Peripheral Nervous System

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Peripheral Nervous System (PNS)
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• PNS consists of nerve fibers coming out
• form CNS and connect all the body parts
• 12 pairs of cranial nerves and 31 pairs of
• spinal nerves.
•  
• According to what body parts they lead to,
• PNS is divided into
• Somatic nervous system
• Autonomic system (ANS).

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Somatic nervous system Nerves that connect CNS
and skeletal muscle, bones, joints, teeth and
skin. Autonomic nervous system Connects CNS
and all the internal organs and glands. ...
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  The somatic nerve system controls voluntary
body function, such as skeletal movement. The
autonomic system controls non-voluntary body
functions such as heartbeat, intestinal movement
and gland secretion.
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Autonomic Nervous System (Chapter 9)
Our surviving is only possible when there is a
homeostatic adjustment, which is partly
controlled by ANS.
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ANS coordinates cardiovascular, respiratory,
digestive, excretory, and reproductive
activities.   The effectors that respond to
autonomic regulation include cardiac muscle,
smooth muscle and glands.  
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Structural difference between ANS and somatic
system In somatic system, a motor neuron can
extend from CNS to effectors.
In ANS, there is always a second visceral motor
neuron between the CNS and the effector. The
second motor neurons have their cell body located
outside of the CNS, and are called ganglionic
neurons.
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The nerve fibers extend from CNS to reach the
ganglions are called preganglionic fibers those
that extend from ganglions to the effectors are
postganglionic fibers. Fig. 9.1
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Two Branches of ANS Sympathetic and
Parasympathetic Nervous Systems Generally,
sympathetic system causes excitatory, and
parasympathetic system causes inhibitory effects
in visceral organs. Homeostasis is based on the
balance of the two systems.
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Structure similarity between the two systems
both have pre- and post- ganglionic neurons
preganglionic neurons all origin from CNS.
They differ in the location of
ganglia. Sympathetic ganglia are near the spinal
cord. Parasympathetic are near the target
organs.
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Most organs are innervated by both sympathetic
and parasympathetic neurons, which run parallel
to each other. Overhead
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Functions of the sympathetic and parasympathetic
nervous systems Generally, the two systems have
opposing effects. When an organ is controlled
by both fibers, the organs activity is
balanced.
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For exp., heart rate is increased when
?-adrenergic receptors are are activated by
sympathetic nerves and is decreased when
muscarinic cholinergic receptors are activated
by parasympathetic nerves. Blood vessels are
dilated when the activity of parasympathetic
system is high, and are constricted when
sympathetic system is activated. Table 9.8
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Some times they may function independently, some
organs are only innervated by sympathetic
others are only controlled by parasympathetic
nerves. Exp. smooth muscle in the uterus is
only innervated by parasympathetic nerves.
When the activity of parasympathetic is high,
the uterus muscle contract. When the nerve
activity is low, the muscle relaxes.
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A. Sympathetic Nervous System Structure of
Sympathetic System The ganglia are located near
the spinal cord. Ganglia on each side of the
spinal cord are connected forming sympathetic
chain of ganglia - paravertebral chain ganglia.
The interconnection between ganglia causes the
whole system to act as a unit.
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Functions of sympathetic nervous system
Prepares the body for crises that may require
sudden and intense response. (fight or
flight effect) Causes increased heart rate and
blood pressure increased breathing rate and
increased skeletal muscle tone
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Neurotransmitters and sympathetic
functions Most postganglionic sympathetic
fibers release epinephrine and NE at the
junction of neuron and effectors.
Preganglionic sympathetic fibers release ACh
at ganglionic synapse.
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The biological effects of epinephrine and
norepinephrine are mostly excitatory but can
some times be inhibitory depending on the
specific cell membrane receptors.   For exp., The
cardiac muscle is stimulated to contract. The
smooth muscles in bronchioles is inhibited from
contraction.  
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  Two classes of adrenergic receptors Alpha
(?1, ?2) - can be found on vessels, in
eyes and skin. Beta (?1, ?2) - heart, smooth
muscles in breath way and liver cells.
(Table 9.8)
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  Activation of ?1 receptors ? vasoconstriction
? increased blood pressure.   Activation of ?1
receptors ? stimulation of cardiac muscle ?
increased hear rate Activation of ?2
receptors ? relaxation of respiratory smooth
muscle ? increase ventilation.  
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• -receptor blocker - a drug used for high
• blood pressure and heart arrhythmia.
• It blocks ?1 receptors to decrease heart
• contraction force and heart rate, so that blood
• pressure can be lowered.
• The side effect is the constriction of
  respiratory
• pipes and short of breath.

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The secondary messenger system of adrenergic
receptors   The action of ? receptors is
mediated by G protein- adenylate cyclase - cAMP
system ? receptors are mostly related with
phospholipase C, IP3 and Ca signaling pathway.
 
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Liver cell
?-receptor
?- receptor
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• Summary of sympathetic functions
• Hightened mental alertness
• A feeling of energy and euphoria (E and NE
• can mimic the action of dopamine)
• Increased cardiovascular activities
• heart rate and blood pressure.
• Increased respiratory activities faster and
• deeper breathing
• Increased skeletal muscle tone
• - Decreased digestive activity.

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B. Parasympathetic Nervous System Structure of
Parasympathetic System Parasympathetic ganglia
are located inside or near the effector
organs. Parasympathetic system is also called
craniosacral division of autonomic system.
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Four of the 12 pairs of cranial nerves contain
parasympathetic fibers III Ocilomotor (eye
movement, iris muscle), VII Facial (facial
expression, also tear and saliva secretion),
IX Glossopharyngeal (vomiting reflex), X
Vagus mixed nerve containing both sensory and
motor fibers, perform general control of
visceral organs.
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Vagus is the major branch of autonomic system
that is involved in the regulation of
cardiovascular, digestive and respiratory
functions.   Overhead
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Function of parasympathetic nervous system
Stimulate visceral activities. Causes rest
and repose effect. (Exp. after dinner
sleepy, relaxed, activated salivary secretion
and stomach movement.)
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Neurotransmitters and parasympathetic
function All parasympathetic functions are
mediated by ACh. Both pre- and postganglionic
parasympa- theticfibers release ACh
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    The effects of ACh released by postganglionic
parasympathetic fibers are generally
inhibitory. In some cases, parasympathetic
activities can be excitatory, depending on the
specific organ and receptors.
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• Nicotinic receptor
• Associated with Na channels
• Located on the skeletal muscles and cell
• surfaces of postganglionic neurons of
• sympathetic and parasympathetic systems.
• Mostly involved with excitation effect.

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• Muscarinic receptors
• Associated with G proteins (?? subunits)
• Located in parasympathetic neuroeffector
• junction.
• Mostly involved in inhibitory effects.

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General functions of parasympathetic
system Activation of parasympathetic system
causes 1). Decreased metabolic rate and mental
alertness 2). Decreased heart rate and blood
pressure, 3). Slowed breathing rate and
constriction of respiratory
passageway, 4). Increased secretion of salivary
and digestive glands,
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5). Increased mobility and blood flow of
digestive tract. 6). Stimulated
defecation. 7). Increased sexual arousal 8).
Relaxed iris muscles and constriction of pupils
9). Contraction of urinary bladder.
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  Generally the sympathetic tone is higher during
daytime. (adrenaline secretion reaches the peak
in the morning). Parasympathetic tone is
higher during the night.
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Control of ANS activities by the CNS The
balance of sympathetic and parasympa- Thetic
nervous systems is mostly controlled by medulla
oblongata. Medulla contains vital centers that
control cardiovescular, pulmonary, urinary,
digestive and reproductive functions.