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
1. 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 effectassuming 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.
a. 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. Theparasympathetic 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 muscleattempting 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
COMT
craniosacral division
dual innervation
epinephrine
homeostasis
hyperplasia
gap junctions
MAO
muscarinic receptor
nicotinic receptor
norepinephrine
parasympathetic nervous system
paravertebral ganglia
peristalsis
postganglionic neuron
preganglionic neuron
stress-relaxation response
sympathetic nervous system
terminal ganglia
thoracolumbar division
vasoconstriction
vasodilation
Practice Quiz
