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
Table 7-1, p. 234
ANS
• Autonomic nerve pathway
– Extends from CNS to an innervated organ
• Ganglion = neuronal cell bodies in the PNS
• Nuclei = neuronal cell bodies in the PNS CNS
– Two-neuron chain
• Preganglionic fiber (synapses with cell body of second
neuron)
• Postganglionic fiber (innervates effector organ)
ANS
Sympathetic
Nervous
System
• Two subdivisions
– Sympathetic nervous Fibers originate in
system
thoracic and lumbar
–
Parasympathetic
Nervous System
Fibers originate from
cranial and sacral areas
• Thoracic and lumbar regions of spinal cord of CNS
Most preganglionic
Preganglionic fibers are
Parasympathetic
fibers are short
longer
nervous system
• Cervical and sacral
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
Brain
ACh
ACh Effector
organs
Terminal
ganglion
Spinal
cord
ACh
Cardiac
muscle
Craniosacral
parasympathetic
nerves
= Parasympathetic
system
= Preganglionic fiber
= Postganglionic fiber
NE
Sympathetic ganglion chain
= Sympathetic system
Smooth
muscle
= Acetylcholine
= Norepinephrine
= Epinephrine
Thoracolumbar
sympathetic
nerves
Adrenal
medulla
= Cell body
= Cell body
= Axon
Blood
E,NE
NE
ACh
Collateral
ganglion
ACh
Terminal
ganglion
Most
exocrine
glands
and
some
endocrine
ACh
glands
Fig. 7-2, p. 235
ANS
• 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
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
Effects of Autonomic Nervous System on Various Organs
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
Autonomic Neurotransmitters
Brain
ACh
ACh Effector
organs
Terminal
ganglion
Spinal
cord
ACh
Cardiac
muscle
Craniosacral
parasympathetic
nerves
= Parasympathetic
system
= Preganglionic fiber
= Postganglionic fiber
NE
Sympathetic ganglion chain
= Sympathetic system
Smooth
muscle
= Acetylcholine
= Norepinephrine
= Epinephrine
Thoracolumbar
sympathetic
nerves
Adrenal
medulla
= Cell body
= Cell body
= Axon
Blood
E,NE
NE
ACh
Collateral
ganglion
ACh
Terminal
ganglion
Most
exocrine
glands
and
some
endocrine
ACh
glands
Fig. 7-2, p. 235
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
– Andrenergic 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
– Andrenergic receptors – bind to norepinephrine and
epinephrine
• G protein coupled
• Alpha (α) receptors - Ca second messenger system
 a 1= stim cyclic amp, sympathetic tissues, vessel
constriction, contraction of smooth muscle, constrictor
 a 2 blocks cyclic amp, decreased 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
Spinal cord
= Acetylcholine
= Norepinephrine
Sympathetic
preganglionic
fiber
= Epinephrine
Adrenal
medulla
Sympathetic
postganglionic
fiber
norepi
Blood
both
epi
Fig. 7-4, p. 239
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
Regions of CNS Involved in Control of
Autonomic Activities
• Can be influenced by prefrontal association complex
through its involvement with emotional expression
characteristic of individual’s personality
• Hypothalamus plays important role in integrating
autonomic, somatic, and endocrine responses that
automatically accompany various emotional and
behavioral states
• Medulla within brain stem is region directly
responsible for autonomic output
• Some autonomic reflexes, such as urination,
defecation, and erection, are integrated at spinal
cord level
Distinguishing Characteristics of Sympathetic and
Parasympathetic Nervous Systems
Outline
• Somatic nervous system
– Anatomy
•
•
•
•
•
Motor neurons
Continuous to effector
Voluntary
Final common pathway
ALS = Amyotropic lateral sclerosis, death of motor
neurons
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
Comparison of Somatic and Autonomic Nervous System
Spinal cord
= Acetylcholine
= Norepinephrine
Sympathetic
preganglionic
fiber
= Epinephrine
Adrenal
medulla
Sympathetic
postganglionic
fiber
Blood
Target organs
Fig. 7-4, p. 239
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
Neuromuscular Junction
• Axon terminal of motor neuron forms neuromuscular junction
with a single muscle cell
• Signals are passed between nerve terminal and muscle fiber
by means of neurotransmitter ACh
• Released ACh binds to receptor sites on motor end plate of
muscle cell membrane
• Binding triggers opening of specific channels in motor end
plate
• Ion movements depolarize motor end plate, producing endplate potential
• Local current flow between depolarized end plate and
adjacent muscle cell membrane brings adjacent areas to
threshold
• Action potential is initiated and propagated throughout muscle
fiber
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
Axon of motor neuron
Action potential
propagation
in motor neuron
Myelin sheath
Axon terminal
Terminal button
Voltage-gated
calcium channel
Vesicle of acetylcholine
Acetylcholine
receptor site
Action potential
propagation
in muscle fiber
Acetycholinesterase
Plasma membrane
of muscle fiber
Voltage-gated
Na+ channel
Chemically gated
cation channel
Motor end plate
Contractile elements within muscle fiber
Fig. 7-6, p. 245
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
– Rigor mortis