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Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 1 of 8 The nervous system coordinates all the functions of our bodies, from running, to digesting, to singing, to breathing. For the sake of discussion and study, the nervous system is divided functionally into the somatic nervous system and the autonomic nervous system, or the ANS. These systems are not separate, but rather different components of the nervous system as a whole. The somatic nervous system may be thought of as the conscious nervous system, responsible for functions of which we are aware. It transmits motor signals that cause our voluntary actions and some reflexive actions, and sensory signals about pain, temperature, touch, and position. In addition to these conscious functions, there is an entire set of functions that our nervous system regulates that we rarely, if ever, notice. These unnoticed functions are regulated by the autonomic nervous system and include all of our homeostatic mechanisms. The autonomic nervous system may be thought of as the unconscious nervous system, responsible primarily for maintaining homeostasis. Functions regulated by the autonomic nervous system include heart rate, respiration rate, blood vessel diameter, sweat gland secretion, digestion, etc. Fortunately, we do not have to consciously regulate all of our organs' functions; otherwise, a person could die merely from forgetting to breathe. This module will focus on the autonomic nervous system, including its functions and its structure. Many students get confused by the autonomic nervous system, perhaps because it does not follow the patterns of the somatic nervous system or because its structure varies from one part of the body to the next. It may help to remember that the autonomic nervous system innervates all the structures that help maintain homeostasis by whatever means necessary. If you take your time, and go through the autonomic nervous system as a whole until it makes sense, it will definitely pay off many times in the future. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 2 of 8 The autonomic nervous system regulates homeostasis via two opposing divisions: the sympathetic division and the parasympathetic division. Both the sympathetic and parasympathetic systems innervate most of the body's organs and act in opposition to one another to maintain normal physiology, including blood pressure, blood oxygen levels, and nutrient levels. The amount of work that a given organ must do to maintain homeostasis differs from one situation to the next. In addition, certain organs need to work to prepare the body for possible future situations. These functions are all regulated by the autonomic nervous system. The sympathetic division of the autonomic nervous system is the division that prepares the body for stressful situations. It is often referred to as the "fight or flight" system. The effects of this system are numerous, but generally include increasing heart rate, constricting blood vessels to the skin and viscera (thereby increasing blood flow to muscles), increasing pupil size and decreasing salivation. These responses all promote survival in a dangerous situation. The parasympathetic division of the autonomic nervous system prepares the body for restful situations and is often called the "rest and digest" system. Effects of the parasympathetic nervous system include slowing heart rate, increasing gastric motility, and increasing salivation. These responses help the body to recover as well as prepare for stressful situations by storing nutrients. The parasympathetic and sympathetic systems do not work entirely separately, but rather work at the same time, often in opposition to one another. For example, while relaxing on a hot day after a meal, your parasympathetic nervous system may predominate as your food digests, but your sympathetic nervous system will be actively innervating sweat glands. On the other hand, if the building you were in suddenly caught fire, your sympathetic nervous system would predominate. The sympathetic and parasympathetic nervous systems may be thought of like a water faucet with hot and cold water balancing one another to make the perfect water temperature for anything from a cold glass of water to drink to a relaxing hot bath. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 3 of 8 The parasympathetic and sympathetic components of the autonomic nervous system (ANS) differ in their structure and in their function, but the two systems have some important features in common. Both the parasympathetic and sympathetic divisions are two-neuron systems with the first neuron named either presynaptic or preganglionic (these terms are synonymous), and the second nerve called postsynaptic or postganglionic. It is important to realize that an autonomic neuron is not called postsynaptic or postganglionic until it has synapsed, regardless of how many ganglia it has passed through. The cell bodies of the presynaptic neurons of both the parasympathetic and sympathetic systems are located within the central nervous system. These presynaptic autonomic neurons synapse only with postsynaptic autonomic neurons. (There is one exception, in the adrenal gland, which will be discussed later in this module.) The cell bodies of postsynaptic autonomic neurons are located in ganglia throughout the body. Recall that a ganglion is simply a collection of neuron cell bodies in the peripheral nervous system. Sensory ganglia, such as dorsal root ganglia, are collections of sensory neuron cell bodies, but NO synapses occur there. A particular autonomic ganglion will be associated with synapses of either the parasympathetic nervous system or the sympathetic nervous system, but it may have fibers from both systems running through it. Remember, a neuron does not necessarily synapse just because it enters a ganglion. Some fibers pass through without synapsing. (The terms preganglionic and postganglionic may be a bit deceptive, but they are often used instead of presynaptic and postsynaptic.) Another important similarity between the parasympathetic and sympathetic divisions of the ANS is that they both use chemical signals to alter the action of the organs they innervate. The organs innervated by the autonomic nervous system are called effector organs. The significant differences between the parasympathetic and sympathetic nervous systems include the location of the presynaptic nerve cell bodies within the central nervous system, and the location of the postsynaptic nerve cell bodies throughout the body. We will discuss the pre- and postsynaptic neurons in detail in this module. Another important difference between the parasympathetic nervous system and the sympathetic nervous system are the neurotransmitters each system uses to effect change. Generally, the sympathetic nervous system releases a chemical called norepinephrine, which is excitatory to neurons, from its postsynaptic neurons. The parasympathetic nervous system releases a chemical called acetylcholine from its postsynaptic neurons. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 4 of 8 We will first spend some time discussing the sympathetic division of the autonomic nervous system. The sympathetic division is sometimes called the thoracolumbar outflow or division of the ANS because the cell bodies of the presynaptic sympathetic neurons are located in the lateral horns (a.k.a. intermediolateral cell columns) of the spinal cord gray matter, which are found in spinal cord segments T1 through L2. There are no sympathetic presynaptic cell bodies above spinal cord level T1 or below spinal cord level L2. The cell bodies of the postsynaptic sympathetic neurons are found in either paravertebral or prevertebral ganglia. The paravertebral ("beside the vertebrae") ganglia are called the sympathetic chain ganglia and will be discussed in more detail later. Prevertebral ganglia, also known as preaortic or collateral ganglia, are located around the major branches of the abdominal aorta and include the celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia. The sympathetic nervous system innervates essentially all the organs of the body as well as blood vessels (smooth muscle in vessel walls), sweat glands, and arrector pili muscles through out the body. (Arrector pili muscles make the hairs on your body stand up, resulting in goose bumps.) Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 5 of 8 The paravertebral ganglia consist of the right and left sympathetic chains or trunks. These ganglia are home to many postsynaptic sympathetic nerve cell bodies, and the site of many synapses between presynaptic sympathetic fibers and postsynaptic sympathetic neuron cell bodies. The sympathetic chains lie next to the vertebral column throughout its length, running across the necks of the ribs in the thorax and along the vertebral bodies in the abdomen. There is approximately one ganglion associated with each spinal cord segment, except in the cervical region and the sacral region. Adjacent ganglia of the sympathetic chain are connected to each other by interganglionic rami which contain sympathetic nerve fibers ascending or descending between ganglia. The presynaptic sympathetic nerve fibers originate in the lateral horns of spinal cord segments T1-L2. From the lateral horns, all of these fibers must reach the sympathetic trunk. Presynaptic sympathetic fibers exit the spinal cord in the ventral roots (because they are motor fibers), pass through the spinal nerves and eventually enter the ventral primary rami of spinal cord segments T1-L2. Shortly after entering the ventral primary rami, the presynaptic sympathetic fibers exit the ventral primary rami via white rami communicantes which carry the presynaptic sympathetic fibers to the sympathetic trunk. The white rami communicantes are so named because they are collections of myelinated (therefore white-ish) axons communicating with the ventral primary rami. White rami communicantes are only found between spinal cord segments T1-L2 because there are no presynaptic sympathetic nerve fibers originating above or below those levels. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 6 of 8 Once the presynaptic sympathetic fibers have arrived in the sympathetic chain they do one of three things: 1. The presynaptic neuron may synapse immediately in the ganglion located at the level it entered. 2. The presynaptic neuron may ascend or descend in the sympathetic trunk before synapsing in a ganglion located at a different spinal cord level. 3. The presynaptic sympathetic neuron may pass through the sympathetic chain ganglia without synapsing at all, synapsing instead in a prevertebral ganglion. In general, sympathetic innervation to structures located in the head, neck, body wall, limbs and thoracic cavity follows one of the first two courses. Sympathetic innervation to internal organs in the abdomen and pelvis follows the third course, primarily. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 7 of 8 Once the presynaptic fibers have synapsed, the postsynaptic sympathetic fibers reach their target structures by the most efficient path for the given region of the body. Postsynaptic sympathetic nerves have several possible routes depending on the location of their target organs. Many postsynaptic sympathetic fibers re-enter the spinal nerve via gray rami communicantes and get distributed to the body with the ventral and dorsal primary rami. Gray rami communicantes carry postsynaptic sympathetic fibers to every spinal nerve, allowing sympathetic innervation to reach all parts of the body, especially the blood vessels, sweat glands and arrector pili muscles. Other postsynaptic sympathetic fibers leave the ganglia where they began and travel directly to their target organs. This is how postsynaptic sympathetic fibers reach the organs of the thorax. Finally, some postsynaptic sympathetic neurons form perivascular plexuses along blood vessels that supply a given area. The nerve fibers then ride along with the blood vessels to reach their targets. This is the method that postsynaptic sympathetic fibers use to reach target organs in the head as well as in the abdomen and pelvis. Copyright© 2002 The University of Michigan. Unauthorized use prohibited. Learning Modules - Medical Gross Anatomy Introduction to Autonomics, Part 1 - Page 8 of 8 So, let's summarize the general plan of the sympathetic nervous system. Presynaptic sympathetic fibers arise within the lateral horn of the spinal cord between T1 and L2 levels of the cord. ALL of these fibers leave the spinal cord in ventral rootlets, travel through the spinal nerve, and then enter the first few millimeters of the ventral primary ramus before leaving to travel to the sympathetic chain within the white rami communicantes. The sympathetic chains are bilateral strings of paravertebral sympathetic ganglia, connected by interganglionic rami, that lie beside or on the vertebral column. Once a presynaptic sympathetic neuron reaches the sympathetic chain, it faces a choice. It may: 1. synapse at the level it entered and then either: a. leave within a gray ramus communicans to reach the ventral ramus at the level at which it synapsed, or b. leave the chain as a direct branch to organs (thoracic visceral nerve, T1-T4/5). 2. travel up or down the chain to synapse at a higher or lower ganglion, where it may either: a. leave within a gray ramus communicans to reach a ventral ramus at the level at which it synapsed, or b. leave the chain as a direct branch to organs (cervical cardiac nerve or sacral splanchnic nerve) or to a perivascular plexus (internal or external carotid plexus). 3. leave the chain without synapsing, to travel within a thoracic (T5-T12) or lumbar splanchnic nerve to either: a. reach a prevertebral (preaortic) ganglion, where it will synapse and then join a perivascular plexus to reach an organ, or b. reach cells of the suprarenal medulla to synapse. These universal truths must be remembered: 1. White rami communicantes only exist to connect T1 to L2 ventral primary rami to the sympathetic chain. The terms "white ramus" and "presynaptic" are not synonymous and interchangeable. 2. Gray rami communicantes exist at ALL spinal nerve levels. EVERY ventral primary ramus is connected to the sympathetic chain by a gray ramus, which carries postsynaptic sympathetic fibers to the VPR. 3. The sympathetic chain exists to carry some of the presynaptic fibers from the T1-L2 levels up to the neck and head and down into the lower abdomen and pelvis. If you find inner peace with these universal truths, then you have passed the first step on your journey to become an Autonomic Master. Copyright© 2002 The University of Michigan. Unauthorized use prohibited.