Transcript Chapter 13 - FacultyWeb Support Center

Chapter 13
The Peripheral
Nervous System And
Reflex Activity
Part A
Shilla Chakrabarty, Ph.D.
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Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
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
Sensory Receptors
• Specialized to respond to changes in their environment
(stimuli)
• Activation results in graded potentials that trigger nerve
impulses
• Sensation (awareness of stimulus) and perception
(interpretation of the meaning of the stimulus) occur in the
brain
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Classification of Receptors
• Based on:
Stimulus type
Location
Structural complexity
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Classification by Stimulus Type
• 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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Classification by Location
1.
Exteroceptors
•
Respond to stimuli arising outside the body
•
Receptors in the skin for touch, pressure, pain, and temperature
•
Most special sense organs
2.
Interoceptors (visceroceptors)
•
Respond to stimuli arising in internal viscera and blood vessels
•
Sensitive to chemical changes, tissue stretch, and temperature
changes
3.
Proprioceptors
•
Respond to stretch in skeletal muscles, tendons, joints,
ligaments, and connective tissue coverings of bones and
muscles
•
Inform the brain of one’s movements
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Classification by Structural Complexity
1. Complex receptors (special sense organs)
•
Vision, hearing, equilibrium, smell, and taste
(Chapter 15)
2. Simple receptors for general senses:
•
Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle
sense
•
Unencapsulated (free) or encapsulated
dendritic endings
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Unencapsulated Dendritic Endings
1. Thermoreceptors
• Cold receptors (10–40ºC); in superficial dermis
• Heat receptors (32–48ºC); in deeper dermis
2. Nociceptors
Respond to:
• Pinching
• Chemicals from damaged tissue
• Temperatures outside the range of thermoreceptors
• Capsaicin
3. Light touch receptors
• Tactile (Merkel) discs
• Hair follicle receptors
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Table 13.1
Encapsulated Dendritic Endings
• All are mechanoreceptors
• Meissner’s (tactile) corpuscles—discriminative touch
• Pacinian (lamellated) corpuscles—deep pressure and
vibration
• Ruffini endings—deep continuous pressure
• Muscle spindles—muscle stretch
• Golgi tendon organs—stretch in tendons
• Joint kinesthetic receptors—stretch in articular
capsules
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Table 13.1
From Sensation to Perception
• Survival depends upon sensation and
perception
• Sensation: the awareness of changes in the
internal and external environment
• Perception: the conscious interpretation of
those stimuli
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Sensory Integration
• Input comes from exteroceptors,
proprioceptors, and interoceptors
• Input is relayed toward the head, but is
processed along the way
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Sensory Integration
•
Levels of neural integration in sensory systems:
1. Receptor level—the sensor receptors
2. Circuit level—ascending pathways
3. Perceptual level—neuronal circuits in the cerebral
cortex
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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
Processing at the Receptor Level
• Receptors have specificity for stimulus energy
• Stimulus must be applied in a receptive field
• Transduction occurs
• Stimulus energy is converted into a graded
potential called a receptor potential
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Processing at the Receptor Level
• In general sense receptors, the receptor
potential and generator potential are the same
thing
stimulus

receptor/generator potential in afferent neuron

action potential at first node of Ranvier
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Processing at the Receptor Level
• In special sense organs:
stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)
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Adaptation of Sensory Receptors
• Adaptation is a change in sensitivity in the presence of a
constant stimulus
 Receptor membranes become less responsive
 Receptor potentials decline in frequency or stop
• Phasic (fast-adapting) receptors signal the beginning or
end of a 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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Processing at the Circuit Level
• Pathways of three neurons conduct sensory impulses
upward to the appropriate brain regions
• First-order neurons
 Conduct impulses from the receptor level to the
second-order neurons in the CNS
• Second-order neurons
• Transmit impulses to the thalamus or cerebellum
• Third-order neurons
• Conduct impulses from the thalamus to the
somatosensory cortex (perceptual level)
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Processing at the Perceptual Level
• Identification of the sensation depends on the
specific location of the target neurons in the sensory
cortex
• Aspects of sensory perception:
 Perceptual detection—ability to detect a stimulus
(requires summation of impulses)
 Magnitude estimation—intensity is coded in the
frequency of impulses
 Spatial discrimination—identifying the site or pattern of
the stimulus (studied by the two-point discrimination
test)
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Main Aspects of Sensory Perception
• Feature abstraction—identification of more complex
aspects and several stimulus properties
• Quality discrimination—the 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., the melody in a piece of music)
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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
Perception of Pain
• Warns of actual or impending tissue damage
• 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
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Structure of a Nerve
• Cordlike organ of the PNS
• Bundle of myelinated and unmyelinated peripheral axons
enclosed by connective tissue
• Connective tissue coverings include:
 Endoneurium—loose connective tissue that encloses
axons and their myelin sheaths
 Perineurium—coarse connective tissue that bundles
fibers into fascicles
 Epineurium—tough fibrous sheath around a nerve
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Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
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Figure 13.3b
Classification of Nerves
• Most nerves are mixtures of afferent and efferent
fibers and somatic and autonomic (visceral)
fibers
• Pure sensory (afferent) or motor (efferent) nerves
are rare
• Types of fibers in mixed nerves:
Somatic afferent and somatic efferent
Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal
nerves
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Ganglia
• Contain neuron cell bodies associated with
nerves
• Dorsal root ganglia (sensory, somatic)
(Chapter 12)
• Autonomic ganglia (motor, visceral)
(Chapter 14)
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Regeneration of Nerve Fibers
• Mature neurons are amitotic
• If the soma of a damaged nerve is intact, axon will
regenerate
• Involves coordinated activity among:
 Macrophages—remove debris
 Schwann cells—form regeneration tube and secrete
growth factors
 Axons—regenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting proteins
that prevent CNS fiber regeneration
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Endoneurium
Schwann cells
Droplets
of myelin
1 The axon
becomes
fragmented at
the injury site.
Fragmented
axon
Site of nerve damage
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Figure 13.4 (1 of 4)
Schwann cell
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Macrophage
2 Macrophages
clean out the
dead axon distal
to the injury.
Figure 13.4 (2 of 4)
Aligning Schwann cells
form regeneration tube
3 Axon sprouts,
or filaments,
grow through a
regeneration tube
formed by
Schwann cells.
Fine axon sprouts
or filaments
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Figure 13.4 (3 of 4)
Schwann cell
Site of new
myelin sheath
formation
4 The axon
regenerates and
a new myelin
sheath forms.
Single enlarging
axon filament
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Figure 13.4 (4 of 4)