Chapter 14

The Autonomic Nervous System Objectives

1. Define autonomic nervous system and list its components.

2. Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways, and neurotransmitters released.

3. Compare and contrast the general functions of the parasympathetic and sympathetic division and the general effects of each on major body systems.

4. Compare and contrast the general anatomical features of the parasympathetic and sympathetic divisions.

5. List the cranial and sacral nerves which carry parasympathetic fibers and the structures they innervate.

6. Describe the sympathetic pathways from origin to sympathetic chain to structures innervated.

7. Explain the mechanism of referred pain.

8. Describe the two types of cholinergic receptors with respect to location of receptors and effect of acetylcholine binding.

9. Describe the two types of adrenergic receptors with respect to location of receptors and effect of norepinephrine binding.

10. Explain the mechanisms by which the drugs neostigmine, tricyclic antidepressants, and beta blockers function.

11. Briefly describe the interactions of the sympathetic and parasympathetic divisions.

12. Compare and contrast the sympathetic and parasympathetic divisions with respect to tone, localized vs. diffuse effects, and unique sympathetic roles.

13. Describe the control centers of autonomic functions.

14. Define the autonomic disorders of hypertension and Raynaud's disease.

AUTONOMIC NERVOUS SYSTEM

Regulates the activities of smooth muscle, cardiac muscle, and certain glands.

Consists of: visceral efferent neurons, visceral afferent neurons with receptors

Functionally it operates without conscious control. The system is regulated by centers in the brain, in particular by the hypothalamus and medulla oblongata.

Comparison of the Somatic and Autonomic Systems

Both have motor fibers.

Differ in their effectors, in their pathways, and in target organ responses to their neurotransmitters.

Divisions of the ANS

Two major arms are the:

1. Parasympathetic

2. Sympathetic

Generally they serve the same visceral organs but cause essentially opposite effects. The two divisions counterbalance each other's activities .

The parasympathetic system is primarily concerned with activities that conserve and restore body energy during times of rest or recovery of the body. ("Resting and digesting system")

Under normal conditions, parasympathetic impulses to the digestive glands and smooth muscle of the G.I. tract dominate over sympathetic impulses. Blood pressure, heart and breathing rates are regulated at low levels, while the GI tract is active after a meal. The skin is warm and the eye pupils are constricted to protect the retinas.

The sympathetic system mobilizes the body during extreme situations of stress. The sympathetic division is primarily concerned with processes involving the expenditure of energy due to either physical or emotional activities. ("Fight-or-flight system") During homeostasis, the main function of the sympathetic division is to counteract the parasympathetic effects just enough to carry out normal processes requiring energy.

During vigorous exercise, the sympathetic division promotes certain adjustments. Visceral and/or cutaneous vessels are constricted, and cardiac and skeletal vessels are dilated shunting blood to the active muscles.

The bronchioles in the lungs are dilated to increase ventilation and the liver releases glucose into the blood.

Temporarily nonessential activities are damped.

Anatomy of the ANS

The two systems are distinguished by:

1. Unique origin sites - parasympathetic fibers emerge from the cranial and sacral regions of the spinal cord. The sympathetic fibers originate from the thoracolumbar region of the spinal cord.

2. Different lengths of their fibers: parasympathetic has long preganglionic and short postganglionic fibers. The sympathetic has the opposite condition.

3. Location of their ganglia: most parasympathetic ganglia are located in the visceral effector organs. Sympathetic ganglia lie close to the spinal cord.

Parasympathetic Division

Originates from cranial and sacral regions of spinal cord (craniosacral).

The preganglionic axons extend from the CNS nearly all the way to the target structures where they synapse with postganglionic neurons located in terminal ganglia that are within or very close to the target organs.

Short postganglionic axons issue from the terminal ganglia and run to synapse with effector cells.

Cranial Outflow

Parasympathetic outflow is contained in several cranial nerves.

1. Oculomotor nerves (III): innervate the smooth muscles within the eyes that cause the pupils to constrict and the lenses of the eyes to bulge.

2. Facial nerves (VII): stimulate the secretory activity of many large glands in the head. Stimulates the nasal glands and the lacrimal glands.

3. Glossopharyngeal nerves (IX): activate the parotid salivary glands which lie anterior to the ears.

These three pairs of cranial nerves supply the entire parasympathetic innervation of the head.

4. Vagus nerves (X): accounts for 90% of all preganglionic parasympathetic fibers in the body.

The two vagus nerves provide fibers to the neck and contribute to nerve plexuses that serve virtually every organ in the thoracic and abdominal cavities. As the vagus nerves pass into the thorax, they branch to the following plexuses located within or near the organs served:

1. cardiac plexuses: supplying the heart- slow heart rate.

2. pulmonary plexuses: serve the lungs and bronchi.

3. esophageal plexuses: supplying the esophagus.

When the main trunks of the vagus nerves reach the esophagus, their fibers form the anterior and posterior vagal trunks. The vagal trunks enter the abdominal cavity and send fibers through the large aortic plexus. The sympathetic and parasympathetic fibers are intermingled in these abdominal plexuses.

Organs receiving vagal innervation include: Liver, stomach, small intestine, kidneys, pancreas, and the proximal half of the large intestine.

Sacral Outflow

Arises from neurons located in S2-S4. Innervates the rest of the large intestine and the pelvic organs.

Sympathetic Division

Originates from the thoracolumbar region of spinal cord. More complex, innervates more organs.

All preganglionic fibers of the sympathetic division arise from cell bodies of preganglionic neurons located in spinal cord segments T1-L2. After leaving the spinal cord, the preganglionic neurons enter the sympathetic chain, which is a sequence of connected ganglia located adjacent to the vertebral column. Although the sympathetic chain extends from the neck to the pelvis, the sympathetic fibers arise only from the thoracic and lumbar cord segments. There are 23 ganglia in each sympathetic chain.

All sympathetic ganglia are close to the cord, and the postganglionic fibers which run from the ganglion to the organs they supply, are much longer than the preganglionic fibers.

Visceral Sensory Neurons

Send information concerning chemical changes, stretch, and irritation of the viscera, and are the first link in autonomic reflexes. Nearly all the sympathetic and parasympathetic fibers are accompanied by afferent fibers conducting sensory impulses from glands or muscular structures.

The fact that visceral pain afferents travel along the same pathways as somatic pain fibers helps to explain the phenomenon of referred pain, in which pain stimuli arising in the viscera are perceived as somatic in origin.

A heart attack may produce a sensation of pain that radiates to the superior thoracic wall and along the medial aspect of the left arm. The same spinal segments innervate both the heart and the regions to which pain signals from heart tissue are referred. The brain interprets most such inputs as coming from the more common somatic pathway.

Neurotransmitters and Receptors

Major neurotransmitters are: Acetylcholine (ACh) Norepinephrine (NE)

ACh is released by:

1. all preganglionic axons of both ANS divisions

2. all parasympathetic postganglionic axons at synapses with their effectors

ACh-releasing fibers are called cholinergic fibers.

Most sympathetic postganglionic axons release NE and are classified as adrenergic fibers.

The effects of ACh and NE on their effectors are not consistently either excitation or inhibition. The response of visceral effectors depends not only on the neurotransmitter but also on the receptors to which they attach.

The two or more kinds of receptors that exist for each of the neurotransmitters allow these chemicals to exert different effects at different body targets.

Cholinergic Receptors

The two types of receptors that bind ACh are named for drugs that bind to them and mimic ACh effects.

1. Nicotinic

2. Muscarinic (mushroom poison)

Muscarine has no effect on nicotinic receptors, and vice versa.

Nicotinic receptors are found on:

1. motor end plates of skeletal muscle cells
2. postganglionic neurons
3. hormone-producing cells of the adrenal medulla

The effect of ACh binding to nicotinic receptors is always stimulatory.

Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers - all parasympathetic target organs and a few sympathetic targets. The effect of ACh binding to muscarinic receptors is inhibitory or stimulatory, depending on the target organ. Binding of ACh to cardiac muscle receptors slows heart activity, whereas ACh binding to receptors on smooth muscle of the G.I. tract cause increased motility.

Adrenergic Receptors

Two classes: 1. alpha 2. beta

Organs that respond to NE display one or both types of receptor. Generally, NE binding to an alpha receptor is stimulatory, while the binding to beta receptors is inhibitory. Notable exception includes binding of NE to beta receptors of cardiac muscle which causes increased heart activity.

Effects of Drugs

Knowing the location of the various receptors is very important clinically because it allows specific drugs to be prescribed to obtain the desired blocking of stimulatory effects on selected target organs.

The anti-ACh drug, neostigmine, inhibits the enzyme acetylcholinesterase preventing the breakdown of ACh and allowing it to accumulate in the synapses. Used as a treatment for myasthenia gravis, a condition in which skeletal muscle activity is impaired for lack of ACh stimulation.

Tricyclic antidepressants help to relieve depression because they prolong the activity of NE on the post synaptic membrane.

Adrenergic blockers that attach specifically to the beta receptors of cardiac muscle (beta-blockers) (acebutolol) are used when the goal is to reduce heart rate and prevent arrhythmias in patients without interfering with other sympathetic effects.

Interactions of the Autonomic Divisions

Most visceral organs receive dual innervation. Both ANS divisions are partially active, producing a dynamic antagonism that allows visceral activity to be very precisely controlled. One division usually exerts the predominant effects in given circumstances. Antagonistic effects are most clearly seen on the activity, of the heart, respiratory system and the G.I. organs.

The sympathetic division increases both respiratory and heart rates while inhibiting digestion and elimination during times of high stress. When the emergency is passed, the parasympathetic division restores heart and breathing rates to resting levels and then attends to processes that refuel the body cells and discard wastes.

Sympathetic and Parasympathetic Tone

With few exceptions, the vascular system is entirely innervated by sympathetic fibers that keep the blood vessels in a continual state of partial constriction. When faster blood delivery is needed, these fibers cause the vessels to constrict and blood pressure to rise. When blood pressure is to be decreased, the vessels are allowed to dilate.

Drugs (phentolamine) that interfere with the activity of these vasomotor fibers are often used to treat hypertension.

During circulatory shock or when more blood must be provided to meet the increased needs of working skeletal muscles, blood vessels serving the skin and abdominal viscera are strongly constricted. This "blood shunting" helps to maintain circulation to vital organs or enhance blood delivery to skeletal muscles.

The parasympathetic effects normally dominate the heart and smooth muscles of the digestive and urinary tract organs. The parasympathetic division prevents unnecessary heart acceleration and also determines the normal activity levels of the digestive and urinary tracts. The sympathetic division can override these effects during times of stress.

Unique Roles of the Sympathetic Division

Regulates many functions not subject to parasympathetic influence. For example, the adrenal medulla, the sweat glands and arrector pili muscles of the skin, the kidneys, and most of the blood vessels receive only sympathetic fibers.

Localized Verses Diffuse Effects

Parasympathetic division exerts short-lived, localized control over its effectors. However, when the sympathetic division is activated, it responds in a diffuse and highly interconnected way. Its effects are also much longer lasting.

This reflects three phenomena:

1. NE is inactivated more slowly than ACh

2. NE which acts through a second messenger system, exerts its effects more slowly.

3. When the sympathetic division is mobilized, small amounts of NE (15%) and larger amounts of epinephrine (85%) are secreted into the blood by the adrenal medullary cells. These effects continue for several minutes until the hormones are destroyed by the liver.

Control of Autonomic Functioning

Although the ANS is not usually considered to be under voluntary control, its activities are regulated. Several levels of CNS controls exist - in the spinal cord, brain stem, hypothalamus, and cerebral cortex..

The brain stem appears to exert the most direct influence over the ANS functions. Most sensory impulses involved in eliciting these autonomic reflexes are delivered to the brain stem via the vagus nerve afferents.

The main or overall integration center of the ANS is the hypothalamus. The hypothalamus contains centers that coordinate heart activity, blood pressure, body temperature, water balance, and endocrine activity. It also contains centers that help mediate various emotional states and biological drives. The hypothalamus serves as the keystone of the emotional/ visceral brain, and it is through its centers that emotions influence ANS functioning and behavior.

Homeostatic Imbalances

The ANS is involved in nearly every important process that goes on within the body. Most autonomic disorders reflect exaggerated or deficient controls of smooth muscle activity. Two examples are hypertension and Raynaud's disease.

Hypertension may result from an overactive sympathetic nervous system response promoted by continual high levels of stress. It is always serious because it increases the workload on the heart, and it increases the wear and tear on the artery walls.

Raynaud's disease is characterized by intermittent attacks during which the skin of the fingers and toes becomes pale, then cyanotic and painful. Commonly caused by exposure to cold, it is thought to be an exaggerated vasoconstriction response in the affected body region.