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- divides nervous system on the basis of anatomical structures that we can easily see.
- 1. CNS - Centrally brain and spinal cord.
- 2. PNS - Peripherally located structures
- (12 pairs of cranial nerves - coming off the brain, 31 pairs of spinal nerves coming off the spinal cord, and all other neural elements)
Somatic Nervous System
innervates the “soma” or outer body wall skin & striated, voluntary skeletal muscle
Somatic Afferent (SA)
somatic sensory fibers from skin & muscle (and some other structures)
somatic motor fibers to striated skeletal muscle.
Visceral Nervous System
innervates viscera or internal organs
Visceral Afferent (VA)
visceral sensory fibers from glands, smooth muscle, & cardiac muscle
Visceral Efferent (VE)
- visceral motor fibers to glands, smooth muscle, & cardiac muscle.
- This is the ANS
The Autonomic Nervous System (ANS)
- technically defined as the motor fibers (visceral efferent = VE) of the visceral nervous system.
- However, some sources use the terms �ANS� and �visceral nervous system� as being interchangeable. We will use the technical definition in this course. As such, the ANS is a visceral efferent (VE) or motor system comprised of 2 divisions: the sympathetic and parasympathic divisions.
Part of ANS concerned with the �flight or fight� response (the response of the body when a person is under threat).
Part of ANS concerned with �calming, resting, and digesting� functions.
The ANS Two-Neuronal System (Motor Pathway)
- ANS is a two-neuronal system. Both, the sympathetic and parasympathetic motor pathways involve 2 neurons: a preganglionic neuron (located somewhere in the CNS) and a postganglionic neuron (located somewhere in the PNS). The preganglionic cell�s axon synapses with the postganglionic cell body. The postganglionic cell�s axon goes to the target organ.
- 1. Preganglionic Cell Body � is located in brain stem nuclei (nucleus = collection of cell bodies in the CNS, ganglion = collection of cell bodies in the PNS).
- 2. Preganglionic Axon � is relatively long.
- 3. Postganglionic Cell Body � is located in the wall (submucosa) of the target organ (for the body below the head) and in 4 special parasympathetic ganglia in the head (ciliary ganglion, submandibular ganglion, pterygopalatine ganglion, otic ganglion).
- 4. Postganglionic Axon � is relatively short.
- Sympathetic Pathway
- 1. Preganglionic Cell Body � is located in the T1-L2 spinal cord segements (specifically in lateral gray horn).
- 2. Preganglionic Axon � is relatively short (because the spinal cord is not that far away from the sympathetic chain)
- 3. Postganglionic Cell Body � is located in the sympathetic chain ganglia or in the pre-aortic (prevertebral) ganglia (celiac ganglia, superior mesenteric ganglion, aorticorenal ganglia, inferior mesenteric ganglion) in the abdomen).
- 4. Postganglionic Axon � is relatively long.
Comparison of the ANS Pathway with the Somatic Nervous System Pathway
- 1. Somatic Afferent/Sensory (SA) Pathway � involves a single cell which is located in the DRGs (dorsal root ganglia). It is a pseudounipolar axon with one process (axon) extending to a peripheral ending and the other extending toward the CNS. There are no synapses in the DRGs. The SA pathway involves a single neuron.
- 2. Somatic Efferent/Motor (SE) Pathway � involves a single cell which is located in the ventral horn of the gray matter of the spinal cord. It is a motor neuron with an axon that extends all the way to the target muscle. The SE pathway involves a single neuron.
- 3. Thus, the Somatic Nervous System Pathway is a single neuron pathway. The ANS motor pathway is a two-neuronal pathway.
- Sympatentic division preganglionic fibers �outflow� from the thoracolumbar segments (T1-L2) of the spinal cord (CNS). This division is associated with the �flight or fight� response.
- dilates the pupils, relaxes bronchi, accelerates the heart, stimulates epinephrine, norepinephrine, & glucose release, and inhibits saliva, digestive, & urinary activity, causes hair to �stand on end� (erector pilli muscles), sweaty palms, vasoconstriction (increases blood pressure). Think about how this relates to the �flight or fight� response. Note: In the sexual response, the sympathetic division is responsible for ejaculation.
- 1. White rami communicantes � The preganglionic fibers exits the spinal cord via the ventral horn and ventral root to the spinal nerve. Here it jumps off to enter the sympathetic chain. These fibers are myelinated (have a myelin sheath which is white) and thus form a connecting branch to the sympathetic chain called a �white ramus communicans (singular).
- 2. Once in the sympathetic chain, a preganglionic fiber can (1) synapse at that level, (2) ascend in the chain to synapse at a higher level, or (3) descend in the chain to synapse at a lower level.
- The preganglionic axons then synapse with postganglionic cells in the sympathetic chain ganglia. Postganglionic fibers then travel out to: (1) viscera in the soma (e.g., blood vessels, sweat glands, erector pilli muscles) or (2) viscera in body cavities (e.g., heart, lungs, esophagus). This is the general case.
- 1. Gray rami communicantes � The postganglionic axons going to the soma are not myelinated and thus have a gray appearance. They jump back to the spinal nerve forming a branch called a �gray ramus communicans�(singular). From the spinal nerve, they travel in both the dorsal and ventral rami nerves out to viscera (blood vessels, sweat glands, erector pilli muscles) embedded within the soma.
- 2. Sympathetic Chain Ganglia (also called sympathetic trunk or sympathetic chain) � is located on the lateral sides of vertebral bodies. It extends superiorly to the cervical region and inferiorly all the way to the coccyx. Thus, there are cervical, thoracic, lumbar, sacral ganglia and the last ganglion is by the coccyx and is known as �ganglion impar.�
- 3. Pathway to Internal Visceral Plexuses � Postganglionic axons going to internal visceral nerve plexuses of internal organs can jump directly off the sympathetic chain to participate in these plexuses (such as the cardiac, pulmonary, and esophageal plexuses).
Sympathetics to the Abdomen
- Symp/pre axons travel to the abdomen in the thoracic splanchnic nerves (greater, lesser, and least). These nerves pierce the diaphragm and then synapse on special pre-aortic (pre-vertebral) ganglia
- 1. Pre-Aortic (Prevertebral) Ganglia � these ganglia in the abdomen contain the symp/post cell bodies whose axons will participate in visceral nerve plexuses that innervate the foregut, midgut, hindgut and their related structures. They are called �pre-aortic� because they sit on the anterior surface of the aorta next to the blood vessels that they are named after.
- a. celiac ganglia � there are two ganglia which sit on the aorta near the origin of the celiac artery (which travels to the foregut and its associated structures).
- b. superior mesenteric ganglion � sits on the aorta near the origin of the superior mesenteric artery (which travels to the midgut and its associated structures).
- c. aortico-renal ganglia � there are two ganglia which sit on the aorta near the origin of the renal arteries (which travel to the kidneys).
- d. inferior mesenteric ganglion � sits on the aorta near the origin of the inferior mesenteric ganglion (which travels to the hindgut and its associated structures).
- �craniosacral outflow.� This is because parasympathetic preganglionic fibers �outflow� from cranial nerves (III, VII, IX, and X) and sacral nerves (S2-4) in the CNS. This division is associated with calming, resting, & digesting functions.
- constricts the pupils, stimulates saliva production, constricts bronchi, inhibits the heart, stimulates the digestive processes of the stomach, pancreas, intestines, and stimulates urinary activity. These processes are more calming, resting, and digesting. Note: In the sexual response, the parasympathetic division is responsible for erection.
The parasympathetic division is also called the �craniosacral outflow� because the parasympathetic preganglionic axons �outflow� in certain cranial nerves (CN III, VII, IX, and X) and from certain sacral segments of the spinal cord (S2-4). Their cell bodies are in certain brain stem nuclei or in the lateral gray horn of S2-4 spinal cord segements. Note: A nucleus is a collection of cell bodies in the CNS (a ganglion is a collection of cell bodies in the PNS).
- The parasympathetic postganglionic (para/post) cells bodies are located in the 4 special ganglia in the head (ciliary, pterygopalatine, submandibular, and otic ganglia) or, in the rest of the body, in the wall of the target organ.
Parasympathetics to the Thorax
- The vagus nerve (CN X) originates off the brainstem, travels through the neck and brings the para/pre fibers into the thorax. Vagal fibers then join visceral nerve plexuses of the various organs (e.g., esophageal plexus, cardiac plexus, pulmonary plexus). Then, these fibers pierce the wall of the target organ to find the para/post cell bodies in the submucosa. Thus, the para/post axons are very short.
Parasympathetics to the Abdomen
- Anterior & posterior vagal trunks bring the para/pre fibers into the abdomen. Vagal fibers then join visceral nerve plexuses (celiac and superior mesenteric plexus) which go to the foregut and midgut (and their associated organs). Para/pre fibers from S2-S4 (pelvic splanchnic nerves) will join the inferior mesenteric plexus to supply the hindgut and its associated structures.
�Visceral Nerve Plexus�
- All of the viscera (organs) of the thorax and abdomen (and pelvis) are innervated by their own visceral nerve plexus. Theses plexuses are comprised of visceral afferent/sensory (VA) fibers and visceral efferent/motor (VE) fibers. The VE fibers are the fibers of the autonomic nervous system (ANS) and are either parasympathetic or sympathetic.
Visceral Nerve Plexus (thorax)
- the VE fibers in the visceral nerve plexuses are para/pre (from the vagus) and symp/post (from the sympathetic chain ganglia). Remember: The VE para/post cell bodies are in the wall of the target organ, so their axons/fibers are not in the visceral nerve plexuses.
Visceral Nerve Plexus (abodomen)
VE fibers in the visceral nerve plexuses of the foregut, midgut and hingut are para/pre and symp/post. The para/pre fibers to the foregut and midgut are from the vagus nerve. The para/pre fibers to the hindgut are from S2-S4 pelvic splanchnic nerves.
Visceral Nerve Plexus VA Cell Body Location
- 1. VA pain fibers travel back with sympathetic fibers and their cell bodies are located in the DRGs.
- 2. VA reflex fibers in the thorax, foregut & midgut of the abdomen travel back with parasympathetic fibers and their cell bodies are located in the vagal sensory ganglia. Those of the hindgut travel back with pelvic splanchnic nerves and are located in S2-S4 DRGs.
Visceral Nerve Plexuses in the Thorax:
- 1. pulmonary plexus (VA, VE-para/pre, symp/post)
- 2. cardiac plexus (VA, VE-para/pre, symp/post)
- 3. esophageal plexus (VA, VE-para/pre, symp/post)