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Chapter 13 Part A
Peripheral
Nervous
System
© Annie Leibovitz/Contact Press Images
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PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
Why This Matters
• Understanding the peripheral nervous system
help you to recognize and treat nerve damage
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The Peripheral Nervous System
• PNS provides links from and to world outside
our body
• Consists of all neural structures outside brain
and spinal cord that can be broken down into
four parts:
Part 1 – Sensory Receptors
Part 2 – Transmission Lines: Nerves and Their
Structure and Repair
Part 3 – Motor Endings and Motor Activity
Part 4 – Reflex Activity
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Figure 13.1 Place of the PNS in the structural organization of the nervous system.
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Part 1 – Sensory Receptors and Sensation
13.1 Sensory Receptors
• Sensory receptors: specialized to respond to
changes in environment (stimuli)
– Activation results in graded potentials that trigger
nerve impulses
• Awareness of stimulus (sensation) and
interpretation of meaning of stimulus
(perception) occur in brain
• Three ways to classify receptors: by type of
stimulus, body location, and structural
complexity
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Classification by Stimulus Type
• Mechanoreceptors—respond to touch,
pressure, vibration, and stretch
• Thermoreceptors—sensitive to changes in
temperature
• Photoreceptors—respond to light energy
(example: retina)
• Chemoreceptors—respond to chemicals
(examples: smell, taste, changes in blood
chemistry)
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Classification by Stimulus Type (cont.)
• Nociceptors—sensitive to pain-causing stimuli
(examples: extreme heat or cold, excessive
pressure, inflammatory chemicals)
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Classification by Location
• Exteroceptors
– Respond to stimuli arising outside body
– Receptors in skin for touch, pressure, pain, and
temperature
– Most special sense organs
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Classification by Location (cont.)
• Interoceptors (visceroceptors)
– Respond to stimuli arising in internal viscera and
blood vessels
– Sensitive to chemical changes, tissue stretch,
and temperature changes
– Sometimes cause discomfort but usually person
is unaware of their workings
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Classification by Location (cont.)
• Proprioceptors
– Respond to stretch in skeletal muscles, tendons,
joints, ligaments, and connective tissue
coverings of bones and muscles
– Inform brain of one's movements
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Classification by Receptor Structure
• Majority of sensory receptors belong to one of
two categories:
– Simple receptors of the general senses
• Modified dendritic endings of sensory neurons
• Are found throughout body and monitor most types of
general sensory information
– Receptors for special senses
• Vision, hearing, equilibrium, smell, and taste
• All are housed in complex sense organs
• Covered in Chapter 15
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Classification by Receptor Structure (cont.)
• Simple receptors of the general senses
– General senses include tactile sensations
(touch, pressure, stretch, vibration), temperature,
pain, and muscle sense
• No “one-receptor-one-function” relationship
– Receptors can respond to multiple stimuli
– Receptors have either:
• Nonencapsulated (free) nerve endings or
• Encapsulated nerve endings
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Classification by Receptor Structure (cont.)
– Nonencapsulated (free) nerve endings
• Abundant in epithelia and connective tissues
• Most are nonmyelinated, small-diameter group C
fibers; distal terminals have knoblike swellings
• Respond mostly to temperature, pain, or light touch
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Classification by Receptor Structure (cont.)
– Nonencapsulated (free) nerve endings (cont.)
• Thermoreceptors
– Cold receptors are activated by temps from 10 to 40C
– Located in superficial dermis
– Heat receptors are activated from 32 to 48C located in
in deeper dermis
– Outside those temperature ranges, nociceptors are
activated and interpreted as pain
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Classification by Receptor Structure (cont.)
– Nonencapsulated (free) nerve endings (cont.)
• Nociceptors: pain receptors triggered by extreme
temperature changes, pinch, or release of chemicals
from damaged tissue
– Vanilloid receptor: protein in nerve membrane is main
player
» Acts as ion channel that is opened by heat, low pH,
chemicals (example: capsaicin in red peppers)
» Itch receptors in dermis: can be triggered by
chemicals such as histamine
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Classification by Receptor Structure (cont.)
– Nonencapsulated (free) nerve endings (cont.)
• Tactile (Merkel) discs: function as light touch
receptors
– Located in deeper layers of epidermis
• Hair follicle receptors: free nerve endings that wrap
around hair follicles
– Act as light touch receptors that detect bending of hairs
» Example: Allows you to feel a mosquito landing on
your skin
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Table 13.1-1 General Sensory Receptors Classified by Structure and Function
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Classification by Receptor Structure (cont.)
– Encapsulated dendritic endings
• Almost all are mechanoreceptors whose terminal
endings are encased in connective tissue capsule
• Vary greatly in shape and include:
– Tactile (Meissner’s) corpuscles: small receptors
involved in discriminative touch
» Found just below skin, mostly in sensitive and
hairless areas (fingertips)
– Lamellar (Pacinian) corpuscles: large receptors
respond to deep pressure and vibration when first
applied (then turn off)
» Located in deep dermis
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Classification by Receptor Structure (cont.)
– Bulbous corpuscles (Ruffini endings): respond to
deep and continuous pressure
» Located in dermis
– Muscle spindles: spindle-shaped proprioceptors that
respond to muscle stretch
– Tendon organ: proprioceptors located in tendons that
detect stretch
– Joint kinesthetic receptors: proprioceptors that
monitor joint position and motion
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Table 13.1-2 General Sensory Receptors Classified by Structure and Function (continued)
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13.2 Sensory Processing
• Survival depends upon:
– Sensation: the awareness of changes in the
internal and external environment
– Perception: the conscious interpretation of those
stimuli
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General Organization of the Somatosensory
System
• Somatosensory system: part of sensory
system serving body wall and limbs
• Receives inputs from:
– Exteroceptors, proprioceptors, and
interoceptors
• Input is relayed toward head, but processed
along the way
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General Organization of the Somatosensory
System (cont.)
• Levels of neural integration in sensory systems:
1. Receptor level: sensory receptors
2. Circuit level: processing in ascending
pathways
3. Perceptual level: processing in cortical
sensory areas
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Figure 13.2 Three basic levels of neural integration in sensory systems.
3 Perceptual level (processing in cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
2 Circuit level
(processing in
ascending pathways)
Cerebellum
Pons
Medulla
Spinal cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
1 Receptor level
(sensory reception and
transmission to CNS)
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Joint
kinesthetic
receptor
General Organization of the Somatosensory
System (cont.)
• Processing at the receptor level
– Generating a signal: For sensation to occur, the
stimulus must excite a receptor, and the AP must
reach CNS
• Stimulus energy must match receptor specificity
(touch receptors do not respond to light)
• Stimulus must be applied within receptive field
• Transduction must occur—energy of stimulus is
converted into graded potential called generator
potential (in general receptors) or receptor potential
(in special sense receptors)
• Graded potentials must reach threshold → AP
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General Organization of the Somatosensory
System (cont.)
– Adaptation: Change in sensitivity in presence of
constant stimulus
• Receptor membranes become less responsive
• Receptor potentials decline in frequency or stop
• Phasic receptors: (fast-adapting) send signals at
beginning or end of stimulus
– Examples: receptors for pressure, touch, and smell
• Tonic receptors: adapt slowly or not at all
– Examples: nociceptors and most proprioceptors
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General Organization of the Somatosensory
System (cont.)
• Processing at the circuit level
– Pathways of three neurons conduct sensory
impulses received from receptors upward to
appropriate cortical regions
– First-order sensory neurons
• Conduct impulses from receptor level to spinal
reflexes or second-order neurons in CNS
– Second-order sensory neurons
• Transmit impulses to third-order sensory neurons
– Third-order sensory neurons
• Conduct impulses from thalamus to the
somatosensory cortex (perceptual level)
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General Organization of the Somatosensory
System (cont.)
• Processing at the perceptual level
– Interpretation of sensory input depends on
specific location of target neurons in sensory
cortex
– Aspects of sensory perception:
• Perceptual detection: ability to detect a stimulus
(requires summation of impulses)
• Magnitude estimation: intensity coded in frequency
of impulses
• Spatial discrimination: identifying site or pattern of
stimulus (studied by two-point discrimination test)
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General Organization of the Somatosensory
System (cont.)
• Processing at the perceptual level (cont.)
– Feature abstraction: identification of more
complex aspects and several stimulus properties
– Quality discrimination: ability to identify
submodalities of a sensation (e.g., sweet or sour
tastes)
– Pattern recognition: recognition of familiar or
significant patterns in stimuli (e.g., melody in
piece of music)
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Perception of Pain
• Warns of actual or impending tissue damage so
protective action can be taken
• Stimuli include extreme pressure and
temperature, histamine, K+, ATP, acids, and
bradykinin
• Impulses travel on fibers that release
neurotransmitters glutamate and substance P
• Some pain impulses are blocked by inhibitory
endogenous opioids (example: endorphins)
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Perception of Pain (cont.)
• Pain tolerance
– All perceive pain at same stimulus intensity
– Pain tolerance varies
– “Sensitive to pain” means low pain tolerance, not
low pain threshold
– Genes help determine pain tolerance as well as
response to pain medications
• Research in use of genetics to determine best pain
treatment is ongoing
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Perception of Pain (cont.)
• Visceral and referred pain
– Visceral pain results from stimulation of visceral
organ receptors
• Felt as vague aching, gnawing, burning
• Activated by tissue stretching, ischemia, chemicals,
muscle spasms
– Referred pain: pain from one body region
perceived as coming from different region
• Visceral and somatic pain fibers travel along same
nerves, so brain assumes stimulus comes from
common (somatic) region
– Example: left arm pain during heart attack
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Figure 13.3 Map of referred pain.
Lungs and
diaphragm
Heart
Gallbladder
Appendix
Liver
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
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Clinical – Homeostatic Imbalance 13.1
• Long-lasting or intense pain, such as limb
amputation, can lead to hyperalgesia (pain
amplification), chronic pain, and phantom limb
pain
– NMDA receptors are activated by long-lasting or
intense pain
• Allow spinal cord to “learn” hyperalgesia
• Early pain management critical to prevent
• Phantom limb pain: pain felt in limb that has
been amputated
– Now use epidural anesthesia during surgery to
reduce phantom pain
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Part 2 – Transmission Lines: Nerves and
Their Structure and Repair
13.3 Nerves and Associated Ganglia
Structure and Classification
• Nerve: cordlike organ of PNS
• Bundle of myelinated and nonmyelinated
peripheral axons enclosed by connective tissue
• Two types of nerves: spinal or cranial,
depending on where they originate
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Structure and Classification (cont.)
• Connective tissue coverings include:
– Endoneurium: loose connective tissue that
encloses axons and their myelin sheaths
(Schwann cells)
– Perineurium: coarse connective tissue that
bundles fibers into fascicles
– Epineurium: tough fibrous sheath around all
fascicles to form the nerve
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Figure 13.4a Structure of a nerve.
Endoneurium
Perineurium
Nerve
fibers
Blood
vessel
Fascicle
Epineurium
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Figure 13.4b Structure of a nerve.
Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
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Structure and Classification (cont.)
• Most nerves are mixtures of afferent and efferent
fibers and somatic and autonomic (visceral) fibers
• Nerves are classified according to the direction they
transmit impulses
– Mixed nerves: contain both sensory and motor fibers
• Impulses travel both to and from CNS
– Sensory (afferent) nerves: impulses only toward
CNS
– Motor (efferent) nerves: impulses only away from
CNS
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Structure and Classification (cont.)
• Pure sensory (afferent) or pure motor (efferent)
nerves are rare; most nerves are mixed
• Types of fibers in mixed nerves:
– Somatic afferent (sensory from muscle to brain)
– Somatic efferent (motor from brain to muscle)
– Visceral afferent (sensory from organs to brain)
– Visceral efferent (motor from brain to organs)
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Structure and Classification (cont.)
• Ganglia: contain neuron cell bodies associated
with nerves in PNS
– Ganglia associated with afferent nerve fibers
contain cell bodies of sensory neurons
• Dorsal root ganglia (sensory, somatic) (Chapter 12)
– Ganglia associated with efferent nerve fibers
contain autonomic motor neurons
• Autonomic ganglia (motor, visceral) (Chapter 14)
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Regeneration of Nerve Fibers
• Mature neurons are amitotic, but if the soma (cell
body) of the damaged nerve is intact, the
peripheral axon may regenerate in PNS; does
not occur in CNS
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Regeneration of Nerve Fibers (cont.)
• CNS axons
– Most CNS fibers never regenerate
– CNS oligodendrocytes bear growth-inhibiting
proteins that prevent CNS fiber regeneration
– Astrocytes at injury site form scar tissue
– Treatment: neutralizing growth inhibitors, blocking
receptors for inhibitory proteins, destroying scar
tissue components
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Regeneration of Nerve Fibers (cont.)
• PNS axons
– PNS axons can regenerate if damage is not
severe
1. Axon fragments and myelin sheaths distal to injury
degenerate (Wallerian degeneration); degeneration
spreads down axon
2. Macrophages clean dead axon debris; Schwann
cells are stimulated to divide
3. Axon filaments grow through regeneration tube
4. Axon regenerates, and new myelin sheath forms
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Figure 13.5-1 Regeneration of a nerve fiber in a peripheral nerve.
Endoneurium
Schwann cells
Droplets of
myelin
Fragmented
axon
Site of nerve damage
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1 The axon fragments.
• The cut axon ends seal
themselves off.
• Axon transport is interrupted,
causing the cut ends to swell.
• Without access to the cell
body, the axon (and its myelin
sheath) begins to disintegrate
distal to the injury.
• Degeneration of the distal
end of the cut axon, called
Wallerian degeneration,
spreads down the axon.
Figure 13.5-2 Regeneration of a nerve fiber in a peripheral nerve.
2 Schwann cells and
Schwann cell
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Macrophage
macrophages clean out the
dead axon distal to the injury.
• Surviving Schwann cells
engulf the myelin fragments
and secrete chemicals that
recruit macrophages.
• Macrophages help dispose
of the debris and release
chemicals that stimulate
Schwann cells to divide.
Figure 13.5-3 Regeneration of a nerve fiber in a peripheral nerve.
Aligning Schwann cells form
regeneration tube
Fine axon sprouts
or filaments
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3 Axon filaments grow through
a regeneration tube.
• Schwann cells release growth
factors and express cell
adhesion molecules (CAMs)
that encourage axon growth.
• Schwann cells line up along
the tube of remaining
endoneurium, forming a
regeneration tube that
guides the regenerating
axon “sprouts” across the gap
to their original contacts.
Figure 13.5-4 Regeneration of a nerve fiber in a peripheral nerve.
Schwann cell
Single enlarging
axon filament
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New myelin
sheath forming
4 The axon regenerates and a
new myelin sheath forms.
• The Schwann cells protect
and support the
regenerating axon and
ultimately produce a new
myelin sheath.