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PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part A
Copyright © 2010 Pearson Education, Inc.
Peripheral Nervous System (PNS)
• All neural structures OUTSIDE the brain and those starting in and
ending outside of the spinal cord
• Sensory nerves contain
• Neurons receiving info in senses, skin, organs, muscles
• Motor nerves contain
• neurons going to muscles and glands
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Structure of a nerve
Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
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Figure 13.3b
How is a nerve different than a neuron?
• A nerve is a group of axons
• Most nerves have both Sensory and Motor neurons
• Cranial nerves carry information to and from the
head and shoulders and brain
• Spinal nerves carry info to and from the body and the
spinal cord.
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Sensory “Receptors”
• Are just modified dendrites of sensory
neurons that respond to things other than just
chemical neurotransmitters
• They respond to changes in their environment
(stimuli) rather than NT.
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Perception:
Modified dendrites respond to many things
• Mechanoreceptors—respond to touch, pressure,
vibration, stretch, and itch
• Thermoreceptors—sensitive to changes in
temperature
• Photoreceptors—respond to light energy (e.g.,
retina)
• Chemoreceptors—respond to chemicals (e.g., smell,
taste, changes in blood chemistry)
• Nociceptors—sensitive to pain-causing stimuli (e.g.
extreme heat or cold, excessive pressure,
inflammatory chemicals)
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Receptors can be classified by Location
1. Exteroceptors
-Respond to stimuli arising outside the body on skin and in
eyes/ears/mouth/nose
2. Interoceptors (visceroceptors)
-Respond to stimuli from organs and blood vessels (low pressure=empty,
high pressure= full)
3. Proprioceptors
-Respond to stretch in skeletal muscles, tendons, joints
-they Inform the brain of where your arms and legs are.
-How do you know where your hand is with your eyes closed?
Proprioceptors!
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Receptors can be classified by how they are made
1. Unencapsulated dendrites
-free dendrites
-temperature, pain, chemicals
2. Encapsulated dendrites
-have specialized coverings
- to distinguish deep touch, light touch
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Copyright © 2010 Pearson Education, Inc.
Table 13.1
How do we perceive the environment?
•
Levels of neural integration in sensory
systems:
1. Receptor level—the sensory receptors
(dendrites or special receptors) (1st order)
2. Circuit level—ascending pathways through
spinal cord/brainstem (2nd order)
3. Perceptual level—neuronal circuits in the
cerebral cortex (3rd order)
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Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
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Figure 13.2
Adaptation of Sensory Receptors
• Have you ever gotten “used to” an itchy
sweater, tight necktie or bra?
• Adaptation is a change in sensitivity in the
presence of a constant stimulus
• Some receptors become less responsive over
time and may turn off.
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Adaptation of Sensory Receptors
• Phasic receptors signal adapt quickly
• only signal the beginning or end of a stimulus
• receptors for pressure, touch, and smell
• Tonic receptors adapt slowly or not at all
• Nociceptors (you want to be aware of pain)
• Proprioceptors (you want to know where
your limbs are)
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How does the brain know what stimuli are
coming in?
• Different senses GO to different places in the brain
• Eye nerves go to back of brain (vision)
• Touch nerves go to side of brain (feeling)
• So different parts of the brain expect different
information
• The more intense the stimulus
• the more action potentials are received
• The bigger the area it is received from
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Processing a the perceptual level
• When information gets to the brain it needs to
decode it:
• By the frequency of APs sent by one or more
neurons (how loud you shout!)
• By the number of neurons sending information from
a narrow or broad area of the body (how many people
are shouting)
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Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
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Figure 13.2
Pain is an important sense
• Warns of actual or impending tissue damage
• Stimuli include extreme pressure and temperature, and
chemicals
• Glutamate and substance P are NT that relay pain info
• Some pain impulses are blocked by inhibitory NT called
endorphins
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Visceral Pain
• Organs have pain, but there are very few sensory
receptors in organs. Stimulation of visceral organ
receptors is:
• Felt as vague aching, gnawing, burning
• Activated by tissue stretching, ischemia, chemicals,
muscle spasms
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Referred Pain
• Referred pain
• Pain from one body region perceived from different
region
• Since visceral and somatic pain fibers travel in same
nerves the brain assumes stimulus from commonly felt
areas, like skeletal muscles or skin
• Ex., left arm pain (commonly felt in life) is perceived during
heart even though the pain is actually from the heart (hardly
ever felt in life!).
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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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Regeneration of a nerve axon in a peripheral nerve. (1 of 4)
Endoneurium
Schwann cells
Droplets
of myelin
Fragmented
axon
Site of nerve damage
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1 The axon
becomes
fragmented at
the injury site.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (2 of 4)
Schwann cell
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Macrophage
2 Macrophages
clean out the dead
axon distal to the
injury.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (3 of 4)
Aligning Schwann cells
form regeneration tube
Fine axon sprouts
or filaments
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3 Axon sprouts,
or filaments, grow
through a
regeneration tube
formed by
Schwann cells.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (4 of 4)
Schwann cell
Single enlarging
axon filament
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New myelin
sheath forming
4 The axon
regenerates and a
new myelin sheath
forms.
Regeneration of damaged axons:summary
• If a cell body of a damaged nerve cell is still
intact, peripheral axon might regenerate
• The axon fragments (Wallerian degeneration)
• Macrophages clean dead axon
• The Schwann cells are still in place and form a
“regeneration tube” or tunnel.
• Axon filaments grow through regeneration tube
• Axon regenerates; new myelin sheath form
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Regeneration of CNS neurons
• Most CNS neurons never regenerate if damaged
• CNS oligodendrocytes bear growth-inhibiting
proteins that prevent CNS fiber regeneration
• Astrocytes at injury site form scar tissue
containing chondroitin sulfate that blocks axonal
regrowth
• Treatment
• Neutralizing growth inhibitors, blocking receptors for
inhibitory proteins, destroying chondroitin sulfate
promising
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