1. © 2016 Pearson Education, Inc.

2. Why This Matters
Understanding the peripheral nervous system help you to recognize and treat nerve damage
© 2016 Pearson Education, Inc.

3. Video: Why This Matters
© 2016 Pearson Education, Inc.

4. 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
© 2016 Pearson Education, Inc.

5. Motor (efferent) division
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
Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
© 2016 Pearson Education, Inc.

6. Part 1 – Sensory Receptors and Sensation
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
© 2016 Pearson Education, Inc.

7. 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)
© 2016 Pearson Education, Inc.

8. Classification by Stimulus Type (cont.)
Nociceptors—sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals)
© 2016 Pearson Education, Inc.

9. Classification by Location
Exteroceptors
Respond to stimuli arising outside body
Receptors in skin for touch, pressure, pain, and temperature
Most special sense organs
© 2016 Pearson Education, Inc.

10. 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
© 2016 Pearson Education, Inc.

11. 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
© 2016 Pearson Education, Inc.

12. 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
© 2016 Pearson Education, Inc.

13. 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
© 2016 Pearson Education, Inc.

14. 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
© 2016 Pearson Education, Inc.

15. 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
© 2016 Pearson Education, Inc.

16. 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
© 2016 Pearson Education, Inc.

17. 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
© 2016 Pearson Education, Inc.

18. Table 13.1-1 General Sensory Receptors Classified by Structure and Function
© 2016 Pearson Education, Inc.

19. 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
© 2016 Pearson Education, Inc.

20. 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
© 2016 Pearson Education, Inc.

21. Table 13.1-2 General Sensory Receptors Classified by Structure and Function (continued)
© 2016 Pearson Education, Inc.

22. 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
© 2016 Pearson Education, Inc.

23. 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
© 2016 Pearson Education, Inc.

24. General Organization of the Somatosensory System (cont.)
Levels of neural integration in sensory systems:
Receptor level: sensory receptors
Circuit level: processing in ascending pathways
Perceptual level: processing in cortical sensory areas
© 2016 Pearson Education, Inc.

25. © 2016 Pearson Education, Inc.
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
Cerebellum
Pons
Circuit level
(processing in
ascending pathways) 2
Medulla
Spinal cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception and
transmission to CNS) 1
Joint
kinesthetic
receptor
© 2016 Pearson Education, Inc.

26. 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
© 2016 Pearson Education, Inc.

27. 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
© 2016 Pearson Education, Inc.

28. 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)
© 2016 Pearson Education, Inc.

29. 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)
© 2016 Pearson Education, Inc.

30. 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)
© 2016 Pearson Education, Inc.

31. 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)
© 2016 Pearson Education, Inc.

32. 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
© 2016 Pearson Education, Inc.

33. 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
© 2016 Pearson Education, Inc.

34. Figure 13.3 Map of referred pain.
Lungs and
diaphragm
Heart
Gallbladder
Liver
Appendix
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
© 2016 Pearson Education, Inc.

35. 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
© 2016 Pearson Education, Inc.

36. 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
© 2016 Pearson Education, Inc.

37. 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
© 2016 Pearson Education, Inc.

38. Figure 13.4a Structure of a nerve.
Endoneurium
Perineurium
Nerve
fibers
Blood
vessel
Fascicle
Epineurium
© 2016 Pearson Education, Inc.

39. Figure 13.4b Structure of a nerve.
Axon
Endoneurium
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
© 2016 Pearson Education, Inc.

40. 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
© 2016 Pearson Education, Inc.

41. 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)
© 2016 Pearson Education, Inc.

42. 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)
© 2016 Pearson Education, Inc.

43. 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
© 2016 Pearson Education, Inc.

44. 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
© 2016 Pearson Education, Inc.

45. Regeneration of Nerve Fibers (cont.)
PNS axons
PNS axons can regenerate if damage is not severe
Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon
Macrophages clean dead axon debris; Schwann cells are stimulated to divide
Axon filaments grow through regeneration tube
Axon regenerates, and new myelin sheath forms
© 2016 Pearson Education, Inc.

46. Figure 13.5-1 Regeneration of a nerve fiber in a peripheral nerve.
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.
Endoneurium
Schwann cells
Droplets of
myelin
Fragmented
axon
Site of nerve damage
© 2016 Pearson Education, Inc.

47. Figure 13.5-2 Regeneration of a nerve fiber in a peripheral nerve.
Schwann cells and
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.
Schwann cell
Macrophage
© 2016 Pearson Education, Inc.

48. Figure 13.5-3 Regeneration of a nerve fiber in a peripheral nerve.
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.
Aligning Schwann cells form
regeneration tube
Fine axon sprouts
or filaments
© 2016 Pearson Education, Inc.

49. Figure 13.5-4 Regeneration of a nerve fiber in a peripheral nerve.
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.
Schwann cell
New myelin
sheath forming
Single enlarging
axon filament
© 2016 Pearson Education, Inc.