Peripheral Nervous System (PNS)

1
Peripheral Nervous System (PNS)

• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings

2
Central nervous system (CNS)
Peripheral nervous system (PNS)
Motor (efferent) division
Sensory (afferent) division
Somatic nervous system
Autonomic nervous system (ANS)
Sympathetic division
Parasympathetic division
Figure 13.1
3
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

4
Classification of Receptors

• Based on
• Stimulus type
• Location
• Structural complexity

5
Classification by Stimulus Type

• Mechanoreceptorsrespond to touch, pressure,
  vibration, stretch, and itch
• Thermoreceptorssensitive to changes in
  temperature
• Photoreceptorsrespond to light energy (e.g.,
  retina)
• Chemoreceptorsrespond to chemicals (e.g., smell,
  taste, changes in blood chemistry)
• Nociceptorssensitive to pain-causing stimuli
  (e.g. extreme heat or cold, excessive pressure,
  inflammatory chemicals)

6
Classification by Location

• Exteroceptors
• Respond to stimuli arising outside the body
• Receptors in the skin for touch, pressure, pain,
  and temperature
• Most special sense organs

7
Classification by Location

• Interoceptors (visceroceptors)
• Respond to stimuli arising in internal viscera
  and blood vessels
• Sensitive to chemical changes, tissue stretch,
  and temperature changes

8
Classification by Location

• Proprioceptors
• Respond to stretch in skeletal muscles, tendons,
  joints, ligaments, and connective tissue
  coverings of bones and muscles
• Inform the brain of ones movements

9
Classification by Structural Complexity

• Complex receptors (special sense organs)
• Vision, hearing, equilibrium, smell, and taste
  (Chapter 15)
• 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

• Thermoreceptors
• Cold receptors (1040ºC) in superficial dermis
• Heat receptors (3248ºC) in deeper dermis

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Unencapsulated Dendritic Endings

• Nociceptors
• Respond to
• Pinching
• Chemicals from damaged tissue
• Temperatures outside the range of thermoreceptors
• Capsaicin

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Unencapsulated Dendritic Endings

• Light touch receptors
• Tactile (Merkel) discs
• Hair follicle receptors

13
Table 13.1
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Encapsulated Dendritic Endings

• All are mechanoreceptors
• Meissners (tactile) corpusclesdiscriminative
  touch
• Pacinian (lamellated) corpusclesdeep pressure
  and vibration
• Ruffini endingsdeep continuous pressure
• Muscle spindlesmuscle stretch
• Golgi tendon organsstretch in tendons
• Joint kinesthetic receptorsstretch in articular
  capsules

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Table 13.1
16
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
• Receptor levelthe sensor receptors
• Circuit levelascending pathways
• Perceptual levelneuronal circuits in the
  cerebral cortex

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Perceptual level (processing in cortical
sensory centers)
3
Motor cortex
Somatosensory cortex
Thalamus
Reticular formation
Cerebellum
Pons
Medulla
Circuit level (processing in ascending
pathways)

2
Spinal cord
Free nerve endings (pain, cold, warmth)
Muscle spindle
Receptor level (sensory reception and
transmission to CNS)
1
Joint kinesthetic receptor
Figure 13.2
20
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

21
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

22
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

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Adaptation of Sensory Receptors

• 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 detectionability to detect a stimulus
  (requires summation of impulses)
• Magnitude estimationintensity is coded in the
  frequency of impulses
• Spatial discriminationidentifying the site or
  pattern of the stimulus (studied by the two-point
  discrimination test)

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Main Aspects of Sensory Perception

• Feature abstractionidentification of more
  complex aspects and several stimulus properties
• Quality discriminationthe ability to identify
  submodalities of a sensation (e.g., sweet or sour
  tastes)
• Pattern recognitionrecognition 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
Cerebellum
Pons
Medulla
Circuit level (processing in ascending
pathways)

2
Spinal cord
Free nerve endings (pain, cold, warmth)
Muscle spindle
Receptor level (sensory reception and
transmission to CNS)
1
Joint kinesthetic receptor
Figure 13.2
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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

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Structure of a Nerve

• Connective tissue coverings include
• Endoneuriumloose connective tissue that encloses
  axons and their myelin sheaths
• Perineuriumcoarse connective tissue that bundles
  fibers into fascicles
• Epineuriumtough fibrous sheath around a nerve

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Axon
Myelin sheath
Endoneurium
Perineurium
Epineurium
Fascicle
Blood vessels
(b)
Figure 13.3b
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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
• Macrophagesremove debris
• Schwann cellsform regeneration tube and secrete
  growth factors
• Axonsregenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting
  proteins that prevent CNS fiber regeneration

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Endoneurium
Schwann cells
The axon becomes fragmented at the injury
site.
1
Droplets of myelin
Fragmented axon
Site of nerve damage
Figure 13.4 (1 of 4)
37
Macrophages clean out the dead axon distal to
the injury.
2
Schwann cell
Macrophage
Figure 13.4 (2 of 4)
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Axon sprouts, or filaments, grow through
a regeneration tube formed by Schwann cells.
3
Aligning Schwann cells form regeneration tube
Fine axon sprouts or filaments
Figure 13.4 (3 of 4)
39
The axon regenerates and a new myelin sheath
forms.
4
Site of new myelin sheath formation
Schwann cell
Single enlarging axon filament
Figure 13.4 (4 of 4)
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Cranial Nerves

• Twelve pairs of nerves associated with the brain
• Most are mixed in function two pairs are purely
  sensory
• Each nerve is identified by a number (I through
  XII) and a name
• On occasion, our trusty truck acts funnyvery
  good vehicle anyhow

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Filaments of olfactory nerve (I)
Frontal lobe
Olfactory bulb
Olfactory tract
Optic nerve (II)
Temporal lobe
Optic chiasma
Infundibulum
Optic tract
Facial nerve (VII)
Oculomotor nerve (III)
Trochlear nerve (IV)
Vestibulo- cochlear nerve (VIII)
Trigeminal nerve (V)
Glossopharyngeal nerve (IX)
Abducens nerve (VI)
Vagus nerve (X)
Cerebellum
Accessory nerve (XI)
Medulla oblongata
Hypoglossal nerve (XII)
(a)
Figure 13.5 (a)
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Cranial nerves I VI
Sensory function
Motor function
PS fibers
I
Olfactory
Yes (smell)
No
No
II
Optic
Yes (vision)
No
No
III
Oculomotor
No
Yes
Yes
IV
Trochlear
No
Yes
No
V
Trigeminal
Yes (general sensation)
Yes
No
VI
Abducens
No
Yes
No
Cranial nerves VII XII
Sensory function
Motor function
PS fibers
VII
Facial
Yes (taste)
Yes
Yes
VIII
Vestibulocochlear
Yes (hearing and balance)
Some
No
IX
Glossopharyngeal
Yes (taste)
Yes
Yes
X
Vagus
Yes (taste)
Yes
Yes
XI
Accessory
No
Yes
No
XII
Hypoglossal
No
Yes
No

PS parasympathetic
(b)
Figure 13.5 (b)
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I The Olfactory Nerves

• Arise from the olfactory receptor cells of nasal
  cavity
• Pass through the cribriform plate of the ethmoid
  bone
• Fibers synapse in the olfactory bulbs
• Pathway terminates in the primary olfactory
  cortex
• Purely sensory (olfactory) function

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Table 13.2
45
II The Optic Nerves

• Arise from the retinas
• Pass through the optic canals, converge and
  partially cross over at the optic chiasma
• Optic tracts continue to the thalamus, where they
  synapse
• Optic radiation fibers run to the occipital
  (visual) cortex
• Purely sensory (visual) function

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Table 13.2
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III The Oculomotor Nerves

• Fibers extend from the ventral midbrain through
  the superior orbital fissures to the extrinsic
  eye muscles
• Functions in raising the eyelid, directing the
  eyeball, constricting the iris (parasympathetic),
  and controlling lens shape

48
Table 13.2
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IV The Trochlear Nerves

• Fibers from the dorsal midbrain enter the orbits
  via the superior orbital fissures to innervate
  the superior oblique muscle
• Primarily a motor nerve that directs the eyeball

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Table 13.2