Autonomic NS
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Fall 1999 Moore, pp 45-52

Lecture 15 Dr. C. Dlugos

OVERVIEW: The autonomic nervous system is the portion of the nervous system which automatically controls the inner workings of your body and the visceral organs that you are now studying. The autonomic nervous system is contained within the central nervous system (e.g. brain and spinal cord) and the peripheral nervous system (e.g. spinal nerves). It is a solely efferent system, however, afferent visceral fibers, associated with the spinal nerve, travel with the efferent fibers and provide information to the CNS concerning the state of the viscera. The autonomic nervous system is subdivided into two parts; (a) the sympathetic nervous system ( the fright, flight, or fight system); and (b) the parasympathetic system (normal body homeostasis system). Sympathetic stimulation requires body energy and is useful in emergency situations while the parasympathetic system conserves body energy and is useful in body homeostasis. Although the parasympathetic and sympathetic nervous systems are antagonistic in some organs (e.g. the heart), they are not dissimilar. In fact the embryologic origin of all autonomic ganglia is from the neural crest. Similarities in the two systems include the fact that both the sympathetic and parasympathetic system are two neuron pathways with the preganglionic neuron in the CNS and the postganglionic neuron in the PNS. The sympathetic system is more extensive than the parasympathetic system, however, since it is responsible for the innervation of blood vessels, sweat glands, and the arrector pili muscles of the entire body. The action of the parasympathetic system is confined to the visceral organs which you are studying.

The Spinal Nerve (M., Fig1.29, p44)

1. Sensory or afferent component:

a. Somatic afferent:pain, temperature, touch, proprioception from body

b. Visceral afferent:pain, pressure from visceral organs, these sensations are not as well localized as somatic afferents.

2. Motor or efferent component:

a. Somatic efferent: These fibers innervate the skeletal muscles of the body.

Visceral efferent: These fibers are autonomic fibers. All autonomic fibers are efferent. In the spinal nerve these comprise mainly postganglionic sympathetic fibers contained within every spinal nerve and its rami and distributed to the hair follicles, blood vessels, and sweat glands. Two other types of autonomic fibers are found in the spinal nerve or its roots. Preganglionic sympathetic fibers also travel in the ventral root of the spinal nerve for a short time before they enter the sympathetic trunk through the white rami communicans. Preganglionic parasympathetic fibers leave the spinal nerve as the pelvic splanchnic nerves (to be learned later) but never reenter it to be distributed to the periphery.

3. Be able to name and identify sensory (dorsal) root, motor (ventral) root, dorsal root ganglia, ventral ramus (larger), dorsal ramus (smaller).

Synapse: site of information exchange within the nervous system.

Two neuron system (one neuron in CNS and one in PNS).

A. Preganglionic neuron and fiber; cell body and preganglionic fiber originate in the brain and the spinal cord.

B. Postganglionic fiber; originates in PNS within a ganglion. Definition: A ganglia generally refers to a collection of neuron cell bodies in PNS, e.g. sympathetic chain ganglia

THE GENERAL ORGANIZATION OF THE SYMPATHETIC OR THORACOLUMBAR PORTION OF THE AUTONOMIC NERVOUS SYSTEM:

A. Neurons involved in this pathway:

Preganglionic neuron: always in the CNS within the lateral horn/intermediolateral cell column of the spinal cord. The lateral horn is present only at spinal cord levels

T₁-L_2,3 meaning that every preganglionic sympathetic neuron originates at these levels. There are no exceptions to this rule.

Postganglionic neuron: always in the PNS and can be located in the sympathetic trunk or the prevertebral ganglia:

The sympathetic trunk- paired chains of ganglia connected by nerve fibers, location of sympathetic ganglia (Moore, Fig.1.33, p48) The sympathetic ganglia trunk;

1. is paravertebral (next to the vertebral column ) in location and extends from the base of skull to the coccyx

2. has a regular arrangement of ganglia including 3 cervical, 11 thoracic, 4 lumbar, and 4 sacral on each side although there is considerable variation. The two sides of the trunks usually unite as one final ganglia in the coccygeal region, the ganglion impar.

b. The prevertebral ganglia are associated with the unpaired visceral arteries and located in front of the vertebral column and cemented on the aorta. They are divided into the celiac ganglia (paired) , the superior mesenteric ganglia (small in humans) , and the inferior mesenteric ganglia (single).

B. Inputs and pathways;

Input to the sympathetic trunk: white rami communicantes (14 pairs) aggregates of preganglionic fibers leaving intermediolateral cell column or lateral horn (T1 -L 2-3) corresponding in number to the levels of the spinal cord containing the lateral horn. Synapse of the preganglionic fibers occurs most often within the sympathetic trunk. All preganglionic sympathetic fibers enter the sympathetic trunk through the white rami communicantes. The fibers are white because they are slightly myelinated.

Output from the sympathetic trunk:

a: Grey rami communicantes: (31 pairs, one/ spinal nerve) postganglionic fibers which are distributed with spinal nerve and supply sweat glands and blood vessels. The grey ramus is named because the fibers appear grey or unmyelinated.

b: Postganglionic sympathetic fibers: travelling to thoracic organs, neck, and head.

c: Preganglionic sympathetic fibers: which travel through the trunk to synapse in the prevertebral ganglia. These comprise most of the splanchnic nerves. The greater, lesser, and least splanchnic nerves and the lumbar splanchnic nerves are formed by these fibers and visceral afferents which travel with them but are not considered to be part of the autonomic nervous system.

Sympathetic neurotransmitters The preganglionic neurotransmitter is acetylcholine. The postganglionic neurotransmitter is norepinephrine (adrenergic). Postganglionic sympathetic fibers.to the sweat glands , however, are cholinergic using acetylcholine as a neurotransmitter.

 

D. Specific pathways or routes of a sympathetic preganglionic fiber (collectivelly known as white rami communicnates). Sympathetic preganglionic fibers can:

1. synapse in sympathetic chain ganglia and postganglionic fibers (grey rami) are then distributed with the spinal nerve

2. ascend and descend to appropriate level of the sympathetic trunk and synapse. This makes it possible for the entire body to receive sympathetic innervation through the spinal nerves . Although the lateral horn (intermediolateral cell column ) extends for only 14 levels of the spinal cord, the sympathetic trunk acts a thoroughfare for preganglionic sympathetic fibers originating in the lateral horn to reach an appropriate level, synapse, and become distributed with each of the 31 pairs of spinal nerves.

3. transverse and exit the trunk without synapsing. These preganglionic sympathetic fibers form the greater (T6-9), lesser (T 10-11), and lowest or least (T12 )splanchnic nerves which synapse in the celiac and superior mesenteric ganglia in the abdomen. Additional preganglionic sympathetic fibers from the upper 2 or 3 lumber nerves (the lumbar splanchnic nerves) synapse in the inferior mesenteric ganglia within the abdomen. All of these ganglia are associated with the celiac, superior mesenteric, and inferior mesenteric arteries. In contrast to the sympathetic trunk, a paravertebral structure, these ganglia are termed prevertebral because their location is directly anterior to the vertebral column. Postganglionic sympathetic fibers from these ganglia are distributed to the abdominal organs.

General Organization of Parasympathetic Nervous System

A. Craniosacral portion of the autonomic nervous system. Parasympathetics are distributed to trunk regions such as thorax, abdomen, pelvis (M 1.35, p50). Parasympathetics also control the purpillary sphincter and have a major effect on salivary glands and GI secretion and mobility.

The cranial portion of the parasympathetic system consists of preganglionic neurons and axons which originate in the brain stem and travel through the cranial nerves.

Cranial nerves III, VII, IX and X all carry preganglionic parasympathetic fibers.

CNIII innervates sphincter pupillae for the light response via the short ciliary nerves

CNVII innervates lacrimal gland via the greater petrosal nerve and the

submandibular and sublingual glands via the chorda tympani

CNX vagus (the wanderer)-very important nerve in thorax and abdomen , carries

parasympathetic innervation all the way down to the transverse colon and plays a

major role in peristalsis and glandular secretion within the GI tract.

2. Sacral portion

a. originates in S2,3,4 in spinal cord

b. in animals called preganglionic parasympathetic fibers comprise the "nervi erigens," energetic little nerves which control the reproductive organs, the sexual response, micturation, defecation, and the other pelvic viscera, assuming the function of the vagus below the transverse colon. In humans, these are called the pelvic splanchnic nerves.

3. The preganglionic fiber is very long and postganglionic fiber is very short because ganglion is usually located near or within the target organ. You will see some of them in histology (eg. submucosal and myenteric plexus in the GI tract, otic ganglia in head, epicardium for heart). Another name for these parasympathetic ganglia lying within the walls of the viscera are the terminal ganglia.

4. Neurotransmitter is acetylcholine in terminals of preganglionic and postganglionic

parasympathetic fibers.

ORGAN PARASYMPATHETIC SYMPATHETIC

Gut

Heart

Sweat Glands

Bronchioles

Eyes

Peripheral blood vessels

Sex organs

Blood vessels to skeletal muscle

Adrenal gland

Salivary Glands

1. Traverse many pathways, many travel with visceral efferents coursing through the sympathetic chain and reaching the spinal nerve by means of the white rami communicans. Afferent fibers travelling with the sympathetic fibers originate as ‘naked endings" in the walls of the of the organs and carry pain almost exclusively. Afferent fibers travelling with parasympathetic systems originate in the walls of the organ and consist of many encapsulated endings.

2. Stimuli do not reach consciousness during normal body homeostasis.

3. Pain or discomfort is perceived as an ache and is difficult to localize until it becomes severe.

the greatest splanchnic nerve:

the lumbar splanchnic nerves:

the vagus nerve:

a grey ramus communicans:

the sciatic nerve:

a white ramus communicans:

the pelvic splanchnic nerves: