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Chapter 7
The Peripheral Nervous System:
Efferent Division
Outline
• Autonomic nervous system
• Somatic nervous System
• Neuromuscular Junction
Outline
• Autonomic nervous system
– Anatomy
• Pre and post fibers, sympathetic ganglia chain, collateral ganglia,
terminal ganglia
– Neurotransmitters
• Pre = Acetylcholine (ach)
• Post = adrenaline (epinepherine) noradrenaline (norepinepherine)
– Sympathetic and parasympathetic branches
• Dual innervation
• Sympathetic dominance “fight or flight”
• Parasympathetic dominance “rest and digest”
– Receptor types
• Cholinergic, muscarinic, nicotinic
• Adrenergic. a1 a2 b1 b2
– CNS control
PNS: Efferent Division
• Communication link by which CNS controls activities
of muscles and glands
• Two divisions of PNS
– Autonomic nervous system (ANS)
• Involuntary branch of PNS
• Innervates cardiac muscle, smooth muscle, most
exocrine glands, some endocrine glands, and adipose
tissue
– Somatic nervous system
• Subject to voluntary control
• Innervates skeletal muscle
ANS
• Autonomic nerve pathway
– Extends from CNS to an innervated organ
• Ganglion = neuronal cell bodies in the PNS
• Nuclei = neuronal cell bodies in the CNS
– Two-neuron chain
• Preganglionic fiber (synapses with cell body of second
neuron)
• Postganglionic fiber (innervates effector organ)
Sympathetic Nervous System Parasympathetic Nervous System
Fibers originate in thoracic and lumbar
regions of spinal cord
Fibers originate from cranial and sacral
areas of CNS
Most preganglionic fibers are short
Preganglionic fibers are longer
Long postganglionic fibers
Very short postganglionic fibers
Preganglionic fibers release
acetylcholine (Ach)
Preganglionic fibers release acetylcholine
(Ach)
Most postganglionic fibers release
noradrenaline (norepinephrine)
Postganglionic fibers release acetylcholine
•
•
•
Most visceral organs innervated by both sympathetic and parasympathetic
fibers
In general produce opposite effects in a particular organ
Dual innervation of organs by both branches of ANS allows precise control
over organ’s activity
ACh
ACh
Autonomic
effectors
Terminal
ganglion
Nicotinic Muscarinic
Cardiac
receptor receptor
muscle
α receptor
Sympathetic
ganglion chain
Nicotinic
receptor
NE
Adrenal
Medulla
E
NE Blood
E Smooth
muscle
β1
receptor
E
Collateral
ganglion
NE
Nicotinic
receptor
E
β2 receptor
ACh
KEY
Parasympathetic preganglionic fiber
Parasympathetic postganglionic fiber
Sympathetic preganglionic fiber
Sympathetic postganglionic fiber
ACh Acetylcholine
NE Norepinephrine
E Epinephrine
Terminal
ganglion
Nicotinic
receptor
Most
endocrine
glands
and some
endocrine
glands
Adipose
ACh tissue
Muscarinic
receptor
Fig. 7-2, p. 241
ANS
• Sympathetic system dominates in emergency or
stressful (“fight-or-flight”) situations
– Promotes responses that prepare body for
strenuous physical activity
• Parasympathetic system dominates in quiet, relaxed
(“rest-and-digest”) situations
– Promotes body-maintenance activities such as
digestion
Eye
Lacrimal gland
Nasal
mucosa
Parotid
gland
Sympathetic
Salivary
glands
Spinal
nerves
Sympathetic
trunk
Liver
Splanchino
nerves
Adrenal
gland
Parasympathetic
Trachea
Lung
Cranial
nerves
Heart
Stomach
Gall
bladder
Pancreas
Spleen
Sympathetic preganglionic fiber
Sympathetic postganglionic fiber
Spinal
nerves
Kidney
Small
intestine
Colon
Parasympathetic preganglionic
fiber
Parasympathetic postganglionic
fiber
Rectum
Urinary bladder
Genitalia
Fig. 7-3, p. 237
Table 7-02a p244
ANS
• Exceptions to general rule of dual reciprocal
innervation by the two branches of autonomic
nervous system
– Most arterioles and veins receive only
sympathetic nerve fibers (arteries and capillaries
are not innervated)
– Most sweat glands are innervated only by
sympathetic nerves
– Salivary glands are innervated by both ANS
divisions but activity is not antagonistic – both
stimulate salivary secretion
ANS
• Adrenal medulla is a modified part of sympathetic
nervous system
– Modified sympathetic ganglion that does not give
rise to postganglionic fibers
– Stimulation of preganglionic fiber prompts
secretion of hormones into blood
• About 20% of hormone release is norepinephrine
• About 80% of hormone released is epinephrine
(adrenaline)
• Broadcast vs. localized
Table 7-1 p242
ACh
ACh
Autonomic
effectors
Terminal
ganglion
Nicotinic Muscarinic
Cardiac
receptor receptor
muscle
α receptor
Sympathetic
ganglion chain
Nicotinic
receptor
NE
Adrenal
Medulla
E
NE Blood
E Smooth
muscle
β1
receptor
E
Collateral
ganglion
NE
Nicotinic
receptor
E
β2 receptor
ACh
KEY
Parasympathetic preganglionic fiber
Parasympathetic postganglionic fiber
Sympathetic preganglionic fiber
Sympathetic postganglionic fiber
ACh Acetylcholine
NE Norepinephrine
E Epinephrine
Terminal
ganglion
Nicotinic
receptor
Most
endocrine
glands
and some
endocrine
glands
Adipose
ACh tissue
Muscarinic
receptor
Fig. 7-2, p. 241
Autonomic Neurotransmitter Receptors
• Tissues innervated by autonomic nervous system have one
or more of several different receptor types for
postganglionic chemical messengers
– Alter tissue/cell response instead of chemical message
– Alter the distribution (localized – varicosity, broadcast –
adrenal medulla)
– Cholinergic receptors – bind to ACh
• Nicotinic receptors – found on postganglionic cell bodies of
all autonomic ganglia
– nicotine
• Muscarinic receptors – found on effector cell membranes
– Mushroom poison
– Adrenergic receptors – bind to norepinephrine and
epinephrine
• Alpha (α) receptors
• Beta (β) receptors
Autonomic Neurotransmitter Receptors
– Cholinergic receptors – bind to ACh
• Nicotinic receptors – found on postganglionic cell
bodies of all autonomic ganglia
– Nicotine
– Respond to Ach, opens Na and K channels, triggers AP
• Muscarinic receptors – found on effector cell
membranes
–
–
–
–
Mushroom poison
Smooth muscle, cardiac muscle, glands
5 subtypes
G protein linked
Autonomic Neurotransmitter Receptors
– Adrenergic receptors – bind to norepinephrine and
epinephrine
• G protein coupled
• Alpha (α) receptors - Ca second messenger system
- a 1= stimulates cyclic amp, sympathetic tissues, vessel
constriction, contraction of smooth muscle, constrictor
- a 2 blocks cyclic amp, decreases smooth muscle
contraction in the digestive tract, inhibitory to the effector
organ
• Beta (β) receptors - cyclic amp
- b1 Found in the heart, increases contractility and rate
- b2 Found in the arterioles and airways, dilator
Table 7-3 p246
Autonomic Agonists and Antagonists
• Agonists
– Bind to same receptor as neurotransmitter
– Elicit an effect that mimics that of
neurotransmitter
• Antagonists
– Bind with receptor
– Block neurotransmitter’s response
Distinguishing Characteristics of Sympathetic and
Parasympathetic Nervous Systems
• Somatic nervous system
– Anatomy
• Motor neurons
• Continuous to effector
• Voluntary
• Final common pathway
• ALS = Amyotropic lateral
sclerosis, death of motor
neurons
Outline
• Neuromuscular junction
– Anatomy
• Muscle fiber, terminal button
• “motor end plate”
– Ach release
• Presynaptic vesicular
• Postsynaptic intracellular
– Ach activation of end plate
potentials
• Activation of nicotinic
receptors
• Inflow of sodium and
potassium
• Ap propagated along the
muscle fiber
– Ach breakdown
• AChe
Somatic Nervous System
• Consists of axons of motor neurons of motor
neurons that originate in spinal cord or brain stem
and end on skeletal muscle
• Motor neuron releases neurotransmitter, Ach, which
stimulates muscle contraction
• Motor neurons are final common pathway by which
various regions of CNS exert control over skeletal
muscle activity
– These areas of CNS include spinal cord, motor
regions of cortex, basal nuclei, cerebellum,
and brain stem
Outline
• Neuromuscular junction
– Anatomy
• Muscle fiber, terminal button
• “motor end plate”
– Ach release
• Presynaptic vesicular
• Postsynaptic intracellular
– Ach activation of end plate potentials
• Activation of nicotinic receptors
• Inflow of sodium and potassium
• Ap propagated along the muscle fiber
– Ach breakdown
• AChe
Axons of two
efferent
motor neuron
Spinal cord
(section)
Axon
terminals
Muscle fibers
innervated by
red motor
neuron
Muscle fibers
innervated by
red motor
neuron
Axon
terminals
Terminal
buttons
Muscle
Neuromuscular
junction
Muscle fibers
Muscle Terminal Neuromuscular
fibers button
junction
Fig. 7-4, p. 251
Axon terminal of
motor neuron
Myelin sheath
Action potential
propagation
in motor neuron
1
Terminal button
Voltage-gated Vesicle of
Na+ channel acetylcholine
Voltage-gated
Ca2+ channel
Plasma membrane
of muscle fiber
8
Action potential
propagation
in muscle fiber
8
Ca2+
Na+
2
6
7
Acetylcholinesterase
Acetylcholine-gated
receptor-channel (for
nonspecific cation traffic)
7
6
3
4
5
K+
Na+
9
Na+
Motor end plate
Contractile elements within muscle fiber
Fig. 7-5, p. 252
Comparison of Somatic and Autonomic Nervous System
Neuromuscular Junction
• Acetylcholinesterase
– Inactivates ACh
– Ends end-plate potential and the action potential
and resultant contraction
• Neuromuscular junction is vulnerable to chemical
agents and diseases
– Black widow spider venom causes explosive
release of ACh
– Botulism toxin blocks release of ACh
– Curare blocks action of ACh at receptor sites
– Organophosphates prevent inactivation of ACh
– Myasthenia gravis inactivates ACh receptor sites
Neuromuscular conditions and compounds
• ALS
– Lou Gehrig’s Disease
• death of motor neurons
– Black widow venom
• Explosive release of Ach
• Paralysis of the diaphram
– Botulinum toxin
• Inhibits Ach release
– Curare
• Ach receptor blocker (chloinergic)
– Organophosphates
• Ache inhibition
– Mysanthia Gravis
• Autoimmune, body attacks Ach receptors