Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Functional Anatomy of the Peripheral Nervous System
Autonomic Nervous System

The autonomic (involuntary) Nervous system innervates Internal Organs, glands, Blood Vessels, and smooth Muscle, performing an adaptive and trophic function. Like the somatic nervous system, it operates through Reflexes. For example, when Stomach receptors are stimulated, impulses travel via the Vagus nerve to the organ, stimulating glandular secretion and enhancing motility. As a rule, autonomic reflexes are involuntary—that is, they occur automatically in response to specific stimuli. An individual cannot voluntarily accelerate or decelerate Heart rate, nor can they stimulate or suppress glandular secretion.

Similarly to a simple somatic reflex arc, an autonomic reflex arc consists of three Neurons. The Cell body of the first (sensory or receptor) neuron is located in the sensory ganglion of a spinal nerve or the corresponding sensory ganglion of a cranial nerve. The second neuron is an associative cell located in the autonomic nuclei of the Brainstem or Spinal Cord. The third neuron is an effector neuron, situated outside the Central Nervous System within paravertebral and prevertebral (sympathetic) or intramural and cranial (parasympathetic) ganglia. Thus, somatic and autonomic reflex arcs differ in the Location OF THE effector neuron: in the former, it lies within the central nervous system (motor nuclei of the anterior horns of the spinal cord or cranial nerve motor nuclei), whereas in the latter, it lies peripherally (in autonomic ganglia).

The Autonomic Nervous System is also characterized by a segmental type of innervation. Autonomic reflex centers have a specific localization within the central nervous system, and impulses to organs are transmitted via corresponding nerves. Complex autonomic reflexes are executed with the participation of suprasegmental structures, which are localized in the Hypothalamus, limbic system, reticular formation, Cerebellum, and Cerebral Cortex. The Role of each of these structures in the autonomic Innervation of the body is detailed in the respective sections.

Functionally, the autonomic nervous system is divided into sympathetic and parasympathetic divisions.

The sympathetic nervous system. The Sympathetic division of the autonomic nervous system comprises central and peripheral parts. The central part is represented by nuclei located in the lateral horns of the spinal cord, spanning from the 8th cervical to the 3rd lumbar segments. All fibers directed toward the sympathetic ganglia originate from the neurons of these nuclei. They emerge from the spinal cord as part of the anterior roots of the Spinal Nerves.

The peripheral part of the sympathetic nervous system includes ganglia and fibers located outside the central nervous system.

The Sympathetic trunk, truncus sympaticus, is a paired chain of paravertebral ganglia running parallel to THE Vertebral Column (Fig. 15.11). It extends from the Base of the Skull to the coccyx, where the right and left trunks converge and terminate in a single coccygeal ganglion. White rami communicantes containing preganglionic fibers pass from the spinal nerves to the ganglia of the sympathetic trunk. Their length typically does not exceed 1.0–1.5 cm. These branches are present only at the levels corresponding to spinal cord segments that contain sympathetic nuclei (8th cervical to 3rd lumbar). Fibers of the white rami communicantes either synapse on neurons in the respective ganglia or pass through them in transit to superior and inferior ganglia. Consequently, the number of sympathetic trunk ganglia (25–26) exceeds the number of white rami communicantes. Some fibers bypass the sympathetic trunk entirely and extend directly to the abdominal aortic plexus, forming the greater and lesser splanchnic nerves. Interganglionic branches run between adjacent ganglia of the sympathetic trunk, facilitating information exchange among its structures. Unmyelinated postganglionic fibers emerge from the ganglia as gray rami communicantes, which return to the spinal nerves, while the bulk of the fibers travel along major Arteries to supply target organs.

The greater and lesser splanchnic nerves pass in transit (without synapsing) through the 6th–9th and 10th–12th thoracic ganglia, respectively. They contribute to The formation of the abdominal aortic plexus.

According to the spinal cord segments, the sympathetic trunk is divided into cervical (3 ganglia), thoracic (12), lumbar (5), and sacral (5 ganglia) regions. The single coccygeal ganglion is usually rudimentary.

The superior cervical ganglion is the largest. Its branches primarily course along the external and internal carotid arteries, forming plexuses around them and providing sympathetic innervation to the HEAD and Neck structures.

The middle cervical ganglion is inconstant, lying at the level of the VI cervical vertebra. Its branches extend to The Heart, thyroid and Parathyroid glands, and neck vessels.

The cervicothoracic (stellate) ganglion lies at the level of the neck of the 1st rib, frequently fusing with the 1st thoracic ganglion, and has a stellate shape. Its branches innervate Organs of the anterior Mediastinum (including the heart), as well as the thyroid and parathyroid glands.

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Fig. 15.11. Autonomic nervous system (diagram):

1 — ciliary ganglion; 2 — pterygopalatine ganglion; 3 — sublingual and submandibular ganglia; 4 — otic ganglion; 5 — celiac plexus ganglia; 6 — pelvic splanchnic nerves

Branches originating from the thoracic region of the sympathetic trunk participate in forming the thoracic aortic plexus, which innervates thoracic organs. Additionally, this region gives rise to the greater and lesser splanchnic nerves, consisting of preganglionic fibers that pass in transit through ganglia 6 through 12. They pierce the Diaphragm into the Abdominal cavity AND terminate on the neurons of the celiac plexus.

The lumbar ganglia of the sympathetic trunk are interconnected not only by longitudinal branches but also by transverse interganglionic branches linking the right and left ganglia. Fibers from the lumbar ganglia join the abdominal aortic plexus, supplying sympathetic innervation to the abdominal wall and lower limbs along the course of blood vessels.

The pelvic portion of the sympathetic trunk consists of 5 sacral ganglia and a rudimentary coccygeal ganglion. The sacral ganglia are likewise interconnected by transverse branches. Nerves branching from them provide sympathetic innervation to the pelvic organs.

The abdominal aortic plexus, plexus aorticus abdominalis, is located in the abdominal cavity on the anterior and lateral surfaces of the Abdominal Aorta. It is the largest plexus of the autonomic nervous system, formed by several large sympathetic ganglia, incoming Branches of the greater and lesser splanchnic nerves, and numerous Nerve Trunks and branches extending from the ganglia. The primary ganglia of the abdominal aortic plexus include the paired celiac and aorticorenal ganglia, and the unpaired superior mesenteric ganglion. These typically give rise to postganglionic sympathetic fibers. Countless branches radiate outward from the celiac and superior mesenteric ganglia like rays of the sun, which explains its historical name—the "solar plexus" (plexus solaris).

Branches of the plexus extend onto arteries, forming secondary Autonomic plexuses of the abdomen (vascular autonomic plexuses) around the vessels. Unpaired plexuses include the celiac plexus (surrounding the celiac trunk), splenic plexus (splenic artery), hepatic plexus (proper hepatic artery), and superior and inferior mesenteric plexuses (along the corresponding arteries). Paired plexuses include the gastric, suprarenal, renal, and testicular (ovarian) plexuses, located around the Vessels of the respective organs. Following the course of the blood vessels, postganglionic sympathetic fibers reach internal organs to supply their innervation.

The superior hypogastric plexus is formed by branches of the abdominal aortic plexus. Shaped like a triangular flat plate, it lies on the anterior surface of the 5th lumbar vertebra, inferior to the aortic bifurcation. Inferiorly, the plexus emits fibers that contribute to the Formation of the inferior hypogastric plexus. The latter is situated superior to the levator ani muscle, near the branching point of the common iliac artery. Branches arising from these plexuses provide sympathetic innervation to the pelvic viscera:

The parasympathetic nervous system. The Parasympathetic division of the autonomic nervous system comprises central and peripheral parts. The central part is represented by the parasympathetic nuclei of cranial nerve pairs III, VII, IX, and X, as well as the sacral parasympathetic nuclei of the spinal cord. The peripheral part includes parasympathetic fibers and ganglia. Unlike those of the sympathetic nervous system, parasympathetic ganglia are located either directly within the walls of the organs they innervate or in close proximity to them.

Fibers from the parasympathetic (accessory) oculomotor Nucleus (cranial nerve III) terminate in the Orbit on the Cells of the ciliary ganglion. Postganglionic parasympathetic fibers originate here, entering the Eyeball to innervate the sphincter pupillae muscle and the ciliary muscle (responsible for accommodation). Sympathetic fibers originating from the superior cervical ganglion of the sympathetic trunk innervate the dilator pupillae muscle.

The parasympathetic nuclei of the Facial Nerve (cranial nerve VII)—specifically the superior salivatory and lacrimal nuclei—are located in the Pons. Their axons branch off the facial nerve and, as part of the greater petrosal nerve, reach the pterygopalatine ganglion situated in the pterygopalatine fossa. This ganglion gives rise to postganglionic fibers providing parasympathetic innervation to the lacrimal gland and the mucosal Glands of the Nasal cavity and palate. A portion of the fibers that bypass the greater petrosal nerve enters the chorda tympani, which carries preganglionic fibers to the submandibular and sublingual ganglia. The axons of neurons within these ganglia innervate the corresponding Salivary Glands.

The inferior salivatory nucleus belongs to the Glossopharyngeal nerve (CN IX). Its preganglionic fibers travel initially within the tympanic nerve and subsequently via the lesser petrosal nerve to the otic ganglion. Branches emerging from this ganglion provide parasympathetic innervation to the parotid gland.

From the dorsal Nucleus of the vagus nerve (cranial nerve X), parasympathetic fibers travel within its branches to numerous intramural ganglia located in the walls of internal organs in the neck, thoracic, and abdominal cavities. These ganglia give rise to postganglionic fibers that provide parasympathetic innervation to the organs of the neck, the thoracic cavity, and the majority of abdominal organs.

The sacral region of the spinal cord is represented by the sacral parasympathetic nuclei located at the level of segments II — IV. They give rise to the fibers of the pelvic splanchnic nerves, which transmit impulses to the intramural ganglia of the pelvic organs. The postganglionic fibers emerging from these ganglia provide parasympathetic innervation to the internal reproductive organs, Urinary Bladder, and rectum.

METABOLISM/2.html">THE CONCEPT OF the metasympathetic nervous system. Relatively recently, scientists identified another division of the autonomic nervous system—the metasympathetic nervous system. This refers to extensive nerve plexuses and microscopic ganglia situated within the walls of hollow organs endowed with motility (the Esophagus, stomach, intestines, urinary bladder, Gallbladder and Bile ducts, and fallopian tubes).

Table 15.1 Changes in the functional states of internal organs under the Influence of the autonomic nervous system

Metasympathetic ganglia differ from parasympathetic ganglia in their histological Structure; their neurocytes are surrounded by a Connective Tissue stroma, and gamma-aminobutyric acid (GABA) or purine bases act as Neurotransmitters. Sometimes these ganglia consist of only 4—5 neurons. These neurocytes are capable of generating impulses and sending them to smooth muscle cells without the involvement of the central nervous system. This mechanism drives organ peristalsis and the contraction of its wall. Neurons of the metasympathetic ganglia maintain connections with the sympathetic and parasympathetic Divisions of the autonomic nervous system, which coordinate the frequency of impulse generation.

Nervous regulation of organ Functions. The autonomic nervous system plays a primary role in regulating The activity of internal organs, the heart, blood vessels, and glands. In this regard, most effects of the sympathetic and parasympathetic systems are antagonistic. Certain organs and Tissues lack either sympathetic or parasympathetic fibers, meaning their activity is regulated by only one division of the autonomic nervous system. Structures that are not subject to parasympathetic influence include, for example, arteries, arrector pili Muscles, the dilator pupillae muscle, and sweat glands.

The primary sympathetic and parasympathetic effects are summarized in simplified form in Table 15.1.

It should be noted that, as a rule, the sympathetic nervous system is activated during stress and high activity. Conversely, the parasympathetic system predominates during rest. Indeed, during sudden environmental changes or in situations requiring significant effort and concentration, heart rate and contractile force increase, breathing deepens, and so forth.

Thus, the combined action of the sympathetic and parasympathetic systems on an organ ensures its appropriate response to changing external conditions.

Review Questions

1. Describe the composition of peripheral nerve fibers.

2. Differentiate between myelinated and Cytology/practical/64.html">Unmyelinated nerve fibers.

3. Name the areas of innervation of the Cranial Nerves.

4. List the branches of the Trigeminal nerve.

5. Indicate the sources of innervation for the facial expression and masticatory muscles.

6. Which nerves provide innervation to the facial Skin, Tongue, and salivary glands?

7. Name the divisions and branches of the vagus nerve.

8. List the branches of the spinal nerve.

9. Name the sources of formation and list the branches of the Cervical plexus.

10. Name the nerves that provide innervation to the muscles and skin of the upper limb.

11. List the branches of the Lumbar plexus.

12. Indicate the sources of innervation for the skin and Muscles of the Lower Limb.

13. Describe The structure of the sympathetic trunk and the abdominal aortic plexus.

14. Name the cranial parasympathetic ganglia.