solely by fibers running forward from the thoracic region. Similarly the posterior lumbar and sacral ganglia are supplied bv preganglionic fibers which reach them by running posteriorly through the sympathetic trunk.
Between each thoracic ganglion and its spinal nerve is a ramus communicans, sometimes spoken of as the autonomic branch of the nerve. This ramus consists of two portions (Fig. 659). Through one part, known as the white communicating ramus because its fibers are myelinated, run preganglionic fibers carrying impulses from the cord to the various autonomic ganglia. Through the other portion, known as the gray communicating ramus because its fibers are unmyelinated, postganglionic neurites run back from the chain ganglia into the spinal nerve, through which they are distributed to sweat glands, arrector pili muscles, and the walls of blood vessels of the body wall and appendages.
As there is no cervical outflow, nerves of this region do not have white rami but they are equipped with gray rami which carry postganglionic fibers from corresponding cervical ganglia. Other postganglionic fibers from these ganglia run to the heart, the salivary glands, and the eye.
Posterior lumbar and some sacral nerves also have gray but no white rami as the thoracico-lumbar outflow ends in the mid-lumbar region.
As previously mentioned, most preganglionic fibers continue beyond the autonomic chain ganglion associated with the nerve through which they leave the spinal cord. Many run anteriorly or posteriorly to distribute to several chain ganglia. These longitudinal fibers, which connect the ganglia together into a chain, form the rami intergangliones (Fig. 662). Other preganglionic fibers may run across to the sympathetic trunk of the opposite side, forming groups known as rami transversarii.
Many preganglionic fibers, continuing through the chain ganglia without interruption, extend to collateral or to terminal ganglia. The largest collateral ganglia are the coeliae, anterior mesenteric, and posterior mesenteric, so called because they lie near the points where similarly named arteries originate from the dorsal aorta. The distribution of postganglionic fibers from these various ganglia is shown in Figure 660.
The visceral nerves, supplying the internal organs, include not only preganglionic and postganglionic fibers of the autonomic nervous system but also visceral sensory fibers which run from these organs to the spinal cord. These sensory fibers, which reach the spinal nerves by way of the white rami, extend from the internal organs to the spinal cord without interruption.
The fibers of the autonomic nervous system are prone to form plexuses by anastomosing together. Chief among these plexuses are the cardiac, coeliac, and hypogastric, the names of which indicate their associations. The largest of these is the coeliac, or solar, plexus in which lie the coeliac and anterior mesenteric ganglia. This plexus includes fibers associated with both the thoracico-lumbar outflow and the vagus nerve as well as visceral sensory fibers. From this plexus, fibers radiate to the stomach, small intestine, liver, pancreas, colon, spleen, kidneys, adrenal glands, gonads, and associated blood vessels. A “blow on the solar plexus,” it is obvious, is likely to have far-reaching repercussions quite as effective as the traditional “monkey wrench thrown into the machinery.”
Double Autonomic Supply
Most of the viscera have a double autonomic nerve supply, each organ being innervated by either the cranial or the sacral outflow in addition to the thoracico-lumbar (Fig. 660). Impulses transmitted to an organ by the thoracico-lumbar division produce effects that differ from those brought about through the activity of the cranial or sacral outflows. Further the cranial and sacral outflows resemble one another, but differ from the thoracico-lumbar, in their responses to certain drugs, for example, adrenalin, atropin, and pilocarpin. It is convenient, therefore, to group the cranial and sacral outflows together as the parasympathetic system, in contrast with the thoracico-lumbar division, or sympathetic system. Examples of this double innervation are the autonomic supplies to the eye, heart, and small intestine.
The parasympathetic innervation of the eye, through the oculomotor nerve, brings about reduction in the size of the pupil and accommodation for near vision. The sympathetic supply, by way of the anterior cervical ganglion, causes dilation of the pupil.
The heart is slowed down (inhibited) by impulses brought to it through the vagus nerve (parasympathetic) but its beat is accelerated and increased in force by sympathetic impulses, which for the most part pass through the stellate ganglion.
Intestinal peristalsis, on the other hand, is increased by the vagus and inhibited by its sympathetic nerves, which come mainly from the coeliac and anterior mesenteric collateral ganglia.
Comparative Anatomy of the Autonomic System
In amphioxus there is no involuntary nervous system, but in cyclostomes ganglia unconnected by means of interganglionic fibers are established in the body region. Elasmobranchs and perennibranchiate amphibians show an advance, in that a network of fibers connects some of the trunk ganglia with each other. In teleosts a thin trunk-line of interganglionic fibers is present with frequent transverse rami, while the Vth, VIIth, IXth, and Xth cranial nerves form a bulbar outflow that extends the involuntary system into the head. In anurans only the Xth nerve is involved in the parasympathetic part of the involuntary system. It is not until the amniotes are reached that the primary involuntary system of the trunk becomes definitely extended to both cranial and sacral regions.
In addition to this nervous regulation of visceral activities, there is considerable non-nervous control through hormones produced by endocrine glands, as when secretin from the intestinal wall stimulates the flow of the pancreatic juice from the pancreas.
Again the close connection between the autonomic system and hormonic control is shown by the fact that at the time when the neuroblasts, or embryonic neurons, migrate out from the thoracico-lumbar region to form the involuntary ganglia, they are accompanied by certain so-called chromaffine cells that have the power to secrete adrenalin. These cells eventually become localized in the suprarenal glands, where they produce adrenalin, a hormone which stimulates fibers of the sympathetic portion of the autonomic nervous system.
The will, or the exercise of the voluntary apparatus, can affect the action of the autonomic system under certain circumstances, as for example, when man, as contrasted with animals, sets up definite arbitrary times for eating and sleeping. This is conscious control of a more or less unconscious performance. Inhibition of the unconscious machinery of digestion can be brought about indirectly by conscious worry originating in the realm of the voluntary nervous apparatus.