Human Anatomy Lesson 5
Lesson 5 - Autonomic Nervous System
Welcome! This is the fifth lesson in a semester-long undergraduate-level course in Human Anatomy. Now that I have gone over the spinal cord, spinal nerves, and somatic nervous system in lesson 4, we can focus our attention on the autonomic nervous system. The autonomic nervous system is the visceral motor division of the nervous system that is active in energy expenditure, energy conservation, and digestion in smooth muscle, cardiac muscle, and glands. I will also briefly cover the visceral sensory system and the issue of referred pain.
Learning Objectives - By the end of this lesson, you should be able to ...
- List the parts of the autonomic division of the peripheral nervous system and what they do
- Explain the difference between preganglionic and postganglionic cell bodies and fibers
- Describe the components of paravertebral ganglia that make up the sympathetic chain, and trace sympathetic nerve impulses from the spinal cord through the sympathetic chain to the paravertebral ganglia in different parts of the body
- Describe how internal organs receive sympathetic innervation via splanchnic nerves
- Describe how parasympathetic nerve fibers hitchhike alongside sympathetic fibers in the sympathetic chain to reach various parts of the body
- Understand how referred pain is the result of mixed visceral sensory and motor sensory impulses in the spinal cord
- Understand that the enteric part of the autonomic division operates outside of CNS control
Autonomic Division of the PNS
Smooth muscle, cardiac muscle, and glands are not derived from dermomyotome. As noted in lesson #4, nerves that innervate these structures are part of the autonomic division of the PNS. The nerve cells in the autonomic nervous system lie outside the CNS, in ganglia and clusters of nerve cells in the walls of glands, smooth muscle, and cardiac muscle. The axons of these nerve cells are called postganglionic autonomic axons, and the neurons themselves are called postganglionic autonomic neurons. The CNS maintains control over these neurons via preganglionic autonomic neurons, cells within the CNS that send preganglionic autonomic axons out to the postganglionic autonomic neurons. In other words, a postganglionic axon is like the cord for a toaster or other electronic device that carries electricity, whereas a preganglionic axon is like the cord that carries electricity in the wall between a breaker box and the electrical outlet. In this example, the postganglionic neuron would be where the toaster cord plugs into the wall.
The sympathetic part of the autonomic division of the PNS is active in moments of great effort or activity, when sweat glands, coronary arterioles, and bronchi dilate and blood is shunted away from digestive organs. The parasympathetic part of the autonomic division is associated with conservation of energy, when pupils and bronchi constrict, the heart slows, digestive glands are stimulated, the bladder contracts, and the anal sphincter relaxes. The enteric part of the autonomic division is involved with direct control of the GI tract, and is a specialized part of the nervous system that operates without input from the CNS.
Sympathetic Part of the Autonomic Division
Postganglionic sympathetic axons reach the structures in the body wall that they innervate by hitching a ride on anterior and posterior rami of spinal nerves. In embryonic development, sympathetic ganglia called paravertebral ganglia develop just lateral to the vertebral bodies in close apposition to each spinal nerve. A small unmyelinated nerve bundle called the gray ramus communicans develops that carries postganglionic axons from the ganglion to the anterior ramus of each spinal nerve. If the axon in a particular gray ramus communicans is destined for a sweat gland or smooth muscle in the anterolateral body wall, then it will run along with the anterior ramus. If, instead, the axon is destined for a sweat gland or smooth muscle in the posterior body wall, the axon will turn proximally, enter the spinal nerve, and pass into the posterior ramus to be distributed with the somatic motor fibers of the posterior ramus.
There is one paravertebral ganglion for every spinal nerve to start, but eventually some of them fuse: ganglia 1-4 fuse to make up a single superior cervical ganglion that sends out four gray rami to C1-C4; likewise, ganglia for C5 and C6 fuse into one middle cervical ganglion, and ganglia for C7 and C8 fuse to form the inferior cervical ganglion. The 12 thoracic ganglia do not fuse, so there are 12 thoracic ganglia in an adult. The 5 lumbar ganglia fuse in a highly variable pattern, as do the 5 sacral ganglia. The coccygeal ganglia from the left and right sides fuse to form the ganglion impar in the midline. Some cells of the superior cervical ganglion give off postganglionic axons that accompany arteries running superiorly into the neck in order to innervate sweat glands, arrector pili muscles, and blood vessels in the head.
The cells in the paravertebral ganglia are controlled by sympathetic preganglionic cell bodies that lie in the thoracolumbar region of the spinal cord, from T1 to L2. These cell bodies reside in the intermediolateral column of gray matter between the anterior and posterior horns. Axons for these preganglionic sympathetic neurons tag along with somatic motor axons leaving the spinal cord in the anterior root. This means that the anterior root and spinal nerves of T1 to L2 contain both somatic motor and preganglionic sympathetic axons. From there, the preganglionic sympathetic axons enter the anterior ramus of the spinal nerve and immediately exit to reach the paravertebral ganglia via the white ramus communicans. All paravertebral ganglia from T1 to L2 are connected to the nearest anterior ramus by a white ramus communicans, whereas every paravertebral ganglion is connected to the nearest anterior ramus by a gray ramus communicans. The ganglia from C1-C8 and L3-Co therefore have gray rami communicans, but no white rami communicans.
The postganglionic cells in the paravertebral ganglia of T1 to L2 are directly controlled by preganglionic neurons from the CNS, but what of the postganglionic cells in paravertebral ganglia above T1 or below L2? Let's follow the movement of a nerve impulse through the sympathetic part of the autonomic division from the thoracolumbar region to the superior cervical ganglion. A preganglionic axon travels out of the anterior root at the level of the 1st thoracic vertebra into the spinal nerve, into the anterior ramus and through the white ramus communicans into the paravertebral ganglion. The axon then ascends without synapse through the inferior cervical ganglion and middle cervical ganglion to reach the superior cervical ganglion, where it synapses. The superior cervical ganglion therefore receives its preganglionic input from axons of cells at the T1 spinal level. The middle and inferior cervical ganglia receive preganglionic input from axons of cells at the T2 and T3 spinal levels. Since sympathetic innervation from the upper limb comes from inferior and middle cervical ganglia, damage to T2 and T3 paravertebral ganglia destroys sympathetic innervation of the upper limb. Preganglionic input below L2 is a mirror image of the above. In fact, each postganglionic cell receives preganglionic input not just from the sympathetic preganglionic cell bodies and axons at its own spinal cord level, but also receives synaptic input from preganglionic axons from paravertebral ganglia one level above and one level below. This integrated system is known as the sympathetic chain.
Sympathetic Innervation of Internal Organs
All internal organs of the body superior to the abdominal diaphragm receive postganglionic axons from cells that lie in paravertebral ganglia. This means that the sympathetic axons that run to the heart, esophagus, bronchi, and other organs above the diaphragm travel in nerve bundles that go directly from a paravertebral ganglion to that organ. These direct postganglionic sympathetic fibers, or sympathetic "organ" nerves, emanate from all paravertebral ganglia above spinal level T6 and are named after the organs to which they run. For example, sympathetic innervation to the heart is delivered via cardiac nerves. The postganglionic sympathetic neurons for abdominal and pelvic organs, on the other hand, lie in subdiaphragmatic ganglia on the anterior surface of the abdominal aorta or in nerve plexuses that extend from the aorta into the pelvis. Subdiaphragmatic ganglia cannot receive preganglionic sympathetic axons from paravertebral ganglia via a white ramus, since there are no white rami below the L2 level. In this case, splanchnic nerves run from the 5th through 12th thoracic paravertebral ganglia, the lumbar ganglia and, in women, the sacral ganglia, and run directly to various organs below the abdominal diaphragm.
In summary, a preganglionic sympathetic axon reaches the paravertebral ganglion corresponding to a given spinal cord segment. These axons do one of 4 things: (1) synapse in that ganglion; (2) ascend and synapse in a higher ganglion; (3) descend and synapse in a lower ganglion; or (4) leave the chain in a splanchnic nerve to synapse in subdiaphragmatic ganglia.
Parasympathetic Part of the Autonomic Division
This part of the PNS innervates smooth muscle, cardiac muscle, and glands internal to the body wall, and is far less complicated than the sympathetic part. All preganglionic parasympathetic cell bodies lie either in the brain, and send their axons out with the cranial nerves, or lie at the S2-S4 spinal level, and send their axons out with the S2-S4 spinal nerves. Cranial nerves III, VII, IX, and X carry preganglionic parasympathetic axons. The axons from III, VII, and IX innervate structures in the head, whereas X carries preganglionic parasympathetic axons for internal organs of the thorax and abdominal organs derived from the foregut and midgut. The postganglionic cell bodies for all of the organs innervated by X are embedded in the organs themselves, not in ganglia. The preganglionic parasympathetic cells in S2 through S4 send axons into the anterior roots of the S2-S4 spinal nerves, which give off preganglionic parasympathetic axons called pelvic splanchnic nerves that pass directly to hindgut and pelvic organs. It is important to understand that pelvic splanchnic nerves are NOT associated with white rami and are NOT associated with sympathetic ganglia like other splanchnic nerves.
Visceral Sensory System
Structures receiving visceral motor input return visceral sensation. Unlike motor sensation, visceral sensation is dull and unlocalized. The cell bodies of visceral sensation are in the posterior horn of the spinal cord. Visceral sensory fibers from blood vessels of the body wall and internal organs travel toward the spinal nerve within nerve bundles that carry sympathetic supply. All visceral sensory fibers therefore hook up with the sympathetic chain, travel from the sympathetic chain to a spinal nerve via a gray ramus communicans, and enter posterior roots between T1 and L2. One exception is for visceral sensation related to pelvic distension, which travels with preganglionic parasympathetic fibers in the pelvic splanchnic nerves, and passes through the ventral rami of S2 through S4 and into the S2-S4 posterior roots.
Referred pain is a sharp, well-localized pain that occurs when a visceral pain impulse from a diseased organ is mixed up with somatic sensation because the visceral and sensory cell bodies are located in the posterior horn of the spinal cord at the same level. The sharp pain occurs in a section of the body wall innervated by the same segments of the spinal cord that received the visceral impulses.
Enteric Nervous System
A third portion of the autonomic division of the PNS is the enteric nervous system, which consists of cell bodies within the gastrointestinal system that operate in isolation from the CNS. We will not cover the enteric part of the autonomic division of the PNS further, except to note in upcoming lessons when function in various organs is under enteric, not CNS, control.
Preview of Upcoming Lessons
In lesson 6, we will finish the back with back muscles and movements of the spine. Then, we will be turning to the thorax, where we will cover the body wall, diaphragm, and lungs in lesson 7 and the mediastinum and heart in lesson 8.
- Human Anatomy Lesson 6
This lesson covers the muscles and movements of the back.
- Human Anatomy Lesson 7
This lesson covers the anatomy of the body wall, thorax, and lungs.
- Human Anatomy Lesson 8
This lesson focuses on the mediastinum and heart - including its chambers, surface blood vessels, innervation, and function.
This is a notoriously difficult part of human anatomy to teach - let me know how I did and any improvements you would suggest!
© 2015 Robert McCarthy