INTRODUCTION TO THE

AUTONOMIC NERVOUS SYSTEM

Dr. James Baker

Dr. Blair Turner

Dr. Robert J. Cowie

Objectives:

The student should be able to:

1. Describe in review the typical thoracic spinal nerve and its four functional components.

2. Compare and contrast terms and concepts related to the sympathetic and parasympathetic systems, including: the central location of cell body of origin, number of synapses between CNS and effector organ, degree of myelination, and general effect on target tissue(s).

3. Define and contrast pre- and postganglionic autonomic neurons, and white and gray rami communicantes.

4. Trace the course of the sympathetic and/or parasympathetic innervation to the smooth muscle of: a cutaneous arteriole in the thoracic wall; or, in a bronchus.

5. Define the anatomical and functional relationships of the autonomic ganglia and plexuses related to the:

a. Paravertebral (chain) ganglia

b. Prevertebral (collateral) ganglia

c. Terminal (enteric) ganglia in or near body organs

Outline:

I. Introduction: Consideration of general terminology and concepts

A. Comparison of important terms

1. nerve vs. tract -

2. ganglion vs. nucleus -

3. receptor vs. effector -

4. voluntary vs. involuntary -

5. convergence vs. divergence -

6. white matter means myelinated

7. gray matter means nonmyelinated

8. "fiber" means axon

B. Compare and contrast concepts of:

1. central nervous system (CNS) vs. peripheral nervous system (PNS)

2. neural signals for specific functional components are carried in PNS fibers comprising any named nerve:

a. General vs. Special components

i. general functions are found in locations throughout body, i.e. cutaneous sensation, or skeletal muscle contraction

ii. special functions are found in a few (cranial) nerves of cephalic origin, ie. related to olfaction, vision, taste, hearing,balance, and branchiomeric tissues

b. Somatic vs. Visceral components

i. somatic tissues are derived from somitic mesoderm, ie. arise from a sclerotome, myotome and dermatome forming the "body wall"

ii. visceral (splanchnic) tissues are derived from splanchnic mesoderm or endoderm, or are of branchiomeric origin (gill arches; see Head & Neck)

c. Efferent (motor) vs. Afferent (sensory) components

i. efferent components carry a signal away from CNS, to an effector organ,

ii. afferent components carry signals toward CNS from receptor organs in skin, joint, muscle or visceral tissue.

C. The Autonomic nervous system (ANS) The original meaning of the word "autonomic" meant self-controlling, independent of outside influences, spontaneous. Experimental research has shown that none of these adjectives is still valid. The "autonomic" nervous system regulates movements of the viscera in response to both external and internal stimuli. Therefore, like the somatic nervous system, peripheral autonomic nerves contain both sensory afferents and motor efferents to form reflexes of the spinal cord and brainstem. Again like the somatic nervous system, these autonomic reflexes are regulated by "upper motor neurons". For the ANS, these are located in the hypothalamus.

Sensory afferents (GVAs) of the ANS sense pain and the amount of stretch in visceral (smooth) muscle, and relay this information to the spinal cord or brainstem to form autonomic reflexes or to transmit the information to higher centers in the central nervous system for further processing. The motor efferents of the ANS are composed of two opposing systems providing general-visceral-effector (GVE) functional components. They function to restore and maintain the "unconscious" homeostasis of internal bodily functions, and to prepare for meeting environmental stresses.

1. Parasympathetic system - provides GVE signals necessary for maintaining "vegetative" behaviors

2. Sympathetic system - provides GVE signals necessary for the preparation for "fight/flight" behaviors

3. Homeostasis is maintained by the appropriate balance of the two systems, under control of higher (CNS) systems

4. We will incorporate the GVA and GVE components into the organization presented below.

II. General characteristics of the A.N.S.

A. The ANS always displays two neurons in the motor pathway from CNS to the effector organ. This contrasts with the situation in the somatic-efferent system where there is one neuron in the path from CNS to a skeletal muscle effector. The two ANS neurons are designated the pre- and post-ganglionic neurons.

1. Preganglionic (pre-g) neurons and their axons (fibers)

a. the cell body is located in the CNS

b. its axon may reach most, or only part, of the distance to target organ

c. it projects to, and synapses on, the postganglionic neuron

2. Postganglionic (post-g) neurons and fibers

a. the cell body is located in an autonomic ganglion (motor ganglion)

b. its axon may be relatively short, or long

c. it projects to, and synapses on, a visceral effector organ

3. some pre-g and post-g neurons use different neurotransmittors

a. all pre-g and post-g parasympathetic, and all pre-g, and sudomotor (sweat-related) post-g sympathetic neurons use acetylcholine

b. the remianing post-g sympathetic neurons use epinephrine

4. Visceral effector organs are always comprised of only one of three tissue types, either:

a. glands of all types which are controlled by a nerve supply (non-endocrine)

b. smooth muscle - ie., arrector pili, in vascular walls, or in walls of ducts & viscera

c. or cardiac muscle - the heart myocardium has an intrinsic pacemaker; but ANS controls rate and force of contraction

5. only one primary afferent neuron provides the pathway to the CNS from visceral and somatic receptors

a. GVA & GSA pseudounipolar cell bodies are located in the (sensory) dorsal root ganglia (DRG) or in cranial nerve ganglia

III. Overview of Characteristics of Parasympathetic and Sympathetic NS.

A. Sympathetic N.S. - The thoracolumbar outflow

1. pre-g GVE neurons are located in intermediolateral (IML) cell column of the spinal gray matter, between T1 and L2 segments only

a. myelinated fibers leave by the ventral root and extend to an autonomic ganglion via the ventral primary ramus of that segment

b. fibers synapse on post-g GVE cells located in the ganglion

2. non-myelinated post-g fibers extend all the way to an effector organ

a. ganglion is typically located some distance from the target organ

3. post-g fibers tend to travel on arteries to reach their targets

4. the number of post-g cells far outnumbers the pre-g cells the anatomical basis for simultaneous divergence of signal to many effectors from a "single triggering" event

B. Parasympathetic N.S. - The craniosacral outflow

1. pre-g GVE neurons located in nuclei within the brainstem project fibers to the periphery via cranial nerves III, VII, IX, and X. (see Head & Neck ANS)

a. Fibers tend to travel with branches of the trigeminal nerve to reach nearly to the effector organs within the head, and they join plexuses to reach vicera in the neck, thorax and abdomen (see Abdominal ANS)

2. Also, pre-g GVE neurons located in the intermediolateral cell column of the sacral cord project axons via spinal nerves S 2,3 & 4

a. fibers join plexuses in pelvic cavity to reach near pelvic viscera (see Pelvic ANS)

3. In all cases, the fibers synapse on post-g cells, located in enteric ganglia, which then send a relatively short, non-myelinated fiber to the adjacent effector organ.

4. Here, the two neuron arc is organized so that pre-g cells project a relatively long, myelinated axon, while the post-g cells lie close to or within the wall of the target viscus.

5. The number of pre-g and post-g neurons is roughly the same.

a. a single pre-g fiber only synapses with one, or a few, post-g cells within the enteric ganglion (non-divergent system)

 

IV. Detailed Organization of the Sympathetic N.S.

A. Pre-g neurons located in the IML (visceromotor, GVE) cell column of each T1 to L2-3 (thoracolumbar) segment project myelinated fibers via its spinal nerve to a paravertebral g. located along lateral aspect of the same vertebral body.

1. White rami communicantes consist of the many myelinated pre-g fibers leaving their spinal nerve.

a. fibers enter the paravertebral g. of the segment of origin (see "chain", below)

b. thus, they are only found between T1 and L2

2. Upon entering, the axon may take one of four courses:

a. It may synapse on post-g neurons located in this paravertebral g.

i. then, all post-g fibers take pathway to body wall or upper extremity (T1)

b. It may ascend to a higher vertebral level within the sympathetic chain, beforesynapsing on post-g neurons in a more superior paravertebral g.

i. this is the pathway to head and neck, and upper extremity (C4-8)

c. It may descend to a lower vertebral level within the sympathetic chain, before synapsing on post-g neurons in a more inferior paravertebral g.

i. this is the pathway to lower extremities and external genitalia

d. Finally, it may pass through this paravertebral g. (without synapse) to course inferomedially via a splanchnic nerve, and synapse on post-g neurons in a prevertebral g. or in the adrenal medulla

i. this is the pathway to abdominopelvic viscera.

 

 

 

 

 

3. Gray rami communicantes consist of many non-myelinated post-g fibers returning to each mixed spinal nerve.

a. to be distributed via its dorsal & ventral primary (1o) rami to effector organs of the "body wall" (including extremities)

i. smooth muscle in blood vessels (vasomotor) throughout the body wall

ii. sweat glands (sudomotor) in the skin.

iii. arrector pili muscles (smooth) of the hair follicles in the skin

b. thus, gray rami may be found at all spinal segments (C1-Co3)

B. Post-g neuronal organization

1. paravertebral ganglia

a. generally, are associated with each spinal nerve, although adjacent cervical and coccygeal gg. may be fused

b. adjacent gg. are interconnected by many longitudinal fibers, forming sympathetic chain (or trunk), which runs from base of cranium to sacrum, bilaterally

i. 3 cervical - from fusion of 8 cervical gg.

ii. 12 thoracic - T1-12

iii. 5 lumbar - L1-5

iv. 5 sacral - S1-5

v. 1 ganglion impar - from 2 or 3 fused coccygeal gg.

2. Splanchnic nerves are composed of pre-g axons (having passed through their paravertebral g.), and connect the spinal segments directly with thoracic and lumbar prevertebral ganglia.

a. 3 thoracic splanchnic nerves

i. Greater splanchnic n. originates from T5-9 segments; distributes mostly to the celiac g. & plxeus

ii. Lesser splanchnic n. orginates from T9-11 segments; distributes to thesuperior mesenteric & aorticorenal gg. & plxs.

iii. Least splanchnic n. originates from T12 segments; distributes to the renal g. & plx.

b. several Lumbar splanchnic ns distribute to inferior mesenteric & pelvic gg.

3. Prevertebral (preaortic) ganglia contain post-g neurons, lie anterior to the vertebral bodies and aorta

a. they surround (and distribute with) the three principal, unpaired arterial trunks from the aorta (See above and Abdominal ANS).

b. these gg. distribute unmyelinated post-g fibewrs to viscera via arterial brs.

4. Adrenal medullary chromaffin cells of the suprarenal medulla (deep portion of adrenal gland) receive direct pre-g fibers, and are considered to be specialized postganglionic cells (See Abdominal ANS).

 

V. Detailed Organization of the Parasympathetic N.S.: Pre-g somata of the craniosacral outflow are located at specific levels of the visceromotor (GVE) cell-column of brainstem and sacral cord.

A. Oculomotor (III) CN (see Head and Neck ANS)

B. Facial (VII) CN

C. Glossopharyngeal (IX) CN

D. Vagus (X) CN

1. The pre-g cells in dorsal motor nucleus of the vagus nerve, in the medulla oblongata, end axons to many, small intramural ganglia located in the walls of cervical, thoracic, and abdominal viscera (form part of the enteric plexuses).

a. post-g cells in thoracic intramural gg. (mostly within the walls of the organ) send fibers to nearby effectors, including:

i. tracheal and bronchial smooth mm. (bronchioconstriction), and respiratory mucosal glands (secretomotor)

ii. cardiac pacemaker (sinoatrial nodal brs. slow rate); smooth mm. decrease in coronary aa. flow (lowering myocardial workload)

E. Sacral parasympathetics (Nervus energens, S2,3,&4; see Pelvic ANS)

 

Copyright©1996-99 B. Turner. All rights reserved.