We’ll revisit the lungs when we begin the Respiratory System unit. For now, the lungs are interfering with our ability to see the important nerves of the mediastinum, so they will need to be removed.

Somatic Nerves

The motor portion of the peripheral nervous system is classified according to the type of tissue being innervated. Motor neurons supplying voluntary, striated, skeletal muscle are somatic efferent neurons. Somatic refers to the body, body wall, or head, neck, trunk, and limbs where these striated skeletal muscles are located.

Left and right phrenic nerves supply the diaphragm (a skeletal muscle, i.e. a muscle which can receive conscious control).  Each nerve arises from the ventral branches of the fifth, the sixth, and usually the seventh cervical spinal nerves.

Each phrenic n. is both motor and sensory to the corresponding half of the diaphragm except at its periphery. This part of the muscle receives sensory fibers from the caudal intercostal nerves.

The recurrent laryngeal nn. carry somatic efferent fibers too. They branch from the vagus nn. and hence are discussed in detail with those nerves.

The Autonomic Nervous System

Introduction

The nervous system is highly organized both anatomically and functionally. It is composed of a central nervous system and a peripheral nervous system. The central nervous system includes the brain and the spinal cord. The peripheral nervous system comprises the cranial nerves, which connect with structures of the head and body, and the spinal nerves, which connect the spinal cord to structures of the neck, trunk, tail, and limbs. The peripheral nervous system is further classified based on anatomy and function. The nerves in this system contain axons myelinated by Schwann cells that conduct impulses to the central nervous system—sensory, afferent axons—and axons that conduct impulses from the central nervous system to muscles and glands of the body—motor, efferent axons. Most nerves have both sensory and motor axons.

The autonomic nervous system consists of components of the peripheral and central nervous systems. Its function is to control involuntary activity, to maintain homeostasis, and to respond to stress. It differs anatomically from the somatic efferent system in having a second motor neuron interposed between the central nervous system and the innervated structures. One neuron has its cell body located in the gray matter of the central nervous system. Its axon courses in the nerves only part of the way toward the structure to be innervated. Along the course of the nerve is a gross enlargement called a ganglion (pl. ganglia). By definition, a ganglion is a collection of neuronal cell bodies located outside the central nervous system. Some ganglia have a motor function, others a sensory function. Autonomic ganglia contain the cell bodies of the second motor neurons in the pathway of the visceral efferent system. Their axons complete the pathway to the structure being innervated. Because of its relationship to the cell bodies in the autonomic ganglia, the first visceral efferent neuron with its cell body in the central nervous system is called the preganglionic neuron. The cell body of the second neuron is in an autonomic ganglion. Therefore, this neuron’s cell body is ganglionic and its axon is postganglionic.

The autonomic nervous system’s visceral efferent system is divided into two subdivisions on the basis of anatomical, pharmacological, and functional characteristics. They are the sympathetic and parasympathetic divisions. (It should be kept in mind that the autonomic nerves we observe also have visceral afferent or sensory axons within them.)

In the sympathetic division the preganglionic cell bodies are limited to the segments of the spinal cord from approximately the first thoracic to the fifth lumbar segments—the thoracolumbar portion. The cell bodies of postganglionic axons are located in ganglia that are usually only a short distance from the spinal cord. At most of the postganglionic nerve endings of this portion of the autonomic nervous system, a humoral transmitter substance, norepinephrine, is released, which causes a response in the structures innervated. The overall effect of this system is to help the body withstand unfavorable environmental conditions or conditions of stress.

In the parasympathetic division the preganglionic cell bodies are located in specific nuclei in the brain stem associated with cranial nerves III, VII, IX, and X and in the three sacral segments of the spinal cord—the craniosacral portion. The cell bodies of the postganglionic axons are often located in terminal ganglia on or in the wall of the structure being innervated. Others are found in specifically named ganglia near the innervated structure. At the postganglionic nerve endings, a humoral transmitter substance, acetylcholine, is released, which causes a response in the structures innervated. This system is associated with the normal homeostatic activity of the visceral body functions—the conservation and restoration of body resources and reserves.

The sensory (afferent) portion of the peripheral nervous system is also classified as somatic or visceral, depending on the structure innervated. Somatic afferent neurons innervate the cutaneous surface of the body, striated muscle, tendons, joints, and two specialized structures in the head: the eye and the inner ear. Visceral afferent neurons innervate the mucosal surface and wall of all tubular visceral organs, exocrine glands, and specialized vascular structures. All of these peripheral sensory neurons are single neurons between the structure innervated and the central nervous system. Their cell bodies are contained in spinal ganglia of all spinal nerves and in cranial nerve ganglia. In these ganglia there is no synapse because the synapse is in the central nervous system.

The anatomy of the sympathetic division of the visceral efferent system requires further description before it is dissected. The cell bodies of the preganglionic neurons are located in the gray matter of the thoracic and first five lumbar spinal cord segments. Their axons leave the spinal cord along with those of other motor neurons in the ventral rootlets of each of these spinal cord segments. Each ventral root unites with the corresponding sensory dorsal root at the level of the intervertebral foramen to form the spinal nerve. The dorsal branch of the spinal nerve branches off immediately. Just beyond this point, a nerve leaves the ventral branch of the spinal nerve, the ramus communicans. It courses a short distance ventrally to join the sympathetic trunk, which runs in a craniocaudal direction just lateral to the vertebral column. A ganglion is usually located in the trunk at the point where each ramus communicans joins it. This is the sympathetic trunk ganglion and contains the cell bodies of postganglionic sympathetic axons.

Each preganglionic sympathetic axon must pass through the ramus communicans of its spinal nerve to reach the sympathetic trunk. Its fate from here is variable and mostly dependent on the structure to be innervated.

The sympathetic trunk is located along the full length of the vertebral column on both sides. Throughout the thoracic, lumbar, and sacral levels, it is joined to each segmental spinal nerve by a ramus communicans. Only those from spinal cord segments T1 to L5 contain preganglionic axons.

Thoracic Autonomic Nervous System

The small enlargements in these trunks at each intercostal space are sympathetic trunk ganglia.

Notice the fine filaments that run dorsally between the vertebrae to join the spinal nerve of that space. These are the rami communicantes (sg. ramus communicans), of the sympathetic trunk.

Follow the thoracic portion of the sympathetic trunk cranially. Notice the irregular enlargement of the trunk near the first intercostal space on the lateral side of the longus colli. This is the cervicothoracic ganglion. It is formed by a collection of cell bodies from a fusion of the caudal cervical ganglion and the first two or three thoracic ganglia.

Many branches leave the cervicothoracic ganglion. Rami communicantes connect to the ventral branches of the first and second thoracic spinal nerves and to the ventral branches of the seventh and eighth cervical spinal nerves. These spinal nerves contribute to the formation of the brachial plexus, which provides a pathway for postganglionic axons to reach the thoracic limb. A branch or plexus from the cervicothoracic ganglion follows the vertebral artery through the transverse foramina. This is the vertebral nerve. This is a source of postganglionic axons for the remaining cervical spinal nerves via branches at each intervertebral space from the vertebral nerve to each cervical spinal nerve. Postganglionic axons may leave the cervicothoracic ganglion and course directly to the heart.

Cranial to the cervicothoracic ganglion, the sympathetic trunk divides to form a loop, the ansa subclavia, around the subclavian artery (ansa [latin], a loop-shaped structure). The two branches of the loop unite at the middle cervical ganglion. This ganglion lies at the junction of the ansa and the vagosympathetic trunk and appears as a swelling of the combined structures. Numerous branches, cardiac nerves, leave the ansa and middle cervical ganglion and course to the heart.

The vagosympathetic trunk in the neck lies in the carotid sheath. Its sympathetic portion carries preganglionic and postganglionic sympathetic axons cranially to structures in the head. The cranial cervical ganglion is located at its most cranial end. This is at the level of the base of the ear, just caudomedial to the tympanic bulla. It will be examined when exposed in the head dissection. The tenth cranial nerve (CN X) or vagus nerve contains parasympathetic preganglionic axons that course caudally down the neck to thoracic and abdominal organs.

On the left side, at the level of the aortic arch, the left recurrent laryngeal nerve leaves the vagus n., curves medially around the ligamentum arteriosum (a cardiac structure which we will observe later) and the arch of the aorta and becomes related to the ventrolateral aspect of the trachea and the ventromedial edge of the esophagus. In this position it courses cranially in the neck to reach the larynx. In this course it reaches the dorsolateral aspect of the trachea.

On the right side, at the middle cervical ganglion, or slightly caudal to it, the right recurrent laryngeal nerve leaves the vagus n., curves craniodorsally around the right subclavian artery, reaches the dorsolateral surface of the trachea, and courses cranially to the larynx. It may be found in the angle between the trachea and the longus colli. Each recurrent laryngeal nerve sends branches to the heart, trachea, and esophagus before terminating in the laryngeal muscles as the caudal laryngeal nerve. These nerves therefore have multiple functions and carry components of the parasympathetic autonomic nervous system and general somatic efferent system.

Branches are supplied to the bronchi as the vagus passes over the roots of the lungs. Between the azygos vein and the right bronchus on the right and the area just caudal to the base of the heart on the left, each vagus n. divides into dorsal and ventral branches. The right and left ventral branches soon unite with each other to form the ventral vagal trunk on the ventral surface of the esophagus. The dorsal branches of each vagus n. do not unite until farther caudally near the diaphragm, where they form the dorsal vagal trunk, which lies dorsal to the esophagus. The termination of these trunks is found in the abdominal cavity.

The Ungulate Perspective

Move on to the instructors’ prosections of the ungulates to once again identify the same nerves and nervous structures described above and dissected out in your own carnivore cadavers.