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The Peripheral Nervous
System (PNS)
Part 1
Peripheral Nervous System (PNS)
 Neural structures outside CNS, although soma may be
w/in CNS
 sensory receptors, peripheral nerves, ganglia, & motor
endings
 Divisions of PNS
 Afferent division (sensory)
 Efferent division (motor)
 Somatic – innervate skeletal muscle
 Autonomic – innervate smooth, cardiac, glands,
visceral organs
 Sympathetic – soma in spinal cord
 Parasympathetic – soma in midbrain or sacrum
Efferent Division
 Motor neurons of somatic division
 Terminate at motor end plate
 neuromuscular junction
 Motor neurons of autonomic division
 Varicosities in smooth & cardiac muscle & glands
Afferent Division - Sensory Neurons
 Sensory neuron termini (dendritic processes)
specialized to respond to stimuli
 Activation triggers impulses to CNS
 Perception in brain
Receptor Classification by Stimulus Type
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Mechanoreceptors – change in neuron shape
Thermoreceptors - temperature
Photoreceptors - light
Chemoreceptors - chemicals
Nociceptors – pain-causing stimuli (chemicals)
Receptor Classification by Location:
 Exteroceptors
 Near body surface
 Respond to stimuli arising outside body
 Include special sense organs
 Interoceptors
 Respond to stimuli arising w/in body
 Found in internal viscera & blood vessels
 Proprioceptors
 Respond to stretch
 In skeletal muscles, tendons, joints, ligaments, &
connective tissue
Receptor Classification by Structure
 Simple
 Dendritic process triggered directly by stimulus
 Encapsulated or unencapsulated
 nociceptors
 touch/pressure
 Complex
 Receptor cells w/in special sense organs
 Sensory neuron stimulated by bipolar neuron
 Photoreceptors
 Mechanoreceptors
 Olfactory receptors
 Gustatory receptors
Table 13.1.1
Table 13.1.2
Adaptation of Sensory Receptors
 sensory receptors subjected to unchanging stimulus
 Receptor membranes become less responsive
 Receptor potentials decline in frequency or stop
 Pressure, touch, & smell receptors adapt quickly
 Merkel’s discs, Ruffini’s corpuscles, & interoceptors
for blood chemicals adapt slowly
 Pain receptors & proprioceptors do not adapt
Structure of a Nerve
Classification of Nerves by Directionality
 Sensory (afferent) – signals TO CNS
 Motor (efferent) – signals FROM CNS
 Mixed –
 both sensory & motor
 most common
 somatic & autonomic signals
Regeneration of Nerve Fibers
 Mature neurons are amitotic
 If soma remains intact, neuron can regenerate
Figure 13.4
Cranial Nerves
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12 pairs of nerves directly from brain
Sensory, motor, or mixed
I - XII according to anterior level of origin
Named by to innervated organs/function
Four cranial nerves carry parasympathetic fibers
serving muscles & glands
Cranial Nerves
Summary of Cranial Nerves
Spinal Nerves
Figure 13.6
Spinal Nerve Roots
(ANS)
Figure 13.7a
Nerve Plexuses
 Interlacing nerve networks
 cervical
 brachial
 lumbar
 sacral
 Branches of plexus contain fibers from several nerves
 Every muscle innervated by multiple spinal nerves
Cervical Plexus
Figure 13.8
Brachial Plexus
Figure 13.9a
Brachial Plexus: Distribution of Nerves
Figure 13.9c
Spinal Nerve Innervation: Back, Anterolateral
Thorax, & Abdominal Wall
Figure 13.7b
Lumbar Plexus
Figure 13.10
Sacral Plexus
Figure 13.11
Reflexes
 Rapid, predictable motor response to a stimulus
 Reflexes:
 Intrinsic or acquired
 Involve only PNS & spinal cord
 Can relay to higher brain centers
 Somatic reflexes – skeletal muscle
 Autonomic reflexes – smooth muscle, glands
Reflex Arc Components
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Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Spinal cord
(in cross-section)
Stimulus
2 Sensory neuron
1
3 Integration
center
Receptor
4 Motor neuron
Skin
5 Effector
Interneuron
Stretch & Deep Tendon Reflexes
 Proprioceptors in tendons & muscle continually
maintain postural contractions & muscle tone
 These effects are via spinal reflex arcs
Muscle Spindles – Stretch Receptors
 Intrafusal muscle fibers
lacking myofilaments in
central regions
 Wrapped by type Ia &
type II fibers afferent
fibers
 Innervated by  efferent
fibers
Operation of Muscle Spindles
Stimulates action potential in
postsynaptic neuron
Does not stimulates action
potential in postsynaptic neuron
Stretch Reflex
Figure 13.17
Flexor & Crossed Extensor Reflex
Figure 13.19
The Autonomic Nervous
System
Autonomic Nervous System (ANS)
 Motor neurons that:
 Innervate smooth & cardiac muscle & glands
 Subconscious control
ANS differs from the SNS
 Effectors
 SNS – skeletal muscle
 ANS – non-skeletal muscle & gland cells
 Efferent pathways
 SNS – single PNS neuron
 ANS – 2 PNS neurons
 Target organ responses
 SNS – contraction of muscle
 ANS – contraction or relaxation, excretion
 Neurotransmitters used
 SNS – acetylcholine
 ANS – acetylcholine, norepinephrine & epinephrine
Distinctions of Efferent Pathways
 SNS motor neurons
 Single neuron extends from CNS to effector
 Heavily myelinated axons
 ANS motor neurons
 Two-neuron PNS chain
 Preganglionic neuron & postganglionic neuron
 Lightly myelinated preganglionic axon from CNS to
ganglion
 Unmyelinated postganglionic axon extends to
effector
Neurotransmitter Differences
 SNS neurons release acetylcholine (ACh), which has
an excitatory effect
 In the ANS:
 Preganglionic fibers release ACh
 Postganglionic fibers
 release norepinephrine or ACh
 effect is stimulatory or inhibitory
 effect depends on neurotransmitter receptor in
cells of effector tissue
Comparison of Somatic & Autonomic Systems
Figure 14.2
Anatomy of ANS
Long preganglionic
Short postganglionic
Short preganglionic
Long postganglionic
Ganglia on/in target
organ
Ganglia close to
spinal cord
Figure 14.3
Divisions of the ANS
 Sympathetic (SANS)
 mobilizes the body during stressful situations
 Parasympathetic (PANS)
 stimulates maintenance activities & conserves body
energy
 SANS & PANS counterbalance each other’s activity
 SANS signals usually override PANS
Examples of ANS Effects
 PANS
 Lowers BP, heart & respiratory rates
 Increases gastrointestinal tract activity
 Superficial arterioles open (smooth muscle relaxed)
 Pupils are constricted/dilated by light level only
 SANS
 Blood flow to organs/skin reduced, flow to muscles
increased
 Heart & respiratory rates increased
 Iris contracts - Pupils dilate
Parasympathetic Division Outflow
Cranial Outflow
Sacral Outflow
Cranial Nerve
Ganglion
Effector Organ(s)
Occulomotor (III)
Ciliary
Eye
Facial (VII)
Pterygopalatin
Submandibular
Salivary, nasal, &
lacrimal glands
Glossopharyngeal
(IX)
Otic
Parotid salivary glands
Vagus (X)
Located within the
walls of target organs
Heart, lungs, & most
visceral organs
S2-S4
Located within the
walls of the target
organs
Large intestine, urinary
bladder, ureters, &
reproductive organs
Parasympathetic Division Outflow
 Longer preganglionic axons
 Ganglion near/on target organ
 Short postganglionic axons
 Vagus nerve (CN X)
innervates all visceral organs
Sympathetic Outflow
 Sympathetic neurons in lateral horns of spinal cord
segments T1 through L2
 T1-T4 preganglionic fibers pass through the white
rami communicantes & synapse in sympathetic chain
ganglia
 T5-L2 preganglionic fibers pass through the gray rami
communicantes & chain ganglia to form splanchnic
nerves & synapse in collateral ganglia around
abdominal aorta
 Postganglionic fibers innervate the numerous organs
of the body
Sympathetic
Outflow
 Sympathetic neurons
in lateral horns of spinal
cord segments T1- L2
 T1-T4 preganglionic fibers
synapse in sympathetic
chain ganglia
 T5-L2 preganglionic fibers
form splanchnic nerves &
synapse in collateral
ganglia on abdominal
aorta
Sympathetic Trunks & Pathways
Pathways to the Head
 T1-T4 preganglionic axons synapse in the superior
cervical ganglion
 Serve skin & blood vessels of the head
 Stimulate dilator muscles of the iris
 Inhibit nasal & salivary gland secretions
Pathways to the Thorax
 T1-T6 preganglionic axons synapse in cervical chain ganglia
 Postganglionic axons from middle & inferior cervical ganglia
enter spinal nerves C4-C8 to innervate the heart, thyroid & skin of
neck
 Other T1-T6 preganglionic axons synapse in nearest chain ganglia
to directly serve the heart, aorta, lungs, & esophagus
Pathways with Synapses in Collateral Ganglia
 T5-L2 preganglionic axons exit sympathetic chain ganglia &
form splanchnic nerves
 Splanchnic nerves form aortic plexus & numerous ganglia
 Postganglionic axons from abdominal ganglia innervate
viscera
Pathways with Synapses in the Adrenal Medulla
 Axons of the thoracic splanchnic nerve go directly to
the adrenal medulla
 Upon stimulation, medullary cells secrete
norepinephrine & epinephrine into the blood
greater thoracic
splanchnic nerve
Visceral Reflexes
 Visceral reflexes have the same elements as somatic reflexes
 Afferent fibers are found in spinal & autonomic nerves
ANS Neurotransmitters
 PANS
 Acetylcholine (ACh) released by pre- & postganglionic axons
 SANS
 ACh released by preganglionic axons
 ACh or norepinephrine (NE) released by postganglionic axons
 Cholinergic fibers – ACh-releasing axons
 Adrenergic fibers –NE-releasing postganglionic SANS axons
 Excitatory or inhibitory effects depend upon the receptor type
Cholinergic Receptors
 Bind ACh
 Nicotinic receptors
 Muscarinic receptors
 Named & distinguished by interaction w/ agonists
 Nicotine
 Muscarine
 Agonist – stimulates effect
 Antagonist – blocks effect
Cholinergic Receptors
 Nicotinic Receptors
 Locations:
 Skeletal muscle motor end plates, CNS neurons
 SANS & PANS ganglionic neurons
 Adrenal medulla cells
 Ion channels
 ACh always stimulatory
 Muscarinic Receptors
 Locations
 Cells stimulated by postganglionic PANS fibers, CNS
 ACh inhibition or excitation depends on receptor subtype
 subtypes – M1, M2, M3
Adrenergic Receptors
 Receptors that bind to norepinephrin & epinephrine
 In cells innervated by SANS postganglionic axons
 Alpha
 subclasses - 1, 2,
 NE is stimulatory
 Beta
 Subclasses - 1, 2 , 3
 NE is generally inhibitory
 Exception – NE binding to  receptors of the heart is
stimulatory
Drugs that Influence the ANS
Table 14.4.1
Levels of ANS Control
Figure 14.9
Interactions of the Autonomic Divisions
 Most visceral organs innervated by both sympathetic
& parasympathetic fibers
 results in dynamic antagonisms that precisely control
visceral activity
 Sympathetic fibers increase heart & respiratory rates,
& inhibit digestion & elimination
 Parasympathetic fibers decrease heart & respiratory
rates, & allow for digestion & the discarding of
wastes