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The Autonomic Nervous System and Smooth Muscle



Nervous System:
The Autonomic Nervous System
Smooth Muscle
Essential Information

Student Performance Objectives - for the lecture
1. Explain the aspects of body function regulated by the autonomic nervous system.
2. List and characterize the two major subdivisions of the ANS.
3. List the effects of each branch of the ANS on heart rate and cardiac output, respiratory rate and depth, coronary circulation, blood glucose level, and gastrointestinal peristalsis.
4. Define the terms dual innervation and antagonistic effects.
5. Explain what is meant by the terms thoacolumbar and craniosacral divisions of the ANS.
6. Explain how ANS motor neuron pathways compare with somatic nervous system pathways to skeletal muscle in terms of number of motor neurons involved.
7. Explain the difference between paravertebral, collateral and terminal ganglia.
8. Compare the lengths of preganglionic and postganglionic sympathetic and parasympathetic fibers.
9. Explain the terms: adrenergic fibers, and cholinergic fibers.
10. Explain why norepinephrine's effects on the body are longer lasting than those of acetylcholine.
11. Describe the similarities and differences between nicotinic and muscarinic receptors in the parasympathetic division of the ANS.
12. Describe the similarities and differences between alpha and beta adrenergic receptors in the sympathetic division of the ANS.
13. Describe the pathways by which ANS reflex circuits may be influenced by conscious, emotional states of being.
14. Compare skeletal and smooth muscle fibers in terms of size, arrangement of actin and myosin myofilaments, and metabolic source of ATP.
15. Define vasoconstriction and vasodilation.
16. Define peristalsis.
17. Compare the contractions of smooth and skeletal muscle fibers with regard to speed of contraction and relaxation, ability to contract when greatly stretched, energy required for a sustained contraction, and resistance to fatigue.
18. Explain the significance of the stress-relaxation response of smooth muscle.
19. Explain the role of hyperplasia in the enlargement of smooth muscle organs.

Lesson Outline

I. The Autonomic Nervous System

A. In General - The autonomic nervous system (ANS) regulates the body's internal environment. Through regulation of blood pressure, heart rate and strength, respiratory rate and depth, body temperature, and digestive processes, the reflexes of the ANS maintain homeostasis, that is, constant satisfactory conditions for the continuation of life. Although autonomic reflexes have both sensory and motor components, the ANS is technically defined as the motor portion of the reflexes that control the internal physiological mechanisms vital for our continued existence.

B. ANS Subdivisions
- the ANS is subdivided into a sympathetic branch and a parasympathetic branch. The sympathetic branch is sometimes called the "fight or flight" branch of the ANS in that it prepares the organism to fight or run effectively from a dangerous or stressful situation. The parasympathetic branch is sometimes called the "relaxation response" branch of the ANS in that it is activated when we eat, when we are relaxed and when we put ourselves into a state that is generally called a "meditative state."

C. Effects of the ANS on Target Organs
When the sympathetic branch of the ANS (abbreviated SNS) is activated, the following are some of the major changes observed in the body. Could you have predicted each effect assuming that the overall action is to promote organismal survival under a time of stress?
         a. Increased heart rate and force of contraction leading to
             (1) Increased blood pressure.
             (2) Increased stroke volume (more blood is pumped from the heart with each beat).
         b. Increased coronary blood flow leading to improved cardiac performance.
         c. Increased breathing rate and depth leading to improved CO2 release from the body and improved O2 intake into the body.
         d. Increased sweating leading to the potential for greater cooling of the active body.
         e. Increased blood glucose levels due to increased breakdown of liver glycogen which gives the muscles and brain a greater supply of energy from glucose breakdown.
         f. Dilation of the pupils presumably to improve visibility in time of danger.
         g. Increased blood flow to the skeletal muscles to maximize chances of success in battle or a successful escape from danger.
         h. Decreased blood flow to the skin and digestive organs to allow a shunting of greater blood flow to the skeletal muscles.
         i. Increased tendency of the blood to clot, presumably to enhance survival if there is injury.
         j. Increased secretion of epinephrine from the adrenal gland which augments all the actions above.
         k. Decreased contractions of the smooth muscles of the urinary bladder and the bowels leading to cessation of urination and defecation. In cases of overwhelming fear (e.g., imminent fear of death) the extremely high levels of norepinephrine released into the hypothalamus and amygdala release the central smooth muscle inhibition (GABA based) and the individual may uncontrollably urinate and defecate.
    2. When the parasympathetic branch of the ANS (abbreviated PNS) is activated, the effects are basically the opposite of those listed above (antagonistic effects). Note that, with some exceptions, internal organs of the body have dual innervation - the organs are innervated by both sympathetic and parasympathetic nerve fibers. Whatever reaction (stimulation or inhibition) one fiber causes, the other induces the opposite reaction. [Some blood vessels, the adrenal gland, sweat glands and piloerector muscles -those attached to hairs in the skin- operate by sympathetic stimulation alone].
D. ANS Anatomy
     1. Anatomical location of the systems
: The sympathetic nervous system's neural pathways are through the spinal nerves of the thoracic and lumbar regions of the spinal cord (T1-L2). This is the reason the SNS is sometimes called the thoraco-lumbar division of the ANS. The parasympathetic nervous system's neural pathways are through cranial nerves III (originating in the midbrain), VII (originating in the pons), IX and X (both originating in the medulla oblongata), and the sacral region of the spinal cord (S2-S4). This is the reason the PNS is sometimes called the cranio-sacral division of the ANS.
    2. ANS motor nerves: The pathway from the central nervous system to the target organs of the ANS is through 2 successive motor neurons - a preganglionic neuron and a postganglionic neuron. The preganglionic neuron travels from its origin in the brain or spinal cord to a ganglion (collection of cytons outside the CNS). The postganglionic neuron begins in and travels from the ganglion to the smooth muscle or gland being innervated.
    3. Anatomical location of the ganglia
a. Sympathetic ganglia are located in two locations: in a connected chain of ganglia lateral to the vertebral column - called the sympathetic chain ganglia (also called paravertebral ganglia), and in a group of ganglia located on the anterior surface of three major abdominal blood vessels (aorta, superior mesenteric, and inferior mesenteric arteries), called the collateral ganglia. The ganglion on the aorta is called the celiac ganglion. Those on the superior and inferior mesenteric arteries are logically called superior and inferior mesenteric ganglia. Preganglionic sympathetic fibers originate from lateral horns of gray matter in the thoracic and lumbar regions of the spinal cord. Their axons travel to and synapse with postganglionic fibers in either the paravertebral or collateral ganglia.
        b. Parasympathetic ganglia are not located near the vertebral column. They are located in ganglia near the target organs - called terminal ganglia.
    4. Length of pre and postganglionic fibers in the ANS.
Sympathetic preganglionic fibers are short because of the close proximity of the ganglia to the vertebral column. Postganglionic sympathetic fibers are long because they must travel from the ganglia all the way to their target organs.
        b. Parasympathetic preganglionic fibers are long because they must travel all the way from the brain or sacral region of the spinal cord to the terminal ganglia near the target organs. The parasympathetic postganglionic fibers are short because they only need to travel a short distance from the terminal ganglia to the organ in question.

E. ANS Physiology
    1. Neurotransmitters released from ANS motor neurons.

a. Sympathetic preganglionic fibers release acetylcholine (Ach) at their synapses in the ganglia. They are called cholinergic fibers because of their release of Ach. Sympathetic postganglionic fibers release mostly norepinephrine (NE) at their synapses in the smooth muscle of the target organs. They are called adrenergic fibers because of their release of NE.
        b. Parasympathetic pre and postganglionic fibers release Ach at their synapses. All parasympathetic fibers are cholinergic because of their release of Ach.
    2. General Effects of cholinergic and adrenergic fibers.
 a. Cholinergic fibers have a generally rapid effect on the body because cholinesterase rapidly breaks Ach down in the synapse after it is released from synaptic vesicles.
        b. Adrenergic fibers have a generally prolonged effect
on the body because NE is either not broken down at all at the synapse or is broken down more slowly after its release from synaptic vesicles.
              (1) Some NE diffuses away from the synapse and enters the blood stream where it mixes with epinephrine released from the adrenal glands and influences target organs for many minutes until it is broken down in the liver.
              (2) Some NE is reabsorbed by the presynaptic membrane and either re-secreted or broken down by the enzyme monoamine oxidase (MAO).
             (3) Some NE diffuses away from the synapse and is broken down by another enzyme, catechol-O-methly transferase (COMT).
    3. ANS receptors
        a. Cholinergic receptors are those to which Ach attaches in the synapse. There are two basic cholinergic receptors - nicotinic and muscarinic.
            (1) Nicotinic receptors are the ones you learned about in the chapter on skeletal muscle. These are the receptors at neuromuscular junctions on the sarcolemma of skeletal muscle fibers to which Ach attaches ultimately resulting in skeletal muscle contraction. Nicotine receptors are also located in all sympathetic and parasympathetic ganglia, and the adrenal gland. All nicotinic receptors are stimulated by Ach.
            (2) Muscarinic receptors are found on the cell membranes of smooth and cardiac muscle fibers, and on glands. Ach stimulates some muscarinic subclasses and inhibits others. You already know what happens in most organs - see part C, above.
        b. Adrenergic receptors are those to which NE attaches in the target organs. There are two basic adrenergic receptors - alpha adrenergic receptors and beta adrenergic receptors. Each basic class has subclasses. NE binding to alpha receptors is usually excitatory; NE binding to beta receptors is usually inhibitory. The distribution of these receptor classes and subclasses on smooth and cardiac muscle and on glands determines NE's effect on these organs. Once again, you already know what happens in most organs - see part C, above.

F. Influences on the ANS
     1. Preganglionic neurons of the ANS, whether in the brain or spinal cord, can be influenced by thoughts and emotions because nerve impulses can travel along conscious pathways in the cerebral cortex, and then pass to subconscious pathways in organs of the limbic system, like the hypothalamus, that stimulate or inhibit the basic ANS reflexes.
    2. Another route for ANS influence is the reticular formation that extends throughout the brainstem and up into the diencephalon: descending pathways from the cerebral cortex and limbic system can influence the nuclei of cranial nerves III, VII, IX, and X that are embedded within the reticular formation and which mediate parasympathetic functions.

II. Smooth Muscle

A. In General,
smooth muscle is a type of involuntary muscle located in the walls of the body's internal hollow organs like those in the digestive system (e.g., esophagus, stomach, small and large intestine), the urinary system (e.g., ureters, urinary bladder and urethra), the blood vessels (e.g., arteries, arterioles, veins, and venules), and the respiratory system (trachea, bronchi, and bronchioles).

B. Smooth muscle fibers possess the following characteristics:
    1. They are small, spindle shaped, and do not possess the striations seen in skeletal muscle.
    2. They generally have one nucleus (single-unit smooth muscle), although some forms of smooth muscle are multinucleate (multi-unit smooth muscle). Single unit smooth muscle fibers have few mitochondria; most of their energy comes from anaerobic metabolic pathways (e.g., glycolysis, or the breakdown of glucose to pyruvic and lactic acids.
    3. Smooth muscle fibers are organized into longitudinal and circular sheets in organ walls.     4. Smooth muscle fibers only possess a thin connective tissue sheath - endomysium - whose connective tissue fibers (collagen and elastin) come from the smooth muscle cell itself, not fibroblasts.
    5. Contractions of sheets of smooth muscles in the walls of the digestive system take the form of peristaltic waves, segmental (mixing) contractions, and, when required, antiperistalsis (for vomiting). In general, smooth muscle contraction and relaxation are significantly slower than those in skeletal muscle.
    6. Contractions of smooth muscle sheets in the blood vessels are described as vasoconstriction and vasodilation. The smooth muscle sheets of the large arteries are the multi-unit type.
    7. Typical neuromuscular junctions, as seen in skeletal muscle, are not observed in single unit smooth muscle- the smooth muscular nerve-muscle junctions have wider synaptic clefts and are called diffuse junctions. More typical neuromuscular junctions are observed in multi-unit smooth muscle of large arteries, and the walls of the trachea and bronchi.
    8. Since the sarcoplasmic reticulum (SR) directly contacts the sarcolemma, there are no t-tubules in smooth muscle fibers. The release of calcium ions from the SR is the stimulus for the ATP-mediated interaction of actin and myosin myofilaments, as in skeletal muscle.
    9. The myosin heads, that attach to and pull on actin, are found along the entire length of the myosin molecule which gives smooth muscle fibers excellent gripping power.
    10. Actin and myosin are wrapped around each other like twisting 2 lengths of wire together. The actin and myosin spiral down the long axis of the smooth muscle fiber. This is a significantly different pattern than that seen in skeletal muscle which has more linearly arranged sarcomeres. An intracellular cytoskeleton of intermediate filaments and dense bodies takes the place of the Z-lines of skeletal muscle in anchoring the actin and myosin myofilaments to the smooth muscle's sarcolemma.
    11. There is a fatigue-resistant locking mechanism that allows smooth muscle fibers, once contracted, to maintain their contraction with little expenditure of energy. This is of importance in the continual contraction required for blood vessel tone that manages our blood pressure for our entire lives. Such a mechanism is also important in the contraction of the uterus during childbirth where a certain degree of continuous pressure is required, without being interrupted by fatigue, for the expulsion of the fetus. The maintenance of tension in smooth muscle requires less than 1% of the energy that would be required by skeletal muscle attempting to accomplish the same task.
    12. Gap junctions between adjacent smooth muscle fibers permits synchronized contractions of smooth muscle sheets which is required for peristaltic waves, uterine expulsion of a baby, urinary bladder expulsion of urine, and vasoconstriction or vasodilation.
    13. As described in the ANS section of this unit, the sarcolemma possesses different receptor types that determine the response (excitation or inhibition) to acetyl choline or norepinephrine being released from postganglionic sympathetic and parasympathetic nerve fibers.
    14. Although stretching promotes smooth muscular contraction, as in skeletal and cardiac mucle, smooth muscle's response to stretch includes a stress-relaxation response that permits organs like the stomach and urinary bladder to slowly fill-up before reaching a critical point where they are stimulated to contract and empty themselves.
    15. If a skeletal muscle is stretched too much, it loses power because of the loss of overlap of actin and myosin in the sarcomeres. But the spiral arrangement of actin and myosin in smooth muscle fibers results in excellent contractile ability even under conditions of great distention. So a very stretched urinary bladder or uterus still contracts very effectively to push out its contents.
    16. The enlargement of smooth muscle as in a pregnant uterus, involves hyperplasia (mitosis to produce additional cells) and not just hypertrophy (enlargement of cells without mitosis to produce additional cells).

Biomedical Terminology
:   Define each term:

adrenergic fiber
adrenergic receptor
antagonistic effects
cholinergic fiber
cholinergic receptor
collateral ganglia
craniosacral division
dual innervation
gap junctions
muscarinic receptor
nicotinic receptor
parasympathetic nervous system
paravertebral ganglia
postganglionic neuron
preganglionic neuron
stress-relaxation response
sympathetic nervous system
terminal ganglia
thoracolumbar division