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The Peripheral Nervous
System (PNS)
Part A
Human Anatomy & Physiology, Sixth Edition
Elaine N. Marieb
13
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